Wheel Fairing Volume

What do you think about this mod? Tomorrow I am going to the airport in the afternoon and remove the landing gear fairings modification fairings for modification. I am going to add foam barriers forward and aft of the main wheels and aft of the nose wheel to reduce the internal volume exposed to outside air flow. Then I will add two layers of fiberglass to seal the exposed surface of the foam and blend it into the internal sidewalls. There are less than two weeks to get this change incorporated but I have the material and it should be fairly easy to implement.

Bob Axsom

This type of thing is done alot on closed wheel purpose built racecars with good effect (see the Audi R8 LeMans race car). Bsically get the fender well as close to the tire as possible. I'm nut sure it'd have the same drag reducing effect on an RV, because the phenomenon is mostly "curing" drag due to tire rotation. Can't hurt, though, and will probabally help some. Although you could argue that w/o the wheel turning (much) the air just gets in there and hangs out, which generally doen'st cause much drag. You might be better off getting the edges of the pants closer to the tire (closing the gap) or getting really nice leg fairing trailing edges.

Wheel fairings

How about this little beauty?

http://www.freepatentsonline.com/7108225.pdf

John Clark
RV8 N18U "Sunshine"
KSBA

Interesting Idea

I'll be really interested to see if you get results by cutting down the internal volume Bob. DO you have the new or old style pants? Mine have a bulkhead just aft of the tire (I'm talking mains of course, having a taildragger), which already blocks off most of the excess aft volume. I think you already know this, but I wouldn't try and cut down the edge clearance between the tires and pants too much - yes, you might gain some speed, but at a significant increase in the risk of cutting a tire. (Been there, done that, and it is no fun!)

Good luck and report back on the results of internal volume reduction!

Paul

I have a mix

the "Pressure Recovery" eggs on the mains and the flat side on the nose. There is no bulkhead in my fairings I'm almost certain but I will check into that of course as I go into this more. I have wrestled with the air flow mentally but without a test ... well there was a sign on the wall in the environmental test building that said one test is worth a thousand expert opinions. If there is any kind of flow inside the things I think there will be drag. Is the flow related to volume? Tom Martin has tried the rubber seal approach to keep the air out and he told me the results were inconclusive. I'm thinking about a combination perhaps, add my bulkheads to limit inflow air chamber volume and reduce the edge gaps with rubber "almost" scrubbers. The brake heat could become a problem but it is hard to tell if you don't try.

Slightly off topic - That solar panel repair reminded me of Pete Conrad's salvage of Skylab - Good show NASA!

Bob Axsom

Two thoughts

Bob,

1. How much speed does each pound cost you? For example, if you lighten the plane by 10 lbs, does that gain you 1 knot? Can you lighten your plane for the race? Remove the baggage bulkhead, right seat, seat bottom, instruments, etc?

2. Regarding the wheel pants. Can you just cut in a exit hole for the air? Will that help?

N941WR said:

Bob,

Can you lighten your plane for the race? Remove the baggage bulkhead, right seat, seat bottom, instruments, etc?

Click to expand...

Don't know about the 9's (for sure), but I'd bet that the baggage bulkhead & seat bottom, to be quite instrumental in holding these machines together!

L.Adamson -- RV6A

My Thoughts

The rule of thumb given to me is 1 kt per 100 lbs. But weight is certainly a factor.

I think the exit hole would promote air flow and increase drag - at least that is what I came up with when I ran it through my mind.

Bob Axsom

Hi Bob,

Forgive me for going off topic here but I'm curious if you have tried some of these things. An old hanger mate of ours would have his father visit often and he had raced in the formula class at Reno and prior to that I guess Mojave and even Cleveland. He is an old time racer is what I'm trying to say. One day when he was visiting he was telling about using tape all over the plane to smooth transitions on the airplane and make them more fluid for the races. Examples would be the outboard ribs on the horizontal would have the lightening holes taped off with aluminum tape to prevents eddy's. The empenage fairing would have tape smoothing the transitions to the fuse and tail. He basically implied that his planes were covered with tape when he would race. I bet it was terribly ugly but try to picture a cowl with tape where the top meets with the bottom half, and all around the firewall joining the fuse to the cowl. Again gear fairings to intersection fairings, etc., etc...

Another thing is gap seals on the top and bottom of the horizontal to elevator gaps as well as both sides of the rudder to vertical. There is a Lancair that I was admiring at Reno two years ago that had thin teflon type plastic used to cover these gaps. It was translucent and unless you were looking for it you might never see it. The surfaces still moved but had a seemless effect to the wind as if they were one piece.

To add credibility to these tape like gaps seals there was a former Air Force pilot and retired Airline pilot in our EAA chapter that had built a Zodiac. He was having some minor pulsing in his control stick that he determined to be caused by airflow on his horizontal. I have no idea how he decided this to be the case but he did the gap seal thing as described above but only on the top of the horizontal and his plane gained 5 full knots in cruise. He was quite proud of this and it might be worth a try on your plane. Again, I don't know if you want to risk your paint to trying any of this but then again you might. If you do let me know what you find. I'm especially interested in the horizontal gap seal on the top of the horizontal.

Best,

Bob have you ever considered/investigated some type of brush seal around inside of wheel pant opening and tire ??

N941WR said:

Bob,

1. How much speed does each pound cost you? For example, if you lighten the plane by 10 lbs, does that gain you 1 knot?

Click to expand...

It depends the calibrated airspeed. At high CAS, the profile drag is much, much greater than the induced drag. The induced drag is decreased if you decrease the weight, but it is such a small percentage of the total drag that the resulting increase in speed is very minimal. At lower CAS, the induced drag becomes more important, so the amount of speed increase for every pound of weight decrease goes up.

I've got a very rudimentary, hypothetical, unvalidated performance model for the RV-8, that I created based on CAFE foundation data, Van's published data, Lycoming power charts and Hartzell prop efficiency maps. It suggests that at sea level, you need to decrease the weight by about 225 lb to increase the wide open throttle (WOT) speed by 1 kt. At 8000 ft, the CAS is lower, and now you need to reduce the weight by about 120 lb to increase the speed by 1 kt. At 12000 ft, you need to reduce the weight by about 80 lb to realize a 1 kt speed increase. These results have not been validated by flight test, so they may be wrong.

Weight reduction has a big effect on take off and landing distance and climb performance, but it has a very minimal effect on level flight speed.

Hey more power to Ya! (A couple of cautions)

Sounds like a lot of work
Sounds like a little more weight

I like the idea of eliminating the air leakage around the tire but not sure volume filling will do it. I suspect air will still go in somewhere and it will go out somewhere (front, side, back). It is just the way it is.

As far as gap seals Van tried that which lead him to the two piece wheel pants v. one piece. Van found gap seals on the wheels add rolling drag and they wear out. Still it has some merit but practical, may be not.

I suspect it may lower drag but not enough the measure.

If you do do it, DO IT WITHOUT ALL THE FOAM, just make the fiberglass inner fender.

Also wheel pant balance is key to avoiding wheel shimmy during taxi. If you put foam in you will make the wheel pants even more tail heavy.

Last will this make the wheel more likely (or less) to get jammed or lock up with foreign debris? I know its a worrywart thing, but mod to a part that "Ain't broke" needs to be thought about.

The Foam Concern and Others

I have 4 blocks of the flue foan left over from previous mod. ideas and it make the web implementation an easy task. I plan to have a thin web of foam to provide support for a semi-conformal fiberglass web of two plys. I'm thinking a 1" gap between the tire and fairing void closure web surface. There will be a pressure relief hole off to the side somewhere.

As we were walking in the park this morning I thought about an active gap closure system with automatic electrical activation self contained in the fairing. It is doable but it is a second or third generation task I think.

Bob Axsom

A walk in the park

Bob Axsom said:

As we were walking in the park this morning I thought about an active gap closure system with automatic electrical activation self contained in the fairing. It is doable but it is a second or third generation task I think. Bob Axsom

Click to expand...

Where you eating the funny mushrooms in the park? Bob I admire your imagination. Keep it up. So would it be a pneumatic seal?

May be some kind of retraction system, where the whole landing gear and wheel are retracted flush into compartments, in the wing or fuselage, so there's NO gear or wheel drag? Oh wait, some one did that. Doha! Oh never mind.

gmcjetpilot said:

Where you eating the funny mushrooms in the park? Bob I admire your imagination. Keep it up. So would it be a pneumatic seal?

May be some kind of retraction system, where the whole landing gear and wheel gets retracted into compartments, flush with either the wing or fuselage so there's NO gear or wheel drag? Oh wait, some one did that. Doha! Oh never mind.

Click to expand...

George,

You bring up a good point. The guy in Germany who built a -4RG reported it only gained him something like 4 kts over his earlier -4 while adding something like 80 lbs. (Those numbers are from memory and they could be wrong.)

What is the best Bob could gain with this wheel mod? .25 to .5 knots?

Not to dissuade you from trying Bob but I doubt you will see any noticeable gain. That said, a bunch of small gains add up to one big one.

Step One

I measured the fairing, tires, etc. and brought home the left main fairing. There is no internal web in the fairing as I thought. It looks like a more difficult task than I thought. I have a bad feeling about the potential on this but I am going to proceed with the idea for a while at least. I will start with cutting foam and fitting it into the rear of the fairing.

Bob Axsom

Looking Good

It may not work but the the foam is fitted for the left rear MLG faring and it looks good. I may do the left fairing first and fly it to see if there is any speed increase and asymetric force. Both would be great. Well up to a point.

Bob Axsom

Bob - Interesting idea. I'm not too optimistic that reducing the internal volume will help, but I'd be happy to be proven wrong. I think there is more potential improvement by reducing the gap between the wheel pants and tire to reduce the amount of air that goes into the wheel pant. Clearly you can't make the gap too small, or you risk the tire hitting the pant when it flexes. But, perhaps you could do what the big aircraft guys do in places like this - they fit stiff brushes. The brush keeps most air out, but will flex out of the way if required.

Fixed gear rocks

N941WR said:

George,

You bring up a good point. The guy in Germany who built a -4RG reported it only gained him something like 4 kts over his earlier -4 while adding something like 80 lbs. (Those numbers are from memory and they could be wrong.)

What is the best Bob could gain with this wheel mod? .25 to .5 knots?

Not to dissuade you from trying Bob but I doubt you will see any noticeable gain. That said, a bunch of small gains add up to one big one.

Click to expand...

I am glad you brought it up. I am totally against retracts on RV's. RV's have very low DRAG fixed gear that is inexpensive, durable, light, simple and no chance of gear-up landings. The 4 kt number gain is about right for a RV, about 6-8 mph TOPS IF and only IF you perfectly get the gear retracted. Fact is most retract gear have doors that don't fit perfect. Stiff is still sticking out with a retract. So for all the cost and expense 6 mph is not worth it, agree. A SJ type cowl will give you that. Now when you get into the 250 mph range, yes retract starts to earn its keep. If you notice the fastest of the fastest piston single engine homebuilts with fixed gear all top out in the 250-260 mph range.

FIX GEAR ROCKS!

Brushes are difficult to implement

Kevin Horton said:

Bob - Interesting idea. I'm not too optimistic that reducing the internal volume will help, but I'd be happy to be proven wrong. I think there is more potential improvement by reducing the gap between the wheel pants and tire to reduce the amount of air that goes into the wheel pant. Clearly you can't make the gap too small, or you risk the tire hitting the pant when it flexes. But, perhaps you could do what the big aircraft guys do in places like this - they fit stiff brushes. The brush keeps most air out, but will flex out of the way if required.

Click to expand...

I had a well thought out response entered but the furum lost it in its processing - seems much more of a hassle lately. Anyway here is a quick summary>

I thought about brushes as has been mentioned from time to time but getting suitable brushes and mounting them correctly is a test of the implementers skill. I don't see an easy way to get it right without a lot of brissle, glue, metal and fairing interface work. In the future perhaps but I've got to fly to Taylor, Texas for the Rocket 100 air race next week Friday so there is not a lot of noodle time. Rubber seals are easier I think but they have problems as well.

I have a foam bulkhead glued in the aft end of the left MLG fairing and I plan to test fly it today then commit to doing them all or tearing this one out.

Optimally I will get a yaw to the right and a slight increase in speed.

Bob Axsom

Bob Axsom said:

I have a foam bulkhead glued in the aft end of the left MLG fairing and I plan to test fly it today then commit to doing them all or tearing this one out.

Optimally I will get a yaw to the right and a slight increase in speed.

Click to expand...

A yaw to the right would certainly be a sign of less drag from the left wheel pant. But, it is quite possible that the total drag in the yawed condition (or from the deflected rudder if you push on it to zero the ball) could be higher than it was when the two wheel pants had equal drag. So don't be discouraged if you can't see a speed increase, or if you measure a speed decrease.

If you've only modified one side, I think the resulting yaw will be a better indication of an improvement than any change in speed.

I test flew this afternoon

I hate when this happens. I reinstalled the left MLG fairing with the bulkhead in the aft portion only. There was no difference that I could detect in flying the plane. The speed at 6,000 ft density altitude using my USAR haddicap procedure and the NTPS spreadsheet for calculation with MAP = 25, RPM = 2730, leaned for best speed was 177.4 kts. That is 1.3 kts faster than my test on the 6th of November after sealing the strut hole in the NLG fairing but it is 0.4 kts less than the previous test after I reduced my wingspan by 3 feet. Intuitively, I don't think this will make the plane fly faster but it is not demonstrably slower either. In fact it is about a half a knot faster than the average of the two previous tests. My belief is that this is just another point in the error range of the test method but I can't take a chance (Mike Thompson are you lovin' this). SO ... I brought all three fairings home and I will install the bulkheads and fillets tonight then take them to the airport for fitting tomorrow. Then I should fly another test to get a speed for the whole mod shape. If it is good I will take them off and bring them home again for fiberglassing. If the results are not good I will cut out the foam and spend the little time remaining before the Rocket 100 race next week washing and waxing the plane (which I would like to be doing now instead of squeezing it in at the last minute).

Now, back to work.

Bob Axsom

The CAFE Foundation flight tests showed more than a 2 kt increase in speed on their Mooney from a wax job. Maybe you should spend more time waxing, and less time adding more weight to those wheel pants.

Mind you, I get the impression that their Mooney really, really needed a wax job. Your paint is probably in better shape then theirs was, so the potential gain is probably less.

Kevin, I have long held the theory that because the Mooney is a laminar flow airfoil (well, more than our fat wing birds, anyway), it would respond more to the wax job than one of ours.

That said, I think a clean bird goes faster anyway. At least it looks like it goes faster. I think.

The Foam is in all three fairings

Plan to test fly tomorrow.

Bob Axsom

Got my fingers crossed for ya'

Two other options for later with more time...

Bob,

I really love reading all about all your never-ending quests for less drag/more speed. I have thought about the air blowing in and out of the wheel pants too. I think there are two options:

1.) Seal the opening between the tire and the wheel pant with brushes as someone has suggested. I'll only add that a series of small paint brushes with the handles cut off might be just the ticket. Or....this is thinking outside the hen house...use feathers to help seal the area. Or feathers and brushes.

2.) Cover the bottom of the tire completely with a nice smooth fairing. This would be like a landing gear door in it's complexity. It would probably be just as tough to fit as a whole canopy. My thoughts were along the lines of something made with unfinished balsa wood. Because the human element enters here. Let's imagine that you made nice smooth covers that covered the tires in flight. They sealed well with the wheel pant and they were smooth and transitioned well with the wheel pant curves. Let's say they produced the desired drag reduction/speed increase. Now you won the race it's time to bring her in and....oops you forgot to retract the tire covers up out of the way. You touch down and hopefully you built these to tear away with very soft shear pins or pop rivets or similar easy tear away construction.

I'm thinking you just left a mess of balsa slivers and tooth picks all over the runway.

I actually want to try the second retractable cover idea. Just getting a good mechanical means to open and close is the challenge. I also thought that using small tabs and aerodynamic forces to cause it to flip up/open and down/closed might also be possible.

Now quit reading all the posts. Go wax the plane and think about good race flying techniques.

Test Flight?

The ceiling was too low to get in a formal test. I did buzz around at ~ 3,000 dodging clouds and SkyVenture Cessnas east of Drake field. The general conclusion I made was it is good enough to go ahead with the fiberglass bulkheads over the foam. It looked quite good actually - a raw single observation three way average at 3000 pressure altitude was 181.6kts. I don't have anything to compare that with and there was no real test method involved. Still, I think I'm on to something.

Bob Axsom

New thought

The foam bulkheads have all been glassed and will be cured tomorrow. Weather permitting I will squeeze in a test flight.

I'm sure you have thought of something and as the idea is implemented a refined idea emerges that was not even conceivable in the original state of things. OK, here it is. I now have these confined areas around the tires in all three fairings. It would not be difficult to install a containment ridge around the bottom of each fairing. An inflatable device could be installed there to seal the gap between the fairings and the tires in flight. I'm not sure how the nose wheel will react in flight but you could just apply the brakes to stop the mains before inflation. You would need at least one valve to control the inflation state and a pump (maybe a hand pump) for inflation. The plumbing would be simple if you didn't have fiberglass strut fairings already installed.

Bob Axsom

Thanks Bob

Bob,

I just wanted to say that I have really enjoyed reading your messages. You have put forth a remarkable effort to gain more speed. Sharing your ideas from conception to implementation and testing has been really fun to read. Thanks for making the effort to share this with us!

Jim

Thanky You Jim

Part of my motive to share and part to keep me from backing out of the experiment.

Bob Axsom

Test Complete

I installed the internally modified fairings and the sky cleared up enough to get in a valid test flight. The USAR Handicap procedure was flown for a 6,000 ft density altitude test with wide open throttle, leaned for best speed and the results were plugged into the NTPS spreadsheet for three track tests. The True Airspeed was 174.6 kts. This is a reduction in speed of 3.2 kts. That is too great a reduction to pass off as a test method artifact. I am going to remove the mains and cut out the bulkheads and try to get a flight in with the modified nose gear fairing as is before I pull it. It looks like an improvement and I want to check it before cutting it. We fly to Taylor, Texas on Friday for the Rocket 100 on Saturday morning. I may have to go there with a dirty bird.

Bob Axsom

Removed the MLG Bulkheads Only

Removed the MLG Bulkheads Only and retested. The speed was up to 176.5 after the removal. I really hated to remove the NLG bulkhead because it looks so good with the nice rounded leading edge etc. but I did. That hasn't been tested yet. Hopefully it will go back up to the 177 range.

I think the reason for the loss of performance is a liniear circulation in the back around the tire circumference conforming bulhkeads and back out the front in to the air below the fairing. With no bulkheads the air goes in pressurizes the interior and resists more inflow. Well that's my thought anyway. I would have liked to work with it but the Rocket 100 race at Taylor is this Saturday (11-17-07).

Bob Axsom

Thanks Bob

I enjoy the fact your are so quick to try things and give your honest evaluation of it. In 20/20 hindsight it seems that having a tight inner fairing around the tire just allows the the air to enter and exit faster. That air squirting out might be causing the extra drag, but geee that was a lot, 3 kts. even half of that would be significant. Well thanks again for the lesson.


NASA tried little holes in wings to control boundary layers. Now this comes under the heading of wild ideas. Back with the wheel fairings. Bob you think air leaks around the gap is bad, so an inflatable seal would solve that. Well that may be true but something passive and simple comes to mind. I was thinking was some small exit holes in the back end the wheel faring, like a pressure relief. Any air sucked in around the tire would exit in a more orderly fashion out the trailing edge of the wheel fairing, in line with the air flow. Also a small hole in the front of the wheel fairing would keep some small positive pressure in the fairing to keep air from even being sucked in & blown out around the tire. Of course the little holes front and back bring air in and out, but more in line with the air flow. More important the idea is keep air flow around the perimeter of the tire to a min.

This has been educational

Everyone knows how much the LG fairings increase the speed of RVs but It was not obvious that the internal features would have such an effect.

Any holes to releive internal pressure will promote air flow through the fairings and I believe will increase drag.

Here are a couple of things to think about:

1 - I think it would be possible the build wheel fairings with a configuration that would allow a laterally moving panel to close the opening over the tire without having a lower bulge on the closed fairing.

2 - The fairings seem to expose much to large an amount of tire. I taxied out behind a Cirrus the other day and you could barely see the black of the nose tire. Mine shows about 1/3 of the tire with the corresponding large hole.

Bob Axsom

Bob its been done "Star Wars"

Bob Axsom said:

Here are a couple of things to think about:
1 - I think it would be possible the build wheel fairings with a configuration that would allow a laterally moving panel to close the opening over the tire without having a lower bulge on the closed fairing.

2 - The fairings seem to expose much to large an amount of tire. I taxied out behind a Cirrus the other day and you could barely see the black of the nose tire. Mine shows about 1/3 of the tire with the corresponding large hole.

Bob Axsom

Click to expand...

Bob you know many race guys use smaller fairings and (lamb) tires verses 5x5, basically the nose tire van sells but on the mains. Look at the racing Formula planes. Of course running smaller tires and lowering the faring closer to the ground has a negative affect on the ability to taxi, land & takeoff on uneven surfaces. Look at the middle pic below with the fairing dragging on the ground.



I agree perfectly sealing the fairing and wheel and reducing the protruding tire will reduce drag. I'd just go with smaller "pressure recovery" fairings, smaller tires, min tire protrusion and min gap around the tire, if going for speed. The active seals and doors sounds too complicated, but the idea of clamshell doors on a fixed gear fairing has been patented! Patents 3750986 and 7108225 look interesting. Patent 3750986 shows a clamshell door on a fixed gear wheel fairing like a RV.

http://patft.uspto.gov/netacgi/nph-...&f=G&l=50&d=PALL&RefSrch=yes&Query=PN/3750986
http://patft.uspto.gov/netacgi/nph-...LL&s1=7108225.PN.&OS=PN/7108225&RS=PN/7108225

Hummm the "sharp ridge on the top of the fairing" is interesting. I gather is a longitudinal ridge especially towards the aft part of the fairing. The 3.5 length to width ratio is also interesting and fairly short. I have to say the door idea looks heavy and complicated. The author is Jim Bede him self. He never used it in a design so.........

Any holes to releive internal pressure will promote air flow through the fairings and I believe will increase drag.

Click to expand...

Bob the idea of allowing a small amount of air to enter or exit the fairing in a "controlled" manner, is to keep the willy-nilly random air being sucked into the fairing. More importantly air shooting out around gaps between tire/fairing cause "interference drag", so a small exit in the trailing edge of the wheel fairing may prove useful as well. Think of it as trading drag for a little less drag.

Allowing a positive air flow into or air flow exit from the fairing, in theory can control air leaking around the tire. Think of it as an AIR SEAL. Is it practical or will it give measurable drag reduction? probably not so the way to go is small tires, small fairing, min gaps and tire protrusion.

Boundary control - The "star wars" missile defense in the future may rely on high flying airplanes with optical sensors looking into to space. The sensors can not project through glass or plexi. They need an open window like at an observatory. On the top of the plane is a 'Cupola' (big hump fairing housing the sensors). The wrinkle is flying at +40,000 feet and 450 kts with an open 'window'. To get laminar air flow over the window, giving the sensor an unobstructed view of space and incoming ICBM's is a challenge. This is done by controlling pressure inside the cupola and the ramps around the opening. The idea of controlling flow using internal pressures has been done.

Showing doors (wide open in flight)

Showing inlets