Do bigger tires effect stall speed?

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MTV

Since a stall is caused by the wing exceeding its critical angle of attack, the tire size shouldn't have anything to do with it aerodynamically. The speed at which the critical angle of attack is exceeded does vary with load, and big tires are heavier than small tires. Any pilot that knows their plane shouldn't have any more trouble with big tires than with other changes in load or configuration. I can imagine a pilot unfamiliar with the load and other configuration changes brought on by big tires inadvertently exceeding the critical angle of attack. A pilot familiar with the plane's performance under a variety of loads and configurations shouldn't have any more trouble avoiding a stall when using big tires than when flying the plane at gross weight.

Bigger tires may add weight but being exposed to the airflow they can also effect the aerodynamics of the airplane. They may not change the stall speed, but they may change the way it handles- esp down around stall speed. I seem to recall an article in the old Northern Pilot magazine of about 15 years ago about real or imagined effect on low-speed handling caused by fat tires, esp in turns -- something about the larger tires possibly blanking out the tailfeathers I think. I guess a pod or other exterior load might possibly do the same thing.

I took my J3 from 600-6" wheels all the way to 29" airstreaks. No observed changes in stall speed or characteristics. Of course other performance factors changed - like cruise and takeoff roll. It's just more drag, IMO. Stall speed is a function of AOA only.

soyAnarchisto wrote:I took my J3 from 600-6" wheels all the way to 29" airstreaks. No observed changes in stall speed or characteristics. Of course other performance factors changed - like cruise and takeoff roll. It's just more drag, IMO. Stall speed is a function of AOA only.

Soy, I'm curious if your take off roll got longer or shorter?
On my citabria it got shorter when I went to 31" tires from 8.00s. This is because it gave me a higher AOA so I was able to rotate sooner. I also had more HP than your cub has...

Longer. But I believe that has far more to do with the tire pressure you run. The lower the pressure, the longer the roll. I'm very lightweight - like my plane. So I keep my airstreaks at about 3 1/2lbs of pressure. If I jumped to 6-8lbs (I started there) I notice not difference in takeoff roll - maybe even shorter. I believe bigger radius rolls over bumps better - but only if you keep the pressures up.

I like em lower cause that really softens up the landings and I like cushy for the old girl.

Just a little perspective. My best takeoff from the line at Llano was 186ft (and I forgot and left the carb heat on on that run). My landings were 201ft if I recall. My obstacle approach was 396ft. Any time I can be within a hundred feet of the overall winner (Randy Goza in a Mackey SQ2), I'll take it. Purty good in my book. I need a lot more practice.

Very good, Anarchisto, on the short field work at Llano. I don't know anything about big tires, except in a Stearman that came with big tires, but I know a lot about booms, spreaders, and stuff hanging out of airplanes. You are pulling a lot more air and it takes power to pull air. The reason things hanging off airplanes has to be STCd is that it most likely will change airflow and airfoils don't generally like changes in airflow.

A spray operation I worked for had an old A-5 Callair that they changed from a single seat with the hopper beside the pilot to a four seat club seating open cockpit airplane. The A-5 had a motorcycle type windscreen in front of the pilot and was also open cockpit, However, when they put a full fuselage width windscreen on the conversion, it almost completely blanked the elevator at descently slow approach speed. They cut the windscreen down and it was workable, if not great. It didn't fly as well and the slipstream kept both front seaters hair vertical throughout the flight. STC? You got to be kidding.

That's great work Soy. I always ran 4.5lbs in my tires. Rolled over rocks nicer on gravel bars that way. Intersting results for sure. Having the extra power to handle the increased AOA makes a difference I would think.

A number of years ago, the FAA Alaska Region decided that big tires cause stall/spin accidents. No data, but it must be obvious......

A DER took a Cub, and did flight test on it, with everything from 6.00 - 6 up to 30 inch tires. I can't recall for sure, but I think he may have also flown with a set of the old 36 inch Airwheels.

In any case, the tires, to the chagrin of the FAA, had no noticeable effect on the stall speed or characteristics of the Super Cub. The tires did reduce climb rate and speed some, but if memory serves, nothing else was significant.

The FAA then changed the subject.......

MTV

....and then changed the subject because it was so obvious that tires so affected stall performance that further study was not needed. Now on to sleep issues, because it is......

Our cub gets off the ground shorter with 31's than it did with 8.50's. It wasn't sorta noticeable, it was significant. As far as stall speed, don't really know, the ASI in the cub is not indicating anything at that point

mtv wrote:A number of years ago, the FAA Alaska Region decided that big tires cause stall/spin accidents. No data, but it must be obvious......

A DER took a Cub, and did flight test on it, with everything from 6.00 - 6 up to 30 inch tires. I can't recall for sure, but I think he may have also flown with a set of the old 36 inch Airwheels.

In any case, the tires, to the chagrin of the FAA, had no noticeable effect on the stall speed or characteristics of the Super Cub. The tires did reduce climb rate and speed some, but if memory serves, nothing else was significant.

The FAA then changed the subject.......

MTV

If I remember correct, they even tried automotive racing slicks (my brain is farting and I can't for the life of me remember the name....Schneider???) at that same time....they were no different either.... Like you say, so they changed the subject.......

soyAnarchisto wrote:Longer. But I believe that has far more to do with the tire pressure you run. The lower the pressure, the longer the roll. I'm very lightweight - like my plane. So I keep my airstreaks at about 3 1/2lbs of pressure. If I jumped to 6-8lbs (I started there) I notice not difference in takeoff roll - maybe even shorter. I believe bigger radius rolls over bumps better - but only if you keep the pressures up.

I like em lower cause that really softens up the landings and I like cushy for the old girl.

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This^

With 5-6 PSI in our 29's on a c-85 PA-11, the takeoff roll is a little shorter than it was with 8.50's, when I get below about 5 PSI I really start seeing the extra rolling drag.


BTW I noticed absolutely no change in stall speed with the bigger tires.

No changes on the stall speed on my avid, but climb with the 30" airstreaks is about 500 FPM less than I get with 850s.