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Single flap locked down - hinge pocket cover jam

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  • Single flap locked down - hinge pocket cover jam

    I really have no idea how this managed to occur, but worth knowing about... If your hinge pocket covers don't go all the way to the front of the flap - then you have a problem waiting to happen.

    We had one of our hinge pocket covers, somehow, jam into the trailing edge of the wing top skin. Our pocket covers were short enough that their leading edge passes beyond the trailing edge of the top wing skin, when the flap is fully deployed.

    Now, the hinge pocket cover didn't come loose. All 4 screws were in tight and stayed tight. The top skin somehow flexed down into the path of the hinge pocket cover. Yes, it is hard to conceive how that can happen - I can only assume it was the high loading on the wing causing the skin to move. With the wing sitting with only self-weight (when the aircraft is sitting on the ground), there is perhaps a 1/4" or more clearance.



    This all took place when I "popped" the flap to take-off from a small airstrip in a strong gusty wind, the flap never retracted past the 3rd (and a half) notch due to the jam. Practically full flap. I can only imagine that the wing skin flexed at the same moment due to a strong gust of wind we experienced (we really leaped into the air when I popped flap).

    The jam did considerable cosmetic damage to the top skin of the wing. It was the pocket where the skins join which jammed, to the combined thickness of the two sheets of 0.025" Aluminium, plus stiffener section, resisted any more serious damage. I was amazed the four small 3/32" self-tapping screws holding the flap hinge pocket cover in place didn't give way, nor did the 0.025" cover itself. I am considerably impressed that this light structure could support the flap at the airspeed we achieved (about 75 KIAS) before we found the problem.

    Naturally I have repaired the damaged metal, removed the crack initiation sites, and smoothed all rough edges to complete a basic field repair. I may consider adding a doubler when I return to home-base. My fear was a crack starting in the under-lapped skin, and moving rivet to rivet unseen beneath the overlapped skin - so I have been very careful to sand and polish everything smooth with Scotchbright. We removed a number of rivets to enable access.

    I will certain be redesigning all the flap hinge pocket covers, so they can slide under the top wing skin in the event the two make contact.

    One very interesting fact:

    The Bearhawk can easily be controlled with 1 flap fully deployed and the other fully retracted, even as fast as 75 KIAS. The control inputs necessary to remain straight and level (or climbing at full power), are only marginally out of balance - just enough that you would certainly notice something is wrong. You most definitely are not fighting for aileron authority, as many would have you believe, all that is required is just a little rudder and a little opposite aileron to continue in balanced flight.
    Last edited by Battson; 08-27-2015, 04:42 AM.

  • #2
    Very interesting! Let's be in touch about including your story in the upcoming Beartracks.

    Comment


    • #3
      The clearance for those screw heads for the flap and aileron gap covers is what I normally look for on preflight inspection. I tried to use the smallest profile round head screws (#4) possible for attaching those little covers. I have bent the upper wing skin up a tiny bit before to give more clearance from those screw heads. But never had a jam like you describe Jonathan. Not ailerons or flaps. Mark

      Comment


      • #4
        Originally posted by Battson View Post
        I really have no idea how this managed to occur, but worth knowing about... If your hinge pocket covers don't go all the way to the front of the flap - then you have a problem waiting to happen.

        We had one of our hinge pocket covers, somehow, jam into the trailing edge of the wing top skin. Our pocket covers were short enough that their leading edge passes beyond the trailing edge of the top wing skin, when the flap is fully deployed.

        Now, the hinge pocket cover didn't come loose. All 4 screws were in tight and stayed tight. The top skin somehow flexed down into the path of the hinge pocket cover. Yes, it is hard to conceive how that can happen - I can only assume it was the high loading on the wing causing the skin to move. With the wing sitting with only self-weight (when the aircraft is sitting on the ground), there is perhaps a 1/4" or more clearance.

        This all took place when I "popped" the flap to take-off from a small airstrip in a strong gusty wind, the flap never retracted past the 3rd (and a half) notch due to the jam. Practically full flap. I can only imagine that the wing skin flexed at the same moment due to a strong gust of wind we experienced (we really leaped into the air when I popped flap).
        I'm thinking that it is more likely that the entire flap was pushed up hard by the wind/air forces, perhaps a little distortion of the hinge cover as well.

        Good to hear that it handled well and you're fine as well!

        Comment


        • #5
          Need more photos. I'm barely comprehending what I'm seeing in this one.

          Comment


          • Battson
            Battson commented
            Editing a comment
            Sorry Z - I only took one photo before we started the repair.

        • #6
          I forwarded Jonathan's posting to Bob Barrows. In the next few days he will write an engineering notice about how to avoid this happening. I will post it here. Mark

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          • #7
            Here is Bob's engineering notice on how to avoid the problem Jonathan had:

            ENGINEERING CHANGE NOTICE
            Subject: Flap and Aileron Hinge Pocket Cover Plates

            Recently a Bearhawk 4-place had a flap retraction problem on one flap because the pocket cover plate jammed into the top wing skin, and would not allow the flap to retract. The plane was still flyable, but this problem needs to be addressed.

            FLAPS: On the top of the flaps the cover plate should only be deep enough to cover the gap from the flap spar to the back of the wing skin, about 1 ¼ inches, with the flap up.
            AILERONS: On the aileron the cover needs to be much deeper, so that with full down aileron, the front of the cover is still under the wing skin.
            Robert Barrows
            9/1/15

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            • #8
              If anyone has pictures, please post when you can

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              • #9
                So has anybody implemented a similar cover for the Patrol?
                David Edgemon RV-9A N42DE flying RV-8 N48DE flying Patrol #232 N553DE in progress ! Plans built.

                Comment


                • dedgemon
                  dedgemon commented
                  Editing a comment
                  thanks Mark, I guess I was just wondering whether they were a part of the design. I dont see them indicated on the plans for the Patrol. Seems pretty simple though. I had thought that maybe a set of mylar seals like the sailplane guys use might work well for the flaps

                • jaredyates
                  jaredyates commented
                  Editing a comment
                  These covers were originally published in the first quarter 2006 Beartracks.

                • dedgemon
                  dedgemon commented
                  Editing a comment
                  ah! thanks

              • #10
                I have to be honest, I am a little confused by Bob's guidance about the flap covers... I think I get it:

                I think he means you only cover the part of the hinge pocket which is visible with flaps retracted, so when you deploy the flaps, the pocket should become visible. In other words, a gap should open up.

                I thought a longer cover would also suffice, as that would prevent the leading edge of the cover from extending past the trailing edge of the upper wing skin, so it would be impossible for the two edges to catch upon each other. Although I guess you would need a cut-out for the hinge assembly, which could still catch on the top skin... so maybe that's a bad idea!
                Last edited by Battson; 09-02-2015, 07:21 PM.

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                • #11
                  I have hurriedly done a crude drawing in my lunch-break in an attempt to illustrate how this happened...


                  image_1523.jpg
                  Last edited by Battson; 09-02-2015, 07:42 PM.

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                  • #12
                    It sounds like Bob is going with the longer cover as you described in the last part of your post Jonathan but for the aileron, & a short cover for the flap. So in other words, the aileron cover overlaps with the skin, but the flap cover does not. Your sketch, and perhaps a couple more pictures, help to illustrate this.
                    Mark
                    Scratch building Patrol #275
                    Hood River, OR

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                    • #13
                      I love the drawing! I've just sent an email version to the Bearhawk Safety Email List, with a link back here for more details. Join the email list at http://bearhawksafety.com/

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                      • #14
                        This was the original cover design from the Beartracks. The new change is to the top right cover, which should now be about 3 inches shorter.

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                        • #15
                          Are these covers required for safe flight of the aircraft? It seems that they would impair your ability to perform a thorough preflight amd be one more item to remove for the annual condition inspection.
                          Joe
                          Scratch-building 4-place #1231
                          Almost Wyoming region of Nebraska

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