Vibration under negative G

[grahambo]

Just wanted to know if anyone has experienced a mild pulsing type vibration when pushing during a slow roll? Been doing them for years and just noticed it on the last flight. Same speeds and G's as usual. Not evident under positive G. Won't have time for a few days to start pulling panels, but nothing unusual noted on post flight. Thanks

RV-4, 730hrs, 180HP, inverted fuel/oil, FP Sensenich

[lowandslo]

I don't have inverted system but in doing full aileron deflection rolls i sometimes get pulsing back through the stick. Got my attention when i first noticed it but from other posts on the forum it apparently is normal for the type of aileron and disappears by using slightly less deflection.

[WillyEyeBall]

Jim, could it be your headset moving? If your post flight didn't reveal anything, you could try pulling up about 45 degrees and then pushing to vary the amount of negative Gs up to about -1.5 and see if it reoccurs. I have a head cap and chin strap to keep mine in place. When I forget to fasten the chin strap, I'll hear a difference with negative Gs with the slight slip of the headset. Also when I wing wag to start a sequence, the sound is very different with the out of trim rolls towards knife edge. Different sounds make me very nervous too.
Bill McLean
RV-4 slider
lower AL

[rocketbob]

Baffling that normally doesn't rub in the cowling rub when subjected to negative g loads.

[grahambo]

Thanks. All good thoughts and will look at each one. Also will get some of the locals to check the play in my elev trim tab. I don't really hear anything, just feel it. I never had the aileron pulsing some pilots report, but maybe, without realizing it, I am flying the maneuver more rapidly or more aggressively. Perhaps it is also happening under positive G, but because they are higher, I just haven't noticed it then. Since it has not occurred before, a wear item, such as the baffling may be a good place to start. Will let you know.

[AndyRV7]

I have noticed the same problem recently so I am watching your thread closely. I don't remember ever feeling the pulsation before recently but I've only owned my 7 for 6 weeks. I only do rolls and it is definitely felt in the stick. It's not some other vibration from a baffle or anything not attached to the control stick. I presumed I had gotten sloppy and was pushing or pulling through the rolls a little. Full deflection or less an it is the same problem. You're not crazy. Oh, and I am never zero or negative G in the roll. I know this because everything is where I left it on the seat next to me.

Edit: My plane flys very tight and precise. I can also fly hands-off once trimmed. This is something I could never really acheive in a Skyhawk.

[Mel]

If you are not going zero or negative, then you are doing aileron rolls or barrel rolls.
The OP is asking about slow rolls. You will definitely go negative during a proper slow roll.

[AndyRV7]

Fair enough. I have the same problem when I do aileron rolls.

[mike newall]

I'm guessing shedding from the undercarriage fairings etc.

When you look at some more advanced aerobatic airplanes, they can get buffet at higher G from turbulence shedding off the undercarriage.

Now look at a negative G perspective on your 7 and wonder where the disturbed air off your gear is going - probably onto your stab, hence the vibration.

Thoughts ?

[grahambo]

Well, finally had time to look at the issue further. Thanks to Len Rulason who has plenty of acro experience and has worked as Kirby Chambliss' Red Bull race mechanic and now does some work for Paul Bonhomme.

He also asked that I check to see if the gear leg fairings had shifted. They had not. Then he noted the counterbalance arm I had built into the top of my rudder. In his experience, he had noted buffet across the top of the rudder when the counterbalance exceeded a critical angle of attack, as it might if you used too much rudder.

After a thorough inspection, off to play with various rudder inputs and too much caused the buffet.

Now the question was why had I never noticed it before? When I went to put my chute into the carry bag the lightbulb went on; for the first time I had flown with the sheepskin pad snapped onto the inside of my Softie Wedge. When I had originally performed a "fit" check with the new chute, it seemed that my hips were pushed uncomfortably forward, so I stored the pad.

This year's "monsoon" season in Phoenix has been unusually muggy and I tried the sheepskin pad and a towel to absorb some of the perspiration. The configuration pushed my hips further forward. When using my "muscle memory" rudder input it was just enough to get the buffet and make the maneuver look not quite right. When either deliberately using slightly less rudder or removing the pad, everything was back to normal.

Thanks for all the input.