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Old 08-05-2020, 07:08 PM
krwalsh krwalsh is offline
Join Date: Nov 2009
Location: San Francisco, CA
Posts: 369
Default Two Things

Originally Posted by walter View Post
I wonder if it's more aerodynamic than our standard wheel pant fairings. Thoughts from the aerodynamically minded?
First, I'm so glad my plane is an experimental, and I am the holder of the Repairman's Certificate for it. For $6000 I can buy all the carbon fiber I want, make my own molds, put wheel pants on my plane, then look around at what to do with the remaining $5800 I would have left over.

Second, in general the form drag of that wheel pant to gear leg intersection fairing would be higher than the most optimized solution. The best way to minimize the interference drag of the wheel pant to gear leg are the following, in roughly priority order:

1) Make the radius between the two start nominally near zero at the leading edge, and grow to be somewhat larger than the thickness of the boundary layer at the end. For the speeds we fly and the sizes of our wheel pants, this means start at the radius of your index finger (~1/4") and increase it to ~2".

2) Do not align the thickest part of the wheel pant and the gear leg fairing. This is much more difficult, as the thickest part of the wheel pant is typically lined up at the tire, and the gear leg fairing is thickest at the axle, so the two thick portions line up. This can be avoided by increasing the thickness of the gear leg fairing and moving it forward or aft. It can also be done with non-traditional wheel pant shapes. There are some great laminar flow airfoils that move the thickest section very far aft. This, combined with a symmetrical airfoil for the gear leg fairing of roughly ~30% chord pushed as far forward as possible helps to not have the two thickest sections align.

3) Realize that there is a point of diminishing returns for the wheel pants. No wheel pants and no gear leg fairings will get you a double-digit hit in cruise speed. Putting a set of wheel pants on will gain most of that. The gear leg fairings will add ~4-5 kts. From there, careful attention to intersections, finishes, not having protruding fasteners, etc is down into the 1 kt or less territory.
Kevin R. Walsh
Cozy Mk-IV
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Old 08-06-2020, 07:32 PM
RV-4 RV-4 is offline
Join Date: Oct 2007
Location: St-Jerome,Quebec,Canada
Posts: 1,147


''I went back to the 5.00X5 and Sam James wheel pants... (I just don't like the looks of Van's wheel pants, but drag rise they are fine.''
How did you find the installation to be?? I looked at them today and like the profile versus the Vans,s One that I find kind of bulky on a -4 with short legs..??

Would you have any pictures of your installation you would like to share?


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Old 08-06-2020, 09:30 PM
BillL BillL is offline
Join Date: Sep 2007
Location: Central IL
Posts: 5,742

I am happy to hear a couple of expert aero guys talk about the intersection radius.

Here is my 7 with the James pants. I was concerned and thinking about fitting the Vans PR pants to get some additional speed.

If the area aft of the intersection is tufted, would they lay flat?

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Lord Kelvin:
I often say that when you can measure what you are speaking about,
and express it in numbers, you know something about it; but when you
cannot measure it, when you cannot express it in numbers, your knowledge
is of a meager and unsatisfactory kind.
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Old 08-07-2020, 01:18 PM
Cumulo Cumulo is offline
Join Date: Apr 2015
Location: KHMT
Posts: 74

Originally Posted by BillL View Post
I am happy to hear a couple of expert aero guys talk about the intersection radius.

Here is my 7 with the James pants. I was concerned and thinking about fitting the Vans PR pants to get some additional speed.

If the area aft of the intersection is tufted, would they lay flat?]
I've read Sig Horner's DRAG cover to cover several times, done wing section optimizing with Xfoil, etc.l and , so I am very familiar with the aero mechanisms at play here. I also find this subject area the be a lot of fun.

Bill. You are right on, I think, suspecting separated flow aft of the gear leg intersection at the wheel pant intersection. The negative factors are low Reynolds Number on the leg, low fineness ratio and boundary layer addition.

It is all but certain that you will have a significant separation bubble near or aft of the section high point. The question would be whether the pressure gradient will be too severe to allow re-attachment and pressure recovery.

Maybe a little boundary layer stimulation at about the 1/3 chord point will help if your tuff tests show separation there. Golf balls have dents for a reason.

I've done napthalene coating for laminar flow, which is easy, but not separation tests. You spray it on , rip around the pattern , land and see what you've got. It might work, but tuffting is standard for indicating separation.


Last edited by Cumulo : 08-07-2020 at 01:29 PM.
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