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substituting streamlined strut material with round tubing

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Kurt Fredrickson
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#1 Posted: 3/5/2010 11:00:00

I am building a Breezy and wanting to substitute the streamlined material with round tubing. At first glance it appears to be about a twenty pound weight penalty if using the round tubing with equivalent compression and tension numbers. I was just browsing the chart on Aircraft Spruce listing the strengths. The price difference for the equivalent round is over $1200 less. I would very much be willing to diet and loose 20 or 30 pounds for twelve hundred dollars (probably should anyway). Have had a few suggestions on the drag penalties and simply fairing the round tubing in some manner to compensate for the added drag. I am wondering if I am looking at this wrong or in  a naive far too simple manner. I see that some high wing designs do use round tubing so am thinking that it can be done with the Breezy as well. Any help or advice is greatly appreciated.  Kurt



Joanne Palmer
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#2 Posted: 3/5/2010 12:02:50

If you want to go with round tubing for the structure, you could then cover the round with a fiberglass fairing shaped like the stramlined tube.  Probably cheaper, and you won't have the drag penalty.



Kurt Fredrickson
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#3 Posted: 3/5/2010 20:27:19

Hi Joanne. Materials and methods for streamlining are readily available to the newbie, I am looking for a heading to find out the correct and SAFE replacement of the tubing itself. If you have any advice on where to aquire this information I would very much appreciate it. Thanks for your input.



Joanne Palmer
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#4 Posted: 3/5/2010 20:49:59

What is the original material specified?  Cross section?  Dimensions?



Cory Gussert
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#5 Posted: 3/5/2010 21:22:14

Kurt,

Joanne is probably on the right page.  In tension, which is how a strut on a high wing aircraft is loaded ordinarily, you're interested in the cross sectional area only... at least as far as strength is concerned.  In compression, or negative g-loading, the shape becomes critical.  With the dimensions the area moment of inertia can be determined, allowing you to select a similarly stiff round section.  Also, as you've already noted, there's a drag penalty that's pretty easily rectified by fairings.

 

Cory

 



Tom Hackel
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#6 Posted: 3/5/2010 21:23:20

I am one to be cost minded also.... $1200 of frustration does not take that long when trying to "make do" with something that can fall even gently from the sky.

You may want to check other metal suppliers, if you have the type (i.e. 6061, 2024 and the shape 2" x 1" oval x how many FT.)    you may be able to find a less costly source. Have you checked A/C salvage yards? Many are on-line. 



Kurt Fredrickson
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#7 Posted: 3/5/2010 23:51:42

Hi Thomas. Just wanted to comment that I have no intention of just "making do" nor am I interested in re using salvaged/used material. Aircraft Spruce and the crotchety old fart in Pennsylvania are the two cheapest in the nation. Damned rare for them to get beat on price and I've looked for a few years, been price shopping and comparing construction costs after looking over several different airplane designs over the last 20 years.  There are a couple of local shops in the Sacramento and Bay area that will fire sale some material once in a great while but it never is the streanlined, always some round stuff they have a abundance of or can't seem to move. There are alot of things that people have to modify or configure to adapt their wing of choice to the Breezy and I am CAUTIOUSLY proceeding down the avenue of avoinding using the streamlined/expensive tubing.



Kurt Fredrickson
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#8 Posted: 3/6/2010 00:01:17

Hi Joanne., the two sizes/dimensions of the streamline strut material used in the breezy are

2.023 in the major axis   .857 in the minor axis      .049 wall thickness

3.372 in the major axis   1.429 in the minor axis    .049 wall thickness

 

Aircraft Spruce states that the equivalent in round tubing for the first is  1-1/2"  tension   1" compression

                                                                                                               second    2-1/2" tension   1-1/2" compression

 

You aren't the first person to remark that the tension and compression numbers aren't as important cross section.  Thanks for your input. Sincerely Kurt



Joanne Palmer
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#9 Posted: 3/6/2010 17:29:13

In order to come even close to the streamlined tube for mechanical poperties you'd have to do something like this:

 

The streamline tube section is in "pink" and the builtup is in mustard yellow.  The builtup consists of two tubes 3/4 Dia and 5/8 dia with .058 wall.  The webs are .100 thick strips.  These would have to be welded full length or at least with 4 inch welds on 8 inch pitch with the last 8 inches welded at each end.  Not easy to do and really easy to get wrong.  This was sized to fit within the profile of the streamline tube but could be adjusted somewhat if you can live with fatter struts.

 

 



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Richard Warner
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#10 Posted: 3/11/2010 19:48:50

Do the plans call for 4130  Steel?  I know that most of the light planes like the Cub, Champ, Taylorcraft, etc., had Mild Steel streamline tubing.  I don't know if Univair still sells strut blanks.  They used to have a lot of various sizes of strut blanks.  They were all welded mild steel streamline tubing.  I'm sure this is the same material they used to make their PMA's lift struts.  My catalog is pretty old, so can't say for sure that they still have the blanks available, but it would surely be worthwhile checking on.  When I built my Kingfisher amphib back in the 80's, I used these blanks to make my struts.  I flew my airplane in smooth to very rough water plus flew it off a not too smooth grass field at gross weights up to 2,000 lbs.  I never had any kind of problem using the mild steel material. The designer recommended using J-3 struts and I know for an absolute fact that they were mild steel and smaller than the material I used.  A couple of years ago, streamline mild steel material was about 1/4 of the cost of 4130.  Again, if the designer specifically says to use 4130, thats what you should probably use.



Richard Warner
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#11 Posted: 3/11/2010 19:59:09

I havew tried to edit my post above, but for some reason the edit won't work.  What I meant to say was I FEEL sure this is the same material they used to make their PMA'd Struts.



Steven Clements
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#12 Posted: 3/12/2010 00:10:12

Round tubing has four times the drag of a streamlined tube.  Years ago our club had an experimental plane that roughly looked like a P-38 Lightning.  Didn't fly well.  They replaced the 447 with a 503 with limited results.  They fussed with the propeller brand and number of blades with limited results.  When they put slip-on farings on the struts and landing gear, the pilot landed with a big smile - THAT was the answer.

So, stay with round tubes with the slip-on farings.  I got mine from Quad City Ultralights in Moline IL.



Lincoln Ross
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#13 Posted: 3/12/2010 20:08:21

Actually, I think the streamlined fairing is better than that, though it depends on the exact shape and the Reynolds number. On a Breezy, although it's hardly worth it, a bit of thick tape that makes a small bump (.01") placed on either side might lower drag on thick struts. (And on round ones too.) I don't know the exact best location. Prevents separation. I wonder if you went around and placed such tape on ALL the round tubes on a Breezy if it would make much difference?

 

As far as the structures go, I have no idea why I did it, but I figured out the moments of inertia, and now I have a little spread sheet to do it with. I approximated the struts by hollow ellipses. In real life their moments of inertia are probably significantly less, I'd guess. Anyway:

big strut long way is 0.397 and short way is 0.104

If one could find some rectangular tubing 2.5 X 1.25 with .065 wall, then the numbers would be 0.385 and 0.129

Given that the first numbers are a conservative approximation, I think the latter size would be an excellent substitute, assuming you could find some.

 

For the little strut, 0.082 and 0.21

A substitute, if you could find it,  might be 1.5 X 0.75 tubing, .065 wall, which gives you 0.078 and 0.025

 

Unless I messed up my math!

 

If the jury struts constrain the tubing fore and aft as well as up and down, you might not need to worry about the higher number.