The Aerodynamics of Wing Strut Fairings

[DanB]

Gents,
Thanks for all of the comments, thoughts and feedback...just what I was interested in hearing. I had a chance at school to talk with a retired aerospace engineer (worked for Boeing). His take was to center them (option b). Many of you were right on target as was one of the "Skunk Works" or Kitfox South engineers (Thanks Bill).

The most important thing to do while centering them is to make certain that each side (both right and left struts) are done precisely the same. As you can imagine, if one side is up or down a little more than the opposite side...you could be building an unwanted roll into the ship.

Thanks again

[ken nougaret]

Whats the best way to align the lift strut fairings so you know all four are perfectly level straight into the wind?
My thought was to clamp a flat sheet of plywood across the top and then place spacers between the trailing edge of the fairings and the plywood. Wings are on the plane and plane is level.

[Micro Mong Bldr]

Lowest drag is the way to go. Fairing chord is short - low Reynolds number flow!

[av8rps]

I remember reading in Horner's aerodynamics book that a very efficient fairing chord should be 4.25 times the diameter of the tube you are trying to reduce the drag of.

Ironically, the old plastic fairings on my Model 4 are 4 inches wide and 1 inch thick. And I have a fast Kitfox. So if it were to do another Kitfox, I'd make them 4.25 inches wide, and then adjust them for least drag.

I would also streamline the horizontal stabilizer struts. I put 1.75 inch wide ones on mine.

For what its worth, the streamlined chromoly struts (big bucks) on my Highlander are only 2 3/8ths wide and 1 inch thick. If they weren't so nice I'd fair them out another 2 inches and I'd bet my Highlander would fly faster...

Worth mentioning, if you want to see the ultimate wing strut idea, check out Dean Wilson's "Ellipse" aircraft design. That was a 150 hp and 150 mph 4 seat folding wing airplane much like the Avid Flyer / Kitfox design. But it was very low drag for a high wing strut braced aircraft that was still STOL. Dean knows how to reduce drag...

[LSaupe]

I have a set of wood fairing that I attached with Duck tape to test the effect before purchasing the PVC type. I positioned them with the trailing edges centered with the struts. I didn't think to try different angles.

TJ:

Did you notice a difference in cruise airspeed with the fairings temp-installed in this manner?

What did you end up with for a final configuration?

[t j]

With the trailing edge of the wood fairing centered with the struts, cruise speed increased from 75 mph to 85 mph. I installed the PVC fairing permanently in that configuration.