Post by Bruce HoultPost by Martin GregorieSure, Cl is dependent entirely on AoA, but is not a linear
- 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.
I'm with you on all that.
Post by Martin GregorieHowever, my understanding is that a stall occurs when the lift
generated by the wing drops below the load the wing is required to
support.
No, a stall is when increasing AoA decreases lift. There might well
still be more lift that the weight of the aircraft, especially at high
I wouldn't describe that as stalled. Separated flow, yes, and hence
high drag, but not stalled.
Post by Bruce Hoult- high drag and thus inefficient
of course
Post by Bruce Hoult- the aircraft is unstable in roll, making it difficult or
impossible to control.
That's not due to the stall, but rather to upper surface flow
separation reducing aileron effectiveness.
Post by Bruce HoultPresumably you've seen aircraft such as the F/A-18 demonstrate a slow
pass at very high and stalled angle of attack? They are getting some of
their support from the downward component of the engine thrust, of
course, but with an AoA of, say, around 30 degrees it would need a
thrust:weight ratio of around 2 in order for thrust to be enough to
support the entire aircraft weight. It would also require *huge* drag
in order to avoid accelerating at such a thrust level. The F/A-18 has
nowhere near that amount of thrust, so the majority of the support is
clearly still coming from the stalled wings. In that situation the
aircraft is unstable, and would probably be improssible to fly like that
without the computer reacting very quickly to unwanted rolls.
IIRC an F-18 has unlimited vertical capability when lightly loaded.
Such vertical flight requires no wing lift and a thrust:weight ratio
of >1:1, so it follows that any other flight regime from normal flight
to high-alpha, fully flow-separated attitudes where the wing is
contributing some lift will require less thrust, not more.
Post by Bruce HoultSo the F/A-18 can be happily flown in steady-state stalled straight and
level flight primarily because of the computer control and also because
the extra drag is less than the engine thrust available.
I think the control regime will be decidedly odd - ailerons should be
pretty ineffective, computer of no computer, and that the aircraft may
well be being flown on a combination of rudder and elevator.
The drag in supersonic flight is so high that almost any Mach 2
aircraft has the thrust to do this. The real issue is control and here
the all-flying tail that the Bell X-1 program discovered also happened
to have the control authority needed to hold the high angle.
Post by Bruce HoultPost by Martin GregorieFor 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.
Well, ... no :-)
If you maintain a fixed AoA, and the speed is such that the lift is less
than the weight of the aircraft then the aircraft will start to follow a
downwards parabolic path (not as sharply downwards as in a zero-G
pushover, but similar).
I think you'll find that's pretty much what does happen if you watch a
stall from the outside, follow the path taken by the CG and ignore the
attitude changes. I think our perceptions from the inside are very
much affected by the pitch-down, but that's designed in by picking
suitable incidence difference and airfoil characteristics for the
tail. When the lift no longer balances the load on the wing the
aircraft will accelerate downward: A = F/M always applies and the
aircraft will accelerate until the lift once again matches the load on
the wing. If you continue to hold the stick back in a fully stalled
glider it will again achieve a steady state, but a very inefficient
one with a largely separated airflow on the wing. Centreing the stick
lowers the AoA and hence the Cl, so the aircraft again accelerates
downward until it has the airspeed necessary for normal flight.
Hmm, it looks like a terminology thing to me. I'd still maintain that
the stall break comes when the wing can't support the load imposed on
it. I suspect there's no disagreement here. However, I'd also accept
that most pilots would call the following highly separated, though
stable steady state rapid descent a "stalled wing" despite the fact
that the wing is supporting the aircraft. Regardless of what its
called, you'd best not try to land in that condition!
BTW I've deliberately ignored accelerated stalls and high-speed stalls
in this discussion because they are so far from steady state that I
think the dynamics of the situation obscures what's really happening.
Post by Bruce HoultWhat happens next depends on what else (if anything) you are holding
constant.
Suppose, for the sake of concreteness, that you are initially flying
straight and level at 60 knots and you then fix the AoA such that the
wings are producing only half the lift required to support the glider.
Normally a glider will accelerate, increasing the lift (and drag, but
not by much). The extra lift will cause the path to become less sharply
curved downward and things will come to equilibrium (or oscillate
around) the point where the combined lift and drag are equal and
opposite to gravity. For a typical glider polar curve this will happen
at an airspeed of around 1/sqrt(0.5) times 60 knots, or 85 knots, plus
or minus a little due to drag.
So all you've acheived is to change the trimmed speed from 60 to 85
knots.
Or, look at it the other way around. Maybe you were flying straight and
level at 85 knots, and then you somehow instantly decrease the airspeed
to 60 knots (maybe a gust up the tail). The lift is no longer
sufficient to maintain level flight. But the glider doesn't stall. It
just drops the nose and accelerates until it has returned to the trimmed
speed of 85 knots.
In no way are the wings ever stalled.
If you stipulate constant speed as well as constant AoA (presumably via
some large and adjustable drag, magical or otherwise) then the flight
path will become steeper until the combined lift and drag vectors are
again exactly equal to and opposite the gravity vector. This will
result in a much steeper flight path, but still stable.
Let's suppose again that you are at 60 knots and reduce the AoA to
produce only half the lift required for flight and then continue to
maintain exactly 60 knots somehow. Alternatively, suppose you're flying
trimmed for level flight at 85 knots and then apply airbrakes to reduce
and maintain 60 knots, while keeping the same trim (AoA).
What happens?
The AoA/speed are insufficient for flight at 60 knots and so the nose
drops. If you draw up the force vectors then you will find that the
glider will stablize in a 60 degree descent at your desired constant 60
knots. Lift (from the wings) is still 0.5 of the weight just as it was
initially (but it's in a funny direction, tilted 60 degrees forward from
vertical). Drag (from the airbrakes) is 0.866 of the weight, tilted 30
degrees from vertical. The horizontal components of lift and drag are
equal and opposite and cancel out. The vertical force to oppose gravity
comes 25% from the wings and 75% from the airbrakes.
In no way are the wings ever stalled.
No matter what you do, if you start with the wings not stalled then
there is nothing you can do that will stall them while all the time
keeping the AoA constant.
If you see the nose drop and don't like it and pull back on the stick to
try to prevent it then that is an entirely different matter -- you're
increasing the AoA which certainly *can* stall the wings.
-- Bruce
--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :