Speed vs. wheel fairing gaps (vs. safety)

[Paul Eastham]

In my rush to get my airplane flying, I ended up with some decent gaps between the wheel fairings and the tire itself. I was shooting for about half an inch (thumb width, the short way), but got a fair bit more in spots. No more than an inch in the worst spots, I believe.

I am curious if anyone has opinions, or better yet data, about whether reducing these gaps will have any measurable speed impact. I know some people recently increased their gaps due to fear about popped tires, did you measure a speed decrease?

[Ron Lee]

I was thinking about this yesterday. Would it be possible to come up with a rubber-like insert that takes up most of the gap yet is pliable enough to allow for tire flexing? Of course it would have to be shown to be beneficial in terms of speed.

[Christopher Murphy]

I know some guys who built a beautiful Lindy award winning Tailwind. Made their wheel fairings so close that on the first landing with them on, the tire grabbed the fairing and pulled it under the wheel. Tore up their nice plane as pieces of the fairing got flung off.

If you want your plane to go fast keep it light!
Chris

[Bob Axsom]

Tom Martin has inserted rubber seals in the fairings of his EVO Rocket. Perhaps he will comment here. He and I have exchanged e-mails on the subject. There are some operational problems caused by the contact interface especially at the rear. There was some agreement that if you pointed the contact edge of the rubber at the center of rotation of the wheel you could overcome that problem. I have had other thoughts as well but have not tried anything in the area of a soft seal. On my airplane I have an older flat sided nose wheel fairing and the more uniform shaped pressure recovery main wheel fairings. After months of thinking and e-mail exchanges with Tom Martin I decided to develop a subfairing for the nose wheel. Originally probably a third of the wheel is exposed and when I had my first encounter with high speed wobble due to low breakout force I thought there was tire to fairing contact so I increased the clearance to about what you describe here. Currently I am flying with the subfairing installed that extends down to approximately 1" above the ground and is trimmed back 3/16" from intimate wheel contact. The actual clearance is much less than 3/16" because of the angles involved but --- THE POINT OF NEAREST ENCOUNTER IS BEYOND THE MAXINUM FLEX POINT OF THE TIRE AND THE FAIRING EDGES ARE ALMOST TANGENT TO THE TIRE. So the side deflection occurs up inside the fairing and any contact (I have not seen any evidence of actual contact yet) is a low conflict rub. I have not done anything with the mains yet and there does not appear to be as much potential for speed gains there.

Bob Axsom

[Ironflight]

Way back in Phase 1, I opened up my clearances from "too tight" (because I cut a tire) to a little more than a thumb's width. Any speed decrease was hidden by the scatter of the speed testing data, maybe a knot or two at most - maybe nothing. I'd rather lose a knot than blow a tire again!

Paul

[Tom Martin]

I have quite a few hours on my plane now with the soft seals in the wheel pants. I used two types of baffle material for the seals. The soft type of baffle material did not last very well. The baffle material, as supplied in Van's kits, seems to wear quite well.
When I first installed the seals I did show a slight increase in speed compared to my normal test plane. However with the wear of the one type of baffle material that difference quickly went away. The seal must be tight or there is no benefit. Bob Axoms idea regarding moving the seal to a lower point on the tire has merit but it probably means that these additions should be easily removable as they certainly would not be suitable for my grass strip or normal flight operations. Also the baffle seals freeze to the tires very easily which means that I would have to remove my wheel pants for a greater part of the year than I would normally, for cold winter conditions.
I am going to do some more experimentation here this year but I would not consider this type of modification unless you are trying to get the last little bit of speed for competition reasons.

[Paul Eastham]

Originally Posted by Tom Martin

.
I am going to do some more experimentation here this year but I would not consider this type of modification unless you are trying to get the last little bit of speed for competition reasons.

Thanks very much for the information.