Hi,

I have been messing with various designs for an aerobaric model and I find myself wondering what difference various wing positions make. Picture an aeroplane from the left side. You have the engine thrust line, the wing centre line and the horizontal stab centre line. Assume that wing and tail do not change and our initial design has all three on the same line. What would be the effect of lowering the wing centre? Or for that matter raising the wing?

I have noticed that most of the current crop of F3A planes have the wing lower than central but the engine appears to be lined up with the tailplane.

I know that this is a very broad question but would appreciate any ideas that you may have. I will be building a few Stik type prototypes but it would be great to have a bit more information before commiting knife to balsa.

Richard.

This is a fun topic and one that I like to discuss. I enjoy pattern flying and have always wondered how the present airpanes come up with the really wierd small compromises they have.

Assume the thing you want in an aerobatic airplane that flies the same upright as upside down the idea configuration is a fully symmetrical fuselage, a midwing, a midwing horizontal, the vertical and rudder equal areas above and below the fuselage center line.

It would not need any trim inputs but would need a little bit of continuous back stick in upright flight and a little bit of forward stick upsidedown.

This is what the control line guys have been doing for years and it gives identical responses in up and down maneuvers. Fully symmetrical aerobatic maneuvers. Oh there is some difference in the horizontal location sometimes but not enough to count.

The CL airplane (without the lines to restrain it) would respond equally to rudder inputs whether right side up or up side down. Remember that the CL guys have a greater feedback though the line pull so go to shorter tail moments than the RC counterpart which has go do snaps so need longer tail moments. But the discussion relative to longitudinal maneuvers is valid.

This is where it gets different though. The CL guys have the beast on line from a handle and have no trouble holding in up or down trim for level flight for a short length of time. Other than that they are always maneuvring so can easily draw a picture in the sky with the setup. They do that the airplane response will be the same no matter what the airplane orientation is.

With RC if we spent identical times with each side up and not a lot of duration between maneuvers we would go to the same thing as the UC guys have done. The up lines and down lines will be the same regardless.

But, in RC we tend to spend a heck of a lot of time right side up and we don't want to hold in the trim through the transmitter stick. So we give a couple of clicks of up trim and have the airplane flying level hands off right side up. What happens when we go upside down. The up elevator trim we put in means the airplane is out of trim upsidedown. It also does pulling to canopy or belly on up or down lines. So we push the stick forward.

Yeccch to holding the forward stick so much.

Maybe if the airplane didn't have to make us hold the stick forward so much, OK, move the CG aft and it is less stable and will need less forward stick. Someone thinks, hummmmm, I bet if we played with the wing downwash and horizontal tail positions I bet we could get some balancing that would auto correct the need for the forward stick upside down which was caused by the aft stick trim so we could fly upright easier. Oh while we are at it lets make the canopy really big to that it might have some effect or the other on the ability of the airplane to do knife edge flight. So the configuration is messed with and the tail ends up lower on the fuselage than it was. Then the airplane looks like the modern ship, low wing, big canopy, low tail. some motor offset, maybe a little wing or tail incidence and still having to hold in some forward stick when upside down. But it flies nicely upright doesn't it.

The guys that win are the ones that have learned by trial and error to balance the little forces. Keep in mind that anything that you do to fix a up right "problem" requires more work when upside down or on up of down lines, knife edge, etc.

But the original fully symmetrical airplane would do the job if we were willing to hold a little forward stick when inverted and aft stick when right side up. There would not be any compromises in the up and down lines, knife edges and the like.

Yes there would still be torque but it is the same both upright and inverted and would require learning to use a little rudder input. Keep in mind that a motor offset that works in upright level flight is twice as effective in the wrong direction when inverted.

Comments???