This summer, we are flying out west for a vacation. We need to sort out lots of details, and tires are one of them. Inspirational backcountry flying videos all show airplanes with giant balloon tires. On the other hand, the guidebooks describe the maintained airstrips as suitable for a standard general aviation plane. Back country pilot has a page dedicated to picking tires. I even found a thread online where someone was asking about flying in Utah with a Maule on 8.50s — the same tires as we have and a very similar airplane to our Bearhawk. With our current 8.50’s we could select safe airports and have fun. Bigger tires cost money, but they open up a lot more airstrips. Tyson and I concluded we wanted the flexibility to fly with other folks we might meet out there.
[Update July 2019] Read about how we actually made out on our trip to Utah and Idaho.
Prior Prep Work
When we bought the Bearhawk, it couldn’t have fit large tundra tires. Two prior modifications were necessary:
- Bigger brakes — twice as big as the original and two pistons per caliper instead of one. The original brakes wouldn’t have had enough leverage on the large tires. Tyson upgraded brakes almost immediately to improve braking on the 8.50s.
- Notching the gear fairing for tundra tires — similar to this forum post. We did this when we made other modifications to the gear for skis.
The new front tires arrived arrived on Thursday — a pair of 31″ Alaskan Bushwheels. The bush wheels are big donuts, completely self sealed. They look like oversized inner tubes with a round cross section and smooth surface. Under the rubber, they have a Kevlar carcass.
Tyson put the new tires on Monday. First he mounted the tires on their rims. The tires bulged out so much the bolts wouldn’t reach through the rims and the brake disks. Working as a team, I was able to thread the nuts on while Tyson pushed the plates together. On the second tire, Tyson squeezed the air out of the tire and closed the valve before trying to mount it. Then the bolts fit through easily. Next we needed to swap tires on the Bearhawk.
When we have changed the Bearhawk tires before with a single car jack, it was a precarious operation. A local Bearhawk builder/pilot had mentioned using a beam between the axles to make it easier. Tyson worked up a plan for two car jacks and a 4×4 supporting both gear legs. At each end of the 4×4, a groove holds the gear leg from slipping off. On the bottom, about a foot in from the ends, the jacks fit into pockets cut with a hole saw.
I came down to the hangar to see how the tire change went. It looked quite simple — unscrew the bolts on the brake pads, take the cotter pin out of the axle nut, unscrew the axle nut, pull the old wheel off. Reversing the process was just as simple. After we return from the Rockies we will only use the big tires for special occasions. I was pleased to see that it’ll only take an hour to change the front tires.
I have never pumped up a completely flat car tire, so I can’t compare. But it took the compressor five minutes to pump each bush wheel up to 12 PSI. Once we were sure they would bear weight, we lowered the plane and removed the jacks.
The 4×4 wouldn’t come out. Two things had changed since Tyson inserted it.
- The smaller 8.50 tires come straight up from the brake disks. The bush wheels balloon out an inch or two beyond the brake disks.
- The gear had been splayed apart from pushing the Bearhawk into the hangar. When Tyson jacked the airplane up, the gear legs sprung together and stayed that way when we lowered the plane.
The obvious, but painful solution was to jack the airplane back up, swap back to the 8.50s, lower the plane, remove the board, and take it to the table saw to shorten. Then start all over with changing tires.
We grumbled at it for a bit. It was time to go eat dinner, so there wasn’t much we could do. I shoved at the tires, but they wouldn’t budge. Then Tyson thought of prying the tires apart with a car jack set sideways and boards a little shorter than the 4×4. Not elegant or stable, but it worked. We ate dinner happy.
The new tires are 36lb heavier than the old ones. Empty weight went from 1,602 lb to 1,638 lb. Still plenty of room for payload and gas given the gross weight of 2,500lb. The center of gravity moved forward less than an inch — from 9.665″ aft to 9.375″ aft. Payload and gas can easily move the CG 5 inches, though always aft.
Tyson test flew the airplane a few days later. It wasn’t a great day for flying, occasional drizzle and overcast, but good enough to fly to Jaffrey. He did several takeoffs and landings and pronounced the airplane good.
Tyson reported that it took more effort to hold the nose up for landing. I am surprised moving the CG less than an inch made that much difference. Another reason the airplane might want to nose forward is additional drag from the bigger tires.
The softer rubber on tundra tires doesn’t hold up on pavement, so Tyson kept his landings on the grass. The smooth wheels slid sideways downhill on the wet grass. Tyson said it was reminiscent of landing with skis.
Tyson said he misjudged the airplane’s height the first few landings. After a few landings, he learned the correct new height.
Otherwise, Tyson didn’t notice much difference. Visibility over the nose on the ground is still dreadful. Cruise speed dropped by about 5-7 kts. Lateral control seemed unaffected.
Tyson thought the tires were stiffer than they needed to be, so after test flying it, he let out 4 pounds of pressure. (Depending on which gage we use, they now have either 10 or 8 PSI.)