Rotax 912 Movement

[skippydiesel]

(Aviation topics few & far between, debate degenerating somewhat. Thought  I would float this matter,  in the hope of returning to more engineering focused conversation.)

 

Sonex aircraft were originally designed to be pulled aloft by an AeroVee (developed from VW) engine. In later years, the factory accepted/supported Jab (4 & 6 cylinders). Rotax installations were not supported at all.

 

Anyone fitting a Rotax 9, was on their own  - builders had to develop their own engine attachment & cooling systems. The primary demand for Rotax power would seem to come from Europe (naturally) and the Antipodes, with some builders in South America & elsewhere.

 

Very recently, Sonex recognition of Rotax demand, took the form of supplying a Rotax /to existing Sonex engine mount, adapter. This was quickly superseded by a second adapter (photo).  A little later, Sonex@ Oshkosh promotion, saw their scratch built kit,  powered by a Rotax 912 ULS - it flew at the end of the event (seems likely it had engine temperature issues - yet to be resolved).

 

 

 

 

As you see the adapter consists of two (poppy red) L shaped brackets. Top attached to engine lower crankcase (4 points). Bottom to Sonex frame (4 points). For vibration/movement control, the adapter uses 4  X Hutchinson/Barry Control 22001-13, mounting rubbers. (I believe that this adapter was developed by a private Sonex/Rotax builder and adopted by Sonex. The original system used 6 x Barry mounts part number ??).

Shortly after supplying this mount, Sonex issued an Advisory Notice: offering two modified spacing washer X 4, to be inserted between the bottom rubber & lower bracket (you can just see them in the photo - not to be confused with the single spacing washer between top rubber and bracket). The reason given was the inner metal sleeve, of the Barry mount, bottoming before rubber compression ie mount not delivering adequate movement control. Sonex also supplied, at the same time, a length of chrome alloy tube, to be cut to length & then welded onto a nominated position on both sides of the Sonex mount, presumably to reduce frame flexing. Note: I have not installed this modification.

 

Subjectively the mounts, even with the addition of the Sonex spacing washers, is not controlling the movement of the engine (mainly @ shutdown & start) - Symptoms : rub/strike marks on the inside of the cowling.

 

I would like to explore the fitting of Barry 22001-14  mounts, the next step up in "stiffness" (same external dimensions) to reduce engine movement to an acceptable level. I am aware that there may be a subjectively noticeable increase in the transmission of engine vibration.  Other than this, I do not know what other negative factors may result.

 

My questions to the Brains Trust are:

 

• What negative impacts, would you  be concerned about, in the fitting of the next level of Barry mount stiffness???
• Would returning to a  x 6 Barry mount system (as per original design) be a "better" solution (may involve softer mounts PN 2201-12 or 11) & why??
• Any other concerns??

 

 

[gareth lacey]

I have made my own mounts and used Nolathene 80, 38 dia and machine and drilled 10mm  28 thickness (different engine , but the stiffness is great

[facthunter]

I'm surprised that anyone would think those crude mounts would work Mounting aero engines is a bit  of science. A big bit. which put's loads on engne cases and has to handle forces in many directions and affected by gravity  and gyroscopics and dynamic engine loads.Nev

[IBob]

Skippy, what weight of prop do you have?
And what are your engine starts and stops like?

 

I ask because I have a reasonably light Bolly 70" prop, and that coupled with the improved Rotax ignition retard, plus a bit of practise and understanding gives me very smoothe starts and stops.
Our club Tecnams, however, have the much heavier prop mandated by the manufacturer and engine start/stops can be quite ugly, no doubt resulting in considerable engine movement.

[skippydiesel]

4 minutes ago, facthunter said:

I'm surprised that anyone would think those crude mounts would work Mounting aero engines is a bit  of science. A big bit. which put's loads on engne cases and has to handle forces in many directions and affected by gravity  and gyroscopics and dynamic engine loads.Nev

I don't disagree with you Nev - but could do with some constructive criticism ie working with what I have.

 

FYI - Rotax Engine Mounting ; General consensus is, that a ring mount is the best. There is a OM Rotax ring mount, however Sonex has yet to adopt it (I live in hope). My last Rotax/Aircraft used an 8 point "rubber" isolator system that worked (very fiddly to get engine in/out).

[skippydiesel]

1 minute ago, IBob said:

Skippy, what weight of prop do you have?
And what are your engine starts and stops like?

 

I ask because I have a reasonably light Bolly 70" prop, and that coupled with the improved Rotax ignition retard, plus a bit of practise and understanding gives me very smoothe starts and stops.
Our club Tecnams, however, have the much heavier prop mandated by the manufacturer and engine start/stops can be quite ugly, no doubt resulting in considerable engine movement.

Prop is relativly heavy - Airmaster CS/ 3 blade. I have little doubt it contributes to the the engine movement on shutdown (not sure about start)

 

I go to a lot of trouble, to try and mitigate the engine movement on shutdown; slow idle - one ignition off - pause to stabilise -  second ignition off.

[skippydiesel]

27 minutes ago, gareth lacey said:

I have made my own mounts and used Nolathene 80, 38 dia and machine and drilled 10mm  28 thickness (different engine , but the stiffness is great

Photo's???

[Blueadventures]

32 minutes ago, skippydiesel said:

(Aviation topics few & far between, debate degenerating somewhat. Thought  I would float this matter,  in the hope of returning to more engineering focused conversation.)

 

Sonex aircraft were originally designed to be pulled aloft by an AeroVee (developed from VW) engine. In later years, the factory accepted/supported Jab (4 & 6 cylinders). Rotax installations were not supported at all.

 

Anyone fitting a Rotax 9, was on their own  - builders had to develop their own engine attachment & cooling systems. The primary demand for Rotax power would seem to come from Europe (naturally) and the Antipodes, with some builders in South America & elsewhere.

 

Very recently, Sonex recognition of Rotax demand, took the form of supplying a Rotax /to existing Sonex engine mount, adapter. This was quickly superseded by a second adapter (photo).  A little later, Sonex@ Oshkosh promotion, saw their scratch built kit,  powered by a Rotax 912 ULS - it flew at the end of the event (seems likely it had engine temperature issues - yet to be resolved).

 

 

 

 

 

As you see the adapter consists of two (poppy red) L shaped brackets. Top attached to engine lower crankcase (4 points). Bottom to Sonex frame (4 points). For vibration/movement control, the adapter uses 4  X Hutchinson/Barry Control 22001-13, mounting rubbers. (I believe that this adapter was developed by a private Sonex/Rotax builder and adopted by Sonex. The original system used 6 x Barry mounts part number ??).

Shortly after supplying this mount, Sonex issued an Advisory Notice: offering two modified spacing washer X 4, to be inserted between the bottom rubber & lower bracket (you can just see them in the photo - not to be confused with the single spacing washer between top rubber and bracket). The reason given was the inner metal sleeve, of the Barry mount, bottoming before rubber compression ie mount not delivering adequate movement control. Sonex also supplied, at the same time, a length of chrome alloy tube, to be cut to length & then welded onto a nominated position on both sides of the Sonex mount, presumably to reduce frame flexing. Note: I have not installed this modification.

 

Subjectively the mounts, even with the addition of the Sonex spacing washers, is not controlling the movement of the engine (mainly @ shutdown & start) - Symptoms : rub/strike marks on the inside of the cowling.

 

I would like to explore the fitting of Barry 22001-14  mounts, the next step up in "stiffness" (same external dimensions) to reduce engine movement to an acceptable level. I am aware that there may be a subjectively noticeable increase in the transmission of engine vibration.  Other than this, I do not know what other negative factors may result.

 

My questions to the Brains Trust are:

 

• What negative impacts, would you  be concerned about, in the fitting of the next level of Barry mount stiffness???
• Would returning to a  x 6 Barry mount system (as per original design) be a "better" solution (may involve softer mounts PN 2201-12 or 11) & why??
• Any other concerns??

 

 

Need longer bolts for the mounts as no thread protruding and what is the spacer under the oil banjo for?  Any images of the mount to firewall fixing?

 

[rodgerc]

For what it’s worth, my AeroVee powered Waiex’s engine mounts were replaced at every annual, because after a year and 25-35h flying they were pretty well shot. 
 

I suspect they were EPDM of around Shore A, 80 hardness. 
 

Given thermoset urethane is readily available and easy to cast in a silicone mould, have you considered casting your own larger mounts from polyurethane to spread the load?

[skippydiesel]

1 minute ago, Blueadventures said:

Need longer bolts for the mounts as no thread protruding and what is the spacer under the oil banjo for?  Any images of the mount to firewall fixing?

 

Thread protruding - You are correct. I have taken delivery of longer bolts however as I may be "tuning" the set up, I felt the originals would suffice in the short term (rear bolts have 3 threads showing).

 

 The spacer under the oil banjo  - This is a factory option to allow for easier installation of the rubber hose & clamps.

 

The mount to firewall -  standard Sonex hard bolted (no rubbers), includes main undercarriage attachment.

 

 

Back to engine mounting please.

[skippydiesel]

6 minutes ago, rodgerc said:

For what it’s worth, my AeroVee powered Waiex’s engine mounts were replaced at every annual, because after a year and 25-35h flying they were pretty well shot. 
 

I suspect they were EPDM of around Shore A, 80 hardness. 
 

Given thermoset urethane is readily available and easy to cast in a silicone mould, have you considered casting your own larger mounts from polyurethane to spread the load?

Hmm! - 25-35 hrs of service seems a tad disappointing .

 

No I hadn't considered making my own "rubber" isolators/mounts. With Barry mounts available in stiffer shore ratings, I felt that this is possibly the most expedient way to go BUT am concerned that I may be causing an unforeseen (by me)  post installation negative effect - your thoughts?

[rodgerc]

I’d be inclined to look at a larger mount rather than a stiffer compound. Simply on the basis of force/area = pressure. (Stress)

 

For information only, the (during construction) pics below are the RANS bed-mount installation in my Raven. 

After ~90h flying over 18 months they are standing up very well with no noticeable compression set/sag. 

 

I suspect they are natural rubber of around 80A durometer. 

[IBob]

27 minutes ago, skippydiesel said:

Prop is relativly heavy - Airmaster CS/ 3 blade. I have little doubt it contributes to the the engine movement on shutdown (not sure about start)

 

I go to a lot of trouble, to try and mitigate the engine movement on shutdown; slow idle - one ignition off - pause to stabilise -  second ignition off.

FWIW:
My idle jet and idle throttle stop settings are as per the Rotax manual. (I checked them at new and one of them was a bit out from the factory, can't remember which one but it's a very quick setup, and evidently worth doing).
With those settings, the tacho drops towards 1400 when pulled to the stop, and I switch off the ignitions almost simultaneously as it is getting there.

 

[skippydiesel]

Veeeery nice Rodgerc. Certainly look like they have been designed/constructed  to aircraft  engineering standards (unlike my Sonex ones)

[skippydiesel]

2 minutes ago, IBob said:

FWIW:
My idle jet and idle throttle stop settings are as per the Rotax manual. (I checked them at new and one of them was a bit out from the factory, can't remember which one but it's a very quick setup, and evidently worth doing).
With those settings, the tacho drops towards 1400 when pulled to the stop, and I switch off the ignitions almost simultaneously as it is getting there.

 

As are mine IBob - your RPM figures are much the same as mine.

 

When you look at Rodgerc's photos ,of his  very nice engine mount (same orientation as mine) in comparison with my very primitive Sonex offering, you can understand why I might think a "stiffer" set of rubbers may be the expedient answer.  

 

Still hoping for some discussion on the potential for negative outcomes, should I fit slightly stiffer rubbers.

[facthunter]

Just more stress in anything involved and more likelihood of vibration and rumbling being noticed in flight. I wouldn't go there. It needs GIVE with a soft stop.  Large cross sectional area in a cup.    Nev

[pluessy]

What compounds the problem on your Sonex is that the angles move the rubber mounts closer together. Even small movements in the mounts will result in large movements of the whole engine.

On my Tecnam, the angles are turned out and the mounts located further apart. The engine still shakes on start-up and shut-down but not touching any fixed parts due to the reduced movements.

[skippydiesel]

4 minutes ago, facthunter said:

Just more stress in anything involved and more likelihood of vibration and rumbling being noticed in flight. I wouldn't go there. It needs GIVE with a soft stop.  Large cross sectional area in a cup.    Nev

Just like Rodgerc's ones - but that's not what I have.

 

Until Sonex come up with a better design, I am stuck making the best of what I have.

[IBob]

What Nev says feels right: stiffen it up and you increase the loads elsewhere.

My first thought was to try and limit movement with smooth running, including starts and stops, hence the the question about prop weight which seems to make a big difference in that respect.

[rodgerc]

Would you be able to crib an extra 1/4” on the diameter of the mount? (I.e. Dimension A)

 

If yes, then personally I’d try that by having a custom set cast in 80A durometer PUR before switching to a stiffer (higher modulus) compound….It would improve the vibration dampening characteristics of the engine mount.

 

When comparing the dimensions of the 22001 rubber components with the Lord mounts used on a (albeit considerably heavier) Conti 0-200, it does seem to be rather a big ask. 

[skippydiesel]

Rodgerc,

 

"................extra 1/4" on diameter..........."

"If yes, then personally I’d try that by having a custom set cast in 80A durometer PUR before switching to a stiffer (higher modulus) compound….It would improve the vibration dampening characteristics of the engine mount."

 

Makes sense to me - essentially a wider cushion =  more control for the same durometer/stiffness.

 

Perchance you have in mind, some possible makers of custom mounts???

 

Edited January 1 by skippydiesel

[skippydiesel]

Rodgerc,

 

What of going to the 22002 series  - No 11.  Larger dimensions - would require drilling out my existing mount brackets.  Axial Load 10LB less Radial Load 10Lb more ?

[Thruster88]

Rogerc's post has given me the idea of having bump stops installed. These could be out board of the isolation, something very stiff with a mm or 2 of clearance. Automotive engine mounts have this design. Isolation when running, travel limited when stopping, win win. 

Edited January 1 by Thruster88

[rodgerc]

If it were me, I’d initially keep life simple by sticking with all 22001 dimensions except the OD. 
 

Some (very) basic stuff assuming a 70kg engine and prop combo….Call it 700 Newtons for simplicity. 
 

Original mount net area 790mm^2 x 4off = 3,160mm^2. 
 

Compressive stress = 790N/3,160mm^2 = 250kPa

 

 

With a 1-1/2” OD the total bearing area increases to 4,216mm^2 or 33% more. So compressive stress decreases to 790N/4,216mm^2 = 187kPa.
 

By implication, the compressive strain on the mounts will also be decreased by about 1/3 and along the way, you’ll have increased the surface area to dissipate all that vibration (kinetic energy) heat into the aluminium mounting rails.
 

Naturally, the back-up washers will also need to be increased in diameter.

 

It’s pretty easy to cast urethane and there’s a bunch of videos on YouTube showing people doing it at home….Search “casting polyurethane bushings” and pour yourself a drink. 
 

If you’re still unsure of your ability (it’s easy) then pop over to 33 Holloway Dr Bayswater and talk to Melbourne Polyurethane. 

[rodgerc]

Just reread my post and realised I used 790N instead of 700N….Despite the error, the outcome remains the same.