Get this sucker to fly right

[Carl]

Since we got the Centre of lift and Centre of mass markers i figued creating a stable aircraft would be a lot easier now. Certinally it helps prevent aircraft going tottally out of control straight off the runway as i saw before.

My main issues:

1. I find it nearly impossibble to trim this ting right, as soon as the nose gets to level/rate of climbs goes zero the whole thing becomes unstable and wants to flip up or down at random. It dosen't like to settle in straight and level flight.

2. If the rate of change of any angle gets too high or it goes past vertical it likes to go into an uncontrolable tumble. I can't figure out why that is or how to get control back.

That said when i can get it vaguely straight and level it flies like a dream and will happily chug along at mach 2 at around 40,000ft and if you baby it enough to keep it near but not past vertical it will happily climb at mach 1 to 55,000ft. It's got plenty of thrust and lift, it just dosen't like to fly stabely.

Hopefully somone can have a look at this and tell me why.

Edited September 25, 2012 by Carl

[trbinsc]

Where is your center of lift in relation to your center of mass?

[EndlessWaves]

Hopefully somone can have a look at this and tell me why.

Difficult to do that without pictures or a craft file.

It may be drag, if you've got more drag at the front (wings) while having engines pushing from behind then that's naturally an unstable combination and it'll want to flip to go tail first.

It could also be angled wings and/or too many control surfaces but they sound less likely from your description.

[Carl]

Pretty much on top of each other, i've re-attatched the craft file as it seems to have got lost when i hit post somehow. Check OP again .

[trbinsc]

You are going to want to move the center of lift back, or move the center of mass forward. The closer the center of lift is to the center of mass, the more maneuverable but unstable your plane will be. If the center of lift is in front of the center of mass, the plane will be extremely unstable.

[Carl]

AHHHH, i was under the ipreshion they had to be on top of each other for ideal balance, not with the Center fo lift behind . Explains a LOT. Is there any sort of too far back?

[EndlessWaves]

I had a quick fly of it and it behaved fine, is it only misbehaving at high altitude or only after flying for some time?

If it's the latter it may be that your fuel tanks are draining from front to back, thus making the front too light. Try connecting them up so they drain from ends at once.

[trbinsc]

Well, yeah, it can be too far back, which results in next to no maneuverability. Also, the best place for landing gear is either one in the front and two just behind the center of mass, or one in the back and two forward of the center of mass.

[Carl]

Ahh cheers, now i understand what center of lift vs center of mass does that helps a LOT. Also i chose the gear layout becuase it was the only way to get the plane to sit level on the ground. Also it does a darned good job of braking it, i can light the engines, hold the brakes down, then when they've spooled up drop the brakes and let it race away.

@Endless Waves: I had serius issues getting it to fly straight and level with no climb/dive on it, it didn't like to settle unles it was climbing or diving.