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Thread: prop clearance advice

  1. #1
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    Default prop clearance advice

    A question for those in the know: Is there a guideline or rule of thumb on prop clearance? or is it just Personal preference?

    I have a set of bush wheels that are worn out. I also have a set of Air Trac 8.5 x 6 i can put on, but it reduces my clearance from 24 inches to 19.5 inches. I don't have a shorter prop, but i'm thinking wheels are generally cheaper than props in the long run. Can somebody lend me their thought here?

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    There is actually a legal prop clearance regulation. With a flat tire and with a blown strut and so on... I had to look it up a few years ago for a seaplane prop while on wheels field approval.

    Sec. 25.925 — Propeller clearance.

    Unless smaller clearances are substantiated, propeller clearances with the airplane at maximum weight, with the most adverse center of gravity, and with the propeller in the most adverse pitch position, may not be less than the following: (a) Ground clearance. There must be a clearance of at least seven inches (for each airplane with nose wheel landing gear) or nine inches (for each airplane with tail wheel landing gear) between each propeller and the ground with the landing gear statically deflected and in the level takeoff, or taxiing attitude, whichever is most critical. In addition, there must be positive clearance between the propeller and the ground when in the level takeoff attitude with the critical tire(s) completely deflated and the corresponding landing gear strut bottomed.
    (b) Water clearance. There must be a clearance of at least 18 inches between each propeller and the water, unless compliance with §25.239(a) can be shown with a lesser clearance.
    (c) Structural clearance. There must be—
    (1) At least one inch radial clearance between the blade tips and the airplane structure, plus any additional radial clearance necessary to prevent harmful vibration;
    (2) At least one-half inch longitudinal clearance between the propeller blades or cuffs and stationary parts of the airplane; and
    (3) Positive clearance between other rotating parts of the propeller or spinner and stationary parts of the airplane.
    Doc. No. 5066, 29 FR 18291, Dec. 24, 1964, as amended by Amdt. 25–72, 55 FR 29784, July 20, 1990]
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  3. #3

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    I am in a hurry but found this thread very interesting---I am flying a cub and 185 part 135.
    I have found that the higher up I can keep that prop the better.
    The 35" tires and 3 " gear on the cub are really good for that.
    On the 185 the 29" tires are great. I know guys that do 31" but that wont work for part 135.
    The bummer for me is, I burn through the 29" bushwheels really quick on the 185. They are soft and dont like that much weight especially on braking.
    I have had some sidewall failures on them.
    Hope this helps, I will check back on the thread ;-) if you need more info

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    wow, thanks Float Pilot. I didn't even think about searching the FARs. I guess as i understand the regs, my champ needs at least 9 inches of clearance in a tail up, takeoff attitude. Additionally, any positive clearance with the tail up, and tires deflated.....so i can take off with flat tires, without wacking my prop into the ground. I'm glad the FAA is looking out for me and my prop.

    Bearsnack, I think airframes sells 29" that are buffed or have a little extra tread. I also found a recapping process on youtube, but i don't think its approved by the FAA, and certainly not for 135 stuff. I might look at recapped tires for my champ. I hate the idea of forking over three grand for a part that inevitable wears down.

    In other news, have any used bushwheels for sale?

  5. #5

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    How about the goodyear 26" tires for your champ? They are pretty reasonable price and give good clearance.
    Not very soft though.
    My first set of 29s went about 450 hours so I guess I can not complain to much. The 2nd set I have now are the extra tread.
    2 positives about the 29" bush wheels-- 1. They are taller than the 8:50 x 10s and other comparable tires (prop clearance) and 2. They are soft and absorb more shock rather than transmitting it to the gear and airframe.

  6. #6
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    35 years ago I was in my old modified J5 with huge tires and my buddy was with his L21 military super-cub on 29s. We landed on a gravel bar in the Skentna River and thought we were hot stuff.

    Then from down the river came the putt-putt sound of a twin exhaust Champ approaching. That Champ came around the corner and set down in the first 6 inches of gravel bar and rolled to a stop in a shorter distance than we had used. The old pilot was actually wearing nice clothes as he jumped out to go fishing. And of course he caught twice as many fish as we had.
    I asked him why he only had 800 size tires. His reply was weight and further that he had never needed anything larger. Then he departed with a nice smooth roll off over the river and he disappeared into the distance. I still remember that lesson. His experience and skill, matched with light weight gave him superior performance.

    That is why I only ever had 26 inch tires on my lightweight non-electric PA-11 90 horse. She was less then 800 pounds empty. So she did not sink into the gravel or sand.

    These days my dang souped up C-180 on 29s weighs darn near 2000 pounds empty and it is like landing a wheelbarrow full of dirt.
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