ASW 20 SPIN CHARACTERISTICS

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 :

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 :

On Fri, 16 Jul 2004 09:22:52 +0200, "Bert Willing"
wrote:

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

I must respectfully disagree - the load being carried by the wing is
at least as important as the AoA.

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

In article ,
Martin Gregorie wrote:

On Fri, 16 Jul 2004 09:22:52 +0200, "Bert Willing"
wrote:

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

I must respectfully disagree - the load being carried by the wing is
at least as important as the AoA.

I'm afraid that turns out not to be the case.

Stalling depends on the AoA, and only the AoA (Reynolds number effects
aside).

The amount of lift generated depends only on the AoA and the airspeed.

The amount of lift necessary to support the aircraft against an
acceleration of 1 gravity depends on the load being carried. For each
load there is a minimum airspeed below which the amount of lift
necessary to support that load against gravity can not be generated.
But if you don't insist on trying to support the load against gravity
(that is, trying to increase the AoA until sufficient lift is generated,
thus stalling the wing) then you can be in perfect control and not
stalled at as low an airspeed as you like.

Which brings us back to: stalling of a wing is connected to the AoA,
nothing else.

-- Bruce

Bruce Hoult wrote:

In article ,
Martin Gregorie wrote:

...the load being carried by the wing is
at least as important as the AoA.

[snippage]

...if you don't insist on trying to support the load...then you can
be in perfect control and not stalled at as low an airspeed as you like.

Bruce, it would appear that you and Martin are in agreement.


Jack

Jack wrote:
Bruce Hoult wrote:

In article ,
Martin Gregorie wrote:


...the load being carried by the wing is

at least as important as the AoA.


[snippage]

...if you don't insist on trying to support the load...then you can

be in perfect control and not stalled at as low an airspeed as you like.

Bruce, it would appear that you and Martin are in agreement.

Appearances can be deceiving...

If you look at the Coefficient of lift diagrams for airfoils, you see
that it is dependent only on AOA, not load. In other words, a wing will
stall at the same AOA at .5 G, 1 G, 2 G, etc. I think this is what Bruce
is saying. Martin is wrong to say the load is as important as AOA, and
that is why some ras posters think we should have AOA indicators in our
gliders.

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

Eric Greenwell
Washington State
USA

On Sat, 17 Jul 2004 10:52:50 -0700, Eric Greenwell
wrote:

Jack wrote:
Bruce Hoult wrote:

In article ,
Martin Gregorie wrote:


...the load being carried by the wing is

at least as important as the AoA.


[snippage]

...if you don't insist on trying to support the load...then you can

be in perfect control and not stalled at as low an airspeed as you like.

Bruce, it would appear that you and Martin are in agreement.

Appearances can be deceiving...

If you look at the Coefficient of lift diagrams for airfoils, you see
that it is dependent only on AOA, not load. In other words, a wing will
stall at the same AOA at .5 G, 1 G, 2 G, etc. I think this is what Bruce
is saying. Martin is wrong to say the load is as important as AOA, and
that is why some ras posters think we should have AOA indicators in our
gliders.

Sure, Cl is dependent entirely on AoA, but is not a linear
relationship throughout the range:

- It is linear at small angles.
- When the AoA is high enough for the upper surface flow
to start to separate the Cl tends to a constant value with
increasing AoA.
- If the AoA continues to increase even further you reach
a point at which the Cl starts to decline, reaching zero
at an AoA of 90 degrees.

However, my understanding is that a stall occurs when the lift
generated by the wing drops below the load the wing is required to
support.

For a given wing the generated lift is proportional to the Cl and to
the square of the speed, so at a fixed AoA you can reduce the speed
until the lift is no longer sufficient for flight, at which point the
wing stalls. If the aircraft weight is reduced then so is the stalling
speed: it doesn't matter whether this reduction is due to dumping
ballast or to pushing over to generate reduced G forces. If you put
water in a glider you raise its stalling speed but you don't
necessarily change the AoA at which it stalls.

Hence my comment that the load on the wing is as important as AoA for
*stalling* behaviour. I was not talking about the aerodynamic
characteristics of the wing section - of course!

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