Ironflight

Ironflight

VAF Moderator / Line Boy
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Since Van's uses pretty much the same wheel pants for all the models, I'll post this in the general section...

I did a couple of speed runs at 8,000' this morning, just to see how things had changed since I opened up the clearances between the tires and the pants. It is hard for me to quantify the clearances, but let's just say that the original clearance was about the thickness of a little finger and now, I can stick my thumb in all the way around. (You'd think that it would be easy to measure, but all the curves and compund geometry make it not so...)

When I ran my initial flight tests a couple months ago, I was getting 187 KTAS at 8,000', wide open and leaned for best power. Today, the best I could get was 185. Now, is that due to the opened up clearances, the engine being slightly more worn-in, slightly less (or more) precise flying, or simply the fact that we're dealing with a 1% variation, which is probably less than the various innacuracies in the overall system? Can't tell you for sure, but something tells me that the opened up clearances cost me a knot or two. Nevertheless, I have no interest in cutting another tire, and if that costs me a nudge of top-end speed (which I won't use very often anyway), then that's the price I pay....

Your speed and mileage may varry.... :p



Paul
 
Interesting info

Paul

Thanks for the info. Since I have not done any fairing work yet, please excuse me if this sounds like non-sense. But, do you think it would be possible/worth it to try and install a flexible rubber type skirt/seal to narrow the opening to almost zero in order to retest your "feeling" and see if the missing knots come back? Heck even mylar tape might work for a temporary experiment.
 
Wheel pant seal.

I Read the same suggestion somewhere else and planned on implementing it. Enlarge the pant clearance to prevent the tire from binding and use some baffle seal material to create a flexible seal to keep the air from flowing up into the wheel pants and creating drag. I would like to hear the results of any testing made.
 
Flexible gap seal on wheel pants

RV8RIVETER said:
... do you think it would be possible/worth it to try and install a flexible rubber type skirt/seal to narrow the opening to almost zero in order to retest your "feeling" and see if the missing knots come back? ...
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Wade, That sounds like a great idea. Could help keep gravel and mud out, too.
 
Paul, First thing that came to my mind are the variables. Outside temperature now vs two months ago. How about bugs and other debris build-up? I noticed one of your prior posts that you are about to get ready to have it painted. Probably don't want to put much of anything on it like wax or the like. All of these and many others (including wheel pants) go into affecting speed.

I was at Van's recently and talking to them learned that the new style wheel pants can make 4 to 8 knots difference. I have the older one with the brake blister on my RV-4. I'm seriously considering changing to the new style even though my plane is pretty quick as is.

My son has a -4 also but with an IO-360 CS prop (mine is an O-320 FP prop). At his cruise settings I keep up with him except in a climb. Of course, if he wants to pour the fuel to it he can run away from me anytime he wants. Another 4 to 8 knots would make it easier to keep up with him. Point being that wheel pants seem to make a big difference in speed. I guess I would talk to Van's to see if they have any data on wheel pant clearance and effects.

The baffling material sounds interesting. I also know of people that have glassed in the pant with a curved bulkhead aft of the tire after they found this area to fill with mud (grass or dirt strips I guess). Mud in that area could add a lot of weight when filled I would guess.

Also because of clearance we tend to fix out pants attitude to ground clearance. I wonder if changing it to water line would make any difference? Of course, you guys with the nose gear can do that easier than us with the wheel in the back.

Anyway, interesting question. Keep us posted on your findings.
 
fairing Gap Seals....

You guys are thinking exactly the same thing I've been thinking....putting some kind of seal there to keep he airflow out. The only thing that bothers me is that I haven't seen it done by anyone - and frankly, it seemed like such a logical next step that I wonder why not? I was thinking of trying some tape first, just to see what it does. I hope that if anyone on the forum has tried it, they'll chime in!

As to temperature and other atmospheric variables Tom, I think I have those corrected out properly ( I always do data massaging before posting numbers - but school was a LONG time ago!)...but of course, that all contributes to the uncertainty. Two knots at that speed is realistically in the noise based on my experience, and a lot of things could throw it off a little one way or another. (And you're exactly right about why I haven't waxed it! Don't want to put anything on that surface until the painters get it....)

Paul
 
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Wheel pant lining

I'm unsure of the posting of pics thing, but if you were to look at the undercarriage of the Canadair Regional Jet you would see that the mains retract into a well which is not sealed by a gear door. Instead the outboard portion of the tire is visible in flight.

A sealing material resembling a course paint brush, or baleen on a whale is mounted to the wheel well and bridges the air gap to the tire when retracted. It actually touches.

I would look into this method because it's light, flexable and probably cheap to replace when it gets dirty or worn.

rd
 
How low will it go

How much tire is sticking out below the wheel pants I think is most critical to Drag. Some wheel pants have little clearance between the bottom of the fairing and ground, while others have lots of tire exposed. Its a trade off with durability, rough surface capabilities and speed. The standard clearance between the top of the tire and wheel pant I think is 1". You could cut that gap down with the worst case being some rubbing on a flat surface. There are variations in tire diameters for differnt brands of tires to keep in mind.

Seals where popular in the 80's/90's when Van sold the early single piece wheel pants. Because the opening was big enough to slide down over the whole tire the fore/aft gaps where large. Van did some tire/fairing seal flight test and recall it was inconclusive. He talked about differnt seal materials. Check old back issues of the RVator, start with 8/93 and 10/93, which I think had some info.

Because air gets into the faring it no doubt needs to get out, that may be the cause of "plume" drag, where the air comes out it interacting with the free airstream, causing drag. What about a small vent on the aft trailing end of the wheel fairing. At least if any air that gets "ingested", it won't blow back out around the tire? Just a thought. I agree the best is to keep air out all together.

One problem with low sitting fairings is the greater tire distortion occurs further away from the rim, so the tire will more likely come in contact with the fairing. Higher up the WF sits the greater the flight drag but better tire/fairing clearance. :confused: What to do? I guess follow the plans and keep the tires inflation in check and hope for the best.

George
 
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RJDickensheets said:
A sealing material resembling a course paint brush, or baleen on a whale is mounted to the wheel well and bridges the air gap to the tire when retracted. It actually touches.

rd
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That is exaclty what I was remembering seeing in the past, but don't know where to find it...(Now someone is going to post the URL from Aircraft Spruce or McMasters within a couple minutes, right?).

I'll have to try and find the previous research that George mentions. I am not surprised that Van's work proved inconclusive, cause frankly, I think that is what my extremely limited test data showed so far....
 
Ironflight said:
That is exaclty what I was remembering seeing in the past, but don't know where to find it...(Now someone is going to post the URL from Aircraft Spruce or McMasters within a couple minutes, right?).

I'll have to try and find the previous research that George mentions. I am not surprised that Van's work proved inconclusive, cause frankly, I think that is what my extremely limited test data showed so far....
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Mcmaster-Carr page 1132. :)
 
Then there is the old Jim Bede trick used on the BD4 factory demonstator of having a retractable door on the bottom of the pant.

The factory BD4 had very large fairings shaped somewhat like the newer pressure recovery wheel pants which thus had room for the door and the mechanism. A simple retract mechanism that was operated by the pilot.

I don't think I ever saw any test data on the door open/closed situation. The BD4 website has most of the original Bede docs online:

http://www.bd-4.org/

I built the large fairings, but never used them due to the seemingly too large size (goes to show you how poor my eyeball aerodynamic judgement is). Ray Ward subsequently used them on his BD4 which was quite fast (almost 200mph on 200hp IO-360). I ended up using Cessna 182 pants which had humonus openings (and a neat mud scrapper in the rear). I got 180mph with my 180 Lyc...not bad for 3 seater (useable).

And BTW, they did test the results of leaving the doors closed on landing. It simply wore a hole in the bottom of the door and the landing completed normally. This was on pavement...

Deene.
 
Paul, what I meant to say in my earlier post about my son's -4 is that he has told me that the difference between his plane being bugged up and clean with wax is about 4 -5 knots.

I'm guessing that is indicated speed he is telling me. You certainly do not want to put anything on you plane at this point as it increase the work in paint prep. After it is painted and polished I wouldn't be surprised that you get your speed back.

I'll still be interested in anyone's research on gap clearance and seals etc.
 
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