Why does the nose pitch down during a stall?

[Roundsounds]

I'm finding some of these answers interesting and amusing!

 

Let's think about what's happening in level flight - lift supports weight (refer to the diagrams you've displayed). If we now progressively reduce airspeed by reducing power we must increase the angle of attack to maintain lift in order to maintain height. As the critical angle (stall angle) is exceeded lift decreases, this results in a change in flight path, as weight now exceeds lift. The change in relative airflow over the tail plane results in a significant reduction in the downforce produced by the tailplane , which was holding the nose of the aeroplane up, therefore the nose will pitch down.

 

This is way easier to describe with the use of a whiteboard and diagrams! This description only addresses the classic stall entry taught by most flying instructors - ie entry from level flight.

 

 

[Romeo Juliet Whiskey]

That's a really good explanation and makes sense.... A diagram would have helped though

 

 

[facthunter]

The classic stall entry and recovery has little relation to what may occur in a real life situation, so has very limited value in the knowledge base of a good pilot. The inadvertent stall will most likely happen in a turn, though you can flick them in a low pass and pull up. Nev

 

 

[Roundsounds]

The classic stall entry and recovery has little relation to what may occur in a real life situation, so has very limited value in the knowledge base of a good pilot. The inadvertent stall will most likely happen in a turn, though you can flick them in a low pass and pull up. Nev

Totally agree, I teach initial slow flight/stall recovery in the context of getting too slow on a turn into final and a bounce / balloon recovery.

After solo circuit consolidation do further stalls, UA recovery, incipient spin (or developed spinning if aircraft type allows).

 

I'm sure current instructors are wary of stalling and don't teach recognition / recovery correctly.

 

I taught my son to fly last year and recently he completed a check out on a new type. The instructor wouldn't let him stall the aircraft, as soon as the stall warning sounded the instructor insisted on him recovering.

 

 

[facthunter]

I've found most U/L pilots I have been involved with are $#1t scared of flying slow or stalling. While that may be considered prudent to be wary It indicates a less that convincing result of the training effort. You can't be watching an ASI and working out bank related stall speeds all the time. That's OK for a BAK exam, but not up there. You should be able to take the plane to the stall in a steep turn with no reference to the ASI and move away from it , same as if you are in a dogfight. Instinctively. Stalling on turn to final is still killing people some of whom have a lot of hours. Hours alone don't make a good pilot. TRAINING does. Nev

 

 

[Garfly]

Obviously a good question RJW.

 

I must admit that my own answer would have been close to the 'obvious' one you first mentioned, i.e, that "there is a loss of lift and the heavy nose of the aircraft makes the whole plane pitch down." And, truth be told, I can't say I'm persuaded otherwise yet.

 

If I imagine a (normally loaded) aircraft suspended at its CG on a very high crane boom and then being dropped from a great height, I imagine it's always going to pitch immediately nose down and - given a bit of height - try to fly. That is, to 'plane'.

 

And if I bring to mind aerobatic displays I've seen, including the occasional tail slide, it always seems to me that the stalled aircraft - like a ridden horse - is always hankering to head for home - nose first.

 

I'd just assume that 'when all (or most) lift goes, down goes the nose' is a basic mantra for all 3-axis aeroplane designers.

 

Is my understanding too simple?

 

 

[Roundsounds]

I'd just assume that 'when all lift goes, down goes the nose' is a basic mantra for all 3-axis aeroplane designers.

Is my understanding too simple?

A common misconception and reflection of very poor training in a such a critical manoeuvre.

You said "When all lift goes", this is not so, when a wing stalls lift decreases and drag increases.

 

 

[Garfly]

Yes, thanks Roundsounds. I edited my post to 'most lift goes' before I saw your reply.

 

(And I certainly wouldn't want to blame my many long-suffering instructors over the years for my poor grasp of aerodynamics. ;-)

 

 

[mnewbery]

It (main wing stalls first) is designed that way (canards too but the torques are the same, to the same effect and the canard stalls first)

 

 

 

It is possible to design an aircraft where the tail stalls first. As noted elsewhere, insurance companies do not like that feature.

 

 

 

If there is ice on the elevator, it can stall first. Look up ice contaminated tail stalls (ICTS).

 

 

 

If the aircraft goes too fast (we are talking jet transports), something called mach tuck can occur and the front wing can mask the tailplane control. The mechanism is a shock wave that extends from the main wing back to the tail plane. This is particularly nasty news if not addressed immediately. Not particularly important for a plank wing at Mach 0.1

 

 

[Romeo Juliet Whiskey]

Obviously a good question RJW.If I imagine a (normally loaded) aircraft suspended at its CG on a very high crane boom and then being dropped from a great height, I imagine it's always going to pitch immediately nose down and - given a bit of height - try to fly. That is, to 'plane'..

You are correctGarfly but its highly simplified. In a straight and level stall the plane is still generating substantial lift (albeit less) and large amounts of drag. Both these factors actually try to pitch the nose of the high-wing plane upwards. Why doesn't it? As a couple of others have pointed out, the centre of pressure shifts aft causing a pitch down moment. In addition, the change in relative airflow due to loss of lift, means the airflow strikes underneath the tailplane at a higher angle adding to the lowering of the nose. At least that's my understanding from reading all these comments:)

 

I'll be stoked if this question is asked in the BAK exam Im doing next week... but I doubt it.

 

 

[Garfly]

Yes, it's a useful exercise to think these things through and I do take the points raised.

 

I understand that at the point of stall, lift/drag forces are still very much in play (on main and tail) but I'm left wondering, at what point does the simple law of the lever take over the show - for most intents and purposes.

 

Any way good luck with your BAK. (Shouldn't you be sitting the ATPL? ;-)

 

 

[Garfly]

This recent article on tailplane stalls is not all that relevant here ... but somewhat:

 

Aircraft Tailplane Stalls - Aviation Safety Article

 

 

 

[flying dog]

If the wing is delivering lift in the "UP" sense(normal to the fore and aft axis) when it stalls, it will not provide sufficient force to support the aircrafts weight as it can't being stalled, with the angle of attack causing turbulent flow and more drag and less lift. The up position of the elevators is not infinite and doesn't cancel out the positive force acting to pitch the nose down. The relative airflow is from a new position, but the back stick may limit the nose down pitch and the aircraft remain stalled, or oscillate in and out of it, as it sinks down. If you build a model and just drop it even with the elevators well up it will initially fall nose down or it's not balanced correctly. Nev

I know I failed English and gramma school but honestly?

 

when it stalls, it will not provide ..... as it can't being stalled, with the angle of attack.....

 

I think there are too many commas in there.

 

I am still trying to make sense of it.

 

 

[flying dog]

On another note:

 

A long time ago, when I was learning, my CFI mentioned an old video "How an aeroplane flies". A video made by Shell "back in the day".

 

It took me MONTHS of searching, but you can get the DVD - but you have to get it from Shell in England - and it costs. ($20 nom)

 

I'm a tenacious dog.

 

I can try to dig out the details, but if you contact Shell in England I am sure you will get there.

 

 

[Garfly]

Is this the one?

 

 

 

[flying dog]

It looks like SOME of it.

 

The DVD is not just a 15 minute thing. It is about 1 hour long.

 

There are 3/4 parts to it. I haven't watched it for years, sorry.

 

I shall try to remember to dig it out (dig - dog... Ha!) and get more details.

 

Update:

 

The DVD is 54:20 long.

 

It starts with a Gull flying through the air with a voice over talking about it.

 

I shall look on youtube too, JUST IN CASE!

 

 

[facthunter]

Fd You can do without the comma I will remove it. Nev

 

 

[Jabiru7252]

Draw the vectors, always helps.Here is the best ever mspaint drawing you will ever see.

 

 

Gravity is pulling down, but the lift generated by the tailplane is pulling up. The aircraft will rotate around its lateral (pitch) axis and hence the nose will drop.

What version of Autocad did you use here? :-)

 

 

[nedpatto]

Flying a high-wing plane like the Jabiru, why does the nose pitch down at the point of stall? The obvious answer is there is a loss of lift and the heavy nose of the aircraft makes the whole plane pitch down. But aerodynamically it doesn't make sense to me. Let me explain:Given the lift moment arm is behind the CoG, any loss of lift due to a stall would cause a pitch up tendency wouldn't it? In fact, in a high-wing aircraft the increase in drag at the stall would also create a pitch up tendency. I presume all other forces (thrust, weight and tailplane) acting around the CoG remain the same at the point of stall (the tailplane is designed not to stall so the elevators remain effective when the wing stalls).

 

 

Hopefully someone can point out where I'm going wrong here.

 

Rich

Rich, Consider where your control column is at the point of stall, and from there, where is the tailplane? Now, with the tailplane fully up, where is the chord line for the tailplane? Now consider the relative airflow over the tailplane which, remember, has a downforce holding the nose up.

 

At the point of stall, the aircraft starts to descend, an immediate change in the relative airflow occurs over the tailplane, resulting in a sudden loss of tail down force. Thus, the nose pitches down. The couple of the forces merely ensures the nose down pitch with the lift behind the weight.

 

An important point to remember is that incorrect loading of the aircraft causing a rearward shift in the weight can result in a severe stall that may not be recoverable.

 

I hope this helps with your thinking. Kevin Patterson.

 

 

[Romeo Juliet Whiskey]

Thanks Kevin...makes a lot of sense.

 

 

[Parkway]

All complicated diagrams and technical words aside... If you suspended an airplane up in the air... Say hanging from a hot air balloon... Not moving at all... Then you drop it... What happens? It would fall straight down but nose first right? Cos it's got a big heavy engine at the front... Someone turn that into something intelligent for me ;)

 

 

[Subsonic]

With the entire aircraft descending vertically, the Centre of Gravity is ahead of the centre of lift of the entire body. Simples.

 

 

[P4D]

Your aeroplane pitches about the Lateral Axis not the center of gravity. See if you can work it out from there.

 

 

[fly_tornado]

Brian Carpenter is doing a series on VGs and their effects on stalls

 

 

 

[Romeo Juliet Whiskey]

All complicated diagrams and technical words aside... If you suspended an airplane up in the air... Say hanging from a hot air balloon... Not moving at all... Then you drop it... What happens? It would fall straight down but nose first right? Cos it's got a big heavy engine at the front... Someone turn that into something intelligent for me ;)

Your right in this scenario but the original question was what happens at the point of stall. At this point the aircraft could be flying at 55kts and not descending at all. The tailplane which is preventing the heavy nose from dropping is still unstalled and functioning by design. So the actual scenario is quite different from the example you give. Still lots of forces acting on the variuos surfaces of the aeroplane.