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Lateral-Directional and Spiral Stability Tests

By Jack Dueck, Chairman - EAA Canadian Council, EAA 337912

At this stage of the flight-test program, stability tests approach an academic status. However, to continue to learn more about the aircraft and its tendencies in reacting to flight disturbances (either by pilot input or by dynamic airflow disturbances), we will still explore directional stability as well as spiral stability tendencies for this RV-9A.

Jack's plane

Static directional stability is the tendency for the aircraft to return to its flight path after yaw forces have been applied by rudder.

We set up for this test with a forward CG, an altitude of 5,000 feet AGL, and a target airspeed of 120 mph and trimmed the aircraft for straight and level flight. Once achieved, we applied left rudder, holding altitude with elevator and maintaining wings level with ailerons. When the rudder was released, the aircraft quickly returned to straight flight.

We repeated this test with right rudder and again observed the aircraft crisply returning to straight flight.

With the large rudder and vertical stabilizer on this aircraft, rudder authority is substantive, and the aircraft likes to fly straight without rudder input.

Lateral-Directional Stability Test

This test is to determine if the aircraft will lift its lowered wing when the ailerons are released after being placed into a side slip. It will also test the rudder's directional control effectiveness.

Since this test can impose high structural loads on the airframe, the tests will be done at or below maneuvering speed.

We started by establishing the flight parameters: forward CG, 5,000 feet AGL, target airspeed at or below 100 mph, and trimmed for straight and level flight. We placed the aircraft in a side slip (left rudder, right aileron) and held our heading with rudder. (It is important to restrain the angle of bank to 10 degrees or less, or any bank angle that requires full rudder deflection.)

Control forces on the ailerons were light, but rudder control forces became heavier with increase in bank angle. Upon release of the aileron control forces, the wing slowly returned to level flight, while directional control with the rudder remained positive throughout the maneuver.

We repeated this maneuver with right rudder and left aileron, and this time the wing did not return completely to its level flight. Again, rudder response was positive throughout the maneuver.

We again noticed that the control forces were asymmetrical, with light forces required on ailerons and substantially heavier forces on the rudder.

Spiral Stability Test

The final test in this series is to determine the aircraft's ability to raise the low wing after controls are released from a banked flight path.

Again, we set up our test with a forward CG, an altitude of 5,000 feet AGL, and a normal cruise speed of 165 mph. We then placed the aircraft in a bank of 25 to 30 degrees, and once established, released the controls.

Our first bank was to the left, and upon release of the controls the aircraft slowly righted itself, showing a slight, positive spiral stability.

However, when we banked to the right and released the controls, the bank increased to about 45 degrees and then remained in this increased bank condition. We repeated this test, and again with the bank to the right, with the aircraft displaying a neutral to negative spiral stability.

The RV series of aircraft are very light on aileron control. This gives them a sporty feel. Neutral or negative spiral stability is not necessarily dangerous; but if the rate of divergence is great, added pilot attention is required, and IFR flight becomes more labour-intensive. My RV-4 had a definite tendency for the left wing to drop and continued to do so until pilot control input would bring it back up to level flight. With these aircraft, it is not advisable to fall asleep on a long, boring cross-country flight, without a wing-leveler. (Tongue in cheek!)

These final tests complete the stability evaluations for the RV-9A. We have not nor do we intend to explore spin recovery or flutter investigations, leaving these maneuvers to the test pilots employed by the kit manufacturer.

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