Why does the nose pitch down during a stall?

[Romeo Juliet Whiskey]

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

 

 

[facthunter]

The centre of lift is not always the same point but more importantly the mainplanes stall and the tailplane doesn't. With most of the lift gone the plane descends abruptly and the "Not stalled" tailplane weathercocks (pitches) the nose downwards aided by the length of the fuselage to provide the necessary "Moment" (turning effect) The extra drag slows the plane and makes the stall more sustained in the early stage until the airspeed builds up because of the "new" pitch attitude. Nev

 

 

[Litespeed]

Well said .

 

Essentially the tail is still creating lift and the wing is not. Wing tips down as tail continues to lift. Like a seesaw.

 

Which is just as it should be. If the tail failed first to provide lift then recovery could be a huge problem and you could plummet tail first to the ground.

 

 

[Romeo Juliet Whiskey]

With most of the lift gone the plane descends abruptly and the "Not stalled" tailplane weathercocks (pitches) the nose downwards aided by the length of the fuselage to provide the necessary "Moment" (turning effect)

That kind of makes sense but is that valid at the point of stall - the plane is not necessarily descending yet you still get a pitch down tendency. What happens in a climbing stall? The plane could still be climbing due to inertia but still produce a nose down tendency when stalling?

 

 

[Romeo Juliet Whiskey]

Well said .Essentially the tail is still creating lift and the wing is not. Wing tips down as tail continues to lift. Like a seesaw.

 

Which is just as it should be. If the tail failed first to provide lift then recovery could be a huge problem and you could plummet tail first to the ground.

Isn't the tail with the elevator up (stick back) at the point of stall creating a nose up tendency. If the tail is still creating lift (or downward force) why would this cause the nose to drop? Its the one thing that is preventing the nose from dropping. Doesn't make sense.

 

 

[bexrbetter]

Isn't the tail with the elevator up (stick back) at the point of stall creating a nose up tendency. If the tail is still creating lift (or downward force) why would this cause the nose to drop? Its the one thing that is preventing the nose from dropping. Doesn't make sense.

You actually answered you own question in the first post and now here again.

 

The tail is still lifting, everything else is falling, it not only makes sense, it's the only conclusion.

 

 

[M61A1]

If your CoG is correct it works like a big weathercock. If you CoG is too far aft, it doesn't and you cant recover.

 

 

[Romeo Juliet Whiskey]

You actually answered you own question in the first post and now here again.The tail is still lifting, everything else is falling, it not only makes sense, it's the only conclusion.

Yes but doesn't the tailplane create negative lift?. At the point of stall this tailplane with the elevator up is trying its hardest to pitch the nose up. Thus, if the tail is lifting (in the negative sense) and elevator up and everything is falling then the nose should pitch up. Check out the diagram below at look at the moment arms acting around the centre of gravity.

 

 

 

[Litespeed]

It matters not if climbing or level it is the difference in lift between the tail and wing is what does it. The attitude of the aircraft is to a degree irrelevant- as long as the tailplane is still generating a greater amount of lift than the mainplane (wing) which is naturally not providing any lift at all in the stall.

 

The ability of the tailplane to overcome the lift of the tail and provide a pitch down movement by applying the elevator full stick back when in a stall

 

a) is overcome by the complete loss of lift from the wing

 

b)and the fact the angle of attack of the tailplane obscures the elevator when in a nose high attitude. The greater the angle of attack the greater the loss of effectiveness of the elevator.

 

That is the way I think about it- but could be wrong

 

 

[facthunter]

The stick will determine the angle of attack, so it will be back at that stage. 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

 

 

[Pearo]

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.

 

 

[ben87r]

The centre of pressure moves rearward after the critical AoA and after there is LESS lift, not no lift, this moves it further aft of the CoG which increases the moment between the two, aiding recovery. Very important with this to understand that it's RELATIVE airflow as well because you can stall inverted in a loop for example and the "nose down" actually becomes a reduction of elevator input or more correctly a reduction of the AoA.

 

There are quite a few different forces going on but as you would have seen the result of the AC wants to correct itself, and is the case in most examples with a few exceptions such as spiral dive.

 

 

[Romeo Juliet Whiskey]

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.

Hey Pearo, love your drawing! You have my talent there :) The tailplane should be generating a downward force not upwards though. See post #8.

 

 

[Cosmick]

If you were able to stop the Aircraft in mid air then let it fall vertically then it would behave along the lines your thinking but at stall the Aircraft still has airflow over the surfaces and the wings stall (lose lift first) causing the weight at the front of the aircraft to become unsupported. (Me Thinks)

 

 

[Romeo Juliet Whiskey]

The centre of pressure moves rearward after the critical AoA and after there is LESS lift, not no lift, this moves it further aft of the CoG which increases the moment between the two, aiding recovery. Very important with this to understand that it's RELATIVE airflow as well because you can stall inverted in a loop for example and the "nose down" actually becomes a reduction of elevator input or more correctly a reduction of the AoA.There are quite a few different forces going on but as you would have seen the result of the AC wants to correct itself, and is the case in most examples with a few exceptions such as spiral dive.

Ahhh... that makes sense. So the wing is still producing lift after the stall which should create a nose up moment. But because the lift moves further aft it creates a bigger moment force (even though there is less lift) than just before the stall and thus causes the nose to drop. Thanks :)

 

 

[facthunter]

Not that I like the method used for demonstrating this but do the standard procedure. Close the throttle and gently ease back with enough elevator to stop the plane losing height. (hold your level not your attitude). At some point you will have full back stick and the nose will drop away near that point. If there is not enough elevator it won't fully stall and will just mush down. This can also happen if you are very nose heavy. Normally the nose will just fall away, sometimes abruptly and other times gently (different aircraft). One wing may go first if you are not fully balanced with the rudder, or the plane is rigged wrong, but that leads into another story. Nev

 

 

[bexrbetter]

Yes but doesn't the tailplane create negative lift?. If the tail is lifting (in the negative sense) and everything is falling then the nose should pitch up.

Ok, you need to know the truth; The Earth is flat, there, I've said it, at great risk to myself.

 

The tail actually pushes down to counter the center of weight that is slightly forward of the center of lift (the main lifting wings).

 

This makes all (standardised) planes fly flat.

 

If the Earth was curved then you wouldn't need the tail to "push down" as you would want the plane to fly in a curved path to suit the "globe"s curvature.

 

If I don't post tomorrow, say goodbye to my kids for me.

 

 

[DWF]

4.9 Pitching moment

 

When using the FoilSim aerofoil flight test simulation program, the static pressures around the aerofoil are given in the output plot that shows the pressure distribution pattern changing with the aoa. It is convenient to sum that distribution and represent it as one lift force vector acting from the centre of pressure [cp] of the aerofoil or wing for each aoa; much the same way as we sum the distribution of aircraft mass and represent it as one force acting through the centre of gravity. The plot on the left is a representation of the changing wing centre of pressure position with aoa. The cp position is measured as the distance from the leading edge expressed as a percentage of the chord. (Please note the diagram is not a representation of the pitching moment.)

 

At small aoa (high cruise speed) the cp is located around 50% chord. As aoa increases (speed decreases) cp moves forward reaching its furthest forward position around 30% chord at 10–12° aoa, which is usually around the aoa for Vx, the best angle of climb speed. With further aoa increases, the cp now moves rearward; the rate of movement accelerates as the stalling aoa, about 16°, is passed. Most normal flight operations are conducted at angles between 3° and 12°, thus the cp is normally positioned between 30% and 40% of chord.

 

The movement of the cp of the lift force changes the pitching moment of the wing, a rotational force applied about some reference point — the leading or trailing edges for example — which, in isolation, would result in a rotation about the aircraft's lateral axis. The consequence of the rotation is a further change in aoa and cp movement that, depending on the cp starting position may increase or decrease the rotation. Thus a wing by itself is inherently unstable and will change the aircraft's attitude in pitch — i.e. the aircraft's nose will rotate up or down about its lateral axis, which may be reinforced or countered by the action of the lift/weight couple — so there must be a reacting moment/balancing force built into the system provided by the horizontal stabiliser and its adjustable control surfaces. This will be discussed further in the Stability and Control modules.

 

The above is from the Tutorials section in this web site. (Part 3 - Airfoils and Wings)

 

It is a bit technical but shows that approaching the critical AoA the centre of pressure is a fair way forward (about 40% of chord) and moves rapidly rearwards at and beyond the stall AoA thus tending to pitch the nose down.

 

 

 

DWF

 

 

[Romeo Juliet Whiskey]

4.9 Pitching momentWhen using the FoilSim aerofoil flight test simulation program, the static pressures around the aerofoil are given in the output plot that shows the pressure distribution pattern changing with the aoa. It is convenient to sum that distribution and represent it as one lift force vector acting from the centre of pressure [cp] of the aerofoil or wing for each aoa; much the same way as we sum the distribution of aircraft mass and represent it as one force acting through the centre of gravity. The plot on the left is a representation of the changing wing centre of pressure position with aoa. The cp position is measured as the distance from the leading edge expressed as a percentage of the chord. (Please note the diagram is not a representation of the pitching moment.)

At small aoa (high cruise speed) the cp is located around 50% chord. As aoa increases (speed decreases) cp moves forward reaching its furthest forward position around 30% chord at 10–12° aoa, which is usually around the aoa for Vx, the best angle of climb speed. With further aoa increases, the cp now moves rearward; the rate of movement accelerates as the stalling aoa, about 16°, is passed. Most normal flight operations are conducted at angles between 3° and 12°, thus the cp is normally positioned between 30% and 40% of chord.

 

The movement of the cp of the lift force changes the pitching moment of the wing, a rotational force applied about some reference point — the leading or trailing edges for example — which, in isolation, would result in a rotation about the aircraft's lateral axis. The consequence of the rotation is a further change in aoa and cp movement that, depending on the cp starting position may increase or decrease the rotation. Thus a wing by itself is inherently unstable and will change the aircraft's attitude in pitch — i.e. the aircraft's nose will rotate up or down about its lateral axis, which may be reinforced or countered by the action of the lift/weight couple — so there must be a reacting moment/balancing force built into the system provided by the horizontal stabiliser and its adjustable control surfaces. This will be discussed further in the Stability and Control modules.

 

The above is from the Tutorials section in this web site. (Part 3 - Airfoils and Wings)

 

It is a bit technical but shows that approaching the critical AoA the centre of pressure is a fair way forward (about 40% of chord) and moves rapidly rearwards at and beyond the stall AoA thus tending to pitch the nose down.

 

 

 

DWF

That's really interesting DWF. I wonder if there is a similar graph that shows the lift vs AoA, especially after the critical angle is exceeded

 

 

[facthunter]

There's lift coefficient graphs for most common airfoils available. Nev

 

 

[Romeo Juliet Whiskey]

Yeah thanks Nev. Just Google them. Wow, didn't realise the main wing was still producing significant lift after it stalls. You kind of assume when the wing stalls it looses most of its lift.

 

 

[bexrbetter]

Wow, didn't realise the main wing was still producing significant lift after it stalls.

They vary depending on shape and loading, some lose lift gradually and some are sudden.

 

 

[facthunter]

It feels that way but the drag increasing as the lift drops off is accentuating it. You can fly the plane beyond the 14-16 degrees AoA but it's not fun nor very safe. The info printed there is of significance to aircraft designers more than pilots. for wing internal bracing (twist) and external strut design. Note you trim back when slowing up but FORWARD at high speed. "Stability and control" is a whole of aeroplane consideration. The tail doesn't always provide down force although that situation is the most stable pitch wise. Nev

 

 

[djpacro]

Suggest reading the relevant chapters of Book #1 at FLY BETTER......

 

or page 2 of the smaller document: Spin notes at http://www.flightlab.net/Flightlab.net/Download_Course_Notes.html

 

A common explanation is simply that the wing centre of pressure moves aft at the stall - seems to be based on the excellent book Aerodynamics for Naval Aviators - see that bit about "... as stall occurs, the drop in suction near the leading edge cause the c.p. to move aft." but that's not true of all aerofoils e.g. NACA 63-415. In any case, the data presented is from two-dimensional wind tunnel tests etc so doesn't really address the real situation of a three-dimensional wing, especially with flow separation originating from the inboard root.

 

 

[Romeo Juliet Whiskey]

Thanks DJP, I've read all the fly better books except the first one so I'll have get onto that one. I love how the author thinks outside the box with his teachings. Highly recommended!