Fairings added.... and now a shake at speed

thinkn9a · #1 09-25-2009, 06:57 PM

ok,... just wondering if someone has some ideas or experience on where to start on sorting out a shake at speed with fairings just added.

Nothing is really noticable until up around 160 or 170mph or so...

thanks for any and all suggestions/questions

iwannarv · #2 09-25-2009, 07:39 PM

Could it be the alignment of the wheelpants? Since their basic shape resembles and airfoil, they could produce lift at certain speeds? My father's -8 seems to get a slight vibration at about 220mph TAS.

Bob Axsom · #3 09-25-2009, 07:55 PM

Did you reach that speed before fairings? If not the vibration may come from another source. Secure mounting and alignment are all that I can think of your you to verify. I have an RV-6A that I have up to 184.4 kts TAS at 6,000 ft density altitude and I have experience no vibration. I think you are right to be concerned until you solve the problem and fix it.

Bob Axsom

thinkn9a · #4 09-26-2009, 04:42 AM

It has been at this speed earlier, without the shake. Certainly the fairings helped to get even faster,.. but shake occurred before reaching the new area.

I believe it is a wheelpant issue,... just wondering if it is simply time to recheck the "final" alignment, .. or a gap,.. or the shape/angle of the tail of the intersection fairing (which I added just a bit to). Also I ended up making the holes in the nose gear slightly larger in order to accommodate my tow bar

other thought was possibly the gear/fuse intersection as there is a small gap there... but I do not hear any banging, or see any cracking around the screws

Bob Axsom · #5 09-26-2009, 05:17 AM

Then I agree the problem is with the fairing installation. You are certainly thinking it through. Try addressing one suspect at a time with a simple temporary fix. I would suggest taping the tow bar openings first, then the gaps etc. I have no towbar holes in my NLG fairing so that is a basic difference. The addition you made to the rear could have aerodynamic and balance effects which I'm sure you have thought about. I modified my NLG fairing mounting to allow simple installation and removal, using gull wing brackets with platenuts on the wings to accept #8 flathead screws with dimple washers. That probably doesn't mean anything to you - I'm sure they improved the installation design by the RV-9A. This is the fun part - I just watched my new DVD "Air Racer Chasing the Dream" for the 3rd time and it is hard to believe how long Jon Sharp and his team struggled with a vibration problem on the Nemesis NXT that started at ~250 kts that was ultimately traced to a right MLG closed position rigging problem.

Bob Axsom

kentb · #6 09-28-2009, 09:45 AM

Wallace, you might try removing some of the fairings to see where the problem lies.

Remove the front wheel pant and fairing and fly.

Remove main gear leg and intersection fairings and fly.

Etc.

Kent

Jamie · #7 09-28-2009, 09:58 AM

I have the same problem. I noticed it at 170kts TAS or higher. I never noticed it until I installed the grove nosewheel. The grove nosewheel has very little rolling resistance as compared to the OEM Matco setup. The problem is the air is hitting the nosewheel and spinning up the nose wheel. I confirmed this by flying in formation with a buddy who observed that my nosewheel would spin at that speed. We marked the wheel with chalk so that the spinning would be obvious.

It is possible that it could be one of your mains spinning. Have you tried hitting the brakes in flight to see if the vibration stops?

Calvin25 · #8 09-28-2009, 10:18 AM

Quote:

Originally Posted by Jamie

I have the same problem. I noticed it at 170kts TAS or higher. I never noticed it until I installed the grove nosewheel. The grove nosewheel has very little rolling resistance as compared to the OEM Matco setup. The problem is the air is hitting the nosewheel and spinning up the nose wheel. I confirmed this by flying in formation with a buddy who observed that my nosewheel would spin at that speed. We marked the wheel with chalk so that the spinning would be obvious.

It is possible that it could be one of your mains spinning. Have you tried hitting the brakes in flight to see if the vibration stops?

I had the same issue on another plane, tap the breaks and it cleaned up.

N941WR · #9 09-28-2009, 11:42 AM

Wallace,

While I agree with the others that the wheels could be spinning I would like to know if you recently removed and re-installed the prop?

If you did, did you track it? That could give you a vibration at some speeds.

thinkn9a · #10 12-21-2009, 05:00 PM

Jamie,

You had it nailed. I finally got to the nose wheel and worked on making sure it was round and balanced.

A good indication that it was the problem was the following: The tire was a bit out of round,... and the "high spot" still had the new tire "nubbies". Appeared that it was out of balance, and just made itself worse with each landing.

taking off the nubbies to keep them from trying to spin up the tire, getting the tire balanced, and "reseating the tire" has taken care of the shake. (I have the Grove wheel also)

Finally made advertised cruise speed without the shake. Woohoo!! Marietta says I have a second RV grin.

Thanks to all,.. as the group knowledge has helped me once again.