Sample Chapter from:

 

"HOMEBUILT AERODYNAMICS AND FLIGHT TESTING"

This is an example flight test, without the flight test card.

 

 

21. Static Directional Stability

 

Directional stability means an airplane will turn its nose into the wind rather than the tail after a small disturbance around the yaw axis. We have to assume that your airplane is directionally stable, which should be the case if it has a vertical surface aft of the CG. The static directional stability test will show you how quickly the airplane returns to straight flight after a disturbance resulting in a yaw angle. The main airplane component responsible for directional stability is the vertical tail. The larger it is or the further from the center of gravity it is, the more stable will the airplane be. Countering the action of the vertical tail is any fuselage or other surface in front of the CG. I once tested two gliders (ASW 22 and ASH 25), which have the same wing and tail. The only difference is the fuselage, one is a single-seater, the other one is a two-seater. Naturally the two-seater has a longer fuselage to accommodate two people in tandem. Most of that extra fuselage is in front of the wing. When comparing the directional stability of both planes, I found that the single-seater was more directionally stable.

 

For this test trim the airplane in the desired configuration and speed. Deflect the rudder fully to one side and hold the wings level with the ailerons.  Release the rudder and hold the ailerons neutral while you observe what is happening. The airplane should immediately swing back to straight flight, but a number of other things can happen. If the motion is not well damped, the nose can overshoot and the plane yaws into the opposite direction, before straightening out. The plane could pitch up or down, and loose altitude. It may even refuse to straighten out and continue flying at a yaw angle, in which case it does not have sufficient directional stability. Note your observations together with the largest yaw angles. Try this test in clean configuration, at different speeds and with flaps & gear down. Slow speed with flaps is likely to be the worst case. The desired result is to have the airplane return to straight flight quickly without oscillations.

 

Configuration Sheet Static Directional Stability

 

Airplane weight: no requirement

Center of Gravity: aft CG is critical

Weather: calm air desirable, some light turbulence acceptable

Altitude: no requirement

Airspeeds: VS to VA

Configuration:  clean, flaps & gear up, flaps & gear down

Power setting: as required for level flight at selected speed

Equipment: j-y-q-sheet

 

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