Glider angle of attack indicator by SafeFlight
Bill Daniels
Magazine".
See: www.safeflight.com
Does anybody know more about this device - especially the price?
Bill Daniels
Ralph Jones
Hope it works out. A design for an AoA indicator appeared in _Soaring_
twenty or thirty years ago, but it was really crude. IIRC, it had its
own separate pitot and static sources, and connected them across a
vertical, tapered tube similar to one side of a pellet vario. A solid
metal ball rode in the tube, its mass serving to sense acceleration,
and its vertical position gave a measure of AoA.
The vane types found on jets work splendidly, but they're mounted high
up on the airplane where people and vehicles don't bump into
them...the equivalent on a glider wouldn't last long!
I presume the Safeflight device uses the pitot/static/acceleration
principle...that should be relatively easy with contemporary sensor
technology and chips.
rj
Ralph Jones
wrote:
Oops, no, I see on the website that it uses a vane. That's likely to
be a problem...
rj
Bill Daniels
"Ralph Jones" <ra...@nomeking.kahm> wrote in message
news:ga1hl35qemt0iqndi...@4ax.com...
Note that they say the vane is removable. You would probably install it as
you would a TE probe just before flight and remove it just after landing.
Bill Daniels
tommytoyz
don't see that they included and audio warning, something I think
would be essential to be useful in stressful situations.
I doubt any pilots deliberately get into stalls and spins by
deliberately flying bellow the required airspeed. Rather, if they had
known or been warned without having to look at the panel, they
probably would have sped up and remained out of the stall range.
So if they didn't notice the airspeed indicator display on the panel,
they probably won't notice an AoA indicator warning them either,
unless there is heads up an audio warning.
Come to think of it, another solution to stall spins could be
installing an audio warning on the airspeed indicator, when the speed
falls bellow a target speed, that could be adjusted based on wing
loading........
These could be simple effective pilot aids......................
Bill Daniels
Bill Daniels
"tommytoyz" <tomm...@yahoo.com> wrote in message
news:9d2b07ec-131c-4e11...@d27g2000prf.googlegroups.com...
Brian
to fly very close to stall a large percentage of the time, i.e.
Thermalling. We don't want the stall warning going off all the way
through the climb.
I agree that most stall spin accidents are cause by inadvertantly
flying to slow and the pilot not realizing they are doing it. I also
agree that an AoA indicator won't help unless it has method to get the
pilots attention.
Perhaps disabling the warning when the gear is retracted would have
some value. Anyone know how many stall spin accidents occur with the
gear extended vs retracted?
The other issue with an AoA indicator is How does it compensate for
Flaps. The Stalling AoA will change radically as flaps are applied. I
am sure this can be done but are any of the AoA indicators available
doing it?
Brian CFIIG
Gary Emerson
know Dick Johnson had a stall indicator mounted over the wing root on
his Ventus. It was a small flat pivoting plate that "flew" in the
airstream and would stall itself close to the wing stall angle. Maybe
Dick can give some more info, maybe a picture, maybe there was an
article someplace.
HL Falbaum
option. I don't know how well it works yet. It is adjustable--(calibrated)
in flight by the pilot.
Cambridge 302 has a "too slow" aural warning, adjusted for mass and g-load
by the internal microprocessor. It does work pretty well.
--
Hartley Falbaum
"Brian" <brian...@msn.com> wrote in message
news:d8e91745-7e68-423a...@e23g2000prf.googlegroups.com...
Martin Gregorie
If its the type with 360 degree rotation, it could easily be using an
optical sensor (Grey encoded rotary position sensor or similar), in
which case the system could be quite robust and friction-free,
especially with a removable vane.
--
martin@ | Martin Gregorie
gregorie. | Essex, UK
org |
gfoster07k
is to be used. If it is intended to be a Go - NoGo stall warning
device, that is one thing. If it is intended to be used to measure
and be proportional to a range of AoA that is quite another. The
mounting becomes quite critical because the airflow may be such that 1
degree of AoA change does not translate into 1 degree of sensor
movement (in the case of a vane for example) . As a young flight
control engineer I learned this the hard way when an AoA vane mounted
near the nose of a test vehicle actually changed 5 degrees for every
degree of AoA change of the aircraft.
Greg
Bill Daniels
"gfoster07k" <gfost...@sbcglobal.net> wrote in message
news:779166bf-2b94-4aa2...@d27g2000prf.googlegroups.com...
Good point.
Lets say that you will be interested in only stall, min sink, and best L/D
at each flap setting. Can it be calibrated for that?
Bill Daniels
Paul Remde
It is a very interesting device. I looked at it very closely at the SSA
Convention in the past. I was impressed with the high quality of the
system. If I remember correctly it includes a low friction rotating vane
that is mounted to the side of the fuselage and a variometer like meter that
is mounted in the instrument panel. I don't recall whether or not it had an
audio output. It looked very cool. I don't remember the price, but I do
remember thinking that very few glider pilots would pay the somewhat high
price. Also, it required a hole in the side of the fuselage.
Paul Remde
"Bill Daniels" <bildan@comcast-dot-net> wrote in message
news:fqydnZDJNOqX6sXa...@comcast.com...
Bill Daniels
device performs in actual use.
However, we seem to pay large sums for safety devices like transponders and
ELT's when it is at least argueable that stall/spin accidents kill far more
pilots than mid-airs or exposure after a landout. It's possible that on a
"bang for the buck" basis, an AOA indicator is a better deal. This is
especially true if the AOA indicator actually improves our soaring
performance while increasing safety.
It's even possible to argue that one offsets the other. i.e. If your AOA
indicator prevents a spin in on the ridge, you have avoided, at least in
that instance, the need for an ELT, saved your glider and maybe yourself in
the bargan.
I can't think of any device that would have more impact on accident numbers
than a really practical AOA indicator.
Bill Daniels
"Paul Remde" <pa...@remde.us> wrote in message
news:aPg6j.222593$Xa3.195325@attbi_s22...
Andy
>
> I can't think of any device that would have more impact on accident numbers
> than a really practical AOA indicator.
Bill, I think you have presented a case for the benefit of an
effective stall warning system but not necessarily a good case for an
AOA indicator. Unless the AOA sensor is linked to an effective
warning system it is more likely to be an added distraction rather
than a benefit.
Now if someone took that sensor and used it to drive an audio
thermalling speed director I could get interested.
Andy
Eric Greenwell
> Cost and installation issues are a real hurdle as is just how well the
> device performs in actual use.
>
> However, we seem to pay large sums for safety devices like transponders and
> ELT's when it is at least argueable that stall/spin accidents kill far more
> pilots than mid-airs or exposure after a landout. It's possible that on a
> "bang for the buck" basis, an AOA indicator is a better deal. This is
> especially true if the AOA indicator actually improves our soaring
> performance while increasing safety.
>
> It's even possible to argue that one offsets the other. i.e. If your AOA
> indicator prevents a spin in on the ridge, you have avoided, at least in
> that instance, the need for an ELT, saved your glider and maybe yourself in
> the bargan.
>
> I can't think of any device that would have more impact on accident numbers
> than a really practical AOA indicator.
I wonder if a stall warning is more complicated than an effective AOA
indicator mounted on or near the fuselage. Sure, it'd work for straight
ahead stalls, but how often do those cause accidents, compared to stalls
while turning? My experience in stall/spins while turning is the root
isn't stalled, it's the outer wing portions (likely from the tip inward
for a few feet) that stalls and precipitates the spin.
And perhaps we are chasing the wrong parameter. Separation of the
airflow is a better indicator of a stall in progress; in fact, I think
Johnson's indicator worked by detecting separation, not AOA. This might
work without the need for sensing flap position, simplifying things.
One way to detect separation would be to flush-mount a microphone out
near the tip of each wing, positioned on the chord where separation
begins as that section gets near stall. The cockpit indicator might just
be the signal from the microphone, amplified and fed to a speaker. LEDs
could be used instead or in addition to the sound, of course.
The microphone could be very small, and a small tube (say, 0.1"
diameter) could lead the sound from the wing surface, through the wing
skins, to the microphone. A thin membrane covering the outer end of the
tube would prevent water and dirt from affecting performance.
--
Eric Greenwell - Washington State, USA
* Change "netto" to "net" to email me directly
* "Transponders in Sailplanes" http://tinyurl.com/y739x4
* "A Guide to Self-launching Sailplane Operation" at www.motorglider.org
Udo
I think what we are after is an effective angle of attack indicator to
optimize performance for a rage of flap settings that will also
coincidently act as a stall warning.
Udo
On Dec 7, 2:53 pm, "Bill Daniels" <bildan@comcast-dot-net> wrote:
> Cost and installation issues are a real hurdle as is just how well the
> device performs in actual use.
>
> However, we seem to pay large sums for safety devices like transponders and
> ELT's when it is at least argueable that stall/spin accidents kill far more
> pilots than mid-airs or exposure after a landout. It's possible that on a
> "bang for the buck" basis, an AOA indicator is a better deal. This is
> especially true if the AOA indicator actually improves our soaring
> performance while increasing safety.
>
> It's even possible to argue that one offsets the other. i.e. If your AOA
> indicator prevents a spin in on the ridge, you have avoided, at least in
> that instance, the need for an ELT, saved your glider and maybe yourself in
> the bargan.
>
> I can't think of any device that would have more impact on accident numbers
> than a really practical AOA indicator.
>
> Bill Daniels
>
> "Paul Remde" <p...@remde.us> wrote in message
>
> news:aPg6j.222593$Xa3.195325@attbi_s22...
>
>
>
> > Hi Bill,
>
> > It is a very interesting device. I looked at it very closely at the SSA
> > Convention in the past. I was impressed with the high quality of the
> > system. If I remember correctly it includes a low friction rotating vane
> > that is mounted to the side of the fuselage and a variometer like meter
> > that is mounted in the instrument panel. I don't recall whether or not it
> > had an audio output. It looked very cool. I don't remember the price,
> > but I do remember thinking that very few glider pilots would pay the
> > somewhat high price. Also, it required a hole in the side of the
> > fuselage.
>
> > Paul Remde
>
> > "Bill Daniels" <bildan@comcast-dot-net> wrote in message
> >news:fqydnZDJNOqX6sXa...@comcast.com...
> >>I was very pleased to see the advert in the December issue of "Soaring
> >>Magazine".
> >> See:www.safeflight.com
>
> >> Does anybody know more about this device - especially the price?
>
> >> Bill Daniels- Hide quoted text -
>
> - Show quoted text -
Bill Daniels
A mere stall warner is just a "stall" or "no stall" binary device. An AOA
indicator, by comparison, gives stall margin or how close you are to a stall
as well as the rate at which you are approaching it. Of course, stall
warning is a part of that.
It also is a performance device that lets the pilot optimize flight. You
can change AOA with the elevator with almost no delay. Airspeed needs more
time to change. Thermalling at a constant, optimum AOA is easy and the
airspeed will be much steadier. The optimum AOA is the same regardless of
wing loading or bank angle.
Unless you have flown with an AOA indicator, it's hard to imagine what it
does. I've never head someone who has used one say they didn' t like it.
Bill Daniels
"Udo" <uru...@reach.net> wrote in message
news:65e6b5fa-b54a-4c82...@d21g2000prf.googlegroups.com...
Andy
> Unless you have flown with an AOA indicator, it's hard to imagine what it
> does. I've never head someone who has used one say they didn' t like it.
Agreed. I last flew with an AOA indicator just a few weeks ago but it
was included in a HUD symbology set, not head down. The last thing we
need is for all contest pilots to have a head down indicator that they
depend on to optimize performance while thermalling.
Make it a director, feed it flap position (if applicable) and make it
audible!
Andy
JMR
Three different versions. Costly, but seems to have all that you guys
are talking about.
We have one in the 737-800 I fly, and never use it unless "heads
down." Captain has HUD with AOA symbology, speed vector and VSI, along
with a bunch of other crap, thats what we need, a HUD.
Justin Rizor
John Galloway
of wool taped to both front/sides of the canopy perform
quite well as angle of attack indications in unflapped
gliders. The angular change between max LD and stall
is surprisingly noticeable. Once 'calibrated' for
the stall position and min sink and best glide positions
they are helpful for optimizing thermalling technique
but don't grab the attention well enough to be a safety
stall warning.
Certainly good value for money.
John Galloway
kirk.stant
John,
I tried this technique on an LS4. While you could see the difference
in angle of attack quite clearly, there was a huge difference when any
yaw was introduced, with the yarns showing significant differences in
relative airflow on opposite sides of the canopy. In effect, with
this system, you are really showing the instantaneous airflow over the
canopy, and deriving AOA from that indication - which is not much
better than just using airspeed, really, and effectively impossible to
use due to yaw effects and the flapping of the yarn masking the small
AOA changes you are interested in.
Having some time in airplanes that used AOA (gauge, lights, and
variable audio cues) in preference to airspeed during low speed flight
(F-4s, in my case) I can vouch that it's a much better way to fly -
especially when the AOA is coupled with an audio signal. But in a
glider, I thing the working AOA range that we are interested in is a
lot smaller than the AOA range that low aspect ratio jets use, so in
practice it may be more difficult to implement.
What I would like to see is a simple AOA indicator that would tell me
when I am at the optimum AOA for efficient thermalling. No fancy
indicator, just a bright LED that would be on steady at thermalling
AOA. Have it start flashing when approaching the desired AOA, and
have a different flash rate when below the desired AOA (which would
also function as a stall warning, since there is really no reason for
ever flying below min sink, especially in the pattern.
To use, pull into the thermal and set your attitude to the approximate
AOA, glance at the LED to adjust to the exact AOA, then fly that,
occasionally crosschecking the light. No figuring for bank angle or
ballast required, the wing takes care of that. Sweet...
AOA is really not that useful in cruise, since we are really more
concerned with the appropriate MC speed, and rarely fly at L/D max.
But again, that would be a single AOA point to measure, and could use
a different color LED.
Obviously, with a flapped ship, flap position would have to be sensed
and compensated for.
Since I'm really only concerned about one angle of attack, a moving
vane should not be necessary - I think it can be done with
differential pressure ports, so no moving parts or drag. Flaps may
complicate things, but even if there has to be a different set of
ports for each thermalling flap position, it may still be easier than
a mechanical AOA vane. Or just pick your favorite flap setting, and
TLAR it from there...
Comments from smarter people out there are welcome!
Kirk
LS6b "66"
Wayne Paul
"kirk.stant" <kirk....@gmail.com> wrote in message
news:b84e329c-84b1-465c...@s19g2000prg.googlegroups.com...Kirk,
I agree with your assessment. A simple LED index system of three lights.
One indicating "fast", another "on speed" (Max Cl) and finally one that
flashes when nearing stall. Sound familiar from your F-4 days?
Wayne
Retired A-6 Bombardier.
http://www.soaridaho.com/Schreder
Marc Ramsey
> I agree with your assessment. A simple LED index system of three lights.
> One indicating "fast", another "on speed" (Max Cl) and finally one that
> flashes when nearing stall. Sound familiar from your F-4 days?
Using either a vane on a shaft encoder, or differential pressure ports,
the hardware and software are pretty trivial (well under $100US in
parts). The aerodynamics, however, are not. Where does one put the
vane or ports such that one doesn't get large errors due to tiny
variations in yaw?
My DG-600 had an AOA meter (it looked to be a modified electronic
variometer) and two cross connected sets of static ports immediately
above the wing root, one set near the leading edge, and the second set
roughly mid-chord. I never got it to do anything useful.
Marc
Bill Daniels
"kirk.stant" <kirk....@gmail.com> wrote in message
news:b84e329c-84b1-465c...@s19g2000prg.googlegroups.com...
> Having some time in airplanes that used AOA (gauge, lights, and
> variable audio cues) in preference to airspeed during low speed flight
> (F-4s, in my case) I can vouch that it's a much better way to fly -
> especially when the AOA is coupled with an audio signal. But in a
> glider, I thing the working AOA range that we are interested in is a
> lot smaller than the AOA range that low aspect ratio jets use, so in
> practice it may be more difficult to implement.
Actually, I think the opposite is more likely. Gliders operate from near
stall to high speed running. Due to the far lower wing and span loading in
gliders, the minimum AOA may be less than a jet. I'd bet that the AOA range
of a glider is greater.
There are a lot of ways to display AOA data and each pilot may have a
different preference. A vertical column of different colored bright LED's
that could be 'read' (i.e. thermal with the green LED lit.) with peripheral
vision might work.
An old Pratt-Read that I used to fly had two very pleasant windsong tones
that changed pitch right at the best thermalling AOA. Of course, that was
before audio varios. Maybe a better idea is to the replace the vario audio
with bright LED lightbar display and replicate the old PR's windsong for
AOA. The LED part should be easy since most electronic varios have a + or -
5V output for a rear seat repeater.
BTW, 'pitch strings' are sensitive to yaw because they have to be mounted on
the outside of the transparent part of the canopy which puts them way above
the 'beltline' of the fuselage. Mounting them lower and further forward
would make them less sensitive to yaw but of course, you wouldn't be able to
see them.
To find the best place for a AOA vane, you could put a bunch of yarns on the
fuselage side and a video camera on the wing aimed at them. A flight test
at various AOA and yaw angles would show the location least sensitive to
yaw.
Still, I think Wayne is on the right track with pressure ports on the top
and bottom of the nose.
Bill Daniels
Eric Greenwell
> What I would like to see is a simple AOA indicator that would tell me
> when I am at the optimum AOA for efficient thermalling.
Does anyone know of documentation that supports the idea showing the
pilot the AOA will actually improve a glider pilot's thermalling? Or
even that the range of AOA needed to be "efficient" is too small for a
pilot to obtain it easily by using airspeed, or by just looking out the
canopy, once he's flown the glider enough to be familiar with it?
For example, I couldn't even find a mention of AOA in "Fundamentals of
Sailplane Design" when discussing thermalling. Circling efficiency is
discussed (page 63-65), but without mention of AOA, which suggests to me
that it's not the important factor. Climb performance, which is what we
really are after, is very dependent on the thermal shape (pages 65-66).
Circling at the best AOA doesn't give you the best rate of climb;
instead, the circling radius is the most important factor.
Look at the "rate of sink versus turn radius" table like the one on page
64 of "Fundamentals...". Does anyone know if the optimum is always at
the same AOA? And if not, what the range of AOA is for the table?
Regardless of the answer is to the question above, what would be useful
would be two additional tables "rate of sink versus turn radius". One
table would use an AOA greater (say, 3 degrees) than optimum; the other
table would use an AOA smaller by the same amount from optimum. This
would give us an idea of how sensitive circling efficiency is to AOA
errors.
If performance is not sensitive to the AOA, there is no need to look for
an indicator of it. A stall warning device would still be useful, but it
doesn't have to be based on AOA: it just needs to tell you when the wing
is getting close to a stall.
SoaringXCellence
> "kirk.stant" <kirk.st...@gmail.com> wrote in message
Everyone is talking about AoA on the fuselage. This is not the AoA of
the wing. As noted the flap position modifies the reading from the
fueslage. What we need is a AoA that is always reading the stagnation
point on the leading edge of the wing and showing that AoA. Then, I
think, the flap is comprehended in the setup.
This could be done with a set of holes, chord-wise around the LE that
feed independent pressure sensors, integrated by a micro-controller,
then displayed and/or driving an audio signal. Very similar to some
projects I've been working on and easily done with about $400 worth of
parts. Accutate, temperature compensated, pressure sensors are
expensive.
Mike
Udo
for the glider. But I still would prefer an A of A indicator. Looking
back, when I was testing my glider, I wish I had an A of A indicator.
It would have made the initial tests to find the right flap settings
and corresponding airspeed faster and easier. Pilots generally do not
what to hang around for test, especially when a home built is nibbling
on there tail.
I knew where I needed to be with my flaps and airspeed, based on the
theoretical polars but making adjustment by adding or subtracting a
degree of flap deflection and making speed adjustments at the same
time you either need a good A of A indicator are a very patient pilot
partner. Hence It took many flights nearly 70 to 90 contest hours to
fine tuning the glider (I am not talking about handling the glider)
Once the parameters had been established there was very little need
for an
A of A. Still I would prefer one for changing ballast and when flying
in marginal conditions at a contest to have a glancing look at it the
odd time.
Udo
Udo
Udo
> Everyone is talking about AoA on the fuselage. This is not the AoA of
> the wing. As noted the flap position modifies the reading from the
> fueslage. What we need is a AoA that is always reading the stagnation
> point on the leading edge of the wing and showing that AoA. Then, I
> think, the flap is comprehended in the setup.
>
> This could be done with a set of holes, chord-wise around the LE that
> feed independent pressure sensors, integrated by a micro-controller,
> then displayed and/or driving an audio signal. Very similar to some
> projects I've been working on and easily done with about $400 worth of
> parts. Accutate, temperature compensated, pressure sensors are
> expensive.
>
> Mike- Hide quoted text -
Marc Ramsey
> This could be done with a set of holes, chord-wise around the LE that
> feed independent pressure sensors, integrated by a micro-controller,
> then displayed and/or driving an audio signal. Very similar to some
> projects I've been working on and easily done with about $400 worth of
> parts. Accutate, temperature compensated, pressure sensors are
> expensive.
Would a series of holes provide any more information than two holes?
What kind of pressure differentials would be involved?
Marc
kirk.stant
> Does anyone know of documentation that supports the idea showing the
> pilot the AOA will actually improve a glider pilot's thermalling? Or
> even that the range of AOA needed to be "efficient" is too small for a
> pilot to obtain it easily by using airspeed, or by just looking out the
> canopy, once he's flown the glider enough to be familiar with it?
Eric, when we fly airspeed while thermalling we are actually trying to
fly AOA. We start with the minimum sink speed (specifically, the
point on the polar we want to thermal at), add speed for ballast, then
add speed for bank angle, then come up with an adjusted airspeed that
approximates our ideal AOA for the selected gross weight and bank
angle.
Using AOA directly (once one has chosen where on the polar one wants
to thermal at) eliminates the need to make all those guesses. The
wing does it all, automatically.
Guess what - when you fly attitude - "what feels right" - in a
thermal, glancing at the airspeed to see what it is - you are flying
AOA!
> For example, I couldn't even find a mention of AOA in "Fundamentals of
> Sailplane Design" when discussing thermalling. Circling efficiency is
> discussed (page 63-65), but without mention of AOA, which suggests to me
> that it's not the important factor. Climb performance, which is what we
> really are after, is very dependent on the thermal shape (pages 65-66).
> Circling at the best AOA doesn't give you the best rate of climb;
> instead, the circling radius is the most important factor.
I disagree. Thermalling at the most efficient bank angle/AOA for the
size of the thermal is the most important factor. Waddling around a
knot above the stall with landing flaps down will give me the smallest
circling radius, but a horrible climb rate.
>
> Look at the "rate of sink versus turn radius" table like the one on page
> 64 of "Fundamentals...". Does anyone know if the optimum is always at
> the same AOA? And if not, what the range of AOA is for the table?
My guess it that the optimum AOA may vary based on turbulence, but
only a very small about - probably less than can be accurately flown
by the average pilot in a typical thermal. And this would only be for
airfoils that are susceptible to turbulent flows. In most cases, the
AOA range for effective Cl max (which I assume is close to the optimum
for min sink and thermalling) is probably big enough to be measured
and flown accurately.
> Regardless of the answer is to the question above, what would be useful
> would be two additional tables "rate of sink versus turn radius". One
> table would use an AOA greater (say, 3 degrees) than optimum; the other
> table would use an AOA smaller by the same amount from optimum. This
> would give us an idea of how sensitive circling efficiency is to AOA
> errors.
>
> If performance is not sensitive to the AOA, there is no need to look for
> an indicator of it. A stall warning device would still be useful, but it
> doesn't have to be based on AOA: it just needs to tell you when the wing
> is getting close to a stall.
If performance is not sensitive to the AOA, we wouldn't need an
airspeed indicator! At low speeds, that old ASI is at best a poor
compromise - the only good thing about it is that is doesn't fail
often (although, the only instrument I've ever had fail in a glider
was the airspeed indicator). And how can anything tell you the wing
is getting close to the stall without measuring AOA? Excessive AOA is
what defines a stall. Airspeed is just an approximation - and can
easily trick you. Try landing back after a low altitude rope break
full of ballast, if you haven't flown wet in a while. Slow to the
airspeed you are used to using to turn back and you will get a big
surprise! In the same situation, slow to the same AOA, and you have
the same margin over the stall you had dry. This isn't opinion, it's
basic aerodynamics.
I think the lack of references to angle of attack in gliding
publications is largely due to the fact that AOA is still mainly
limited to military jets and expensive airliners/biz jets. Most
general aviation pilots never have a chance to be exposed to the joys
of knowing exactly what their wing is doing. Or not doing, as the
case may be! Funny thing is, the common Cezzna uses a crude AOA
sensor for it's stall warning (the little paddle on the leading edge).
Kinda like audio varios - once you try it, you'll never want to go
back to airspeed as a low speed control instrument.
Any real aero majors lurking out there, please join in!
Cheers,
Kirk
Bill Daniels
No need. That was a damn good explanation.
Bill Daniels
hans
think about ways to implement an effective AoA indicator for my flapped
glider.
I have the AoA string on the right side of my canopy, but it is not very
helpful, as it has to be calibrated for every flap setting.
kirk.stant
> We start with the minimum sink speed (specifically, the
> point on the polar we want to thermal at), add speed for ballast, then
> add speed for bank angle, then come up with an adjusted airspeed that
> approximates our ideal AOA for the selected gross weight and bank
> angle.
>
> Using AOA directly (once one has chosen where on the polar one wants
> to thermal at) eliminates the need to make all those guesses. The
> wing does it all, automatically.
I realized a bit late that what I really meant to say is that when
using airspeed, we find the performance point on the polar we want (L/
D max, min sink, stall, whatever), then move the polar for ballast and
bank angle, and use the resulting adjusted airspeed. This is made
necessary when using the common sink rate vs airspeed polars for all
the desired conditions of ballast and bank angle. Using AOA directly
(which would require the polar in sink rate vs AOA and L/D vs AOA)
eliminates the need to move the polar (and refigure the resulting
airspeed), as the AOA for a specific flight condition is not affected
by ballast or bank angle.
Has anyone seen glider polars with sinkrate plotted against angle of
attack? That would be interesting. I've seen plots for aircraft of
Cl vs AOA, and L/D vs AOA, neither of which is very useful in this
discussion.
I should have listened up more in aero classes, long time ago...
Kirk
jcarlyle
describe the sink vs airspeed performance curve. As far as I could
determine, it's aerodynamic's tribute to Lilienthal. His original Cl
and Cd curves for a bird's wing were plotted only against AOA using
polar coordinates, and the curves were called Lilienthal's Polar. As
aerodynamics developed it became easier to drop the polar plotting and
only use cartesian coordinates. Perhaps this came about because it was
easier to measure airspeed rather than AOA, and thus speeds were of
more practical use?
The AOA is of course embedded into the common cartesian performance
curve. It's theoretically possible to identify specific AOA points
along a performance curve, starting at high AOA values at the stall
and progressing to low AOA values at high airspeeds. I've never seen
AOA angles superimposed onto a performance curve, but I imagine that
the spacing between a AOA degree is closer near the stall end than it
is near the high speed end. If a practical AOA meter were to be
developed, I imagine such curves would be published.
I've never flown a plane with an AOA meter, so I don't understand how
it would respond during changing conditions:
(1) In landing through a wind gradient, I assume the AOA suddenly
increases as you drop into the slower wind? I assume this because I
know you're closer to a stall when the wind suddenly slows and the ASI
decreases, but I don't understand the relative wind change needed to
cause an AOA change in this situation.
(2) What happens in a turbulent thermal? When you fly a thermal using
a constant attitude it's common to see fairly definite ASI changes -
what would an AOA meter show during this situation? I assume the AOA
would change as the relative wind changes, and that you would still
fly a constant attitude much as you do with an ASI.
In both of these situations the ultimate question is: why would an
AOA meter be better than an ASI? I'm guessing the AOA would be good
upon entry so you would know what attitude to maintain, but I think it
wouldn't necessarily be any better than an ASI once you were into the
changing situations described in (1) and (2).
Can someone enlighten me? Thanks!
-John
On Dec 9, 1:03 am, "kirk.stant" <kirk.st...@gmail.com> wrote:
>> Has anyone seen glider polars with sinkrate plotted against angle of
> attack? That would be interesting. I've seen plots for aircraft of
> Cl vs AOA, and L/D vs AOA, neither of which is very useful in this
> discussion.
>
> Kirk
Michael Ash
> I've never flown a plane with an AOA meter, so I don't understand how
> it would respond during changing conditions:
I haven't either, so the below is all theoretical, take it with a grain of
salt.
> (1) In landing through a wind gradient, I assume the AOA suddenly
> increases as you drop into the slower wind? I assume this because I
> know you're closer to a stall when the wind suddenly slows and the ASI
> decreases, but I don't understand the relative wind change needed to
> cause an AOA change in this situation.
Right, the AOA increases. On a very short timescale, when you drop into
slower wind, your airspeed decreases. This then decreases the amount of
lift your wings are developing. The lift they create is now less than your
weight, so you begin to drop. As you drop, the relative wind becomes more
vertical, increasing your AOA and the lift created by the wings. Once the
AOA increases to the point where the lift balances out your weight, you
reach a steady state again at a slower airspeed and higher AOA. If the AOA
reaches the stall angle before the lift increases to match your weight,
you'll stall, and this is why you should carry plenty of extra speed in
that situation.
> (2) What happens in a turbulent thermal? When you fly a thermal using
> a constant attitude it's common to see fairly definite ASI changes -
> what would an AOA meter show during this situation? I assume the AOA
> would change as the relative wind changes, and that you would still
> fly a constant attitude much as you do with an ASI.
Turbulence would bounce it around similar; if you get hit with a gust from
below the AOA will momentarily become larger, increasing the lift from the
wings. You then begin to climb until your vertical speed matches the gust
and your AOA goes back to what it was. Similarly with a gust from above,
where you start to drop.
> In both of these situations the ultimate question is: why would an
> AOA meter be better than an ASI? I'm guessing the AOA would be good
> upon entry so you would know what attitude to maintain, but I think it
> wouldn't necessarily be any better than an ASI once you were into the
> changing situations described in (1) and (2).
I think the advantage in a thermal is that the optimal airspeed changes
with bank, whereas the optimal AOA doesn't change. Instead of trying to
figure out the best airspeed to track as you keep altering your bank to
center the thermal, you can just track a single AOA all the time.
For landing I think the advantage is just that you can stick a warning
signal around the stall AOA to remind you to stop hauling back on the
stick when you get too close to a stall.
--
Michael Ash
Rogue Amoeba Software
jcarlyle
is needed!
The only thing I'm left unsure about is AOA behavior vs. ASI behavior
in a turbulent thermal. I buy that AOA eliminates worrying about bank
angle and ballast - just set the attitude for best min sink AOA and
try to keep it there. But I wonder how much an AOA meter (say one
based upon pressure ports) would vary in a thermal vis a vis an ASI.
I've seen plus/minus 5 kts in ASI. If this translated in several
degrees in AOA, it might make the AOA as useless as the ASI.
-John
Michael Ash wrote:
> Right, the AOA increases. On a very short timescale, when you drop into
> slower wind, your airspeed decreases. This then decreases the amount of
> lift your wings are developing. The lift they create is now less than your
> weight, so you begin to drop. As you drop, the relative wind becomes more
> vertical, increasing your AOA and the lift created by the wings. Once the
> AOA increases to the point where the lift balances out your weight, you
> reach a steady state again at a slower airspeed and higher AOA. If the AOA
> reaches the stall angle before the lift increases to match your weight,
> you'll stall, and this is why you should carry plenty of extra speed in
> that situation.
>
> Turbulence would bounce it around similar; if you get hit with a gust from
> below the AOA will momentarily become larger, increasing the lift from the
> wings. You then begin to climb until your vertical speed matches the gust
> and your AOA goes back to what it was. Similarly with a gust from above,
> where you start to drop.
>
Wayne Paul
Check your gmail account for an offline note.
Wayne
http://www.soaridaho.com/Schreder
"jcarlyle" <jmca...@gmail.com> wrote in message
news:1dd3155b-d4b7-46b4...@e1g2000hsh.googlegroups.com...
Bill Daniels
"jcarlyle" <jmca...@gmail.com> wrote in message
news:1dd3155b-d4b7-46b4...@e1g2000hsh.googlegroups.com...
> is needed!
>
> The only thing I'm left unsure about is AOA behavior vs. ASI behavior
> in a turbulent thermal. I buy that AOA eliminates worrying about bank
> angle and ballast - just set the attitude for best min sink AOA and
> try to keep it there. But I wonder how much an AOA meter (say one
> based upon pressure ports) would vary in a thermal vis a vis an ASI.
> I've seen plus/minus 5 kts in ASI. If this translated in several
> degrees in AOA, it might make the AOA as useless as the ASI.
>
> -John
>
Many pilots who have tried the "pitch strings" report that they give advance
warning when entering an area of lift. When you enter the edge of a
thermal, the strings show a sharp increase in AOA several seconds before the
vario shows lift. This improves the efficiency of "dolphin flying" by
providing an earlier signal of when to start a zoom. One pilot on a
marginal final glide told me, "Without them, I wouldn't have made it home."
An ASI gives the same signal but it's weaker and harder to interpret.
Most AOA sensors have some damping to smooth the signal in rough air.
Bill Daniels
Eric Greenwell
> Many pilots who have tried the "pitch strings" report that they give advance
> warning when entering an area of lift. When you enter the edge of a
> thermal, the strings show a sharp increase in AOA several seconds before the
> vario shows lift. This improves the efficiency of "dolphin flying" by
> providing an earlier signal of when to start a zoom. One pilot on a
> marginal final glide told me, "Without them, I wouldn't have made it home."
> An ASI gives the same signal but it's weaker and harder to interpret.
And yet, I never see any gliders with "pitch strings" on them, not even
on contest winners' gliders, presumably the pilots most interested in
getting the most from their efforts. If it's effective, why hasn't
something so cheap and easy spread throughout the fleet?
I did try these on a Std. Cirrus 30 years ago, and eventually concluded
they didn't help in any way. Maybe it's time to repeat the experiment,
but I'm skeptical about the string's value.
jcarlyle
but I've not seen this behavior. I'll make an effort to look carefully
the next time thermals appear in Pennsylvania! As was said earlier in
the thread, though, strings on the canopy are greatly affected by yaw.
They might help indicate lift, but they won't really be useful AOA
indicators. And if AOA instruments average out rough air fluctuations,
they won't be too useful as incipient lift indicators. Tough problem!
-John
Bill Daniels
strings" are very sub-optimum AOA indicators. I don't use them because
it's such a hassle closing the canopy without trapping the strings between
the canopy frame and fuselage.
I think it's likely that the SafeFlight vane-type AOA indicator works fine
as long as you are fairly good at keeping the yaw string centered.
SafeFlight has been around selling these things to airplane owners for a
long time. It's nice to see them offering a product for gliders.
Bill Daniels
"jcarlyle" <jmca...@gmail.com> wrote in message
news:93dea195-3679-4840...@p69g2000hsa.googlegroups.com...
jcarlyle
out this site:
http://www.adventureairsports.com/aoa.htm
and pay particular attention to the 3rd photograph. Yaw string and AOA
in one, and not very expensive!
-John
Bill Daniels
cone. Laminar airflow around a nose cone will be nearly parallel to the
skin at any AOA of interest. That's why the AOA sensor has to be on the
sides of the fuselage.
Bill Daniels
"jcarlyle" <jmca...@gmail.com> wrote in message
news:82ee9ec8-a296-4e14...@s19g2000prg.googlegroups.com...
kirk.stant
> I think that probably works on an ultralight without an aerodynamic nose
> cone. Laminar airflow around a nose cone will be nearly parallel to the
> skin at any AOA of interest. That's why the AOA sensor has to be on the
> sides of the fuselage.
>
> Bill Daniels
Good point. They can also be located on a boom extending away from
the fuselage or wing - as usually seen on prototypes, where accurate,
"true" AOA is needed. Often combined with a yaw sensor (just a fancy
yaw string, really) to measure true sideslip.
I wonder if an AOA vane could be mounted on a modified tail TE/combi
probe? The probe would have to be a lot stiffer to work (at least
that's what she said...).
Kirk
Udo
shaft attached, line up with the wing chord. I had several strings
attached on top of each other about half inch apart and different
length. Very interesting to see how the air responds well ahead of the
leading edge but useless for any useable information. With flaps, the
changes I was interested in were to small in any case.
Udo
> > long time. It's nice to see them offering a product for gliders.- Hide quoted text -
Sarah Anderson
Why do you say it was interesting but useless? Did it not give you AoA information?
Interesting you say changes with flaps were too small.. to see?
I wonder if a piano wire probe like
http://www.adventureairsports.com/aoa.htm
but on the nose would work. How far out on the wing was your cuff?
Sarah
Udo
You would need something a lot more precise to be of use for flying
efficiently day to day
I could see a resolution of a 1/2 degree or better. with adjustable
dampening. I was trying to see if the theoretical match the real
thing.
To optimize performance you should have something to compare it with
and record it. That is why the annual Idaflieg is such a big deal in
Germany it takes a lot of effort and equipment.
I found that the theoretical for me was close enough and all I needed
was time to fine tune my settings and speed vis a vis other gliders.
When the big day came I gave R.Johnson the speed and flap setting I
thought best and it work out not bad.
Udo
> >> - Show quoted text -- Hide quoted text -
Chip Bearden
> And yet, I never see any gliders with "pitch strings" on them, not even
> on contest winners' gliders, presumably the pilots most interested in
> getting the most from their efforts. If it's effective, why hasn't
> something so cheap and easy spread throughout the fleet?
Eric,
I installed "pitch strings on my ASW 24 last year at Uvalde when my
ASI failed on takeoff. I landed quickly, swapped an old ASI into the
panel (which didn't work either) and taped two long (2 ft.) strings
low on the canopy. I could barely see the strings flapping wildly
around but discovered 40+ years of flying allowed me to do OK at
thermaling anyway, even with full water. But when I got back that
night, I removed the "whiskers"--the strings were so long that they
trailed on the ground and gave the airplane a very odd look--and
substituted short (~4") ones higher on the canopy following the advice
of ex F-14 driver Bif Huss, who has them on his Discus 2. I took a
grease pencil with me and marked the positions for min. sink and best
glide on the inside of the canopy.
They work fine. They're not revolutionary: they given different
indications when I'm skidding or (more of a problem) intentionally
slipping slightly, the range of movement is relatively small, and of
course they're sensitive to being knocked off during canopy cleaning.
It's just one more input. Interestingly, I'm confident I could fly the
glider at slow speeds very efficiently if my ASI fails again, which is
reason enough to keep them. And a quick glance at them from time to
time does reassure me I'm not cutting it too close trying to slow the
glider down. I refer to them a little more when I'm low, just to be
safe. I haven't noticed them responding early as I enter an updraft
but will pay attention the next time I fly, most likely not until next
spring. :(
I do get a lot of questions about them but, as you note, they haven't
spread throughout the fleet. Maybe if I started winning, that might
change! :)
Chip Bearden
Eric Greenwell
> Does anyone know of documentation that supports the idea showing the
> pilot the AOA will actually improve a glider pilot's thermalling? Or
> even that the range of AOA needed to be "efficient" is too small for a
> pilot to obtain it easily by using airspeed, or by just looking out the
> canopy, once he's flown the glider enough to be familiar with it?
This document addresses the "best" thermalling speed, as part of an
effort to include climbing ability in handicapping gliders:
http://www.tux.org/~milgram/papers/thermaling2005.pdf
It's written by Judah Milgram, the translator for "Fundamentals of
Sailplane Design". I'm still digesting it, but it seems to say the
theoretical speed is slower than you would deduce from the straight
flight minimum speed; in fact, for bank angles less than 30 degrees, the
best speed is actually lower than the straight flight minimum sink speed.
There are some caveats: without polar measurements in a turn, you have
to make some assumptions that may not be true, and ignore real world
handling issues from flying slowly.
Practically speaking, I suggest that circling "efficiently" (regardless
of the value of an AOA meter in achieving it) only has value in weak,
smooth thermals; otherwise, where you are in the thermal is more
important than minimizing your glider's sink rate, so you have to fly
fast enough to maneuver well and accept losses from the excess speed and
bank angle changes.
Mike Borgelt
If the gliders we fly are longitudinally statically stable what you want is
a head up elevator deflection angle indicator.
That gives you the pilot commanded angle of attack.
Mike
Bill Daniels
"Mike Borgelt" <mbor...@borgeltinstruments.com> wrote in message
news:13lrgua...@corp.supernews.com...
Many high performance gliders have their CG set near the rear limt so they
are only mildly stable. i.e. small elevator deflection angles result in
large airspeed and AOA changes.
In transient events like gusts, AOA may not track elevator deflection angle.
In the case of winch launch, the pitch-up, and therefore AOA, may be due to
the placement of the CG hook as well as elevator control inputs.
Bill Daniels
Denis
> If the gliders we fly are longitudinally statically stable what you want is
> a head up elevator deflection angle indicator.
> That gives you the pilot commanded angle of attack.
hum... if you're really convinced by what you say, I suggest you fly with
notches on your stick like there are on the flaps, since you need only a few
fixed elevator positions : one for best climb, one for 1,3 Vs on approach, one
for best L/D, and one for each McCready setting you use ;-)
Denis
> Does anyone know of documentation that supports the idea showing the
> pilot the AOA will actually improve a glider pilot's thermalling? Or
> even that the range of AOA needed to be "efficient" is too small for a
> pilot to obtain it easily by using airspeed, or by just looking out the
> canopy, once he's flown the glider enough to be familiar with it?
Hi Eric,
Optimum thermalling implies flying near minimum turn radius, thus near Clmax.
Modern profiles (beginning at Discus or ASW 24) have flat Cl curves in order to
be less sensitive to stall. Thus you can fly a little beyond maximum lift
without stalling or even "feel bad". That means that at 2 differents AOA (each
side of ClMax AOA) you have the same lift (Cl) thus the same airspeed !
Therefore it is absolutely *impossible* to use airspeed as a mean to control AOA
(even let away the influence of load, bank, bugs, etc.) in that range, as you
might do in approach, at smaller AOAs.
The only difference is the drag (thus the sink rate), which will be
significantly increased beyond the ClMax AOA. There may be a very light
buffeting noise and/or feeling but you will not be aware of it if you don't pay
a very close attention to it.
Try a piece of wool on the side of your canopy as an AOA indicator, as already
suggested, and you will be convinced. Put both of them each side to see the
influence of yaw if you wish. Get a pencil and a tape on the inner side of the
canopy to mark AOAs. The best way is to try first in straight flight, and look
for AOA that gives you minimum speed (better use GPS speed than pneumatic speed,
since the latter is getting pessimistic at high AOAs due to pitot masking). That
will be the AOA-to-fly for best climb. Then continue to put the stick back and
watch the AOA. There should be a noticeable AOA range whithout significant speed
change nor near-stall warnings. Then do the same in steady turning flight: it
should not change a lot except for higher speeds and yaw influence. You may try
different flap settings also.
I did first this experiment when I began to fly Crystal (a glider with a profile
similar to the Discus) in which it made a dramatic climb improvment not to fly
at too high AOAs. I tried it also in my ASH 26, as I would in any new glider
until I get accustomed to it (I don't need using it after a while, though I
would if a better AOA indicator was available).
Denis
BTW: yes you can "see" AOA - at least in no headwind - it's the angle between
the nose of your glider and the point of the ground you are aiming for in final
approach... it's not easy to convert that into degrees, but who cares ? What
matters is to get used to it
--
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