View Full Version : Pitching Moment

23rd May 2002, 21:46
Can someone tell me if a wing sections pitching moment is proportional to speed or is roughly constant with speed?

I'm trying to understand if a marginally stable aircraft can be pushed into pitch instability by a modest increase in speed.

24th May 2002, 10:26
Speed is of no interest for aerodynamics, only dynamic pressure is. This depends on air density (altitude, pressure, temperature) and speed of couse. This means high sped in high altitude might be equal to low speed at low altitude.

What you are probably thinking about is that for most wing sections (airfoils) the pitching moment coefficient is constant over a wide AOA-range. While lifting force coefficient is nearly linearly increasing with AOA in the same range, this still means different center of lift and therefor a total aircraft moment change.
(got it ? The airfoil moment is equal to the moment produced by the distance between center of gravity and center of lift, therefor changing lift means changing this second moment and therefor changing total moment away from the equilibrum)
Therefor any airfoil with a negative moment coeficient is instable althouh the moment coeficient is constant.

So the most interesting parameter for stability is the angle of attack (AOA), you always must have a negative slope of the total aircraft moment versus AOA curve. The AOA change is always depending on a dynamic pressure change for unacelerated flight because lift and weight must be equal. Changing speed alone always results in changing AOA and therefor changing moment. If the plane is instable, this results in a moment changing AOA in the same direction as the speed change did leading to the end of the flight.

24th May 2002, 19:04
Thanks for the reply. Perhaps I should explain my interest...

In WW1 Aero magazine a writer is proposing a theory that early/vintage aircraft had a 'diving tendency' that resulted in many crashes. I believe he blames several accidents on the then practice of diving with power on during approach.

I'm reasonably familiar with early aircraft and some did indeed have...

a) lots of camber
b) A rearward CoG
c) small tailplanes

A combination that doesn't exactly make for excess stability.

Assuming these aircraft were just on the right side of neutrally stable... could an increase in speed cause them to become unstable by increasing the pitching moment so much that the small tail could no longer cope?

I've seen one reference to pitching moment being proportional to the square of speed. However I have an airfoil analysis program (Javafoil from Martin Hepperle) and I don't see anything like this magnitude of change I increase the Reynolds number.

Perhaps many of these aircraft just needed the pilot to have a heavy lunch for them to be unstable and didn't need to be dived to push them over the edge.

Later edit: Now that I read your post again I think what you are saying is that the pitching moment is proportional to lift which in turn is proportional to the square of speed.

27th May 2002, 08:21
Now your idea is much clearer to me.

The problem for these planes is the very large (negative) momentum coefficient of the high cambered airfoils combined with the torsional soft wing structure with very thin airfoils and soft wire bracing design.
This is a classical problem of static aeroelasticity. Increasing speed (or increasing dynamic pressure) the wing twists nose down at the wing tip producing an overall decrease in angle of incidence difference between wing and stabilizer. This is equal to a nose down elevator deflection and causes speed to increase even more.
So pulling out of a dive just lifts the fuselage nose but not the wingtip, making it imposible to end the dive.
About 10 Jears ago a test pilot was killed while doing high speed testing a composite microlight plane with the wing skin fibres orientated 0/90 (for cheaper production) which resulted in a far to low torsional stiffness of the wing. He was able to tell what was wrong with the plane on the radio until the very last moment, he had fully nose up deflected elevator but the plane dives steeper and steeper while the torsion of the wing became spectacular visible.
Might have been similar in the early WWI planes.

27th May 2002, 12:00
Thanks again for the reply - I really should have thought of this
explanation myself - I once made my own wings (for RC aircraft) and put the weave at 45 degrees for this very reason!