jrf

My Feedback: (551)

I'm new to gas engines. Am building a P-51 giant scale with a DLE 55RA. In order to get the prop to firewall distance correct, I will have to use 3 3/4" standoffs. That is the 2 3/4" standoff that come with the engine plus another one inch standoff.

It seems to me that the torque of the engine will be constantly trying to twist it off the firewall and the longer the standoffs, the bigger the problem. Can I get away with the longer standoffs, or should I be figuring a way to move the firewall?

TIA,
Jim

flyinwalenda

My Feedback: (5)

If the existing standoffs are through-hole then I would cut a 1"x1.375" hardwood block for each side of the firewall and glue them in place and drill the mounting holes. Get some 4.25" socket head bolts and bolt the engine on using the standoffs with washers and lock nuts on the inside of the firewall.

Propworn

My Feedback: (3)

Do you need the standoffs to clear a carb or something? If not do like I do and build an extension of the wood motor box and attach it to the stock one eliminating standoffs completely. If the engine has a rear carb I cut a clearance hole and access holes for the adjustment screws. Several lightening holes in the box allow plenty of air in. You can even use some filter media over the holes to create a large volume air filter box. I have even gone so far as removing the old motor box and building a new one eliminating the standoffs completely. Usually there is enough room inside to house the ignition and battery neatly out of sight.

Dennis

Truckracer

My Feedback: (19)

I'm with Dennis. I wouldn't use standoffs longer than provided with the engine. If at all possible extend the firewall or make a one piece spacer out of sandwiched hardwood or plywood to make up the difference in length. If necessary, a hole in the middle will allow for any required carb clearances, etc.

jrf

My Feedback: (551)

Thanks for the input guys. I seems like we are all in agreement that the extended standoffs are a bad idea. I will either use hardwood spacers or build an engine box.

Jim

Propworn

My Feedback: (3)

Last time I ran into a poor fit for my chosen motor I was able to remove the box right down to the firewall. I sanded it smooth then attached the cowl as I would like to see it when it was finish mounted. I measured the distance from the front of the cowl to firewall.

I made an over long motor box and mounted the engine without any stand offs. I trimmed the bottom of the box to get the down and offset thrust exactly as I needed them. I carefully measured and brought the distance from the prop back plate to the firewall face of the motor box to within 1/4 inch of the measured distance from the firewall to front of the prop opening on the cowl.

Rigging a wooden fixture to hold the fuselage vertical nose up I removed the cowl and placed the motor and motor box on the firewall. Screwed the cowl back on and centered the spinner to the cowl. Measured the gap between spinner back plate and cowl, figured how much to remove to keep the minimum clearance. Removed cowl then marked the base of the motor box and trim right to size. Assemble and check for perfect fit then take cowl off mix up a batch of 30 min epoxy and coat the firewall and reassemble motor/motor box and cowl. Line up for perfect fit to cowl and let sit overnight. Motor should be in perfect position with the cowl, now all you have to do is epoxy the re-enforcements in place and your motor box is finished.

Dennis

jrf

My Feedback: (551)

Thanks Dennis. That is the most precise way to do it that I have seen. Unfortunately the Top Flight Giant Scale p-51 cowl is built around the engine after it is mounted. I am going to have to trust the measurements given in the manual. I have built engine boxes before, but never for an engine this large. I think I will use the hardwood block method. The DLE 55 is a light engine so I will probably need the nose weight also.

Jim

MTK

The 55 puts out about 5 HP on muffler. A 3/16"- 1/4" aircraft ply box provides adequate strength. You can even cut holes all over the thing for lightness if desired. OR, why not a second firewall forward of the first. The P51 should have a narrow fuse so a second firewall might be easier to build in ( I think). But I don't know what kind of room you really have there to access things. A center hole in the firewall will enable better carb environment and more consistent running.

BTW, my firewalls for the 55 are 1/4" cross grained balsa that's been carbon fabric laminated. Very light but strong and tough. I always soft rubber iso mount my engines up to 60cc so the front end doesn't need much beef in my set-ups. Actually soft rubber isolation makes sense for any engine; I just don't have anything larger than 60cc. Add a 3 blade prop and the set-up becomes pretty quiet too.

jrf

My Feedback: (551)

MTK:

What do you use to soft mount your engines?

Jim

Propworn

My Feedback: (3)

Be careful with soft mounting many engine and exhaust manufacturers don't recommend soft mounting. One exhaust manufacture I know of will refuse to warranty the exhaust if the engine is soft mounted. Check with your engine and exhaust manufacturer about soft mounting. I have run a lot of gas and I haven't soft mounted anything since the old Quadra days.

Dennis

dirtybird

My Feedback: (5)

Merle Hyde makes great soft mounts. They are not cheap. You can get them from central hobbies.
I just use cheap mufflers.
A good soft mount is a help to prevent flutter and lose bolts. It will also reduce your battery drain.

MTK

It's a double flange design similar to the Hyde. The back flange is secured to the firewall while the front flange carries the engine and is isolated from the back flange with a rubber tube. I wrote about how to build them in the Pattern Forum. Look for Soft Mounting the ZDZ40 or something like that.

These are easy to make with basic hand tools. There is a learning curve of course but after the first couple, they have become elementary builds. Every one of my gas engines is soft mounted. Over the years I have made many improvements to simplify the build and reduce weight. Mounts for the 35cc class engines weigh about 2 1/2 ounces. And 3 1/2 ounces for the 55/60 cc. Or one can buy one from Hyde but he is very pricy. If interested, i can tackle a mount for you. PM me. They are not cheap but are much less than the Hyde.

I have found that folks poo poo soft mounting but have never actually done it. Just regurgitate what they heard from other sources.

In addition to nearly eliminating the high frequency vibrations (the low frequency shake is still there but is far less destructive to the airframe) which truly save airframes and radio equipment, soft mounting reduces noise considerably. If your flying field neighbors are fussy like mine, all measures should be taken to reduce noise as much as possible.