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 “washâ€. 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 1/8â€) 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
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 “washâ€. 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 1/8â€) 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
