Forging ahead...spar tubes and fittings

[Tree top pilot 13]

I went to a new metals place in NC today. Felt like crap and didn't really want to ....but there is an airplane to build. Picked out the best 24' spar tube they had { the one with the least amount of abuse from shipping}. It will clean up nice with a touch of sanding here and there. They had 6 pieces 24 feet long....2.5 " X  .065"  T-6. Also many sizes of German made 4130 tubing. These guys treated me like I was someone. I was impressed. Very clean site and nice folks. Check them out online....Race City Steel . They even have airfoil shaped tube in some sizes I didn't know was available...shame on you Aircraft Spruce. Fair priced as well. They told me they didn't speak chinese and seen no reason to have materials from there. I did check at another place- Stock car steel and there's was .080" thick but had a lot of scratching and gotchas on them.

 

On another topic...the spar attach fittings I have made were from .040" 4130 sheet. Is that the thickness most have here? I spent a lot of time in making the fixtures to bend them correctly. Just want to make sure I don't need to make another set from .050". I will calculate the cross sectional areas and see what kind of loads each are capable of this weekend.

 Anyhow the questionable spars will be replaced. No worries then. Monday the doctor will see me again and I will return useless and in pain for a while. Need to make this weekend count. Finished most everything but the wings.

So what are your spar attach fittings thickness?

[EDMO 1078]

TTP,  Did you measure the old Spar Attach Fittings?

       My Kitfox 1 manual gives .040 for the Jury Strut attach angles - My new extended ones will be .060.

No thickness given for the Spar attach Fittings, or the Strut Fittings, but I can measure mine tomorrow if you don't get a better answer.   My strut fittings also have a thin washer bonded to the attach hole in each side.  Manual don't say that, but that was how my first Kitfox was built, so I copied it - Been too long for me to remember thickness.

What are you going to use for Spar Inserts?

EdMO 

Edited 4 Jul 2014 by Ed In Missouri

[Tree top pilot 13]

The old ones were made from .040". Just wanted to see what else was out there. There is a lot of stresses where these make the bend coming off the tube. As for the inserts.....a suggestion was made here to make them from 1/4" plywood. So I drilled and "V" shaped the ends and sealed with epoxy clear-coat. That should do the job of keeping the tube in shape and from collapsing under all those + G forces. I mean if .040" will safely handle a 1350 MTOW on an Avid I would like to know that it has been proven to be safe. There was a pair of washers welded on the bolt hole side to spread the load on the bolt. It was to be finished reamed to .250" before bonding to the spar.

Thanks for looking into this.....

Edited 4 Jul 2014 by Tree top pilot

[EDMO 1078]

TTP,

      I believe that there is more stress on the spar strut fittings than on the spar attach fittings - I think I made the curved portion of my strut fittings out of .060 - still going to shop today to double-check sizes just for curiosity.

     Later,

I checked as best as I could, and all of my spar fittings measure about .070 to .075 with the paint on them,  Sooo, guess I used .060 for everything - Not saying that is what Kitfox used, but probably what I had at the time.

EDMO

Edited 4 Jul 2014 by Ed In Missouri

[Tree top pilot 13]

Just to make sure we're both on the same page......made from .040 molly sheet. Rusty ones were the poorly made parts as delivered. No pride in the workmanship......not to mention no concern for safety. I powder coat most all small parts that will fit in the shop's oven. Girly pic is my nose art I am working on / editing as time gets closer to finish.

Edited 4 Jul 2014 by Tree top pilot

[dholly 718]

I would help if I could but everything is riveted on and/or under fabric. Good to see you up and working on your kit.

[EDMO 1078]

Since I am not a mechanical engineer, I cant even guess at the difference in strength/stiffness between .040 and .050 - Guess if it don't bend or break it will work either size.   Looks Good.  I added 6 of the 8 additional rivets that the Magnum specifies for each attachment fitting, but Leni said it was not needed because there has never been a failure reported there.

Sand-blasted and painted, the old ones should be OK to use.  I would pay the postage if you are going to scrap them.  I have a Kitfox 1 clone that could use them.

EDMO

Edited 4 Jul 2014 by Ed In Missouri

[Tree top pilot 13]

Still would like to know what thickness have been used out there....my reasoning below....partially

 

OK for the math.....4130 N steel has a yield of near 63100 psi. The cross sectional area of the narrow portion that bears most of the load is .040" thick x 4" long = .16 sq. in. per side of the fitting. Both sides combined = .32 sq. in.  So then take sectional area  x  area of the part......63100psi  x .32 = 20192 psi. Divide by a safety factor of 2 =10096psi. That is a lot of force to just get this to start to tear open. That is for pure tension only...no bending moments. The math gets a little scary for the real stresses that are present in these little pieces so I won't take you on that trip. There is a small component of compression...but those loads are transferred directly to the spar tube and the wing moves up. Not enough to be an issue....but the force is there at times. You have to look at everything a part can encounter alone and in the place it will work with other parts as a whole. Time and math is the only way to be certain provided the materials have no flaws to skew your calculation. That is why I like to use a safety factor of 2 instead of 1.5. Errors on the strong side only adds a tiny bit of weight.....but a ton of confidence. So for my little bird that would mean about =7 G's before the fitting would start to fail with correct FEA load testing. The spar would probably give up the ghost first. But this is not taking into consideration the two sides are tied together as a unit of three pieces....that's when the game starts getting more complicated. The stainless rivets are good for over 400 psi in shear....

 As for the original fittings....my attorney said to keep them. The reason being is that I am going to make a full disclosure of my experience. We both have photo documentation and the original parts as delivered on the following day to cover any issue of a slander suit in any form by the manufacturer. I just want the truth to be known. It may keep someone from being hurt. I would be willing to loan out my fixtures if you want to make a set.

 

Your mileage may vary...I am on medication.

Have a happy and safe 4th....and thank you to all who have served to keep freedom of all we have to enjoy. I lost a good friend in Iraq.

Edited 5 Jul 2014 by Tree top pilot

[EDMO 1078]

TTP,  Aren't you concerned that the powder coating will interfere with the 9460 bonding them to the spar?

The builder's manual does not say to paint them before bonding and riveting.

If I had to worry with lawyers, I wouldn't even build a plane - just buy a spam can or someone's built bird.

Thanks for the engineering data.   I already have mine on the spars, and spare parts to start another set.

I lost fellow soldiers in Germany, and no telling how many of my outfit died in Nam.

My family has served since the Colonial Wars and Revolutionary Wars.

BTW: Most folks think that the Revolutionary war in 1776 only lasted until we defeated the British the first time - Truth is, that they fought against our freedom for nearly 40 years before finally giving it up as a lost cause in the war of 1812, which stopped with their defeat at the Battle of New Orleans in 1814.  

Wish we could have kept Canada too! 

 But, Canada and Australia are two of our best Allies.

God bless those who served and will serve to keep this country free.

Happy 4th!

EDMO

Edited 6 Jul 2014 by Ed In Missouri