fellow builders,

Has anyone used an aluminum engine mount for OS.91FS for the Super Skybolt ARF? The Skybolt firewall holes do not match the 1 15/16 " measurements of the OS engine mount shown on tower hobby. Am I limited to the the adjustable engine mount that comes with the Super Skybolt ARF ?

Thanks

Drill new holes.

It's really, really hard to find a motor mount that works with ARF's predrilled holes. That is, excluding the one that comes in the ARF box.

There really is something else you need to look at about that firewall.

That firewall is made out of LitePly. LitePly is excellent stuff for fuselage bulkheads and sides (when it's got great big lightening holes cut in it.)

Liteply doesn't stand compression very well. When you tighten down the motor mount bolts, the T-nuts put a lot of compression on 4 small areas of the LitePly. And the motor mount puts the same pressure on the front of that firewall.

It seems that the firewall isn't supported from it's centerline up. OK, it is, but it's by the balsa sheeted foam that forms the top front of the fuselage.

The firewall will benefit greatly from an additional layer of "real" plywood. Airply works great. It's hardwood and compression ain't no problem for it.

It's a sensible idea to sheet the front of that firewall with a layer of Airply (any LHS has it) to spread the support out over more of the firewall and to resist the compression of mounting the motor mount. And when you do that, drilling new holes will be needed anyway.

What's wrong with the included mount?

I have the .91 in mine and the mount has worked great.

If you do want to change mounts though, re-drilling is probably the only choice.

Thank you MinnFlyer, DaRock, and ChuckW...

I actually have seen the firewall pictures DaRock posted in the Skybolt build thread, and thought a "made for OS" engine mount would distribute the compression and releve stress on the firewall ( well presented by DaRock in the skybolt thread). I now agree that beefing-up the firewall is the way to go. How thick should the Airply be? Won't adding thickness to the firewall make the engine stick out of the cowl a little farther?

AND the adjustable mount was UGLY.

ChuckW, thank you.

Done properly, you should not see any difference.

If you put a 1/8" ply sheet over the existing one, merely decrement the firewall to prop driver distance by 1/8" as well.

As DaRock has pointed out, you should also add tri-stock to the area behind the firewall along the areas the firewall joins the fuselage ply.

The idea is to distribute any torsion or stress over a greater area.

While you are in there also consider adding some ply or hardwood stock behind and against the existing firewall for good measure.

The additional ply doesn't have to be very thick. The real stuff stands compression excellently. I used 1/16" on the new H9 Corsair and the same has just cured on the H9 Spitfire.

Yeah, adding thickness makes the engine stick out more, but you simply subtract the added thickness from the suggested thrust-washer-to-firewall measurement and you're good to go.

That Great Planes motor mount is good enough that I use it on the H9 ARFs. I find it's easier to use than 2 piece mounts, very much easier to use. And it does distribute the compression over even more area than the 2 piecers, but LitePly's ability to stand compression varies a lot more than other plywoods.

Thank You Da Rock, OPJose, ChuckW, and MinnFlyer, for your valuable guidance.