The Mid-wing Model

[Richard Miller]

Dave,

...and of course anyone else who'd care to comment;

I begin with those three representations of stability in mind, the ball in the shallow bowl; on the flat plate; on the bottom of the inverted shallow bowl - positive, neutral, negative - and with a pretty good understanding of how these relate to longitudinal stability - decalage n' like that.

Now the vertical/lateral. I've stayed safely in area of high wings, which is kind of cowardly on my part I think. That's the positive case, when there's vertical separation of the lift centroid and the CG. The model *wants* to do right, keep the wings level mainly, and save me the trouble of sweeping up the remains and gluing the pieces back together.

Now, please tell me about the middle ground, when the lift centroid and the CG are at the same level, and that with special reference to the effects of prop wash over the single, central-mounted fin. Do twin tails mitigate the problem.

I do see your photos regularly Dave, and druel.

-Richard

[Dave Robelen]

Hello Richard,
You are asking for a simplified answer to a pretty complex question (but I bet you knew that). I treat the vertical wing location as a situation where the dihedral can compensate to the point where a low wing model "feels" very much like a high wing. I developed such a model, the Pronto, in 1970 that handled a great deal like a high wing trainer due to a moderate dihedral angle. Actually, it handled considerably better, in that the forces were grouped much closer in the vertical plane. (See also Mega Pepper published in MAN). It has been my experience that within the range we work with, the vertical CG has about the least effect of all of the items mentioned.
The effect of slipstream on the vertical tail can be "tuned" by the arrangement of the area above/below the centerline. Take note of the numerous aerobatic designs and how they have a large portion of their area below the C/L. The twin tail option can be a bit of a mess at very low speeds, mainly due to a complete lack of slipstream impnging on the rudder(s). Without a steerable nosewheel, a twin fin airplane is just about uncontrollable on the takeoff roll. Small variations in the thrust angle, and sometimes a change of props will alter the amount of turning effect on the early takeoff roll.
In summary, a low wing model with generous dihedral and a vertical tail that has ample are below the C/L can be one of the most pleasant designs around for "laid back" sport flying.
Take care, Dave