No, I'm not talking about how we looked when we were young, and how we look now.

I'm talking fluid dynamics here (or whatever it's called) as it relates to tire sizes and drag.

Surely we have a fluid dynamicist among us.

Assuming identical surface areas, is there any difference in drag between a tall skinny tire and a short fat one?

If you're worried about the drag of tires, buy a set of 6:00 X 6:00 and wheel pants.

The size and shape of off airport tires is dictated by the landing surface, not the drag.



MTV

Did you say Fluid Dynamics?

Ok - so you said surface areas. That makes for an interesting mathematical optimisation.
Perhaps you were thinking of the area when viewed from the air's direction, or perhaps volume?

The best way to determine it would be a test. My first thought is that the interface with the brakes / gear leg, the roundness of the 'edges' of the tire, and the profile of the side wall are going to have a considerable effect - probably quite a lot.

So in that line of thinking, assuming a fixed hub size, a longer skinnier tire will have a shape more like a Frisbee, whereas a squatter fatter tire will have a shape more like a rounded cylinder or ball, with a big divot missing from one side.

You would think a taller skinnier tire is better, at an educated guess, but it's really hard for sure to say without doing a simple test. Of course a CFD model is only a good as the base assumptions, boundary conditions, and user configuration you feed it AKA "sh*t in = sh*t out", so I prefer a real life test.

It's an easy thing to model in CFD, and it would only require a few minutes to run. But better yet, four seconds of google-fu beat everyone to the punch, without all the fancy, unnecessary software:

http://naca.central.cranfield.ac.uk/reports/1935/naca-report-485.pdf

Drag power is proportional to the cube of the speed, just to add to the drama.

I read a report the FAA had done but that was back when fluid dynamics was fresh in my mind. It studied the effects of large tires on GA airplanes. Had Reynolds numbers for different sizes and shapes, the whole shebang. I'd try to find it but even if I did it would mean much to me anymore. But the info is out there.

lesuther wrote:It's an easy thing to model in CFD, and it would only require a few minutes to run. But better yet, four seconds of google-fu beat everyone to the punch, without all the fancy, unnecessary software:

http://naca.central.cranfield.ac.uk/reports/1935/naca-report-485.pdf

Drag power is proportional to the cube of the speed, just to add to the drama.

That is a fantastic find!! It certainly helps you to see how the GeeBee could be so fast with those massive wheels and fairings - some of the lowest drag penalties with that configuration. This was really, really cool. Awesome!

Seems like you could do a pretty good job of streamlining even a set of 31" BW's, with a properly-shaped set of "spats" like Maule used to install. Get that GeeBee-type wasp shape going on.

If you can keep vortex streets from forming you'll reduce a lot of drag...

I asked the original question to our applied aero master here (he literally wrote most of our books!). He said yes, tall and skinny is more aerodynamic but for practical application you're limiting available braking power with the reduced contact patch.

I've seen a trailing teardrop fairing on a supercub with bushwheels. It was cantilevered off the axle and strut. The guy said he got almost 10 mph back...they looked like a lot of work to make though.

lesuther wrote:I've seen a trailing teardrop fairing on a supercub with bushwheels. It was cantilevered off the axle and strut. The guy said he got almost 10 mph back...they looked like a lot of work to make though.

8.6kts! That is almost worth the awful sight of those fairings

whee wrote:I read a report the FAA had done but that was back when fluid dynamics was fresh in my mind. It studied the effects of large tires on GA airplanes. Had Reynolds numbers for different sizes and shapes, the whole shebang. I'd try to find it but even if I did it would mean much to me anymore. But the info is out there.

Are you referring to the report that Penny Nixon did for the Fish & Wildlife Service, Alaska Region on big tires on Super Cubs? I've got a copy around here somewhere. I don't recall the details, except that, unlike the FAA was suggesting at the time, big tires didn't adversely affect the aerodynamics of the Super Cub, other than slowing it down some.

MTV

I found bush tires significantly reduced the size of my wallet. Therefore decreasing range.

Battson wrote:...Of course a CFD model is only a good as the base assumptions, boundary conditions, and user configuration you feed it AKA "sh*t in = sh*t out", so I prefer a real life test.

Of course. (actually I have no idea what a CFD model is but I don't want to appear uneducated in front of my friends...too late. I do get the whole "sh*t in = sh*t out" thing though.)

lesuther wrote:http://naca.central.cranfield.ac.uk/reports/1935/naca-report-485.pdf

Seriously cool document, thanks. Neat to see how gear design and placement has such an affect on drag. Next rainy day I'll sit down with about 10 cups of coffee and read the entire thing. But a cursory glance led me to believe that Battson is correct, the best thing is a real life test.

hotrod150 wrote:...Get that GeeBee-type wasp shape going on.

THAT was a seriously cool plane. Glancing through the document lesuther pointed us to, and thinking of the airplane development that took place in the 1930's, those had to be amazing times in aviation.

So you are probably wondering what was the motivation behind my original question? Okay maybe you're not wondering, but I'm gonna tell you anyway.

Since getting the SQ2 with its 35's, the mission of my 185 with its 29's has morphed, not as much off-airport activity - in fact almost none, more hauling me long distance to hunting and fishing destinations. Thinking of going back to 26's to get back as much of my lost 15mph cruise as possible. Which raises the question "which 26?".

(a) Bushwheel 26x12x6: three inches shorter but only one narrower then my 29's, so a lot of the 29's floatation is retained, the question is how much cruise speed do I get back?

(b) Goodyear 25.5x10.5x6: nearly the same height as the 26 bushwheel but an inch and a half narrower, and two and a half inches narrower then my 29's. And this was the tire I was running before going to the 29's, the tire that gave me 15mph better cruise.

Seems like a clear choice doesn't it, go back to the Goodyears. It was a clear choice, that is until I got my SQ2 stuck. My Alaska friends found it amusing that someone could actually get stuck with 35's, but I managed to do it. And when I stepped out to ponder my situation, in my size 13's, I penetrated the surface and sunk to my crotch, and at 6' 5" tall that's a significant distance to sink. I took off my shoes so I wouldn't leave them at the bottom of a 3 foot hole, but that made the penetration situation even worse. I was surprised at the loss of floatation from shoes to bare feet. Standing there, ass deep in wet sand, a question came to my mind; how can a plane weighing about 1,600 pounds (me, a pocket knife, and some fuel), stay on top of the sand (until I penetrated of course) while my 275 pounds with size 13's penetrated more often then not?

So perhaps the extra floatation of the 26 bushwheel over the goodyear would be nice as I sometimes follow wagons around outfitted with 29's. I also prefer the tubeless nature of the bushwheel, I can air the tire down lower, and won't matter if it spins on the rim a bit.

Another question that comes to mind is do I really need to gain back some/all of the cruise I lost? I mostly fly the 185 from Texas to New Mexico and Colorado. No, I don't need the extra speed for these trips. I do occasionally go to Idaho, the extra speed would be nice here. And next year I'm going to Alaska. The extra speed would be REALLY nice for this trip.

Any other advantage to going back to 26's? Much better braking. Does that really matter? Probably not with my current mission, although I'm headed to Arkansas tomorrow with a bunch of CarbonCub and Katmai guys, I'd dearly love to best the Katmai's. But I have to admit my landing distance in the 185 has grown since I've gotten the SQ2, the nature of her becoming a trip airplane I guess. I'll work on that during this trip.

So right about now you are probably saying "wait just a minute, this thread was all about identical surface areas either tall & skinny or short & fat, comparing a 26 Goodyear to a 26 Bushwheel doesn't fit this scenario" - and of course you would be right. But look at the cool stuff we've learned by approaching the question in this manner.

Barnstormer wrote:

Battson wrote:...Of course a CFD model is only a good as the base assumptions, boundary conditions, and user configuration you feed it AKA "sh*t in = sh*t out", so I prefer a real life test.

Of course. (actually I have no idea what a CFD model is but I don't want to appear uneducated in front of my friends...too late. I do get the whole "sh*t in = sh*t out" thing though.)

lesuther wrote:http://naca.central.cranfield.ac.uk/reports/1935/naca-report-485.pdf

Seriously cool document, thanks. Neat to see how gear design and placement has such an affect on drag. Next rainy day I'll sit down with about 10 cups of coffee and read the entire thing. But a cursory glance led me to believe that Battson is correct, the best thing is a real life test.

hotrod150 wrote:...Get that GeeBee-type wasp shape going on.

THAT was a seriously cool plane. Glancing through the document lesuther pointed us to, and thinking of the airplane development that took place in the 1930's, those had to be amazing times in aviation.

So you are probably wondering what was the motivation behind my original question? Okay maybe you're not wondering, but I'm gonna tell you anyway.

Since getting the SQ2 with its 35's, the mission of my 185 with its 29's has morphed, not as much off-airport activity - in fact almost none, more hauling me long distance to hunting and fishing destinations. Thinking of going back to 26's to get back as much of my lost 15mph cruise as possible. Which raises the question "which 26?".

(a) Bushwheel 26x12x6: three inches shorter but only one narrower then my 29's, so a lot of the 29's floatation is retained, the question is how much cruise speed do I get back?

(b) Goodyear 25.5x10.5x6: nearly the same height as the 26 bushwheel but an inch and a half narrower, and two and a half inches narrower then my 29's. And this was the tire I was running before going to the 29's, the tire that gave me 15mph better cruise.

Seems like a clear choice doesn't it, go back to the Goodyears. It was a clear choice, that is until I got my SQ2 stuck. My Alaska friends found it amusing that someone could actually get stuck with 35's, but I managed to do it. And when I stepped out to ponder my situation, in my size 13's, I penetrated the surface and sunk to my crotch, and at 6' 5" tall that's a significant distance to sink. I took off my shoes so I wouldn't leave them at the bottom of a 3 foot hole, but that made the penetration situation even worse. I was surprised at the loss of floatation from shoes to bare feet. Standing there, ass deep in wet sand, a question came to my mind; how can a plane weighing about 1,600 pounds (me, a pocket knife, and some fuel), stay on top of the sand (until I penetrated of course) while my 275 pounds with size 13's penetrated more often then not?

So perhaps the extra floatation of the 26 bushwheel over the goodyear would be nice as I sometimes follow wagons around outfitted with 29's. I also prefer the tubeless nature of the bushwheel, I can air the tire down lower, and won't matter if it spins on the rim a bit.

Another question that comes to mind is do I really need to gain back some/all of the cruise I lost? I mostly fly the 185 from Texas to New Mexico and Colorado. No, I don't need the extra speed for these trips. I do occasionally go to Idaho, the extra speed would be nice here. And next year I'm going to Alaska. The extra speed would be REALLY nice for this trip.

Any other advantage to going back to 26's? Much better braking. Does that really matter? Probably not with my current mission, although I'm headed to Arkansas tomorrow with a bunch of CarbonCub and Katmai guys, I'd dearly love to best the Katmai's. But I have to admit my landing distance in the 185 has grown since I've gotten the SQ2, the nature of her becoming a trip airplane I guess. I'll work on that during this trip.

So right about now you are probably saying "wait just a minute, this thread was all about identical surface areas either tall & skinny or short & fat, comparing a 26 Goodyear to a 26 Bushwheel doesn't fit this scenario" - and of course you would be right. But look at the cool stuff we've learned by approaching the question in this manner.

Barnstormer,

I think that the 26" Goodyears are an excellent choice for a Skywagon. They are probably the best choice of all other options when considering longevity. There is no noticeable speed penalty over 8.50 x 6 tires. Don't let them get below about 12 - 14 psi unless you plan to glue them to the rims. I recently cut a valve stem in a remote location after making this mistake. Had to hike several miles out...

It would be a shame to scrub away a set of Bushwheels in a couple of seasons worth of pavement landings and taxiing for fuel. The sound of bushwheels on a Skywagon spooling up and rolling along pavement is how I imagine the sound that money makes when it evaporates. If you have the luxury of mostly operating from dirt, grass or gravel, the 26 inch Bushwheels are probably the better choice.

My $.02

Scolopax wrote:Barnstormer,

I think that the 26" Goodyears are an excellent choice for a Skywagon. They are probably the best choice of all other options when considering longevity. There is no noticeable speed penalty over 8.50 x 6 tires. Don't let them get below about 12 - 14 psi unless you plan to glue them to the rims. I recently cut a valve stem in a remote location after making this mistake. Had to hike several miles out...

It would be a shame to scrub away a set of Bushwheels in a couple of seasons worth of pavement landings and taxiing for fuel. The sound of bushwheels on a Skywagon spooling up and rolling along pavement is how I imagine the sound that money makes when it evaporates. If you have the luxury of mostly operating from dirt, grass or gravel, the 26 inch Bushwheels are probably the better choice.

My $.02

Worth at least a nickel I'd say.

Like you, I saw no speed loss going from 8.50's to Goodyears 26's. When I ran them I did consider drilling out the wheel cover screw holes and using an old sand dragster trick of running sheet metal screws into the tire bead. Glue would certainly help and be a bit less intrusive.

I am fortunate to fly on/off turf, and most Muni airports have turf that's landable someplace, at least long enough to get the wheels spinning. I have to admit I've gotten lazy lately on a few of my trips (like landing at Taos) where I just landed pavement, even though I know better. (this seems to be a recurring thread, me having gotten lazy with the 185 - shame on me) As a result I've got a couple of those cross-tire divots that I would guess come from the ply layups separating when the tire goes instantly from 0 to 40 once it touches pavement.

Hmmm, when it comes time to head for Alaska maybe I should just put some old 8.50's on until I get to Birchwood, pick up some 26's from AFA, and have Mark Sheets swap my tires out so I can play, and just fly the 26's back. Save the shipping charges. Hmmmm.

The 26's are bulletproof.
No tread is nice if you are operating off of gravel or dirt. No rocks/pebbles into the back of your propeller blades.
They are stiff though, nothing like Bushwheels.
Day to Day, I run the 8.50's.
Wilkerson?? will recap 8.50's smooth (no tread). To me, that would be the best all round!

237

10 X 10 Alaska Bush Wheels and 8.50 X 10's. Excellent wear and floatation, buries the brakes inside the wheel so speed loss is minimal, and a solid "8" on the cool scale. Best thing out there for mixed use, and I've run every combination.