Tyre Pressure for Rough, or Sand, Strips

[APenNameAndThatA]

Hi, I can't find any information about this anywhere. I am wondering what tyre pressure is best for operating on rough, sandy airstrips. For example, ones with some rocks rough ground and dirt, please. This relates to how much difference it makes if you have 8.00-6 tyres or 6.00-6 tyres.

 

I am thinking of the A22LS Foxbat tricycle gear LSA, which has a MTOW of 600 kg. Those are the two tyre sizes. An A32 Vixen would become experimental if you added 8.00-8 tundra tyres. The Foxbat can do a lot to avoid using its nose wheel. It even has a tail wheel incase people over rotate.

 

I am posting here because some of the same questions will arise about other airplanes. The Goodyear Databook_7_2016.pdf is where I am getting my info. It says that the C206 main gear can be 6.00-6 or 8.00-8 and that the main gear is 5.00-5. The nose gear of a C441 is 6.00-6!

 

My thinking is that there are two main reasons to fit bigger tyres: a) larger rolling circumference, giving a smoother ride over rocks, and b) the opportunity to have a larger footprint/less PSI on soft ground. Also, possibly the bigger tyre will be able to cushion a larger bump better, and give slightly slightly better prop clearance.

 

Based on my four wheel driving experience, 16 PSI is fine for soft sand, and usually allows a 4WD to move from standstill, applying drive force via the tyres. In a 4WD, if you have a (tubeless) tyre part company from a rim it is a nuisance, in an airplane it would be worse than that.

 

The Goodyear 8.00-8 has a shoulder diameter of 17.04 inches and a shoulder width of 6.75 inches.

 

The Goodyear 6.00-6 has a shoulder diameter of 15.45 inches and a shoulder width of 5.35 inches.

 

Calculating with just the main gear, at maximum LSA weight, you have 300 kg per wheel, or 660 lb.

 

660 / 16 = 41.25 square inches for 600 kg at 16 PSI.

 

41.25 / 6.75 = 6.11. So, the contact patch on a 8 tyre, would be 6 inches long.

 

41.25 / 5.35 = 7.71. So, the contact patch on a 6 tyre would be 8 inches long. I think I can remember that...

 

 

[spacesailor]

As a side note, Ian's permission please, Aldi's have a CO2 cartridge powered, tyre inflater, tomorrow the 12 th July.

 

spacesailor

 

 

[Geoff13]

I carried one on my triathalon bike for years. Very handy.

 

 

[APenNameAndThatA]

Hi, Penname, great post! Maybe a simpler question would be "what is the minimum safe tyre pressure?"

 

 

[Downunder]

One important factor is how many "ply" the tyre is and resistance to puncture.

 

The "turf glide" 8.00x6 on the foxbat is a soft 4 ply tyre with not much "meat" on the tread.

 

Foxbat max pressure 23 psi (tyre states 20 psi)

 

The 4 ply gives softer cushioning and thinner rubber thickness makes it deform easier, thereby making it softer again. Making a larger footprint.

 

A few owners have changed them over to more conventional tyres because of punctures. Even at mtow (not really that heavy)and 6.00x6 tyres, at the landing speed of a foxbat few seem to have and issues on rough ground. Maybe with sand you might want the larger footprint of the 8.00.

 

A number of tyre mfgs make the same tyres in different ply, generally 4 and 6 ply but up to 12.

 

 

[Soleair]

I run my 4 ply 15" x 6.00 - 6 tyres at 8 psi. But then, my MTOW is 300kg and I have no suspension

 

Bruce

 

 

[fallowdeer]

Aero Classic 21/800 on the mains and 600x6 Condor nosewheel. Run the mains at minimum pressure that they don't rotate on the rims.

[johnm]

Waignawa fallowdreer ?

 

.............. fotgetting how to spell the rivers

 

 

[APenNameAndThatA]

[ATTACH=full]51129[/ATTACH] Aero Classic 21/800 on the mains and 600x6 Condor nosewheel. Run the mains at minimum pressure that they don't rotate on the rims.

Wow, Fallowdeer, that Savannah VG has a MTOW of ? about 550 kg. You were almost certainly lighter in the picture. The tyres have not deflected much at all.

 

 

[Downunder]

Wow, Fallowdeer, that Savannah VG has a MTOW of ? about 550 kg. You were almost certainly lighter in the picture. The tyres have not deflected much at all.

Without anyone in it and an unknown quantity of fuel, it would be 300 and something kilo's sitting there probably.

 

As "force= pressure x area" you probably don't need the psi (pressure) in larger diameter tyres to some degree due to the larger internal tyre area. And as air is compressable, you get the spongy cushion on landing.

 

 

[fallowdeer]

Hi John M

 

Close, in the Ruamahanga near Papawai. Yep, 305kg plus another 30kg of fuel plus me equals about 420kg.

 

Peter

 

 

[APenNameAndThatA]

I run my 4 ply 15" x 6.00 - 6 tyres at 8 psi. But then, my MTOW is 300kg and I have no suspension Bruce

When I was looking up Aero Classics, I saw on a Maule forum that someone runs 9 PSI. It seems that the advantage of the tundra tyres is the soft ride, rather than having a large enough contact patch to not have excessive drag.

 

The trade off between ply and puncture resistance seems to be important. A lower ply rating should give a softer ride, and the tyre should absorb less energy during takeoffs.

 

The tyres should not be running so long that overheating would be a problem.

 

If you get a puncture on takeoff, and land with a flat tyre, you could bend your airplane, could you not? Or, do people generally get away with it? Unlike a car, the air is not being squashed out of the tyre during travel. Of course, it would eventually get down to zero PSI. I suppose that a lower tyre pressure would increase your puncture resistance, rather than increase it. I imagine you could get sand inside the tyre if you ran it at low pressures (I did in my 4WD) which might cause problems.

 

Using Pythagoras's theorem, by my calculation, the radius of a 6 tyre, at MTOW would be 6.69", or about 7". If you take away 3" for the wheel the tyre is on, there is a bit less than 4" between the ground and the wheel.

 

With the 8 tyre, there would be 7.95" radius at the middle of the contact patch, giving about 5" between the ground and the wheel.

 

 

[Soleair]

The tyres should not be running so long that overheating would be a problem.

On takeoff, my roll lasts perhaps 10 seconds. Landing maybe 20 seconds. So little danger of overheating there.

 

If you get a puncture on takeoff, and land with a flat tyre, you could bend your airplane, could you not? Or, do people generally get away with it?

I'll let you know when it happens!

 

 

[Yenn]

I have landed with a flat tyre and no problems. Of course I can't tell what pressure it had as I first touched down. It did take a lot of power to move it off the strip.

 

 

[APenNameAndThatA]

I have landed with a flat tyre and no problems. Of course I can't tell what pressure it had as I first touched down. It did take a lot of power to move it off the strip.

It is reassuring to know that you can land on a flat tyre without problems. I suppose that tyres are designed that way on purpose. Given that Goodyear supplies its 6 and 8-sized tyres for C206's, there should not be any problem landing a LSA on a flat tyre even if it was only 4 ply? Goodyear makes 6 tyres in 4 to 8 ply.

 

 

 

 

[APenNameAndThatA]

I have landed with a flat tyre and no problems. Of course I can't tell what pressure it had as I first touched down. It did take a lot of power to move it off the strip.

How heavy? How many ply?