Kitfox Aircraft Stick and Rudder Stein Air Grove Aircraft TCW Technologies Dynon Avionics AeroLED MGL Avionics Leading Edge Airfoils Desser EarthX Batteries Garmin G3X Touch
Results 1 to 5 of 5

Thread: Torquing prop bolts

  1. #1

    Join Date
    Sep 2009
    Location
    Sioux Falls, SD
    Posts
    63

    Default Torquing prop bolts

    Was surfing around and found this little note on another forum. It applies to a Zenair but struck a note with me. Every time I go to retorque my GSC 66" adjustable pitch prop, the bolts are loose. The prop was just inspected and overhauled by GSC. My torque wrench is calibrated, etc, etc.

    Anyway, here's the topic:

    -----------------------------------------------------------------------------------

    Our CH601HD Zodiac with a Rotax 912S 100 HP engine has a 68-inch, 3-blade
    GSC wooden prop. In our installation, there is a prop flange extension.
    The prop is held on by 6 1/4-inch bolts (3 through the aluminum blade
    roots), and by 6 5/16-inch bolts onto the flange extension. GSC Systems
    says use 100 inch pounds of torque for the 1/4 inch bolts, and 140 for the
    5/16; these values are critical. Normally, with a washer under both the
    head and the nut, one holds the bolt head stationary, and torques the
    nylon lock nut; these nyloks are the preferred nuts recommended by GSC.
    Because of the design of the prop flange extension, it is not possible to
    get the torque wrench onto the nuts. Thus, one holds the nut stationary,
    and torques the bolt head. THIS IS NOT THE SAME THING. My son Peter
    (co-builder of our aircraft, and a professional mechanical engineer) knows
    from practical experience that putting the torque wrench on the bolt head
    can undertorque the nut by possibly 20-30%, or even more. It stands to
    reason: the bolt shaft (or grip) inevitably experiences some friction in
    passing through the parts being bolted together on the way to the nut.
    There is a little part called a "crow's foot" to overcome this difficulty,
    which goes over the nut, extends radially outward for about an inch, and
    has a square hole for attachment of the torque wrench. It happened that I
    had always torqued the prop bolts on our aircraft. In setting out to
    retorque the bolts yesterday because the weather is now turning cold,
    Peter realized right away that there was a problem. I had never used a
    crow's foot (I did not know of their existence!). I'm guessing that many
    others using this Rotax/GSC prop installation were also unaware of this
    problem. What have others done about this? Were they aware of the
    problem?

    -----------------------------------------------------------------------------------

    Anyone else run into this problem?

    Chris
    Model 2, 582
    Sioux Falls, SD

  2. #2
    Super Moderator Av8r3400's Avatar
    Join Date
    Jan 2009
    Location
    Merrill, WI
    Posts
    3,044

    Default Re: Torquing prop bolts

    By using a crow-foot socket, you are extending the lever of the torque wrench and the torque applied is greater than the setting on the wrench.

    (A good read on the subject.)

    Torque Calculator
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
    YouTube Videos

  3. #3
    Senior Member
    Join Date
    May 2009
    Location
    Lethbridge, Alberta
    Posts
    270

    Default Re: Torquing prop bolts

    I have always wondered about this. I have an IVO in flight adjustable and the bolts are about 4 inches long and go thru a number off pieces with the nut being held on the back side. Because the bolts go thru all the pieces there is definitly some resistance on the bolts. What I did is set the torque wrench to 0 and try each bolt without the nut on and slowly increase the torque setting till the wrench will not release. It seems to average around 3-5 foot pounds. I take the highest reading and added it to the IVO recommended of 30 foot pounds giving me 35 foot pounds.
    Since this is a new prop with less than 10 hours I check it after every flight. Have had one bolt that needed about a 16th of a turn to bring it back to the 35 foot pounds. The silver tape is still intact and all appears good.

    yours thoughts on my methods would be welcomed....

    mic

  4. #4

    Join Date
    Sep 2009
    Location
    Sioux Falls, SD
    Posts
    63

    Default Re: Torquing prop bolts

    Mic...I think this is as valid a method as any, especially since it applies to your specific situation and doesn't rely on guessing of unknown factors.

    My issue with the crow's foot is that it's imprecise. The reason you should torque on the nut instead of the bolt head is that there are no other resistances. If you're on the bolt head, one has no way of knowing how much resistance there is to torquing along the length of the shaft. Therefore, it's anyone's guess as to just how much you should compensate. I think that the 20-30% advocated in the original thread is an estimation and probably ok for non critical/structural applications. Holding the propeller on the airplane is not one of these however.

    My A&P mechanic buddies say that a crow's foot is not designed for this. They also say that a little loosening of the prop flange bolts is normal and 'figured in' when determining the correct torque settings, especially with a wooden prop where expansion/contraction has to be taken into account.

    Chris

  5. #5

    Join Date
    Oct 2008
    Location
    WA
    Posts
    6

    Default Re: Torquing prop bolts

    On torquing with a crowsfoot. You should allways turn the crowsfoot 90deg to the torque wrench. That keeps the torque the same as if you used a socket. If the crowsfoot is in line with the wrench there is a mathamatical formula to determine the correct torque. At least that's how it's done in the Boeing bolt installation spec. Actually used a crowsfoot to torque some jam nuts for an F-22 a couple hours ago.

    And we do have a different torque requirement for torquing the head vs the nut. Torque from the head is only slightly higher.

    Scott

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •