Tailwheel pressure

Farnold · Post by **Farnold** » Tue Dec 31, 2002 3:37 am

Wondering what other members run for air pressure in their tailwheels. My C170 POH says 24 lb for the mains but doesn't specify for the TW. A friend with a Citabria with the Scott 3200 tells me his POH calls for 40 psi.

Post by **GAHorn** » Tue Dec 31, 2002 4:32 am

The correct air pressure for the Scott 3200 mounted on a 170 is 34 psi.

N1478D · Post by **N1478D** » Tue Dec 31, 2002 4:33 am

Run 30 PSI in the tailwheel, but it seems to wear in the middle. Thinking about dropping down to 28.

blueldr · Post by **blueldr** » Tue Dec 31, 2002 5:15 am

The reason the POH fails to specify a tail wheel pressure is probably because the stock tail wheel was a solid rubber tire. Many of the old original advertising photos show the aircraft withthe small solid rubber tired tailwheel. The Scott tailwheel apparently was either optional or aftermarket.

Dave Clark · Post by **Dave Clark** » Tue Dec 31, 2002 12:20 pm

I suppose the tire pressures should depend on loading. I keep the mains between 28 and 32, topping them to 32. If I'm heavy I'll see to it that I have at least 30lbs. More than 32 seems to make it a bit squirrly.

The tailwheel I like at 35 but I fill it to 40. I put a new tire and tube on last summer and it's the first tube I've had in 25 years that actually holds air for some time! Don't know why. The important thing on the tailwheel is not to let it get low on air or it can cause shimmy and can rip the valve stem out. I admit it, that's why I replaced the set last summer.

It seems I need to inflate the tires every two or three weeks.

zero.one.victor · Post by **zero.one.victor** » Tue Dec 31, 2002 5:31 pm

I run 40 in the tailwheel and about 20 in the mains (800's).

Eric

Post by **GAHorn** » Tue Dec 31, 2002 11:13 pm

The Scott air pressure vs weight graph stipulates 34 psi but the 170's that left the factory with the Scott 3200 option were equipped with two placards, one inside the glove box door, and one inside the left engine access door, which stipulated tire pressures (and oil grades). The placard states tire pressures to be "Mains - 24 psi" and "Tailwheel - 35 psi".

zero.one.victor · Post by **zero.one.victor** » Wed Jan 01, 2003 12:51 am

I'd be interested in seeing the Scott tire-pressure vs weight graph,George. My ragwing has 180 gear legs,which are raked farther farward than stock 170 legs,which in turn makes the tail heavy. Does the Scott graph use tail weight as "weight",and if so,does it stipulate tail weight in 3 point attitude or tail weight in level attitude(as in weight-and-balance tail weight)?

Eric

Post by **GAHorn** » Wed Jan 01, 2003 1:31 am

Send me a self-addressed stamped envelope with a note reminding me what you want, and I'll copy it for you. But I can tell you that 150 lbs of wt on the tailwheel should be supported by 35 lbs of air in the tire. It makes absolutely no difference if the wheel is on the ground or 3 feet above the ground on a table. (And the difference between the top of the tire to the axle and the bottom of the tire (the supporting surface) and the axle should be between 1/4 and 1/2 inch! (?) I have no idea what use that last bit of data is, ...nor what to do about it should yours not agree with that data! It seems to me to simply be a goofy way to check tire pressure without a tire pressure gauge! In other words, the bottom of the tire should show about 1/4-3/8 inch deflection, regardless of the tire pressure or the weight upon the tire.)

zero.one.victor · Post by **zero.one.victor** » Wed Jan 01, 2003 2:28 am

I've never compared them,but I would think that the tail weight would be different weighed in 3 point attitude,than weighed in level attitude. I thought that's why the factory W&B sheet sez "level at door header" (upper door sill) when weighing for the weight and balance calculations. I would guess that the tail weighs more in the 3 point position,the mains are farther forward & therefore ahead of more of the airplane's weight.
But you didn't answer my question about whether Scott stipulated attitude (level or 3-point) one way or the other.

Eric

zero.one.victor · Post by **zero.one.victor** » Wed Jan 01, 2003 2:34 am

Just for kicks,several years ago I weighed my 170 on some (questionable) scales the local EAA chapter had bought. Partially full of fuel,I had 660# on the right main wheel,600# on the left,and 200# on the tail. That's in the 3 point attitude. I figured I had about 160 lbs of fuel & stuff aboard,for an empty eight of around 1300#. The original factory W&B sheet showed 1230# empty weight.

Eric

Dave Clark · Post by **Dave Clark** » Wed Jan 01, 2003 2:10 pm

Eric
I would guess you'd want to figure the weight with the tail on the ground because in the level attitude it is immaterial. If you've ever weighed the airplane by lifting the tail to level attitude you know that indeed it does get lighter as it goes up. I think Scott is concerned about the actual loaded weight on the ground.

My higher (30lb) mains pressure is with 6:50x6 tires. I usually fly off the island light and return pretty heavy so I tend to keep the tires a bit overinflated. At 24lbs in the mains with any weight in the airplane it gets really hard for this old man to push around. I like the smaller tires but I guess no Capalis beach for me. But maybe with the 180hp I could just power myself out of the sand???

Post by **GAHorn** » Wed Jan 01, 2003 2:52 pm

Awww, Dave! You take all the fun out of this! I was developing a long response about moment-angles and the rate of descent of the tailwheel (dependent upon actual Lattitude, as it affects the distance from the Earth's core and therefore the acceleration of gravity) when you posted that. Damn!

Eric, it makes absolutely no difference how the airplane is positioned... as to the 150 lbs and 35 psi relationship. Nor does it matter whether the airplane is taxying UP a steep ramp, ...or DOWN a steep slope,...or if the fuel has sloshed to the rear of the tanks while accelerating... or....

What they want is a slight 1/4"-3/8" deflection on the bottom of the tire at whatever weight you're operating. That should correspond to about 35 psi on a Cessna 170. The graph would be useful if you also operated a Scott 3200 on a Beaver or Otter or AN-2 with tailwheels supporting weights ranging up to 485 lbs...so you could confirm that the 70 psi it took to produce the 1/4"-3/8" deflection you observed at 485 lbs ... was within the Scott specification. In other words, inflate the tire so that the portion against the ground appears "flat" or out-of-round by approx. 1/4"-3/8" and you'll have it correct. (And if you are doing this on a Cessna 170 you'll be operating at about 35 psi.)

Post by **GAHorn** » Wed Jan 01, 2003 3:07 pm

N1478D wrote:Run 30 PSI in the tailwheel, but it seems to wear in the middle. Thinking about dropping down to 28.

The McCreary (Aircraft Specialties) tire is constructed from a round profile tire, and has the tread design applied as an afterthought. So if you take a new tire, mount it and inflate it, the center tread will appear shallower than the shoulder tread....as if it's been run over-inflated. So, I doubt if the pattern you are observing is due to overinflation.
If you continue to run this tire underinflated it will be distorting the bias plys and will cause failure at the shoulder. This tendency to fail at the shoulder has been reported in several articles over the last few years and is universally attributed to running underinflated. It's better to be overinflated on these tailwheels. (The tire is rated all the way up to 70 psi, and 30 psi is the absolute minimum pressure allowed by Scott.)

Farnold · Post by **Farnold** » Thu Jan 02, 2003 3:13 am

Thanks to all for the prompt replies. I guess I really have been running mine low for a looong time. I had gotten lazy about checking the TW pressure and apparently allowed it to drop even lower and must have pulled the tire off the rim taxing. No damage to rim but have ordered up a new tire & tube. This one will be kept at 35 lb.