Flying Wing Aerodynamics
William D. Allen Sr.
particular are they severely limited in pitch authority for recovering from
steep dives? And are they effectively limited to subsonic speeds?
Yours truly,
William D. Allen Sr.
end
Bev Clark/Steve Gallacci
William D. Allen Sr. <ball...@home.com> wrote:
>Looking for sources regarding the aerodynamics of flying wing aircraft. In
>particular are they severely limited in pitch authority for recovering from
>steep dives? And are they effectively limited to subsonic speeds?
>
pitch and yaw contol are very dependent on wing sweep and airfoil.
On things like the YB35/49, the modest sweep didn't help pitch or yaw any.
Some of the Horton wings had both more sweep and better control. Their
airfoil section and such might have been more helpful too.
As for Mach, I don't really know, but the one thing a supersonic plane
doesn't need is oodles of wing area, and the nature of most flyuing wing
configs are in conflict with neat supersonic aerodynamics.
df
>particular are they severely limited in pitch authority for recovering from
>steep dives? And are they effectively limited to subsonic speeds?
There is a Flying Wing bibliography and some pointers on my website.
Note especially the Nurflugel (all-wing) page, which is pretty much
the gold standard on all-wing aircraft.
Agmessier
>particular are they severely limited in pitch authority for recovering from
>steep dives? And are they effectively limited to subsonic speeds?
>
William,
You asked a very tricky question. I received my B.S. in aeronautical
engineering last year, and my senior design project happened to be a tailless
flying wing unmanned strike aircraft. I was the stability and control engineer
on the design team, so I had the chance to learn first hand about all the
problems encountered in flying wing design.
I have an excellent source that describes all the performance characteristics
of flying wings and how to design a stable and controllable flying wing
aircraft. I don't have it with me, though. I will have to dig it up later.
I don't know how much you know about flight dynamics, so I'm going to try to
explain the basics in very simple terms. Most aircraft have tails that exert
an aerodynamic force downward to 'trim' the aircraft in flight. This is partly
because in order to be stable, the center of gravity has to be relatively far
forward of the aerodynamic center of the wing, giving it a natural tendancy to
pitch downward. This effect must be accounted for in one of two ways on a
flying wing design. A reflexive airfoil can be used, which has a trailing edge
section that 'bends' upward. This is very different than most cambered
airfoils which have a tendancy to pitch the plane downward when they produce
lift. Another solution is to use wing washout (twist) with a swept back wing.
A twisted wing has its wing tips at a lower angle of attack than the root
section. If the wing is swept back sufficiently, the wingtips are further back
than the rest of the plane, so this section can act to trim the aircraft.
The problem with these two designs is that they can only trim the aircraft if
it is flying at its design speed. If it's flying faster, it keeps wanting to
pitch upward (trading speed for altidude, tending toward its design speed.),
and just the opposite if it's going slower. And flying wings by nature don't
have much control authority to counter this effect. The only way around this
is to have a plane with very little static stability. If you are willing to
accept a more unstable aircraft, you can have one that can fly trimmed at all
angles of attack, provided it is still controllable. This is only possible
with a fly-by-wire control system, because the faster an airplane goes, the
faster the response time needed to control it.
Basically what it all boils down to is this. A low-speed flying wing has a
large aspect ratio, because it can fly efficiently at low speeds. This type of
airplane can have very good flight characteristics. In order to fly
supersonic, though, it needs a large sweepback angle. Most supersonic aircraft
have a shock wave coming off the nose, slowing the flow down before it reaches
the wing. Not true with a flying wing. This makes the plane very draggy
unless it has a large sweepback angle. The consequence of this is a very low
aspect ratio, and a plane with a greater length relative to span. This is not
as efficient in flight as a high aspect wing, and is not very stable laterally
either because the control surfaces are not far enough from the centerline of
the aircraft. A plane like this would fly supersonically just fine, but would
not be as controllable or as maneuverable at low speeds due to its high inertia
about the pitch axis, and the fact that the control surfaces are not very far
back ( they're stuck on th wing, no tail). This airplane would have to have a
fairly high landing speed, and would not be able to use any high lift devices
like landing flaps very effectively, because they would pitch the aircraft in
the wrong direction.
There are a lot of different possibilities and configurations possible with
flying wing design. However, it is MUCH more constrained to flying at the
flight conditions for which it is designed and flying wings are less practical
for high-speed aircraft. The basic aerodynamic rule of thumb is the higher
the aspect ratio, the more efficient the wing. The basic supersonic rule of
thumb is the faster it goes the longer and more slender the aircraft is.
Conventional aircraft can manage to have both a moderately high aspect wing and
a long, slender shape. Look at the F-105 or F-16. With only a wing, high
aspect ratio directly correlates with small sweepack angles and higher
supersonic wave drag.
I was surprized to learn in my design class how difficult it is to make a
flying wing with the desired flight characteristics. I was also surprized to
learn all the clever ways they've found to make such a plane controllable.
More and more research is being done, though, and I think flying wings are
going to become a big trend in the aircraft industry. Low speed flying wings
can fly more efficiently than conventional aircraft, but the only reason you
would want a supersonic flying wing is for stealth reasons (unless landing
speed and maneuverability are of absolutely no concern). Hope you've found
this useful. I'll try to find the name of this one book I used, too. It
focuses on low speed designs, but it would give you a good idea of the general
characteristics of flying wings.
Andy
RotorDyne
This one went to the hard drive =)
Al Bowers
bev...@netcom.com (Bev Clark/Steve Gallacci) writes:
> In article <HJoT2.185$mB5...@news.rdc2.occa.home.com>,
> William D. Allen Sr. <ball...@home.com> wrote:
> >Looking for sources regarding the aerodynamics of flying wing aircraft. In
> >particular are they severely limited in pitch authority for recovering from
> >steep dives? And are they effectively limited to subsonic speeds?
> I don't know of any formal data on such, but from what I understand,
> pitch and yaw contol are very dependent on wing sweep and airfoil.
> On things like the YB35/49, the modest sweep didn't help pitch or yaw any.
> Some of the Horton wings had both more sweep and better control. Their
> airfoil section and such might have been more helpful too.
> As for Mach, I don't really know, but the one thing a supersonic plane
> doesn't need is oodles of wing area, and the nature of most flyuing wing
> configs are in conflict with neat supersonic aerodynamics.
There are a couple of good books in the area, one being by Wohlfahrt
and Nickel (Dr Karl Nickel was one of the principals at Horten
Flugzeugebau), "Tailless Aircraft: In Theory and Practice" by the AIAA
in the USA and by Arnold in England. There is also the book on model
flying wing theory by Bill and Bunny Kuhlman (aka B^2) called "On the
Wing..." published by them in Seattle WA USA.
It should be noted that while the theory of this class of flying wings
has been written about to some extensive detail, there are few (if
any) good books on the theory of flying wings as practiced by the
Horten brothers. There are tremendously significant differences
bewteen flying wing aircraft as practiced by the Hortens and nearly
everyone else. The Hortens were much more interested in flying wings
("Nurflugel" in German for "wing only") as _integrated_ aircraft, a
rather modern concept (system integration). They were also very aware
of span loading as we think of the term today prior to 1940 (possible
as early as 1933). You'll have difficulty finding a through
explanation of their complete theory (in any language), but I'll take
a stab at a quick summary:
Let us assume you begin with a given span. It is well known that the
minimum induced drag for a given span and a given lift coefficient is
an elliptical span load. Prandtl formulated that theory and one of
the his students, Max Munk, did the optimization for this problem back
in 1918. However, this isn't necessarily the optimum solution (and in
fact this is where most aerodynamic theory books stop). Using that
elliptical span load, you need integrate the span load to find the
wing root bending moment. Given that elliptical span load's
associated wing root bending moment, what would happen if you
unconstrain span, holding the total lift and wing root bending moment
constant?
In fact, two people have done this, Ludwig Prandtl (Ref 1) and R T
Jones (Ref 2). Both found the same result. If you stretch the span,
you can achieve an 11% DECREASE in induced drag with a 22% INCREASE in
span (now remember, we held lift and wing root bending moment
constant) over an equivalent elliptical span load. This is an
important finding, because if you introduce the structural constraint,
then the elliptical span load is not optimum.
This isn't the entire story, however. It gets better. Reimar Horten
developed an entore line of aircraft based on this theory (Ref 3). If
the wing has an increased washin initially, this will increase the
upwash farther out towards the tip. Then if the washout is displaced
to the tips, the resultant lift vector of the tip will be FORWARD of
the average angle of attack vector. This implies that as you increase
the lift on one wing, the lift will pull that wing FORWARD. Think
about what this implies for flying wings in the area of adverse yaw.
With a FORWARD component of an increased lift, the wing traveling UP
will move FORWARD also. The span load of Prandtl and Jones negates
adverse yaw.
In fact, this was the basis of Dr Reimar Horten's PhD dissertation
which he completed while in prison in late 1945. Horten had known
this for years and had been using in his sailplane designs, but he
didn't publish the work until 1945 (Ref 4).
It should be noted that the traditional "linear" twist distribution of
washout in swept wings will NOT produce the needed upwash at the wing
tips to overcome adverse yaw. this twist distribution is the one that
most people use with a rectangular wing planform to achieve a near
elliptical span load.
The only conclusion I can draw is that Horten was a genius in the best
sense of the word. Horten solved several very difficult and sticky
issues for flying wings with his span load work; he solved adverse
yaw, and wing root bending moment, while minimizing induced drag (for
the given structure).
Ref 1: Prandtl, L.: "Uber Tragflugel des kleinsten unduzierten
Widerstandes"; Zeuts. Flugtechnik und Motorluftschiffahrt, Vol 24, pp
305-306, Nov 1933.
Ref 2: Jones R T: "The Spanwise Distribution of Lift for Minimum
Induced Drag of Wings having a Given Lift and a Given Bending Moment";
NACA TN 2249, Dec 1950.
Ref 3: Horten, Reimar, and Selinger, Peter: "Nurflugel: Die Geischicte
der Horten-Flugzeuge 1933-1960"; Weishaupt Verlag, Graz, 1983.
Ref 4: Myhra, David; "The Horten Borthers: and their all wing
aircraft"; Schiffer, Atglen PA, 1998.
Al Bowers
df
<bow...@orville.dfrc.nasa.gov> wrote (a long & detail post on all-wing
aerodynamics):
Thank you, Al. What a refreshing sight after wading through
screens-ful of Kosovo flames.
Agmessier
aerodynamic theory and flight mechanics to consider structural constraints to
increase the efficiency of a wing. It makes a lot of sense, though. A couple
of questions(by the way, I intend to read more about Horten after reading your
post, but I don't want to wait for answers.):
> If you stretch the span,
>you can achieve an 11% DECREASE in induced drag with a 22% INCREASE in
>span
1. How badly does this affect parasite drag? 22% longer span would have to
increase your parasite drag substantially. If this is significant, wouldn't
this only benefit low speed performance? This would lower your optimum
cruising speed for a given aircraft(all other things being equal), but would it
ultimately improve the range of such a design?
>If
>the wing has an increased washin initially, this will increase the
>upwash farther out towards the tip. Then if the washout is displaced
>to the tips, the resultant lift vector of the tip will be FORWARD of
>the average angle of attack vector. This implies that as you increase
>the lift on one wing, the lift will pull that wing FORWARD. Think
>about what this implies for flying wings in the area of adverse yaw.
>With a FORWARD component of an increased lift, the wing traveling UP
>will move FORWARD also. The span load of Prandtl and Jones negates
>adverse yaw.
2. Is adverse yaw a problem with flying wings? I had always understood that
adverse yaw was an attribute of forward swept wings, which is why they're so
unstable. I know that wing sweep and tail fins both contribute to roll-yaw
coupling in a favorable direction, right? Do you mean that N sub p (Yaw due to
roll rate), N sub beta(yaw due to slideslip or roll angle(indirectly)), or N
sub p-dot(yaw due to roll acceleration or rolling moment) is adverse? Your
discussion leads me to believe it is N sub p-dot(not N sub beta, which is
traditionally studied as the definitive derivative for roll-yaw coupling). Why
is this adverse? I'm thinking it may be the products or moments of inertia of
your typical long span, high aspect flying wing, but I'm not sure. Please
explain.
Thanks in advance for your response.
Andy
wal...@oneimage.com
>>bev...@netcom.com (Bev Clark/Steve Gallacci) writes:
>
since I scanned it in store I unfortunately do not recall the title nor the name of
the mag itself. But I gathered the machine wasn't very stable in pitch - as one can imagine.
The problem as I see it is reconciling the need for low speed lift to get airborne with
the shift in aerodynamic center with change of angle of attack due to the necessity for
low speed lift. A symmetrical wing doesn't have that shift but then it doesn't have much
low speed lift either. In addition because of the short moment arm in pitch the CG must be kept
within very close limits. Tough problem in operation!
Yaw control using drag rudders is really 'no problem' any more. Granted, it may entail use of
a powered damper system but hey, there's at least one of these as a STC for the venerable Bonanza!
Biggest problem for airline use would be getting into a gate somewhere without folding the wings.
Another problem one doesn't think of is ground effect which would commence about semi-span altitude.
This coupled with loss of pitch control effectivity at low speed could cause real probems trying to
control touchdown attitudes.
Lots of things to juggle during the design of flying wings!
Walt Bj ftr plt ret
df
When I was editing the diaries of Glen Edwards, who was killed in the
crash of the YB-49 Flying Wing bomber, I interviewed two of the men
who'd flown it, (General) Robert Cardenas and former Northrop test
pilot Charlie Tucker. Naturally they had directly opposed
recollections of its stability. I've posted extensive notes on those
interviews at http://www.danford.net/edwards.htm
Another good place to look is the Nurflugel Page; there's a pointer to
it on my website. (I apologize for earlier posting a note to this
effect and neglecting to include the url; in order to cut through the
Kosovo Krap I've adopted a new newsreader and haven't yet whipped it
into shape.)
All the best - Dan Ford
See "Nothing New About Death" at http://www.danford.net
and the message board at http://www.delphi.com/annals/
Al Bowers
agme...@aol.com (Agmessier) writes:
Thanks to Andy, Dan Ford, and Walt for their kind words...
> Al, I found you post very interesting and informative. I'm usually
> too bent on aerodynamic theory and flight mechanics to consider
> structural constraints to increase the efficiency of a wing. It
> makes a lot of sense, though. A couple of questions(by the way, I
> intend to read more about Horten after reading your post, but I
> don't want to wait for answers.):
I think the Hortens had a very fresh look at the problem. On of the
things that took me _YEARS_ of digging to resolve was how Reimar
Horten, the protoge, could throw out the elliptical span load created
by his own mentor, Ludwig Prandtl? It made no sense to me. In fact,
it was work that Prandtl did in exploring the span load that led to
the work of the Hortens. But it was the Hortens that found the
adverse yaw benefits and did the flight mechanics work to expand the
theory of these span loads.
>> If you stretch the span, >you can achieve an 11% DECREASE in >
>> induced drag with a 22% INCREASE in span
> 1. How badly does this affect parasite drag? 22% longer span would
> have to increase your parasite drag substantially. If this is
> significant, wouldn't this only benefit low speed performance? This
> would lower your optimum cruising speed for a given aircraft(all
> other things being equal), but would it ultimately improve the range
> of such a design?
This is a problem. One of the things the Hortens did NOT have access
to during WWII was a wind tunnel. So all their work had to be solved
through flight experiments. And the only way to effectively make
changes in flight experiments was to make them _small_. This allowed
the changes to be easily made and the results could be tested rather
quickly. There are corollaries to this, but the opposite approach was
taken by Jack Northrop with his flying wings. I think that had
Northropo stuck to smaller aircraft for longer he might have solved
more of the problems in his airplanes 9though this may not have been
true). But it IS a fact that changes to large aircraft are difficult
to effect.
There is an optimum at which the span load works. In some simple
analysis I've done on a couple of Horten designs (with generous help
from David Lednicer and Reinhold Stadler; both to whom I am deeply
indebted) shows that the benefits are not across the entire envelope.
As a lower lift coefficient is trimmed to, the span load is less and
less effective than at higher lift coefficients. Unfortunately, this
goes hand in hand with lower and lower directional stability. So it
is the worst of both worlds at low lift coefficients because the
directional; stability is decreased and the adverse yaw arises again.
Now my analysis was done for a simple elevon configuration (if you are
familiar with Horten wings, my analysis was for the Horten H Xc
configuration). A full trailing edge multiple elevon configuration
_might_ be able to solve the trimmed lift coefficient problem and
maintain the Horten span load. I don't know, I haven't done that
analysis. I suspect that was part of the complexity of the control
system used on the Horten H IX and Horten H XI sailplanes. But
aeroelastics start to affect the validity of the results as David
Lednicer found.
> >If
> >the wing has an increased washin initially, this will increase the
> >upwash farther out towards the tip. Then if the washout is displaced
> >to the tips, the resultant lift vector of the tip will be FORWARD of
> >the average angle of attack vector. This implies that as you increase
> >the lift on one wing, the lift will pull that wing FORWARD. Think
> >about what this implies for flying wings in the area of adverse yaw.
> >With a FORWARD component of an increased lift, the wing traveling UP
> >will move FORWARD also. The span load of Prandtl and Jones negates
> >adverse yaw.
> 2. Is adverse yaw a problem with flying wings? I had always
> understood that adverse yaw was an attribute of forward swept wings,
> which is why they're so unstable. I know that wing sweep and tail
> fins both contribute to roll-yaw coupling in a favorable direction,
> right? Do you mean that N sub p (Yaw due to roll rate), N sub
> beta(yaw due to slideslip or roll angle(indirectly)), or N sub
> p-dot(yaw due to roll acceleration or rolling moment) is adverse?
> Your discussion leads me to believe it is N sub p-dot(not N sub
> beta, which is traditionally studied as the definitive derivative
> for roll-yaw coupling). Why is this adverse? I'm thinking it may
> be the products or moments of inertia of your typical long span,
> high aspect flying wing, but I'm not sure. Please explain.
Yes, adverse yaw is a problem in flying wings without verticals (the
"classic" Horten design). It needs to be noted that proverse yaw (the
opposite of adverse yaw) is also not desirable. In fact a pilot can
tolerate more adverse yaw than proverse yaw (this is documented in Mil
Std 1797a for handling qualities).
Adverse yaw is usually explained as an artifact of induced drag. The
wing the pilot commands to rise increases the lift. This increases
the adverse yaw, which drags the up-moving wing aft, opposite to the
desired yaw direction. This isn't dependent on wing sweep (if I
misunderstood this part of your question, and I suspect I did, please
let me know).
So far the work I have done has been on the initialization of the
roll, so the proverse and adverse moments are (in my notation) Cnda
(or coefficient of yaw due to delat aileron; which is differential
elevon; in your notation it would be N sub da?). Once beta is
introduced, then Cnb (N sub beta) and Clb (L sub beta) drive the
dynamics. And once a roll is estabilshed, there are additiional Cnp,
Clp, Cnr and Clr terms that come into play. As for the acceleration
terms, Clpdot, Cnpdot, Clrdot, and Cnrdot, it get REALLY confusing
because you really should break those out into betadot and pdot and
rdot terms (please, lets ignore this, it is hard enough trying to
explain this in English without waving my arms and hands in the air!).
All the moments I've dealt with are for static moments. To introduce
moments of inertia for the actual accelerations (steady state
rotations don't need inertias) in transient motion. I'm trying to
solve the simple static probalmes first. there are others who have
worked some of the dynamic transient stuff (Gregg MacPherson of New
Zealand, and Robert Osbourne of England, are two).
Al Bowers
Al Bowers
wal...@oneimage.com writes:
> Al Bowers <bow...@orville.dfrc.nasa.gov> wrote:
> >>bev...@netcom.com (Bev Clark/Steve Gallacci) writes:
> There was a recent articel oncerning flying the Horten glider in an
> aviation mag - since I scanned it in store I unfortunately do not
> recall the title nor the name of the mag itself. But I gathered the
> machine wasn't very stable in pitch - as one can imagine.
Actually, pitch stability of the Horten wings was always very strong.
Their span load creates a very strong pitch trim moment, which can
only be countered with a rather far forward CG. However, in years of
work, the Hortens never were able to achieve a sustained spin or
tumble in their designs. The two most serious losses were in
departures from their traditional work. A short synopisis:
H IVb: used a laminar flow wing at the wrong Reynolds numbers. the
resulting separation resulted in a spin which was unrecoverable. The
pilot was fatally injured.
H IX V2: this was the Horten twin jet fighter prototype. the engine
out characteristics with insufficient procedural background (in the
early development of jets, this was pretty endemic). Loss of
directional stability resulted in ground impact during an aborted
single engine landing. Fatal to the pilot.
many other Hortens were lost due to ground handling or landing
accidents (overshot and flew through trees, lost in hail storms, PIO
into the ground) but most were due to errors on the part of the pilot
or pilot technique (particularly the PIO problem wich was aeroelastic
coupled with novices); not due to poor design (which you may have
gathered, I am biased towards the Horten aircraft!).
> The problem as I see it is reconciling the need for low speed lift
> to get airborne with the shift in aerodynamic center with change of
> angle of attack due to the necessity for low speed lift. A
> symmetrical wing doesn't have that shift but then it doesn't have
> much low speed lift either. In addition because of the short moment
> arm in pitch the CG must be kept within very close limits. Tough
> problem in operation!
This is another design departure between Horten/Lippisch and the
approach by Northrop. Northrop felt that symmentic airfoils would
allow him to achieve the performance he desired. As a result, in many
photos, you'll note the outboard elevons deflected slighty up in the
Northrop wings. In the case of the Hortens and Lippisch, both used
cambered reflexed airfoils (mostly Goettinen developed airfoils) to
achieve postivie lift coefficients at zero pitching moment.
> Biggest problem for airline use would be getting into a gate
> somewhere without folding the wings.
There was a good article in "Flight International" (Apr 98 Vol 54 No
4) about flying wing airliners. I'm afraid that is all I can say
about that. The article is worth looking up and reading.
> Another problem one doesn't think of is ground effect which would
> commence about semi-span altitude. This coupled with loss of pitch
> control effectivity at low speed could cause real probems trying to
> control touchdown attitudes.
Porposing is a problem because of the close coupling too. there have
been lots of "wrong" answers to the ground handling and power approach
work done on flying wings. Eventually, someone will figure them out.
Best regards,
Al Bowers
Agmessier
Thanks for clarifying. I was thinking of 'adverse yaw' in terms of free body
roll-yaw coupling (in which sweep IS a factor), not as a control response.
Thanks for clarifying, it all makes sense now.
Andy
Maury Markowitz
Whoa, I missed this one and it's one I always wanted to know. What is the
cause of roll-yaw coupling? And if it's sweep related, why was the X-3 one
of the classic examples for it?
Maury
Agmessier
Imagine an aircraft with swept-back wings flying AT AN ANGLE OF ATTACK (you may
need to use your hand or something as a visual aid). If it happens to roll to
the right, the right wing moves forward a bit into the oncoming wind, while the
left wing moves backwards, away from the wind. This puts the aircraft in a
sideslip, where, in an aircraft-fixed reference frame, the airflow is coming
across the aircraft a little bit from right to left. Because of this, the flow
is approching the right wing's leading edge more head-on, so this wing appears
longer than the left wing perpendicular to the direction of flight. This has
two effects. It first increases the lift on the right wing, providing a
stabilizing left roll. It also increases the drag on this wing causing a right
yawing into the wind. This second effect is what's known as roll-yaw coupling,
because an initial roll to the right has essentially caused the aircraft to yaw
to the right as well. This is a stabilizing effect in both roll and yaw, and
is one reason swept back wings may be used in low speed aircraft. The same
effect can be achieved with dihedral as well. Also, the presence of a tail fin
can cause a significant amount of roll-yaw coupling.
Now imagine a forward-swept wing. It is a completely different story. In
fact, forward swept wings can be very unstable laterally because any deviation
from the straight and level can be compounded if its not corrected by an
adequitely sized vertical tail or fly-by-wire control computer. This is what
is called adverse roll-yaw coupling.
This is not the end of the story, however. As our sweptback winged aircraft
begins to roll to the left and yaw to the right, the left wing begins to start
moving faster than the right wing, now causing the aircraft to roll right
again and yaw to the left. This goes back and forth a couple of times, damped
a bit with each sway from side to side, and is referred to as a dutch roll
oscillation. The F-14 is known to have a substantial dutch roll when landing,
and can often be seen rolling back and forth on its approach.
Any aircraft with a wing or a tail is going to have some interaction between
roll and yaw. Our minds tend to try to seperate the two(e.g. ailerons control
roll and fins control yaw), but they are very closely coupled and always affect
each other in flight.
Regretfully, I am not very familiar with the X-3, so I cannot comment on its
roll-yaw coupling characteristics, but I hope this answers your question.
Andy
Agmessier
I just took a look at the X-3 and I think I know why it had roll-yaw coupling
problems(keep in mind, this is only an educated guess, but it makes sense).
The plane is obviously a high speed aircraft judgeing from its long body and
very short wings. This means that in order to fly slowly and take off and
land, it must fly at a very high angle of attack, and its narrow shape would
tend to roll very easily. If this aircraft is flying at a high angle of attack
and rolls, a significant sideslip can occur like I described. Now the tail has
a very long lever arm, being as far back as it is on such a long aircraft.
This may cause the aircraft to overcorrect in the yaw direction, causing a
large dutch roll oscillation that is poorly damped, if it doesn't actually
diverge. I can see why this aircraft had such poor handling qualities. Hope
this helps.
Andy
Michael Lundahl
The Nurfugel-site contains much information about flying-wing aircraft.
Among others it has alot of information about the Horten Nurfugels
and I must say I was very impressed with those birds, especially
Horthen IX (Ho 229).
Enjoy :)
/Michael
Maury Markowitz
> stabilizing left roll. It also increases the drag on this wing causing a
> right yawing into the wind.
Got it.
> This is not the end of the story, however. As our sweptback winged aircraft
> begins to roll to the left and yaw to the right
Uhhh, you lost me, we had the plane rolling and yawing to the right before.
Is this a new case? If so, is the yaw in this case induced via some other
method?
> oscillation. The F-14 is known to have a substantial dutch roll when
> landing, and can often be seen rolling back and forth on its approach.
So does a Cessna actually. One good kick in a 150 will give you a couple
of orbits.
> Regretfully, I am not very familiar with the X-3, so I cannot comment on its
> roll-yaw coupling characteristics, but I hope this answers your question.
Well yes and no. It did point out the difference between straight and
swept wings for stabilization, which I hadn't considered before.
The main issue here is that long skinny planes have a specific coupling
that is dangerous. Both the US and UK spent a lot of time and money
investigating the problem, but I'm still not sure what the problem is. I
don't believe it's specifically related to dutch roll, I believe this is a
different effect (I could be wrong).
Maury
Gord Beaman
--cut--
>This may cause the aircraft to overcorrect in the yaw direction, causing a
>large dutch roll oscillation that is poorly damped, if it doesn't actually
>diverge. I can see why this aircraft had such poor handling qualities. Hope
>this helps.
>
>Andy
Andy, I like the way you describe things aeronautical and wonder if you'll
help me convince a friend here about something. He's of the opinion that the
horizontal stabilizer on any a/c provides lift during flight and I cannot
convince him otherwise. My understanding is that it provides a downward force
which provides fore and aft stability by tending to raise or lower the nose
when the airspeed tends to increase or decrease because of slight
irregularities in the airmass that it's traversing.
I also understand that this stability can be traded for better cruise economy
on some a/c that employ a variable horizontal stabilizer as the fore/aft trim
mechanism, by substituting weight to replace the aerodynamic down force. This
is accompolished by pumping fuel into a tail mounted trim tank then setting
the horiz. stab. to ~zero. (less drag - better economy). This sound about
right?.
--
Gord Beaman
PEI, Canada
Maury Markowitz
> Andy, I like the way you describe things aeronautical and wonder if you'll
> help me convince a friend here about something. He's of the opinion that the
> horizontal stabilizer on any a/c provides lift during flight and I cannot
> convince him otherwise.
It should be pretty easy if you live near any airport with small planes.
Walk up to any small aircraft and you'll notice that the stab is an inverted
airfoil. That pretty much says it all, curved side down, it pulls down.
Small planes balance near the 1/3rd point of the chord, and if you look at
the plane you'll notice that practically all of the weight is in front of
that line - the engine. Thus in order to keep the nose up, the tail provides
a counter force by pulling down on the tail. By sticking it way out on the
end of a long tail, you don't have to push as hard, the lever effect is
working for you.
The reason for doing this is exactly as you noted, it's to provide
automatic pitch stability. Consider the case when the nose is deflected up
for instance. In this case the tail moves down, and relative to the overall
airflow the tail is now seeing a reduced angle of attack (as Andy noted, this
is easier on paper). That makes it generate less lift, thus less downward
force on the tail, thus less to ballance the downward pull of the nose, and
thus the nose moves down and back towards normal. In the other case the tail
moves up, which increases the angle of attack on the tail, increases lift,
increases downward force, and pulls the nose back up to normal.
You gotta admit, it's pretty clever.
> I also understand that this stability can be traded for better cruise
> economy on some a/c that employ a variable horizontal stabilizer
There's all sorts of ways to help, but frankly I don't know if they
_really_ do anything. For instance Piper's have flying tails for which the
claim is that the overall drag is less for any given configuration - but even
if it is, it weighs more. Is there a gain? I don't know.
Canard planes have long claimed to be much better in this regard because
the "tail" (in this case on the front) is also a lifting surface. That means
that while all these lifting surfaces are still generating drag, in this case
the stab is at least helping to lift the plane too. Once again though the
difference appears to diappear in the wash, and there's no clear performance
or economy differences between, say, a EZ and a Katana that I can see.
A few years back USask won the weight/power showdown with a conventional
layout. They said that they started with a canard design, but it became clear
it wasn't a winner.
> is accompolished by pumping fuel into a tail mounted trim tank then setting
> the horiz. stab. to ~zero. (less drag - better economy). This sound about
> right?.
Most airliners do this in fact, by taking off with the fuel in the wings
and then pumping it into the vertical tail once they reach cruise alt. On
those planes it makes a big difference though and is clearly worth the extra
weight and complexity. I think the later DC-10-30's introduced this,
although it might have been the MD-11's.
Maury
Al Bowers
maury@remove_this.istar.ca.invalid (Maury Markowitz) writes:
> In <19990427191151...@ng-cs1.aol.com> Agmessier wrote:
Classical "roll-yaw" inertia coupling (of which the X-3, X-1A, F-100
(early), and F-101 had problems) is caused by angle of attack, and the
distribution of mass.
For a really GOOD treatise on this, read:
Day, Richard E.: "Coupling Dynamics in Aircraft: A Historical
Perspective", NASA SP-532.
Coupling dynamics can produce either adverse or beneficial stability
and controllability, depending on the characteristics of the
aircraft. This report presents archival anecdotes and analyses of
coupling problems experienced by the X-series, Century series, and
Space Shuttle aircraft. The three catastrophic sequential coupling
modes of the X-2 airplane and the two simultaneous unstable modes of
the X-15 and Space Shuttle aircraft are discussed. In addition, the
most complex of the coupling interactions, inertia roll coupling, is
discussed for the X-2, X-3, F-100A, and YF-102 aircraft. The mechanics
of gyroscopics, centrifugal effect, and resonance in coupling dynamics
are described. The coupling modes discussed are interacting multiple
degrees of freedom of inertial and aerodynamic forces and moments. The
aircraft are assumed to be rigid bodies. Structural couplings are not
addressed. Various solutions for coupling instabilities are discussed.
Available on-line at:
http://www.dfrc.nasa.gov/DTRS/
And search for "Day".
Al Bowers
Al Bowers
d93...@efd.lth.se (Michael Lundahl) writes:
> This is to everyone in the thread (and others).
> The Nurfugel-site contains much information about flying-wing aircraft.
> Among others it has alot of information about the Horten Nurfugels
> and I must say I was very impressed with those birds, especially
> Horthen IX (Ho 229).
Dr David Myhra is about to release a new book (actually a three volume
set) on the Horten H IX/Ho-229. I was able to see a pre-print of it,
and it should prove to be the definitive work on the subject. From
Schiffer Military Publishing, perhaps late this summer or this fall...
Al Bowers
Agmessier
>aircraft
>> begins to roll to the left and yaw to the right
>
> Uhhh, you lost me, we had the plane rolling and yawing to the right before.
>Is this a new case? If so, is the yaw in this case induced via some other
>method?
Let me try to explain this better. The dutch roll mode is the hardest to
describe, especially when limited to writing as a means of communication. The
original right roll (gust? a passenger dropping a heavy bag on the floor...
whatever) puts the aircraft in a sideslip. This sideslip induces a right yaw
due to wing sweep and tail, and a stabilizing left roll, due to the increased
lift and drag on the right wing. This left roll eventually passes up the
equilibrium (overcorrects) and eventually you end up with the airplane banked
to the left. This is the exact opposite case as the original right roll. This
continues back and forth until the plane settles back to straight and level.
The oscillatory mode is admittedly difficult to understand. The important
thing tounderstand is that it is perpetuated by the interaction between roll
and yaw. There are three 'separate. oscillations taking place as the aircraft
flies: oscillations in roll, yaw, and actual movement of the plane itself.
These oscillations happen in a regular matter, all at the same frequency, but
at different amplitudes and phase shifted. A right yaw would precede a left
roll by almost 90 degrees IIRC.
Each aircraft has its own dutch roll behavior, which can be calculated
relatively simply if you assume everything behaves linearly. The actual
oscillatory mode can vary from aircraft to aircraft in frequency, in phase
shift between the rotations, and in the proportions of the various rotations
that come into play. My intuition (I could be wrong) tells me that aat low
speeds, an X-3 would have a dutch roll mode that oscillates in a much greater
amplitude in yaw than in roll. Since its slender shape can roll very easily, a
moderate roll can put it into a very severe oscillation. I would be interested
to find out more about its actual behavior if you find anything out.
Andy
Mike Kelly
On Wed, 28 Apr 1999, Maury Markowitz wrote:
[an excellent post snipped]
>
> Most airliners do this in fact, by taking off with the fuel in the wings
> and then pumping it into the vertical tail once they reach cruise alt. On
> those planes it makes a big difference though and is clearly worth the extra
> weight and complexity. I think the later DC-10-30's introduced this,
> although it might have been the MD-11's.
Maury,
It was the MD-11. This was done of course to save weight for the
reasons you mentioned above. However, unlike almost all other Douglas
airliners, which were known for having heavy controls, the MD-11's
controls were very light for an aircraft of that size. Combined with a
poor placement and design of the slat extention lever there were a number
of inadvertant deployments in cruise which caused a few fatalities. My
best friend's dad used to be a very senior captain for an airline over
here that operated both 10's and 11's. He was one of the first guys
selected to fly the MD-11. He hated it so much that he requested to be
transfered back to DC-10's. The MD-11 in-flight upsets are well
documented and have lead to a number of AD's being imposed on the MD-11.
Michael Kelly
>
> Maury
>
>
>
>
Maury Markowitz
> http://www.dfrc.nasa.gov/DTRS/
>
> And search for "Day".
Thanks Al!
Maury
Maury Markowitz
> Let me try to explain this better. The dutch roll mode is the hardest to
> describe, especially when limited to writing as a means of communication.
No,, I'm familiar with Dutch roll, it's easy to induce in a Cessna due to
their excellent rudder authority. What I'm looking for is the gyroscopic
effects in that paper that Al pointed out.
Maury
Maury Markowitz
> 'aircraft' rotates. Centripetal force tends to widen the circle. As this
> happens aerodynamic forces can exceed
> control surface restoring moments and the circle(s) widen, or diverge, and
> now the aircraft can end up going
Well _that's_ a simple explaination! Thanks!
Maury