Thrust angle setup on a rear mounted engine

[Vecchio Austriaco]

As some of you may know I am working on the Bettini Extra Super, a rather historic kit with very poor instructions.

 

I start thinking how to fix the engine to the firewall and how to set up the necessary thrust angle. The engine will be a 45 cc petrol engine, fixed with round aluminium spacers to the firewall. 

 

The firewall itself doesn't have any inclination - neither down nor sideways.

I suppose downthrust will not be such a problem as the plane is a low wing plane.

 

For the sidethrust I expect 2 degrees will be necessary. But how or where to add them?

 

If I do it at the firewall I will have 2 problems -

 

A) the tip of the engine will move to the right - this is the smaller problem - as I expect 2 degrees I can shift the position of the holes for the bolts at the firewall to the left. 

 

B) as the bolts will see a small inclination of said 2 degrees they will press on the plywood only in one point.  - Can this be neglected or is somebody using inclined washers?

 

If I do the correction at the engine itself -  

 

A) the problem of moving the tip to the right as described above will remain but has less influence as only the engine itself wil be moved to the side.

 

B) The problem stays the same, may be worse as now we have the force in one point to the casing of the engine - again the question - can it be ignored or you are using inclined wahsers?

 

Are there any better or simpler ideas how to do it?

Tried to find something in the forum but without sucess.  

 

Thanks

VA

 

Edited By Vecchio Austriaco on 01/06/2010 09:58:55

[Doug Ireland]

Can the firewall be moved?

[Braddock, VC]

SLEC (I think) manufacture plastic thrust wedges in various sizes from 1 to 5 degrees inclination. They have a 4mm hole at one end.

They were intended for beam mounted engines and I have used them in the past.

If you go down the route of inclined or tapered washers you will have to remember that the starboard engine offset mounting spacers will need to be shortened, on an engine of the dimensions you show this amount could be 2 or 3 mm.

If I were doing it, and I may yet have to as I have a Mantua Models kit with equally minimal instructions, I would be tempted to securely fasten the alloy standoff spacers to the engine and mark the firewall ends to allow the offset and cut the standoffs accordingly.

 I would use blind nuts at the rear of the firewall which will quite easily take up the offset, fastening them to tha rear of the firewall with epoxy putty then pull them up tight with the engine in place. You can then use sturdy washers between the front of the firewall and the alloy standoffs. 

 

[Vecchio Austriaco]

Thanks guys.

 First of all I have to say that i didn't look too close - there is a 2 degree side thrust angle already at the fuselage. The Italians are better than I thought...

 Doug,

The firewall cannot be moved as the end of the fuselage has a flange where the firewall has to be glued around. Not easy to describe, I think I get the camera out.

 

The firewall consists of 2 pieces - both ply - one outside and one inside. they will be glued together with epoxy with the flange inbetween.

 

 

John,

thanks for the hint - I may need to change something even if the 2 degrees are already built in. For the aluminium spacers: no problem. Have a lathe.

VA

[Engine Doctor]

Hello VA . To calculate the offset of the mounting holes  draw a line the same lenght as from the firewall to the back of the spinner /front of prop driver and add 3mm for running clearance( engine lenght) . Using a protractor from the front end   mark 2 deg and extend this to the end of your engine lenght line. The distance between the two will give you your offset for the mounting holes . If the model is designed for this size of engine you should have plenty of room . The engine  can then be mounted directly to the firewall/flat surface  without the chance of distorting the crankcase with packing washers under the stand offs . To prevent the nuts and studs pulling through or crushing  the fire wall  I make washers out of 2p peices drilled to the correct size . I dont know wether its legal or not but its cheaper than  buying washers . lets see some more pics as the build progress ?

 

Good luck

 

E.D.

Edited By Engine Doctor on 22/06/2010 11:21:44

[Vecchio Austriaco]

Thanks for the hint E.D. I have a separate thread running for the build which you find here Extra Super Bettini . I am still struggling with the wings and the tail - so to fix the engine will take a while. But I think it is always useful to start thinking a bit in advance.

VA