I am new to the game and just crashed my LT 40 on the third flight after my solo when the rudder came off at takeoff. Things happen fast when the rudder departs the fus, and none of them are good I think. I fractured both motor mounts, SIG type, and I want to replace them and go to a smaller motor. I know the mounts should be centered left and right on the firewall, but how about vertically? What happens if the mount places the motor slightly above where it was before? Thanks

A little higher or a little lower, no problem, but make sure it is as close to center as possible, and I mean the tip of the engine shaft, not the mount itself, remember that most airplanes call for a couple of degrees of right thrust, in other words, when looking at the aircraft from the front, the motor mount will be a tad to the right of the vertical center line of the firewall. Although I've seen engines pointing at every which direction, they flew good enough to learn on. But try to be as close to center as possible, it will fly better.

I've had tons of answers on this subject,but I've rasised or lowered the engine an 1 1/2" on some planes because of larger engines and fuel draw,trim changed a tad but other than that I could not see any difference,would be nice to have more answers on fact.Best I can answer.

Assuming that you're mounting the engine upright, the top of the mounting rails should be on the thrust line. Engines are designed with the bottom of the mounting flange at the thrust line

I just mounted by trainer engine with plug on top but I have seen some planes where the engine is mounted sideways. Can the engine be rotated any direction on the firewall as long as the thrust line stays in the correct orientation. I assume the answer is yes since plances can fly upside down but does the orientation effect things like idleing and fuel flow?

answer is yes, but I'm like bulletbob,have never got an answer as to why, or what changes in flight when the thrust line is not used,because there are a lot of r/c planes and full size planes that the engine is not on the center of the thrust line,yes I can see it on the prints but that isn't a very good answer.Hey Ollie,need your help, where are you?

engines can run in any orientation to the ground but they run best when upright.

when they are inverted you can have a problem with flooding the top of the cylinder which can cause it to not start at best and damage the engine at worst.

alot of times mounting and engine sideways or inverted is done for cosmetic reasons (to fit inside a cowl) or so that the carb is not too high to aid in fuel flow.

bullet when you mount your engine you want to check were the carb is going to end up.

if it is too high above the centerline of the fuel tank then you will have trouble getting the engine tuned (it will want to lean out alot when fuel gets low) too low and you will have a problem with flooding and too rich.

Thanks for the assistance. I have placed the new, smaller engine about where the old one was on the LT. I am finishing a Armidillo trainer now and find with the engine on its side that your comments will also be usefull. Onward and upward!

Actually the sidesaddle mount (cylinderhead 90 to the right) as far as engine function goes is the best possible position. this allows positioning the tank in the best position for the most uniform engine run throughout the run with the tank centered on the spraybar. Its for this reason virtually all pylon racing airplanes are sidesaddle designs. Another very good reason for this is safety. This mounting will put conventional needles straight up and a much better position for adjusting.

Concerning vertical and lateral thrustline changes, both will affect flight slightly with vertical changes actually effecting a more obvious change than a lateral change. All airplanes have a vertical CG as well as a longtitudinal one this is the point where the lateral axis runs through the airplane. If the thrust line is below that point (as most high wing cabin types are) then when power is increased the ship pitchs up and down when reduced. Down thrust is designed in to combat these pitch changes with power changes but this only occurs at one power setting.
Raising the thrustline can reduce these pitch changes or lowering it can aggravate it. In the case of low or mid wing aerobatic type there is no or very little pitch change with power because the thrustline is very close to that vertical CG if you will or lateral axis.

Lateral thrustline changes. Most high wing types have right thrust designed in to combat the left yawing tendency at low airpeeds and high deck angle caused by among other things 'P' factor which is a disymetry of thrust across the propellor as a result of that high deck angle. Now moving that thrust line to the left (not changing the angle ) will have a slight effect of increasing the right thrust and moving the whole thrustline laterally to the right without changing the angle will have the effect of reducing the right thrust. These lateral displacements of the thrustline (not thrust angle changes) has only a marginal effect because of the confines of the nose.

Sorry for motor mouthing

John

Thanks again. Your explanation makes sense and fits with what a real pilot, my son. tried to explain to me. The armidillo indeed has a sidesaddle mount and I feel better about it now. These theoretical discussions must be about subtle effects which would explain why so many differently constructed models all seem to fly acceptably. Or may explain why there is so many crashes in this consarned sport. Happy Hunting

I am building my first plane its a Sig LT 40. I have seen numerous comments from other fliers on this forum state that they had to put triangle re-enforcements along the bottom of the stab. I have'nt got that far on mine yet but your story convinces me its the way to go...sorry to hear about your plane.

I now have the LT back together with the smaller engine but do not really have a cure for the stab falling off and had not heard others with a similar problem. I am reinforcing the front of the stab since it drops slightly into the fus at that point. Other suggestions welcomed. This is my first kit and I spent so much time on it that I fell in love with it. wrong thing to do with a trainer. I just finished the Armidillo, coroplast and all, and it took no time at all and I am more than ready to risk it in the learning process and I suspect it is a whole lot more durable when its flight path intersects mother earth.

Bob, just cut some balsa triangle stock and place it under the stab where it meets the fuse and on eash side of the vertical fin where it meets the stab