distance btw the firewall and the Rotax engine attachment points

[wantobe]

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

[MPerkins]

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

[wantobe]

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

[MPerkins]

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.

[Matt Gonitzke]

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.

[wantobe]

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!