THE CARE AND FEEDING OF TIRES AND TUBES…(110500)
By Steve Wilson <SteveWlson@aol.com>
Tube type tires, which are original equipment on our Swifts, slowly leak air. It’s normal! It is a fact of life you need to get used to! Aircraft tires are one of the most important components which are routinely ignored until they “look” too low. If you allow that condition be your signal to check the air pressure, you’re late! Your tires are probably not broken, but they are overdue for your attention.
The failure mechanisms of an aircraft tire are complex. Let’s just say that the tire’s worst enemies are heat and over-stress caused by under-inflation. The hysteretic energy resulting from the rapid elastic stress-cycling of a rotating tire can generate enough heat to literally burn the tire. Remember seeing the smoke coming from the under-inflated semi-truck tire? The rolling resistance of a tire significantly influences the length of the takeoff roll. While the rolling resistance is not directly proportional to inflation pressure, it can increase as much as 50% for an under-inflation condition of 15%.
The inflation pressure recommended by the manufacturer is a compromise which weighs three fundamental targeted design performances of aircraft ground handling characteristics, dynamic spring rate, and service life. There may be other performance characteristics that are functions of inflation pressure, i.e., footprint shape and area, bead seating force, etc., but I believe the above three are more easily recognized by most of us. As the tire loses air pressure, its performance changes are gradual and subtle. You might not sense the degradation of handling performance of your airplane during takeoffs and landings until it’s too late. Sure it’s a bother to check the tire pressure regularly, say monthly or whenever the ambient temperature changes significantly. The alternative is not very attractive either. The dynamic spring rate will come into play, big time, if you over-inflate in order to avoid having to check the pressure as frequently.
You say, you know all this stuff already… Then just heed this reminder: “Your tires won’t let you down, if you don’t let them down!” — Steve Wilson
SMALL TIRES AND STALL STRIP…(120200)
Subj: Small tires, stall strip
From: Dick MacInnes <firstname.lastname@example.org>
In one of the October updates from Arbeau you mentioned an STC that included a machined stall strip as well as the small wheels, etc. I bought the tires and tubes from Swift Parts some time ago and they sent me a copy of Charlie Cummings 1986 337 on his N96CC along with his print. Also a letter from Charlie Nelson to Jack Nagel dated 15 Sept 1993 authorizing him to use this info. Also in the letter to Nagel Mr Nelson stated that “it is our intention to have the installation of the 15X600X6 tires…” added to our type certificate data as Item #201, and #603. My questions are 1. Is there an STC or are we referring to the same data, and 2. did Swift ever add the mods to the Type Certificate data. I’m not quite up to going back to work on the bird yet but am trying to gather data, money! and misc parts, like a more or less usable cowl (Have a Swift Tech) Appreciate your work and insight Dick MacInnes N80923 email@example.com
Dick, What I was referring to was the same thing you have. It is not a full STC but to my knowledge the FAA has not turned anyone down on the use of it. I have typed up a 337 and sent it in for prior approval without any problem. I don’t think Swift has been successful in getting that data included in the aircraft spec. Many Swifters are using the 15:6:00×6 tires now. I had them on one Swift I had about 12 years ago and didn’t like them. It was like landing with the tires full of concrete. Maybe if your landings are better than mine it would be OK! — Jim
NEW TIRES REQUIRE RETRACTION TEST…(OCT 02)
Editor says… Here’s a good one. Many thanks to Bill Tiley for sharing it with us.
From: Bill Tiley <Yakflyer@aol.com>
Subject: Fitted Gear Well Mods
AARRRRGGGGGHHHHH!! During the last annual inspection, the IA said “your tires are okay, but worn… you might want to replace them”. I said “Yeah, I’ll do that later”. We completed the annual, including the retraction test and emergency crank-down, etc. A few weeks went by and I decided to change the tires… replacing them with the new diamond pattern tires I purchased from Swift Parts in Athens. No, I did not do another retraction test. Wrong decision! On the 10th landing, the gear failed to extend. Following procedures, I tried the emergency extension. No luck. I then went into a shallow dive and pulled up to about 4.5 to 5 G’s. The gear came down and locked. Funny thing about those diamond tread tires (Specialty Tires of America), they are nearly 5/16 greater in diameter than the standard 600 X 6. My fitted gear mod was accomplished some time ago by using the standard 600 X 6. Damage was restricted to the “enclosure mod”, bending it outward, and popping some rivets. I learned a lesson: Do a retraction test after replacing the tires! Bill Tiley
BIGGER IS BETTER??? (SEPT 03)
Subject: Re: data plate
What’s your opinion about the savings in drag/lift or whatever with going to 5″ wheels instead of 6 inch wheels. I am operating off grass that is not the smoothest in the world and am considering switching back to 6″ wheels. Not sure it would make much of a difference at all in the performance since my Swift is slow anyway…..big tailwheel, slots in the wings, metal cowling off a C-310 with too much cooling drag, etc. etc. Sincerely, Phillip
The smaller wheels & tires do help speed “a little”, especially if the opening is closed up around them. Every little bit helps but if you put the 6:00×6 tires and wheels back on I doubt if you would see much loss. Be sure you check clearances on jacks afterward! I don’t like the small tires, it’s like landing with the tires full of concrete. I think they are harder on the airplane. I think they need about 50 psi. — Jim
BIGGER IS BETTER??? PART DEUX… (SEPT 03)
Subject: Re: data plate
The wheel wells are closed up around the smaller tires at this moment but I think I agree with you about the hard feeling. The Swift that went to France had the big tires at first and then the switch was made to smaller wheels and tires when it left my place and headed towards Europe. I don’t remember how much air the bigger tires needed but I assume you could get by with the old Piper formula of one pound for every hundred pounds of gross weight. That was published in the Piper Stinson manuals and also in the L4J manual that the Army Air Corps had with each Piper Cub they used during the war. So…….twenty pounds or so should do it with a 1970# gross weight Swift. Add a few pounds for the wife and you’d be around 25lbs or so. Does that sound right? Sincerely, Phillip
25 pounds or so is Ok for the standard 6:00 x 6’s but the low profile 15x600x6 tires need about 50 psi. — Jim
TIRE TALES… (SEPT 03)
From: Dick Aaron <firstname.lastname@example.org>
I saw your comments about the 5.00 size tires and pressures in the last newsletter. I’m getting close to needing new tires (6:00’s) and was wondering what you recommend between McCreary’s Air Trac, Air Hawk 4 or 6 ply. The Aircraft Spruce catalog shows the 6 plys to be 0.2 lbs heavier and for the Air Tracs to need 42 psi vs 29 psi for the Air Hawks. Regards, Dick
Get the lightest tires you can for a Swift. A pound or so might not be much in the overall weight of a Swift, but the load on the hydraulic actuator when you figure the moment is impressive. The lightest tires are usually the cheapest. Get light, cheap tires and if you have to replace them every few years, so what? I remember comparing cheap McCreary’s to some deluxe 6 ply Goodyears once and the weigh difference was several pounds. — Jim
TIRE PRESSURE… (FEB 04)
Subj: Low Profile Tire Pressure
From: Mike Foushee <mike.j.foushee@Cummins.com>
The original Swift manual calls for a tire pressure of 20 PSI as I recall. My recently acquired Swift (N711E) has “low profile” tires (15 – 6.00 x 6). A logbook entry for an annual inspection done several years before I purchased the plane indicated that they “aired” the tires to 50 PSI which seems excessive to me. The tires LOOK properly inflated at about 30 PSI. Is there an official tire pressure level for the low profile tires on a Swift? Thanks for any information you can give me. Mike Foushee N711E
I had the 15:6:00×6 tires on one Swift I had and I believe I used 45 psi. I would use the lowest pressure possible. The tires absorb a lot from a hard landing. I have previously heard of excess premature wear when using lower tire pressures in the little tires. — Jim