Why are many of the latest fighter aircraft designs aerodynamically unstable?
the_dr
work. The obvious answer is because it gives the aircraft the
potential to be more agile (when artificially stabilised by
computer)...but is the fundamental reason fuel economy? Basically the
premise of the latter argument was :
In *very* general terms, when an aircraft goes supersonic, the
aerodynamic centre moves rearwards. This requires a large trim change
to maintain stability. This in turn generates a large amount of trim
drag, which in turn greatly increases fuel consumption. If however
the aircraft is designed to be stable at supersonic speeds it will
require less or no trim drag, and use far less fuel. It will however
be unstable at subsonic speeds, hence the requirement for computer
stabilisation. The corresponding fuel increase at subsonic speeds is
the lesser of the two evils. It was proposed that an aircraft such as
the Typhoon would be incapable of supercruise unless it was designed
to be inherently stable at supersonic speeds, and the agility benefits
of its instability at subsonic speeds are nothing more than a positive
side effect.
Discuss!
Roger Conroy
"the_dr" <dr_nic...@yahoo.com> wrote in message
news:335bff4a-604e-4248...@j4g2000yqh.googlegroups.com...
Start by putting a "citation needed" tag on the implied claim that the
Typhoon is in fact inherently stable at supersonic speeds.
Ed Rasimus
While there is a trim change to compensate for the shift of C/L, the
wing is reducing AOA. The pitch trim is a slab adjustment, and not a
dragging elevator with trim tabs. The "flying tail" reduces the
parasite drag of a trim control service.
In any event, the significant drag on a supersonic aircraft is the
induced drag, not the parasite or "form" drag.
I've never heard of a relationship between subsonic instability and
supersonic stability. The principle role of instability is related to
agility--the capability of rapidly diverging from a given flight
condition. Agility is a requirement for air combat and that is
accomplished at sub-sonic speeds generally.
Ed Rasimus
Fighter Pilot (USAF-Ret)
www.thundertales.blogspot.com
the_dr
wrote:
> "the_dr" <dr_nichol...@yahoo.com> wrote in message
Fair enough, but I think many of the latest fighters are? I was just
outlining the two sides of the argument and the Typhoon was the
example used.
Jim Wilkins
> ...
Why was the Sopwith Camel intentionally unstable?
jsw
the_dr
> On Mon, 5 Jul 2010 18:26:40 +0200, "Roger Conroy"
>
>
>
>
> >"the_dr" <dr_nichol...@yahoo.com> wrote in message
Ed - That's very interesting: I asked this same question to another
pilot, and he also reckoned the fundamental reason for instability was
agility, but IIRC he also said that the time spent supersonic in most
combat scenarios would not *in itself* justify compromising stability.
BTW I put the arguements on here to get opinions, not to try and prove
anyone right or wrong. The guy who proposed the 'drag' thesis seemed
to know what he was on about, and it seemed odd that what he said was
totally wrong.
Next question: With fly-by-wire, how come you need instability to get
the necessary agility? Couldn't you increase the 'gain' on the contols
of a 'stable' design to such an extant the you'd get more pitch rate
(or whatever) than the pilot could handle? Is this something to do
with the drag associated with moving the control surfaces to such a
degree would result in the speed of the aircraft reducing too much
during the manouver?
I'm interested in this stuff partly because I've been building a
'pitcheron' model glider. There are no tail controls at all (it's a
fixed 'v'-tail) and the wings don't have control surfaces, but the
entire wing panel twists to control pitch and roll. It seems to retain
speed in manouvers far better than a similar sized conventional model.
I was thinking that even at much lower speeds, the low-drag elevator
'slab' adjustment you mentioned might also apply to twist wing
aircraft, but obviously give a bigger drag reduction advantage with
the entire wing moving rather than just the 'flying tail'?
Thanks for your time.
Ken S. Tucker
> On Jul 5, 5:26 pm, "Roger Conroy" <rogercon...@nospam.hotmail.com>
> wrote:
>
>
>
> > "the_dr" <dr_nichol...@yahoo.com> wrote in message
>
> >news:335bff4a-604e-4248...@j4g2000yqh.googlegroups.com...
>
> > >A while back there was a discussion about this on a forum, and also at
> > > work. The obvious answer is because it gives the aircraft the
> > > potential to be more agile (when artificially stabilised by
> > > computer)...but is the fundamental reason fuel economy? Basically the
> > > premise of the latter argument was :
>
> > > In *very* general terms, when an aircraft goes supersonic, the
> > > aerodynamic centre moves rearwards. This requires a large trim change
> > > to maintain stability. This in turn generates a large amount of trim
> > > drag, which in turn greatly increases fuel consumption. If however
> > > the aircraft is designed to be stable at supersonic speeds it will
> > > require less or no trim drag, and use far less fuel. It will however
> > > be unstable at subsonic speeds, hence the requirement for computer
> > > stabilisation. The corresponding fuel increase at subsonic speeds is
> > > the lesser of the two evils. It was proposed that an aircraft such as
> > > the Typhoon would be incapable of supercruise unless it was designed
> > > to be inherently stable at supersonic speeds, and the agility benefits
> > > of its instability at subsonic speeds are nothing more than a positive
> > > side effect.
> > > Discuss!
Thanks.
> > Start by putting a "citation needed" tag on the implied claim that the
> > Typhoon is in fact inherently stable at supersonic speeds.
Yeah, I'd like some kind of ref.
Like to know more about the alleged effect, (the C/L variation).
> Fair enough, but I think many of the latest fighters are? I was just
> outlining the two sides of the argument and the Typhoon was the
> example used.
I'm in agreement with Ed Rasimus, tho C/L can vary significantly near
stall, I don't know it varies (much) during the transonic transition.
Ken
Alan Dicey
>> Start by putting a "citation needed" tag on the implied claim that the
>> Typhoon is in fact inherently stable at supersonic speeds.
http://www.eurofighter.com/capabilities/performance/aerodynamic-characteristics.html
There you go, Roger :)
Ed Rasimus wrote:
>
> While there is a trim change to compensate for the shift of C/L, the
> wing is reducing AOA. The pitch trim is a slab adjustment, and not a
> dragging elevator with trim tabs. The "flying tail" reduces the
> parasite drag of a trim control service.
>
> In any event, the significant drag on a supersonic aircraft is the
> induced drag, not the parasite or "form" drag.
>
> I've never heard of a relationship between subsonic instability and
> supersonic stability. The principle role of instability is related to
> agility--the capability of rapidly diverging from a given flight
> condition. Agility is a requirement for air combat and that is
> accomplished at sub-sonic speeds generally.
> Ed Rasimus
> Fighter Pilot (USAF-Ret)
> www.thundertales.blogspot.com
>
I'll have to go away and look for the references to back this up, but
The Dr. has it broadly correct. By allowing a smaller, lighter airframe
with smaller control surfaces, the aircraft's drag is reduced in all
areas of the flight envelope, improving acceleration, top speed and fuel
consumption. Although there is an improvement in instantaneous turn
rates, it is not startling compared to the equivalent statically-stable
airframe. The Jaguar fly-by-wire demonstrator was one of the few test
aircraft for the concept that was actually trialled in both regimes:
weights above the leading-edge extension were gradually removed and
weight added to the tail. Jaguar is not supersonic, of course.
http://www.rafmuseum.org.uk/cosford/collections/aircraft/sepecat-jaguar-act-demonstrator.cfm
NASA started digital fbw tests earlier, with an F-8
http://www.nasa.gov/centers/dryden/pdf/182985main_DFBW_rev1.pdf
but this airframe was never modified to a statically unstable
configuration,like the FBW Jaguar. the USAF fly-by-wire F-4 test
aircraft did acquire canards at one stage, but I don't know when or how
much testing was done in the unstable regime.
It is best to say that benefits are gained in all areas, both airframe
size reductions and improvements in turn rates.
the_dr
Ken,
Don't laugh at the reference, but I found this on Wikipedia:
"When supersonic, a negatively stable aircraft actually exhibits a
more positive-trending (and in the 4th gen aircraft case, a net
positive) static stability due to aerodynamic forces shifting aft
between subsonic and supersonic flight. At subsonic speeds, however,
the fighter is constantly on the verge of going out of control."
Ref:
the_dr
wrote:
> > "Roger Conroy" wrote:
> >> Start by putting a "citation needed" tag on the implied claim that the
> >> Typhoon is in fact inherently stable at supersonic speeds.
>
>
> There you go, Roger :)
Roger, I think that may be wrong:
http://www.scorpionaviation.com/html/Eurofighter.html
4th paragraph.
Then again it might be right...:-)
Ed Rasimus
I'm feeling like "Bones" on Star Trek here...."God damn it, Jim, I'm
not an engineer, I'm a doctor (operator)"
If one's goal is to fly comfortably straight and level, then you build
a positive stability airplane with an asymmetrical airfoil wing. If
you want an acro airplane that will fly almost as good inverted as
upright, you use a symmetrical airfoil. You lose some effeciency, but
you gain capability in the inverted position.
If you want agility then you want to easily displace from stable
flight with minimum flapping around and drag-inducing control
deflections. Lots of stuff happens that is unwanted with huge control
movements--think about adverse yaw and the departure characteristics
of aircraft like the F-100 and hard-wing F-4.
One of the fascinating things I encountered when I was at Northrop,
long enough ago that this is not classified info, was that there was a
relationship between stealth and instability. As you made the airframe
stealthier you reduced the stability. That meant you began getting
more control flapping to maintain control and each control movement
was a factor in increasing radar reflections. The trade off point of
optimum stealth without being essentially uncontrollable was a
delicate balancing act.
Dan
<snip>
> Next question: With fly-by-wire, how come you need instability to get
> the necessary agility? Couldn't you increase the 'gain' on the contols
> of a 'stable' design to such an extant the you'd get more pitch rate
> (or whatever) than the pilot could handle? Is this something to do
> with the drag associated with moving the control surfaces to such a
> degree would result in the speed of the aircraft reducing too much
> during the manouver?
>
Think of an inherently stable small aircraft like the Cessna 172. It
is stable because of wing area, wing span and the distance between
center of gravity and tail control surfaces. OK, no matter how much
input you put into any control the aircraft isn't agile. Look at roll,
to put it simply you have to swing a lot of wing area around the
longitudinal axis. A shorter wing would be easier to swing, think of
leverage, but is less stable in straight and level flight. Similarly
pitch and yaw.
As for computer controlled flight in unstable aircraft, corrections
have to be made several times a second and a human just can't keep up
with it. In WW1 the Germans flew the Dr.1 tri-plane which was impossible
to fly hands off, but had very nice roll, pitch and yaw rates. It could
be exhausting to fly.
Dan, U.S. Air Force, retired
Ed Rasimus
<al...@diceyhome.free-online.co.uk> wrote:
>I'll have to go away and look for the references to back this up, but
>The Dr. has it broadly correct. By allowing a smaller, lighter airframe
>with smaller control surfaces, the aircraft's drag is reduced in all
>areas of the flight envelope, improving acceleration, top speed and fuel
>consumption. Although there is an improvement in instantaneous turn
>rates, it is not startling compared to the equivalent statically-stable
>airframe. The Jaguar fly-by-wire demonstrator was one of the few test
>aircraft for the concept that was actually trialled in both regimes:
>weights above the leading-edge extension were gradually removed and
>weight added to the tail. Jaguar is not supersonic, of course.
>
>http://www.rafmuseum.org.uk/cosford/collections/aircraft/sepecat-jaguar-act-demonstrator.cfm
>
>NASA started digital fbw tests earlier, with an F-8
>
>http://www.nasa.gov/centers/dryden/pdf/182985main_DFBW_rev1.pdf
>
>but this airframe was never modified to a statically unstable
>configuration,like the FBW Jaguar. the USAF fly-by-wire F-4 test
>aircraft did acquire canards at one stage, but I don't know when or how
>much testing was done in the unstable regime.
>
>It is best to say that benefits are gained in all areas, both airframe
>size reductions and improvements in turn rates.
The F-4 had full three axis stability augmentation. It could be hand
flown with stab-augs off, but it was very sensitive and exhibited
negative stability. Of course it was not a FBW system and primitive
when compared to flight control system of teen-fighters and newer.
For an example of the instability one need only refer to the speed
record attempt of the Navy F-4B during early developement. At low
altitude, the test pilot experienced a failure of the pitch axis stab
aug. He elected to continue the speed run believing his skill level
adequate to control the aircraft. The aircraft began to porpoise and
the rapidly increasing amplitude of the PIO was such that the aircraft
disintegrated within three oscillations.
the_dr
Mmmm, but Alan said re, the Jaguar:
"Although there is an improvement in instantaneous turn
rates, it is not startling compared to the equivalent statically-
stable
airframe. The Jaguar fly-by-wire demonstrator was one of the few test
aircraft for the concept that was actually trialled in both regimes"
Isn't a shorter wing "easier to swing" due to inertia rather than
'leverage' : A larger span with ailerons at the ends would give more
leverage than a corresponding short span would it not?
Cheers
Dan
Inertia, yes, but try moving what is essentially a flat surface
through a fluid, in this case air, normal to the surface. Try an
experiment where you insert a ruler into some water and estimate the
resistance when you move it through the water with the flat surface
facing the direction of movement. Now try it with twice as much of the
ruler inserted into the water. Try it both ways with different rates of
speed. Now try imagining a wing rotating around the longitudinal axis
similarly.
You are correct in that ailerons closer to the wing tips provide more
leverage than a similar size closer to the root. That's why they are
there in the first place. The problem is there is a limit to how big one
can make an aileron with respect to the wing.
In all fairness I am oversimplifying a tad. Wing shape, air speed,
angle of attack and so on all influence roll rates.
Paul J. Adam
<B24...@aol.com> writes
> Think of an inherently stable small aircraft like the Cessna 172. It
>is stable because of wing area, wing span and the distance between
>center of gravity and tail control surfaces. OK, no matter how much
>input you put into any control the aircraft isn't agile. Look at roll,
>to put it simply you have to swing a lot of wing area around the
>longitudinal axis. A shorter wing would be easier to swing, think of
>leverage, but is less stable in straight and level flight. Similarly
>pitch and yaw.
I clocked a dozen hours in a two-seat Robin, which was agile enough (at
least in roll) to bounce your head off the canopy and to be fully aero
qualified: but the fundamental rule of flying her was that if it got
scary, let go and let her settle down. As long as you had altitude,
she'd take care of the rest. Even spin recovery was "let go and wait,
and never push her into that corner below two thousand feet".
Plenty of control authority to let you get yourself into trouble or to
throw her around the sky (roll rate was _fast_, I'd guesstimate 360
degrees a second if you went into it right - I was hanging on while my
instructor showed me what the aircraft could do so I didn't have to
worry about breaking her) but as soon as you eased off the controls, she
started trying to get back to sensible straight and level flight.
A good aircraft to start to learn in, if you had the right teacher (and
I liked and respected mine).
--
He thinks too much, such men are dangerous.
Paul J. Adam
the_dr
Dan,
I see, you meant 'leverage' required to overcome of the drag on the
wing in rotation, not of the aileron on the aircraft.
Cheers.
Paul J. Adam
Dicey <al...@diceyhome.free-online.co.uk> writes
>Jaguar is not supersonic, of course.
Yes it was, clean and special occasions only - but still topping out at
Mach 1.6 best effort with no payload and no mission except "generate a
headline speed for the coffee-table books to print". I don't think a Jag
tooled up with any sort of operational capability ever busted Mach,
though.
Dan
My instructor made note of how I said it was a gentle flier. He asked
me if I remembered saying that. Before I could answer he darn near gave
me a coronary demonstrating "what it could do." One of the interesting
things about taking lessons on base is one periodically has a fighter
pilot for an instructor.
I have never flown a fighter, but I have flown F-4E, F-15, C-130 and
KC-135 flight simulators. I won't say how well I did. Let's just say the
fighter roll rates were a bit faster than the cargo ones.
Remember how you could land the little Cessnas by tickling the
throttle, weather permitting, on final? It works for KC-135 and C-130,
but not fighters. It's amazing the assorted angles and velocities with
which one can find the ground in a fighter.
I have a video clip shot small airplane where one can see the shadows
move vertically across the pilot's face then a dog float up into the air
behind him. The dog didn't seem overly bothered and settled down rather
nicely when things leveled out.
Dan
I never claimed to be good at explaining things. There are all kinds
of books on basic aerodynamics, but basic is relative. The math can be
entertaining.
Alan Dicey
>
>
> Roger, I think that may be wrong:
>
> http://www.scorpionaviation.com/html/Eurofighter.html
>
> 4th paragraph.
>
> Then again it might be right...:-)
The original design was certainly stable supersonically. I can't yet
find anything online to prove it, though.
He's comparing it to Rafale, which has a coy little canard (as does
Gripen) acting as a trim surface. The canards on Eurofighter are big
enough to participate in roll control (never mind pitch) and to act as
airbrakes once weight is on the wheels.
Eunometic
Was it deliberatly unstable or did it just end up that way?
Certainly the pilot seemed to be manipulating a gyroscope and needed a
deep sense of the property of gyroscipic precesion.
The Wright brothers aircraft were in fact deliberatly unstable as part
of a somewhat misguided philosophy that this would aid
controllabillity. They in fact had to develop a crude autopilot using
a pendulum for roll and an angle of attack vane for pitch controll to
make their aircraft marketable.
Jim Yanik
news:1KWdneq3gc9Iyq_R...@giganews.com:
IOW,if you want quick response,it's far easier with a vehicle that's
already near changing it's course and being held back by a fast computer.
"stable" means it's harder to change it's path.
--
Jim Yanik
jyanik
at
localnet
dot com
Jim Yanik
news:TMWdnfOifere_q_R...@brightview.co.uk:
On the topic of Rafale,how is "Rafale" pronounced? ra-fa-lay,ra-falll,what?
Dan
No fair, you simplified my simplification. Of course you are correct.
Eunometic
> A while back there was a discussion about this on a forum, and also at
> work. The obvious answer is because it gives the aircraft the
> potential to be more agile (when artificially stabilised by
> computer)...but is the fundamental reason fuel economy? Basically the
> premise of the latter argument was :
>
> In *very* general terms, when an aircraft goes supersonic, the
> aerodynamic centre moves rearwards. This requires a large trim change
> to maintain stability.
The automatic "Mach trimmer" was already part of aircraft in the 1950s
and 1960s. These aircraft had irreversible hydraulic controls: ie
cables/rods opperated servo valves on the hydraulic actuators which
were mounted near the flight control surface and these also had
mechanical feedback from the flight control surface.
The mach trimmer would adjust pitch trim, usually by the slab/
stabilator with the mach trim mechanically added to the cable/rigging
by mechanical means.
Note these are not hydraulically 'boosted' controls that assist the
pilot, they are all powered because the possibillity of any 'feedback'
from the flight control surface must be eliminated due to the
possibillity of flutter (this needs regidity that only powered
surfaces can provide) combining with the phenomena of 'pilot coupled
oscilations'.
Artifical 'feel' was restored by tensioning the pilots controls with a
bellows that provided force feedback according to dyanamic pressure
(square of air velocity)
"Rigging" was starting to get complicated with 'trim' adjustments
added in mechanically.
Another 'automatic' system in early aircraft was the 'yaw damper' to
stop a sort of fish tailing that is a problem in jet aircraft. The
Germans started experimenting with them on the Me 262. Basically its
a gyroscope that opperates on the rudder. It was indispensible on the
Skyhawke. Early experimental aircraft used the autopilot/inertial
reference gyros but this nearly lead to a crash in sideways flight as
the yaw damper fought the pilot. Seperate rate gyros were used from
thence forward. B-52 initially used a tuned pendulous mass to
opperate the rudder to provide yaw damping but this once caused a tail
to be ripped of due to the difficulty of maintaining the bearings: any
stiction causes
As you can see with Mach trimmers and Yaw dampers the 'need' for fly-
by-wire was already there in the 1940s.
> This in turn generates a large amount of trim
> drag, which in turn greatly increases fuel consumption. If however
> the aircraft is designed to be stable at supersonic speeds it will
> require less or no trim drag, and use far less fuel. It will however
> be unstable at subsonic speeds, hence the requirement for computer
> stabilisation.
This is also true. FBW makes a huge difference in delta winged
aircraft stall and takeoff speed I believe eg Mirage III and Mirage
2000.
The concord had 6 elevons these adjusted to to give optimal cruise
camber rather than adjusting pitch. The trim changes were handelled
by moving fuel around.
Another 'problem' solved is the issue of inertia coupling in roll.
An aircraft performing a roll it doesn't roll about its principle axis
but about the principle axis of flgiht. The result is that the mass
at the ends of the aircraft tend to create centrifugal forces that
create serious handling problems.
A Fly By Wire control system can trim that out as well, as it does on
the F-18E/F.
Then we also have the classic trim adjustments neccessary to
compensate for engine thrust, plus reducing authority of controls in
flight.
The corresponding fuel increase at subsonic speeds is
> the lesser of the two evils. It was proposed that an aircraft such as
> the Typhoon would be incapable of supercruise unless it was designed
> to be inherently stable at supersonic speeds, and the agility benefits
> of its instability at subsonic speeds are nothing more than a positive
> side effect.
I don't think that FBW makes a big difference in speed, acceleration,
manouverabillity: such things still get down to power to weight ratio
and wing loading etc.
It does make a big difference in low speed handling.
However FBW can dramatically improve handling and eliminate bad or
dangeous handling. It can provide 'envelope protection' and trim and
adjust and limit half a dozen flight control surfaces in a multitude a
pilot could never do. It can also prevent the airframe from being
over-stressed, allow carriage of outsize load etc.
It also makes possible to produce airframes with a great deal more
freedom: no need to line up the principal axis of drag, thrust and
flight to get predictalbe handling. Airliners clearly pitch up when
throttled however FBW trims this unpleasant distraction out.
And of course many of the things done by FBW were already done by
extremly elaborate mechanical systems.
Eunometic
Amazing how engineering gobblygook can obscure rather than aid
comprehension.
At supersonic speeds the fighter is stable because it is nose heavy
(like a dart).
At subsonic speeds, because the center of lift has shifted forward,
the aircraft is now unsable (like an unweighted dart or a badly
diesigned dart/arrow with the 'quils' mounted on the head.
Formalism is of course important but as a supplement.
Eunometic
> On Mon, 5 Jul 2010 14:41:21 -0700 (PDT), the_dr
>
>
> One of the fascinating things I encountered when I was at Northrop,
> long enough ago that this is not classified info, was that there was a
> relationship between stealth and instability. As you made the airframe
> stealthier you reduced the stability. That meant you began getting
> more control flapping to maintain control and each control movement
> was a factor in increasing radar reflections. The trade off point of
> optimum stealth without being essentially uncontrollable was a
> delicate balancing act.
Which is probably another reason for thrust vectoring on the F-22.
Paul Saccani
<al...@diceyhome.free-online.co.uk> wrote:
Mach 1.6 is generally regarded as supersonic...
>http://www.rafmuseum.org.uk/cosford/collections/aircraft/sepecat-jaguar-act-demonstrator.cfm
>
>NASA started digital fbw tests earlier, with an F-8
The Poms did analog FBW decades earlier, FWIW. Well, actaully, they
even put it into production. Not for the same purpose, of course.
>http://www.nasa.gov/centers/dryden/pdf/182985main_DFBW_rev1.pdf
Interesting report.
Cheers,
Paul Saccani,
Perth,
Western Australia
Paul Saccani
<ne...@jrwlynchANDNOTTHIS.demon.co.uk> wrote:
>In message <lMmdnceS6onTzq_R...@brightview.co.uk>, Alan
>Dicey <al...@diceyhome.free-online.co.uk> writes
>>Jaguar is not supersonic, of course.
>
>Yes it was, clean and special occasions only - but still topping out at
>Mach 1.6 best effort with no payload and no mission except "generate a
>headline speed for the coffee-table books to print". I don't think a Jag
>tooled up with any sort of operational capability ever busted Mach,
>though.
The SOAF ones did, in the air defence role with two over the shoulder
missiles.
Paul Saccani
<rasimus...@verizon.net> wrote:
>One of the fascinating things I encountered when I was at Northrop,
>long enough ago that this is not classified info, was that there was a
>relationship between stealth and instability. As you made the airframe
>stealthier you reduced the stability. That meant you began getting
>more control flapping to maintain control and each control movement
>was a factor in increasing radar reflections. The trade off point of
>optimum stealth without being essentially uncontrollable was a
>delicate balancing act.
Not on subject, but I think one of the most fascinating aspects of the
US low observable program was its origin in, and extensive use of
(indeed, fundamental reliance on), Russian science, published in open
sources. They didn't realise that it had real world utility, so it
wasn't classified.
Roger Conroy
"Alan Dicey" <al...@diceyhome.free-online.co.uk> wrote in message
news:TMWdnfOifere_q_R...@brightview.co.uk...
Alan, have you seen the Gripen in action? Those "coy little" canards have a
whole lot of authority. They definitely do their share of braking and their
pitch authority is anything but coy. I'd say the Gripen carries a fair dose
of "genes" from Heineman's Scooter as a hot little plane that punches well
above its weight.
Roger Conroy
"the_dr" <dr_nic...@yahoo.com> wrote in message
news:bee5178c-05fc-412e...@j4g2000yqh.googlegroups.com...
<sermon>
See why editing Wikipedia isn't easy?
Folks should try doing it themselves before disparaging WP as "unreliable".
</sermon>
Roger Conroy
Ken,
Ref:
http://en.wikipedia.org/wiki/4th_generation_jet_fighter
==================================================
Unfortunately the references provided for the quote from WP are incomplete.
They give only author, date and page number but leave out the title of the
work!
Hoh and Mitchell 1983, pp. 11ff.
Aronstein and Piccirillo 1996, p. 21.
Does anyone here recognise these authors?
Daryl Hunt
The one of the most successful fighters from WWI was
definately unstable. It was said that if the Camel didn't
kill you you killed the enemy.
I don't think you can have a dog fighter that is that stable
and still have the maneuverability that is world class. Of
course, controls have come a long way since the Sopwith Camel.
coffelt2
> The F-4 had full three axis stability augmentation. It could be hand
> flown with stab-augs off, but it was very sensitive and exhibited
> negative stability. Of course it was not a FBW system and primitive
> when compared to flight control system of teen-fighters and newer.
>
> For an example of the instability one need only refer to the speed
> record attempt of the Navy F-4B during early developement. At low
> altitude, the test pilot experienced a failure of the pitch axis stab
> aug. He elected to continue the speed run believing his skill level
> adequate to control the aircraft. The aircraft began to porpoise and
> the rapidly increasing amplitude of the PIO was such that the aircraft
> disintegrated within three oscillations.
>
> Ed Rasimus
> Fighter Pilot (USAF-Ret)
> www.thundertales.blogspot.com
>
Ubon Airfield... mid 1960's.... during low level, high speed pass
down runway, there was a slight, visible, "bobble" which I understood
was the three axis stab aug doing it's thing. We flight line grease monkeys
were told that with any stab aug gyro out, the pass was not possible. Those
suckers made a smoke dot you could see coming 15 miles out!
Emotions ran high!
Old Chief Lynn
the_dr
Dan: It wasn't a criticism - more my misunderstanding! I've done some
aerodynamics, and designed a couple of models using these principles,
but just when you think you've figured things out, something comes out
of the woodwork and bites you!
Thanks for all the information guys - there's a lot to digest here.
The instability = better manouverability was never in question, but I
think my supplementary question still has no concensus:
Can a stable aircraft be as (or more) manouverable as an unstable one
if the gain of the FBW is such that a tiny control input causes enough
control rate to overcome the pilot?
The "Jaguar" comment would appear to agree somewhat, but...not exactly
conclusive.
the_dr
> Unfortunately the references provided for the quote from WP are incomplete.
> They give only author, date and page number but leave out the title of the
> work!
>
> Hoh and Mitchell 1983, pp. 11ff.
> Aronstein and Piccirillo 1996, p. 21.
>
> Does anyone here recognise these authors?
No, but if you put the names into Google multiple papers / articles on
aerospace subjects come up.
Dan
I didn't take it as such.
the_dr
Paul Saccani's reference:
http://www.nasa.gov/centers/dryden/pdf/182985main_DFBW_rev1.pdf
Probably answers the question indirectly: If you're using digial
control for all the other reasons apart from the 'agility' issue ie
weight reduction, stealth, fuel reduction, 'feel' when varying stores
are deployed etc etc, then the aircraft design is probably unstable
anyway, in fact it would be a distinct disagdvantage on many other
levels (quite apart from agility) to it being designed as inherently
stable.
Thats if I understood the bits of the article I read correctly...
Ken S. Tucker
Thanks for the ref.
> Amazing how engineering gobblygook can obscure rather than aid
> comprehension.
>
> At supersonic speeds the fighter is stable because it is nose heavy
> (like a dart).
>
> At subsonic speeds, because the center of lift has shifted forward,
> the aircraft is now unsable (like an unweighted dart or a badly
> diesigned dart/arrow with the 'quils' mounted on the head.
>
> Formalism is of course important but as a supplement.
One easy refinement is to twist the main wing, so at high speed
and low AoA's the wing root has zero AoA, but the wing tip has
a degree or so. That system 'programs' the C/L to move rearward
as the speed increases.
A detail is the increase in the air resistance the vertical stab has
in the supersonic which causes a bit of extra pitch up.
At 'supercruise' one would want to have a minimum of parasitics,
if that's a design requirement, and sounds to be for Eurofighter.
Ken
kirk....@gmail.com
> One easy refinement is to twist the main wing, so at high speed
> and low AoA's the wing root has zero AoA, but the wing tip has
> a degree or so. That system 'programs' the C/L to move rearward
> as the speed increases.
> A detail is the increase in the air resistance the vertical stab has
> in the supersonic which causes a bit of extra pitch up.
> At 'supercruise' one would want to have a minimum of parasitics,
> if that's a design requirement, and sounds to be for Eurofighter.
Wrong - that would lead to nasty stall and departure characteristics
at low speeds and high angles of attack.
Look at the wing of an F-15/16/18 and you can see the exact opposite -
lots of washout (wing twist, tips at lower AOA than root) for good
departure resistance during hard maneuvering.
Supersonic maneuvering requires either a huge tail (to get any
effectiveness with shock waves present) or relaxed stability at lower
speeds (since, as others have explained, modern jets tend to be more
stable at supersonic speeds).
This is basic aero - may I suggest you go back to designing "tanks"?
Kirk
Ken S. Tucker
wrote:
> > One easy refinement is to twist the main wing, so at high speed
> > and low AoA's the wing root has zero AoA, but the wing tip has
> > a degree or so. That system 'programs' the C/L to move rearward
> > as the speed increases.
> > A detail is the increase in the air resistance the vertical stab has
> > in the supersonic which causes a bit of extra pitch up.
> > At 'supercruise' one would want to have a minimum of parasitics,
> > if that's a design requirement, and sounds to be for Eurofighter.
> > Ken-
>
> Wrong - that would lead to nasty stall and departure characteristics
> at low speeds and high angles of attack.
No, cuz you're using a canard compensator.
(I've built and tested them).
As I wrote it is a 'programable' based on how to do wing twist.
Read this guy,
http://en.wikipedia.org/wiki/Rutan_VariEze
Tho subsonic he put in -4 degree twist on the wing, ask me why :-).
> Look at the wing of an F-15/16/18 and you can see the exact opposite -
> lots of washout (wing twist, tips at lower AOA than root) for good
> departure resistance during hard maneuvering.
That's NOT a canard.
> Supersonic maneuvering requires either a huge tail (to get any
> effectiveness with shock waves present) or relaxed stability at lower
> speeds (since, as others have explained, modern jets tend to be more
> stable at supersonic speeds).
You're incorrectly generalizing, look at the F-104.
> This is basic aero - may I suggest you go back to designing "tanks"?
Yes you may.
Is it ok if I approve a 2020 gen mock-up?
Ken
kirk....@gmail.com
> No, cuz you're using a canard compensator.
> (I've built and tested them).
> As I wrote it is a 'programable' based on how to do wing twist.
> Tho subsonic he put in -4 degree twist on the wing, ask me why :-).
Oh please, spare me the VariEze bs. That plane is a POS that sucked
in a lot of gullible builders. And WTF is a "canard compensator"?
While you are at it, how about a cite for the "-4 degree twist"?
Since a canard cannot afford to let the main wing stall before the
canard (or even stall at all - again, basic aero) twist is less of a
factor, and it's possible that Rutan may have designed in some twist
to optimize local airflow in an attempt to justify a basically flawed
concept by increasing efficiency at all costs. That's not the issue
here.
> > Look at the wing of an F-15/16/18 and you can see the exact opposite -
> > lots of washout (wing twist, tips at lower AOA than root) for good
> > departure resistance during hard maneuvering.
>
> That's NOT a canard.
Duh....
> > Supersonic maneuvering requires either a huge tail (to get any
> > effectiveness with shock waves present) or relaxed stability at lower
> > speeds (since, as others have explained, modern jets tend to be more
> > stable at supersonic speeds).
>
> You're incorrectly generalizing, look at the F-104.
I've flown in an F-104, have you? First, it's not a "modern jet", and
secondly it has a large tail stuck up high on the tail in order to
provide adequate supersonic pitch control. Unfortunately, this also
means it's susceptible to low speed pitchup at high AOA, and requires
other fixes to solve that little problem.
Kirk
Alan Dicey
Ahem.
I think you'll find that was me . . .
> Probably answers the question indirectly: If you're using digial
> control for all the other reasons apart from the 'agility' issue ie
> weight reduction, stealth, fuel reduction, 'feel' when varying stores
> are deployed etc etc, then the aircraft design is probably unstable
> anyway, in fact it would be a distinct disagdvantage on many other
> levels (quite apart from agility) to it being designed as inherently
> stable.
>
> Thats if I understood the bits of the article I read correctly...
If you design for static stability the airframe is larger because the
control surfaces must be bigger and/or further away from the center of
mass. Depending on just how statically stable you make it, the moment
arms must be large enough to generate the instantaneous turn rates you
require, the "agility" you want.
So yes, a stable airframe can be built with enough control authority to
generate the equivalent agility (in terms of being able to point the
nose where you want to) to an unstable airframe. It will be bigger,
heavier and more draggy and will run out of fuel, speed, height and
ideas first ;)
Ed Rasimus
<al...@diceyhome.free-online.co.uk> wrote:
> It will be bigger,
>heavier and more draggy and will run out of fuel, speed, height and
>ideas first ;)
Ahh, I can do that even in a airplane designed for instability and
agility. Any plumber can dissipate fuel, airspeed, altitude and ideas
quickly. Ideas run out first.
the_dr
>
> Ahem.
>
> I think you'll find that was me . . .
Sorry!
> If you design for static stability the airframe is larger because the
> control surfaces must be bigger and/or further away from the center of
> mass. Depending on just how statically stable you make it, the moment
> arms must be large enough to generate the instantaneous turn rates you
> require, the "agility" you want.
>
> So yes, a stable airframe can be built with enough control authority to
> generate the equivalent agility (in terms of being able to point the
> nose where you want to) to an unstable airframe. It will be bigger,
> heavier and more draggy and will run out of fuel, speed, height and
> ideas first ;)
Understood. Thank you.
the_dr
> On Tue, 06 Jul 2010 22:30:58 +0100, Alan Dicey
>
> > It will be bigger,
> >heavier and more draggy and will run out of fuel, speed, height and
> >ideas first ;)
>
> Ahh, I can do that even in a airplane designed for instability and
> agility. Any plumber can dissipate fuel, airspeed, altitude and ideas
> quickly. Ideas run out first.
>
> Ed Rasimus
> Fighter Pilot (USAF-Ret)www.thundertales.blogspot.com
Thanks for your contributions Ed.
BTW why doesn't your website link work?
the_dr
Ah , it does: for some reason my browser puts a google address infront.
Ken S. Tucker
wrote:
> On Jul 6, 2:49 pm, "Ken S. Tucker" <dynam...@vianet.on.ca> wrote:
>
> > No, cuz you're using a canard compensator.
> > (I've built and tested them).
> > As I wrote it is a 'programable' based on how to do wing twist.
> > Read this guy,http://en.wikipedia.org/wiki/Rutan_VariEze
> > Tho subsonic he put in -4 degree twist on the wing, ask me why :-).
>
> Oh please, spare me the VariEze bs.
Ok
Ken
Eunometic
The 1950s developed F-104 starfighter had a "SAS" Stabillity
Augmentation System. Think about what this means:
Mechanical linkages from the joystick etc connected to the hydraulic
valves mounted directly on the hydralic actuators.
Rigging ensured such delicacies such as "differential aileron
opperation" ie a downward deflected airleron causes proportionatly
more drag (causing proverse yaw or a tightening circle) and is more
likely to stall than a upward deflected one, the solution is
differential airlerons that deflect upward more than downward. This
can be accomplished mechanically and was in WW2 aicraft.
Now consider the SAS system. It obviously intercepts the mechanical
linkages and adds in roll, pitch and yaw corrections generated by a
electronics using rate information from gyroscopes, possibly attitude
information and angle of attack sensors and probably Mach/IAS
sensors. Without the SAS the F-104 was limited to 300knots.
In reality fly by wire has been in service aircraft since the mid
1950's its just that its been inserted in a failsafe fashion in the
mechanical linkages. Digital Fly By wire is simply a more convenient
system that keeps down complexity and enables far greater subtelty and
thoroughness in controll law design.
the_dr
> more drag (causing proverse yaw or a tightening circle)
Shouldn't that be "adverse yaw", causing a larger circle?
I have had this problem (adverse yaw) on a Fournier Motor Glider
model, and used the programmable differential function on the R/C Tx
to eliminate it (more or less).
Jim Wilkins
> On Jul 6, 11:11 pm, the_dr <dr_nichol...@yahoo.com> wrote:
> > On Jul 6, 10:48 pm, Ed Rasimus <rasimusSPAML...@verizon.net> wrote:
> > > Ed Rasimus
> > > Fighter Pilot (USAF-Ret)www.thundertales.blogspot.com
>
> > Thanks for your contributions Ed.
>
> > BTW why doesn't your website link work?
>
> Ah , it does: for some reason my browser puts a google address infront.
I see that too. This may work better:
http://www.thundertales.blogspot.com/
jsw
Daryl Hunt
Yupper, that worked.
Remind me to take alternative transportation.
Dweezil Dwarftosser
>
> The 1950s developed F-104 starfighter had a "SAS" Stabillity
> Augmentation System. Think about what this means:
>
> Mechanical linkages from the joystick etc connected to the hydraulic
> valves mounted directly on the hydralic actuators.
>
> Rigging ensured such delicacies such as "differential aileron
> opperation" ie a downward deflected airleron causes proportionatly
> more drag (causing proverse yaw or a tightening circle) and is more
> likely to stall than a upward deflected one, the solution is
> differential airlerons that deflect upward more than downward. This
> can be accomplished mechanically and was in WW2 aicraft.
>
> Now consider the SAS system. It obviously intercepts the mechanical
> linkages and adds in roll, pitch and yaw corrections generated by a
> electronics using rate information from gyroscopes, possibly attitude
> information and angle of attack sensors and probably Mach/IAS
Yes; such ANALOG 'augmentation' systems were commonplace in
most combat aircraft designs through the late 1970s.
> In reality fly by wire has been in service aircraft since the mid
> 1950's its just that its been inserted in a failsafe fashion in the
> mechanical linkages.
No. Analog-augmented systems cannot be considered 'Fly-by-wire'
(the most bastardized term in aviation today), under any circumstance.
In the strict technical sense, FBW is nothing more than replacing
the cables, rods, bellcranks, weights, and piano wire (whether
augmented or not) that run directly from the control stick to the
various hydraulic actuators - with electronic control wires. There
is no requirement (other than redundancy) for either an analog or
digital processor of any type in fly-by-wire. However, choosing
to ignore a bad signal in a redundant system is extremely difficult
with an analog computer - and well-designed digital systems can do
so quite simply. (In some cases, so simply that no digital processor
is necessary to 'think' it over; fault-rejection can be accomplished
solely via the use of fixed-logic gating.)
Of course, using a digital computer for the purpose has many benefits
because of its ability to be reprogrammed; so much so that additional
facilities can be installed, well beyond the identification and
rejection of faulty signals in a redundant system.
The installation of programs that blazingly-fast correct overly-
strong pilot inputs is the point where fly-by-wire becomes fly-by-
computer, as well. Aircraft that are inherently unstable in any
flight regime can become controllable in almost all of them.
I don't pretend to know about the aerodynamics of it all (or why
any aircraft other than Have Quick/F-117/B-2 would absolutely
require fly-by-computer), but much of it comes from stick-jockeys
wanting to be able to fly up their own butts, instantaneously.
In some very small percentage of engagements, this is highly
desirable, I guess - but not particularly necessary for the vast
majority of the time.
> Digital Fly By wire is simply a more convenient system ...
True, but misleading. The only reason it is more convenient is
that programming can install new features without rebuilding the
hardware.
> ... that keeps down complexity ...
Definitely untrue. There is nothing faster, more accurate, or
less complex in a control system than analog computation - though
all the hardware must change to accomplish a modification of its
operational modes. Mechanical complexity is higher in an analog
system (and requires no signal redundancy) - but electronic
complexity is at least an order of magnitude greater in a digital
system - and software is many orders of magnitude greater in
complexity.
> ... and enables far greater subtelty and thoroughness in controll
> law design.
Agreed. With proper programming (which is very rare in non-military
aircraft), a digital fly-by-computer system can be far superior to
its analog ancestors and fly-by-wire precursors.
the_dr
Have Quick? Have Blue?
Dweezil Dwarftosser
> Have Quick? Have Blue?
Yes, Have Blue. Please excuse my 'senior moment'.
I still remember everything ... three days later !!
kirk....@gmail.com
Well, you are probably technically correct in stating that it would
take fly-by-wire to make a Have Quick radio fly! Or you could just
drop it out the cargo door...
Kirk
the_dr
No worries - I thought it might have been a secret project!
Ken S. Tucker
I've designed and built digital FBW and the software and it's simple,
(I 'was' an authority on the subject).
As one needs to interface to a glass cockpit anyway, you need to
digitize for the displays.
It's all very systematic.
Ken
Dan
Have Quick was secret, for awhile, then it "wasn't discussed."
Eunometic
> ie a downward deflected airleron causes proportionatly
>
> > more drag (causing proverse yaw or a tightening circle)
>
> Shouldn't that be "adverse yaw", causing a larger circle?
Yes you are correct, adverse yaw is turning against the banked turn.
IE if you bank left in order to turn left the downward deflected
airleron on the right would tend to cause more drag than the upward
one on the left and thereby turn against the desired turn (adversely)
However provers yaw (ie tightening of the turn) is also a problem and
I believe more annoying and dangerous. It might be caused by
excessive differential opperation of ailerons or friese ailerons. As
the speed changes the degree of differential opperation would ideally
change as well.
>
> I have had this problem (adverse yaw) on a Fournier Motor Glider
> model, and used the programmable differential function on the R/C Tx
> to eliminate it (more or less).
Adverse yaw is reduced by selecting more appropriate wing sections:
those with flat bottoms, such as clarke-y tend to have more adverse
yaw.
Another way is to program the rudder to counter the yaw.
Deflecting of a rudder will cause roll, so aileron action can be
programmed to counter it as well.
Most of these methods were already in use well before any form of FBW.
Eunometic
>
> > The 1950s developed F-104 starfighter had a "SAS" Stabillity
> > Augmentation System. Think about what this means:
>
> > Mechanical linkages from the joystick etc connected to the hydraulic
> > valves mounted directly on the hydralic actuators.
>
> > Rigging ensured such delicacies such as "differential aileron
> > opperation" ie a downward deflected airleron causes proportionatly
> > more drag (causing proverse yaw or a tightening circle) and is more
> > likely to stall than a upward deflected one, the solution is
> > differential airlerons that deflect upward more than downward. This
> > can be accomplished mechanically and was in WW2 aicraft.
>
> > Now consider the SAS system. It obviously intercepts the mechanical
> > linkages and adds in roll, pitch and yaw corrections generated by a
> > electronics using rate information from gyroscopes, possibly attitude
> > information and angle of attack sensors and probably Mach/IAS
> > sensors. [...]
>
> Yes; such ANALOG 'augmentation' systems were commonplace in
> most combat aircraft designs through the late 1970s.
>
> > In reality fly by wire has been in service aircraft since the mid
> > 1950's its just that its been inserted in a failsafe fashion in the
> > mechanical linkages.
>
> No. Analog-augmented systems cannot be considered 'Fly-by-wire'
> (the most bastardized term in aviation today), under any circumstance.
Strictly yes BUT I was emphasising that electronic intervention or a
sort of "fly by computer" was already essential to aircraft designed
in the 1950s. The rate gyros and other sensors providing yaw, roll
and pitch damping were evaluated at least partially electronically and
inserted in the control paths.
DD In the strict technical sense, FBW is nothing more than replacing
DD the cables, rods, bellcranks, weights, and piano wire (whether
DD augmented or not) that run directly from the control stick to the
DD various hydraulic actuators - with electronic control wires.
The mechanical linkages did however perform a crude mechanical
computing function such as providing differential airlerons, roll
compensation when the rudder is used, aileron deflection increases at
low speed (eg when Mitsubishi A6M zero undercarruage was down) or
switching to spoiler control instead of aileron control in big
Boeings.
So strictly a transfer to an immagined pure "Fly By Wire" without "Fly
By Computer" could never have existed in a practical sense because "
Fly By Computer " was already implemented and essential in the 1950s
and earlier: only the computer was a hybrid analog mechanical
incorporated in the rigging and supplemented by electronic stabillity
augmentation systems added in to the mechanical linkages.
There
> is no requirement (other than redundancy) for either an analog or
> digital processor of any type in fly-by-wire. However, choosing
> to ignore a bad signal in a redundant system is extremely difficult
> with an analog computer - and well-designed digital systems can do
> so quite simply. (In some cases, so simply that no digital processor
> is necessary to 'think' it over; fault-rejection can be accomplished
> solely via the use of fixed-logic gating.)
Its hard to see faults developing in electrical/electronic systems (ie
one can notice sloppy rigging easier than a loose or corroded wire)
and electrical systems tend to fail suddently rather than
progressively. There is often no warning.
>
> Of course, using a digital computer for the purpose has many benefits
> because of its ability to be reprogrammed; so much so that additional
> facilities can be installed, well beyond the identification and
> rejection of faulty signals in a redundant system.
> I don't pretend to know about the aerodynamics of it all (or why
> any aircraft other than Have Quick/F-117/B-2 would absolutely
> require fly-by-computer), but much of it comes from stick-jockeys
> wanting to be able to fly up their own butts, instantaneously.
> In some very small percentage of engagements, this is highly
> desirable, I guess - but not particularly necessary for the vast
> majority of the time.
>
> > Digital Fly By wire is simply a more convenient system ...
>
> True, but misleading. The only reason it is more convenient is
> that programming can install new features without rebuilding the
> hardware.
The complexity of control laws is such that producing a competive
practical analog electric 'fly by computer' is likely to be prohibitve
in terms of cost and weight.
Try build a self adaptive lypunov non linear controller in analog or a
kalman filter in analog or fuzzy logic. It is probably provably
impossible.
>
> > ... that keeps down complexity ...
>
> Definitely untrue. There is nothing faster, more accurate, or
> less complex in a control system than analog computation -
In terms of 'control' I've never seen that. Sure if you need to do
'signal processing' eg multiply two baseband microwave radio signals
together analog is for the momment possibly the only way but at the
level of speed required is only to move a flight control surface
digital systems must be more accurate, with speed not even an issue
given the mega flop processing speeds.
The douglass skyshark tried to use electrical controls but these were
too slow and a swtich to hydraulics was made. The speed issue related
to the servo motors not the control electronics.
though
> all the hardware must change to accomplish a modification of its
> operational modes. Mechanical complexity is higher in an analog
> system (and requires no signal redundancy) - but electronic
> complexity is at least an order of magnitude greater in a digital
> system - and software is many orders of magnitude greater in
> complexity.
>
> > ... and enables far greater subtelty and thoroughness in controll
> > law design.
>
> Agreed. With proper programming (which is very rare in non-military
> aircraft), a digital fly-by-computer system can be far superior to
> its analog ancestors and fly-by-wire precursors.- Hide quoted text -
Paul Saccani
<roger...@nospam.hotmail.com> wrote:
>"When supersonic, a negatively stable aircraft actually exhibits a
>more positive-trending (and in the 4th gen aircraft case, a net
>positive) static stability due to aerodynamic forces shifting aft
>between subsonic and supersonic flight. At subsonic speeds, however,
>the fighter is constantly on the verge of going out of control."
>
>Ref:
>
>http://en.wikipedia.org/wiki/4th_generation_jet_fighter
>
>==================================================
>
>Unfortunately the references provided for the quote from WP are incomplete.
>They give only author, date and page number but leave out the title of the
>work!
I would suggest the following;
>Hoh and Mitchell 1983, pp. 11ff.
Hoh, Roger H. and David G. Mitchell. "Flying Qualities of Relaxed
Static Stability Aircraft - Volume I: Flying Qualities Airworthiness
Assessment and Flight Testing of Augmented Aircraft." Federal Aviation
Administration (DOT/FAA/CT-82/130-I), September 1983.
>Aronstein and Piccirillo 1996, p. 21.
Aronstein, David C. and Albert C. Piccirillo. The Lightweight Fighter
Program: A Successful Approach to Fighter Technology Transition.
Reston, VA: AIAA, 1996.
>Does anyone here recognise these authors?
Yep, they are pretty well known, I should have thought.
Cheers,
Paul Saccani,
Perth,
Western Australia
Paul Saccani
<euno...@yahoo.com.au> wrote:
>In reality fly by wire has been in service aircraft since the mid
>1950's its just that its been inserted in a failsafe fashion in the
>mechanical linkages.
Vulcan didn't have mechanical reversion.
Roger Conroy
"Paul Saccani" <sac...@pc.jaring.my> wrote in message
news:7afa369ooc3qih5nu...@4ax.com...
Thanks! Now I can fix those refs.
the_dr
> A while back there was a discussion about this on a forum, and also at
> work. The obvious answer is because it gives the aircraft the
> potential to be more agile (when artificially stabilised by
> computer)...but is the fundamental reason fuel economy? Basically the
> premise of the latter argument was :
>
> In *very* general terms, when an aircraft goes supersonic, the
> aerodynamic centre moves rearwards. This requires a large trim change
> to maintain stability. This in turn generates a large amount of trim
> drag, which in turn greatly increases fuel consumption. If however
> the aircraft is designed to be stable at supersonic speeds it will
> require less or no trim drag, and use far less fuel. It will however
> be unstable at subsonic speeds, hence the requirement for computer
> stabilisation. The corresponding fuel increase at subsonic speeds is
> the lesser of the two evils. It was proposed that an aircraft such as
> the Typhoon would be incapable of supercruise unless it was designed
> to be inherently stable at supersonic speeds, and the agility benefits
> of its instability at subsonic speeds are nothing more than a positive
> side effect.
>
> Discuss!
I found the original text from the forum. No idea who wrote it, maybe
some kind of lecturer from the way it's written:
"If I had a servo for every time I’ve heard someone say “modern
fighters are made unstable to make them more agile” I’d have crow-
brakes on everything! It seems every single model flyer knows about
this, which would be great were it not for the annoyingly inconvenient
detail that it isn’t actually true. Which is a shame, given how often
it’s said. I’m hoping that in writing this piece people will
understand WHY it isn’t true!
The concept that instability leads to high agility is a fallacy that
dates back over 40 years to the days when autostabilisers were being
developed to address the handling deficiencies of various early
American jets and persists to this day. Let's start with the basics:
1. Many modern supersonic jets have a negative Static Margin (ie they
have the C of G behind the Neutral Point) which makes them
aerodynamically unstable, and they are made controllable by use of a
full-authority autostabilisation system. This is true.
2. These modern supersonic jets are designed with inherent instability
to give enhanced agility. This is NOT true. Not only is it not the
reason why the aircraft are designed in this way, it is also not true
that an unstable aeroplane is more agile.
I know this is contrary to the received wisdom, but let's just examine
what's going on. Most of the following is grossly oversimplified to
avoid the use of mathematics, and is also rather over-generalised, but
it is valid and accurate for the purposes of this discussion.
Professional aerodynamicists are requested to stop reading this now
and pop into the forums to see how Phil is getting on with those
wonderful Fw190 and B26 builds.
"Agility" of an aircraft in the pitching plane is determined by how
quickly it can apply the lift forces to pull the 'G'. This in turn is
dependant on how quickly the angle of attack can be increased - the
pitch-plane angular acceleration, or more to the point the
INSTANTANEOUS pitch-plane angular acceleration. Now a clever chap
called Newton once showed that in any constant mass situation the
acceleration of an object was dependant solely on the mass of the
object and the sizes of the forces applied to it. The same is just as
true for angular accelerations, except that we substitute "moments of
inertia" for mass and use the "moments" of the forces as any attentive
GCSE science pupil will be able to tell you. You will note that
nothing has made reference to the "angular stability" of the object,
because it's irrelevant and so we have just demonstrated that
instability does NOT increase agility (QED – not that hard, was it?).
Fine, so why DO we bother with all this negative-stability-and-fly-by-
wire cockamamie? After all, it would be so much simpler, cheaper and
more reliable to simply connect a conventional aircraft hydraulic
system (or even a pushrod) between the stick and the control surfaces!
The answer is simple - it reduces the supersonic fuel consumption.
[What? Where did that come from? What's this guy been smoking!? I mean
one minute I was dozing through a bit about stability and the next
thing I know you're blathering on about fuel consumption. How can
these be related?? - Ed].
To understand this we need to briefly look at another bit of
aerodynamics, the concept of "Trim Drag". We all know that with a
stable aeroplane you place the CG in front of the "centre of
pressure" (I'd prefer to use "Neutral Point" but let's keep it simple)
which makes the aeroplane pitch downwards. We oppose this by having a
tailplane to push the tail down or a foreplane to lift the nose up and
voila! We have a stable aeroplane.
The actual amount of effort the tailplane/foreplane has to exert to do
this depends on how far the CG is from the centre of pressure, and we
call this the "Static Margin". Those who have paid a bit of attention,
rather than chatting up the totty at the back, will also know that the
act of generating lift inherently generates drag. Indeed the more
pretentious will point out that it has to because lift and drag are
the hydrostatic and the hydrodynamic manifestations of the same
phenomenon (at which the rest of us will jeer, throw ink bombs and put
sand in their PE kit because we can't STAND a smarty-pants).
Now if we put these two things together we will see that the lift
generated by the tailplane/foreplane to stabilise the aeroplane will
inherently result in some extra parasitic drag which, since it is the
result of trimming the aeroplane, we call "trim drag". The amount of
drag generated is proportional to the size of the static margin. This
is important, and I’ll be coming back to it in a minute.
Now we need to look at what happens when an aeroplane goes supersonic.
When an aeroplane flies at moderate speeds the “aerodynamic centre”
can be considered to be at 25% mean aerodynamic chord. It isn’t
really, but this assumption works for the purpose of this discussion
and where required the differences are corrected for other purposes
with what is technically known as “complicated mathematical stuff”.
Now if the pilot pushes the throttle into afterburner and leaves it
there the aircraft accelerates until it goes supersonic, whereupon
something interesting happens; the aerodynamic centre moves back to
50% mean aerodynamic chord. This is why the early attempts at
supersonic flight often ended up in ever-steepening dives because
there wasn’t enough elevator authority to pull out.
At this point I would be grateful if any professional aerodynamicists
who didn’t pop over to the forums would do so now (that B26 really IS
worth looking at) or perhaps go off and make themselves a coffee. Have
they gone? Good! Now as far as the rest of you are concerned, the
aerodynamic centre can be considered to be bolted to the “centre of
pressure” via a short piece of carbon-fibre rod and a pair of high
quality metal clevises, so that when the AC moves aft the CP moves
with it – got that? Great! Oh here they come – nice coffee? No, you
haven’t missed anything; we were just talking about transonic AC
shift.
Now we can start to put it all together. The AC shifts aft, and
increases the static margin so the tailplane/foreplane has to produce
more lift to hold the aircraft level, and this substantially increases
the trim drag. So when the aircraft is trying to fly as fast as
possible there’s an inherent and unwelcome increase in drag, which
increases the supersonic fuel consumption and limits the cruise speed.
This is technically referred to as a “bad thing”(tm) so there are a
number of techniques used to overcome it, and this is where we get
back to where we started.
If you design the aeroplane so that the static margin is just
sufficient for stability when the aircraft is supersonic you
significantly reduce the supersonic trim drag and thus get lower fuel
consumption, longer range and higher cruising speeds. Unfortunately
when you slow down again the AC moves forwards and ends up in front of
the CG, so the static margin is actually negative and the aeroplane is
unstable. This is unpopular with the pilots because they’re
essentially pretty lazy types who don’t like to work for a living, so
the aircraft manufacturer reluctantly installs an expensive and
complex autostabilisation system to allow the pilots to catch up on
their sleep. And so there you have it, without so much as a whiff of
an agility pixie. Pitch instability isn’t the only issue incidently -
as the AC moves aft it affects directional stability as well, which is
why supersonic aircraft have such vast fins (and often more than one).
Negative stability isn’t the only solution, there are others. Concord
pumps fuel around to keep the static margin under control, and you
could in principle use variable geometry, although this would require
the wings to be moved forwards at supersonic speeds and you have
trouble keeping the structure inside the Mach cone. A particularly
elegant variable geometry solution was used on the cancelled XB-70
Valkyrie supersonic bomber project, which drooped its outer wing
panels through 60 degrees at supersonic speeds. This did three things:
• It provided a “tunnel” to trap the shock-lift at speeds above mach 2
• It reduced the wing area behind the CG, moving the AC forwards and
overcoming the supersonic trim drag issue
• It substantially increased the fin area when supersonic, overcoming
the directional stability problems due to the AC shift.
Much more than anyone really wanted to know, I’m sure. But hopefully
this note might go some way towards dispelling the myth that negative
stability is used to increase agility. As professional control system
designers will tell you, agility is controlled by agility pixies. But
that is a subject for another day.
PS. Don't shoot the messenger!
Cheers,
the_Dr.
Tinzinious Nicklefritz
>
> read more »- Hide quoted text -
>
> - Show quoted text -
Fabulous post. Thanks!
the_dr
Sorry it's so long, and it might be a load of crap, but I liked
it...kind of.
Tinzinious Nicklefritz
> On Jul 8, 2:02 pm, Tinzinious Nicklefritz <1978...@gmail.com> wrote:
>
No need for apologies whatsoever; it's on topic, informative and
written in grand style.
That post is an excellent example of exactly what this newsgroup needs
- fresh, intelligent, objective content with the purpose to inform
rather than the usual thinly-veiled hash designed to either self-
aggrandize or prop up some gibberish agenda.
Good stuff - enjoying your posts immensely.
the_dr
Thanks very much ... by the way I hope I made it clear *I * didn't
write it - I just copied it here. Unfortunately I have no idea who or
where it originated from, but apparently it's been posted on model
aircraft forums before.
It certianly made me think anyway!
Ed Rasimus
<euno...@yahoo.com.au> wrote:
>
>Yes you are correct, adverse yaw is turning against the banked turn.
>IE if you bank left in order to turn left the downward deflected
>airleron on the right would tend to cause more drag than the upward
>one on the left and thereby turn against the desired turn (adversely)
>
>However provers yaw (ie tightening of the turn) is also a problem and
>I believe more annoying and dangerous. It might be caused by
>excessive differential opperation of ailerons or friese ailerons. As
>the speed changes the degree of differential opperation would ideally
>change as well.
>
>those with flat bottoms, such as clarke-y tend to have more adverse
>yaw.
>
>Another way is to program the rudder to counter the yaw.
>
>Deflecting of a rudder will cause roll, so aileron action can be
>programmed to counter it as well.
>
>Most of these methods were already in use well before any form of FBW.
You make adverse yaw sound much more benign than it is in swept wing
aircraft. What you say is certainly true with regard to straight wing,
subsonic general aviation types like a Cessna 172. It is the reason
for a "coordinated" turn; one in which you input a bit of rudder to
accompany aileron deflection.
In a swept wing airplane, particularly at high AOA, the adverse yaw
can be quite violent and result in "departure from controlled
flight"--a term that essentially connotes snap-rolling into a spin.
What is happening is that the adverse yaw causes the aircraft fuselage
to partially blank the outside wing while moving the inside wing more
prominently into the relative wind. The immediate effect is to "lift"
the airplane opposite the direction of turn quite violently.
Early supersonic aircraft such as the F-100 were notorious in this
regard. The airplane was basically flown with pitch control and
rudder. Aileron deflections were to be avoided.
The hard-wing F-4 was not quite as bad, but had serious adverse yaw
issues at high AOA. (Read about Robin Olds' departure during F-4
checkout with Bill Kirk at Davis-Monthan in 1966 enroute to the 8th
TFW at Ubon. It's covered in "Fighter Pilot")
The F-105 didn't exhibit negative adverse yaw characteristics as it
employed a combination of ailerons and spoilers for roll control. The
rudder, however, could be used quite effectively in high AOA maneuvers
for rolling the aircraft.
Even the docile T-38 had a aileron-rudder interconnect built into the
flight control system. It also had a rudder deflection limiter that
cut rudder authority with the gear up. The airplane still could be
flown quite aggressively with the rudder.
Ed Rasimus
Fighter Pilot (USAF-Ret)
www.thundertales.blogspot.com
Ed Rasimus
Alarms should go off when the author dismisses "aerodynamicists" to
another room while he is carbon rod and clevis pin bolting AC to CP!
Until the advent of fifth generation fighters with actual super-cruise
capability, excursions into the supersonic realm were done with full
A/B power which effectively increases fuel flow by factors of four or
more over engine static military thrust.
To assert that "supersonic trim drag" is a governing factor in overall
design when placed in relation to the total fuel consumption is
ludicrous on its face.
To accept instability to save tenths of a percent in fuel consumption
during the relatively rare and brief excursions into supersonic makes
no sense.
Might be good to define stability at this late point:
Stability is tendency of the aircraft to deviate or return to
stabilized flight. Positive stability means an aircraft displaced from
stable constant state flight conditions will return to those
conditions naturally--Pulse the stick and you'll get decreasing
oscillations and return to original conditions.
Neutral stability means displacement from stabilized steady state will
simply result in a new stabilized steady state.
Negative stability means an aircraft displaced from steady state will
oscillate away from neutral. When large negative stability is
involved, the oscillations can be violent.
For a tactical aircraft, you want your control input to be effective
and you don't want the basic stability of the aircraft to resist what
you are trying to do.
the_dr
>
> To accept instability to save tenths of a percent in fuel consumption
> during the relatively rare and brief excursions into supersonic makes
> no sense.
>
> Ed Rasimus
> Fighter Pilot (USAF-Ret)www.thundertales.blogspot.com
Ed,
I had a discussion about this subject with the chief test/development
pilot for the Super Hornet program. He said exactly the same thing as
you regarding the time spent supersonic vs fuel economy.
I am surprised that whoever wrote that article did so in such an
uncompromising manner: If you are going to be contraversial, then
you'd better make sure you're right, otherwise you lose all
credibility. Maybe this is the reason I can't find out anything about
the author?
Cheers.
Dweezil Dwarftosser
> Dweezil Dwarftosser <f4...@yahoo.com> wrote:
> > Eunometic said...
> > > In reality fly by wire has been in service aircraft since the mid
> > > 1950's its just that its been inserted in a failsafe fashion in the
> > > mechanical linkages.
> >
> > No. Analog-augmented systems cannot be considered 'Fly-by-wire'
> > (the most bastardized term in aviation today), under any circumstance.
>
> Strictly yes BUT I was emphasising that electronic intervention or a
> sort of "fly by computer" was already essential to aircraft designed
> in the 1950s.
"Sort of" fly-by-computer? That's like saying that biological
brains (analog neural networks) are digital, because software
on a digital computer can produce a grossly-granular simulation
of learning patterns (to a very minor degree).
Please note (at this point, since it is the foundation of this
discussion) the very major differences between analog computing
and the highly-complicated, ingenious methods required in
digital software just to simulate an analog response.
An analog system _continuously combines_ all available control
and reporting inputs (as well as internal/external feedback),
arriving at an instantaneous 'analog state' of the entire system
at all times. In noisy or highly irregular evironments, damping
is built into the unit (often using proportional, integrated, and/or
derivative measures which provide correction on the basis of recent
events). Change a feedback resistance dynamically - or a capacitor
(statically) and you change the computer's continuous, insantaneous
solution.
A digital computer must read (and store) the state of its sensors
_sequentially_ across time (all the while being subject to temporarily
delay that process to accomodate a higher-priority interrupt), then
_sequentially_ 'sum' them (one at a time) into a format to be
_sequentially_ processed by math software capable of simulating
the analog process - but only in chunky bits, grossly representative
of the actual magnitude of the signal.
> The rate gyros and other sensors providing yaw, roll and pitch
> damping were evaluated at least partially electronically and
> inserted in the control paths.
> DD In the strict technical sense, FBW is nothing more than replacing
> DD the cables, rods, bellcranks, weights, and piano wire (whether
> DD augmented or not) that run directly from the control stick to the
> DD various hydraulic actuators - with electronic control wires.
>
> The mechanical linkages did however perform a crude mechanical
> computing function such as providing differential airlerons, roll
> compensation when the rudder is used, aileron deflection increases at
> low speed (eg when Mitsubishi A6M zero undercarruage was down) or
> switching to spoiler control instead of aileron control in big
> Boeings.
>
> So strictly a transfer to an immagined pure "Fly By Wire" without "Fly
> By Computer" could never have existed in a practical sense because "
> Fly By Computer " was already implemented and essential in the 1950s
> and earlier: only the computer was a hybrid analog mechanical
> incorporated in the rigging and supplemented by electronic stabillity
> augmentation systems added in to the mechanical linkages.
It's a real stretch to call a simple variable-length (or fulcrum
point) lever a "mechanical computer" - as it also is to name
hydraulic actuators (that can be electronically biased by dynamic
sensors) 'fly-by-wire'. Solid push/pull rods, cables, and pulleys
were still used to transmit the pilot's intentions to the control
surfaces; very safe, very reliable. (Barring catastrophic material
failure or battle damage.) If the electronic augmentation failed
in some way, the crew could immediately switch it off; they still
retained mechanical control of the aircraft.
'Fly-by-wire' is nothing more (and nothing less) than replacing those
cumbersome, internally-located cranks, cables, pulleys, and rods
with electronic control wiring which can be routed anywhere in the
fuselage - perhaps adding more internal fuel or a larger-diameter
jet engine where the old hardware used to be.
[ snippage ]
> > > Digital Fly By wire is simply a more convenient system ...
> >
> > True, but misleading. The only reason it is more convenient is
> > that programming can install new features without rebuilding the
> > hardware.
>
>
> The complexity of control laws is such that producing a competive
> practical analog electric 'fly by computer' is likely to be prohibitve
> in terms of cost and weight.
High cost and weight are certainly problem areas for analog
systems (as is large physical dimensions) - but the point you
are missing here is that the whole paradigm of complex (digital)
'control laws' was created (by true genuises) in order to
recreate a digital format that can come close to simulating the
resultant output of a millisecond-fast analog system.
> Try build a self adaptive lypunov non linear controller in analog
I'm unfamiliar with Lyponov - but the whole point of an analog
computer is that it was non-linear at all times; _everything_
was based upon dynamic (which you can consider 'adaptive') non-
linear calculus equations.
> or a kalman filter in analog or fuzzy logic.
That's really funny; Kalman was the true genius that finally
was able to raise the bar in software, so that the digital
machines could begin to approach the predictive nature of the
analog machines. Without him (and a few lesser beings) there
would be armies of college-boy computing geeks with absolutely
no sound, massive 'control math' libraries to include into
their bloated coding - and dangerously-deficient weapons-system
mysteriously dropping out of the sky all over the world.
> It is probably provably impossible.
Even funnier. Initially, Kalman was met with extreme skepticism
that his math could be used to improve digital processing so much
that it could approach the accuracy of analog systems. His stuff
was/is extremely complex; not truly understandable by most.
However, the proof was that it worked famously in real designs.
Remember: ALL of the digital control designs came about to simulate
(as much as possible) the RESULTS readily available from
instantaneous, continuous-solving (and simple) analog designs.
The RESULTS are what count in this digital/analog comparison - since
the discrete-level nature of a digital design cannot - ever - match
the instantaneous infinite-granularity accuracy of a functional analog
system.
Ken S. Tucker
Instead of the Horizontal Stablizer pushing the tail down,
there is efficiency to be gained by loading ~equal to the
positive lift of the main to have the HS pushing the tail up,
per unit of area, when flying level.
To pitch-up the HS is neutralized for lift, so it has no induced
drag, of course at a harder pitch up, the HS creates a heavier
tail that requires more lift (and induced drag) from the main,
to compensate for extra weight on the tail.
Clearly the 'negative stability' required for the above does
both enhance agility and reduce fuel consumption, (or permits
a smaller main, same thing).
Ken
Roger Conroy
"Ed Rasimus" <rasimus...@verizon.net> wrote in message
news:00mb36ts942t3872p...@4ax.com...
>>> > . It reduced the wing area behind the CG, moving the AC forwards and
>>> > overcoming the supersonic trim drag issue
IIRC When I was a kid my Dad demonstrated neutral stability during a flight
with just the two of us on board (Mom would have objected to such silliness)
by banking his Beech Bonanza A36 (can't speak for other versions) and taking
his hands and feet off the controls - the plane simply stayed banked.
kirk....@gmail.com
Well, it all sounds good, but....
Look up "decalage" and you will quickly understand that you do not
need downforce on the tail for stability - it's just one way to do
it. My 15 M racing glider (LS6-b) does not have downforce on the tail
when trimmed for cruise, as far as I can tell - or at low speeds,
either. Go to http://www.av8n.com/how/htm/aoastab.html#sec-pitch-equilibrium
for a good explanation.
And, although I'm not a physicist, I question the glib dismissal of
relation of stability to agility. Agreed that you can have low
stability and low agility (my glider in roll, for example - long wings
plus small control surfaces equals low agility i.e. low roll rate, but
it also has neutral roll stability; left alone it will diverge in
roll).
I think the truth is more complicated: relaxed static stability,
coupled with FBW - in a fighter - gives you lower weight (smaller tail
surfaces, smaller actuators, etc), higher performance, more agility
(takes less force to accelerate the airframe in different direction,
since you are not fighting as much natural stability), and allows
design for "carefree" handling. All very nice to have in a fighter.
In a bigger plane, such as an airliner, fuel savings become more
important, especially if the CG can be moved aft to reduce trim drag,
and the tail surfaces made smaller and lighter.
Kirk
Jim Wilkins
wrote:
> ...
> Look up "decalage" and you will quickly understand that you do not
> need downforce on the tail for stability - it's just one way to do
> it. My 15 M racing glider (LS6-b) does not have downforce on the tail
> when trimmed for cruise, as far as I can tell - or at low speeds,
> either. Go to
> http://www.av8n.com/how/htm/aoastab.html#sec-pitch-equilibrium
> for a good explanation.
> Kirk
Thanks, that really is a good explanation.
When I was little I found that my home made model sailboats would
automatically hold course if a small jib rigged far forward was
trimmed tighter than the main. That is apparently the canard version
of decalage, with a higher AoA on the forward airfoil.
When trimmed such that the course made good was at right angles to the
wind they would run back and forth along a line and usually return to
the dock.
jsw
Ed Rasimus
Actually the term originally referred to differences in the angle of
incidence between the two wings of a biplane. With monoplanes, it has
become the description of difference between angle of incidence of
main and supplemental wing surfaces. As you correctly note, it fits
canards as well as conventional empennage.
The Vari-Eeze aircraft used it quite effectively to make them idiot
proof for home-builders.
kirk....@gmail.com
> The Vari-Eeze aircraft used it quite effectively to make them idiot
> proof for home-builders.
> Ed Rasimus
But not quite idiot-proof enough for Colorado-based folk singers,
apparently!
Nasty little beast...a good RV4/6 will do all and more, off a shorter
strip, and be aerobatic to boot!
Kirk
Ken S. Tucker
wrote:
> > The Vari-Eeze aircraft used it quite effectively to make them idiot
> > proof for home-builders.
> > Ed Rasimus
>
> But not quite idiot-proof enough for Colorado-based folk singers,
> apparently!
> Nasty little beast...a good RV4/6 will do all and more, off a shorter
> strip, and be aerobatic to boot!
> Kirk
Kirk, what Rutan did is to twist the main swept wing (-4 at tip)
so it caused the CL to move rearward near stall speed, thus
placing more weight on the canard, to make sure it would
stall 1st, then it does a slight dip, gains speed and recovers.
It's the CL control we are discussing, try to keep that in mind.
Ken
Dan
Denver's aircraft had a manual fuel valve in an odd orientation. He
had to contort himself to operate it which apparently contributed to the
crash. One of the problems with homebuilt aircraft is the builder can be
as illogical as he wants. Denver was a competent pilot, Vari-Eze is a
good design, but Denver made a bad decision when he bought the airplane
as is.
kirk....@gmail.com
> On Jul 9, 10:11 am, "kirk.st...@gmail.com" <kirk.st...@gmail.com>
> wrote:
>
> > > The Vari-Eeze aircraft used it quite effectively to make them idiot
> > > proof for home-builders.
> > > Ed Rasimus
>
> > But not quite idiot-proof enough for Colorado-based folk singers,
> > apparently!
> > Nasty little beast...a good RV4/6 will do all and more, off a shorter
> > strip, and be aerobatic to boot!
> > Kirk
>
> Kirk, what Rutan did is to twist the main swept wing (-4 at tip)
> so it caused the CL to move rearward near stall speed, thus
> placing more weight on the canard, to make sure it would
> stall 1st, then it does a slight dip, gains speed and recovers.
> It's the CL control we are discussing, try to keep that in mind.
> Ken
In other words, a tweak to avoid the nasty problem inherent to all
canards - main wing stalling before canard. If you look carefully at
a Vari (or Long-) Eze, there are all sorts of aero tweaks to make it
acceptable; compare that to a classic, conventional configuration such
as any of the RVs, or a Mustang 2, etc. A lot of the mystique of
Rutan's canards was due to the unconventional (and to some, exotic and
"sexy") appearance of the VariEze and LongEze, as well a some really
good marketing by Rutan and his followers. And if all you want is a
small crosscountry airplane that can only fly off hard, long runways
and not be acro capable but looks real cool, then it's OK.
I guess you can tell I'm not a fan of canards - they are a solution to
a problem that doesn't exist. And to me, the best proof is that in
the world of gliders, which have to be efficient and maneuverable at
the same time, Rutan's attempt at a canard glider (the Solitaire) was
an abysmal failure.
I find the Euro Canards (and the Chinese newcomer) interesting - a
fad, perhaps? I can see the logic for the Viggen and Gripen, where
the canard is used to satisfy the Swedish need for STOL capablility,
but it's interesting that neither the Soviets/Russians, or the US,
have gone down that path, despite lots of testing of canard
prototypes.
Cheers,
Kirk
Jim Wilkins
> ...
> Denver's aircraft had a manual fuel valve in an odd orientation. He
> had to contort himself to operate it which apparently contributed to the
> crash. One of the problems with homebuilt aircraft is the builder can be
> as illogical as he wants. Denver was a competent pilot, Vari-Eze is a
> good design, but Denver made a bad decision when he bought the airplane
> as is.
>
> Dan, U.S. Air Force, retired
The way I understood it the builder didn't want fuel lines in the
cockpit and so located the selector on the rear bulkhead, in a
position I can't reach without unbuckling the seatbelt.
jsw
Ken S. Tucker
wrote:
> On Jul 9, 1:01 pm, "Ken S. Tucker" <dynam...@vianet.on.ca> wrote:
>
>
>
> > On Jul 9, 10:11 am, "kirk.st...@gmail.com" <kirk.st...@gmail.com>
> > wrote:
>
> > > > The Vari-Eeze aircraft used it quite effectively to make them idiot
> > > > proof for home-builders.
> > > > Ed Rasimus
I fairly much agree (I've built many flying models of canards).
They can be quite aerodynamically superior, but that is at the
sacrifice of forward visibility. A big canard can do amazing
things, like when set at 30 degs can do a controlled descent
(slowly) at about 45 then level off real quick when the canard
stall is corrected, but the problem is a trade-off: IMHO I favor
visibility, a key factor in any kind of normal flying, including
combat flying.
Ken
Ken S. Tucker
Just to add, I used a large hard packed sand pit and drew the
front part of the a/c on it to scale then sat in the cockpit
location,
to actually get a view of obstructions,.
Personally I value visibility so the canard obstruction was a no-go
for me.
Ken
Eunometic
> On Wed, 7 Jul 2010 17:00:30 -0700 (PDT), Eunometic
>
>
>
>
>
> <eunome...@yahoo.com.au> wrote:
>
> >Yes you are correct, adverse yaw is turning against the banked turn.
> >IE if you bank left in order to turn left the downward deflected
> >airleron on the right would tend to cause more drag than the upward
> >one on the left and thereby turn against the desired turn (adversely)
>
> >However provers yaw (ie tightening of the turn) is also a problem and
> >I believe more annoying and dangerous. It might be caused by
> >excessive differential opperation of ailerons or friese ailerons. As
> >the speed changes the degree of differential opperation would ideally
> >change as well.
>
> >Adverse yaw is reduced by selecting more appropriate wing sections:
> >those with flat bottoms, such as clarke-y tend to have more adverse
> >yaw.
>
> >Another way is to program the rudder to counter the yaw.
>
> >Deflecting of a rudder will cause roll, so aileron action can be
> >programmed to counter it as well.
>
> >Most of these methods were already in use well before any form of FBW.
>
> You make adverse yaw sound much more benign than it is in swept wing
> aircraft. What you say is certainly true with regard to straight wing,
> subsonic general aviation types like a Cessna 172. It is the reason
> for a "coordinated" turn; one in which you input a bit of rudder to
> accompany aileron deflection.
I've read that the use of 'analog fly by wire' for the ailerons
greatly reduced the possibillity of adverse yaw on the F-15 whose
handling was highly rated I believe.
>
> In a swept wing airplane, particularly at high AOA, the adverse yaw
> can be quite violent and result in "departure from controlled
> flight"--a term that essentially connotes snap-rolling into a spin.
>
> What is happening is that the adverse yaw causes the aircraft fuselage
> to partially blank the outside wing while moving the inside wing more
> prominently into the relative wind. The immediate effect is to "lift"
> the airplane opposite the direction of turn quite violently.
So its an issue of low aspect ratio aircraft when they have long
noses.
>
> Early supersonic aircraft such as the F-100 were notorious in this
> regard. The airplane was basically flown with pitch control and
> rudder. Aileron deflections were to be avoided.
>
> The hard-wing F-4 was not quite as bad, but had serious adverse yaw
> issues at high AOA. (Read about Robin Olds' departure during F-4
> checkout with Bill Kirk at Davis-Monthan in 1966 enroute to the 8th
> TFW at Ubon. It's covered in "Fighter Pilot")
One feature absent from the f-4 is the ventral fins seen on many
other. Perhaps this might have helped a little.
>
> The F-105 didn't exhibit negative adverse yaw characteristics as it
> employed a combination of ailerons and spoilers for roll control. The
> rudder, however, could be used quite effectively in high AOA maneuvers
> for rolling the aircraft.
Showed up on the Blackwidow nightfighter.
The famous 'row' between willy mersserschmitt and the head of the
luftwaffe, erhard milch was over a stall occuring on a
>
> Even the docile T-38 had a aileron-rudder interconnect built into the
> flight control system. It also had a rudder deflection limiter that
> cut rudder authority with the gear up. The airplane still could be
> flown quite aggressively with the rudder.
I don't know who did the first aileron rudder cross link, it was
definetly showing up immediatly post war, but the mechanical problem
was surely solved by the time the 'V' or butterly tail was developed.
Eunometic
wrote:
>
> In other words, a tweak to avoid the nasty problem inherent to all
> canards - main wing stalling before canard.
There is possibly another issue: lack of stall warning eg pre-stall
buffet.
The Curtiss XP-54 never seems to have gotten past this issue.
Of course this can be dealt with by a stick shaker.
Ed Rasimus
AFAIK the main wing stalls before canard OR horizontal stab in all
aircraft. Since it is the primary lifting surface, it would define the
aircraft's aerodynamic stall.
Swept wing aircraft generally have a very broad band of pre-stall
buffet and in fact during maneuvering flight most of them exhibit
considerable buffeting. Lift curves of swept wing aircraft are
generally quite rounded while straight wing lift curves are peaking
with an abrupt reversal of lift generated for increased AOA at the
stall point.
konakover
If done properly the canard will have a higher wing loading and will
stall first at a given airspeed, allowing the nose to drop, the craft
to gain airspeed and thus prevent the main wing from stalling at all.
During the early days of the ATF program some AF brass said "the best
place for a canard, was on your enemies plane" or words to that
effect. I always assumed it was due to visability or weapon
obstruction.
Ed Rasimus
wrote:
>
>During the early days of the ATF program some AF brass said "the best
>place for a canard, was on your enemies plane" or words to that
>effect. I always assumed it was due to visability or weapon
>obstruction.
ATF was to be optimized for stealth. (YF-23 was arguably a stealthier
offering while -22 was more agile.)
Lumps, bumps, flapping surfaces, articulating nozzles, etc. all create
flash-back points for sensors. Moving control surfaces to aggressively
maneuver your aircraft is something avoided until within visual range.
Until that time, mission planning would consider minimum maneuver
routes so as to control where reflected energy off your aircraft is
going to be focussed.
Canards are exposed, moving surfaces. That makes them significant
additonal reflection points. Love to see them on opposition. Hate to
have them on your system.
Ken S. Tucker
Thanks Ed for that insight. I think the generation 2020 fighter will
be
able to fulfill that using thrust vectoring, apart from 'articulating
nozzles'.
(It's a bit nutty to try to sequence a/c generations, better to shoot
for
what we could get operation at such n such a date like 2020).
Very interesting requirement.
> Ed Rasimus
> Fighter Pilot (USAF-Ret)www.thundertales.blogspot.com
Cheers
Ken
Roger Conroy
"Ken S. Tucker" <dyna...@vianet.on.ca> wrote in message
news:b2f4601b-94fc-4254...@j13g2000yqj.googlegroups.com...
What makes canards so much worse than any other flapperey bits?
I don't really buy the visibility argument as canards (in their modern
military manifestation) are all well behind the cockpit.
Dweezil Dwarftosser
> Ed Rasimus <rasimusSPAML...@verizon.net> wrote:
[ snip ]
> > Early supersonic aircraft such as the F-100 were notorious in this
> > regard. The airplane was basically flown with pitch control and
> > rudder. Aileron deflections were to be avoided.
> >
> > The hard-wing F-4 was not quite as bad, but had serious adverse yaw
> > issues at high AOA. (Read about Robin Olds' departure during F-4
> > checkout with Bill Kirk at Davis-Monthan in 1966 enroute to the 8th
> > TFW at Ubon. It's covered in "Fighter Pilot")
>
> One feature absent from the f-4 is the ventral fins seen on many
> other. Perhaps this might have helped a little.
I know nothing about aerodynamics, but ventral fins on an
F-4 may have been too much with four AIM-7s missiles
'submerged' into its underside - particularly with large-
finned -7E-2s, (the 'dogfight' missile).
BTW - none of these missiles were perfectly aligned with
'straight-and-level' direction of flight; they all 'toed-in'
toward the aircraft centerline a bit.
Ed Rasimus
The ventral doesn't deal with adverse yaw, it deals with longitudinal
stability. The only ventral fins I can think of, are the stub on the
F-104 and the fairly prominent one on the F-105. The one for the 105
was added to correct a tendency to dutch roll or "snake" in flight.
The characteristic was still observable when carrying a fully loaded
C/L MER which blanked the ventral fin. On refueling the airplane would
start a slow swinging back and forth while on the boom. Oscillations
would increase until you were simply slung off at the lateral boom
limits.
Solution was to cross-control after hook-up. Some rudder to yaw and
opposite aileron to counter. The airplane would sort of cock sideways
a few degrees and be as stable as a rock.
The F-4 had no such problem. Lateral stability was enhanced by the
turned up wing tips, but adverse yaw at high AOA was still an issue.
It was discovered that the inboard station stubs enhanced lateral
stability and so they were always carried, even on a "clean" airplane.
Ed Rasimus
The issue is managing your spikes (reflections of sensor hits).
Trailing edge control surfaces on the wings are somewhat shielded from
some viewing aspects and carefully faired into the curves of the wing.
Canards are protrusions from an otherwise smooth fuselage.
When you start multiplying the control surfaces that are flapping
about you get many more spikes to consider.
Dan
We flew slick F-4E at Zaragosa and Incirlic without inboards if
memory serves. Was it just earlier models that had the problem or were
we just being different?
Ed Rasimus
If they were LES (soft-wing) that might have been allowable. On the
hard-wing, it was a TO requirement to carry the stubs even without
stores. The LES birds were after my time.
Dan
OK, that explains it. We had the slats. This was 1978 - 1980.