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Cant open Avid Manual series...Can Someone give strut attach measurements?

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Posted

I have never been able to open any of the Avid Build Manual Files - Just for my info, can anyone give me the dimensions from the wing root to the hole in the strut attach fitting on the spars?   I have model 1 and 2, and model 4 Kitfox dimensions, and there is about 10 inches difference between them - Just wondering how this compares to the different Avid models from A to MkIV.

EDMO

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Posted

I downloaded one of the mauals from the site a while back. Not sure which model it is for (doesn't seem to say), but I would guess this measurement is the same for all Avids except the Magnum.

 

There is a diagram that shows 80" from the end of the tube to the center of the bolt hole. Hope that's what you were looking for.

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Posted

Thanks Luke,

     The Kitfox 1 and 2 show 86 5/8 same locations, and the Kitfox 4 shows 96 3/4 - The Speedster 97 3/4.

If I was an aero-engineer, I might know whether it makes any difference.

EdMO

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Posted (edited)

Ed,

 

My manual shows 80" as well on the HH Wing and 80" on the Aerobatic Wing too. This is on an Avid.

Edited by Wild Bill

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Posted

Thanks Wild Bill and Luke,  I don't know who or why the Kitfox struts are farther out on the wing than Avid's 80 inches - Guess it is because the wing is longer - But then, why was it changed from 86 5/8 to 96 3/4 on the Kitfox 4 when the wing stayed the same?

Wish we had Dean Wilson here to talk to about it.

EdMO

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Posted

Hey Ed,

 

I'm not an expert, and I don't know what the thinking was in the design decisions. Something may have lead them to re-think their earlier assumptions. Here is my understanding of a strut braced wing in simple terms off the top of my head.

 

The part of the wing past the upper strut attachment is the easiest to understand because it's just cantilevered. All things being equal, the longer that section becomes, the more bending stress it will experience near the strut attach (the point of highest bending for that section). So, if you make the wing longer but leave the strut attached at the same distance from the root, the spar gets higher bending stress at that point.

 

The part of the wing that goes from the wing root to the strut attach point is a little more complicated. It's like a simply supported beam except that it has that cantilever section extending out to one side, and the strut is applying a compression force to it (when G load is positive). To get into the stresses on this part of the wing, you would want to do a shear and bending diagram (and free body diagram showing the strut forces), but basically, all things being equal, you can say the following. If the length of this section is increased, bending stress at the middle will be increased, and resistance to compression (buckling) will decrease.

 

The last thing to think about is the strut angle. As you move the strut upper attach point farther from the wing root, the angle of the strut becomes flatter. This part is hard to explain because you can't make as many simple blanket statements about it. A flatter strut angle means that a larger component of the force in the strut is compressing the wing spar, but as you move the strut out on the wing, it decreases the vertical force component needed to support the plane's weight because more of that vertical load is shifted to the wing root attach point. Also, the longer the strut, the less resistance it has to compression loads (negative Gs). It's kind of a balancing act.

 

Add to this that airplanes are loaded in many different ways as they fly through the air and make different maneuvers. The ailerons (flaperons for us) add additional up or down force at times for example.

 

Anyway, I've rambled enough. I hope I don't come off sounding like too much of a know it all jerk. This stuff is probably not that interesting to a lot of people. I like to talk about it though, and learn all I can. I probably misstated one of two things, so please let me know if I did. Also, keep in mind that the stuff above is very general concepts. Engineers consider many more factors in greater detail when actually designing an airplane.

 

Luke D.

 

 

 

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Posted (edited)

Luke,

     I understand what you are saying - I have studied this somewhat - an engineer with all the figures could probably come up with some positive figures on it.  I have talked to one of the design engineers who did some work for Dan Denny on the early Kitfoxes, but we didn't get into this in our discussions.  So far, I have never heard of a gull-wing Avid or Kitfox!  :lol:

EdMO

Edited by Ed In Missouri

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