The Aerodynamics of Wing Strut Fairings

[DanB]

I launched into the blue without my strut fairings so I'm now zeroing in on them. As I contemplate horizontal positioning between the trailing edge of the front one and the leading edge of the aft one, my question is one of best aerodynamic performance...
Q: Should the trailing edge of the front fairing be positioned in relation to the aft fairing leading edge
a. slightly to the top?
b. center?
c. slightly to the bottom?
d. other

I have seen this done all three ways and I would like to get a known physics answer. Any aeronautical engineers out there that could help? :confused:

[JimS]

Not an engineer Dan, but when I did mine I figured the spars are like a glider wing,(long and have a short cord), so I positioned the trailing edge of my fairings a tad low figuring this would give a bit of angle of attack and result in some lift being generated. Can't really say for certain that it was a good idea but it hasn't seemed to hurt.
JimS

[HighWing]

A couple of ideas. One guy elliminated the glue between the fairings and strut so the fairing would find it's own best position. No lift, but max drag reduction and that's the way he flew. I recall another guy from Florida who flew with the fairings and somehow discovered their preferred location in cruise and glued them in that position. I have wondered about an epoxy that would set in about an hour - place some in the unfinished fairings and go fly. When back on the ground do the finishing. Speaking of lift, the Florida guy talked about the lift and thrust being generated by the angle of the exhaust pipe.
Lowell

[t j]

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.

[rogerh12]

Not an engineer Dan, but when I did mine I figured the spars are like a glider wing,(long and have a short cord), so I positioned the trailing edge of my fairings a tad low figuring this would give a bit of angle of attack and result in some lift being generated. Can't really say for certain that it was a good idea but it hasn't seemed to hurt.
JimS

Jim, I agree a little positive angle of attack would be better than a little negative. Also, as I recall from the wing design section of an aircraft design book I read a few years back, a symmetrical airfoil acts as any other airfoil with a positive angle of attack, it provide lift and does so with little drag if the angle of attack is not too great.
Roger

[jiott]

I can't imagine needing any more lift on a Kitfox. The only condition it might help a little is on very slow flight; at cruise there is already more than enough lift from the wings and flaperons to hold the plane in the air. Therefore I think I would go for minimum drag at cruise, where you spend most of your time, rather than a little help at very slow speeds. Any increase in lift is always accompanied by an increase in drag, even if its very small. So I will set mine up for a neutral position in the cruise airstream and go for the most increase in speed possible. After all, isn't that the whole purpose of the strut fairings anyway.

Jim

[Dorsal]

Jim,
I believe your thinking is quite correct as any lift generated by the fairings would be inefficient relative to the main wing and would increase drag. I used option B and second the notion that fairings should be tuned only to minimize drag.

[Esser]

A couple of ideas. One guy elliminated the glue between the fairings and strut so the fairing would find it's own best position...

The exhaust thing is interesting. My girlfriend told me that on her plane the two exhausts add the equivalent of almost 10% in horsepower. It's almost 120hp from the exhaust thrust alone. This is a turbine engine mind you but still interesting. I had never really thought of it before she mentioned it to me

[Peteohms]

Just make the strut fairings extra wide and call it a biplane. I'm sorry I just can't help myself.

[Dorsal]

Would increase your dihedral a bit :D

[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...:cool:

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.