Is a real airfoil on the empennages a benefit?

Saw your posting on ezone about airfoils and thought you might be just the guy to ask the following if you don't mind spending the time.

In a typical RC model with flat vert and horizontal stabs. would the drag be much reduced if these were replaced by fatter more rounded shapes? Is the difference worth worrying about? I have seen charts showing the effect of the LE and TE shape, but can't recollect anything about the bit in between. I am thinking of speeds up to say 70 mph.

From : Don Stackhouse

It depends, but from what I'm seeing, there are several reasons why a "real" airfoil is better.

I generated a 4% thick "flat-plate" airfoil with an elliptically radiused leading edge and a straight tapered trailing edge, and compared it in my airfoil program to an 8% thick SD8020 airfoil (a very common section for foam-cored tails). The program didn't like the flat airfoil at all; in fact it disliked it so much that I don't trust the results, I think they're overly pessimistic. Even so, over the entire range of Reynolds numbers from 60K to 300K, the difference in drag was very significant, even allowing for an overly pessimistic computer code.

Possibly even more important, the max lift of the flat-plate airfoil was much lower than the thinned SD8020, and the lift and drag characteristics vs. angle of attack were downright squirrely! This means that the flat tail would have to be larger to generate the same maximum control force. It could also be prone to porpoising and other control abberations due to hysteresis and/or deadband in the airfoil characteristics. All this means that you might need to have a larger tail to compensate for the lower lift capabilities, and that there could be some handling problems that absolutely refuse to go away no matter how large you make the tail.

A two-element tail surface (like a stabilizer/elevator, as opposed to an all-moving tail) helps this situation somewhat. The introduction of camber with a control input helps stabilize the airflow across the tail, as well as increasing the max lift capability of the tail surface. This last item is one of the main reasons I usually use two-element tail surfaces on my designs; in addition to the structural benefits, a two-element (or even a three-element) tail doesn't need to be as big to generate the same maximum control force. On the other hand, the damping requirements for dynamic stability often dictate the tail area required, so in that case you don't get to save on tail area. You do, however, usually get an increase in control authority and allowable c/g range in that situation.

Overall, a properly airfoiled tail section with a hinged control surface will usually give the best combination of low drag and predictable handling. In particular, if your flat-plated-tailed model is showing some quirky oscillations or refusal to hold a trimmed pitch attitude, you might try changing to a "proper" airfoil section. If you want to save every possible bit of drag, then it's probably worth changing to a better airfoil on the tail.

OTOH, if the performance and handling of your flat-tailed model are living up to your expectations, and durability and ease of construction and repair are important to you, then a flat-plate tail airfoil might still be a good choice despite the theoretical drawbacks. It's like the old adage about the bumblebee; if your flat tail doesn't know that theoretically it can't possibly fly well, it just might fly well anyway!

Don Stackhouse @ DJ Aerotech
djarotec@bright.net
http://www.bright.net/~djwerks/