ptxman

My Feedback: (2)

I have some questions for the aerodynamic theorists. It occurs to me that the horizontal stabilizer has a very limited function in a pylon racing application. It has to (1) counter pitching moment from the wing (2) provide necessary ‘up’ force for 30 pylon turns (3) provide enough throw for takeoff & landing control. So why do most racing stabs have a standard symmetrical shape? Unlike aerobatic planes, they never (intentionally) fly inverted or have to use the same degree of down throw. Wouldn’t some sort of dedicated airfoil & incidence combo be more efficient towards providing a less drag? If so, what sort of engineered airfoil & arrangement would be best? What is a typical tail drag component % relative to overall model drag? Is it worth worrying about? Do stabs with very sharp L.E. radii have any disadvantages in terms of pitch sensitivity to the wings wake or buffety air for example? If so, would a blunter nosed shape of the same % thickness be better? What about hinge line, would a top skin hinge be better vs a mid or bottom hinge assuming all were sealed & there were otherwsie no construction concerns?

banktoturn

ptxman,

The horizontal stabilizer on a pylon racer has a limited function most of the time, just as most horizontal stabilizers have limited function most of the time. Generally, the amount of downforce required of the stab is small, as long as the CG is in a fairly neutral position. You are right, since the plane never flies inverted, it could give a small reduction in drag to give the stab a bit of 'upside down' camber, especially if you could get it right on so that level flight would require no elevator deflection. This makes me wonder whether there is more potential gain in optimizing for level flight or for turns. I think the gain would be fairly small in either case, but that is a mostly uneducated guess. I don't think there is any special sensitivity to the LE radius. I think the main issues involving LE radius kick in in the neighborhood of stall, and there is no reason that the stab should be near stall. I think the best choices for LE radius and hinge line are the lowest drag choices.

FWIW,

banktoturn

BMatthews

Good post. I've often thought about doing the same thing for sailplanes. At lest until I did some calculations using the old David Fraser software that told me the tail works with 0 load when the CG is optimized. The only thing I'm not sure of is whether or not I could comfortably fly the model with a 0'ed stab load.

Ollie

A stab airfoil is really a series of different airfoils, one for each angular position of the elevator. Stabs generally operate at very low lift coefficients and there is usually a big stall margin. The drag characteristics depend on the smoothness of the hinge line on the convex side when the elevator is off center. For most thin sections and elevator deflections of up to about 3 or 4 degrees the drag remains low because the flow remains attached. Beyond that small deflection, the drag increases abruptly as the flow seperates and continues increasing with increased deflection as the turbulent wake thickens. For speed models, it is important to keep the pitch stability close to neutral so that the horizontal tail load and elevator deflections are minimized so that horizontal tail drag is minimized during the high speed mode of flight. For take off and landing more elevator deflection can be tolerated.