Desert Aircraft vibes

[stangflyer]

Displacement also plays a huge factor in the vibes of a motor. Not just compression. Something else I have noticed...I have seen many lesser quality airframes nearly shook to pieces from motor "x". However upon installing that same motor on a higher quality and stronger built airframe, the motor ran silky smooth with nary a wiggle at the tail surfaces.

 

[49dimes]

Displacement also plays a huge factor in the vibes of a motor. Not just compression. Something else I have noticed...I have seen many lesser quality airframes nearly shook to pieces from motor "x". However upon installing that same motor on a higher quality and stronger built airframe, the motor ran silky smooth with nary a wiggle at the tail surfaces.

Agreed. The EF 104 is really a beefy but light airframe. Just like the DA 120! BEEFY and Light! Need to start eating some "light" cheeseburgers cuz I am getting rather beefy!lol

 

[49dimes]

Hmn....double post. Must have been me double chin!lol

 

[Ohio AV8TOR]

I can't speak to why one engine vibrates more than another, but I will relate my experience with 2 DA120's in 2 completely different planes.

My first 120 went into my 3DHS 103 (mod'ed to 105") Extra 330SC. It ran very smooth on 2 or 3 blades and of course had gobs of power. Later I got a Beast and bought another DA120 for it. I was surprised that it really shook the crap outta that plane, especially at low rpms. You could barely focus on the tail feathers as it shook the whole rear of the fuse so bad.

Thinking I had a rough running engine, I swapped the DA's in both planes. Low and behold, the Extra ran just as smooth with the other engine and the Beast shook just as bad!!! I was surprised to see this result, but it answered my question about my "shakey" 120.

@Pistolera
Interesting results, do you have any feelings on what difference made the change between airframes, short coupled, landing gear height etc.? Where both using the same mount?

 

[Pistolera]

@Pistolera
Interesting results, do you have any feelings on what difference made the change between airframes, short coupled, landing gear height etc.? Where both using the same mount?

Really couldn't say. It was definitely shorter than the Extra, and longer aluminum gear vs CF. Motor mounts were basically the same, ply box and short (about 1/2") standoffs. Same canisters. Who knows....don't have either ship now. Crashed the Extra and sold the Beast.

 

[-Rick-]

Is it possible that the motor is ever so slightly twisting the motor box and releasing that energy to the rest of the plane?

 

[49dimes]

Is it possible that the motor is ever so slightly twisting the motor box and releasing that energy to the rest of the plane?

I agree that is a possibility coupled with the prop load and weight. Torque and inertia.

 

[Pistolera]

The motorbox on the Beast is very short....maybe about 3" sticking out from the first former on the fuse. Way shorter than the Extra, which was what you typically see on aerobatic planes. maybe the longer one absorbed more energy before transferring it to the rest of the fuse structure???

 

[49dimes]

The motorbox on the Beast is very short....maybe about 3" sticking out from the first former on the fuse. Way shorter than the Extra, which was what you typically see on aerobatic planes. maybe the longer one absorbed more energy before transferring it to the rest of the fuse structure???

Just my opinion... But I think the way the motor box gets coupled to the wing tube aids in dissipating vibration. The Beast is a biplane and it does not have the typical "truss formers" that plank the motor box sides and run back to be bored out for the wing tube.

 

[Terryscustom]

Not that any of us will solve this mystery but I have also read other places that are in line with Stangflyers point in Post #21, only opposite. I can vouch for my own 120 rattling pretty hard on one high quality brand airframe that I know to be very strong......I recently mounted it on another name brand GQ plane that is known to be built substantially weaker and it vibrates less. Both planes are very similar wing area, wing span and weight. Both planes have similar engine box construction and length. Only variable is quality of materials and type / thickness of plywood used in the fuse. I had this exact same engine on one of my own custom built MXS's with foam core wings and it rattled pretty bad at low RPM's.

So, maybe it's the weaker brand "X" that actually absorb the vibes better until they live out their lifespan. Will tell this year, I plan to get a lot of time on this airplane this year and with the quality issues already I don't intend to get another one from the same company.

 

[-Rick-]

Sometimes I wonder about "boxing" or stiffening the motor box right back to the wing tube to let it carry the vibes right to the wings ,that would make it better able to release the vibration, or energy. From what I have seen is most motor boxes are built solid to the firewall, but after the firewall the two main motor box supports are more tried to the outside of the plane. Seems to me that the vibration is carried to the outside skin of the plane this way. Maybe for a reason?

If that makes any sense ?