I have been a great fan of the Dubro Motor mounts http://www3.towerhobbies.com/cgi-bin...&I=LXD630&P=SM , and have them on most of my planes. An interesting issue came up on my Ruperts Dad.

I found myself doing donuts if I got on the throttle to quickly. However the donut is to the right, not the left as I would expect. I first chased a radio issue, then just scratched my head for lack of a possible cause. During my last day flying, I peaked the needle valves and was testing the transision on the Mag 70 Four stroke. I noticed that when I punched it, the engine twisted to the right, to the maximum move of the rubber mounts. I fly a 13x4W APC prop and the engine/prop gives a lot of torque, apparently to much for the mounts.

I can cut some hard rubber replacement groments, but I was wondering if any one had any other ideas or had experienced this. I flew these mounts on my trainer, and a couple scratch builds, all with O.S. 52 four strokes truning a 12 1/4 x 3 3/4 APC, and my 4*60 with a Mag 91 turning a 14x4W APC, and never saw this issue before. Or at least they didn't behave the same. The Ruperts Dad has 2 degrees right built in, and I'm guessing the mount is shifting another three to four degrees when I punch it.

Don

The only way to prevent changing thrust issues and still have a soft mount is to use a nose ring. This is the norm in pattern where the mounts are used to reduce noise and to extend the life of the airframe and gear, generally a Hyde mount with a nose ring (various makers.) The soft mount allows mainly axial absorbtion, but will allow the thrust line to move. The nose ring stops the thrust line from moving and keeps all absorption of the mount axial. A stiffer mount will help reduce the changing thrust line, but ultimately the solution is to use a nose ring.

John, thanks for the info. This is the first I've heard of a "nose ring" A google showed a couple examples, although one looked like it just captured the nose bearing housing on the crank and let it float in the hole. I suspect there is some details that I'm missing from the photos.

Lets see if this solution might work. What if I but a thin, SS bar behind both the upper and lower firewall mounting lugs. If I then used a piece of Pull-Pull cable from this bar, both upper and lower to an extended front motor lug screw, on both sides of the engine, and put adjustable clevisis on all four wires, would that hold things in alignment OK? I suspect that if the engine is pulling the mount to the right, it may also be pullind down or up a like amount. This is an interesting problem.

Do you by any chance have a photo or two of a nose ring, and do you have a comercial supplier or do you make your own?

Thanks.

Don

This is the first time I have seen one of the Dubro mounts flex that much, I even run A small RCS 1.40 gasser with one of there mounts without problems, I think I have 6 or 7 of them??
I'm going to show you an old mount I have from the pattern days, I have A couple of these too. They don't say who made them on the casting but I "think" they are Dave Brown anti vibe rail mounts, these don't flex and there is no vibration. I know I got them through Central some time ago. The Hyde nose ring mounts work great but that would be the first Magnum 70 I have ever seen needing one. I also have the solid OS mount for my OS 1.20 and I use the Jtec anti vibe on my YS 1.40, I love that mount but it's not an easy one to install on your fire wall but it sure works!!
There really are A lot of good mounts out there to choose from. Have you mounted that Dubro some way different?? Just wondering, A .70 Mag pulling it over???[:-]

Grey Beard, the only thing unique about the mount on this plane is that I first set it up for Mag 52. I found I needed around 7 oz up front to balance, so seeing that the Mag 70 is just about 7 oz heaver than the 52, and I had one, I decided to use it. I didn't want to put a second set of holes in the firewall and the differece in widtth wasn't that great, so I milled out a set of the mounts to keep the 52's firewall pattern but accept the 70 without distortion.

It's kind of like the mount has some overly soft rubber gormlets. The first thing I thought of was, was that the firewall screws were not tight. This turned out not to be the problem. I've got a couple more of these mounts that are still mounted on firewalls without fuselages. I'm going to pull down one and swap the rubber and see if it makes a difference.

You got it, it is nothing more than a bushing that the front bearing housing sits in. It forces the motor to only move on the axis of the crank, hence no thrust changes from the soft mount. If you think about it, to reallly get any benifit from a soft mount it must be soft enough for the engine to move, but we only want movement along the axis of the crank. The unwanted soft mount movement will casue thrust changes. A nose ring fixes the "naughty" bits from soft mounts.

You can make your own from any suitable stock, such as ply, phenolic, metal, etc. You can use fuel tubing as the bushing. You can also buy them comerically made, but you will need to be sure they will fit snugly to the bearing housing on your engine. You will also need to be sure the nose of your plane can handle the nose ring load. Any plane with a removable cowl will present problems as the cowl won't be strong enough to handle the ring loads.

http://www.centralhobbies.com/engines/nose_rings.html

IMHO a nose ring for a .52 four-stroke engine is not needed. Could it be that the rubber bushings are worn out and need to be replaced? I have used the DuBro soft mounts on engines ranging from .46 OS two-strokes, .72 and 1.00 Saitos and also 1.8 Super Tigre two strokes. None of these engines has exhibited the behaviour you describe.
It is important to put some oil (castor is fine) on the rubber bushings when installing them in order to prolong their life.

Thanks for all this great information. I've been flying RC now for about 8 years or so. I am putting together a Super Star 120. The engine I have selected, so far that is, is the OS 1.60 FX. The engine is new.

I have been considering using a flex mount for this installation and have read your posts with interest.

I have read, on several different posts as well as other places on the net, about the nose rings. It may be real simple to most of you, but I am somewhat confused about how this is installed and how it works.

I presume that, being a nose ring, that it goes on the prop shaft or on the spinner. Is that correct? Or does it go on the Drive Washer? I've not seen one, so I am both interested in curious.

Thanks.

CGr.

http://www.probuild-uk.co.uk/factsheets/nose_ring.php

http://www.wtp.net/DBEST/bestbuilt/nosering/nr1.html

Hope this helps, they even have pictures..

So, the nose ring does not go on a rotating part. I misunderstood, I guess. I was under the impression that it went on the 'nose' or the spinner or something. It looks like it goes behind the drive washer. Is that correct? (see picture).

CGr.

Red, the 52 isn't a problem. It's my Mag 70 that is testing the mounts. Like you, I have used them on several engines and this is the first issue I've had.

The engine has had less than a gallon of fuel through it. The mounts were new on this engine. I did lube the bushings per the instructions when I installed them. I'm pulling down the engine mount this morning and I'll compare the bushings with another mount I have in hand. I'll post some photos.

Don

Mystery Solved. I pulled the motor off and checked the screws into the fire wall. All were tight. I then noticed that the compression on the bushings on the top mount were defferent side to side. Photo 1. and Photo 2. I pulled that mount off and the rubber bushings were quite different looking. Photo 3.

The problem was that one of the aluminum bushing centers was short by a tad under 2mm. This gave me a three leg mount and was pulling the top right mount away from the firewall when the motor was locked down tight to the mount legs. There was less compression on the top right, so when I goosed the power there was less resistance to movement.

So, before mounting your Du-bro mount check to see that the eight aluminum spacers are all the same length.

Don

OK, I pulled the bushings and spacers out of another mount I have and made sure all four were the same and re-installed the motor mount and engine. I can't get the twisting of the engine that I could orginally. Off course, there is a slight movement, but no noticable movement to either side like I had before. It is going to be interesting to fly it tomorrow and see the difference.

My guess is that if I had the short sleve outboard from the firewall that I would never have seen the problem.

Don

I need to get this right too please! Looks like the nose ring is holding the non moving part of the engine just before the drive washer .Wouldn’t that though put a lot of stress on the engine body? I used the same Dubro soft mount for an OS70 4s on a trainer last month. Well the trainer did go down in an uncontrollable fashion and crashed .I believed at the time that that was due to elevator’s clevise/ horn clip got undone. In that flight I had used the Futaba 7c 2.4 GHz system, the soft mount and the new trainer all for the first time. It couldn’t have been due to the vibrating soft mount could it?

The problem with the mount was when the plane was on the ground. A sudden application of power caused the plane to do donuts to the right because the engine thrust line shifted to right offset. I didn't notice any issues in the air. I highly recommend the soft mounts for a trainer. They save a lot of damage to the motors when the planes crash.


Don

Just got back from the field. The plane handles much better now. I still can see a bit of cock off to the right just before the engine gets up to full RPM. Before, it was an imeadiate snap to the right and a much higher magnatude movement.

What I may do is to cut some shims and put over the bushings and stiffen the mounts a bit for front to aft movement. that should remove all of the twist. A nose ring just isn't practicatal on this plane, is has an engine setup like a stick, everyting in the open. Actually it isn't bad now the way it is. I have to be in the top 10% of the RPM range before it kicks and that is in a static setup. My guess is that there will be no movement in the air.

Humm, My son got me a FlyCam2 for fathers day and I haven't mounted it yet. Maybe a shot out over the prop from the cockpit would show if I am still having a problem that I can't see.

Don

Glad you got it working better by reworking the mount... much simpler than adding a nose ring. And to confirm, if you should ever use a nose mount, yes, it supports the non-spinning portion of the motor at the nose where CGRetired indicated in the picture. If it is kicking on the ground, I'd say it is kicking in the air too. But this is probably not a big deal if the movement is small and you aren't trying to fly super precisely. Fuel up and enjoy. - Cheers

<Edited for typo>

Are the mounts tightened on the photos you are showing?? Just looked loose to me.
Only plane I have the Dubro mount on right now is A very old Bridi Kaos 120. I removed the engine and mount from an Extra I sold. This is A small 1.40 22cc gasser from RC Showcase, you can see where I had drilled/cut out the side of the mount for throttle or choke clearence. With this engine in both planes there was no flex or any problems. After I get the mount snugged down onto the fire wall I do use A torque wrench just to get it all equal. Can't remember the torque right now but I do have it writen down. It isn't sinched way down but there is some squish in the rubber.

Grey Beard, the design of the mounts gives you a hard mount to the firewall, the two aluminum centers and the screw. The mount floats, but is trapped on the aluminum centers. I think the big difference is that the four stroke mounts are quite a bit longer than the 2 storke ones. That and the fact that the carb will interfre with one of the screw heads requires the engine mass to be an inch or inch and a half out front of a 2 stroke. It gives a little extra leverage, along with the low pitch prop and the torque from the four stroke.

I kind of agree that the rubber isn't tightended down, but if you notice that in the photo of the long and short aluminum centers the one rubber bushing was damage from being trapped between the spools because they were to short. I don't think shortening the aluminum centers is the answer. Maybe a shim between the aluminum center hubs and the bushings would work OK. Although I think I'm Ok with what I have now. It's not giving the snap to the side I saw eariler but a gentle change at full throttle, and then only during the spool up to the final 500 rpm or so.

Back when I was into street rods, you could get standard rubber bushings or neoprene bushings for the suspension. The later were quit a bit stiffer than the stock bushings. I suspect I need some Neoprene bushings here. Not realy though, as this isn't a pattern plane where precession is mandatory. It's just a knock around fun fly.

At least I can get onto the next step of the tuning process now. It is still a little nose heavy but for now, I like that as when I had it balanced out correctly, it was a hand full. I've got about two ounces one the nose now and will trim it down over the next week or so until I get more comfortable with it. Not having it go nuts on power on is a big plus.

Don