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Vibration

n348pb

Member
I am about 20 hours into Phase 1 flying and I am getting a vibration that I can't find a source for. During climbout (or at level off) with the power set at about 2500 rpm I feel/sense a vibration that seems to be coming from the right side cowl area. There is no vibration in any of the controls but I have found it subsides somewhat if I add enough right rudder to move the ball out about a quarter of a ball width to the left. I have noted the vibration since the first flight and have checked all the control surfaces, the cowl, spinner, etc. and haven't noted anything out the ordinary. I have checked the clearance around the exhaust pipe (there is plenty) and the lower cowl around the exhaust is secured with screws into plate nuts. Airspeed does not seem to be a factor although I have not checked it over a wide range - just 120 to 140. I will try other speeds on the next flight. When I pull the power back to 2300 or so it goes away and when I at max rpm I don't notice it. Any ideas would be appreciated.
 
Same vibration

Interesting,

A friend of mine just started flying his RV-10 and reported a vibration very similar to what you describe.
After eliminating fairings and checking just about everything he noted that pushing on the right skin panel below the air vent made the vibration stop.
 
Bill,

I had an almost identical problem with my Rocket. I am about 30 hrs into Phase 1, and any of the higher power settings caused a vibration. I tried duplicating the problem on the ground, and had the prop dynamically balanced. No luck.

I checked all the other usual culprits: floor boards, exhaust, loose engine hardware, etc. I finally took a very close look at the exhaust as it was exiting the cowling, and found witness marks that could have only been caused by the exhaust rubbing on the cowl. Since I couldn't duplicate the vib on the ground, I speculated it could be flow induced. Or possibly a sweet spot of flow and a mechanical resonance. And since I was also having cooling issues, I built a stiffener for the lower cowl that also included a diverter, much like you see on the Cessna amphibs. Took about 2hrs, and can be removed with three screws.

Vibration is now completely gone, and the cooling issues are gone (but no 90 OAT, either). The result was pretty dramatic. In fact, it is the difference in cooling performance that I am still scratching my head over.
 
And since I was also having cooling issues, I built a stiffener for the lower cowl that also included a diverter, much like you see on the Cessna amphibs. Took about 2hrs, and can be removed with three screws.

Vibration is now completely gone, and the cooling issues are gone (but no 90 OAT, either). The result was pretty dramatic. In fact, it is the difference in cooling performance that I am still scratching my head over.

Any chance you could post some photos????

Please.......
 
I could, Mike, but I've already been ostracized by my Rocket colleagues because I haven't documented how much speed I have lost (if any);).

Let me go find the pics...
 
CowlStiffenerDam003.jpg


CowlStiffenerDam001.jpg


The assembly consists of a .040" aluminum strip on the lower cowl. But I wanted some out of plane stiffness as well, and decided to create the flow diverter as an experiment to see if I could also improve cooling. It's about an inch wide, and out of plane about 30 degrees.

Caution is in order here. Although I have eliminated the vibration and improved cooling, I haven't yet convinced myself that the diverter alone improved cooling. It may be that the vibration of the lower cowl was inhibiting exit air flow, and that a robust stiffener without the diverter would have been sufficient. More testing is in order.
 
Paul
I would explore other methods of stiffening the cowling, this flange must be costing you knots. Most rocket cooling problems that I have encountered have been with the inlets rather then the outlet. Call me if you would like to discuss this issue. 519-631-1369
I think it would be a good idea if all racers, myself excluded, had to install an Areonca Champ cooling flange like you have on your rocket :)
 
Ah, I knew as soon as I used "Rocket" and "speed" in the same paragraph I would get your attention, Tom.:D

I don't want to hijack this thread, but just to be clear--the intent was to get a quick and dirty stiffener to bracket my vib problem, and as I was building the stiffener I made the out of plane portion wide to see how much I could affect cooling (but it was purely a secondary issue). I already have a modified inlet diffuser that will be tested this winter. I don't intend on leaving this on; but I do intend to trim it and sequentially record the data, so that I can compare inlet and outlet fixes. But it has allowed me to complete all the Vx and Vy testing, and to finally start flying at full power.

Thanks again, Tom, I'll give you a call tonight when I get back.
 
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