Can't find fuselage structural diagram

[EDMO 1078]

Thanks Larry,

      That is just what I wanted to see.   I think most of us forget that the 7/8 struts were tested to 15 G's - So our 1" struts may be a little overkill - but it makes us feel better since Kitfox used 1" on the later models.

EDMO

Edited 3 Oct 2014 by Ed In Missouri

[Av8r3400 1179]

Dave's use of .065" wall tube was probably WAY overkill...

[Luked 71]

Thanks for the pictures Larry. That helps a ton. It will be a little while before I get to the struts, but I'll be coming back to this thread when I do.

 

Ed, I would have just used the stock Avid struts if I was going to go with the standard length Avid wings. The KF wings are longer though, and the struts attach farther out. This changes the angle, of the struts, so the forces should change. It also makes the struts longer, so they will buckle more easily in compression (negative Gs). That's my thinking anyway. If 1" x .049" tubing worked for the IV-1200, then I would think it would be adequate for my plane.

 

The lower attachment pictures are very interesting. I remember my engineering instructor saying that connections (vs. the members themselves) are usually where people run into problems.

[dholly 718]

Somewhere I saw a diagram showing the fuselage tubing that indicated where weaknesses were found. Seems like the uprights to the front spar carry through were one area that was marginal. It was a side view. Does anyone know where this diagram can be found? I think it might have been a hand sketch.

 

Thanks,

Luke D.

 

Luke-

 

I've posted these diagrams before, perhaps it is what you were thinking of? You will definitely need to enlarge for clarity and take a few minutes of close study to understand. As I was told, these fuselage modifications were stipulated for EU built/registered Avids back in the day.

 

Cheers.

 

[Luked 71]

Hi Doug,

 

Thanks for posting this. I think that might be the diagram I was thinking of. My memory of it is a little foggy. I thought I saw one that looked very similar to this (fairly rough hand drawing), but it seems like it showed several areas where failures occurred under testing. For some reason I'm thinking that the tubes that go up to the front spar carry through were a weak point. I don't want to start any rumors or mis-information though, so don't quote me on that.

 

I'll definitely be taking a look at this one though.

 

Thanks again,

Luke D.

[EDMO 1078]

Luke,

   The 1" x .049 struts should be more than enough for your wings.   When welding fittings, it is important not to weld directly across the tube - AC 43-13 shows all tube welds to be either on a 30 degree scarf angle, or a double-30 degree V .

EdMO

[Luked 71]

Thanks Ed, I'm confident in the strut material. I was thinking of the vertical tubes in front of the doors that go up to the carry through.

 

Just to clarify. I'm not trying to find fault with the Avid structure. I know it's proven. I'm just kind of interested to see this drawing.

Edited 8 Oct 2014 by Luked

[1avidflyer 1983]

Are you worrying about those tubes in compression or in a pull?   In other words, a nose over or an extreme wing load.  I don't believe there has ever been an inflight failure of Avids or Kitfoxs, but I do know that those tubes will buckle in a nose over.  Don't ask me how I know that.  Never got hurt though.  Jim Chuk 

[Luked 71]

Ha ha. I won't ask Jim. I'm not really worried, just curious. Remember though that I'm not talking about a stock Avid of Kitfox here. I'm talking about an Avid MK IV that was designed for 12' wing panels, but will instead have the longer KF wing panels. I  know of one Avid that has already done this successfully (Larry's friend).

 

Edit: Sorry Jim, I realized that I didn't really answer your question. Like I said, I'm not really worried about any area in particular. I just like to get all the information I can on a subject. That was just one area that I think I remember seeing on that diagram. I'd like to see what it had to say about other areas too.

Edited 9 Oct 2014 by Luked

[EDMO 1078]

Luke,

    "When in doubt:  Cut and Weld"!   Or, how about just skip-welding another tube to those tubes inside the cabin?  A small tube adds lots of strength, but also a little weight.

EDMO

[Bandit 286]

I will add one more thing to Lenies post. He is right they do float like rocks. Add power on your flare or you will be fixing your gear and fuselage no matter how big of tubes you welded in.

[Luked 71]

Okay, I found the mysterious diagram I thought I had seen somewhere. It's not quite what I thought it was, but this is definitely the one I remember. It was tagged onto the end of the "History of Pursang" PDF file. Not much explanation was given except that it appears to be a MKIV fuselage.

 

Jim appears to be right in thinking this was only an issue in a nose over since that's what the notes on the picture indicate. I took a screen grab of the image from the PDF file.

 

 

 

[Av8r3400 1179]

Luke, I've been cleared to tell you that Dave (He's with me at LEAF 912 School) dropped in that plane a couple of times resulting in landing gear damage (crappy Airdale "bush" gear) which would have shown the weakness in the wing support structure.  Absolutely no damage, except for the gear tubes.

 

You have absolutely nothing to worry about strength wise with the Kitfox wings on an Avid IV.

[1avidflyer 1983]

Another thing I would mention, is I think the lift struts carry most of the weight of the wings in flight.  If you get a chance to look at pictures of a wing/fuselage upside down loaded with sandbags, it's amazing how they bend in ways you may not think they would at first.  Jim Chuk

[Luked 71]

Thanks guys for the information and for putting up with all my questions. Larry, tell Dave thanks for the information about his plane. It has all been really helpful. I think I have enough confidence to proceed on the big stuff. I just started having some last minute second thoughts about the stuff I was doing, and kind of wanted to reassure myself that I'd be okay. 

 

I'm sure I could have attached the wings with no modification to the fuselage. Dave's plane has proven that. I'm already past the point of no return though. The upper part of the cabin area is removed. I just wanted my plane to be a little closer to the configuration KF uses for this wing (angle of attack wise). While I'm changing that, I decided to make a few of the structural changes at the same time. It will be interesting to compare notes with Dave when my airplane is flying. I'm sure he'll win on who has a shorter build time though! 

 

As of right now, this is my plan (along with my thinking):

 

- I'll use 1" x 035" spar carry throughs - Probably not needed,  but this is what KF changed to when they went to the model III

- I'll use 1/2" square tube for the uprights in  front of the door - Probably not needed, but this is what was used in the Later model KF I looked at, and I have to replace them anyway.

- 1" x .049" struts - This may be overkill because the model III only had 7/8", and was tested to something like 14Gs (1050 GW I think).

 

 

 

[Bandit 286]

If I landed hard I would rather bend the crappy Airdale bush gear over the airframe.

[Av8r3400 1179]

Very true, Mark.  However...

 

The landing in question was not what I would consider hard enough to bend any part of the airframe or landing gear.  Your plane will most likely thrive with the Airdale gear due to it's light weight and large soft tires.  Dave's plane is 670# empty with rather stiff 8.50-6 tires.  This has proven itself to be too heavy for the cabine style gear.  When the steel die springs bottom out from their limited travel, something will bend.  This is inevitable. 

 

He bent the gear legs once, rebuilt them heavier and shortly thereafter bent them again - also bending the lower fuselage longeron this time.  The fuselage was repaired and a Grove spring gear was installed and there have been no problems since.

Edited 13 Oct 2014 by Av8r3400

[Bandit 286]

Yes that would make a difference.

[EDMO 1078]

Luke,

     If you haven't bought your material, consider using 1" x .049 for the carry-thru tubes, the same as the struts..

EdMO

Edited 13 Oct 2014 by Ed In Missouri

[Luked 71]

Yeah, I already bought the 1" x .035" tubing a while back when I started thinking about these changes. I really hated to add the weight, but these are fairly short pieces, and it's not much different than the 7/8" x .035". Also, I figured if I'm putting in new pieces, I might as well use what KF used.