PPRuNe Forums - View Single Post - Turn Rate Indicator / Turn Coordinator / Looping Error
Old 21st Nov 2013, 23:07
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flyer101flyer
 
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observations

Well, that was interesting!

I don't know anything about how the dynamics would be different in faster aircraft, or how older turn rate indicators might have differed from what is currently in use, but here's what I observed:

Aircraft -- 1956 Cessna 172
Age of turn rate indicator (needle and ball style)-- the instrument is of recent manufacture, not original.
Also on panel-- portable piezoelectric 1-axis turn rate indicator (sensing yaw only)

Summary-- pulling extra G's causes more of an increase in turn rate indication in a right turn than in a left turn. Pushing to reduce the G-loading seems to cause more of a decrease in the turn rate indication in a right turn than in a left turn.

Nothing I observed helped me understand why a turn coordinator might be less useful than a turn rate indicator for partial-panel recoveries from unusual attitudes-- except for the inverted spin issue as noted in post #3. I'll repeat these tests some day in an aircraft with a turn coordinator. In general, I still feel like the roll sensing inherent in a turn coordinator usually helps a pilot bring an aircraft smoothly to wings-level with less of a tendency to overshoot into a bank/ turn in the opposite direction.

Based on what I observed, my own partial-panel recovery technique for light aircraft will not include pushing the stick forward to reduce the G-loading to 1 to eliminate errors in the reading of the turn rate indicator or turn coordinator.

(Edit: see post #5 for some more thoughts on possible reasons for the differences between what I observed, and the observations / partial panel recovery techniques that others have posted.)

Details:

Enter coordinated, constant-speed, constant-altitude standard rate right turn at high airspeed. Then pull extra G's.

Turn rate needle shows a strong increase in turn rate, significantly sooner than, and more than, the increase in turn rate shown on the piezoelectric sensor.

Enter coordinated, constant-speed, constant-altitude standard rate left turn at high airspeed. Then pull extra G's.

Turn rate needle seems to lag in showing an increase in turn rate, compared to the increase in turn rate shown on the piezoelectric sensor.

Enter coordinated, constant-speed, constant-altitude standard rate right turn at low airspeed. Then push to reduce the G-loading.

Turn rate needle seems to show a strong decrease in turn rate, which seems to be significantly sooner than, and more than, the decrease in turn rate shown on the piezoelectric sensor.

Enter coordinated, constant-speed, constant-altitude standard rate left turn at low airspeed. Then push to reduce the G-loading.

Turn rate needle seems to show a slight increase in turn rate in some tests, and in other tests, seems to show much less decrease in turn rate than is shown on the piezoelectric sensor.

With this new information in mind, I did some more experiments with spiral dives. The errors in the turn rate indication due to G-loading seemed insignificant compared to the overall dynamics of the spiral dive and the phugoid dynamics that accompanied the entry and recovery to the dive. It still seemed that the basic rule I posted in my first post applied well:

Basic rule for partial-panel recoveries-- use rudder or coordinated rudder/aileron to roll against the deflection of the turn rate indicator. Simultaneously, apply aft stick pressure whenever the airspeed is increasing, and forward stick pressure whenever the airspeed is decreasing, with the amount of stick pressure proportionate to the rate of movement of the airspeed needle (NOT to the actual value of the airspeed.) Once the airspeed is frozen, ease it slowly back to trim speed. Avoid exerting strong aft stick pressure at very low airspeed-- but this situation will rarely arise because the airspeed will never be very low, and rapidly increasing, at the same time unless perhaps the aircraft has whipstalled!

Details of two spiral dive recovery tests:

1) Trim full nose-up which yields 62 MIAS wings-level at 1950 rpms, which yields a constant altitude. Roll to 60 degrees bank, exerting no pitch pressure. Making no pitch inputs, hold the bank angle until the airspeed stabilizes. Airspeed eventually stabilizes at 97 MIAS. Turn rate indicator is pegged. Begin partial-panel recovery.

One effective partial-panel recovery technique at this point is to use rudder or coordinated rudder and aileron to roll opposite the turn rate needle deflection, stopping the roll at wings-level (some overshoot is likely--correct back the other way as needed until arriving at wings-level), while simultaneously feeding in forward stick pressure as needed to freeze the airspeed or allow only a slow decrease in airspeed, so that the roll to wings-level doesn't cause the nose to pitch up steeply. Continue to apply forward stick pressure whenever the airspeed is increasing, and aft stick pressure whenever the airspeed is decreasing, until the airspeed is frozen. Then ease the airspeed slowly down to trim speed.

No obvious differences were observed in trials involving left and right turns, nor was there any obvious oversensitivity of the turn rate indicator when the G-load was greater than one or greater than "normal" for the bank angle -- whatever gyro errors may have been present seemed to be relatively insignificant compared to the overall dynamics of the maneuver.

Another experiment, initiating the recovery sooner:

2) Trim full nose-up which yields 62 MIAS wings-level at 1950 rpms, which yields a constant altitude. Roll to 60 degrees bank, exerting no pitch pressure. Making no pitch inputs, hold the bank angle constant as the nose drops. This time begin the partial-panel recovery sooner, at the point of maximum nose-down pitch attitude and maximum rate of increase of airspeed, well before the airspeed peaks or stabilizes.

Again, the turn rate indicator is pegged. One effective partial-panel recovery technique is to use rudder or coordinated rudder and aileron to roll opposite the turn rate needle deflection, stopping the roll at wings-level (some overshoot is likely--correct back the other way as needed until arriving at wings-level), while simultaneously feeding in aft stick pressure as needed to help arrest the ongoing increase in airspeed. At some point before wings-level is reached, the airspeed may begin to decrease again, indicating that the nose has pitched up above horizontal due to the decrease in bank angle. If so, apply forward stick pressure as needed to arrest the increase in airspeed. Continue to apply forward stick pressure whenever the airspeed is increasing, and aft stick pressure whenever the airspeed is decreasing, until the airspeed is frozen. Then ease the airspeed slowly down to trim speed.

Again, no obvious differences were observed in trials involving left and right turns, nor was there any obvious oversensitivity of the turn rate indicator when the G-load was greater than one or greater than "normal" for the bank angle -- whatever gyro errors may have been present seemed to be relatively insignificant compared to the overall dynamics of the maneuver.

In actual practice of course it would be better to initiate the recovery even sooner-- but I wanted to test some extreme cases.

Please don't construe anything in this post to be a critique of some other partial-panel recovery technique for some other aircraft equipped with instruments of an older vintage-- I'm simply reporting what I observed during a couple of hours in the air this morning!

Last edited by flyer101flyer; 10th Dec 2013 at 00:01.
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