Thrust line question...
 

Started the Super Aeromaster this afternoon. Naturally, a problem reared its head almost immediately.

With the engine on the mounts (to determine bolt spacing), and the mounts aligned on the thrust lines, the top mount overhangs the top edge of the firewall by 1/8". With the mount flush against the top edge of the firewall, the engine crank center is now below the thrust line by 1/8". (See photo)

The structure that will be built above the firewall doesn't look substantial enough to support the engine mounts, blind nuts, etc.; so moving the mount up is not an option.

So my question is, with the engine 1/8" below the thrust line, how much, if any, will that affect the flying characteristics?

RE: Thrust line question...
 

How does the engine and mount look if you lay them on the plans? Same problem?

RE: Thrust line question...
 

Yup.

RE: Thrust line question...
 

That's weird! Great Planes usually makes pretty decent stuff. Gotta be a reason why you can't get the engine/mount assy to fit properly. Sorry, but I'm afraid I can't be of much help. I'd be calling GP looking for answers if no one else know.

RE: Thrust line question...
 

Got it scoped. You need the GP adjustable mount, which has plenty of "meat" to move the mounting holes if necessary. The Sig mounts are smaller along the mounting beam, which is why I wanted to use them; those GP mounts have a lot of unecessary bulk. I have one set up for a 61 FSR, which has the same "footprint" as the 91 Surpass. It fits the AeroMaster firewall fine, albeit with some overhang off the top of the firewall. No worries; there is some additional structure up there to cover it. Just hog out the mounting holes to move the upper bolts down a tad...

Now, to get a new mount for the 91 Surpass. Usually, the LHS has a wall full of these. Naturally, they are out of of the 60-90 size...:eek:

RE: Thrust line question...
 

Mr. Campbell, One of the 3 Aeromasters I have built had a small ply box to move the motor mount out from the fire wall to get enough space for prop clearance. It also over hung the balsa on top of the fire wall. It flew that way for almost 10 years, until a low level dumb thumb attack. After it landed wing tip firt, the last thing I was worrying about was the motor mount box being to high. The second one I built I used a 3/16 inch ply plate that went over the balsa, the ply also held the blind nuts for the motor mount. I have yet to put an engine in the third one I have built. It is all ready for covering and power plant but to many other projects in line before the Aeromaster.



Dru.

RE: Thrust line question...
 

Dru, the engine box is for two stroke engines; I'm using a 91 Surpass.

The problem was that with the Sig mounts on the thrust line, there wasn't enough "meat" left in the top of the firewall to hold the upoper blind nuts. The adjustable GP mount will allow moving the mount holes down enough so the blind nuts will be fully supported by the five-ply firewall.

Using an auxillary ply plate like you mentioned would either; 1) move the mount forward enough so that the thrust washer would stick out too far, necessitating modifying the cowl, or 2) placed on the back side, it would cut down on space for the fuel tank, which is already tight.

Thanks for the ideas, though. I have three very well-stocked hobby shops within a 60-mile radius of me, and not one of them has this mount. Gotta call The Empire this morning and order one...