Thrustline vs angle of engine, wing and tail surfaces.

[indoruwet]

In making a plane from scratch, is there an advantage to making sure that the thrustlines of the engine, wing and the horizontal are all on the same surface ??
If not, is there an optimum placement of these three parameters, in order to have a "true" 3 D plane, not a stable upright only one.
Did take a look at the Sirex Wasp (all in line), Dazzler (horizontal stab slightly above wing centerline), Twist (same as Dazzler).
Looks like the engine is "usually in line with the wing CL.

Come to think of it, if one would like the plane to behave right side up the same way as upside down, then everything should be symetrically formed and placed .

Appreciate help here.

[Ollie]

"Come to think of it, if one would like the plane to behave right side up the same way as upside down, then everything should be symetrically formed and placed ."

Right on!

[indoruwet]

Thanks Ollie.
When I am all done, meaning, found the exact location of where the engine should be placed for optimum CG, I get you a copy. Until then, the design now has a fairly long nose and long engine carrier rails.

Still the question: why do some designs have the horizontal tail surface slightly above the wing thrust line to all the way up to the top of the wing. Does this have anything to do with "aerodynamic wash" ?? Is there a magic number about how far back the tail surfaces have to be from the main wing, in order NOT to be affected by areodynamic forces ??

TIA

[Ollie]

Start with the CG forward. Fly it. Move the CG a bit aft between flights. It moves the CG a bit so it makes the plane less stable. When you get to neutral stability, then don't ever change CG. You have found the best CG for maneuverability. Adjust throw controls for worst maneuvers. The plane points where you point it. Adjust the thrust line so the throttle control range doesn't dive or climb. Only climb or dive under elevator control. You might hold the elevator just a bit for level flight upright up and inverted with a bit of down.

[indoruwet]

Hej Ollie, I think you misunderstood me, since you do not know the whole picture yet. I have done the numbers to get where the CG is supposed to be, with reference to the main wing. So that is not the problem. I did a layout (ACAD 2005) and I have most of the plane layed out, but I do not know yet where to mount the engine so I get the CG correct for "SO SO OK" flying, without adding useless weights. I have made the nose section long enough to be able to move the engine back and forth on the rather long engine mounting rails. So when I get to that point I can mount the engine at the right place and then redesign the nose section. I have placed the tail section in about the same place where "everybody else" has it. Once that is done I will fly the bugger and then tweak the CG back, by moving the battery back, to get 3 D flying capabilities. I like to fly a bit CG back anyway. In my initial design, the engine is mounted right side up, just to get the mounting position for proper CG. In the end I will mount the engine different, like I do in all my other planes. I hold the engine by the prop and see how it "hangs" naturally. I then mount the engine in that particular position. I also use only Sullivan mounts. Those thin plates that weigh almost nothing.
Sorry for long post.

[Ollie]

You must find every item of weight which sums to the total flight weight. The position of item distance from plane CG multiplied by item weight is item moment. The sum items moments fore the CG and sum items moments aft the CG equals zero. Design arranges every item so the balance is zero. You want to position the heavy items close to the CG. You want pack heavy weights close to the CG. Start with the motor, prop and motor mounting. Etc, etc. Last make the tail moment and tail area so the plane balances. You might redesign the tail a by a better structure weight. You might redesign the fuselage structure. You might use a micro for throttle servo to save weight. You might change the radio battery type to save weight. You might change the wheel type to save weight. You have control of the design process. Don't paint in the corner by a rigid design process.