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Desired shape of a fairing?

claycookiemonster

Well Known Member
What guidance is there for the "radius" of the fairing between two surfaces? Is this just artistry? Does it come down to "That Looks About Right?"

We are not in "area rule" territory, so I'm thinking our fairings are much like the molding at the base of a wall; they're just there to make an awkward intersection more pleasing to the eye.

I can understand why large radii between VS and HS (for example) would be impractical and block airflow, etc. Our eyes would reject that. But what about really small, tight fairings that simply close the gap with a tiny radius of, say, 1/2" or less? Sometimes "Less is More?"

Is it different for wing fairings, with the pressure differential generated by the airfoil, as opposed to the tail fairings which shouldn't have such differentials?

Inquiring minds want to know.
 
Interference Drag

My understanding is that this is a tough one to try to finesse without a wind tunnel and a lot of experimentation. IF you have a big velocity delta then a larger radius is likely to help. But it's hard to know by looking.
 
My vote: TLAR, definitely.

I made wing root fairings for my 6A with a radius of about a gallon paint can. They were not bad looking, garnered a few compliments along the way, but very labor-intensive to lay up by hand and sand, sand, sand. I never established that they added any speed, just weight and cost and build time.

Jesse in Florida has wing root intersection fairings he sells for the -10. There is a claimed speed increase, but I've seen the results disputed. At the price point, I've never been tempted.

Van, even with his commitment to "Total Performance," seems content to tolerate the interference drag penalty of wing dihedral making a less than 90 degree wing to fuselage angle across the upper surface, no factory-standard fillet fairings, so that says something. He's also content to leave open holes in the belly of the 10, and through the V-folds in all of our baggage bulkheads, and concave end-ribs in all the control surfaces, so it's a given that some of the detail work is left to us, whether it is aerodynamically significant or not.
 
Dan, good reference to Fluid-Dynamic Drag: page 8-12 explaining the desired radius as a percent of chord. BTW, that document is available online in a scanned format for reference.

https://archive.org/details/FluidDynamicDragHoerner1965/page/n146/mode/1up?view=theater&q=radius

When I did my gear leg fairings, I consulted a well known designer who is familiar with the RV's. His recommendations ended up being along the lines of TLAR as Bill mentioned, but something around of 1 - 1 1/2". Mine ended up being a radius of about 1", much more than the 4-8% from the book.

Looking back, a radius of 8% on the upper main leg fairing would be about .5". That seems really small! but may be the right answer ... image a 1" diameter ball nestling into the fairing curve.

Interesting discussion!
 
"Is this just artistry? Does it come down to "That Looks About Right?""

you nailed it in first place.
 
Trivia

"Is this just artistry? Does it come down to "That Looks About Right?""

you nailed it in first place.

Theoretical Fluid dynamics is incredibly difficult. Even the starting equation (“Navier-Stokes”) used for a first principles calculation is lacking a mathematical proof of certain fundamental properties. Said proof is one of the ‘Millenium prizes in mathematics’, worth a cool $1M to the first person to produce such a proof.
 
But there is plenty of entirely useful test data available, as DanH pointed out, in the book "Fluid-Dynamic Drag," by Hoerner. You can get lost in the theory, but the practical application involves only looking up the topic in this book and paying attention to the details. KISS!

Dave
 
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