Plane cannot be stalled

[photofly]

I don't know who's arguing what, any more, but for taking off at a high angle of attack: you're in ground effect which reduces the induced drag. In a stall it's the induced drag that goes up. So it's feasible that you can take off with an AoA that's officially stalled and not suffer the same drag/performance penalty that you'd have with the same AoA away from the ground.

Also ground effect might change the critical angle of attack. I can't find a reference for that, but my intuition says that flow separation would be delayed by the ground effect, so the critical angle of attack could be considerably higher, so 16 degrees might not even be stalled in that situation. I guess you'd have to lower the nose shortly after leaving the ground to get any climb performance though.

[Doc]

Okay, hate to be just learning to fly and read all this! So what do you call it when you're maintaining a nose up attitude, cruise/max power, and you're descending? I always thought you were stalled here.

[photofly]

You almost certainly are stalled in that situation (like the guy that asked the question in the first place).

[System Message]

The angle of decent and therefore the relative airflow across the wing would have to be greater than the critical angle for the airfoil.

[photofly]

If you're descending (and moving forward) then the angle of attack is higher than if you were flying level with the same deck angle: 1000 fpm down at 70 kts adds 8 degrees to your alpha over straight and level with the same attitude. And the 1000fpm descent with full power is a good indication that something is creating a LOT of drag.

(oops, got my math wrong the first time)

[Frank Uckër]

1000 fpm down at 70 kts adds 8 degrees to your alpha

Assuming you did your trig right, you add that to the 8 degrees AOA that a taildragger has in the three point attitude, and there's your "full stall" three point landing (or thereabouts - 16 deg AOA).

Most people don't touch down at 1000 fpm! That's a REALLY hard landing. Given how fussy everyone is about their hemorrhoid-sensitive gentle touchdowns, there is NO WAY they are doing a full-stall landing after flaring to level before touchdown.

A carrier landing is only 600 to 700 fpm. And you expect me to believe that everyone here is routinely touching down harder than a carrier landing?! WTF?! If anyone here actually ever did a "full stall" three-point landing, they would run crying to their AME for a CAR 625 App G abnormal occurrence inspection.

[trey kule]

So, if I understand what most of that which has been posted, it goes something like this.

Control column full aft, aircraft in a descent. Keep all the control imputs as they are while calcualating alpha angles, considering center of pressure movements, airflow theory, and of course the effect that will occur when the aircraft when the plane enters ground effect, as well as transmitting last goodbyes to all . Edited...to also consider the relative comparisons to a carrier landing.....a most necessary consideration in the proper PDM process.

Or....put the nose down, get the plane flying and stop losing sltitude, and , if necessary, get speed at least to Vx and start to climg.

Which do you think is a better course of action?

It boggles my mind sometimes.

[Frank Uckër]

Results are in. I measured the wing angle in the three-point attitude of NINE different types of parked certified tailwheel aircraft. In all cases they were parked on level concrete (check). All tires were nominally inflated (check - no unslightly bulges). Make of tailwheel? Get a life.

Rise in 48 inches - Degrees in three-point attitude
6.75 - 8.0
8.25 - 9.7
8.25 - 9.7
8.50 - 10.0
8.75 - 10.3
9.25 - 10.9
10.125 - 11.9
10.750 - 12.6

Lowest was 8.0 degrees, highest was 12.6 degrees. All of them a long way from a stalling AOA in ground effect after a flare to level.

To get ANY of the above to perform a real "full stall" landing, you would need a carrier-style high descent, very hard landing with no flare, likely damaging the aircraft.

[photofly]

I don't think anyone's saying that you have to calculate angles of attack etc. while flying. Some of us (possible just me) like to understand what's going on in and around the airplane from a more theoretical point of view, it helps cement the more practical aspects of (in this case) how to recognise a stall and what to do about it.

If you measure wing angles, you need to consider the difference between two definitions of angle of attack - the angle the chord makes with the horizontal, and the angle the chord makes with the "zero-lift" angle (zero alpha = zero coefficient of lift.) Most airfoils stall about 15 degrees relative to the zero-lift angle, which is a few degrees different to 15 degrees of chord angle. To put it another way - because the airfoil is symmetric, when the chord is horizontal you already have a positive angle of attack of a few degrees.

As for full stall landings - I'm pretty sure at the point of touch down, without using the flaps, the nose of my 182 is 15-16 degrees up in the air, if I'm concentrating on it, so it doesn't seem entirely implausible that I'm landing with the wing just about stalled.

[Frank Uckër]

how to recognise a stall

Airplane goes down when yoke pulled back

what to do about it

Yoke goes forward.


Colgan Air and Air France pilots have difficulty with the above.

[Beefitarian]

Colgan Air and Air France pilots have difficulty with the above.

Maybe they were told that stalls are a different thing from when the wing stops creating enough lift to hold the plane up and they wanted to investigate some theories.

I don't think anyone's saying that you have to calculate angles of attack etc. while flying. Some of us (possible just me) like to understand what's going on in and around the airplane from a more theoretical point of view, it helps cement the more practical aspects of (in this case) how to recognise a stall and what to do about it.

I'm interested in learning about what happens when the plane flys, and how things transition when it's wings stop flying.

[Frank Uckër]

As for full stall landings - I'm pretty sure at the point of touch down, without using the flaps, the nose of my 182 is 15-16 degrees up in the air

Nowhere near that. I just measured a C172. 20.5 inches vertically to the center of the main gear mount point. 173 inches from the longitudinal center of the main gear mount point to the tail tiedown ring.

When a tail strike occurs, 20.5 inches is the opposite, and 173 inches is the hypoteneuse.

The arcsin of 20.5/173 is 6.8 degrees at the time of tailstrike. You claim to have your nose in the air 10 degrees higher than a tailstrike, which is nonsense.

You have never performed a full-stall landing, and you'd better hope you never do, because the no-flare rate-of-descent required to generate the additional 10 degrees AOA will result in a brutally hard landing.

[Beefitarian]

So now we have guys with tape mesures and levels but no one has an answer for this question so I can quit calling it a stall.

When you slow down to a high taxi speed in a nose dragger, the wings are still producing lift. Maybe not enough to lift the aircraft, but they are still producing lift. If you raise the nose gear off the ground (like soft field) they will produce more lift, reducing the weight on the tires.

This will happen until a certain speed that is too low. What's it call when that happens?

[Frank Uckër]

I don't think anyone's saying that you have to calculate angles of attack etc. while flying

No, but while you're flying you had darned well better be able to understand and precisely control the aircraft's angle of attack so that you don't exceed Clmax. A stall at low altitude (eg 50 feet) will likely destroy your aircraft, and potentially kill you.

This summer I watched as a homebuilt aircraft lost power, and the pilot tried to stretch the glide. He stalled, and destroyed the aircraft. I can't believe he lived through it, but he did.

He did a full-stall landing.

[Beefitarian]

Here's a serious potential problem here.

A pilot doesn't think his nose is high because he's thinking of where it is relative to level ground but due to decent, it's beyond the angle of attack relative to the imaginary lines representing the actual direction of airflow. The wing is both stalled and going too slow to create enough lift. Here comes the ground. You might have a hard time convincing yourself to point the nose at it, even at 1000 feet above it.

Regardless of what that extra 300 did powering up at the airshow. Full C-172 power isn't going to fix it.

[iflyforpie]

Results are in. I measured the wing angle in the three-point attitude of NINE different types of parked certified tailwheel aircraft. In all cases they were parked on level concrete (check). All tires were nominally inflated (check - no unslightly bulges).

But you didn't measure the chord line. I take it you've never used rigging boards before, or know why you would need to use them?

See a discrepancy in this picture of a typical rag wing airfoil between where you measured and the actual chord line?



What good is your 'fancy' mathematics if you can't even obtain the raw data correctly?

Lowest was 8.0 degrees, highest was 12.6 degrees. All of them a long way from a stalling AOA in ground effect after a flare to level.

If you add the 2.5 degrees you missed between the chord line and the flat bottom, the last one is right in the typical stalling AOA regime. I am not aware of ground effect changing angle of attack. It would make it very difficult to land a plane if all of a sudden there was a vertical component to the airflow any time you came near the ground.

Get a life.

Classy.

[Frank Uckër]

I am not aware of ground effect changing angle of attack

I don't find it surprising that you are unfamiliar with ground effect.

http://en.wikipedia.org/wiki/Ground_effect_(aircraft)

Flying close to a surface increases air pressure on the lower wing surface, (the ram or cushion effect) improving the aircraft lift to drag ratio. As the wing gets lower the ground effect becomes more pronounced. While in the ground effect, the wing will require a lower angle of attack to produce the same amount of lift. If the angle of attack and velocity remain constant, an increase in the lift coefficient will result,[5] accounting for the "floating" effect.

So any pilot that actually flares to land a tailwheel aircraft - instead of smashing it on - will have a lower angle of attack to generate the same lift as he would have at altitude. He won't be stalled.

[photofly]

Beefitarian says

This will happen until a certain speed that is too low. What's it call when that happens?

I think I see what you're saying.

Here's my description of landing, give or take. You fly straight and level just above the runway, at 1.3VS0 or 1.3VS1. To do so requires you to have say 4 degrees alpha. To maintain altitude as you slow down you must raise the nose to increase the angle of attack (and so increase the coefficient of lift). When you reach an angle of attack that you're happy with you stop raising the nose. The decreasing speed is no longer compensated for by an increase in CL - lift decreases. You descend and touch down.

The speed at which you touch down depends on the angle of attack you reach when you stop raising the nose.

What is the slowest speed at which you could possibly touch down? The one where you continue to raise the nose until the wing achieves maximum coefficient of lift - which is (by definition) the critical angle of attack. That is, just on the edge of the stalled regime. Landing at the edge of the stalled regime gives you the slowest touchdown speed.

At the critical angle of attack, the graph of coefficient of lift is "flat", because it has reached a maximum. Anywhere (say) between 14 and 16 degrees gives you very little change in lift. What that feels like is that pulling back on the yoke doesn't make the plane rise any more, which is why it "feels" like a stall. In fact you're exactly on the edge of the stalled regime, where the lift the wing generates remains constant - but it hasn't suddenly decreased.

I think that was the link between landing and stalling that you were looking for.

Frank says:

You have never performed a full-stall landing, and you'd better hope you never do, because the no-flare rate-of-descent required to generate the additional 10 degrees AOA will result in a brutally hard landing.

I trust your calculations; however I usually land with 20 or 40 degrees of flap; that drops the trailing edge and increases the angle of attack by (I guess) 5-10 degrees or more. That means I can reach the critical angle of attack in level flight without a tail-strike. I'm pretty sure I do land close to, or at the critical angle of attack because the stall warning has gone off some time earlier.

Can I do a full stall landing without flaps? I don't do enough landings without flaps to remember if the stall horn goes off or not. Your contention that it would result in a tail-strike seems plausible though.

[Frank Uckër]

Can I do a full stall landing without flaps?

Sure, but I don't think you'd enjoy it. You would need to have a massive rate of descent at touchdown with no flare to generate the additional AOA to exceed Clmax.

You would likely also have a tailstrike. Remember that the effect of flaps during landing is to lower the nose. This is because the flaps pump up the Cl curve of the wing, allowing you to produce the same lift at the same airspeed with less AOA.

[KK7]

It's painful to read this thread.

I think a thorough review of the definition of "stall" is in order for most. For a simple well laid out explanation, check Wikipedia:
http://en.wikipedia.org/wiki/Stall_(flight)

This may also be useful for many,
http://en.wikipedia.org/wiki/Lift_(force)

Angle of Attack indicators on more aircraft might be a good idea.

[Frank Uckër]

It's painful to read this thread

and scary, to think that so many licensed pilots understand so little about Theory of Flight.

[iflyforpie]

I am not aware of ground effect changing angle of attack

I don't find it surprising that you are unfamiliar with ground effect.

http://en.wikipedia.org/wiki/Ground_effect_(aircraft)

Flying close to a surface increases air pressure on the lower wing surface, (the ram or cushion effect) improving the aircraft lift to drag ratio. As the wing gets lower the ground effect becomes more pronounced. While in the ground effect, the wing will require a lower angle of attack to produce the same amount of lift. If the angle of attack and velocity remain constant, an increase in the lift coefficient will result,[5] accounting for the "floating" effect.

So any pilot that actually flares to land a tailwheel aircraft - instead of smashing it on - will have a lower angle of attack to generate the same lift as he would have at altitude. He won't be stalled.

Frank, you forgot this....

If the angle of attack and velocity remain constant, an increase in the lift coefficient will result,[5] accounting for the "floating" effect.

Our angle of attack and velocity aren't remaining constant, they are increasing and decreasing respectively. The result is our stall speed will be lower in ground effect but at the exact same angle of attack.


Really, you have no understanding of how a stall works. You say that you would need a carrier landing rate of descent to produce a stall when landing. Do we need that same carrier rate of descent to produce a power off stall when we are flying? No, of course not. We just reduce power and hold altitude with attitude until we can't do it any more.

Why would it be different for landing? Say you had a 20,000 foot long runway and you flew six inches above it. Reduce power to idle and simply prevent the aircraft from landing. If you've got a plane with a lot of elevator authority, the only way it is going to land is if it stalls, or if you like many other pilots simply 'give up' and hold attitude or check forward.

Landing in a full stall is pretty nasty from five feet up, but hopefully you don't land like that. Even landing before the stall but on the back side of the power curve without checking it with power or attitude is going to be nasty from five feet up. Yes, because of ground effect it will be slower, but that is what we want.

But you don't need a 20,000 foot runway. We are only supposed to be 1.3 Vso on final, that speed goes out the window (or should) as we cross the fence and reduce power to idle. It doesn't take long to bleed off to the stall.

Like I said before, you can't (or shouldn't) land all aircraft in a full stall. But if it has a shallow curve on the top of the Cl chart, and if it has the elevator authority to do it, there is no problem with it.

[Chuck Ellsworth]

This is one of the most disturbing discussions regarding flying I have read here in a long time, this argument has become circular with no real agreement as to the actual behaviour of a wing in the stall regime.

So can you explain this bit Frank?

So any pilot that actually flares to land a tailwheel aircraft - instead of smashing it on -

You can land a tailwheel airplane without flaring first?

[costermonger]

You have never performed a full-stall landing, and you'd better hope you never do, because the no-flare rate-of-descent required to generate the additional 10 degrees AOA will result in a brutally hard landing.

You would need to have a massive rate of descent at touchdown with no flare to generate the additional AOA to exceed Clmax.

Or, you know, you could flare.

[trampbike]

Okay, hate to be just learning to fly and read all this! So what do you call it when you're maintaining a nose up attitude, cruise/max power, and you're descending? I always thought you were stalled here.

I call it not having enough lift to oppose the weight, and not enough power to counter the drag. There is a very very very high probability that you are stalled at this point, but it is not certain (certainly there exists some low-powered aircrafts without too much elevator authority and very efficient wings at high AoA that would allow you to fly it in such a way without stalling it).

Control column full aft, aircraft in a descent. Keep all the control imputs as they are while calcualating alpha angles, considering center of pressure movements, airflow theory, and of course the effect that will occur when the aircraft when the plane enters ground effect, as well as transmitting last goodbyes to all . Edited...to also consider the relative comparisons to a carrier landing.....a most necessary consideration in the proper PDM process.

Or....put the nose down, get the plane flying and stop losing sltitude, and , if necessary, get speed at least to Vx and start to climg.

Which do you think is a better course of action?

I hear you about the practical aspect of it: of course all the maths are useless and the best course of action is always to reduce the AoA ASAP. I totally agree. Still, I think it's dangerous that people believe that a wing is stalled when the aircraft is parked in the hangar, or rolling down the runway, or that as soon as there is a nose up attitude and a descent associated to it it means you are stalled. Believing any of the three things I just named means one does not understand at all what a stall is.