Spar Inserts

[TJay 1041]

 I'm not sure I trust the stock spar insert that comes with the kitfox model 1, It is a Z bar shape, So I decided to talk to kitfox today about buying there I Beam insert they said it comes in 60" length and there $150 EACH! Ouch what do you all think,

[EDMO 1078]

For that price, I would think about using or modifying the one you have, or making my own - several different ways to do that.  How thick is the one you have?  The 1700gw Avid Magnum uses a .090 thick aluminum insert, and only .270 wide on the bottom edge where it is glued/riveted to the spar, but you should be fine with at least .060.   Are your spars .065 wall?

I would use aluminum, and not plywood like some early Avids - Later Avids (Airdale / Magnum) used aluminum.

EdMO

Edited 18 Aug 2015 by Ed In Missouri

[TJay 1041]

.035 aluminum Z shape, I had a thought here about buying 2/1/2 6061 t6 I Beam material with a top and bottom flange of about 1/4 inch then chuck it up in the lath and turn it down to fit the shape of the inside of the spar.

[EDMO 1078]

I had the same idea, with a light I-beam - not everyone has a lathe, but there are usually shops that will do it for you.  Or, you could rivet another vertical piece to the Z-bar, and have the same vertical strength as a beam - the flanges only serve to hold it in place - your spars become the top and bottom of the beam.

I have no idea of the engineering figures of the spar with the insert, but it works.

 

EdMO

Edited 19 Aug 2015 by Ed In Missouri

[Luked 71]

I had sticker shock when I priced the KF stiffeners too, but being a small business, I understand it. They probably have these made in small runs (compared to what the aluminum extrusion companies usually run). Their setup cost for each run probably account for a big part of the the cost on each part.

 

When it comes time to do my wings, I plan to buy the KF inserts. If I were doing Avid length wings, I'd just use the plywood inserts like many Avids have done. Since I am doing the longer KF wings, I'm going to go with what is proven. I don't really want to screw around when it comes to my wing spars.

[EDMO 1078]

I wonder what the web thickness is on the Kitfox inserts.  I have the original Kitfox spars with the full-length, about .060 extruded web - I think they may be stronger/stiffer than the inserted ones.  Again, I have no engineering numbers to prove that.

EdMo

Edited 19 Aug 2015 by Ed In Missouri

[Av8r3400 1179]

Guys.  Remember you are building an airplane.  

 

 

Are the spar inserts somewhere you want to go the cheapest route?

[EDMO 1078]

I agree Larry - Plywood may have worked - but I wouldn't go that route if I can get aluminum - $600 is cheap insurance compared to a failed wing - but I think you can make them a lot cheaper than that!

 

TJay,  I hope you haven't attached your strut fittings before putting the spar inserts in.....

EdMO

Edited 19 Aug 2015 by Ed In Missouri

[TJay 1041]

Oh Of course there installed Ed and the stock inserts are already riveted in Ha, when I get started on this thing again It is a little drilling first.

[1avidflyer 1983]

TJay, as I remember, you have thicker walled spars.  Also, if your wings are shorter, they would be stronger than if they were longer.  Not sure what length you ended up going with.  I guess what I'm getting at is your wings maybe are pretty strong already.   Avid was able to demonstrate  5.7 Gs of load on their stol wing with the .065 wall spars with no damage to the wings.  Their gross weight was 911 lbs.  X 5.7 equals 5193.  If one used a gross weight of 1150, and divided that into 5193, you are at 4.51 Gs.  That's pretty strong.  I would worry about drilling out your stiffeners and maybe scratching the inside of the spars in trying to slide them out.  That could cause cracks to start from the scratches.  Just some things to consider..... Jim Chuk

Edited 20 Aug 2015 by Jim Chuk

[EDMO 1078]

Jim,

      Does TJay have the .083 wall spars?   If so, they are plenty strong even with thin inserts.

TJay,

      I have to agree with Jim - You could do more harm than good by drilling out rivets, but that's your choice to make.  Kitfox inserts wont fit .083 wall spars anyway, I don't think....My book says you may have to squeeze the sides of their .065 spars to slide the inserts in, since they fit so tight.

EdMO

Edited 20 Aug 2015 by Ed In Missouri

[TJay 1041]

I do have the .083 spars with the kitfox I beam inserts I would just turn them down a few thousands, shouldn't hurt them, Hey Jim I am going to stick with the 12' wings and am keeping the under camber rib profile, should perform well. I would love to see 70mph on the ASI I'm not holding my breath on that one though ha.

[EDMO 1078]

TJay,

     I'm not trying to tell you how to build your bird - You got to go with whatever you are satisfied with - I like your idea of turning a light I-beam instead of paying such a high price for Kitfox inserts, and having to turn them anyway - But again, that is a choice to make.

     On one Kitfox I helped build, the inserts fit loosely, and we took gobs of 9460 on a long pvc pipe with cloth on the end of it, and swabbed the glue on the inside of the spar at about the location of the insert, and slid the insert in, using a slit in a pvc pipe to keep the insert vertical.

Good Building,

EdMO

Edited 20 Aug 2015 by Ed In Missouri

[1avidflyer 1983]

TJay, I don't think you will have any trouble seeing 70 MPH. Probably will do that at 5000 RPM or just a bit more. Jim Chuk

[EDMO 1078]

Speed is just a relative term - I have flown at an indicated 100 mph, and the cars below me on a two-lane highway with a 55 mph speed limit were going faster than me.

EdMO

Edited 20 Aug 2015 by Ed In Missouri

[TJay 1041]

how Many kitfox 1 airplanes With the Z spar insert you think are flying today, I would guess 10 maybe. Thats my thinking on the newer stiffeners also I will be flying at 900 to 1000 lbs all the time i'm sure. Then again I do have the heavy spars and short wings. That all helps too. I talked to the guy with all the tooling today he says it would be really hard to turn down a 5 foot ibeam and keep it from bending in the lath.

[EDMO 1078]

TJay,

     I don't know where Jim got the .083 spars and insert, but the original KF1 had the extruded integral web .065 spars and no inserts.

I don't know when they quit making that type of spars, but my KF2 also had them - I don't know about the KF3, but when the KF4 came out, it had inserts to be put into the spars.

EdMO

Edited 21 Aug 2015 by Ed In Missouri

[Bandit 286]

I wouldn't make that big of a deal out of the inserts. I have the wood ones and they have worked great. Where do you stop trying to make things stronger on a already proven design?

[IFMT 36]

Just aircraft was able to supply the insert for 0.83 wall spar. 

[1avidflyer 1983]

TJay, I would think that if a person was determined to use an aluminum I beam for the spar stiffener, they could cut the top and bottom of the I beam on a table saw to more or less round it off. Carbide tipped blade will cut aluminum quite well, although you want to wear a good face shield as chips will be flying around. Might want to make several passes at slightly different angels to get the rounded effect. On the other hand, what you have now is most likely just fine. Jim Chuk

[EDMO 1078]

Here is the page from the Avid Magnum 1700gw wing insert that the builder makes for the .083 wall spars. 

Not as impressive as the fancy Kitfox inserts, but guess it works OK.  I don't know if Airdale was the same.

Note that it is shown downside-up, and is only .090 thick at the top with no flange.

If I was designing it, I would have used a narrow I-beam, Z-bar, Lightening holes in a 3/8" thick bar, or angles riveted to top and bottom of the vertical, and probably would have made it longer - but I am not an engineer.

EdMO

Edited 21 Aug 2015 by Ed In Missouri

[TJay 1041]

Ok guys I bit the bullet and ordered the I beam stiffeners today, only not from Kitfox. I Bought them from Just Aircraft, and here is why, kitfox price quote $150 each total $600 bucks Just Aircraft price quote $28 each total $112 bucks, Now I'm all about a business making a profit to keep the lights on but when I have to pay the whole years electric bill, now that just pisses me off. I am for once really disappointed with Kitfox.  That's one point for Just Aircraft.

[EDMO 1078]

TJay,

     the Kitfox inserts might not fit your spars anyway - have you measured your spar wall thickness/ID?

Hoping you can post some photos and dimensions of the Just inserts when you get them.

     I wonder if the Just inserts are the same as what you have?

Although I usually feel that Kitfox parts are overpriced, they may have paid more wholesale than Just for the inserts and have to charge more?   But, That's a huge difference in prices!

EdMO

Edited 26 Aug 2015 by Ed In Missouri

[TJay 1041]

The outside diameter of the Just inserts are 2.365 since I have .083 spar wall thickness I will probably have to do some vixen filing and sanding to make a nice fit but for $112 bucks to feel 100 times safer I'll spend some time doing what I have to.

[Luked 71]

Ok guys I bit the bullet and ordered the I beam stiffeners today, only not from Kitfox. I Bought them from Just Aircraft, and here is why, kitfox price quote $150 each total $600 bucks Just Aircraft price quote $28 each total $112 bucks, Now I'm all about a business making a profit to keep the lights on but when I have to pay the whole years electric bill, now that just pisses me off. I am for once really disappointed with Kitfox.  That's one point for Just Aircraft.

 

That's very interesting. Are they the same I beam as the KF inserts? I'd be interested to see some pictures when you get them. If they're the same, I'll be buying mine from Just.