But if an airfoil has two states, stalled or flying, how can you land
without a stall?



"Todd Pattist" wrote in message
...
Peter's comments on this are right on.

I'll add a bit to the difference between a landing and a
"level" stall in flight. As you may recall, the total drag
on an aircraft is the sum of induced drag and parasitic
drag. The induced drag is high at low speeds and parasitic
is high at high speeds.

Anyway, as you slow in your attempt to produce a level
stall, induced drag rises very quickly (by a factor
proportional to one over the airspeed squared.) This rapid
drag rise causes a descent that quickly increases the angle
of attack of the wing to above the critical angle and thus
you quickly get to the stall and beyond it., producing a
loss of lift and the continuation of the descent.

During landing, the increased induced drag tries to cause
the same descent you experienced aloft, but fortunately your
wheels are there to catch you, and you never get the rapid
AOA increase that you got aloft, so you never get a true
stall.

"Andrew Sarangan" wrote in message
. com...
You guys are challenging my understanding of landings :-)

Challenge is good for the soul.

The landing technique, as taught by many before us, is to
progressively increase elevator deflection to maintain zero vertical
speed.

That's the ideal. In practice, it's nearly impossible to obtain exactly
zero vertical speed, and it's bad form for your vertical speed to go
positive (i.e. start to climb). In a properly executed landing, vertical
speed is always negative (i.e. a descent), and one typically reaches the
runway before reaching the critical AOA.

I suppose it is possible that you can reach max elevator
without reaching critical AOA.

Certainly once the main gear is on the ground, it is. I commonly
continue
to increase elevator back pressure after touchdown, so as to allow the
nosewheel to touchdown gently, and may well reach max elevator travel
before
allowing the nosewheel to touch. But this is a red herring in any case,
as
there is no requirement nor even a recommendation to reach max elevator
travel during a landing.

But I think that is unlikely, because
that would mean you will never be able to perform power-off stalls in
level unaccelerated flight.

At least one plane does have this characteristic (Ercoupe). Landings in
that airplane, one in which it is impossible to stall (in level
unaccelerated flight, anyway, such as one would experience during a
landing), are pretty much just like landings in any other airplane.

A typical "normal" landing involves flying a slightly fast approach speed
(1.2 to 1.3 Vs0), and then flaring and touching down while still above
Vs0.
One hopes that during the flare, airspeed is reduced to as close to Vs0
as
possible so as to minimize touchdown speed. In optimal conditions, a
well-executed landing will even be done with the stall warning going off.
But touchdown itself should still occur prior to the stall occurring
(which,
of course, prevents the stall from occurring at all).

Bottom line: just as George said, "no normal landing involves a stall".

Pete


"It is possible to fly without motors, but not without knowledge and
skill."
Wilbur Wright