Funjet conversion

[doctormike]

I am converting a Multiplex Funjet to a 64mm EDF. My main concern is how to line up (parallel) the EDF with the wing chord. It appears that the bottom of the fuselage is parallel to the wing. I'm going to try placing it on a level surface and then place a level on top of the EDF. After all this, should the EDF BE parallel to the wing? How much down thrust or up thrust? Any input is certainly welcome. Doc

[KJD]

You will love this answer, it depends.

Remember that a wing flies at an angle of attack to the air flow. What angle that is highly dependent on the speed and weight of the ship. So to aim the thrust vector in line, or slightly down, with the direction of flight you would want a thrust line that is a little low in front. Now the tricky part, this assuming the thrust passes in front or below the center of mass. Now if the thrust line passed behind and above the center of mass the thrust line is often higher in the front. This is so the thrust is is actually pushing slightly up on the rear of the ship. In effect giving you some down thrust, very helpful at launch.

So, have you flown the Fun jet as a rear driven prop plane? Are you happy with the placement of the thrust line? If so mimic the same line with the fan. If not make adjustments with the prop thrust line before adding the EDF unit. If you have to change the placement of the EDF (higher or lower to the centerline of the wing) make the adjustment with an eye to the center of mass (often highly dependent on battery placement, up or down in the fuse).

Yes, the center of pressure will have much the same effect on the trust line (thrust induced pitch changes). But by then the plane is often at flying speeds so aerodynamic forced can be used to trim the plane.

[doctormike]

Thanks, KJD! Yes, I have a prop Funjet and it flies like a dream. The EDF center is about 1" above where the prop center would be. I assume that would push the nose down. I have put in a tad up thrust to counter this. Does that sound right to you? Doc

[KJD]

Higher thrust lines do tend to push the nose down.

[gooniac33]

A thrust line above the centerline will need up thrust to keep the plane from nosing over. In my experience with pusher (prop or edf) the thrust line must go through the CG for best performance. If the thrust line is low enough there will be no need to tilt the motor to get through the CG. But if it is above or below the centerline you will need to adjust...