What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Wheel pant intersection fairing - interesting design

walter

Well Known Member
Was browsing the net and found a photo of the WAT (Whelen Aerospace Technologies) wheel pants for the Cirrus SR series aircraft. The wheel pant intersection fairing looks interesting. I wonder if it's more aerodynamic than our standard wheel pant fairings. Thoughts from the aerodynamically minded?

Wheel pant/Fairing:
6968bf_c8477ce9e4584ef6a920142b36c3b497_2048x2048.jpg


Website Link: https://flywat.com/products/main-pants-cirrus-iceskates
 
The trade-off here between the two concepts (the one pictured vs a typical RV fairing) is between intersection interference drag and wetted area and form drag.

It is generally not too good to have an airfoil-shaped strut intersect a body at an acute angle. The boundary layers from the two bodies merge and aggravate each other, and it is possible to get some separation. This is less of a problem if the strut is not lifting, and if the strut is pretty thin (in percentage of thickness to chord). Both of these factors make the normal RV intersection fairing work pretty well.

The pictured fairing has gone to great lengths to ease the intersection angles so that the boundary layers on the two surfaces have much less interference. But in exchange for that, the designer has created a very thick cross section with a lot of wetted area and frontal area.

I would be very surprised if the pictured fairing has less total drag than a typical RV type fairing, well executed.
 
The trade-off here between the two concepts (the one pictured vs a typical RV fairing) is between intersection interference drag and wetted area and form drag.

It is generally not too good to have an airfoil-shaped strut intersect a body at an acute angle. The boundary layers from the two bodies merge and aggravate each other, and it is possible to get some separation. This is less of a problem if the strut is not lifting, and if the strut is pretty thin (in percentage of thickness to chord). Both of these factors make the normal RV intersection fairing work pretty well.

The pictured fairing has gone to great lengths to ease the intersection angles so that the boundary layers on the two surfaces have much less interference. But in exchange for that, the designer has created a very thick cross section with a lot of wetted area and frontal area.

I would be very surprised if the pictured fairing has less total drag than a typical RV type fairing, well executed.

Steve,
Agreed on the Interference, wetted, and form drag comments, but the WAT pants will produce far less induced drag as your pocketbook will be $6,000 lighter. What a luxury experimental aviation is!

Peter
 
A More Minimalist Approach

The timing of this thread coincides nicely with development of an integrated gear leg & lower intersection fairing for the RV-6 and RV-7.

The design approach I chose is a fun counterpoint to the approach WAT chose for their Cirrus fairings. You can see that the intersection fillet is much smaller resulting in less wetted area and less frontal area.

Another factor in sizing of the intersection fillet was my desire to not have so many "extra" screws just for attachment of the intersection fairing to the wheel fairing. Only one of the five screws attaching the intersection fairing is not already part of either holding the front and rear wheel fairing halves together or a bracket attach location.

Gotta admit that the little door in the WAT fairing for airing up the tires is pretty nifty...just not $5,995 nifty :eek:
 

Attachments

  • DSC_1097.JPG
    DSC_1097.JPG
    788.9 KB · Views: 720
  • DSC_1099.JPG
    DSC_1099.JPG
    635.9 KB · Views: 696
Nice looking fairings. Are you planning to offer these for other RV models?

Jerre
 
Are you planning to offer these for other RV models?

Yes, the wheel fairings you see in the photos are designed to allow those wanting the greater rolling diameter and width of 6.00-6 tires to have an easy-to-install and lightweight wheel fairing option. We have wheel fairing kits for RV-6 & 7, RV-8, and RV-12.

Matco offers a nice 6 inch wheel & brake that is perfect for RV-12 - it fits on the standard axle, is lightweight, and the tires can be used "tubeless". http://www.matcomfg.com/MHE6BESERIESforA3AAxle-idv-2336-30.html
 
Ken,
Don't most people glass their intersection fairings onto the wheel fairing and split it along the same parting line as the front and rear half of the wheel fairing?

So no screws at all to attach it. The intersection fairing becomes integral to the wheel fairing. That's what I did.

The timing of this thread coincides nicely with development of an integrated gear leg & lower intersection fairing for the RV-6 and RV-7.

The design approach I chose is a fun counterpoint to the approach WAT chose for their Cirrus fairings. You can see that the intersection fillet is much smaller resulting in less wetted area and less frontal area.

Another factor in sizing of the intersection fillet was my desire to not have so many "extra" screws just for attachment of the intersection fairing to the wheel fairing. Only one of the five screws attaching the intersection fairing is not already part of either holding the front and rear wheel fairing halves together or a bracket attach location.

Gotta admit that the little door in the WAT fairing for airing up the tires is pretty nifty...just not $5,995 nifty :eek:
 
WHEELS FAIRINGS

Hi Ken

''Are you planning to offer these for other RV models? ''
--------------------
If you ever have a kit for the RV-4, I will be very interested.

Thanks

Bruno
[email protected]
 
I guess if you own and fly a $175K to $500K (not kidding) Cirrus single engine plane you can afford $5995 for wheel pant intersection fairing...:rolleyes:

As was said above, just eyeball engineering the fat gear leg to wheel pant intersection fairing does not appear to be better than sliced bread (greater wetted area, frontal area). Intersection/interference drag is life when ever anything intersects, like wing to side of body. Van's wing to body sheet metal and rubber seal is easy to install, but my RV4 had fiberglass fillet fairings... Did it help? Don't know. However my 150HP C/S Hartzell, SL top speed of about 206 mph, 75%@8000ft cruise 194 mph, which is about the same or a few MPH than 160HP RV-4.

However you will get slightly lower drag with good DETAILS. As was said having smooth gap free, edge free fairing, as in glassing in integral to wheel pant to gear leg fairing verses screws is likely give you a tiny advantage.

Another way to lower drag is used the smaller LAMB tire and smaller fairing. I went with smaller tires/fairing initially. However before installing it 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.) I chickened out on the smaller LAMB tires. The 5.00x5 is pretty small for mains on an airplane that can weigh 1800 lbs, on soft fields. Just me... Speed is fine, but if you flip on landing the fun stops...

I also went with wider fiberglass gear leg fairings (RV-7). My RV-4 had very short chord aluminum ones.... The later fiberglass ones from Van are wider. The fiberglass ones from Team Rocket are wider still.... Keep in mind Van has cast some shade on wider gear leg fairings due to possible changes in stability and/or spin characteristics. Nothing has ever been determined, but just be aware.
.
 
Last edited:
Expensive, ugly, and really not a discernible drag reduction. Any one of the three separately would keep me from buying.
 
Two Things

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.
 
SAM JAMES WHEEL PANTS

GMCJETPILOT

''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?

Thanks

Bruno
[email protected]
 
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?


IMG_4545.jpg
 
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.

Ron
 
Last edited:
Back
Top