View Full Version : Static margin %

December 6th, 2008, 17:17
Ok, what is it and how do you figure it out?:banghead::banghead::banghead::banghead:

And adjust idle rpm for cfs3????

December 7th, 2008, 07:20
Static margin is a measure of the static stability of an aircraft. Positive values mean the static stability is positive; and conversely negative values indicate negative stability.

When the angle of attack is increased, an airplane with positive static stability will tend to return to the original angle of attack. For example: from straight and level flight, pull back on the stick and the nose starts rotating up, increasing the angle of attack. But as soon as the stick pressure is released, the nose stops rotating, and the angle of attack returns to zero. In the end, the only thing changed is the attitude.

In an unstable plane, i.e. one with negative static stability, the rotation doesn't stop when stick pressure is released. The angle of attack will continue to increase until the stick is pushed forward past the center position. This doesn't mean an 'unstable' plane won't fly, it just means the pilot has to work constantly to maintain control.

Adjust the idle RPM by trial and error until you get the value you actually want.

December 7th, 2008, 09:53
So, it is like caster in a car. the more positive caster you have the faster and more predictable is the return to straight after releasing the wheel. Got it!

Shane Olguin
December 8th, 2008, 18:18
Sounds a lot like the cm_adot stability derivative to me. :-)

The larger this value, the more the tendency of the aircraft to remain at its current pitch. This value can be very useful if you want to simulate fly-by-wire in FS9.

December 8th, 2008, 19:14
Sounds a lot like the cm_adot stability derivative to me. :-)

No, cm_adot is pitch damping due to pitch acceleration, which tends to smooth out jerky pitch motions. (think of it as a shock absorber coefficient)

With respect to stablity derivatives, static margin is the slope of the CM-AoA curve (dCM/dAoA). Increasing the static margin increases the strength of the spring that returns the pitch to the neutral position.

One other flight characteristic controlled by static margin: Aircraft with higher values of static margin need more elevator trim adjustments with airspeed than aircraft with lower values of static margin.

Shane Olguin
December 8th, 2008, 19:32
Oh, I see, you're talking about table 473. I guess I was confused about what you were referring to. I get a real nice break from a stall by inserting a "bump" at the radian value for the corresponding AoA° that I want using that table.