Engine mount position

[Hogster]

Many years ago I built a Citabria from a kit. It is the only kit built plane I have ever done and as it was my pride and joy i was to scared to learn to fly on it. However upon returning to the hobby I have flown ARTFs which I didn`t  have such an emotional attachment to, However, I now want to ressurect my citabria and I need to fit a new engine and mount. The existing mount is too small for my Irvine53 and in any case I want to mount the engine at 45 degrees if possible rather than vertical as at present. My question is what is the best way to ascertain the correct position of the mount onto the firewall. Is it a case of simply rotating the mount around a centre line?

[Biggles' Elder Brother - Moderator]

Basically, yes!

 

You need to try to establish a point on the firewall that is on the thrustline of the engine. You then rotate the new mount about this point. One note of caution, don't assume that the existing mount holes are equally spaced around the thrust line - sometime they are and sometimes they're not!

 

If they are then the job is simple - just draw two diagonals between the holes on the firewall - where these lines cross is the thrust line.

 

If they're not the job is more complex. . It is usually the case that the lower mounting surfaces of the engine lugs are on the thrust line for the engine - so with the old mount in place draw a horizontal line on the firewall in line with the engine mounting lug surface on the mount (IFSWIM!). Then draw a vertical line that is equispaced beween the mounting holes on the firewall. The intersection of these lines should be on the thrust line. If you have one of the few engines where the mounting lugs are not on the engine thrust line (such as the RCV CD series for example) then you will have to add the necessary offset distance. You can usually find this on a drawing of the engine in the manual. But as I say this is pretty rare and most engines place the mounting lug lower surfaces on the thrust line.

 

Hope all this is clear! If not just ask!

 

BEB

Edited By Biggles' Elder Brother - Moderator on 14/12/2010 13:21:45

[Hogster]

Thanks BEB.  The original mounting holes are equally spaced. So assuming they were in the right position in the first place ( it did have few trimming flights as I recall and all was well) then your first method is the one to go for and this would give me a guide for the 45 degree mounting as well as the original holes were in a square pattern.

 

Many thanks

[Steve Hargreaves - Moderator]

Remember to "fill" the old mounting screw holes in the firewall.....a method I have often used is to drill the holes out to 6.5mm & use short lengths of 1/4" wooden dowl pressed into the holes after smearing with epoxy.

 

If you have good access then cut overlong lengths of dowl, smear with epoxy & press home, then trim flush with the firewall when the glue is set. If access is more restricted then cut the dowel to the thickness of the firewall (usually 6mm) & again smear with epoxy & press home. A bit messier/tricker this way but can be done. Doing this will prevent oil getting past the firewall & restore strength to it as well.

 

Note, that the dowels must be flush with the front face of the firewall to allow the mount to sit flat. The back face is less important unless you are going to use a captive nut near the existing (repaired) hole....

 

Hope this helps.....

[Stephen Grigg]

This sounds quite exciting,how long ago did you build it Hogster?

[Hogster]

Ahh yes good advice Steve, I hadn`t considered that.

 

Stephen: I built it about 14 years ago. I went to my LMS (which closed down shortly after) and took his advice on my first foray into R/C planes. I enjoyed the building process but it took about 4 months. It was a slow process as I remember. I had to keep asking for advice along the way. But I must have done something right because its still in the same good condition as when I put it in the loft all those years ago.

[Biggles' Elder Brother - Moderator]

ARTF's are great all that - but for shear flying pleasure and general satisfaction you can't beat one you built with your own fair hands! You'll love it Hogster! Get to it.

 

BEB