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vibration causing rpm loss?

Old 03-27-2012, 06:13 AM
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flybyjohn
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Default vibration causing rpm loss?

I recently built a twin seamaster and it has 2 - 25 size magnum engines with front mounted needle valves, each controled by its own servo y harnessed together to one channel. I have taken the advise I read here about tuning each engine by itself, one at a time and got each to run good (400 rpm below peak, and transision smoothly). The problem I am having is when both engines are running, I loose another 200 - 400 rpm off each engine at full throttle, the right engine being worse. I also get a hesitation from the right engine from idle to 1/3 throttle when both engines are running, but when only the single engine is running it transitions great. I was looking for airbubbles in the fuel lines and did not see any at any time.

Are there any remedies to solve this problem of mine or should I just live with it.
Old 03-27-2012, 01:05 PM
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mboland
 
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Default RE: vibration causing rpm loss?

A few more details would help, even a picture or two.
Are the engines mounted upright, inverted or what?
Position of tank?
On board glow?
props used?
Are they balanced?
Spinners?
Does the plane feel like it's vibrating? More with one engine than the other?

Might help elicit a response.
Old 03-28-2012, 04:23 AM
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flybyjohn
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Default RE: vibration causing rpm loss?

Sorry, lets get a few more details. Engines are mounted about 100 deg. counter clockwise from upright, muffler facing down. The tanks centered on spray bar if not just slightly above by 1/4" or less. No on board glow system. Props are 9x5 MA s2 design, balanced as close as I could with that small of prop on my magnetic balancer. Dubro 2" plastic spinners. At one rpm range about mid throttle with both engines running, the wing starts to vibrate like a drum, then just above that setting it smooths out again. It does not do this with one engine running.
Each engine alone peaks out at 14,900 - 15,000 rpm and is richened up to 14,500 rpm, then when both engines are started they only are taching at 14,000 to 14,300. I increased the idle set from 2500 to 3200 and it helps a little bit with 1/3 throttle hesitation but it is still there.

In the third picture, I have since taken the spring clip off the high speed needle and placed a piece of silicone tubing over the needle and housing to help seal the needle and to prevent sideways pressure on the needle.
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Old 03-28-2012, 06:21 AM
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MajorTomski
 
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Default RE: vibration causing rpm loss?

There is a very minor possiblity that the wing vibrating is affecting what your tach sees as the prop blades go by. I ususally rest my hand on the model so the tach is shaking at whatever rate the nacelle is shaking. It often changes what the tach reads.

MTC just an idea.
Old 03-28-2012, 11:18 AM
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Villa
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Default RE: vibration causing rpm loss?

Hi flybyjohn
I too had a twin, but with .28 size Magnum engines. I purchased a tach for it and used it in the beginning. After a few flights I stopped using the tach. I just tuned each engine separately, as you have learned. I would not worry about the small RPM difference you have noted. Your plane is very nice looking.
Old 03-28-2012, 01:50 PM
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Default RE: vibration causing rpm loss?

Good advice.
The slight variance in tach readings you are seeing is probably a mixture of things, including the tach as described.

Twins setup in sidewinder mode do seem to also have some strange tendencies.
I have a P-61 with twin Saito 125's in sidewinder mode, and although set correctly on the ground the right engine leans out in the air.
There is also an rpm setting where the is harmonic addition of vibration.
I have read some articles warning against sidewinder mounting due to these problems but mine flies ok, and I can cope with the slight problems, and the vibration is not damaging, so ... I fly it and enjoy it.

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