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Thread: Load Considerations for Doors

  1. #1

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    Load Considerations for Doors

    What are the critical loads for doors in an unpressurized airplane?

    When the airplane has no side slip, the pressure difference between the two sides of a door is pretty small, especially for our slow experimental LSA. With side slip it will increase, but how can we estimate this pressure difference?

    Another critical load is the "bang" in abusive closing of a door. (The 172 I flew needed a big bang to lock, I hated it and did not understand why some designers would design like that.) Again I am pretty clueless in putting a numerical value to this load.

    Any other loads should be considered?

  2. #2

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    How are you at trigonometry? Will try to simplify the calculation.

    So you are looking for the psi load on a door surface when you say skid 10 degrees in level flight at a certain airspeed at sea level on a standard day?

    You first need to figure out the air pressure on the profile of the door that is presented to the airstream by the slip or skid. So start by looking up how to calculate what is called "dynamic Q" for a sea level standard day air density. If you fly in Death Valley or Nome Alaska, you can get more dense air, but start with the easy calculation. That should get you a value in pounds per sq inch (psi), or your favorite metric equivalent.

    Now we look at the height and length of the door, hinged at its front edge 10 degrees into the wind. Think right triangle. The door is the hypotenuse of the triangle. The line from the aft edge of the door forward that is parallel to the relative wind is a base leg of the triangle, and the short leg from the forward door hinge shows you the width of the flat plate that wind sees as the door. The height of that effective flat plate is the height of the door.

    So now we have a representative flat plate size and the air pressure on that flat plate. We multiply area time psi to get the load on that representative flat plate.

    Now we need to translate back to the load on our real door. Well essentially the load we calculated for our representative flat plate is spread across our entire real door. You can divide the value of the area of the real door into the total load calculated for the representative flat plate to get a psi value.

    In reality, most light airplane (Cessna/Piper/Mooney) doors are not designed with the air loads as the important consideration. The limit loads are stuff like people slamming the doors, using the doors to pull themselves into the seat, etc. Think about human abuse when you do your design. And many airplanes are structurally fine for flying with the doors removed. Cessna's for example can fly without doors so that skydivers can do their thing. No structure mod required, just a permission slip from the FSDO.

    Hope this is helpful,

    Wes
    N78PS

  3. #3

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    Quote Originally Posted by wantobe View Post
    Another critical load is the "bang" in abusive closing of a door. (The 172 I flew needed a big bang to lock, I hated it and did not understand why some designers would design like that.)
    I think that indicates more of a maintenance problem than a design problem.

  4. #4

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    Assuming that one doesn't put "suicide" doors that open towards the front of the aircraft, the positive pressure of a slip would tend to keep it shut rather than force it open....one is still moving forward, after all.

    The opposite side won't have very much negative pressure (if any at all), so if it is secure by any reasonable standard should remain shut.
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  5. #5
    Matt Gonitzke's Avatar
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    Quote Originally Posted by martymayes View Post
    I think that indicates more of a maintenance problem than a design problem.
    I have yet to fly a Cessna that didn't have this problem.

  6. #6
    steveinindy's Avatar
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    Quote Originally Posted by Matt Gonitzke View Post
    I have yet to fly a Cessna that didn't have this problem.
    Same here. It's another reason why I prefer Pipers and Beechs.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



  7. #7

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    Quote Originally Posted by Matt Gonitzke View Post
    I have yet to fly a Cessna that didn't have this problem.
    I have flown quite a few Cessna's that didn't have this problem.

  8. #8

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    You mean that Cessna's are supposed to have doors? They get in the way of my going skydiving! And they are also in the way when on floats and trying to dock. They do help when sailing a float plane though.

    Cessna door latches have evolved through the years from pretty minimal to much more substantial. We can only guess that the more junior engineers were assigned to work on them. After all, who aspires to go home each day and say "I'm the engineer in charge of the door latch!".

    That said, Piper door latches work well but some other parts of the airplane are really really cheezy so I will suggest that it is seriously questionable for a Piper driver to point fingers....

    Best,

    Wes
    N78PS

  9. #9
    steveinindy's Avatar
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    That said, Piper door latches work well but some other parts of the airplane are really really cheezy so I will suggest that it is seriously questionable for a Piper driver to point fingers....
    No doubt. There's a number of reasons why I want to build my own aircraft and a lot of them have to do with trying to do away with the shortcomings of Pipers and Cessna especially when it comes to comfort and related topics.
    Unfortunately in science what you believe is irrelevant.

    "I'm an old-fashioned Southern Gentleman. Which means I can be a cast-iron son-of-a-***** when I want to be."- Robert A. Heinlein.



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