Simple angle of attack sensor

[Bill D]

http://cumulus.atmos.und.edu/winds_introduction.php

Note first picture.

I think it's easy to see how a pair of AOA sensing ports could be added to a glider nose.

[Eric Greenwell[_4_]]

Bill D wrote, On 7/29/2013 8:16 AM:
>
> http://cumulus.atmos.und.edu/winds_introduction.php
>
> Note first picture.
>
> I think it's easy to see how a pair of AOA sensing ports could be
> added to a glider nose.

There are a number of AOA systems using that technique already on the
market, but no glider pilots have reported on them that I can remember.
It's time to stop talking about them and instead, install one and try it.

I am considering installing an AOA probe when I get my new motorglider
(Phoenix U15) next year, using one of these unheated probes:

http://dynonavionics.com/docs/SkyView_AoA.html

They are cheap at $200, and there are no additional costs for the panel
indicator, as it displays on the Dynon SkyView panel display the glider
will have. At that cost, I'm willing to experiment without knowing that
it will be useful.

Has anyone used one of the Dynon AOA probes? How well did it work? Did
it seem suitable for gliders?

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

[Dan Daly[_2_]]

According to my SZD-55 manual (issue IV, March 1993), such an AOA sensing device is mandatory in Canada (page 12a of the Flight Manual/POH). The SP3 system works on the pitot and pressure from another port just below the nose pot (about 1.5" down). It works reliably giving audio warning 3-5 kts above the stall. Once properly adjusted (one-time deal) it is reliable, and has a very low draw (higher when it is alerting - which usually only happens in the flare - or if you're about to stall).

"Electronic STALL WARNING SYSTEM designed to meet the requirements of JAR 22.207 and FAR 23.207 as well. Awarded the O.S.T.I.V. prize, being installed on gliders since 1991 and certified in Poland. Easy to install. No aerodynamic burden. Only one additional pressure sensing hole in the nose of the glider, or in the lower surface of the wing of the aeroplane. Ideal for sailplanes, motogliders and ultralight aeroplanes. The device signals close to stall condition in the range between 5 and 10 percent over the stall speed in straight and circling flight over the whole loading and ballast range. You set the alarm threshold only once, individually for your glider. How does it work ? The difference between pressures Pt and Pc is the function of the two variables : speed and angle of attack. The device measures, calculates and detects the moment when the combination of these two parameters informs you about an approaching stall."
see - http://www.olk.com.pl/indexen.php?bo=prod&prx=products&lpx=26

There is a discussion on this topic on the DG website at:http://www.dg-flugzeugbau.de/ueberziehwarnung-e.html.

Tested and available since 1991 - why re-invent the wheel?

2D

[Sean F (F2)]

On Monday, July 29, 2013 11:16:55 AM UTC-4, Bill D wrote:
> http://cumulus.atmos.und.edu/winds_introduction.php
>
>
>
> Note first picture.
>
>
>
> I think it's easy to see how a pair of AOA sensing ports could be added to a glider nose.

Simple? lol! ;-)

[[email protected]]

The Butterfly Vario will include an AOA sensor soon.

[[email protected]]

On Monday, July 29, 2013 10:16:55 AM UTC-5, Bill D wrote:
> http://cumulus.atmos.und.edu/winds_introduction.php
>
>
>
> Note first picture.
>
>
>
> I think it's easy to see how a pair of AOA sensing ports could be added to a glider nose.
>
> and others:

The photo shows in essence a 5-hole hemispherical probe using the nose of the glider as the probe. 5-hole probes are fairly common instruments in wind tunnels and sometimes even flight.

The center port measures total pressure. Total pressure is accurately measured for angles up to about 15 degrees off axis, and even for angles up to 20 degrees, the error is a small percentage.

The other four ports will likely trigger transition locally on the nose. They will each create about a 7-degree wedge of turbulent flow, which could/would significantly reduce the amount of laminar flow on the fuselage.

I would suggest using a conical 5-hole probe (google it). These can be quite small in size and are easy to calibrate. One can determine total pressure, static pressure, yaw angle, and angle of attack. I have calibrated probes of this type to angles up to 45 degrees. A decent set of calibration curves can be made from CFD calculations which model the probe and the nearby portions of the glider.

As mentioned above, the probe can be quite small, a 3/16" probe could work well. It can be mounted to the fuselage, or possibly better, the leading edge of the vertical fin.

If five pressure transducers are used, one for each point, aoa, yaw, airspeed, and altitude could be calculated from a single probe. Since both airspeed and altitude are calculated from the connected flight data computer, it is possible to electronically calculate the climb rate corrected for total energy. One could also alert the pilot either visually or audibly for excessive aoa or yaw.

As with any probe of this type, it could be contaminated by rain, which is why most aircraft pitot-static probes are heated. The Kulite Flow Angle Probe FAP-250 (google it) is only 0.25" in diameter and may be more resistant, since the sensors are quite small and are nearly surface mounted. This probe is calibrated to up to 35 degrees off axis.

That's my 2 cents.

[Steve Leonard[_2_]]

But, if your plane has flaps that deflect much, the "approaching stall angle" for flow relative to that fixed probe changes. A bunch. I would hazard a guess that if your plane has flaps that go 30 degrees or more, and you set the "warning" point for adequate warning with thermaling flap, it will never go off with flaps 30 or more. The calibration shifts whith flaps.

Not saying it can't provide some warning some of the time, but if your ship has flaps, expecially larger deflection flaps, this 5 hole probe is NOT going to do what you want it to without some flap position input to the system for sounding the alarm.

That's my nickel's worth (Inflation, Neal.) :-)

[kirk.stant]

On Wednesday, August 21, 2013 7:34:46 AM UTC-5, Steve Leonard wrote:
> But, if your plane has flaps that deflect much, the "approaching stall angle" for flow relative to that fixed probe changes. A bunch. I would hazard a guess that if your plane has flaps that go 30 degrees or more, and you set the "warning" point for adequate warning with thermaling flap, it will never go off with flaps 30 or more. The calibration shifts whith flaps.
>
>
>
> Not saying it can't provide some warning some of the time, but if your ship has flaps, expecially larger deflection flaps, this 5 hole probe is NOT going to do what you want it to without some flap position input to the system for sounding the alarm.
>
>
>
> That's my nickel's worth (Inflation, Neal.) :-)

Of course, but that really isn't a big deal, really - you establish the calibration curve for a few selected flap positions and display accordingly. No hardware change in the probe or display, a flap position switch, and some software in the computer/display.

Way better than what we have now!

Kirk
66

[Jim White[_3_]]

At 12:49 21 August 2013, kirk.stant wrote:
>On Wednesday, August 21, 2013 7:34:46 AM UTC-5, Steve Leonard wrote:
>> But, if your plane has flaps that deflect much, the "approaching stall
>an=
>gle" for flow relative to that fixed probe changes. A bunch. I would
>haza=
>rd a guess that if your plane has flaps that go 30 degrees or more, and
>you=
> set the "warning" point for adequate warning with thermaling flap, it
>will=
> never go off with flaps 30 or more. The calibration shifts whith flaps.
>>=20
>>=20
>>=20
>> Not saying it can't provide some warning some of the time, but if your
>sh=
>ip has flaps, expecially larger deflection flaps, this 5 hole probe is
NOT
>=
>going to do what you want it to without some flap position input to the
>sys=
>tem for sounding the alarm.
>>=20
>>=20
>>=20
>> That's my nickel's worth (Inflation, Neal.) :-)
>
>Of course, but that really isn't a big deal, really - you establish the
>cal=
>ibration curve for a few selected flap positions and display accordingly.

>=
>No hardware change in the probe or display, a flap position switch, and
>som=
>e software in the computer/display.
>
>Way better than what we have now!
>
>Kirk
>66

I have the best stall warning device ever - the glider. Nose high attitude,
low airspeed, rearward stick position, shuddering, reduced effectiveness of
controls. Zero cost and complexity.

[Steve Leonard[_2_]]

On Wednesday, August 21, 2013 7:49:04 AM UTC-5, kirk.stant wrote:

Of course, but that really isn't a big deal, really - you establish the calibration curve for a few selected flap positions and display accordingly. No hardware change in the probe or display, a flap position switch, and some software in the computer/display. Way better than what we have now! Kirk 66

For planes like your LS-6, Kirk, you might even get adequate warning with a single set point. A bit earlier warning with no flap, and a little less warning with thermal or landing flap. Planes like my Zuni, where you don't set and forget the flaps, the system becomes more complicated.

Not well done, but my comment was intended to say "It may seem simple for some cases, but it is not a simple solution that will work just that easily for all sailplanes."

I tend to add the audio airspeed (open the side scoop) when I enter the pattern. Helps with ventilation, and adds an extra airspeed sensor that doesn't require a look inside. Not AOA, but another reference.

Steve

[Bill D]

On Tuesday, August 20, 2013 9:47:32 PM UTC-6, wrote:
> On Monday, July 29, 2013 10:16:55 AM UTC-5, Bill D wrote:

> The other four ports will likely trigger transition locally on the nose. They will each create about a 7-degree wedge of turbulent flow, which could/would significantly reduce the amount of laminar flow on the fuselage.


Not sure I follow this. The pressure taps would be 1mm holes. How could they significantly reduce laminar flow on the fuselage? Many gliders already have multiple static port holes on the sides of the cockpit ahead of the panel.

[Eric Greenwell[_4_]]

Jim White wrote, On 8/21/2013 6:33 AM:
> At 12:49 21 August 2013, kirk.stant wrote:
>> On Wednesday, August 21, 2013 7:34:46 AM UTC-5, Steve Leonard wrote:

>>
>> Way better than what we have now!
>>
>> Kirk
>> 66
>
> I have the best stall warning device ever - the glider. Nose high attitude,
> low airspeed, rearward stick position, shuddering, reduced effectiveness of
> controls. Zero cost and complexity.

My glider does exactly the same thing, but only in straight ahead
flight; in fact, it is very difficult to force a stall while flying
straight ahead. But, in a gentle turn of 20 degrees from downwind to
base it can smoothly, quietly drop the inside wing without a
significantly nose high attitude or rearward stick position. There is no
shuddering, and the controls and "picture out the window" seem normal.
They aren't, of course, but the signs of an impending stall are very
much muted compared to flying straight ahead.

This behavior in a gentle turn is not specific to my ASH 26 E, but also
appeared in most of the other gliders I've flown. And sure enough, it's
stalls in turns where we have the most accidents, not straight ahead
stalls. This lack of sufficient warning is what drives the interest in
stall warning systems, with angle of attack (AA) being one method.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

[Soartech]

Eric,
I am more of a novice but have noticed the same thing. Straight ahead there are all kinds of warning signs before a stall as mentioned. Yet I have been surprised while thermalling as the ship suddenly and quietly takes a bit of nose down pitch (with easy recovery). What's up with that?
Is this the reason sailplanes have a dangerous reputation for spinning in?
Why does this happen?

[kirk.stant]

On Wednesday, August 21, 2013 8:37:35 AM UTC-5, Steve Leonard wrote:
>
> For planes like your LS-6, Kirk, you might even get adequate warning with a single set point. A bit earlier warning with no flap, and a little less warning with thermal or landing flap. Planes like my Zuni, where you don't set and forget the flaps, the system becomes more complicated.
>
>
>
> Not well done, but my comment was intended to say "It may seem simple for some cases, but it is not a simple solution that will work just that easily for all sailplanes."
>
>
>
> I tend to add the audio airspeed (open the side scoop) when I enter the pattern. Helps with ventilation, and adds an extra airspeed sensor that doesn't require a look inside. Not AOA, but another reference.
>
>
>
> Steve


I've mentioned this before, but I'm less concerned with stall warning that with providing an easy to use indication of the correct speed to fly when thermalling and in the pattern. The airspeed indicator is a crude way of doing that, and requires one to look at the darn thing instead of outside. Having flown aircraft with audio AOA indicators, it's a much nicer way to fly slow. For thermalling, (due to the vario audio) I would settle for some bright LEDs on the panel at the thermalling AOA (for the flap setting). But for landing (gear down) I would switch off the vario and use the AOA to give me speed cues. If you have never tried this, it may sound funny, but trust me it works great!

Of course, when approaching the stall, you can always have the lights flash and Bitchin' Betty yell at you "Stall Stall!!!".

Kirk
66

[Jim White[_3_]]

At 14:52 21 August 2013, Eric Greenwell wrote:
>Jim White wrote, On 8/21/2013 6:33 AM:
>> At 12:49 21 August 2013, kirk.stant wrote:
>>> On Wednesday, August 21, 2013 7:34:46 AM UTC-5, Steve Leonard wrote:
>
>>>
>>> Way better than what we have now!
>>>
>>> Kirk
>>> 66
>>
>> I have the best stall warning device ever - the glider. Nose high
>attitude,
>> low airspeed, rearward stick position, shuddering, reduced
effectiveness
>of
>> controls. Zero cost and complexity.
>
>My glider does exactly the same thing, but only in straight ahead
>flight; in fact, it is very difficult to force a stall while flying
>straight ahead. But, in a gentle turn of 20 degrees from downwind to
>base it can smoothly, quietly drop the inside wing without a
>significantly nose high attitude or rearward stick position. There is no
>shuddering, and the controls and "picture out the window" seem normal.
>They aren't, of course, but the signs of an impending stall are very
>much muted compared to flying straight ahead.
>
>This behavior in a gentle turn is not specific to my ASH 26 E, but also
>appeared in most of the other gliders I've flown. And sure enough, it's
>stalls in turns where we have the most accidents, not straight ahead
>stalls. This lack of sufficient warning is what drives the interest in
>stall warning systems, with angle of attack (AA) being one method.
>
>--
>Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
>email me)
>- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
>http://tinyurl.com/yb3xywl
>
Never flown a 26. My 27 gives just as much warning at all wing loads and
angles of bank. Just ease the stick forward when it goes mushy. Maybe that
1500 hours on type helps a bit?

[Eric Greenwell[_4_]]

Jim White wrote, On 8/21/2013 2:49 PM:
> At 14:52 21 August 2013, Eric Greenwell wrote:
>> Jim White wrote, On 8/21/2013 6:33 AM:

>>> I have the best stall warning device ever - the glider. Nose high
>> attitude,
>>> low airspeed, rearward stick position, shuddering, reduced
> effectiveness
>> of
>>> controls. Zero cost and complexity.
>>
>> My glider does exactly the same thing, but only in straight ahead
>> flight; in fact, it is very difficult to force a stall while flying
>> straight ahead. But, in a gentle turn of 20 degrees from downwind to
>> base it can smoothly, quietly drop the inside wing without a
>> significantly nose high attitude or rearward stick position. There is no
>> shuddering, and the controls and "picture out the window" seem normal.
>> They aren't, of course, but the signs of an impending stall are very
>> much muted compared to flying straight ahead.
>>
>> This behavior in a gentle turn is not specific to my ASH 26 E, but also
>> appeared in most of the other gliders I've flown. And sure enough, it's
>> stalls in turns where we have the most accidents, not straight ahead
>> stalls. This lack of sufficient warning is what drives the interest in
>> stall warning systems, with angle of attack (AA) being one method.
>>
>> --
>> Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
>> email me)
>> - "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
>> http://tinyurl.com/yb3xywl
>>
> Never flown a 26. My 27 gives just as much warning at all wing loads and
> angles of bank. Just ease the stick forward when it goes mushy. Maybe that
> 1500 hours on type helps a bit?

I doubt that your hours have anything to do with how the plane flies in
the situation I described. I have 3400 hours in type, and it's handled
that way since I began flying it, as have other gliders I've flown,
including the Blanik L13 our club used to train students. I'm sure it
depends on CG position, perhaps other things.

Here's how I do it to get the smooth, quiet entry into dropping the
inside wing:
- begin a gentle, coordinated turn with 15 to 20 degrees of flap in
neutral flap (flap 3 in the 26 E)
- slowly pull the stick back while maintaining that smooth, coordinated
15-20 degree bank
- as the speed diminishes, it will require more and more "top aileron",
and soon the inner wing will drop
- just moving the stick forward and centralizing the stick is usually
enough, but also moving the flaps forward a notch or so makes recovery
even quicker.

Give it a try next time you fly. A lot of pilots don't make such shallow
turns (at least not while also slowing down), and don't ever experience
this situation. It's important the bank be shallow - if it's too steep,
say over 35 degrees, the glider may not stall at all.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

[Jim White[_3_]]

At 23:19 21 August 2013, Eric Greenwell wrote:
>Jim White wrote, On 8/21/2013 2:49 PM:
>> At 14:52 21 August 2013, Eric Greenwell wrote:
>>> Jim White wrote, On 8/21/2013 6:33 AM:
>
>>>> I have the best stall warning device ever - the glider. Nose high
>>> attitude,
>>>> low airspeed, rearward stick position, shuddering, reduced
>> effectiveness
>>> of
>>>> controls. Zero cost and complexity.
>>>
>>> My glider does exactly the same thing, but only in straight ahead
>>> flight; in fact, it is very difficult to force a stall while flying
>>> straight ahead. But, in a gentle turn of 20 degrees from downwind to
>>> base it can smoothly, quietly drop the inside wing without a
>>> significantly nose high attitude or rearward stick position. There is
no
>>> shuddering, and the controls and "picture out the window" seem normal.
>>> They aren't, of course, but the signs of an impending stall are very
>>> much muted compared to flying straight ahead.
>>>
>>> This behavior in a gentle turn is not specific to my ASH 26 E, but
also
>>> appeared in most of the other gliders I've flown. And sure enough,
it's
>>> stalls in turns where we have the most accidents, not straight ahead
>>> stalls. This lack of sufficient warning is what drives the interest in
>>> stall warning systems, with angle of attack (AA) being one method.
>>>
>>> --
>>> Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
>>> email me)
>>> - "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
>>> http://tinyurl.com/yb3xywl
>>>
>> Never flown a 26. My 27 gives just as much warning at all wing loads
and
>> angles of bank. Just ease the stick forward when it goes mushy. Maybe
>that
>> 1500 hours on type helps a bit?
>
>I doubt that your hours have anything to do with how the plane flies in
>the situation I described. I have 3400 hours in type, and it's handled
>that way since I began flying it, as have other gliders I've flown,
>including the Blanik L13 our club used to train students. I'm sure it
>depends on CG position, perhaps other things.
>
>Here's how I do it to get the smooth, quiet entry into dropping the
>inside wing:
>- begin a gentle, coordinated turn with 15 to 20 degrees of flap in
>neutral flap (flap 3 in the 26 E)
>- slowly pull the stick back while maintaining that smooth, coordinated
>15-20 degree bank
>- as the speed diminishes, it will require more and more "top aileron",
>and soon the inner wing will drop
>- just moving the stick forward and centralizing the stick is usually
>enough, but also moving the flaps forward a notch or so makes recovery
>even quicker.
>
>Give it a try next time you fly. A lot of pilots don't make such shallow
>turns (at least not while also slowing down), and don't ever experience
>this situation. It's important the bank be shallow - if it's too steep,
>say over 35 degrees, the glider may not stall at all.
>
>--
>Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
>email me)
>- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
>http://tinyurl.com/yb3xywl
>
That sounds like a pretty standard spin entry technique that we use during
instruction all the time. If you kick the rudder out of the turn when it
goes mushy it will make a dramatic entry in the opposite direction. My
glider will spin reliably using this technique too, but it tells me when it
is going to stall a wing and autorotate.

[Squeaky]

Eric,
I am more of a novice but have noticed the same thing. Straight ahead there are all kinds of warning signs before a stall as mentioned. Yet I have been surprised while thermalling as the ship suddenly and quietly takes a bit of nose down pitch (with easy recovery). What's up with that?
Is this the reason sailplanes have a dangerous reputation for spinning in?
Why does this happen?

From Tom Knauff:

Here is another interesting exercise:

Roll into a modest bank angle of 30 degrees, and then pull back on the stick to bring the nose of the glider above the horizon. Watch carefully what happens.

If one wing stalls, the glider will roll as it stalls. If both wings stall, the glider will pitch nose down.
(You really must try this.)

With few exceptions, what you will observe is the glider neither rolls nor pitches, but yaws.

What happens is as the glider loses airspeed, the lift produced is not adequate to hold the glider up and it begins to fall. Since the glider is tilted, the falling glider causes the airflow to strike the side of the fuselage, causing the tail of the glider to be pushed up, causing the yawing motion.

There is no rolling. No pitching.

The wing is not stalled.

Because the wing is not stalled, the ailerons and rudder work properly.

The steeper the angle of bank, the more stall-proof the normally certificated glider becomes.

In a very steep angle of bank, (60 degrees) at a stable airspeed, the control stick may be fully back. At this attitude, how would you stall the glider? Since the angle of attack control is used up (fully back) it is not possible to increase the angle of attack and thus, the glider becomes stall-proof.

Thus, from the standpoint of stalling, steeper turns are safer than shallow turns.

Just as important: If you are performing a steeper angle of bank turn, and the glider seems to be falling/stalling, your instinctive reaction of applying opposite aileron will be OK. Since the wing is not stalled, opposite aileron works fine to level the wings.

The same instinctive reaction in a shallow angle of bank turn can create a stall/spin.

This is so contrary to what pilots are taught, that I expect lots of mail from you all.

Before you send me mail, please go try it yourself. Let me know what you experience.

Stall prevention includes:

* Know the signs of a stall in the order they occur.

* Keep the nose of the glider below the horizon.
* Fly an appropriate, stable airspeed.
* Keep the yaw string straight.
* Use steeper bank angles. (30 – 45 degrees)

All of the above is especially important during landings and all low altitude turns (rope breaks above 200 feet, low altitude thermalling.)

During training, we teach the student pilots to, “Watch the airspeed indicator.” What we mean is verify you are flying the correct airspeed by looking at the airspeed indicator every few seconds when landing or during rope break practice..

We demand the yaw string stay straight during low altitude turns

[Bill D]

Nice discussion about impending stall recognition but that isn't really the point. Pretty much anybody from student pilots on can recognize an impending stall - if they're looking for it. You see what you're looking for and sometimes don't see what you're not looking for.

The need for a Stall warning device arises when the pilot isn't expecting a stall - perhaps when fatigued, dehydrated, hypoxic, distracted or just in over his head and overwhelmed by the pace of events. It's this pilot a stall warning is intended to save. Who among us can swear it's never happened to them? A stall warning system isn't there to take over and fly the aircraft or to diminish the skill required, it's there to watch a pilots back.

The AoA portion of the system is icing on the cake which helps a pilot an aircraft more accurately and extract more performance from it with greater confidence.

[jfitch]

On Thursday, August 22, 2013 9:58:53 AM UTC-7, Squeaky wrote:

> From Tom Knauff:

>
> With few exceptions, what you will observe is the glider neither rolls
>
> nor pitches, but yaws.
>
>
>
> What happens is as the glider loses airspeed, the lift produced is not
>
> adequate to hold the glider up and it begins to fall. Since the glider
>
> is tilted, the falling glider causes the airflow to strike the side of
>
> the fuselage, causing the tail of the glider to be pushed up, causing
>
> the yawing motion.
>
>
>
> There is no rolling. No pitching.
>
>
>
> The wing is not stalled.
>
>
>
> Because the wing is not stalled, the ailerons and rudder work properly.
>
This cannot be a technically correct explanation. And it isn't what any glider I have flown in recent memory does. If there is inadequate lift, and "the glider begins to fall" without pitching the AoA is very quickly going to exceed the stall angle. If it is sliding sideways enough due to falling that a large yawing moment is produced, the AoA is way past the stalling angle. A spin departure can produce pretty high yaw accelerations, but that ain't the mechanism.

[Gilbert Smith[_2_]]

jfitch > wrote:

>On Thursday, August 22, 2013 9:58:53 AM UTC-7, Squeaky wrote:
>
>> From Tom Knauff:
>
>>
>> With few exceptions, what you will observe is the glider neither rolls
>>
>> nor pitches, but yaws.
>>
>>
>>
>> What happens is as the glider loses airspeed, the lift produced is not
>>
>> adequate to hold the glider up and it begins to fall. Since the glider
>>
>> is tilted, the falling glider causes the airflow to strike the side of
>>
>> the fuselage, causing the tail of the glider to be pushed up, causing
>>
>> the yawing motion.
>>
>>
>>
>> There is no rolling. No pitching.
>>
>>
>>
>> The wing is not stalled.
>>
>>
>>
>> Because the wing is not stalled, the ailerons and rudder work properly.
>>
>This cannot be a technically correct explanation. And it isn't what any glider I have flown in recent memory does. If there is inadequate lift, and "the glider begins to fall" without pitching the AoA is very quickly going to exceed the stall angle. If it is sliding sideways enough due to falling that a large yawing moment is produced, the AoA is way past the stalling angle. A spin departure can produce pretty high yaw accelerations, but that ain't the mechanism.

Tom Knauf is right of course (as always), in a spin one wing is
producing lift, the other is not. This is not the case in the
"falling" scenario.

Whether the ensuing yaw changes this is debatable, but I find that
unloading the controls prevents any departure.

[jfitch]

On Wednesday, August 28, 2013 3:52:39 AM UTC-7, Gilbert Smith wrote:
>
>
>
>
> >On Thursday, August 22, 2013 9:58:53 AM UTC-7, Squeaky wrote:
>
> >
>
> >> From Tom Knauff:
>
> >
>
> >>
>
> >> With few exceptions, what you will observe is the glider neither rolls
>
> >>
>
> >> nor pitches, but yaws.
>
> >>
>
> >>
>
> >>
>
> >> What happens is as the glider loses airspeed, the lift produced is not
>
> >>
>
> >> adequate to hold the glider up and it begins to fall. Since the glider
>
> >>
>
> >> is tilted, the falling glider causes the airflow to strike the side of
>
> >>
>
> >> the fuselage, causing the tail of the glider to be pushed up, causing
>
> >>
>
> >> the yawing motion.
>
> >>
>
> >>
>
> >>
>
> >> There is no rolling. No pitching.
>
> >>
>
> >>
>
> >>
>
> >> The wing is not stalled.
>
> >>
>
> >>
>
> >>
>
> >> Because the wing is not stalled, the ailerons and rudder work properly..
>
> >>
>
> >This cannot be a technically correct explanation. And it isn't what any glider I have flown in recent memory does. If there is inadequate lift, and "the glider begins to fall" without pitching the AoA is very quickly going to exceed the stall angle. If it is sliding sideways enough due to falling that a large yawing moment is produced, the AoA is way past the stalling angle. A spin departure can produce pretty high yaw accelerations, but that ain't the mechanism.
>
>
>
> Tom Knauf is right of course (as always), in a spin one wing is
>
> producing lift, the other is not. This is not the case in the
>
> "falling" scenario.
>
>
>
> Whether the ensuing yaw changes this is debatable, but I find that
>
> unloading the controls prevents any departure.

If the glider is "falling" without pitching down, the wing is stalled. The whole wing. The rudder and ailerons might still work a bit even above the stall angle. When he says "The wing is not stalled" - I just can't see that.

In a sustained autorotating spin, one wing is operating at a higher AoA than the other, above the max lift AoA for the section. Above that point, lift reduces and drag increases with increasing AoA. In a spin departure, both wings might be stalled, the wing going down is at a higher AoA and higher drag, therefore producing a net yawing component.