Why does the nose pitch down during a stall?

[Jay_1984]

Try this for size...

 

Found it while studying CPL Aero and helped. Combine this with the point that at the stall the wing is now producing less lift and unable to maintain straight and level

 

https://www.youtube.com/watch?v=P_0R0eWOKns

 

 

[facthunter]

That could have been presented more clearly in 1/3rd the time. Why do they show the "weight" acting through a point well ahead of the LE of the wing ?

 

OK .. the up or down loads from the horizontal stabiliser add or subtract from the amount of lift required in stabilised unaccelerated flight.

 

Noseheavy requires the most down force so the highest stall speeds happen then, as the wing must provide lift for the weight of the plane PLUS the downforce of the elevators. IF you are very noseheavy you may run out of "back"stick, (up)elevator if you get slow on landing. The plane may be "impossible" to stall and is very pitch stable.

 

At the other end of the scale the tail heavy plane is kept level inflight by a lifting force provided by the horizontal stabiliser assy.. This lifting force reduces the amount of lift required from the wings, and for small values there is an improvement in efficiency, overall, but if the tailplane is lifting a large amount it won't be as efficient as the wing is at producing lift, nor will the plane be safe to fly. At very low speeds the tailplane may stall before the wing does and this is about the worse thing that can happen to a plane. Usually the only way that happens is for a load to shift in flight or a gross error in loading on the ground.

 

The big consideration is not movement of the centre of lift on the wing. The distance to the horizontal "control" surfaces is so much larger that the main control and stability effects are caused by the tail feathers. If there's a large moment arm and plenty of area your Cof G position is more flexible.(less critical).

 

Anyhow don't fly a plane where the CofG is outside the designers allowable range. EVER. Nev

 

 

[djpacro]

At very low speeds the tailplane may stall before the wing does and this is about the worse thing that can happen to a plane. Usually the only way that happens is for a load to shift in flight or a gross error in loading on the ground.

.... or tailplane icing ... or a "moose stall" .....After one accident report I tried to replicate the scenario - yep, an aeroplane with a very benign stall in all situations suddenly pitched extreme nose down. Added a warning in the flight manual about combinations of flap, power and airspeed to avoid, especially in a turn.

 

Anyhow don't fly a plane where the CofG is outside the designers allowable range. EVER. Nev

Excellent advice, but I have had to do it (and I am sure that others here have had to do it also).Whenever I do any analysis of flying qualities I put the axes through the CG, not only by convention but it just makes the equations and arithmetic more simple. Velocities and accelerations etc are therefore defined wrt those axes through the CG. Back seat of a Pitts S-2 is way behind the CG so quite a different feeling in snap rolls compared to a Pitts S-1. Aeroplane going up but, initially, the rear seat is going down.

 

 

[diesel]

Elevators can and do stall. Several aircraft have had their flap movement reduced because of this.

 

A stall proof aircraft can be made by stopping the pole comming right back. Ercoupe etc.

 

If the elevator falls off you will slam onto your back knocking you out before the not nice decent.

 

At high aa the weight moves forward on the aerofoil while the lift thats left moves aft.

 

There is your pitching moment.

 

If the usually undersized elevator stalls first it will happen quicker and be much more exciting

 

 

[facthunter]

Rudders also stall on multi engine planes, at times. Horizontal stab stalls mean no control of mainplane' s AoA. Nev

 

 

[DWF]

.... At high aa the weight moves forward on the aerofoil ....

How does the "weight move forward on the aerofoil" by changing the AoA?

 

 

The Cessna Cardinals have slots in the all-flying tailplane to prevent the it stalling, particularly on round-out for landing.

 

 

[Roundsounds]

How does the "weight move forward on the aerofoil" by changing the AoA? 

 

The Cessna Cardinals have slots in the all-flying tailplane to prevent the it stalling, particularly on round-out for landing.

When they were first released to the market they didn't! The reverse slots were then retrofitted and all subsequent production aircraft built with them.

 

 

[djpacro]

Whenever I do any analysis of flying qualities I put the axes through the CG, not only by convention but it just makes the equations and arithmetic more simple. Velocities and accelerations etc are therefore defined wrt those axes through the CG. Back seat of a Pitts S-2 is way behind the CG so quite a different feeling in snap rolls compared to a Pitts S-1. Aeroplane going up but, initially, the rear seat is going down.

I initially had a comment about different readings on the g meters between the front and back cockpits but edited it out as I wanted to quote a reference.Here's a good one: http://atlantis-press.com/php/download_paper.php?id=2599 A Method for Correcting The Error in Indicated Normal Acceleration Due to G-Sensor Location. SAE PAPER 700222 (Loading conditions measured during aerobatic maneuvers in flight test to determine structural design requirements for aerobatic-type aircraft) has some typical data for Pitts and Decathlon but the document is not available for free online. See extract below.

 

The bottom line is that the G meter must be located very close to the aircraft CG.

 

 

[facthunter]

Yes assuming the movement is about the CofG might be a convenient "wish". There's a difference between a balance point of the mass and having ALL the mass there, especially in a dynamic situation. Having large amounts of mass at the extremes of the fuselage makes a very unpleasant aeroplane to manoeuver. Nev

 

 

[DrZoos]

The simplest answer I can think of to the original question is as the aircraft slows, the Angle of Attack (A of A) increases and the center of pressure (C of P) moves. The aircraft slows and the elevator and stabiliser have changed shape (due to up elevator - back pressure on controls), producing negative lift so that the tail drops and the nose pitches up as aircraft pitches around lateral axis. As the aircraft slows this A of A increases more and more until the stall occurs.

 

While the wing and elevator/stabiliser continue to produce lift this works, the aircraft continues to fly in a nose high attitude. The aircraft continues to seem to fly normal, even though it may actually begin descending due to lift and thrust being less than weight and drag....ie total reaction formula....Some aircraft will descend all the way to the ground without snapping forward. Others will snap on or after stall as described below.

 

Eventually the aircraft exceeds A of A to point where wing rapidly develops turbulent airflow, produces significantly less lift and significantly more drag at the same time.....total reaction, significantly changes for the worst. Thrust and weight stay the same, but lift significantly drops at the same time as drag significantly increases.

 

The aircraft which was being held in an artificially nose high position from pilot input, reacts, with both a drop due to aircraft now in descent and a nose drop as aircraft rotates around lateral axis .... this combination of a loss of lift at the front of the aircraft, and a rotation is what gives the snap some describe at stall or shortly after.... for the most part its the rotation that the pilot notices most and describes as the snap. Its possibly the only time a beginning or non aerobatic pilot will experience a sudden rotation around the lateral axis other than a landing as they stall it on.

 

It is, just like when you hold a kids swing up and let it go, it drops around the axis suddenly, when the force holding it up is released suddenly.

 

The rotation can be exaggerated if the pilot simultaneously releases the back pressure and the "rear wing " (elevator and stabiliser) goes from producing negative lift to neutral, or positive for a moment due to its A of A.

 

Far from a perfect answer, but good enough to help, in some basic language i hope...