Any vibration engineers about?

[pgkevet]

I was toying with the feasibility of the following..but not sure whether that's thr right approach or not:

There are some lightweight, cheapish and quite powerful plank ei petrol engines about but their use in helis is limited by the inherent vibration.

Exceptions might well be the radikal and the use of rx engines and PUH engines..but they weight way more.

It struck me that if one isolated the whole engine mounting and primary power train then one gets around that. So could one mount the engine on a separate base on rubber isolators and then use a first stage belt (as in vario or avro) to then drive the pinion?

Could this be effective or just change the vibration frequency and amplitude?

TIA

[Craig33]

IN theory it should work as long as the operating frenquncy is greater (i think) than the natural frequency of the mount.

engine rpm/60 = frequency

If you are buying proper mounts you should be able to get some data on the frequncy range of the rubber.

[xygax]

Check out the PUH engine on the Bergen there is an online manual on their site its all rubber mounted... pay attention to how the clutch mounts its not strightfoward and its really important that the clutch alignment is spot on... I think the engine basicly hangs on the clutch shoe when its in flight....

Steve

[trillian]

That's essentially how the Robbe Futura engines are mounted in that they have a clutch where the bell fits over the clutch and forms a unit with a pulley and the engine delivers power to the drivetrain via a belt.

There is no bearing block above the clutch, no dail indicating involved etc.

The difference from your description being that the engine is still mounted rigidly to the frame. But this could be modified.

[bolders]

This is a complex issue which I dont think can really be answered without some imperical data and is difficult to explain the theory without some pretty graphs.

What craig said is correct in that you do not want the frequency of vibration to be near to the natural frequency of the mount as resonance will occur (it does not matter if the vibration is of a higher or lower than the natural frequency though - as long as it is not the same), although this can be minimised with the correct damper selection (but not eliminated). Furthermore it is likely that there will be more than one frequency of vibration.

If you are operating at a frequency which is different to the natural frequency of the mount then the transmissability of the system will be determined by the stiffness of the structure or the mass of the system (dependant on what side of the natural frequency you are operating).

Further to this, there is the consideration of how many natural frequencies the mount has got etc.

I know this doesnt answer your question but it should hopefully show that without actual measurement data it will be difficult to get the correct results.

I think your best option would be to try it and see - the results may be satisfactory but they may not. I really could not say tbh.
Your only other option is to properly analyse and design the system around the operating characteristics of the engine which would be difficult - its certainly well beyond my understanding at the present time.

[pgkevet]

I suppose I got the mixed bag of answers I expected.. it isn't an easy one.

I'm not an engineer(except where it applies to orthopaedics and locomotion) but always understood that vibration could only be dealt with my eliminating it's source, mass dampening or converting it to other frequencies.

Where the engine is isolated with a belt drive first stage one avoids the 'hanging from the clutch'. Mounting it on isolators struck me as an option .. what i was thinking was say using a donor Trex6/700 and mouning the engine below the baseplate on it;s own baseplate then attaching the 2 together with a bunch of skid-damper type rubbers. that ought to allow for some 'de-tuning' because one could then adjust the number of dampers or their hardness and perhaps even add/remove small weights on rods to that base??

[trillian]

I suppose I got the mixed bag of answers I expected.. it isn't an easy one.

I'm not an engineer(except where it applies to orthopaedics and locomotion) but always understood that vibration could only be dealt with my eliminating it's source, mass dampening or converting it to other frequencies.

Where the engine is isolated with a belt drive first stage one avoids the 'hanging from the clutch'. Mounting it on isolators struck me as an option .. what i was thinking was say using a donor Trex6/700 and mouning the engine below the baseplate on it;s own baseplate then attaching the 2 together with a bunch of skid-damper type rubbers. that ought to allow for some 'de-tuning' because one could then adjust the number of dampers or their hardness and perhaps even add/remove small weights on rods to that base??

Rubber mounts are used on cars and other engines all the time. The prevailing goal is to keep from transitting all the vibrations to the rest of the airframe and I am sure that some kind of flexible mounts will do this.
I think the stiffness of the rubber mounts will be determined by how much the engine can be allowed to move around and still keep the belt on its pulleys and not start to wear prematurely or try to ride off.

[Craig33]

Well said Bolders. I only have to dabble in the subject to choose mounts for motors that operate at set speeds or within a fairly tight range.

There is a huge amount of information on this specialist subject if you care to go down that route. I suspect you might just have to suck it and see then tweak if needed.

[bolders]

Rubber mounts are used on cars and other engines all the time. The prevailing goal is to keep from transitting all the vibrations to the rest of the airframe and I am sure that some kind of flexible mounts will do this.
I think the stiffness of the rubber mounts will be determined by how much the engine can be allowed to move around and still keep the belt on its pulleys and not start to wear prematurely or try to ride off.

This is true and to some degree it would be true with a heli also, however, the thing that imediately springs to my mind is that the mass of a heli is much less than a car and it is not grounded in any way.
Im thinking along the lines of newtons second law and every action has an equal and oposite reaction - the rubber mounts would need something to react against which in this case would be the heli airframe.

Well said Bolders. I only have to dabble in the subject to choose mounts for motors that operate at set speeds or within a fairly tight range.

There is a huge amount of information on this specialist subject if you care to go down that route. I suspect you might just have to suck it and see then tweak if needed.

You are not wrong.. Dynamics and specifically vibration analysis is a subject in itself - one which involves a lot of maths .

[trillian]

This is true and to some degree it would be true with a heli also, however, the thing that imediately springs to my mind is that the mass of a heli is much less than a car and it is not grounded in any way.
Im thinking along the lines of newtons second law and every action has an equal and oposite reaction - the rubber mounts would need something to react against which in this case would be the heli airframe.

Well, think of it this way. If you mount the engine rigidly to the frame you will transmit quite efficiently any and all vibrations that the engine generates at their original frequency (whether the airframe resonates or not it is still receiving the vibes at whatever frequency they are.)

If you rubber mount and isolate the engine you have reduced the efficiency of transmisson of vibrations. So some of it will go bye bye because the engine will simple oscillate back and forth on its own. More than likely the high frequencies will not transmit very well through the rubber at all.

What the belt accomplishes is removing another transmission point for vibrations because the belt is very flexible compared to a metal pinion against a nylon gear.

It's like you do with sound waves, if you want to cut down on reflections in a room you use baffles made from a softer material and what happens is the sound waves simply lose energy and before that same vibration can be reflected it is essentially 'absorbed' due to the material being very inefficient at transmitting vibrations.

Anyway, my guess is that depending on how softly or rigidly you can get away with mounting the engine
you will get less high frequency energy to the airframe and maybe lose some frequencies entirely, whether
that ends up being very helpful remains to be seen.

[Gareth-71]

Hi

Hmmmn! The science behind this is broad and complex. There is a far easier solution to this....remove the vibration at source. The single 2t engine used is inherintly unbalanced as it has no counter balance shaft, however this is not the only major source of vibrations (the engine can also be tuned in such a way that vibrations are minimal with a limited range or rpm and load).

The major source of the vibrations are mostly secondry, this is in the flywheel / fan and more importantly in the cdi pickup / reluctance sensor / stator which cannot be statically balanced because it effectively an eddy current brake, even dynamic balancing will not correct this effect.
So all those 'blue printed and balanced' engines may be a whole lot better than stock, but not quite balanced

Solution? loose the fan, loose the cdi and use an optical pickup with a battery powered cdi system, pu a knock sensor on the engine and dynamically adjust ignition advance. this will seriously increase power output and reduce vibrations.

replace the mechanical fan with a temperature controlled electric fan....

As the distributor for heli bug kits, I am developing packages for gasser engines where we will loose a lot of weight and seriously increase power output. I am also good friends with some ex 2stroke gp engine designers and they don't have much work on anymore......

I have a 700 LE gaser conversion as the test rig and have advanced data logging electronics, including high freq acceleromoters for vibration.

It is a complex problem, but while perfection may be a while away, there certainly is a lot to be gained over what is available now.

Watch this space....

[pgkevet]

There's no argument that avoiding source of vibration is ideal..electric options exist.
You cannot totally lose vibration in any reciprocating system although once again I fully agree that engineering for our specil application helps. It sadly comes with a cost and I was brainstorming for a way to improve the end result but with a stock powerplant.
Amongst those options the trsuty zenoah has ruled but there are other gas engines..many lighter and with more power an cheap..but designed for planks and therefore lower revving...more vibe..and the perpetual cooling issue.
Cooling one can get around..electric fans should be the answer..just then how best to use them. It was the best way forward to handling the vibe one couldn't lose that i was after...

..probably a case of go play and see....and if doing that without the fancy measuring stuff then suspend the base and muck about...

[trillian]

There's no argument that avoiding source of vibration is ideal..electric options exist.
You cannot totally lose vibration in any reciprocating system although once again I fully agree that engineering for our specil application helps. It sadly comes with a cost and I was brainstorming for a way to improve the end result but with a stock powerplant.
Amongst those options the trsuty zenoah has ruled but there are other gas engines..many lighter and with more power an cheap..but designed for planks and therefore lower revving...more vibe..and the perpetual cooling issue.
Cooling one can get around..electric fans should be the answer..just then how best to use them. It was the best way forward to handling the vibe one couldn't lose that i was after...

..probably a case of go play and see....and if doing that without the fancy measuring stuff then suspend the base and muck about...

I think rubber mounts and a belt drive is the way for what you're talking about, using an off-the-shelf engine and just reducing the transmission of vibrations.

[J-S-Q]

Could be worth looking at the Kasama system of dampers on the engine mount (although this is still with a pinion drive).

[cbergen]

Just a point if I may...We do NOT rubber mount our engine as posted earlier, it IS hard mounted at 7 points into our G10 frames.

It has always been my contention that we have LESS of a vibration problem due to the use of the G10 frames, vs aluminum or Carbon Fiber.

Of course this is predicated that the engine is tuned properly, and the rest of the drivetrain is balanced and running true as well.

Rubber mounting an engine MAY be the answer, but if the engine is vibrating due to mistuning or components out of whack, how long do you think the rubbers will last?