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'[OT]: Re: [PIC]: Autopilot landing system for mode'
2002\06\27@215543 by Tony Nixon

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Brendan Moran wrote:
>
> > As my areobatics instructor stresses, pull the stick back to position
> > 'X' at any speed and the wings will stall, so you may be better
> > monitoring the back pressure on the "stick" to detect when a stall is
> > imminent.
>
> I think that there may be two separate discussions in progress here.  When
> you mention stalling, Tony, you are speaking specifically of a wing stall,
> rather than not having lift at all.  They are two different things.

I understand that.

zero speed  = zero lift, no stall
some speed + zero attack = zero lift, no stall
any speed + overly high attack (>16') = zero lift, stall
some speed + good attack = no stall, lift (up)
speed + straight up + good attack = no stall, lift (horizontal)
speed + straight down + good attack = no stall, lift (horizontal)
speed + upside down + good attack = no stall, lift (down)

Just because a wing is not producing lift doesn't mean it is stalled.
When the wing loses the 'abillity' to produce lift, then it is stalled.

Wings will generate lift in any forward aircraft direction with a good
angle of attack, because all they act on is the parcel of air that
surrounds them at the time. In a straight up climp, forward stick
pressure may be needed to stop the plane looping over backwards. ie.
less lift force needed from the wing, because of engine thrust, so you
have to counteract it somehow.

> I know of one Russian prop stunt plane that had so much prop force that it
> could literally hover using its prop.  Not to mention that many fighter jets
> will not stall going straight up unless the throttle is low (for going
> straight up, less than 90% counts as low) but by that point, the wings are
> no longer causing any lift, and therefore are stalled, while the aircraft
> clearly continues to have lift, and is therefore not stalled.

I've seen Pip Borman do that at Tyabb airfield. Very very impressive,
but the planes weight is being conteracted by the propellor/jet thrust.
The wings are not stalled because they are in a static position and
obviously producing no lift. Can you consider a wing stalled while just
sitting on the tarmac.

My instructor has actually seen a plane lift off the ground purely from
wind gusts. Scared the bejeezes out of the pilot.

> The other guy (sorry forget the name and it's waaayyyy back in my PIC box)
> who was talking about all the different factors was, I think, talking more
> about when the aircraft would cease to gain lift at all.

> What I'm trying to say is that a wing can stall and the aircraft can
> continue on un-stalled (though if the aircraft is stalled, I think you're
> almost guaranteed to have all wings stalled).  It depends on all the factors
> originally mentioned.

Of course an aircraft can continue on if the wings are stalled, but
probably not where you want it to go, and will require a pilot with some
knowledge to recover lest he/she finds themself in an inadvertant spin.
A stalled wing is a departure or loss of control whether intentional or
not.

> If you've built a model jet fighter, and it can climb
> straight up, chances are that if you punch the throttle, you'll never stall
> the little beggar, though the wings may only be serving as fins at that
> point.

It will stop when it reaches full throttle height :-)

As I mentioned, in this case weight is counterbalanced by the engine
thrust. The wings are producing no vertical component of lift at all,
but they could be producing horizontal components of lift and as such
are still subject to stalling.

Interestingly, as you can see, more lift is required by a wing in
straight and level than in any climb.

I could be misunderstanding your reasoning, but you seem to be confusing
aircraft speed with stalling. Speed has nothing to do with stalling, and
I don't think you can say an aircraft has stalled just because the wings
are producing no lift or it has stopped mid air.


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Best regards

Tony

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2002\06\28@041811 by Andrei B.

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>
> zero speed  = zero lift, no stall

> some speed + zero attack = zero lift, no stall
Depends on the airfoil. Some airfoils generate lift even at small
negative attack.

> My instructor has actually seen a plane lift off the ground purely
> from
> wind gusts. Scared the bejeezes out of the pilot.
Should have made it a glider!



=====
ing. Andrei Boros
Centrul pt. Tehnologia Informatiei
Societatea Romana de Radiodifuziune

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2002\06\28@042429 by Jafta

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I have to agree with Tony - a wing stalls only when a specific angle
(14-16deg?) to the RELATIVE airflow is exceeded - whether it is in a
dive, straight-and-level or a climb.  Has nothing to do with speed
etc.  Only lifting devices can stall, and not an aircraft, although it
is spoken of as if the AC stalls.  When the wings stall, the AC can
only go one way - down.

Regards

Chris A

{Original Message removed}

2002\06\28@052859 by Attila Muhi

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When talking about stall, we must remember that not only the wings can stall. The stabilizer can also stall, and because it normally produces negative lift, the result will be stabilizer up - nose down. Nasty !  A common cause of stabilizer stall is ice buildup on the stab. The effects of that ice are often not obvious until you extend flaps or something when you are about to land, with not enough altitude to recover... So don't extend flaps if you have ice on the stabilizer :-)

Deep stall is even more complicated. Often seen on jets with T-tail and all engines aft, like BAC 111, DC-9.  Wings go into stall, and the attack actually blocks the airflow around the stabilizer at the same time. With that engine placing there's no slipstream around the stabilizer either... and thus the elevator doesn't do what it is supposed to. Difficult to recover from. I think they crashed the first BAC111 prototype that way .

Regards

Attila - SM4RAN

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2002\06\28@053805 by Pic Dude

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Yep -- wing stall indicators are little tabs on the leading
edge at a specific angle.  When the angle of attack changes
such that that angle is reached and exceeded, that airflow at
the leading edge pushes the tab upwards, and triggers the
stall warning horn in the cockpit.  These are not speed
dependant (above a certain very-low speed to have enough
force to move the tab).

> "Only lifting devices can stall, and not an aircraft"

Technically, an aircraft (fuselage et al) can be a lifting
device.  Example: many military jets.

Cheers,
-Neil.



{Original Message removed}

2002\06\28@115329 by Brendan Moran

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At 11:50 AM 28/06/2002 +1000, you wrote:

>I could be misunderstanding your reasoning, but you seem to be confusing
>aircraft speed with stalling. Speed has nothing to do with stalling, and
>I don't think you can say an aircraft has stalled just because the wings
>are producing no lift or it has stopped mid air.

The definition of stall: "A condition in which an aircraft or airfoil
experiences an interruption of airflow resulting in loss of lift and a
tendency to drop"

If you ask me, being at a dead standstill seems to indicate an interruption
of airflow, and a tendency to drop.

I think it is actually possible to stall an older aircraft by going too
fast.  The air rides down the front of the wings, so that you lose the
airfoil effect, but you're moving very fast.  Like you said, the angle of
attack has a large effect.

However, an aircraft could be said to have stalled if it has hit a brick
wall and still has wings.

Just my thoughts...

Regards,
Brendan

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2002\06\28@123408 by Sean H. Breheny

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Hi Brendan,

I must preface this by saying that I'm not an aerodynamics expert, only a
model aircraft enthusiast who also does research in controlling small aircraft.

At 08:54 AM 6/28/2002 -0700, you wrote:
>The definition of stall: "A condition in which an aircraft or airfoil
>experiences an interruption of airflow resulting in loss of lift and a
>tendency to drop"
>
>If you ask me, being at a dead standstill seems to indicate an interruption
>of airflow, and a tendency to drop.

There is no such thing as angle of attack when you are at a dead
standstill, so I don't think we can argue one way or another there.
However, I have always heard that stall was a very specific condition which
occurs when the airflow over the wing separates from the surface and
becomes very turbulent. If you look at a graph of wing lift vs. angle of
attack at a constant airspeed, you usually see lift increasing with AOA up
to a point, and then a sharp drop-off in lift. The region beyond this point
is called stall, AFAIK. If you make a graph of lift vs. airspeed at a
constant AOA, you simply see a monotonically increasing roughly quadratic
curve (at least in the subsonic region), there is no precipitous drop at
any airspeed, simply a gentle decrease toward 0 lift at 0 airspeed. There
are many reasons why a wing could be generating inadequate lift to
counteract gravity, but only one specific such cause is called stall.


>I think it is actually possible to stall an older aircraft by going too
>fast.  The air rides down the front of the wings, so that you lose the
>airfoil effect, but you're moving very fast.  Like you said, the angle of
>attack has a large effect.

I don't know about this effect of airflow being deflected at very high
speeds but you may well be correct.

Sean

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'[OT]: Re: [PIC]: Autopilot landing system for mode'
2002\07\01@185234 by Tony Nixon
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Brendan Moran wrote:

> I think it is actually possible to stall an older aircraft by going too
> fast.  The air rides down the front of the wings, so that you lose the
> airfoil effect, but you're moving very fast.  Like you said, the angle of
> attack has a large effect.

Possibly true, I know airflow patterns change at high speed, but know
nothing about the subject. Sounds like Vne is totally exceeded in this
case, which is when you start crossing your chest :-)

On a brighter note, I sat and passed the practical flying test on Sunday
so I'm now fully licensed - well, as soon as CASA gives the final OK on
paper.

What a day. 9AM till 6PM, followed by the aeroclub bar ;-)

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Tony

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2002\07\02@122018 by Brendan Moran

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----- Original Message -----
From: "Tony Nixon" <Tony.NixonspamKILLspamENG.MONASH.EDU.AU>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Monday, July 01, 2002 3:50 PM
Subject: Re: [OT]: Re: [PIC]: Autopilot landing system for model aircraft


> Brendan Moran wrote:
>
> > I think it is actually possible to stall an older aircraft by going too
> > fast.  The air rides down the front of the wings, so that you lose the
> > airfoil effect, but you're moving very fast.  Like you said, the angle
of
> > attack has a large effect.
>
> Possibly true, I know airflow patterns change at high speed, but know
> nothing about the subject. Sounds like Vne is totally exceeded in this
> case, which is when you start crossing your chest :-)

I get the info on that one from a documentary on the invention of the Avro
Arrow.  What they ended up doing was taking the wings, and chopping them,
and offsetting the chopped parts slightly higher so as to create a channel
that forced airflow down itself.

> On a brighter note, I sat and passed the practical flying test on Sunday
> so I'm now fully licensed - well, as soon as CASA gives the final OK on
> paper.

Congrats!

--Brendan

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