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Thread: Prop tip to ground distance question

  1. #1
    Alex Clark's Avatar
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    Prop tip to ground distance question

    What does statically deflected mean???
    On my nose-dragger I still have 7.5 inches of clearance from my prop tip to the ground if the nose wheel tire is totally flat and the strut is completely empty with the rubber snubber removed. Are they talking about that ???.... or ....just the strut being all the way down?

    I have 16.5 to 18 inches with normal inflation on both.



    §23.925 Propeller clearance.

    Unless smaller clearances are substantiated, propeller clearances, with the airplane at the most adverse combination of weight and 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, normal takeoff, or taxing attitude, whichever is most critical. In addition, for each airplane with conventional landing gear struts using fluid or mechanical means for absorbing landing shocks, there must be positive clearance between the propeller and the ground in the level takeoff attitude with the critical tire completely deflated and the corresponding landing gear strut bottomed. Positive clearance for airplanes using leaf spring struts is shown with a deflection corresponding to 1.5g.

    (b) Aft-mounted propellers. In addition to the clearances specified in paragraph (a) of this section, an airplane with an aft mounted propeller must be designed such that the propeller will not contact the runway surface when the airplane is in the maximum pitch attitude attainable during normal takeoffs and landings.

    (c) Water clearance. There must be a clearance of at least 18 inches between each propeller and the water, unless compliance with §23.239 can be shown with a lesser clearance.

    (d) 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. 4080, 29 FR 17955, Dec. 18, 1964, as amended by Amdt. 23-43, 58 FR 18971, Apr. 9, 1993; Amdt. 23-51, 61 FR 5136, Feb. 9, 1996; Amdt. 23-48, 61 FR 5148, Feb. 9, 1996]

  2. #2
    skywagon8a's Avatar
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    Alex, You have a Part 3 airplane.

    3.422 Propeller clearance. With the airplane loaded to the maximum weight and most adverse center of gravity position and the propeller in the most adverse pitch position, propeller clearances shall not be less than the following, unless smaller clearances are properly substantiated for the particular design involved:

    (a) Ground clearance.
    (1) Seven inches (for airplanes equipped with nose wheel type landing gears) or 9 inches (for airplanes equipped with tail wheel type landing gears) with the landing gear statically deflected and the airplane in the level, normal takeoff, or taxiing attitude, whichever is most critical.
    (2) In addition to subparagraph (1) of this paragraph, there shall be positive clearance between the propeller and the ground when, with the airplane in the level takeoff attitude, the critical tire is completely deflated and the corresponding landing gear strut is completely bottomed.


    3.422-1 Propeller clearance on tricycle gear airplanes (CAA interpretations which apply to sec. 3.422 (a) (1)). In determining minimum propeller clearance for aircraft equipped with tricycle gear, dynamic effects need not be considered.

    The following is from the Type Certificate 3A12 for your 172N:
    Propeller and Propeller Limits
    1. Propeller
    (a) McCauley 1C160/DTM 7557
    Static rpm at maximum permissible throttle setting: Not over 2400, not under 2280 No additional tolerance permitted.
    Diameter: not over 75 in., not under 74 in.
    2. Propeller (seaplane only)
    (a) McCauley 1A175/ETM 8042
    Static rpm at maximum permissible throttle setting: Not over 2570, not under 2470 No additional tolerance permitted.
    Diameter: not over 80 in., not under 78.5 in.

    Your airplane meets the requirements of CAR 3.422 for a land plane. However the type certificate has a limitation of a maximum of 75" diameter for a land plane. When Cessna sells a 172 with the seaplane option it is delivered with a special flight permit to the location where the floats will be installed. (This is because the 80" prop diameter exceeds the approved diameter for a land plane). The type certificate is the limitation in your case. Not the CAR.

    I'm assuming that the reason for your question is that you wish to use your 80" seaplane prop while on wheels. Given the above information I would suggest that you will need a field approval from the FSDO on a 337 to use the 80" on wheels. This "should" be a doable simple procedure (particularly since you are in Alaska) since you do comply with the appropriate CAR 3.422 and 3.422-1
    N1PA

  3. #3
    www.SkupTech.com mike mcs repair's Avatar
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    i would think you could keep the rubber on bottom of strut, since it will always be there...?

  4. #4
    skywagon8a's Avatar
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    Quote Originally Posted by mike mcs repair View Post
    i would think you could keep the rubber on bottom of strut, since it will always be there...?
    The 172 does not have a piece of rubber on it's strut as part of it's approved equipment.
    N1PA

  5. #5
    Aviator's Avatar
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    I agree. I think “statically deflected,” here, could be interpreted as “collapsed” strut and tire. A “collapsed” rubber, if any, suggests its compressed dimension at the most unfavorable (nose heavy) CG. Probably a negligible change from relaxed.

    A challenge: define “ground” when on skis in 8” deep snow...
    Last edited by Aviator; 01-19-2015 at 10:12 AM.

  6. #6
    skywagon8a's Avatar
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    Quote Originally Posted by Aviator View Post
    I agree. I think “statically deflected,” here, could be interpreted as “collapsed” strut and tire. A “collapsed” rubber, if any, suggests its compressed dimension at the most unfavorable (nose heavy) CG. Probably a negligible change from relaxed.

    A challenge: define “ground” when on skis in 8” deep snow...
    Alex will have to chime in with what he means by "rubber snubber". In some case folks have been known to place (hose clamp), around the oleo, a section of hard rubber "piece of hose" to prevent the strut from bottoming with a loss of air and/or fluid.

    The regulations are silent relative to your question of 8" snow. I would think that for approval purposes that the FAA would consider the prop clearance dimension to be the top of the ski?
    N1PA

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    Let's look at it this way.
    static
    adj. adjective

    • Having no motion; being at rest; quiescent.
    • Fixed; stationary.
    • Of or relating to bodies at rest or forces that balance each other.



      So "staticly deflected" would mean the aircraft and landing gear at rest on the ground in its normal position with all parts installed, all fluids at the correct level, and all pressures correct.. That would mean if you had the airplane jacked off the floor and then lowered it, if the landing gear was displaced because the friction of the tires did not let the gear spread to the normal relaxed position because the tires didn't slide out, you would roll the airplane until the gear spread to its normal relaxed position. Did I make any sense? jrh
    You can't get there from here. You have to go over yonder and start from there.

  8. #8
    Aviator's Avatar
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    Quote Originally Posted by N86250 View Post
    Let's look at it this way.
    static
    adj. adjective

    • Having no motion; being at rest; quiescent.
    • Fixed; stationary.
    • Of or relating to bodies at rest or forces that balance each other.



      So "staticly deflected" would mean the aircraft and landing gear at rest on the ground in its normal position with all parts installed, all fluids at the correct level, and all pressures correct.. That would mean if you had the airplane jacked off the floor and then lowered it, if the landing gear was displaced because the friction of the tires did not let the gear spread to the normal relaxed position because the tires didn't slide out, you would roll the airplane until the gear spread to its normal relaxed position. Did I make any sense? jrh
    I agree and stand corrected. Thanks. With the oleo and tire collapsed, only positive prop-to-ground clearance is required.

  9. #9
    hotrod180's Avatar
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    I was told to check prop clearance with mains on greased plates (or equivalent) and the airplane loaded to gross weight.
    Cessna Skywagon-- accept no substitute!

  10. #10

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    Thats right hotrod, it reads at gross weight, with most adverse C of G, so I would interperet that as forward edge of tge envelope for a nose dragger. Taildragger too for that matter, all the weight on the mains...

  11. #11
    Alex Clark's Avatar
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    I should have mentioned my reason for asking.

    1. I now have a larger Cessna Heavy Duty nose fork installed via and approved Field Approval.

    2. I have 800 tires on the mains and a 700 tire on the nose with a second field approval.

    I am going to submit a third field approval request to be able to install the 80 inch seaplane prop while on wheels.
    ( it would be handy in late spring when I may need to fly to to Mackey Lakes for the float install on short notice)


    We installed the 80 inch prop the other day just for measurements. With everything inflated I have 16.5 to 18 inches of prop clearance at gross weight.

    With my rubber snubber removed, ( Mine is a solid piece of heavy duty hose so you have to slide if off the strut) and with the strut bottomed out metal to metal, I still have over 12 inches of prop clearance.

    If I have the strut bottomed out and the nose wheel tire removed, ( like it blew off) I still have 7.5 inches of clearance.

    I was asking what they meant because I wanted to see if ;
    A. It was worth trying for an approval
    and B. to get an idea of how to explain the measurements on the 337.
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  12. #12

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    Sounds like you have way more than enough clearance. With the tire off you have more than 7 inches? Sounds like positive clearance to me

  13. #13
    skywagon8a's Avatar
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    Alex, The measurement is taken at the prop tip to the ground.
    N1PA

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    Alex Clark's Avatar
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    skywagon8a

    Alex, The measurement is taken at the prop tip to the ground.
    Roger that, I just can't get the darn computer to post photos correctly.
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  15. #15
    www.SkupTech.com mike mcs repair's Avatar
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    I wonder if ANOTHER field Approval is even needed, too bad you didn't mention this on fork approval...

    log book entry.....
    "because larger nose fork gained adequate tip to ground clearance over original small fork and tire, there is no issue with running approved sea plane prop on wheels now, tip to ground measurement increased X.Y inches and the propeller radius only increased Z" ......"

    ....why I go down this thought process was on last -12 was asking my PMI about Field approval for fuel system changes since I put the huge cargo front door on left side and that nessitated some major fuel line routing changes to be made, and he said none needed, it was required change, do to the door install, which he was field approving the installation as a whole....

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    Alex Clark's Avatar
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    The guy at FISDO seemed to think we needed another field approval.
    I thought that the big fork and big tires could have been all on one 337 request as well, but they denied it in that form.

    The big Cessna fork was an approved OPTIONAL item on many models of C-172s and 175s. But for some reason my particular year and model was not mentioned. ( The 1977 model ( but made in 1976) 172N was the last year with a 12 volt system and 40 degrees of flaps. So it was a one year oddball. But we still referenced the parts manuals for other models on the 337. It is not like there is any difference on a M or N model nose wheel.

  17. #17
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by Alex Clark View Post
    I thought that the big fork and big tires could have been all on one 337 request as well, but they denied it in that form.
    ...
    things seem to be going backwards... we used to do them all separate, then they wanted them all on one 337/field approval say all your fuselage mods, was 8 PAGES long....

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    Quote Originally Posted by Alex Clark View Post
    The guy at FISDO seemed to think we needed another field approval.
    I thought that the big fork and big tires could have been all on one 337 request as well, but they denied it in that form.

    The big Cessna fork was an approved OPTIONAL item on many models of C-172s and 175s. But for some reason my particular year and model was not mentioned. ( The 1977 model ( but made in 1976) 172N was the last year with a 12 volt system and 40 degrees of flaps. So it was a one year oddball. But we still referenced the parts manuals for other models on the 337. It is not like there is any difference on a M or N model nose wheel.
    Hi, I came across this old thread while looking for info on installing the 80" prop on a wheeled 172. Have you had success in doing this? If so, how is the performance??

  19. #19
    Alex Clark's Avatar
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    After I took all sorts of photos and measurements, for the field approval request, my IA was called off on another project and we lost all the info. Thus he never submitted the paperwork. After all the measurements and photos we re-intalled the LAND PROP. I was not going to run a big prop without it being approved. My IA has been talking about it ever since.

    I have flown another plane which is similar to mine, except he has a 150 horse converted to 160 with jugs,instead of a stock 160hp and he has a slightly larger Landis nose fork. He has the 80 inch prop on his and it certainly takes off and climbs at a steeper angle. It also slows down better when throttling back to idle since the flat prop acts more like a brake. His top speed is about 15-20 knots slower than mine on wheels.
    So better take-off and landing performance but less cruise speed. And less miles per gallon.

    Since most of my winter flying is up to Kenai or Soldotna or Anchorage for meetings or training, I am not so sure the long prop is what I need in the winter. In fact my land prop does not have enough bite since it is a re-furb.
    If I had not just given our thieving SOB family dentist $5,000 I would have bought a new land prop with a speed twist rate for winter zoom-zoom.

    If I was going to run on wheels in the summer, I would do the long prop deal.

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    Hi Alex, thanks for the reply. I have a 172K down here in Idaho. It has the Landis fork with an 8.00 X 6, and 8.50's on the mains. It sure looks like I'd have plenty of prop clearance if I lost air pressure in the strut. Do you know if they had to go through a similar process to get that 80-40 prop on the other plane? Sounds like it would be a great prop for backcountry type stuff. I don't have the high compression Pistons, but have the power flow exhaust and sportsmans STOL.

    Thanks,
    Bob

  21. #21
    skywagon8a's Avatar
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    Quote Originally Posted by 7Bravo Skyhawk View Post
    ... It sure looks like I'd have plenty of prop clearance if I lost air pressure in the strut. Do you know if they had to go through a similar process to get that 80-40 prop on the other plane?
    CAR 3 is the same.
    23.925 Propeller clearance.

    Unless smaller clearances are substantiated, propeller clearances, with the airplane at the most adverse combination of weight and 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, normal takeoff, or taxing attitude, whichever is most critical. In addition, for each airplane with conventional landing gear struts using fluid or mechanical means for absorbing landing shocks, there must be positive clearance between the propeller and the ground in the level takeoff attitude with the critical tire completely deflated and the corresponding landing gear strut bottomed. Positive clearance for airplanes using leaf spring struts is shown with a deflection corresponding to 1.5g.
    Last edited by skywagon8a; 02-03-2017 at 12:45 PM.
    N1PA

  22. #22

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    Quote Originally Posted by Alex Clark View Post
    Roger that, I just can't get the darn computer to post photos correctly.
    I was taught in A&P school, strut collapsed, any bumpers removed and flat tires, then measure. Its to simulate having a worst case scenario and still have prop clearance.

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