Static Lateral Stability

Went out to formally run through the Stability tests at low airplane weight. Results are as expected for all tests except one: I think my technique is very incorrect for the Static Lateral Stability test. From a few sources online (including EAA), Static Lateral Stability is tested by:

1. Trim for some airspeed, level flight
2. Bank in one direction and maintain heading with opposite rudder
3. Release stick, use rudder input if necessary to raise low wing level

Basically, go into a side slip, then release the stick. When I release the stick, instead of wings returning level, I feel an immediate loss of G force (not quite negative G) and the airplane rapidly loses altitude (-3000 fpm). Let's just say it got my attention. I will not repeat this test until I learn the proper way of performing it. How aggressive does the slip need to be in order to verify stability?

something sounds very wrong, likely with technique. If you trim for a given airspeed and bank 30 degrees with opposite rudder to hold heading (i.e. slip), it shouldn't take much up elevator to hold it and assuming for the test you should not add elevator and let it sink a bit (not sure though). Assuming you are just adding bank and opposite rudder, no elevator, struggling to see how returning aileron to neutral with some rudder can cause a 3000 FPM dive. What airspeed are you doing this at?

Try lesser amount of sideslip initially.
For more aggressive sideslip, you should probably re-trim in pitch so it doesn't go nose-down when you release the stick. Or release the lateral stick pressure but maintain pitch attitude with fore/aft stick pressure as needed.

A little bit indirect way to do the same test is to just qualitatively assess stick pressure as you increase the slip. Starting with a very slight slip, it should take progressively more stick pressure to increase the slip (and of course progressively more rudder pressure and deflection as well).

At a constant speed, if it takes increasing aileron pressure and/or deflection to fly at greater slip angles, then you are laterally stable.

I repeated the test with more gentle control inputs and everything went as expected. I guess I was in too extreme a slip and possibly not trimmed level prior.

I'm going to go out on a limb here... When we did static directional stability in commercial aircraft flight test, it was to obtain rudder pedal position and force, and stick position / force as a function of beta. I think the test ends if you cannot keep the roll angle under 10 deg or something. It's been a few years since I did flight test. What you don't want is lightening of the controls as the slip is increased.

However, by definition, this is a static test and a release of the stick and response is not part of this test. This test is performed with minimum essential crew due to the hazards associated with stall and spin of the aircraft. What is it that you are trying to get out of this test?

I'm on a test right now, but somewhere I have a paper called "Static Directional Stability - Give It the Respect it Deserves" or something like that. I'm trying to recall which company lost a crew doing this test (either Bombardier or Lear sometime around 1994ish).

MTCW

Ezburton said:

I'm trying to recall which company lost a crew doing this test (either Bombardier or Lear sometime around 1994ish).

MTCW

Click to expand...

Yes. A steady heading sideslip on the first Canadair Regional Jet SN 7001. It "departed" as in departed controlled flight and entered a deep stall. The deployment of the spin chute didn't save them and they crashed, killing all on board. We don't risk deep stalls on the RVs but I don't think a departure from a mishandled SHSS is beyond the realm of possibility.