Help someone please!! I have built probably a half dozen or so planes by now, and am just finishing up a stick build "roadrunner" from Bill Carpenter plans and am wondering about the placement of the gasser Mac 28. I actually had the plans reduced 12% so the 28 would fly it better. I want to mount the motor upright, but in doing that it needs to be lowered on the firewall, which means that the crankshaft/prop line is lower than the centerline on the plans. It will still mount on center line left and right, but not up and down. WILL THAT MATTER. I would guess it would not, as the motors for some of the old Mariners/seaplanes were mounted very high above the wing to keep out of the water i suppose. Any thoughts on this. Thanks much in advance from some of you guys that know bunches about building model rc aircraft.
building planes should be in my blood. go to www.aerofiles.com and then look up 962N, or STOELK--HACKER there is a plane listed there that my "namesake" grandfather designed in the way early years. I found the data and pictures in some of my fathers things and sent them in for additions to the AEROFILES.com They already had some of the data, but were glad to get the additional.
Regards: William E. Stoelk II

No major impact will result from moving the engine down the firewall. Watch for prop clearance. You may need to add a bit of down thrust, but not more than say 1/2-1 degree (due to the vertical couple established by the low engine and higher drag profile).

Go for it!

Bedford

Andy Lennon says in RC Model Aircraft Design that a high thrust line is stabilising and that a low thust line is destabilising. However I think this really depends on the aircraft in question. My personal preference is for engines whose thrustline is directly through the wing and tail - they seem to be the most controllable, neutral and flyable.

I doubt whether the change you are talking about will have a significant impact on flying the model.

As beepee says to ensure that the thrust line goes through the centre of drag with a lower thrust line you could add some downwash.

....or you could build it with the correct thrust angle without any downwash and see how it flies. If you need corrections you could mix it in your radio. I can't remember if it is Andy Jesky or Jason Shulman who advises no firewall correction for torque/P factor, but use mixes instead. How much correction are you making for the engine to fit?

Thanks guys for your input. That makes me feel much more at ease. I just checked it again, and appears that I will only be moving down about 1/2" max. I think I'll give that a try. Its only a plane. I can always build another if something goes wrong. At least it wouldnt be getting a virgin. I've rebuilt them before. In fact, I have always contemplated folding up one of those plastic sacks you take your groceries home in, and put it in the tail of the plane when I build it. Then I would be able to carry the plane home easier if something should go wrong. Or is it when something goes wrong. thanks again for the input.
Bill