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Old 03-03-2012, 01:47 PM   #1
CKeller
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Now that my upper wing is rebuilt I'm tackling rigging the airplane properly.

I've corrected incidence and symmetry problems with the one-piece upper and both lower wings. Can you guess what my next hurdle is? Yup, fabricating new I-Struts.

We are working with a fairly old plan set and “builder’s notes” so there may have been some new light shed on this issue over the past years.



The plans call for +1.5 degrees of incidence (all wings), top wing flat/level with built in sweep, lower wings square to the fuselage with 2.5 degrees dihedral (as you can see...OLD plans). We have a few questions that will have to be answered before we start any work to the I-Struts.



The plan set requires that the I-Strut be built in between the space formed by the rigged wings. We currently have the wings supported by an external structure. This will be not much of a problem except for a couple of things that don’t make sense.



The I-Strut fabrication drawing basically says that the curved square tubes that fit to the front and rear spar attach bolts are bolted directly to the top spars (front and rear), and bolted directly only to the lower rear spar with a spacer fixturing block 1 5/16” thick between the front lower wing spar (top side) and the foot on the front of the curved square tube. Tack it into position, remove it and weld it into one solid, non-adjustable I-Strut. Of course there are a few support diagonals welded into the structure as well, but the basic strut shape and dimension is fixed when the streamline tubing is welded to the square tubing. We got that. No problem. Here are the questions……..



• When the 1 5/16” fixturing block is no longer in play and the I-Strut is built, that will now leave a gap between the front lower foot of the I-Strut and the spar attach point. That gap will certainly close and the foot can be bolted to the spar, but it will also raise the lower wing dihedral to over 3 degrees and the fixed “wash in” (+ incidence) will be dramatic. Is this correct?



We realize there has to be some “wash in” but this seems excessive, only increasing drag and provides no means of decreasing the “wash”. The A&P I am working with has experience with quite a number of biplane aircraft types. He says a well-rigged Bucker, for instance, requires only a couple of degrees. Travelaires, Stearmans and Wacos have similar amounts, and Staggerwings not nearly so much. Normally the lower wings should have the same incidence throughout the span as much as possible, only moving the incidence as little as necessary to correct any objectionable roll.



• Since the 3 ¾” streamline tubing is no longer available through ACS or Wicks, is it possible that there is another standard dimension tubing (3 3/8”?) that has been substituted for use in the I-Struts?



• Versus the aluminum leading edge skin - has anyone compensated for the thickness of a wooden leading edge (about 1/8”) when using that 1 5/16” fixture block inasmuch as the front lower I-Strut foot lands directly on top of that leading edge skin?



I hope all of this makes sense as I am trying to pose the question for my coworker here. The bottom line is we'll be definitely making new I-Struts, and we're obtaining a more modern set of plans to do the latest design (which I think might be lighter?). The plane was seriously out of rig, which was why it flew so slowly. Incidence on the top wing was 2.8 deg, and one tip was an inch closer to a spot on the rudder than the other. New attach brackets have solved that problem. On the bottom, one wing was 1.9 and the other was 0.8, and one was not square to the fuselage. We've relocated the holes on the lower wing attach brackets and will be welding on doubler plates for those new holes.

Oh, and obviously, we'll be refabricating aileron slave struts.

The lesson here: build it straight and true the first time.

Any clarification on wash in the wing due to the I-strut design would be most appreciated. Thanks~

Edited by: CKeller



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Old 03-03-2012, 02:09 PM   #2
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Hi Colleen,
Glad to see you making progress. More knowing than I will weigh in. No wash out/in required on the 'Bolt except as a final rigging during test flight thing. That type of thing is taken care of by the swept upper wing and the different airfoils.




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Old 03-03-2012, 02:16 PM   #3
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I built my own, but it was a couple of years ago. Your reference to a 1 5/16 fixturing block doesn't ring a bell, for me. I believe that 1 5/16 number is only referencing the highest point of the square tube apex to ensure the sq tube doesn't dip below the rib line. I heat formed my sq tubes on rib shaped templates. Both sq tubes are then bolted through the spar holes, streamline tubing tacked in place, along with the bracing tubes. I added 1/8 shims to the lowers to compensate for weld shrinkage.
The Skybolt has no wash, in or out.
The older plans call out a very old streamline tube spec that is no longer available, the newer plans call out what is available, it is slightly different. They do end up slightly lighter.

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Old 03-03-2012, 02:53 PM   #4
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Generally speaking, symmetrical (aerobatic) airplanes do not have wash in/out. Not being a Sky Bolt builder, I won't speak of particulars on the Sky Bolt.

A common rigging error that occurs in swept wing airplanes is that the level of the top wing and dihedral of the bottom wing is set with the fuselage leveled on the top longeron. What will happen is the swept wing will end up having a small amount of dihedral because the trailing end (tip) of the top wing is raised by the sine of 1.5 deg x the amount of sweep. The wings should be leveled on the cord line of the airfoil when rigging. In the case of an airplane with 1.5 deg incidence the tail will be raised until the fuselage is 1.5deg nose down.
This is why when you look head on at a swept wing tail wheel airplane as it is parked, they appear to have anhedral.

Perhaps the best approach is to fixture the wings into their proper position and build the struts to accommodate that position.



Edited by: Neil
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Old 03-03-2012, 11:17 PM   #5
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The I struts are very easy to make. You can read up on it in the Skybolt news to get some pointers. I believe the newer plans call for one degree dihedral on the lower wings. Put one 970 washer between each attach point between the square tubing and the spar. Just fit the tubes to your rigged wings and tack the tubes in place on the wings. You will have to protect the spars some how to keep from burning them. Remove the struts from the aircraft and finish weld. That is it. Your done.

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Old 03-03-2012, 11:18 PM   #6
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Neil, can we use a fan laser across the top of the upper wing to level it, then follow your instructions?

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Old 03-03-2012, 11:25 PM   #7
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Ok we think we're all set. Thanks guys; well post some photos later.Edited by: CKeller

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Old 03-04-2012, 12:30 AM   #8
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Quote:
Originally Posted by Vic Zubot
The I struts are very easy to make. You can read up on it in the Skybolt news to get some pointers. I believe the newer plans call for one degree dihedral on the lower wings. Put one 970 washer between each attach point between the square tubing and the spar. Just fit the tubes to your rigged wings and tack the tubes in place on the wings. You will have to protect the spars some how to keep from burning them. Remove the struts from the aircraft and finish weld. That is it. Your done.
Sounds easy! ;0), Vic is right though..the steps are easy,but cutting everything to fit requires some patience and tenacity. I still have no idea regarding the 1 5/16 thingy u referenced, but discount it. Here are a few final pointers:
-If you have access to the equipment, TIG or MIG weld the tacks...much easier (burning wood potential) and less steel movement than gas.
-When finish welding, TIG results in less distortion to the struts, whoever welds, tell them to weld one side of a joint then the other side in sequence..this helps to bring things back after the initial welding on the other side...
Here's my I-strut welding experience:
http://65degrees.net/index.asp?idPage=301
couple of pics:http://65degrees.net/pics/Dscfboxsection0333.jpg
http://65degrees.net/pics/Dscf0finalstrut368.jpg

Colleen, you ended up building a biplane afterall, I see :0)


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Old 03-04-2012, 03:01 AM   #9
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Not at all familiar with the laser level you mention. I'm pretty low tech. I've always just used a smart level or a long carpenters level or a really long length of vinyl tubing with water to find where I need things to be. I'm sure that properly applied, the laser will work.

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Old 03-04-2012, 03:08 AM   #10
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I tried a fan laser level and had issues; it was mounted on a tripod, there was a mini bubble level on the deal to level the laser on the tripod, trouble is, you are then trying to level something over a 6 inch span or so, then using that for the laser beam....meaning, if your bubble is off by a few thou' over 6 inches...then how much off would it be over 24 feet? I ended up using a water level....

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Old 03-04-2012, 03:21 AM   #11
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A simple, longpiece of clear plastic aquarium tubing is perfect for making sure one side is the same as the other. It worked great when I built my I struts for the Pitts. The digital levels work great on the rigging boards but are not that great for setting the top wing level. Keep it simple.
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Old 03-04-2012, 11:59 AM   #12
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ckeller...I've been using a laser level a lot in my construction/rigging. It's a very useful tool. Here are two examples... canopy installation and planing trailing edge parallel with leading edge.Edited by: chef

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Old 03-04-2012, 12:59 PM   #13
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Those pics are straight line scenarios, perfect for a laser. My issue was, getting that perfectly straight very long laser line level.

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Old 03-04-2012, 01:09 PM   #14
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This long piece of aquarium tubing everyone is talking about... any pictures of it in use ? I'm pretty sure I think I know what everyone is talking about...

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Old 03-04-2012, 01:45 PM   #15
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RE leveling laser line....That's the beauty of it. the laser is in fluid so it's always level. as long as the fuselage is leveled correctly, level is level. When I rigged my wings,I was able adjust the wires so I had half the line along the top of the wing(laser pointed towards tail from the nose) and half on the wall. Then checked with digital protractor and got 0.0. Lower wings with Steen struts within .1 degree of each other and with proper dihedral. Also good for putting up shelves

Edited by: chef

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Old 03-04-2012, 01:54 PM   #16
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Just a long piece of clear tubing. Ours at the shop is about 30 ft. Add water to all but about 3 feet and let all the air find it's way out and hold the ends up. The water at one end will be at exactly the same level as the water at the other end.

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Old 03-04-2012, 02:03 PM   #17
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In Colleens' case, the fuselage will still need to be jigged 1.5deg. nose down for the level, laser or other wise, to tell you the top wing is flat.

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Old 03-04-2012, 02:06 PM   #18
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Low tech upper wing level -- a string level hung on a tightly stretched string taped to the aft end of the wing tip bows. Worked pretty well, especially if you turned the level around and averaged the readings.

Higher tech solution from the olden days -- a transit reading on scales taped to the leading edge at each tip. I'm not sure how much better the results were but you felt like you went the extra mile.

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Old 03-04-2012, 02:12 PM   #19
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All neat solutions. Edited by: chef

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Old 03-24-2012, 04:50 AM   #20
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We've acquired the 4130 steel, and last weekend we bent the 1-in square tubing, trimmed it off, and drilled the mounting holes. We're ready to cut and tack weld streamlined tubing this weekend.
There are two methods for marrying the streamlined tubing to the square tubing. One approach cuts a channel in the streamlined tubing so it fits over the square tubing. The other approach requires the builder to narrow the end of the streamline tubing so it matches the 1-in tubing, and sits on top of it for welding.
What are the pro's and con's of each approach? We're open to suggestions here.
Thanks,C

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Old 03-24-2012, 12:08 PM   #21
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I would rather flatten the end of the tube to match the square tube. I think you get a cleaner look and a more straightforward joint to create and weld.

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Old 03-24-2012, 02:44 PM   #22
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i agree with bill, just painted my struts and transition looks good. i just squeezed the streamline with 2 plates in my vice.
Dennis in Chicago



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Old 03-26-2012, 02:43 AM   #23
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Thanks, guys...the "squeezed in a vise" worked just fine for us and made sense. Tack welding tomorrow, hopefully. One step closer...

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Old 03-30-2012, 07:47 PM   #24
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A quick clarification. The I-struts are tacked with the square tubing directly on the spars, unless you add something thin to protect from heat. The I-struts will shrink slightly during finish welding (approximately 1/8" top to bottom with TIG welding, more with oxy-acetylene). The AN970 washers mentioned by Vic are added during final assembly (one between I-strut and spar at each bolt, 8 total). This keeps the wings right where they were when you started tacking. I'm doing mine now. I'd like to see some pictures of your progress.
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Old 03-30-2012, 10:41 PM   #25
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Ed, I understand. We're welding this Sunday. I'll take lots of photos and post a couple for you.



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