ASW 20 SPIN CHARACTERISTICS

In article ,
Andreas Maurer wrote:

On 13 Jul 2004 14:33:53 -0700, (Andy Durbin)
wrote:

Just catching up with this thread and no-one seems to have mentioned
the effect of the flexible wings. I don't have experience in the 20
but I do have a series of photos of a fatal accident that started
with a contest finish pull-up and quickly ended up in a spinning
impact with the ground. I believe that the increased angle attack
caused by the wings returning to normal deflection contributed to the
accident.

Flexible wings do NOT change their AoA while they are bending -
otherwise flutter would start immediately.

It's not the amount of bend, it's the change in the bend.

Anything that makes the wings move vertically changes the angle of
attack to generate a lift force that opposes the movement. That's why
gliders roll so slowly, for example.

The exception to the above is if the extra angle of attack is sufficient
to cause the wing to stall, in which case the lift becomes less and
rolling/bending becomes anti-stable.

That's what the original poster is talking about, not about twisting of
the wing or some such thing.


I'd be very surprised though if that was a big enough effect to *cause*
the accident. Especially given that things are normally designed so
that the wing roots stall considerably before the tips.

-- Bruce

On Thu, 15 Jul 2004 09:02:20 +1200, Bruce Hoult
wrote:

Anything that makes the wings move vertically changes the angle of
attack to generate a lift force that opposes the movement. That's why
gliders roll so slowly, for example.

Now I start to see the light...


I'd be very surprised though if that was a big enough effect to *cause*
the accident. Especially given that things are normally designed so
that the wing roots stall considerably before the tips.

Not necessarily in a 20 with flap setting 4...


Bye
Andreas

Andreas Maurer wrote in message
Flexible wings do NOT change their AoA while they are bending -
otherwise flutter would start immediately.


Bye
Andreas

Hi Andreas,

Wing flex must not result in twisting and wing flex does not change
the angle of incidence of any part of the wing. I don’t think
this means that wing flex does not change angle of attack.

Assume a glider is static on the ground and has the tail raised so
that the mean chord is horizontal. Now flex the wings upwards and
release them. The wings move downward through the air. The relative
air motion is at 90 deg to the mean cord so the angle of attack at the
tips is approximately 90 degrees while the wings unflex.

Now assume a flight condition that resulted from a high g pull up that
approached stall speed. I’ll assume the speed is 40kts, that
the wing tips flexed up 6 feet, and that as the pilot pushes forward
to avoid stall the wings return to normal deflection in 1 second. The
wing tip angle of attack change due to the downward motion can be
calculated from the forward speed of 40kts = 67.5 ft per second, and
the downward speed of 6 ft per second. Inverse tan of 6 / 67.5 is 5
so the tip angle of attack was increased by 5 degrees as the wings
unflexed. The effect reduces to zero at the root where there is no
deflection.

If the numbers are valid then it remains to be decided if wing flex
induced angle of attack changes of this magnitude would have an effect
on stall and stall recovery characteristics. I expect that they
would. Others disagree with me.

Note to other posters - I didn't say this *caused* the accident. I
said I believed it was a contributing factor.


Andy

Andy Durbin wrote:

Now assume a flight condition that resulted from a high g pull up that
approached stall speed. I’ll assume the speed is 40kts, that
the wing tips flexed up 6 feet, and that as the pilot pushes forward
to avoid stall the wings return to normal deflection in 1 second. The
wing tip angle of attack change due to the downward motion can be
calculated from the forward speed of 40kts = 67.5 ft per second, and
the downward speed of 6 ft per second. Inverse tan of 6 / 67.5 is 5
so the tip angle of attack was increased by 5 degrees as the wings
unflexed. The effect reduces to zero at the root where there is no
deflection.

I think the problem here might be picking the right numbers: A high G
pull up would no longer be "high G" at 40 knots (near stall speed), as
the G loading would already be reduced to 1 G. At one G, the wings will
not be flexed upwards. So, I think the wings will return to their normal
position during the speed reduction that occurs after the pull-up is
initiated; that is, more slowly than the 1 second used in the calculation.

To get a 2 G load (a guess - I don't know how much it takes to bend the
wings up 6 feet) on the wing that stalls at 40 knots would require 56
knots. Perhaps there would still be some effect, but it would also be
reduced by the increased speed used (56 knots).


If the numbers are valid then it remains to be decided if wing flex
induced angle of attack changes of this magnitude would have an effect
on stall and stall recovery characteristics. I expect that they
would. Others disagree with me.

It might be a difficult effect to determine experimentally: a pilot
would have detect that the tip stalled when he reduced the G loading.
--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

On 14 Jul 2004 15:22:13 -0700, (Andy Durbin)
wrote:

Now assume a flight condition that resulted from a high g pull up that
approached stall speed. I’ll assume the speed is 40kts, that
the wing tips flexed up 6 feet, and that as the pilot pushes forward
to avoid stall the wings return to normal deflection in 1 second. The
wing tip angle of attack change due to the downward motion can be
calculated from the forward speed of 40kts = 67.5 ft per second, and
the downward speed of 6 ft per second. Inverse tan of 6 / 67.5 is 5
so the tip angle of attack was increased by 5 degrees as the wings
unflexed. The effect reduces to zero at the root where there is no
deflection.

Hi Andy,

In my last posting I was thinking about what you described and decided
that you had been talking about wing twist due to wing bending.


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).

But it's an interesting theory anyway.


One more thought: At 40 kts a high g pullup in an ASW-20 is not
possible anymore - 40 kts is close to its stall speed.
Stall speed of a 20 is about 38 kts, so at 40 kts it wil be able to
generate only 1.02 g without stalling, therefore at 40 kts you simply
do not get any extraordinary wing bending.

For a pullup with 2 g you need at least 53 kts in an empty 20, and for
one of 3 g you need 66 kts. And from 66 kts even a 20 will gain
perhaps 100 ft. How high was that pilot when he started to spin? I'm
pretty sure it was a "standard" spin, caused by too little airspeed
and possibly wrong flap setting.


If the pull-up has been executed at higher speed and the pilot is
pushing the nose down to level flight after a straight climb (with 1g
and loss of speed), the only danger is an inverted stall of the wing
if he tries to push too many negative G's at low speed, but this is
independent of wing bending.



The effect that you describe is imho what causes the smooth ride in a
20: If the wing tips are accelerated upwards, they reduce their AoA,
dampening their movement, and vice versa.





Bye
Andreas

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

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.

Andreas Maurer wrote in message . ..
On 13 Jul 2004 14:33:53 -0700, (Andy Durbin)
wrote:

Just catching up with this thread and no-one seems to have mentioned
the effect of the flexible wings. I don't have experience in the 20
but I do have a series of photos of a fatal accident that started
with a contest finish pull-up and quickly ended up in a spinning
impact with the ground. I believe that the increased angle attack
caused by the wings returning to normal deflection contributed to the
accident.

Flexible wings do NOT change their AoA while they are bending -
otherwise flutter would start immediately.


Bye
Andreas

(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

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