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Pressure recovery Wheel Pants

N395V

Well Known Member
Catchy name but I am confused as to what it means and the physics behind it.

Can someone explain (in small words) exaxctly what pressure recovery means relative to wheel pants.
 
pressure recovery

Fundamentally, it means the shape is designed to minimize flow separation on the downstream side of the fairing. Minimum form drag occurs if laminar (non-separated) flow can be maintained along the entire surface of any body moving though the air.
 
LOL - That was sorta tongue in cheek but thanks for the replies - makes more sense now to this non-engineer !! :)

My understanding is that these are smaller and tighter fitting - any issues with grass or unimproved strips?

T.
 
TShort said:
LOL - That was sorta tongue in cheek but thanks for the replies - makes more sense now to this non-engineer !! :)

My understanding is that these are smaller and tighter fitting - any issues with grass or unimproved strips?

T.

I just replaced my old-style RV-6 pants with the newer PR pants, and the new ones are most assuredly not smaller! I was actually surprised at the improved performance because the new ones look so much larger than the old ones.

There's a side-by-side comparison here:

http://n466pg.blogspot.com/2006/06/time-to-put-on-my-pants.html

Tight fit is up to you. I trimmed mine such that I don't have to worry about tire rubbing if the tire pressure is a little low. I have heard, though, that the newer ones can collect grass and stuff back behind the wheel. I haven't had that happen, but I haven't been flying with the new ones very often yet.
 
Dgamble said:
I just replaced my old-style RV-6 pants with the newer PR pants, and the new ones are most assuredly not smaller! I was actually surprised at the improved performance because the new ones look so much larger than the old ones.


There is actually a noticeable difference in performance between the two styles?
 
skelrad said:
There is actually a noticeable difference in performance between the two styles?
They say there is, but my only real measurement I have is between no pants and PR pants. The old pants got a little, uh, worn during my TW transition, so I've left them on the work bench for quite awhile. The difference between no pants and PR pants has been unscientifically measured as 9 knots on a 150hp RV-6.
 
History of these things

I'm trying to remember... did the PR wheelpants start out as some team rocket option and then Van's picked them up?
 
any issues with grass or unimproved strips?

Yep,

Shattered one hitting a bump at Mallards Landing fltin last year and the othe retained enough dirt/grass to chew up a disc.

Cut more clearance in the next set and it cost a little speed.
 
More Info Please

I would like to get more information through this forum on the original question. DGlaeser's answer provides the best information so far but the name is so universally accepted that there must be someting special about the shape that relates directly to the words "pressure" and "recovery". I can imagine all sorts of "bar of soap squeezing" anologies and I can remember seeing what I think were pressure recovery wheel pants as early as the Formula 1 airplane Nemesis. I believe they are characterized by: a shape that expands uniformly in all directions from the front with a maximum cross section near the axle location except for the wheel cutout, then starts to reduce cross section in a non-uniform manner such that the sides actually curve in more quickly and have some reflex or concavity resulting in a trailing edge that is a vertical line instead of a point.

If I think of the the wing and lift explaination and apply it here I could generally rationalize some accelleration of the air relative to the surface and reduction of pressure then after passing the maximum crossection it recovers to ambient and effects a little push to the shape. I think there is more to it than that and there must be a lot of experimental development behind the shape that is now so familiar. I wonder which ones I see are actually optimized and which ones are just general copy-cat shapes that do not qualify as pressure recovery wheel fairings?

My plane has the destinction of being caught in the development phase at Van's. The mains are pressure recovery fairings but the nose gear is not. I question whether there is truely anything to be gained by switching the nose wheel fairing to the new design notwithstanding the cost, paint and other negative factors? I also wonder about the exact effect of the gap around the tire? Like others I had some early rub experience and trimmed them back a bit before painting.

Bob Axsom
 
From what I understand, the point of pressure recovery wheel pants is to have the airflow on both sides moving as parallel as it can when it comes off the back. Cessna has been using this style on the 182 since 2001, I don't know how much gain it affected on the 182, but I'd like to find out ;).
 
Any Recent Aero Graduates?

Bob - I think that you and I both suffer from having been out of school too long! The textbooks I had (and still have on my shelf) were all first printed in the late 40's, when they were just discovering that supersonic flight was posible - but didn't even know about the area rule!

My guess is that the modern tesxtbooks might not even include the concepts of presure recovery pants, becasue in the real world of aero engineering these days, the experimenters are out ahead of the academics....

I just did a search of some of my favorite Aero web sites,and didn't find anything on the topic...but maybe we've got some young guys lurking that can educate us!

Paul
 
Old Pant vs PR Pant

I have the older, non-PR wheel pants and have occasionally thought about doing the conversion. Anyone out there that did the conversion and has actual comparisons as to the performance impact they received? ie performance with old pant compared to performance with the PR pant?
 
Speed difference of fairings

In Van's accessory catalog and online webstore the following is shown under Sensenich fixed pitch prop section:

"Cruise speeds for RVs with older wheel fairings (narrow cross-section w/ brake blisters) will be 5-6 mph slower and require a prop with approximately 2" less pitch."

Since I haven't got to the wheel fairings yet I am definitely going with the newer ones.
 
Per Bob Axom's request here is what I know about the pressure recovery wheel pants. First let me clarify that I am the person that on another thread posted that...I don't fly or have built an RV...but would like to in the future...and that I have scratch built a one of a kind that sort of looks like a small RV4 (tubing fuselage, aluminum wings, C85 engine) One of the things that was scratch built was a set of pressure recovery wheel pants. I have the "regular" wheel pants on the plane now and will soon be testing the PR type. I will let you know, if you are interested, the difference between the two. The following is what I know about PR pants. First it helps to have an inventor buddy that did all the leg work. This inventor explained the theory PR in layman's terms this way to me.... 4 air molecules meet the PR pant at the tip one goes over...one goes under...one goes right ...one goes left. They all remain attached and meet at the back. They don't swirl or meet each other on the sides top or bottom....thus you have no turbulence or separation of flow and less drag (the flow remains attached). This inventor made different pants and flew them until he came up with a design. To make the pants a symmetrical air foil is used. The air foil does not have to have a good coefficient of lift but a good coefficient of drag. If one Looks DOWN at the wheel pants one can see the full air foil...looking from the side, the air foil has been split at the chord. The "Sam James" PR pants are designed the same way....why it's called "pressure recovery" I don't know....I hope this helps....Ivan/Sugar Land, Tx.
 
Pressure recovery

For streamline shapes, the air pressure changes as it flows around the body, initially decreasing as it is accelerated toward the point of maximum thickness, then increasing again toward its original pressure as the air is decelerated during its flow to the aft edge of the body. This increase in pressure along the aft portion of a body is referred to as "pressure recovery".

If this deceleration (and resulting pressure gradient) is not gentle enough, the flow can seperate from the body, leading to increased drag. If everything is done just right, the pressure can be fullly recovered, and the flow remain fully attached (and mostly laminar), all the way to the training edge of the body.

One characteristic of low drag laminar shapes is a reflex curve on the aft portion of the body, such as can be observed in the shape of Van's "pressure recovery" wheel pants.
 
Tufting, Existing Knowledge, Experimentation, etc.

Some good information has come out but we don't have it locked in yet. Probably not many remember a video magazine called Aileron which contained an feature in the second and last issue where Barry Shiff (sp?) tufted a wing of a Piper Dakota to help viewers visualize the flow of air across it. I would be interesting if someone would tuft an RV pressure recovery wheel fairing and non-pressure recovery wheel fairing and record the flight observations. There has got to be more specific hard knowledge about this if it is as much of an improvement as it seems. In the history of aircraft development I'm sure the best and brightest shifted to other performance limiting problems after effective retractable landing gear were developed but I would be surprised if the NASA small aircraft studies of recent years would have ignored this if it is as good as everyone says. Does anyone at Langley or Lewis have anything they could report? What about Lancair and Cirrus? What about van's? Are there any wind tunnels still around where full scale or smaller accurate model landing gear fairings could be tested? Are there people who would and could do the research? The knowledge is out there I'm sure of it but it is something that can be effectively re-discovered from scratch if necessary. I will continue my cooling drag modification experiments for some time into the future but when I have completed that I could buy a new nose gear fairing and do comparative tests.

Bob Axsom
 
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