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  #1  
Old 04-27-2019, 05:02 PM
Ed_Wischmeyer's Avatar
Ed_Wischmeyer Ed_Wischmeyer is offline
 
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Location: Savannah, GA
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Default RV-9A longitudinal dynamics

So a buddy and I were out fooling around today, trying to excite the phugoid.

We excited the short term response three times, but were fooled because it was oscillatory and lightly damped, maybe five cycles to dampen out. Data after the fact showed a period of about 22 seconds. Only got the phugoid going once, period about 60 seconds. And even though the phugoid is supposed to be constant AOA, the AOA was oscillating with the same period as everything else.

Anybody else got any numbers?

And the light damping may be part of why the -9A is not all that friendly to hand fly IFR.

Ed
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  #2  
Old 04-27-2019, 05:48 PM
Finley Atherton Finley Atherton is offline
 
Join Date: Nov 2005
Location: AUSTRALIA
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Longitudinal stability for the 9A was tested in this CAFE report pages 3 and 5. The graphs on P 5 include the RV 8 for comparison.

Try searching CAFE APRS if the link does not work.

Fin
9A

Last edited by Finley Atherton : 04-27-2019 at 06:10 PM. Reason: Bad link?
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  #3  
Old 04-27-2019, 05:55 PM
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Ed_Wischmeyer Ed_Wischmeyer is offline
 
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CAFE measured different phenomena than I did. And when they say that the damping was deadbeat, they're obviously not referring to the phugoid.

Thanks, though.
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ATP CFII PhD, so I have no excuses when I screw up
Vaccines kept me out of the hospital but COVID still cost me a month of living, all told...
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  #4  
Old 04-27-2019, 08:28 PM
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vlittle vlittle is offline
 
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My 9A was difficult to trim due to the phugoid response. What fixed it was careful elimination of all control system friction.

The biggest contributor was friction in the elevator hinges, including the main bearing that connects the elevators.

If any of the bearings exhibit side load from the hinge brackets they will bind. Carefully choosing shim washers at the hinge points eliminates this. It's meticulous work but it makes a difference.

Disconnect the main pushrod from the elevator bellcrank while doing this. When reconnecting the pushrod, make sure there are no side loads on the bearing as well. There should should be just enough shim washers in place so that there is no distortion of the horns when torquing the through bolt.

V
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  #5  
Old 04-28-2019, 12:08 AM
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rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Quote:
Originally Posted by vlittle View Post
My 9A was difficult to trim due to the phugoid response. What fixed it was careful elimination of all control system friction.

The biggest contributor was friction in the elevator hinges, including the main bearing that connects the elevators.

If any of the bearings exhibit side load from the hinge brackets they will bind. Carefully choosing shim washers at the hinge points eliminates this. It's meticulous work but it makes a difference.

Disconnect the main pushrod from the elevator bellcrank while doing this. When reconnecting the pushrod, make sure there are no side loads on the bearing as well. There should should be just enough shim washers in place so that there is no distortion of the horns when torquing the through bolt.

V
This is very good advice and is relevant to all the RV models except the 12.

Pitch system friction can have a serious influence on pitch stability if it is bad enough.

Besides the elevator hinge points, another common place for system friction (on side by side models) is the control column pivot bearings. The brackets are very stiff, and if the bearings are not shimmed carefully it is easy to induce a side load on the bearings which will add a noticeable amount of friction to the system.

A properly built and adjusted pitch system will have elevators that flop around easily when pushed with your hand, with no hint of stopping abruptly.
Some brands of autopilot servos induce enough friction to influence the test so it needs to be disconnected to get a true reading.
If you have one of the brands of servos that does add some system friction it is even more important to have a totally friction free system because the added friction of the servo is unavoidable.
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  #6  
Old 04-28-2019, 04:03 PM
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Ed_Wischmeyer Ed_Wischmeyer is offline
 
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Did some more research. The 22 second period was in fact the phugoid with all the correct indications. No idea what the longer period phenomenon was, and my FAA DER friend has no idea either.
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Previously RV-4, RV-8, RV-8A, AirCam, Cessna 175
ATP CFII PhD, so I have no excuses when I screw up
Vaccines kept me out of the hospital but COVID still cost me a month of living, all told...
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