On Sat, 17 Jul 2004 17:13:42 -0700, Eric Greenwell
wrote:

Martin Gregorie wrote:
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.

This is the usual result of a stall, and is what occurs in the typical
training situation, but it isn't the definition of a stall. Generally, a
stall begins when the airflow starts to separate from the wing at
increasing AOA. It is this separation that keeps the lift from
increasing and sets the maximum lift coefficient.

Usually major airflow separation coincides with a stall and the drag
increase ensures that a stall will happen because of the associated
loss of airspeed. However, flow separation is not the same as a stall.
Many aircraft have quite a high degree of flow separation during low
speed flight. In the model world we assume separation always occurs at
about 60% chord at min.sink and this would appear to be close to the
mark for sailplanes judging by Will Schumann's experiments.

A wing can be stalled and still produce plenty of lift; for example, in
a high speed pull up done with too much elevator can stall the wing, but
the stalled wing will still have more lift than the weight of the
aircraft because of the high speed.

I would normally call that a high drag flight regime rather than a
stall.

In a high speed climb after rapid pull up, pushing the stick enough to
give zero G will reduce the lift to zero, but the wing is not stalled
(the airflow is well attached - no separation) even though it can not
support the glider.

Sure - and I don't think a wing can be stalled in a zero-G situation,
e.g. a model flown in ISS. It probably can't be stalled in vertical
flight either. In both cases the generated lift is necessarily zero
and so is the opposing load on the wing.


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.

Perhaps I don't understand this correctly: by load, do you mean
different G loads, or just different aircraft weights? By *stalling
behavior*, do you mean how rapidly the aircraft responds as it stalls,
the amount of buffeting, how the nose is, or...?

By 'load' I mean the instantaneous load applied parallel to the wing's
lift vector. It will be the vector sum of the weight and acceleration
at that instant.

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