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Cozy MKIV - Eighteenth Flight Test

Date: September 22, 2002

So, Sunday I headed out to the airport to test the turbulator effectiveness, in concert with the oil cooler air scoop. I took about 100 lb. out of the rear seat, so the CG stayed in about the same place. I took off and headed west, climbing at about 115 mph. The oil temp. seemed to climb more slowly than the day before, but still higher than before the air scoop. So, I resolved to leave the turbulators and remove the scoop, and see what happens.

Anyway, I climbed up to 6500 ft., leveled off, and did the longitudinal dynamic stability tests. This involves trimming to 170 mph and stabilizing. Then, raise the nose 3 degrees, lower the nose 3 degrees (back to where it was) and releasing the stick. Next, observe whether the aircraft regains pitch attitude, and in how many oscillations. In my case, the aircraft did so in 1 - 2 oscillations. That's the "short period" dynamic stability.

Next, at the same trim point, pull back to 165 mph and release the stick. Observe whether the aircraft regains pitch attitude, and in how many oscillations. Then, do the same when pushing to 175 mph. In both cases, the aircraft did regain pitch attitude and trim, within 2 - 3 oscillations. That's the "long period" dynamic stability.

The next step is Lateral Stability. There are three parts to this.

First, the "dihedral effect". Here, I trimmed to 130 mph straight and level, with both pitch and aileron trim, so the plane would fly S&L hands off. Next, start a slip with full left rudder and enough right aileron to maintain the original flight direction. Observe that no force reversals occur - force should be necessary to maintain the slip, both on the rudder and the aileron. After than, release the aileron and hold the rudder - observe whether the low wing will come up and return to level or beyond. Reverse the slip, and perform the same observations. In all cases, the aircraft did the right thing, and was deemed stable in "dihedral effect".

Second, the "Static Directional Stability". In this test, maintain the same trim point, and then slowly add rudder, keeping the wings level with ailerons. Now, release the rudder - observe whether the aircraft returns to straight flight. My aircraft did so in both directions.

Lastly, the "Spiral Stability". Keep the same trim point, and enter a coordinated, level 20 degree bank. Release the controls and observe the aircraft's reaction. Does it return to level? Does the bank angle decrease, or increase?

In this test, I found that because of the slight roll trim issue that I've got, where I need a small amount of left trim at low speeds, and about 2/3 right trim at higher speeds, I have some spiral stability if I turn to the right, but spiral instability if I turn to the left. According to AC90-89A (and other references), spiral instability is not necessarily a dangerous thing - it depends upon the time constant. In my case, in my plane, it takes about 10 - 15 seconds for the turn to go from 20 degrees to 30 degrees - this is NOT a safety issue. When the plane is trimmed for straight and level flight, it will maintain that attitude hands off for minutes at a time with no tendency to roll whatsoever - it's only an issue when the speed changes due to large bank angles.

So, that's the stability tests.

Since all the stability tests were done at 130 mph at 6500 ft., the oil was cool enough to attempt a high speed run up to Keene, NH (EEN) at 3500 ft. I pushed the throttle in and leaned for max RPM, and let it stabilize at just about 192 mph IAS. plugging the OAT and IAS into the Garmin 195 gave me a TAS of 204 mph. Woo, hoo. That's with no wheel pants, no spinner, and no gear leg-fuselage fairings. I SHOULD be pretty close to the 220 TAS advertised speed, eventually, and 204 mph in the current configuration is nothing to sneeze at. One landing at Keene, and then back to FIT.

On the way there and back (at 3500 ft., so I'll do them over again up high) I did flutter tests at 180, 185 and 190 mph. No worries.

I finished the flight with 24.9 hours on the Hobbs, so it's time for the second oil change (and a filter change), and the removal of the oil cooler scoop/plenum/SCAT tube.

End Date: September 22, 2002

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Copyright 2002, All Rights Reserved, Marc J. Zeitlin