Is a normal landing a controlled stall?
 

Had an argument with a guy (none aviation) the other week, who said "yeah, landing is just a controlled stall" - I kind of didn't know what to say, other than I disagreed with him, yet I found that I couldn't quite explain it properly. My thoughts are that if you stall on landing, depending on which part, you can go down with a bump, and that you shouldn't exceed the 15 degree critical angle of attack :confused:

I feel that there may be mixed views on this, so I welcome any thoughts :ok:

Maz

Yes, there will be.

Lee, some landings are, some aren't, and there might be no rights & wrongs, too many variables in the landing appropriate to the aircraft type and/or the experience of the driver. As your Diary progresses (with your experience) and also after some experience in different types, you will become edumacated.

But your chap is wrong in generalising so much.

Rob

It's a controlled stall if you do it right - the aim being to stall a fraction after the mains touch the ground.

The word I'd dispute is "just" !

G

My view is that GENERALLY your friend is correct.

In nearly all cases (tricycle or tailwheel) the last second or two of a landing aircraft should involve the aircraft gently stalling onto the ground.

In a tricycle type this involves the mainwheels touching down and the P1 holding off the nose until it drops through lack of elevator influence. In a tailwheel configuration, the aim is to touch down all three wheels at the same time.

In certain conditions a tailwheel pilot will "wheel it on" which means he will be landing main wheel first (not stalled) and will then allow his groundspeed to decay enough for his airspeed to ensure that his tailwheel drops onto the ground. In this scenario he/she is not stalling the aircraft during landing.

On all cross wind "flares" I do believe that the aircraft is still stalled although it may well be with one wing down to counteract the drift effect.

To summarise, I still believeyour friend is correct.

DubTrub has a point however. I am not qualified to go into further detail on the areas I believe he is referring to. For the time being, I suggest you aim to stall your ship onto the hard and not do what too many do these days which is a flat "three wheel" splat that does nothing but keep Mr Front Oleo in the manner in which he ahs become accustomed.

Interesting thread, I'm interested as to how this develops

Landing your average, tricycle GA-aircraft (which is all I have experience from) should always be done stalled. The target will be to stall the aircraft as you touch down. Ideally you should be flying level so close to the runway surface that when you do stall you will touch down very gently, at the lowest possible groundspeed. Such a touchdown is very, very satisfying but happens rarely, at least for me.

You can go down with a bump, of course, but that's quite normal. All it takes is that you round out just a little to high and then when you stall you drop into the runway. That's not a problem. Indeed, you can easily make perfectly smooth landings by touching down with all three wheels at the same time, but that's obviously not a good landing. On the centerline, on the planned touchdown point and fully stalled - that's an excellent landing, whether or not you experience a "bump".

Should be: yes. Is: most certainly not always. Therefore to say a landing is just a controlled stall is incorrect. You can definitely land a plane without stalling it first - you just need the ground between where you are and where you were heading.

I would have said so, certainly I can get Jemima's nose high enough that there is an actual feelable 'break' when she drops that last foot, (OK , usually rather more) onto the runway.

BUT, if you think about it the usual effect is just that of pre-stall sink. Do you get buffeting? stick shake? do you have to use rudder instead of aileron to prevent wing drop? I don't, therefore I deduce that I am not actually stalling, just pronounced sink, which is good enough because clearly there is not enough lift to maintain height so she stays down.

Depends on the type. Also somewhat depends on what you mean by "stall", since a stall isn't really an absolute thing but is actually progressive.

In a tailwheel plane doing a three-point landing, the answer is a pretty definite "yes". Of course in a wheel landing the answer is an equally definite "no" - the plane is very much still flying when the mains touch the runway.

In a high-wing tricycle plane, the answer is a definite "sort of". A nice landing in a Cessna is done with the stall horn sounding for the last second or two. However trying to land in a true stall attitude is a good recipe for a tail strike (guess how I know). And the stall horn sounds quite a bit before the wing actually stalls (obviously, or it wouldn't be of much use as a warning).

My experience of low-wing planes (Cirrus, Cherokee, Mooney) is that you want a flatter attitude than a stall - the plane is still flying at touchdown.

So I think the general answer is "it depends".

n5296s

Not stalled !
 

A landing should not be made in a stalled condition as we all know as an aircraft stalls the C of P moves aft and the nose will drop out of control as all lift is lost. Roll control would be marginal to say the least ( within one or two knots of the stall most light aircraft can be rolled to the right with small amounts of left aileron & vice-versa) so recovering a wing drop would be interesting !.

The airspeed at the touchdown may well be below the normal stall speed but this is due to Ground effect and the aircraft is not stalled.

Once the aircraft is on the ground stalling the wing might be an advantage as the drag rise in a stalled condition will help slow the aircraft and the stalled wing will not be suporting the aircraft putting more weight on the wheels thus increasing braking action. Only aircraft with powerfull pitch control can take advantage of this so you can forget it in the PA28 but the DR400 and the PA38 can use this aerodynamic brake to an advantage on slippery runways.

Without wishing to turn this into a debate, I disagree.

I do not believe that at a couple of feet above the ground with continuous back pressure to bleed speed that it is possible to get a nose drop. The aircraft will sink onto its mains well before the nose drops. Even when the wings have stopped producing enough lift to keep you in the air the elevator will normally be authorative enough to keep the nosewheel out of trouble.

Just my view

I agree most with A & C. All you experience when landing is not enough lift to maintain height. The wing probably isn't at or past critical alpha. In my limited experience the stall warner may be sounding on some a/c in the flare, but the stall had yet to take place.

If the question is interpolated to heavies, then my personal interpretation of the theory is that the wing certainly isn't stalled on touch down!

I do disagree with n5296s a little. My understanding is that a stall is an absolute thing and is related only to the AoA, is measurable and has a defined constant start point for a set configuration; the critical AoA.

If the wing were near the stall with a wing down landing, would there not be a severe risk of a sudden wing drop on the up wing as the down aileron would effectively increase the AoA on that tip.

I tend to agree with Monococks understanding. In the rare event that I do a "greaser" the aircraft touches down with the stall warner buzzing, with a lot of aft elevator input. The elevator on most aircraft will retain authority even if the wing is stalled. Its important on some types not to relax back pressure too quickly as the nose will drop hard onto the ground (Seneca springs to mind) which is due to the aft CoP due to the stalled wing.

Stall recovery is often to "relax back pressure", and in types (and many others) like the PA28's, you can hold the aircraft stalled indefinitely (height permitting) with full back pressure, but keep a fairly level or slightly nose up attitude without it suddenly dropping until you relax back pressure.

Big jets have spoilers with spring up on contact with the ground. These have the same effect as stalling the wing I suppose (destroying lift).........

Just a humble opinion ;)

It's a handy simplification but not really true. There's a range of aoa during which airflow separation is taking place. Separation starts at the trailing edge and as aoa increases it moves forward along the upper surface, reducing lift. In addition the stall does not happen equally along the span of the wing - depending on flaps, washout, dihedral, stall-modifying devices such as fences and vortex generators. Thus there is quite a range of aoa where some of the wing is stalled but not all of it.

That said, from a pilot's perspective the plane stalls when you go over the hump of the C/L curve such that increasing aoa reduces lift rather than increasing it. That's what causes the nose to drop. But as another poster says, you can fly a plane in a fully stalled condition - you will lose altitude but it is completely controllable (sometimes called a "falling leaf"). It's an excellent exercise in control.

I suspect that in a "fully stalled" landing (e.g. in a taildragger) the wing is somewhat stalled but typically there is still some back elevator left - not much, but some.

John

My understanding is that, ideally, you should stall on landing. Especially so when 3-pointing a tail-dragger, but ideally a landing in a tricycle should be no different. (We'll disregard wheel-landings in tail-draggers for now, because they are completely different.)

I base this on the fact that, at the point of landing, I am unable to get sufficient lift from the wings to continue straight and level flight. Even if I move the elevator further back (and yes, in most aircraft I can still move the elevator further back) the amount of lift I get does not increase, and the aircraft settles onto the runway.

However, we all know that when we stall an aircraft, the nose will drop, sometimes sharply, and a wing may drop too. This doesn't usually happen when you land, fortunately. I've never had anyone give me an explaination for this which I like. The best explaination I have is that it's something to do with ground effect - ground effect changes the characteristics of the stall. Whether this is true or not, or how it works, I don't know, unless anyone else can explain it?

FFF
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Not in a helicopter .

A correctly held off landing is a stall on to the ground in most aircraft. You keep pulling back with the mains an inch or so above the runway (ideally!) trying to prevent it from touching down (tailwheeler or trike, though many pilots of the latter don't bother) untill either you reach the back stop, or more usually the wing stops flying and the aeroplane sinks onto the main wheels at the absolute minimum possible speed. That's why the nose dosn't drop as it does in a stall at height - the mains releive the wing of any loading by touching down before things get that far. A (gentle) stall at height results in (initially) a high sink rate, followed by nose and maybe a wing dropping. When we perform a max performance fully held off landing (and except in strong and cross winds you always do don't you???), the touchdown happens during the very early stages of 'sink'.


SSD

How to do it properly........... ;)

My normal landings are uncontrolled stalls........ but another 7,500 hours or so and I'm sure I'll crack it :p

Thanks for all the great responses, I have been reading today and have been waiting for someone to mention "ground effect" - still a phenomenon in my mind, yet I think must have something to do with the landing, and how it is different from a full on stall.

When I practice a stall, I cut the power, hold the nose level, hold it, hold it, hold it, and this usually entails maximum pullback on the controls, just to keep the aircraft level (from which it eventually sinks and pitches forward). What I am trying to say is that, so far, I haven't been able to compare this stalling practice to a good landing, if you know what I mean, i.e I don't have to use full back pressure to stop the aircraft smacking the tarmac, just a significant amount to hold it gently as the aircraft slowly sinks. Having said that, occasionally I do hear the stall warner, especially flying dual due to the added weight.

I am starting to think that, after all, stalling is not an exact science, and can vary in lots of different ways, in different conditions, as already pointed out.

Thanks again (and for the vid BRL),

Maz :ok:

I find it difficult to see how any aircraft I have flown could be stalled during the landing. As several people have pointed out, the stall occurs at the critical angle of attack, and all the tailwheel aircraft I fly sit on the ground at much smaller angle of incidence. Now, angle of attack depends on the direction of the airflow past the wing, but during the landing (i.e. in ground effect) it is difficult to see how this will be significantly different from horizontal unless the aircraft is decending very rapidly, certainly not something it should be doing in the flare.

I believe it was Alan Bramson who measured the angle of incidence (wing) of lots of tailwheel aircraft when they were on the ground, and concluded that the Dragon Rapide was one of the few that could be stalled in the three-point attitude.

I haven't tried pushing the tail of a Cessna 150 down to the floor to see if it could reach the critical angle of attack in an extreme nose-high landing, but I don't teach people to land at such an extreme angle anyway. As far as I can see, when I land an aircraft it is not stalled, and I don't want it to be.

Just think of the angle of the nose of the aircraft to the horizon when you practice a normal (1g) stall. You won't see the nose at anything like that angle to the horizontal when you are landing (I hope).

NB. I know purists will point out that the angle of incidence really refers to the angle of the wing wrt the aircraft, but in the above context I am using it wrt the ground for simplicity.