Hi all!

I flew my Seagull Ultimate 90 last weekend (yay!). I have the DLE-20 on the front and I used the stock 'nylon' mount that came with the bird. Everything was nice and rigid before flying, but now I have some very noticeable 'flex' in the mount. I've not really experienced this before (flew .40 size in the past) and I'm not sure what is acceptable. Is this normal for a plastic mount with a gasser on it? When should I worry about it? There are no cracks that I can see and it really just seems like the plastic softened up a bit, but I'm just not sure.

Any info would be appreciated!

Thanks!

Z

I have a 20cc RCGF in the nose of a Seagull Decathlon. It's very similar in power to the DLE, on a stock nylon mount, and I can move mine visibly as well. It's probably got 2 gallons of fuel through it now, and still hangin in there!

Not sure about 20 size gas; but on a 50-60 size gasser, a guy had a flex mount of some sort, and the vibration that was created broke the muffler, it was explained that the engine should be mounted solidly onto the airframe, and that the wings of the plane act as a vibration dampener, in short the harmonic vibration that the engine creates is transmitted and dispersed through the wings, to test this, try running an engine mounted solidly on fuselage without the wing on and it will vibrate and bounce around wildly; on the smaller size gasser it may not be a problem, if it does change the mount out for some metal mounts.

I wouldn't tolerate that kind of flex and don't especially care for any of the molded mounts. Hanger 9 makes a decent universal aluminum mount at a fairly good price. These have worked well for me in the past and are the same as supplied in many of their ARFs.

Yes, a flexible mount can be very hard on your muffler and airframe.

Guys, it's not like the engine is jumping around while running at all. Mine isn't moving around at all actually. I can though, grab the prop and move the engine visibly, as seen by looking at the spinner - cowl gap.

Regarding solid mounting, Dubro supplies a mount I see a few guys using - that uses rubber bushings? How is it that those mounts work, when you're suggesting a nylon mount that allows some movement will not? Not following that logic, but I could darn sure be missing something! Don't forget, we're using beam mounted engines here!

At least with the DuBro mount the engine lugs are bolted to a solid piece aluminum that doesn not flex under the engine lugs. The flex area is limited to the firewall area only and not across the whole length of the mount beam. This has to be better for the engine I would think. The DuBro mount would have to be a far cry better than the ARF supplied, almost rubber like mounts. But then, I don't really care for the DuBro mount either. Just personal preference here.

I'm only stating my opinion of the types of mounts I prefer. As always anyone is free to use whatever they see fit to use.

Is this the mount that you're talking about?

http://www.horizonhobby.com/products...61-2-2-HAN2033

I was considering changing to this when I first noticed the movement on my current mount. I just wasn't sure about the slotted mounting style? Has an engine ever moved on you?

Z

I would bet if you put a strobe light on it your opinion that it's not jumping around would change.

Thats the Horizon Mount. No movement so far. I use heavy flat washers on the bottom of the mount. I'd prefer tapped holes but the mount works well as is and the price is reasonable.

Most "flex" or "soft" mounts are junk. They simply transmit too much energy into the airframe and really don't do much isolation.

In my experience the only such mount that makes sense is a Hyde mount. I make my own mounts in Hyde's style for either beam mounts or stand offs. I've soft mounted a DLE55 this way and it truly works wonders. Of course every other smaller gassie is also soft mounted in my planes. BUT I don't use mufflers in mine...only headers and tuned pipes, also soft mounted. And BTW- these soft, rubber isolation mounts are not that heavy-for 30cc class, the mounts weigh around 2 1/2 ounces. The one for the 55 weighs 4 ounces.

I have various threads in RCU that discuss this. Even have one that teaches you to make your own. I abhor hard mounted engines unless they are smaller than 40s

For lack of properly designed soft mounts in the marketplace and because most of us are more than a bit lazy, thankfully, hard mounting works fairly well.

I compliment you on engineering your own solutions.

Truckracer, others, just trying to pick your brain(s) to see if I'm missing something? ;^)
MTK, I know you run the flex mounts, and the fact you run headers, was hoping you would chime in with your thoughts.

My engine mounting bolts are threaded into the mount, haven't moved since I noticed they were (really!) loose after the second or 3rd flight (heat set?). I've considered that mount squirming under the engine, but that doesn't seem to be the case because if it were, it would be oozing black crud at the joint, and the bolts would be loosening all the time?

I have read somewhere that these mounts have cracked/broken, so we're on a wait and see basis there. No doubt keeping a close eye on it. Obviously all bets off at that point. I do have a heavier (literally, and also nylon) mount on standby. I chose to use the stock one because of that weight difference actually.

When this type mount first became popular many years ago, I didn't trust them. Had to have alum. Today, maybe trust them too much? If I break it, so be it. It's not like I wasn't warned.

Point regarding muffler weight being flung around by any engine movement taken and I agree. If I were running a wrap around I would be concerned. Especially one of those expensive/comparatively heavy ones. To date though, this stock muffler has stayed in place nicely, again loosening once very early on, but never since. Nothing too unusual there either in my experience.

I don't think I'm hurting the airframe either. There's no unusual "buzz" when handling it, and no "resonance" noise at any speed in the air like you hear on some planes.

Bottom line, I'll be staying with it until I see or read about a good reason not to. Your mileage may vary....

One benifit of the "nylon" type of mount is it is better to have one of them break on a bad landing ect....than to break off a engine lug mount. Less expensive to replace. Capt,n

My thoughts on it, if the engine seems to be flexling and moving about slightly, then something may be amiss. A engine that is slightly loose( screws mounting it down, broken or cracked mount lugs), a cracked or broken mount, a firewall that is coming loose, mount to firewall screws and or blind nuts that are loose, or a firewall/fuselage that is flexing. I have seen where the blind nuts pulled into the wood and crushed it and ran up against the motor mount and wound up being loose in the wood although they were very tightly screwed down. The firewalls on some ARF's are a little on the thin side in my opinion and made from a softer lower quality wood too, plus the glue they use to gue it together is very minimal. I have reinforced the front end on several ARF planes so far because of that.

Of course if all that checks out, scratch your head and just go with it then, Happy flying.

Not so, they do a great job.

So your homebrew, TLAR mount works? Good on ya. The soft mounts have been around since the '80's at least, and still do well in the market. They transmit LESS energy in a less violent form to the airframe. Hard to figure how your homemade version could be much different.
 
I've had a couple H9 ARF nylon fiber mounts flex, plus their thin firewalls flex. My Tango mount luckily broke on landing rollout, second one to fail. Going to a cast mount now.

Nonsense!!

Care to elaborate beyond a one-sentence statement? I only presented plain facts. 

This is the mount I've had trouble with:

http://www.horizonhobby.com/webapp/w...pe=productgrid

Make sure your prop and spinner is properly balanced.
My gassers have stand off mounts, there is no flexing at all.
But I never had problems with the firewall or any of the plane's structure.
Just make sure you use loctite on all screws

Don't use locktite on the screws that thread into a plastic motor mount as the solvent in the locktite attacks the plastic.
If the screws aren't directly threaded into the motor mount then by all means use some. But I would suggest the non-permanent locktite though.

On plastic mounts I always drill through and use nuts. And loctite, of course.
This guy here has a 20 cc gasser, plastic threads won't work here for very long.
Engine standoffs are made of Aluminum.
But for a rail mount, I suggest to get an aluminum version, they just hold up better.

I looked it over some more. It's not the firewall or screws coming loose. It's literally movement in the mount itself. I guess I'm going to swap it out. I'll give the aluminum Horizon mount a try and see how that does. I just hate putting more holes in my firewall, haha! For no more than I flew it, the flexing of the mount is bothering me. I guess I'll miss this weekends flying

Thanks for all the input!

Z

I think all this is pretty interesting.... would like some thoughts about something I keep thinking about. Maybe we could get away from planes for a second and talk about our automobiles? It uses rubber motor mounts to isolate engine vibration from the rest of the car, right? So take those out and replace them with steel and bolt it direct for a second. You would imagine lot's of vibration, right? Now let's install some plastic mounts. Would you imagine you would have the same vibration level as the solid mounts, more than , or maybe, just a little less? That would be my bet, a tad less, but probably not a lot. Certainly though, not more than the solidly mounted engine?

Bringing this thought back to planes, wondering how the aluminum mount might differ from a solidly mounted automobile engine?

<hr />You must have the engine way out towards the end of the mount too get flex. If you got room...cut a block/piece of oak wood and insert it between the arms. Use wood screews through the arms and into the wood. That may stiffen it up. Its wort a try if it can be done. Capt,n

I have several threads in RCU where I discuss and describe soft mounting techniques I use in various projects. I will not rehash old material here. Search and find if you want. Study what I teach and learn if you want. Either way, no skin off my nose