I am building a 1/4 scale Sopwith Triplane with scale (under cambered) wing ribs. I will be fitting an OS 120 4 stroke to power it.

I am now ready to fix the fire wall.

My question is should I add any right and down thrust, the plans (Jerry Behrens) do not show any thrust, but having recently made a flair Taube and flair Pup both of these had significant thrust angles.

You might check the incidence of the tail plane and the wings on your plans. The full size planes use incidences instead of down and side thrust. My SE5a (Duncan Hutson design) uses this method and the plane flies very well.

For example: if the wings are +3 degrees and the stab is +3 degrees this effectively gives you 3 degrees of down thrust when the plane is flying level.

I have done some research on the incidences, and found that there is 0-1/2+ on the top wing and 0 on the centre, lower and tailplane, so It doesn't look as though there is any "engine thrust lines" built in to the airframe.

Any advice would be appreciated
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Thanks


Paul

Build the firewall perpendicular to the thrust line and use shims (washers) under the engine mount to adjust the thrust. I'd start out with a couple degrees of both, maybe even 3 down and 2 right. You've got a big round hole in the cowl to stick the crank shaft out of, so you've got plenty of room for adjusting these lines. Maybe offset the engine mount upwards and to the left slightly to compensate for the down and right. I'd fly and see how things work, before installing a mock rotory.

It's interesting, thrustlines used to be discussed much more then they are now. Maybe this is because in the world of ARFs, we don't have to think, anymore!

Thanks for the help, I am a way off flying yet, but as you say I can try it, and then add any shims should I need too. The plan instructions suggest bolting the stab on and adjusting the flight using that.

I am not happy adding nuts and bolts that far backwards of the C of G, so intend to fix the stab at 0 degrees, and add the shims as sugested or simply add elevator trim, or maybe adjust the 6 ailerons.

I woudn't worry too much about adding nuts and bolts behind the cg. They aren't likely to weigh much PLUS your engine will look right sticking out of the cowl. To me that's more important than adding a lil extra nose weight.