rsieminski

Scratch built, low drag, sholder wing. If the wing is slightly above the thrust line, how much downthrust? There must be some rules of thumb on this.
Thanks,
--Rick

rsieminski

WOW! I posted this question 2 months ago and no replies. Now, In my search for the answer, it comes back to me again. I was going to post this question again. At full throttle, the plane needs a little down elevator. I've checked the incidence, and they appear to be at 0/0. As near as I can tell, the thrust line is parallel to the chord.

Down-thrust or upthrust?

Mike James

The answer you seek is dependent on too many unknown variables to answer.

My advice is to first set up your airplane per the manufacturer's specs, then go fly it. (If it's an original design, setting everything to zero degrees is a safe place to start.)

There are many "trimming charts" available online, mostly created by Pattern flyers, which will help you then decide what needs to be changed, whether it be the incidence of the flying surfaces or the engine thrust line.

You can download one of these trim charts at http://www.nsrca.org/

Ollie

When the thrust line passes through the center of drag of the whole plane, no up or down thrust is required. When the thrust line passes above the center of drag it forms a nose down couple with the drag force and requires up thrust to compensate. When the thrust line is below the center of drag the couple produces a nose up moment which requires down thrust to compensate.

Just don't ask me to tell you how to locate the center of drag. An eyeball estimate of the center of drag location should be close enough for initial flight testing to prepare for fine tuning. As Mike said, the best way to find the thrust offset is by flight testing. Flight testing to arrive at thrust, CG and decalage adjustments is part of the design and development process. It should be done on the prototype before plans or kits are made available to the public.