What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

shake it, baby!

HansLab

Well Known Member
I have an interesting experience, but let me share some figures first:
TT70 hours
new aluminium Cummins Spinner since 6 hrs. Beautifully made, nearly-perfect mounted per manual: even less movement of the pitot than with the original spinner.
New spinner is not balanced, yet.

The spinner-info is added because I never had this issue before I mounted that beauty..
Engine runs fine, temps where they should be. Taxiing for some minutes, run-up flawless. Takeoff nice as ever. Then the prop begins 'shaking' as if it isn't centered. changing RPM always solves it, and the flight is effortless.
Changing RPM's afterwards never ever got me shaking again...

This happens at some 100ft height, and no (read my lips: NO) shaking whatsoever at run-up.
Checked of course:
No loose motor mounts, engine rubbers - nothing.
No wear or tear on the engine-axle (as far as I can feel/see without deep examination).

Today I noticed a little gas stain in the drip pan of one of the carbs.
I thought it was my turn for sunk floats, but one Rotax guru (Roger Lee) said to be careful with quick actions, for sometimes '..it makes the carbs shake so bad the floats and needle valve can't control the fuel level and it vents out the tube'.

That shake is definitely what I got - but how?
Might the (slightly heavier) spinner be the culprit?
And how do I check that?

Any ideas highly appreciated!
 
Did you remove the prop bolts to install the spinner? I would check the tracking of the propeller to insure that it is within 1/8" or less. With the spinner being on center, I would not expect it to affect the balance, but perhaps it isn't on straight or the bolts are not torqued equally....
 
tracking

thank you - will check.
Bacplate had to be swapped for a Cummins (more sturdy!) one, so yes, prop has been dismantled.
Bolts are toqued equally I'm sure - but even so I have this issue, so: I'll check.
 
Last edited:
This happens at some 100ft height, and no (read my lips: NO) shaking whatsoever at run-up.

This sounds like nose wheel coasting down. Imbalance of wheel at certain speeds will cause entire engine to jump around on rubber mounts...
 
What I do; tip the plane so the nose wheel is off the ground, easiest way is to put weight on the tail anchor. Remove the rear nose wheel faring. Spin the wheel to 1200 rpm and let it coast to zero. What you see will surprise you. If the wheel is out of balance the whole plane will shake at several different rpm’s.

To spin the wheel I put a 8” rubber backing disk on the electric drill and run the edge of the disk against the tire tread.
 
Do you tap your brakes during climb out? I suspect this since you say your at 100'....could be wheels..
 
Prop Shake

When I swapped out the nose fork per the SB; liftoff and 100’ altitude and a significant spinner/prop vibration relative to the cowl. I static balanced the nose wheel (same assembly as prior) and the vibration is gone.
I concluded the new reinforced fork introduced different harmonics.
 
I have a similar vibration and I'm 99% sure it's the nose wheel out of balance. The degree of shake, well it's more like a heavy wobble, actually changed whether I have the nose fairing on or off. I plan to static balance the nose wheel at the next annual or next time I put the plane down for MX.

Try short field takeoff technique and see if it affects the magnitude and duration of the shake. Get the nose off the ground early, like as soon as 25-30kts indicated and wheely down the runway until liftoff. Compare that to rotating at 55-60 kts with the nose wheel spun all the way up.
 
great suggestions, thank you all. The shake is quite severe to be originated by the frontwheel, but I've listened and will try the 1200RPM trick.
I do hope it is this simple, because anything else I could think of is try a TestFlight without spinner (but with backingplate), but that means without pitot as well. Didn't figure out how to solve that one - although I think I can fly the thing without pitot as well.
I'll keep you posted. DO keep sending ideas!!
 
At 1200rpm an out of balance nose wheel will shake but not as bad as when it decelerates around 1/2 that RPM and hits the natural frequency of the nose gear strut. Do it yourself and you will be a believer in wheel balance.
 
it is your front wheel out of balance. mine did the same thing until i tried the early lift of nose on takeoff to minimize the wheel speed....no vibration! common problem.
 
The shake is quite severe to be originated by the frontwheel

On several occasions, I thought my engine was going to shake right off my airplane. Getting the nosewheel up right away and doing the wheelie takeoffs solved the issue.
I confirmed this was causing the issue by intentionally holding my nose wheel on the ground longer than normal. It shook everytime I did this...
 
Last edited:
Chapeau!!

Still cannot believe the results, but: after having balanced the frontwheel the shaking was gone.
Entirely.
30 grams of lead made all the difference. And concerning the small diameter of the wheel that is quite a lot, but the results count.

How I love this forum.
Let's continue it for a few decades.
 
Still cannot believe the results, but: after having balanced the frontwheel the shaking was gone.
Entirely.
30 grams of lead made all the difference. And concerning the small diameter of the wheel that is quite a lot, but the results count.

How I love this forum.
Let's continue it for a few decades.

What device did you use to balance the wheel? I’ll be doing it this winter as well.
 
balancing

I just went to a motorcycle workshop, where they had the appropriate gear to balance the frontwheel (Automotive did not work for the rim was too small for the gear they use there). Just stick-on 5 grams lead pieces. OUtbound pressure keeps it tight, the man said when I figured they' d be gone with a flight or two...
We'll see.
 
It is quite possible to install the stick-on wheel weights inside the wheel halves. Turn the wheel on the motorcycle wheel balancer and mark the heavy spot. Once the tire is deflated, remove the hardware and separate one wheel half from the tire, tube, wheel assembly without disturbing the clocking of the tire and apply weights inside the wheel halves opposite the heavy spot. Reinstall hardware loosely and repeat as necessary to achieve excellent balance.
Clean the places inside the wheel where the weights will go first with a suitable solvent, rubbing alcohol works well.
We no longer have lead weights available here in the US, and the weights shown are a lighter metal so it took several of them to balance the tire assembly.
This is the nose wheel under discussion. On the mains the weights can go on the outside.
 

Attachments

  • IMG_3232.jpg
    IMG_3232.jpg
    417.5 KB · Views: 92
Last edited:
Back
Top