Problem with my BD-5J design

[Speedfreak]

Here is the link to my thread, figured I'd get a better response from you guys.


http://www.rcgroups.com/forums/showthread.php?t=677616

any help would be great!

[Sparky Paul]

How scale is it?
ISTR those things have a very small horizontal. You may be running out of elevator power.

[Salto]

The two problems you report, not looping and porpoising, sound to me to like symptoms of a CG that is too far forward. Have you performed a dive test to determine what pitch stability it's flying at? This will tell you how much further you can shift the CG back.

I'm not familiar with the planform, but as Sparky says if the elevator is small that will exaggerate problems caused by a forward CG. You might also experiment with coupling a bit of elevator into the ailerons to act like elevons. In other words, think of it as a flying wing. This may help with pitch control.

Graham.

[flybd]

See my post in original thread:

http://www.rcgroups.com/forums/showthread.php?t=677616

Joe

[Speedfreak]

I have coupled the ailerons to the elevator, still hasn't cured the looping problem but I can almost do vertical landings with them mixed and full up. :D

[Salto]

Speedfreak,
Have you done a dive test? I still think CG location may be the cause of your problems.

Graham.

[Brandano]

I realized a detail reading the other linked post. Too large throws on a flying tail will stall the elevator, and make it ineffective. Ideally you'd have a system to control pitch relative to the airflow rather than relative to the airframe, failing that you should have the elevator range somewhat restricted a few degrees from the AOA range of the plane. Also, with a high set tail and a low set wing at some AOA the wing will blank the tail, and you might even risk a superstall, a problem known in the F104, for example.