Newbie Qs on stalls and spins

Andrew Sarangan wrote:

You guys are challenging my understanding of landings :-)

The landing technique, as taught by many before us, is to
progressively increase elevator deflection to maintain zero vertical
speed. I suppose it is possible that you can reach max elevator
without reaching critical AOA.

Ah, yeah. See Cessna 177A. They had to add slots to the stab so the
tail wouldn't stall before the wing.

"Morgans" wrote in message
...

"Cub Driver" wrote

I was really astonished, the first time I landed as a passenger in a
small plane, to be told by the pilot that the horn that blared just
before touchdown was a stall warning. I assumed the pilot had made a
mistake (in his landing technique, not in his explanation for the
horn)!

all the best -- Dan Ford
email: (put Cubdriver in subject line)

But remember, stall horns are usually 8 mph before stall.
--
Jim in NC

Careful, Jim. Depends on your flight regime. The stall horn is actually
set a couple of DEGREES above stall angle of attack. In the landing
configuration that may equate to about 8 mph, but that's not how it's set.

Shawn
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"Andrew Sarangan" wrote in message
om...
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

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

Cub Driver wrote in message . ..
On 17 Nov 2004 12:12:55 -0800, (John Galban)
wrote:

(2) Spins and other flight maneuvers required by the regulations for
any certificate or rating when given by?

(i) A certificated flight instructor; or

So ... the instructor who spun the Cub with me in the back was in
violation of the FARs? The maneuver was required for my certificate!


I don't see how. What I posted above is an exception to the
parachute requirement. If he was a CFI, I don't see how he could have
been in violation of the regs, regardless of whether or not you were
wearing chutes.

John Galban=====N4BQ (PA28-180)

"ShawnD2112" wrote
Careful, Jim. Depends on your flight regime. The stall horn is actually
set a couple of DEGREES above stall angle of attack. In the landing
configuration that may equate to about 8 mph, but that's not how it's set.

Shawn

True, but you can NEVER make a statement that does not have any exceptions.
:-)
--
Jim in NC


---
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No, not a trick question...

You CAN'T take off without a stall, if an airfoil only has two states:
flying or stalled.

If the airfoil is flying you cannot take off, and if it's not flying it's
stalled.

(There's a whole bunch of physics involved here that I don't yet know, so
anyone, please feel free to correct whatever I get wrong.)

You stated: "It's flying as soon as you start moving on the runway". That is
not correct. It doesn't begin to fly until you develop enough relative wind
to create enough lift to overcome drag. If an airplane is only moving at 1
kt. down a runway, it is probably not flying.

Forward motion of the aircraft is not required. Given a strong enough
headwind, an airplane will readily fly backward; just ask some J3 drivers.

And an aircraft will not land until it has reached a "stalled" state.




"Todd Pattist" wrote in message
news
"Bill Denton" wrote:

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

Is this a trick question? For the same reason you can take
off without a stall. It's flying as soon as you start
moving on the runway and it's flying while you're in the air
and it does the same thing in reverse on landing.
"It is possible to fly without motors, but not without knowledge and
skill."
Wilbur Wright

"Morgans" wrote
But remember, stall horns are usually 8 mph before stall.

Section 23.207: Stall warning.

(c) During the stall tests required by §23.201(b) and
§23.203(a)(1), the stall warning must begin at a speed
exceeding the stalling speed by a margin of not less
than 5 knots and must continue until the stall occurs.

"Bill Denton" wrote in message
...
You CAN'T take off without a stall, if an airfoil only has two states:
flying or stalled.

First of all, there is a continuous regime of "flight" between stalled and
not stalled. It's not binary. But secondly, even if you assume the airfoil
has just the two states, the rest of your conclusion regarding that is
incorrect...

If the airfoil is flying you cannot take off, and if it's not flying it's
stalled.

"If the airfoil is flying you cannot take off". Care to rephrase that? At
best, I can assume you meant to write "if the airfoil is not flying you
cannot take off". Which would be true (inasumuch as I might assume what you
mean by "flying"), but not particularly germane. Your second clause, "if
it's not flying it's stalled" seems to get to the heart of your
misunderstanding however.

"Flying" is not a technical aerodynamic term, and in particular you cannot
say that "flying" is the opposite of "stalled". The opposite of "stalled"
is "not stalled".

As has already been pointed out, "stall" simply means that the airfoil's
angle of attack is greater than the critical angle of attack. An airfoil
that has no relative wind has NO angle of attack, and the term "stall" is
meaningless in that context. Once the airfoil has relative wind (e.g. you
start your takeoff roll), you can then look at the angle of attack and
compare it to the critical angle of attack. Looking at the example of a
takeoff roll, the wing's angle of attack remains below (and generally, WELL
below) the critical angle of attack at all times.

No stall at any point in time during the takeoff roll.

Same thing applies to most landings. As the airplane slows after touching
down, the amount of lift being generated is reduced, but this is compensated
for by the wheels providing the balance of the required support. At no
point does the wing wind up with a higher angle of attack than the critical
angle of attack, and thus there is no stall.

(There's a whole bunch of physics involved here that I don't yet know, so
anyone, please feel free to correct whatever I get wrong.)

We're trying.

You stated: "It's flying as soon as you start moving on the runway". That
is
not correct.

It IS correct. Well, inasmuch as you've failed to define "flying" for us,
and inasmuch as "flying" has no predefined aerodynamic definition. The
instant there is ANY relative wind, the wing is creating lift (since its
angle of attack is below the critical AOA). That's my definition of
"flying": "creating lift". What's your definition?

It doesn't begin to fly until you develop enough relative wind
to create enough lift to overcome drag.

Lift overcomes gravity. Thrust overcomes drag. In order to lift off from
the ground, you do need enough relative wind to allow the wing to generate
enough lift to overcome the force of gravity. But if by "flying" you simply
mean "to have lifted off from the ground", then it's especially true that
"flying" is in no way the opposite of "stalled".

If an airplane is only moving at 1
kt. down a runway, it is probably not flying.

Again, you'll have to define "flying". But the wing certainly is developing
lift, and certainly is NOT stalled.

Forward motion of the aircraft is not required. Given a strong enough
headwind, an airplane will readily fly backward; just ask some J3 drivers.

Forward motion through the air mass IS required. Given a strong enough
headwind, an airplane may well depart from the ground, but as soon as it's
no longer tied to the ground, it will slow relative to the airmass and fall
back to the ground. Probably in a stalled state, even.

And an aircraft will not land until it has reached a "stalled" state.

Simply untrue. Virtually all of my landings involve touching down and
coming to a stop without ever exceeding the wing's critical AOA. I hesitate
to claim that I've *never* stalled the wing during a landing, but I sure
don't do it intentionally.

Pete

Todd Pattist wrote in
news
"Bill Denton" wrote:

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

Is this a trick question? For the same reason you can take
off without a stall. It's flying as soon as you start
moving on the runway and it's flying while you're in the air
and it does the same thing in reverse on landing.
"It is possible to fly without motors, but not without knowledge and
skill." Wilbur Wright


That is the best explanation I have seen. Thanks!