ASW 20 SPIN CHARACTERISTICS

(Chris OCallaghan) wrote in message . com...
In fact, if you think about it, there would be a change in AoA as the
wings returned to their normal 1g state. The AoA increase at the tips
would be greatest and negligible at the roots. How large an increase
are we talking about? Pretty darn small. An amusing exercise though. A
friend once figured out how thick a layer of material a tire leaves on
the road, given normal wear. This seems on the same order.


According to Thomas, Fundamentals of Sailplane Design, the wing twist
of the ASW-20 is 2.5 deg (page 210). Isn't twist designed into a wing
to prevent the tip stalling before the root? If my numbers were
derived for 68 knots instead of 40kts they give a result that is
similar to the designed-in wing twist. In other words, the wing flex
effect appears to completely offset the protection provided by the
wing twist.

Andy

Andreas Maurer wrote in message . ..


I don't think that the scenario you describe can lead to a wing stall:
The cause that returns the wing to normal deflection is of course that
the pilot reduces AoA by pushing the stick forward. The instance the
pilot reduces g-load this way he also reduces his stall speed - I
doubt that it's possible to stall if the pilot was able to pull a
high-g pull-up only one second before without having a highspeed
stall. Of course the relative AoA-rise indeed occurs when the wing
tips are moving downwards, but the overall AoA is reduced a lot more
with the elevator (otherwise he woudn't lower the AoA enough to cause
the rapid unbending of the wing).

I couldn't find the photos last night so I can't attempt any
measurements of wing deflection. The scenario I imagine is this. The
pilot makes an agressive contest finish pull up. The pull up starts
at over 100kts, the pilot continues to pull as the speed decays to say
60kts where the wing experiences an accelerated stall. The reduction
in lift causes the wings to start to unflex. What happens next
depends on how well the glider is coordinated and how quickly the
pilot pushes forward to exit the accelerated stall. Isn't it possible
that the push forward makes the wings unflex at a rate that leaves the
tips stalled?

Andy

On 15 Jul 2004 06:43:05 -0700, (Andy Durbin)
wrote:

(Chris OCallaghan) wrote in message . com...
In fact, if you think about it, there would be a change in AoA as the
wings returned to their normal 1g state. The AoA increase at the tips
would be greatest and negligible at the roots. How large an increase
are we talking about? Pretty darn small. An amusing exercise though. A
friend once figured out how thick a layer of material a tire leaves on
the road, given normal wear. This seems on the same order.


According to Thomas, Fundamentals of Sailplane Design, the wing twist
of the ASW-20 is 2.5 deg (page 210). Isn't twist designed into a wing
to prevent the tip stalling before the root? If my numbers were
derived for 68 knots instead of 40kts they give a result that is
similar to the designed-in wing twist. In other words, the wing flex
effect appears to completely offset the protection provided by the
wing twist.

If the pilot is pushing over hard the wing will be carrying a reduced
load. As a result the stalling speed will be reduced: remember that a
stall occurs when the wing fails to generate the lift needed to
support the current load on the wing and is only indirectly connected
with the AOA and Cl figures. In the case we're considering the stall
speed will be reduced below normal because the push-over is creating a
reduced G situation.

I haven't noticed you mention this factor. How does its inclusion
affect your calculation?

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Andy Durbin wrote:


According to Thomas, Fundamentals of Sailplane Design, the wing twist
of the ASW-20 is 2.5 deg (page 210). Isn't twist designed into a wing
to prevent the tip stalling before the root?

It is also used to adjust the lift distribution for decreased drag, and
since the airfoil also changes from root to tip, it might be hard to
determine (just from the numbers) why the designer chose to put twist
into the wing.



--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

Andy Durbin wrote:


I couldn't find the photos last night so I can't attempt any
measurements of wing deflection. The scenario I imagine is this. The
pilot makes an agressive contest finish pull up. The pull up starts
at over 100kts, the pilot continues to pull as the speed decays to say
60kts where the wing experiences an accelerated stall.

If he waits to 60 knots, he is well into a loop. Based on my contest
finishes, I'd guess he'd be back at 1 G before the speed decreases to
85-90 knots. The high G part of the pull-up is very short - just long
enough to get the glider aimed upward.


--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

Eric Greenwell wrote
Andy Durbin wrote:


I couldn't find the photos last night so I can't attempt any
measurements of wing deflection. The scenario I imagine is this. The
pilot makes an agressive contest finish pull up. The pull up starts
at over 100kts, the pilot continues to pull as the speed decays to say
60kts where the wing experiences an accelerated stall.

If he waits to 60 knots, he is well into a loop. Based on my contest
finishes, I'd guess he'd be back at 1 G before the speed decreases to
85-90 knots. The high G part of the pull-up is very short - just long
enough to get the glider aimed upward.

I've read most of these posts and now I'm thoroughly confused:
1. isn't it true that unless the pilot aggressively pushes over, the wings will unflex too slowly to cause a large AOA increase? And even then, the increase is relatively small - at most about 1 degree for realistic speeds of travel and rates of unflexure?

2. isn't it more likely that the pilot tried to initiate a low speed, initially low G turn, then as the load factor increased due to the increasing angle of bank the wing AOA went past the stalling angle and so the (then inevitable) spin became a fact?

3. everything I've ever read and learned about wing theory states that a stall results when the AOA is greater than the stalling angle - it doesn't matter what the G loading is or what the speed is.

Rgds,

Derrick Steed

On 15 Jul 2004 18:17:04 GMT, Derrick Steed
wrote:

Eric Greenwell wrote
Andy Durbin wrote:


I couldn't find the photos last night so I can't attempt any
measurements of wing deflection. The scenario I imagine is this. The
pilot makes an agressive contest finish pull up. The pull up starts
at over 100kts, the pilot continues to pull as the speed decays to say
60kts where the wing experiences an accelerated stall.

If he waits to 60 knots, he is well into a loop. Based on my contest
finishes, I'd guess he'd be back at 1 G before the speed decreases to
85-90 knots. The high G part of the pull-up is very short - just long
enough to get the glider aimed upward.

I've read most of these posts and now I'm thoroughly confused:
1. isn't it true that unless the pilot aggressively pushes over, the wings will unflex too slowly to cause a large AOA increase? And even then, the increase is relatively small - at most about 1 degree for realistic speeds of travel and rates of unflexure?

2. isn't it more likely that the pilot tried to initiate a low speed, initially low G turn, then as the load factor increased due to the increasing angle of bank the wing AOA went past the stalling angle and so the (then inevitable) spin became a fact?

3. everything I've ever read and learned about wing theory states that a stall results when the AOA is greater than the stalling angle - it doesn't matter what the G loading is or what the speed is.


I'm not certain any of us know enough detail about the accident to
make a sensible guess about its cause.

FWIW, on Tuesday evening I decided to investigate turning stall/spin
behaviour in my '20 at a sensible altitude. It was calm and with
little air movement under a high overcast. With the aircraft clean and
flaps at zero (setting 3) I flew some moderately steep turns - about
45 degrees of bank and at speeds ranging down to about 43 kts. This
was completely uneventful - no buffeting, burble or hints of
departure. In short, it flew like a pussycat. I'll try this again by
myself in a turbulent thermal next time because all that series of
turns told me was that in nearly still air my '20 can fly uneventful
turns at stupidly slow airspeeds. By comparison I typically fly at
48-50 kts for that steep a turn in zero flap during normal thermalling
turns.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Isn't it possible
that the push forward makes the wings unflex at a rate that leaves the
tips stalled?


No.
The wings unflex as a response to reduced angle of attack and thus load. It
does not make sense that the wing could unflex faster than the load being
removed. If that was the case, then the wings would not flex in the first
place.
-
Mark Navarre
2/5 black ace
LoCal, USA
remove brain to reply
-

Andy Durbin wrote:


Isn't it possible that the push forward makes the wings unflex
at a rate that leaves the tips stalled?

They unflex because the load is removed. In order to stall they would
have to unflex faster than the load is removed. If the pilot is pushing,
the stall speed can go _way_ down. It's when he starts to load the wings
again that his technique, or failure to track his loss of airspeed in
the pull/push, can bring about the stall.


Jack

Stalling of a wing is connected to AoA in the first place, nothing else.

--
Bert Willing

ASW20 "TW"


"Martin Gregorie" a écrit dans le message de
...
On 15 Jul 2004 06:43:05 -0700, (Andy Durbin)
wrote:

(Chris OCallaghan) wrote in message
. com...
In fact, if you think about it, there would be a change in AoA as the
wings returned to their normal 1g state. The AoA increase at the tips
would be greatest and negligible at the roots. How large an increase
are we talking about? Pretty darn small. An amusing exercise though. A
friend once figured out how thick a layer of material a tire leaves on
the road, given normal wear. This seems on the same order.


According to Thomas, Fundamentals of Sailplane Design, the wing twist
of the ASW-20 is 2.5 deg (page 210). Isn't twist designed into a wing
to prevent the tip stalling before the root? If my numbers were
derived for 68 knots instead of 40kts they give a result that is
similar to the designed-in wing twist. In other words, the wing flex
effect appears to completely offset the protection provided by the
wing twist.

If the pilot is pushing over hard the wing will be carrying a reduced
load. As a result the stalling speed will be reduced: remember that a
stall occurs when the wing fails to generate the lift needed to
support the current load on the wing and is only indirectly connected
with the AOA and Cl figures. In the case we're considering the stall
speed will be reduced below normal because the push-over is creating a
reduced G situation.

I haven't noticed you mention this factor. How does its inclusion
affect your calculation?

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :