Zero Pitching Moment

[Martinuk]

I've read that my Moth glider is a 'Zero Pitching Moment' design. What does this mean?

Thanks,

Martin
UK

[Tom Hunt]

It means that the center of pressure of the airfoil is right over the CG of the model for all angles of attack.

don't believe it... it's like the perpetual motion machine.

You can minimize it, but it will still shift enough to cause a moment which will have to be trimmed elsewhere on the aircraft (elvator or elevons)

Tom Hunt

[Martinuk]

thanks for the reply Tom, but I am fairly ignorant about aerodynamics! What I want to know is the significance of the Moths' zero pitching moment. The implicaction is that this will lead to a lack of stability/increased manouevrability compared to a larger pitching moment plane.

Am I on the right lines?

Martin

[Sparky Paul]

Originally posted by Martinuk
I've read that my Moth glider is a 'Zero Pitching Moment' design. What does this mean?

Thanks,

Martin
UK
.
It may mean nothing more than it uses a symmetrical airfoil, which has no pitching moment.

[Tom Hunt]

Originally posted by Sparky Paul
.
It may mean nothing more than it uses a symmetrical airfoil, which has no pitching moment.

Sparky, Martin

A symmetrical airfoil only has zero pitching moment at 0 lift. The pitching moment (usually resolved in 2D testing at the 1/4 chord) can be significant even on a symmetrical airfoil.

Airfoils that have low Cm/Cl slopes (pitching moment vs Lift) are often used in flying wings to make them more manueverable, as the moment arm from the elevons to the CG is poor. This also causes less trim drag, affording higher L/D ratios. Stability, has very little to do with the airfoil. A poor choice of airfoil usually only turns into a "trim drag problem" or lack of control authority. which should not be confused with static stability margin.

The Clark YH (used on the Hurricane and other WWII aircraft) is a high lift but low pitching moment airfoil (basically a Clark Y with TE reflex). Ever wonder why the Hurricane got away with such a small tail?.... yes, Reynolds number did help, but I'm sure the use of the Clark YH airfoil had a lot to do with it too!

I'm sure there are some good resources out there on the net.... just try some searches on flying wing design.

Regards
Tom

[Sparky Paul]

"center of pressure" is an outmoded concept.
The theory stated that at zero lift, the c.p. would have to be an infinite distance behind the wing.
Obviously, whatever force might have been there, couldn't work -on- the wing.
When measuring the pitching moment in the tunnels, this distance away from the wing would vary, as if something a distance away was pushing the wing via a lever.
Air can't do that!
C.p. itself waa mathematical construct, found by measuring the force on the wing (in the tunne) and dividing that by the lift.
Even at zero lift, it was found that "something" was still acting on the airfoil. Division by zero isn't permitted.
The motion of the c.p. itself depended on the airfoil, some profiles having no c.p. travel, others having it move forward, and others having it move aft..
It was discovered with good wind tunnel data that a symmetrical airfoils "center of pressure" was a constant value for all angles of attack, and had a value of zero.
This discovery led to the more correct theory of the aerodynamic center, which states "The pitching moment for all unstalled symmetrical airfoils is zero when measured at the 25% chord line.
It was further found that for cambered profiles, the pitching moment is essentially constant, and negative (nose down) in direction.
Previously thought to be a function of the c.p. moving aft, it was determined that the camber itself generated the moment.
The point at which the moment operates is close to the 25% chord line in every case.
For the reflexed airfoil, the pitching moment can be positive.. nose up. Flying wings exploit this by placing the c.g. -ahead- of the 25% chord position. The c.g. wants to pull the nose down, the airfoil wants to pull the nose up. A happy medium between reflex and c.g. results in a stable airplane.
MOF, a "normal" airfoil when inverted has a nose-up pitch. Anyone that's seen a model lose its horizontal in flight.. and pitch down until it (missing the ground of course) can be recovered when it stabilizes upside down! The c.g. has moved forward, the pitching moment is still nose down, but the plane is upside down, so the nose goes up!
I watched Walt Good land a plane that did exactly this many years ago. And a time or two since.
*ref: Martin Simons, "Understanding Sailplanes", R/C Soaring Digest, July 1994.. pp 38.45