ASW 20 SPIN CHARACTERISTICS

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 :