distance btw the firewall and the Rotax engine attachment points

I am designing my first airplane, an LSA. My questions are what are the considerations in determining the distance btw the firewall and the Rotax engine attachment points, and how much it should be.

I plan to use composite for both the airframe and the firewall, and have not decided to use 912 or 914. I assume the 4 attachment points for Rotax 912 and 914 are the same, with a vertical distance of 270mm between upper attachment points and lower attachment points and a horizontal distance of 210mm between left attachment points and right attachment points.

I would like to have the spacing btw firewall and the attachment points as short as possible to keep the airplane compact. What are the limiting factors and possible values?

Jeffrey

Aside from the obvious clearance issues for accessories, hoses, and tool clearance to work on those accessories, the biggest factor is weight and balance. A few pounds one way or another in the aft fuselage area can be compensated for by a few tenths of an inch in motor mount length. Flexibility of a few inches in motor location can prevent having to move the battery late in the project to an ugly location, or worse yet, upping the battery size/weight to compensate for weight/bal.

Mike Perkins

Mike, Thanks for the reply.
So there are two factors in determining the distance, one is clearance/accessibility, the other weight/balance. I have a couple of questions here:
(1) What is the shortest distance practical for clearance/accessibility ? No weight/balance consideration here. I need a number (like 5.3 inches) to start.
(2) Is the motor mount structure also a determining factor? If it is too short, I am afraid part of the structure like some of the diagonal struts will not be strong.

Jeffrey

Most issues with access to accessories for maintenance are overcome by requiring that the engine be removed from the aircraft. There are some pretty good photos of 912 installations on the Zenith site: http://www.zenithair.com/stolch701/7-photo18.html . Also, the Rotax site has outline drawings of their engines. As far as structure strength, a structural engineer would be able to help with that.

I'd suggest getting ahold of the engine's installation manual from Rotax, as it has a lot of information that will probably be useful to you.

thanks for both replies. I did download the installation manual. It does have a lot of info, but does not give suggestions for spacings. Zenith site has good pictures. However, as they stated, the airframe is not tailored to any specific engine, and the installation of 912 does not look compact. I think I should check other airplanes that are engine specific in their design. Again, thanks for the tips!

In an Lyco or Continental installation, the dimension that requires the most firewall clearance between the engine and firewall is the space required to remove an oil filter, magneto, or alternator/generator. I will offer that 6" is a good place to start. Why you ask. For those engines with an oil filter mounted on the read of the accessory case, with the oil filter oriented fore-and-aft, there is a central threaded tube that mates to the internal thread of the oil filter. You have to unscrew then move the oil filter straight back to clear that tube and remove the filter. Magnetos are removed by two nuts but then you have to pull them back so that the drive gear clears the accessory case. The same is true for rear mounted alternators or generators. I will guess that maybe a minimum of 4" is needed there. And remember that after you have unbolted those accessories and lifted them away from their spot on the back of the engine, you have to remove those components by wiggling them around the other stuff that is still bolted on.

You also need room to use your tools. Think of a 1/4" ratchet with a 4" drive extension and a socket on the end, plus space for your hand to operate the ratchet. You can use Snap-On sockets that have a universal joint built in, but you won't enjoy working on the back of the engine with that little space.

Design engineers should be required to do some maintenance on the type of equipment that they design before they are turned loose on the computer or drafting board.

Best of luck,

Wes
N78PS