Rotax 912 Movement

[skippydiesel]

On 01/01/2024 at 8:11 PM, Blueadventures said:

How would some purpose made cup washers go at retaining the rubber shape and compressibility better?

Have come back to your KISS suggestion Blue - not so hard or expensive to do and I can see that IF the problem is the bottom rubbers are allowing too much movement, by "cupping" them the effect is likely to be less movement due to the rubbers being contained.

 

This concept, l retains the  original dimensions & the vibration damping of the, under constant compression rubbers above.

 

So far my search for suitable "cup washers" has come to naught - so once again your suggestion of making my own may be the way to go.

[turboplanner]

23 minutes ago, skippydiesel said:

Have come back to your KISS suggestion Blue - not so hard or expensive to do and I can see that IF the problem is the bottom rubbers are allowing too much movement, by "cupping" them the effect is likely to be less movement due to the rubbers being contained.

 

This concept, l retains the  original dimensions & the vibration damping of the, under constant compression rubbers above.

 

So far my search for suitable "cup washers" has come to naught - so once again your suggestion of making my own may be the way to go.

Sounds good; you’re already close so little bites still give you a good indication of improvement. 

[skippydiesel]

Hi Turbs; The following is the essence of my letter to Sonex technical - I would be intersted in your thoughts around using  40 mm ID  OR 35 MM ID Cupped Snubbing Washer??

 

"My agriculture  interpretation of how the Hutchinson Barry Controls 22001-13 mounts works:

 

(For the purpose of discussion, I am separating & exaggerating, the function of the upper t & the lower mount. Realise that when correctly installed they work together)

• The upper part of the mount is responsible for vibration control.
• The lower part (donut) for movement control.

If I contain the lower mount (donut), in a “cupped washer”  ie flat washer with vertical sides of suitable dimension. The donut movement would be better restricted ie resist the lateral movement of the mount and thus the rotational movement of the engine?

 

The Barry 22000 Snubbing Washer PN 9810145-01804 is 1.56” (39.624mm) x .391”(9.9314mm) x 0.90” (2.286 mm) - I can source the metric equivalent locally (40 x 10 x 2.6mm). To this I can weld a ring 8mm high (donut is .485” (12.319mm) high/thick), thus making a Cupped Snubbing Washer.

 

Alternatively:

 

 The pre installation dimensions of the donut,  is 1.31” (33.274mm) diagonally – I can make a  Cupped Snubbing Washer, using a smaller 35 mm OD washer. This would, on installation/tightening up, restrict the sideways spread of the donut, resulting in some pre load/compression, that may further reduce the movement of the lower mount."

[turboplanner]

2 hours ago, skippydiesel said:

Hi Turbs; The following is the essence of my letter to Sonex technical - I would be intersted in your thoughts around using  40 mm ID  OR 35 MM ID Cupped Snubbing Washer??

 

"My agriculture  interpretation of how the Hutchinson Barry Controls 22001-13 mounts works:

 

(For the purpose of discussion, I am separating & exaggerating, the function of the upper t & the lower mount. Realise that when correctly installed they work together)

• The upper part of the mount is responsible for vibration control.
• The lower part (donut) for movement control.

If I contain the lower mount (donut), in a “cupped washer”  ie flat washer with vertical sides of suitable dimension. The donut movement would be better restricted ie resist the lateral movement of the mount and thus the rotational movement of the engine?

 

The Barry 22000 Snubbing Washer PN 9810145-01804 is 1.56” (39.624mm) x .391”(9.9314mm) x 0.90” (2.286 mm) - I can source the metric equivalent locally (40 x 10 x 2.6mm). To this I can weld a ring 8mm high (donut is .485” (12.319mm) high/thick), thus making a Cupped Snubbing Washer.

 

Alternatively:

 

 The pre installation dimensions of the donut,  is 1.31” (33.274mm) diagonally – I can make a  Cupped Snubbing Washer, using a smaller 35 mm OD washer. This would, on installation/tightening up, restrict the sideways spread of the donut, resulting in some pre load/compression, that may further reduce the movement of the lower mount."

I'd let them amswer first. They may know an owner who has a successful combination.

 

I don't think its as simple as some people say because you've got all the different forces taking place within the engine to deal with and the harmonics vary with rpm, so you cam fix one area and make another worse.

 

My own experience with race car installations went from several cracks after each race meeting to a race meeting with no cracks and then, by solid mounting the engine to the frame a season with no cracks and years with no cracks, but I would not recommend solid mounting in the much lighter aircraft frame. It's as I said before, small changes, then see what happens at all RPM, then another etc. With small changes you can see if you're getting anywhere, but make big changes and you have to start the refining process all over again.

[skippydiesel]

"I'd let them amswer first. They may know an owner who has a successful combination."

 

In the last 12-18 months I have tried to get advice from Sonex, the result has been the same each time - can't comment on something we have not tested.

 

The reality is Sonex have only recently "recognised" Rotax, so have little factory understanding/experience of the engines characteristics, other than the one aircraft built at last years Oshkosh (stil on the ground possibly due to cooling issues). The few Rotax "accommodations" that they offer, seem likely to have come from the homebuilt community. This is fine but I am disappointed that they won't even enter in to well reasoned comments on my suggestions.

 

In short - nothing from Sonex .

[skippydiesel]

Discretion being the better part of valour - I will proceed with the 40mm ID home made Cupped Snubbing Washer.

 

Should they prove insufficiently effective, will move to the 35 mm ID Cupped Snubbing Washer.

 

In thinking about this idea (retaining rings/shoulders) I have suggested what I believe would be an even better modification - attach the rings to the underside of the lower rail. The snubbing washer would still be used but without cupping ring. This way the movement of the bottom of the Barry mount (donut) will be better contained and the donut less likely to distort.

 

 

[IBob]

Continuing to follow (and enjoy) your Sonex voyage of discoveries here, Skippy. It continues to raise interesting stuff I'd never thought about.

I know where there's as Sonex parked at the back of a hangar...it almost makes me want to look at tinkering...........)

[Thruster88]

If those rubber mounts are 40mm that puts the bolt spacing at about only 110mm.  I think this will always be a problem.  

[BurnieM]

Have you looked at the mounting systems on other 912 equipped aircraft ?

 

Something like the Evektor or Sling who have been using 912 for a long while.

[skippydiesel]

2 hours ago, Thruster88 said:

If those rubber mounts are 40mm that puts the bolt spacing at about only 110mm.  I think this will always be a problem.  

The  rubber start out/preassembly with a 32 mm diameter . The mount is compressed (to an inner steel tube length) by tightening the bolt/nut. The upper/lower donuts (my technical term)  are squeezed, causing outward expansion.

 

The snubbing washer  at the bottom (see photos at start of conversation) has a 40 mm diameter

[skippydiesel]

1 hour ago, BurnieM said:

Have you looked at the mounting systems on other 912 equipped aircraft ?

 

Something like the Evektor or Sling who have been using 912 for a long while.

Hi Burnie,

 

If you had read through the conversation, you would know that I am fully aware of the alternative (better?) Rotax mounting systems - this is the one that I have "inherited" & is supplied/endorsed by Sonex. Whatever it shortcomings may prove to be, I am bound to try and make the system work , at least until Sonex comes up with a better engine mount.

[turboplanner]

7 hours ago, skippydiesel said:

"I'd let them amswer first. They may know an owner who has a successful combination."

 

In the last 12-18 months I have tried to get advice from Sonex, the result has been the same each time - can't comment on something we have not tested.

 

The reality is Sonex have only recently "recognised" Rotax, so have little factory understanding/experience of the engines characteristics, other than the one aircraft built at last years Oshkosh (stil on the ground possibly due to cooling issues). The few Rotax "accommodations" that they offer, seem likely to have come from the homebuilt community. This is fine but I am disappointed that they won't even enter in to well reasoned comments on my suggestions.

 

In short - nothing from Sonex .

OK, 

[skippydiesel]

Moving forward:

Cupped Snubbing Washers have been installed and the engine ground run - everything looks good/hopeful.

 

[Blueadventures]

On the way, looking good; patent pending🙃

Edited January 6 by Blueadventures

[LoonyBob]

Nice!

[danny_galaga]

Im no expert, but just a slightly left field idea- could it help if, instead of those 8 rubber bushes, you had 4 complete 'blocks' of rubber instead? Maybe with an extra bolt each side to increase to 6 bolts? Thereby not having to make any new brackets etc.

Edited January 7 by danny_galaga

[facthunter]

It's too narrow. Wider and on an angle please.  Nev

[skippydiesel]

3 hours ago, danny_galaga said:

Im no expert, but just a slightly left field idea- could it help if, instead of those 8 rubber bushes, you had 4 complete 'blocks' of rubber instead? Maybe with an extra bolt each side to increase to 6 bolts? Thereby not having to make any new brackets etc.

Its really 4 rubber assemblies.

I believe the original design, by a Sonex home builder, has 6 rubber mounts. They may have been softer, than the 4,that Sonex (the factory) supply at the moment.

I hope that Sonex will adopt one of the better designs that various Sonex/Rotax builders have come up with - some real beauties, mostly incorporating the Rotax ring mount or making their own variant- they  "look the goods"!

 

[skippydiesel]

"...........complete 'blocks' of rubber ............."

 

I am sure Nev or one of the other aircraft experts could wax lyrical on this topic, however I give it a shot;

 

Engine mounts for aircraft must contend with far more than a similar sized (weight/power/etc) automotive engine.

 

There are the rotational forces generated by both engine & prop that must be controlled (lateral movement) -Vibratiofrom engine/prop ,which is not just annoying to the pilot can/will destroy an airframe - Longitudinal movement due to weight shift from power on/off landings & high speed taxi. - Changes in orientation (deliberate/accidental aerobatics) All of this in very light chassis/airframe that does not have the added damping of ground contact.

 

The aircraft engine mount must control all these, while being a light as possible, hence the popularity of various Ring mounts, giving 360 degree controle and some Angle mounts, possibly not quite as effective but a potentially lighter option, if aircraft unlikly to be used for aerobatics. 

 

 

[facthunter]

The mounts are also a design failure point  IF too strong some other part will have to give in an overload situation.  Vulcanised rubber in  tension can't be part of it reliably. It's also carrying the mass of the exhaust system , torque, gyroscopic and airframe "G" loads.   Nev

[danny_galaga]

3 hours ago, skippydiesel said:

"...........complete 'blocks' of rubber ............."

 

I am sure Nev or one of the other aircraft experts could wax lyrical on this topic, however I give it a shot;

 

Engine mounts for aircraft must contend with far more than a similar sized (weight/power/etc) automotive engine.

 

There are the rotational forces generated by both engine & prop that must be controlled (lateral movement) -Vibratiofrom engine/prop ,which is not just annoying to the pilot can/will destroy an airframe - Longitudinal movement due to weight shift from power on/off landings & high speed taxi. - Changes in orientation (deliberate/accidental aerobatics) All of this in very light chassis/airframe that does not have the added damping of ground contact.

 

The aircraft engine mount must control all these, while being a light as possible, hence the popularity of various Ring mounts, giving 360 degree controle and some Angle mounts, possibly not quite as effective but a potentially lighter option, if aircraft unlikly to be used for aerobatics. 

 

 

Hence why I thought instead of those bushes, which obviously must have been thought to be at least nominally ok, you replace the bushes with a block. You would at least have more longitudinal rigidity without having to change anything else. I'm not sure people are picturing what I mean so I'll give an example. PLEASE these are just numbers for an example.

 

Say you have a mount like yours. With 4x rubber bushes of dimensions 50mm diameter and 25mm thick. The distance between the bolts is 200mm. Instead of those bushes, you have a 'block' of rubber that is 250mm long, 50mm wide and 25mm deep. 

 

That seems to me that it would have more rigidity than the 50mm bushes.  I'm not saying it's the solution, but you also haven't said anything I didn't already know.

 

The blocks must surely be somewhat better than the bushes, which remember are considered ok else no one would fly with that setup. Again I don't know that it's the answer, but an improvement on something already being accepted, whilst not having to change anything else.

[facthunter]

Shock absorbers use exotic bushes on the Pin end. ( Hole in the middle). Truncated cones, small ends facing each other  would be OK. .  Nev

[skippydiesel]

Danny-G You also must have some way of holding the engine down, while still allowing some movement hence the "rubber" is on both sides of the mounting system. My (crude) understanding of how it works:

• The upper part of the mount is responsible for vibration control.
• The lower part (donut) for movement control.
• The above combination, with a steel tube & nut/bolt through the middle & fixed to the top plate, keeps the whole caboodle in place, while allowing a certain amount of movement.

[danny_galaga]

Sorry I thought it went without saying that you are replacing each pair of bushes with a block. I think there's 4 pairs of bushes in your photo? So 4 blocks. Instead of washers you would sandwich the 'outside' bushes with a strip of metal 250 X 50. It all adds up to just more of what you wanted (vibration absorption) in the same space. Did you say that mounting system was lasting 30 hours or something? With all that extra rubber sandwiched in there, maybe it could make it to the 100 hour mark...

[skippydiesel]

Danny_G

 

My first instinct was to replace the existing mounts (bushes) with the, same design/dimensions, next level (durometer) stiffness - go from a Barry Controls Mounts 22001-13 to a 22001-14. My only concern with this idea , was the potential for the stiffer mounts to generate unforeseen (by me) problems (I was ready to accept some small increase in vibration- already very low)

 

One of the respondents (Rodgerc), pointed out that the Barry mounts are narrow/diameter and a wider mount (at the top/bottom) of the same material would have the effect of reducing lateral movement, while retain the same vibration damping. Great idea -but  where to source - possibly from a custom maker - potential high cost, compounded by possible need for several sets of different durometer to find best set.

 

No, the 30 hr lifespan was Rodgerc with an AeroVee engine - hopefully different problem - my engine/mounts already have close to 60 hrs without any sign of wearing out.

 

I would hope to get, at the very least, 5 years (Rotax rubber replacement) and 500 hrs or more from a set of mounts.

 

The difficulty with your idea (for which I thank you) is that there must be a way of having a through fixing/bolt, that is not in direct contact with the plate it is going through. In the Barry mount this is achieved by having the bolt inside a steel tube, that is bonded to the top structure and has a thick layer of rubber around it isolating it from the bottom plate. The steel tube also acts as a spacer, the bolt/nut being torqued up tight (30ft/lb)

 

Edited January 8 by skippydiesel