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-   -   Elevator Pulsing/Hunting (https://vansairforce.net/community/showthread.php?t=169447)

Jared_Solomon 03-07-2019 02:35 PM

Elevator Pulsing/Hunting
 
Hi All,

Looking for some thoughts from the VAF brain trust. I have put my 14A through it's paces and have about 230 hrs on it now . I'm puzzled by a "problem" I keep experiencing in flight. It appears that when the trim tab is in the neutral position, or close to neutral, the control stick "pulses" back and forth at about 1-2hz. I've spoken to Van's about it in the past and they believe the elevator is "hunting" when the elevator and the trim tab align just right. There is advice is to "trim it out", which does work, just seems like a bit of a cop out. For the past year I have been testing various theories as to why this might be happening, some theories related to the trim tab and others not. Below is a summary of what I've tried so far
  • Checked all linkages (no slop found)
  • Removed all slop from the trim tab (machined bushings)
  • Taped on reinforcement plates anywhere I found oil canning (removing oil canning areas from the elevators has no effect)
  • Elevators balanced (no effect)
  • Applied speed tape to the outboard lightening holes in the HS

Couple of questions:
  • Other RV-14 drivers, have your experienced anything similar?
  • If it is the trim tab / elevator alignment, any suggestions on how to fix it?

Thanks,

Nova RV 03-07-2019 02:56 PM

Does the pitch change with the hunting, or does it stay stable and the elevator assembly is "floating" as in moving but not making any pitch change? I know back in my R/C days we would make the trailing edges thick like the 14 has to make the sensitivity less so at neutral like you describe. I never noticed this on my 14A but will try to pay close attention to replicate what you describe.

Bavafa 03-07-2019 03:07 PM

Interesting to hear this.

Obviously I don't have nearly as many hours on my 14A as you do but so far, twice I have noticed a very short "vibration/pulsing" in my stick just right after rotating (during take off) that last about 1sec to 1.5 second. I have not noticed it in the air at all. I do believe the pulsing during rotation is related to where exactly my trim tab is which I set it to neutral position but It is not a precise setting. I have, likewise, checked for any 'play' in the control surfaces and can say there is about 1/32" (as best as I can measure it) of play in my trim tab but zero on my elevators in relation to one another and I did balance my elevators each individually.

AlexPeterson 03-07-2019 06:57 PM

Jared, could you fasten something like a rubber strap, temporarily with tape, to keep the trim tab always against one side of whatever small slop might be in the system? Some way to preload it to one direction. Seems it could be related to that. Putting a small amount of trim in and the putting a small pressure on the stick does a similar thing.

vlittle 03-07-2019 08:44 PM

Here's an experiment: Try putting a 30-40 lb weight in the baggage compartment. See if you get an improvement when you fly.

If so, you may have to adjust the incidence of the HS.

V

Jared_Solomon 03-07-2019 08:52 PM

No pitch change
 
Quote:

Originally Posted by Nova RV (Post 1329957)
Does the pitch change with the hunting, or does it stay stable and the elevator assembly is "floating" as in moving but not making any pitch change? I know back in my R/C days we would make the trailing edges thick like the 14 has to make the sensitivity less so at neutral like you describe. I never noticed this on my 14A but will try to pay close attention to replicate what you describe.

Hi Chris,

No perceptible pitch change. I've asked passengers while it's happening and they can't feel it. It would appear to be "floating".

Thanks,

Jared_Solomon 03-07-2019 08:55 PM

Quote:

Originally Posted by AlexPeterson (Post 1330009)
Jared, could you fasten something like a rubber strap, temporarily with tape, to keep the trim tab always against one side of whatever small slop might be in the system? Some way to preload it to one direction. Seems it could be related to that. Putting a small amount of trim in and the putting a small pressure on the stick does a similar thing.

Hi Alex,

The trim tab has 0 slop. I machined SS bushing sleeves that go over the clevis pins to remove all slop.

Thanks,

Jared_Solomon 03-07-2019 08:57 PM

Quote:

Originally Posted by vlittle (Post 1330045)
Here's an experiment: Try putting a 30-40 lb weight in the baggage compartment. See if you get an improvement when you fly.

If so, you may have to adjust the incidence of the HS.

V

Hi Vern,

Good idea. Easy to do. I'll give it a shot!

Thanks,

rvbuilder2002 03-07-2019 09:51 PM

Quote:

Originally Posted by Jared_Solomon (Post 1329952)
I've spoken to Van's about it in the past and they believe the elevator is "hunting" when the elevator and the trim tab align just right. There is advice is to "trim it out", which does work, just seems like a bit of a cop out.

Cop out seems just a bit harsh considering the circumstances. Troubleshooting problems such as this via e-mail or phone call is about the same as using a phone consult with a Dr for an unusual medical condition. It could be a lot of things......

This is not something that has been common with RV-14's so there is no "yea we have heard that a bunch of times.... just do this...."

Regardless, in the interest of trying to help you out, please explain what you mean by "trimming it out does work". (to stop the pulsing?)

lr172 03-07-2019 10:54 PM

Quote:

Originally Posted by Jared_Solomon (Post 1330048)
Hi Alex,

The trim tab has 0 slop. I machined SS bushing sleeves that go over the clevis pins to remove all slop.

Thanks,

Seems like you have identified the one thing that is different from all of the others and possibly why yours is doing what all of the others are not. Sometimes "improving" something doesn't always deliver the results you were hoping for. I can think of numerous mechanical systems that have "slop" designed in, escpecially in dynamic applications. Not saying the slop was designed in in this case, but it still may account for the performance difference. In cases of unique performance problems, you usually start with what might be different from the norm.

Think of the poor guy that said "look at that ugly rivet for a static port; I can make something much better" and proceeded to make a nice flat one that looks just like the expensive certified planes then wondered why his airspeed readings were so far off.

Larry


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