Question: I am installing my engine and I'd like to shift the thrust line. Is this OK?

Answer: Well... it depends! The factory location for the vertical and horizontal thrust lines is fairly important and the best option is to not depart from the factory specs regarding thrust lines. However, we recognize that some engines and some engine orientations may look better aesthetically if they are shifted somewhat from the factory thrust line locations.

Our advice in this regard is as follows:

1) Do not shift the left and right location of the engine from the location of the vertical thrust line recommended for the model. i.e. keep the engine centered on the firewall with respect to left and right. If you need a bit of left or right thrust, use the power module nut and bolt spacing system or the engine mounting angle to change the thrust angle but do not offset the engine itself to the left or right.

2) You can shift the engine slightly higher or lower on the firewall than indicated by the location of the horizontal thrust line on the firewall. Don't go crazy... a centimeter maybe is a good maximum guideline depending on the model and how you plan on using it. The more you depart from the recommended location the more risk you are taking that this will adversely and significantly affect the flying characteristics of the model. Again, if all you want to do is induce some up or down thrust, use the the power module nut and bolt spacing system or the engine mounting angle to change the thrust angle rather than offset the entire engine up or down.

To understand what is happening when you shift the thrust lines by moving the engine up or down, left or right on the firewall.... it helps to take things to an extreme in your head. Consider nailing a 48" long 2x4 plank to your firewall so that the plank is pointing up and down and centered over the firewall so that 24" is above the center of the firewall and 24" is below the center of the firewall.

Now imagine mounting your engine to the front of the top end of the 2x4 and think what would happen when you rev up the propeller. The engine thrust would pull the top of the 2x4 forward and torque (twist) the nose of the model downwards. So applying power would result in the model pointing down... not really what you expected but that's what will happen! This will require aggressive amounts of up elevator to correct and the degree of up elevator required will constantly change not only with the engine thrust per RPM and prop selection but with the degree of effectiveness of the elevator depending on the air speed of the model. So every time you change the tprop or the throttle setting (RPM) of the motor or the models air speed you will need to adjust the elevator trim. Talk about a pain in the butt!

Now let's go the other way... imagine mounting your engine to the bottom end of the 2x4 and think would would happen when you rev up the propeller. The engine thrust would pull the bottom of the 2x4 forward and torque (twist) the nose upwards... i.e. applying power will result in the model pointing up. This will in turn require lots of down elevator to compensate for and the amount of down elevator needed will constantly change depending upon engine thrust and the effectiveness of any down elevator setting will in turn vary with the air speed of the model. Every time you change throttle setting of the motor or the prop or the air speed of the model, you will need to adjust the elevator trim. Big time headache!

So taking our imaginary extreme analogy and putting it into context of a typical model without a big honking 2x4 nailed to the front of the firewall... here are the results of moving the engine up or down from the recommended thrust line location:

a) Moving the motor below the recommended thrust line will pitch up the nose somewhat and require varying amounts of down elevator trim depending upon RPM, prop selection and air speed of the model.

b)  Moving the motor above the recommended thrust line will pitch down the nose somewhat and require varying amounts of up elevator trim depending upon RPM, prop selection and air speed of the model. 

c) The pitch up and pitch down forces will increase as you move the engine further from the recommended thrust line.

d) The further you shift the thrust line the more "snaky" and jumpy the model will tend to be as you vary throttle settings and the more difficult it will be to fly in a controlled and predictable manner.