Up thrust makes plane unmangeable?

Well, I finished an electric plane and found that I have some up-thrust to the prop due to the down- swoop of the front end of the plane. Took it out to a nearby field and flew it. The first take off was uneventful except for the needed adjustment to the aileron. Landed and corrected, then took off again. This time the plane was extremely sensitive. All flying surfaces seemed to be correctly adjusted and the plane was balanced.

Could this be because of the up-thrust?

Wan

Reply to
wanjung
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Up thrust would cause the plane to climb more as power is applied. There wouldn't be any way to trim it for level flight at different throttle settings. If it's too sensitive, it's either tail heavy, you have too much throw, or need to add some more expo. What model is it? Some are designed to be more sensitive...

PCPhill

Reply to
PCPhill

PC, It is an 46" wing span electric model I designed and patterned after a photo I saw in an ad, looks like a jet plane. Therefore the down swoop on the nose section. I thought I compensated with the datum line, but the drawing may have been a bit off, the price of sketching a building plan. Would a swept wing contribute to the sensitivity of the plane?

Do you think shimming the motor to tame the up-thrust and moving the CG forward would help? Though shimming may spoil the nice fitting of the spinner. I spent time to make this plane look nice. Maybe better handling is better than appearances?

Wan

Reply to
wanjung

I would try to move the Bal point forward first. mk

Reply to
Storm's Hamburgers

If you get it running at some constant throttle setting and it's too sensitive then up thrust isn't causing the stability problem -- thrust line problems show up as a coupling between throttle and pitch.

If you trim it for straight and level flight and every little gust makes it want to shoot up or down, then the center of gravity is too far back.

If you can trim it to fly hands off, but hands-on it wants to dart every which way, then the throws are too big (or you need more expo, if you have a fancy radio).

The stability problem will magnify it's response to the thrust line. _After_ you have the stability problem dealt with, then start playing with the up thrust issue. If it were me I'd take the spinner off, get the motor oriented right with shims made out of whatever is handy (I've used dirt before). Then I'd take it back to my shop and hack into the motor mounts to get the centerline back where it should be to look good.

If this thing is going to spend much time on its back then you probably want a thrust line that's closer to level than down (and you want a CG that's farther back than on a trainer). If you're at that level of flying make sure to do some inverted maneuvers when you're assessing the thrust angle. If you're _not_ at that level of flying then keep the canopy up and point the propeller down.

Reply to
Tim Wescott

Tim, I like the you way you put the "if", "then" The way I see it, you explained the various problematic possibilities and their solutions. I have been hacking at the thrust line problem and have the up- thrust pretty well moved into neutral. Then I was able to easily readjust the CG by moving the battery. The nose section didn't look as pretty as before, but what the heck, whose going to notice. Anyway, the plane is still in the experimental stage.

Remember, I took the plane off only twice and never was up more than a minute or so each time, before I encountered the touchiness of the plane.

Now I am ready to give it another try. If the plane handled as well as my other ones, I will report.

Thanks to the others who made commentaries. I take every suggestion with appreciation. Wan

Reply to
wanjung

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