Rotax 912 Movement

[skippydiesel]

Have done two 1.5 hr flight. Checked for excessive movement - unfortunately still too much.

 

Next step- will purchase the Barry 22001-14 mounts.

 

To try & preserve as much of the Sonex recommended system (22001-13) I will start by replacing the lower donuts with the  14 donuts  and see if that has the desired effect.

[skippydiesel]

1. "To try & preserve as much of the Sonex recommended system (22001-13) I will start by replacing the lower donuts with the 14 donuts  and see if that has the desired effect."

 

As a first step, I think the above is small,  therefor may not result in sufficient movement reduction, so planning for the future (if needed);

 

2. The lateral movement, viewed from the cockpit, is to the left. With this in mind, could step two be replacing the two - 13 mounts, on the left side only, with the stiffer -14's. What might be the negative effect of mixing two diffrent mount stiffness (Note: All 22001 mounts have the same dimensions)???

 

3. It seems that the rear/aft mounts are being subjected to more movement that the front two - Should I consider replacing the aft mounts with -14's leaving the front two -13's? As in 2, what might be the negative effects??

 

4. I will be purchasing 4 X - 14 mounts, so if step two/three don't have the desired effect OR are skipped (due to advice/reflection) I will just replace all -13's with -14'. Aside from checking for desired movement reduction, I would appreciate any advice on what I should monitor ??

 

 

Edited January 16 by skippydiesel

[LoonyBob]

8 hours ago, skippydiesel said:

1. "To try & preserve as much of the Sonex recommended system (22001-13) I will start by replacing the lower donuts with the 14 donuts  and see if that has the desired effect."

 

As a first step, I think the above is small,  therefor may not result in sufficient movement reduction, so planning for the future (if needed);

 

2. The lateral movement, viewed from the cockpit, is to the left. With this in mind, could step two be replacing the two - 13 mounts, on the left side only, with the stiffer -14's. What might be the negative effect of mixing two diffrent mount stiffness (Note: All 22001 mounts have the same dimensions)???

 

3. It seems that the rear/aft mounts are being subjected to more movement that the front two - Should I consider replacing the aft mounts with -14's leaving the front two -13's? As in 2, what might be the negative effects??

 

4. I will be purchasing 4 X - 14 mounts, so if step two/three don't have the desired effect OR are skipped (due to advice/reflection) I will just replace all -13's with -14'. Aside from checking for desired movement reduction, I would appreciate any advice on what I should monitor ??

 

 

Mixing 14's with 13's will "detune" the system, giving two close but separate natural frequencies - just like dynamic counterweights on a Lyconental crankshaft. This can only be good (see below). Apart from that issue, the 14s will transmit a tad more force, which is what you need to limit engine motion, etc.

 

Replacing all the 13's with 14's will not give you that cunning detuning turnip, but will push the natural frequency up maybe 10~15%, which should make naff all difference (if there was something else on the donk that might be triggered to resonate by a 15% higher excitation frequency, it woulda resonated by now (lumps of rubber being not quite the same as tuning forks). Remember, I'm talking about an idle rpm resonance, or less, of the engine & prop.

 

From memory, some Ponk conversions in Cessnas offer "standard" or "heavy duty" engine mount rubbers...

 

[skippydiesel]

So LB, on balance you approve of my, if needed, three step approach and can see no problems - is this correct?

[LoonyBob]

6 minutes ago, skippydiesel said:

So LB, on balance you approve of my, if needed, three step approach and can see no problems - is this correct?

Totally. I think you've set out a very sensible approach. Go for that 50 hours of satisfactory operation!

 

[skippydiesel]

Have fitted Barry 22001-14 mounts ( flown 1.4 hrs & 2 engine starts).

 

The suppliers of the mounts Shock & Vibration Technologies (SVT), with 30 years experince in the field, strongly recommended fitting the whole mount, without the Sonex washers,  rather than the incremental approach, I proposed.

 

If there is an increase in noise & vibration I didn't realy notice - if pressed, I might say that HF vibration , felt through  control stick, may have increased very slightly.

 

The stiffer mounts have reduced lateral movement significantly BUT I am still getting a "brush" (no longer a dent) marking on my main concern point.

 

Next step? - refit the Sonex supplies spacing washers to increase compression of rubbers, there bye further stiffen the mounting system.

[turboplanner]

19 minutes ago, skippydiesel said:

The stiffer mounts have reduced lateral movement significantly BUT I am still getting a "brush" (no longer a dent) marking on my main concern point.

 

Next step? - refit the Sonex supplies spacing washers to increase compression of rubbers, there bye further stiffen the mounting system.

You now have an accurate data point. How about measuring from whatever is brushing to the brushing marks, and check with the Manufacturer if that movement is within expected tolerances. If "No", more work needed on the mounts. If "Yes", can you put a bulge in the brushed item (if non-structural)?

[skippydiesel]

Thanks Turbs - I appreciate your input and support.

 

A flying colleague has already suggested the application of a ball peen hammer. Unfortunatly the main concern point, is a section of engine support frame. I don't think denting/bending would be appropriate, as frame (designed for the AroVee engine) is , in my view, marginal.

 

The left exhaust pipe (header?) is the offending engine part -  the only long term solution may be to remake this part, with a greater radius bend (than already has) to clear the frame. In the first instance, I try for the minimum effort & cost solution - this approach is possibly close to exhausted.