Soft Mounts

Dose anyone have experience with soft mounts? Are they any good? do they reduce vibration? Do they cause any problems given out high pitch rates by gyroscopically twisting or pitching the engine on its mounts?

Windy and Bob Hunt have experimented. I believe Windy was not impressed.

Al - As I recall, Bob Hunt used them - possibly on his Saturn. I think he removed the lugs and used some sort of radial mount.

Bob Z

I used soft mounts on all of my RC Pattern planes with the .91 and 1.20 four stroke engines. They worked very well in that application.

The most important part of the mount was to make a soft ring around the front of the engine. This lessened the gyro movement of the prop in some of the violent snap maneuvers and allowed you to run softer cushions at the firewall. We all used firewall maounts - no beam mounts. My design came from Dick Hansen, who produced some great aircraft kits in the 70's and 80's.

If you are interested in the details, just drop me a note ([email protected]) and I'll try to explain it. It's not difficult to do, but it's easier to "show" than to "tell".

Hi Al,

I know that Werwage used them on one of his GeoXL planes. A side mount job. He was, according to the clever Werwage I got from Bob Hunt, not impressed. Said, it just didn't work out as planed.

Be an interesting experiment with a 4 Srroke.

The problem with Bob Hunt's system, as I understood it, was the grommets that created the "cushion". Seems the right "rubber" composition was not found. Wore out rather quickly.
Maybe Bob will comment?
Bill

Al,

I used them on my Golden Falcon for a short while. They really allowed the engine to move around with an ST 60. A large clearance between the spinner and cowl is necessary. Harder rubber compound mounts became available but I gave these to Windy, and I think he gave them to Billy.

I ended up replacing the rubber with four aluminum spools. I had eight but didn't need that many points. Windy eventually put those I had left over on his Sidewinder which had an OS 46VF. I think the prop disc moves in a tilt, as if the crankshaft axis was moving around a small 360 cone while it is running even in level flight, or static for that matter. When high G and GP is introduced in a corner these forces might be multiplied by the elasticity of the rubber mounts. This is probably a transient trim change that we don't need.

I replace the rubber Lord mounts on my Pitts Special every year, you can see a noticable sag between the spinner and cowl after about 30 hours of acro. It might be a complexity to a stunt model that is not really warranted.

Al,

I, like Ron King, have plenty of experience with soft mounts in several aircraft from R/C pattern days as well in my full size aerobatic planes. In the pattern planes we used a multitude of different soft mounts (Lord mounts as some called them also) of different hardnesses. As you can imagine the higher the G load and the more frequent the load, the faster the soft ones would tear or rip. So, the harder you flew, the harder the mounts ended up needing to be. As Ron mentioned you could use a soft ring to limit the deflection of the motor and go with softer mounts. I never used that set up on my planes but have seen them and it worked. I used them on everything from YS 60's to the larger 4 strokes. Most set ups were done for radial mounts but I have seen a few beam mount set ups... although that was many years ago. I think that given the right hardness there would be know reason that it should not work.

On another note or side as it may be. I have seen the results of prolonged G's on the lord mounts in larger acro planes too. I was flying a friends extra 300 and was on a vertical down line and was pushing hard at the bottom to a inverted recovery. It was only around a -6 g push and what I saw in front of me about made my heart stop. I was looking at almost the complete backplate of the spinner throughout the negitive push. I at that point I put it on the ground to be inspected! It was found that 3 of the four rubber mounts were torn and the forth not to far behind. Now I had seen the plane flown from the gound a hundred times but never saw the engine move. I just wonder how much a good stunt ship might displace the motor flying a good pattern?

Moral of the story...You can use a rubber mount but check them as they will fail at the most critical moment!!!!

I guess if you read that without a word or two and it still holds true. Hmmmm???

Godd vibrations to all,

Al

I asked Richard Oliver this and he said that if he started flying RC pattern again he would use them because he would be forced to. Otherwise he won't use them.

My soft mounted RC motors would shake softly around the crankshaft centerline at idle, then would go through a kind of reasonant period before smoothing out. The main advantage was that my RE piped 60s were barely audible in flight. And the radio gear was treated kindly. But I can't see it working on a CL stunter, with the concerns about precession amd trim.

R/C uses these for a couple of reasons, none of which is voluntary. First and foremost was to reduce noise for FAI limits. After exhaust has been all but silenced it was found to not be enough. There were 3 other factors causing as much noise. Prop, airframe, intake. The soft mount was brought about to reduce the airframe noise. A sideline effect was that the airframe lasted longer and radios suffered less vibration damage. 4 strokes solved the prop noise because of lower RPMs and since all of them are also rear intake the carb was now inside the airframe and included a natural muffling effect. These guys are paying 200.00+ for their mounts. The one real draw back was the high mortality rate of exhaust systems, headers and pipes. There is a lot of movement even with the front ring tie down. The idea is to have the motor torque sideways(axial) and not up and down. It is a maintenance item and I do not see many people in Stunt finding enough benefit to warrant the trouble. But hey if it becomes the thing to do I will be in line with the rest of Ya.
Richard Oliver

LAST EDITED ON Oct-23-03 AT 10:35 PM (CDT)
 
>Dose anyone have experience with soft mounts? Are they any
>good? do they reduce vibration? Do they cause any problems
>given out high pitch rates by gyroscopically twisting or
>pitching the engine on its mounts?

The precession issue is minor compared to the really big problem - inconsistent runs. David had a airplane with a radial soft-mount (on a PA51) and despite all sorts of things (like varying the stiffness of the rubber), it ran absolutely terribly - weak, never the same from flight to flight, really frustrating. He finally replaced the rubber gommets with hard aluminum standoffs, and it *instantly* ran perfect. He used it to win the 99 and 2001 NATs, I think. At this point it still flies and runs well, but the front end construction is really not what you want.

Another interesting item. The mount was an aluminum plate that bolted on with the backplate screws, and spread the mounting distance to about 1 3/4" square pattern. Once he hard-mounted the engine, he started breaking the plates, including some .100 6061 T6 plate. I beleive the current plate is 1/8" *steel*, and that seems to be lasting.

I'm not sure why something that works on R/C is a disaster in C/L, but that happens often. My own recent experience leads me to believe that soft mounts on standard C/L airplanes are a problem.

After many years being out of flying, I started again with some left over stuff. The airplane was a Goldberg Buster and the engine was an OS Max S 35. Just to be sure that I could survive the first flight I installed Brodak nylon shims under the engine mounts to provide a 1 degree engine offset. On the first flight, the vibration was so bad that I could see the line shaking, and could feel it in the handle. I balanced the prop and tried it again with the same result. Lacking any other lever, I removed the nylon shims and replaced them with phenolic pieces with the same engine offset. It was a whole different experience! The engine was smooth, the airplne flew well, and I found out how much my reflexes had deteriorated over the 40 years since I stopped flying. At least in a profile, softer mounts don't work. I have since found that I don't need as much engine or rudder offset as I remember.
Steve

The backplate mount in the Golden Falcon is .100 T6 and I have never had a problem with cracking on the Tigre 60. I have drilled a lot of lightening holes in it too.

I never got a good run on my 60 until I put the hard spools on it either. Windy had more success with the OS 46 VF but it ultimately ended up hard mounted so maybe run characteristics was the motivator there too.

Agreed that this is not the ultimate type of mounting. I have gone back to maple bearers.