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Spar blank dimensions and bending

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  • Spar blank dimensions and bending


    I'm preparing some aluminum to be used for my aileron spar webs (starting small, so the screwups, er, learning, costs less), and am running the numbers to cut the needed material for the blanks. I have found a disagreement with what appears to be standard practice for bending vs. what is included on page seven of "The Bearhawk Book" as well as what appears in Eric Newton's wing build manual.

    I am using the Bingelis Sportplane Construction Techniques book to calculate bend allowance, setback, etc. I've included a picture of my calculations for the aileron spar web with the dimensions in the plans (my aileron spars will need to be a bit taller to account for my as-built rib heights, and the fact that I'm not planning to joggle). Basically, the Bingelis method says that the aileron spar webs (per the Model B plan dimensions) need to be made from a blank that is 5.230" tall, whereas the Bearhawk Book method indicates that the aileron spar web blank should be 5.433" tall.

    I went to a third source - - and used the "J-chart" method. That gives me a spar blank height of 5.175", which is admittedly a little error-prone since I'm interpolating from the chart.

    Am I missing something? Does anyone know why "The Bearhawk Book" uses its particular approach? I suspect it is for simplicity, and errs on the side of slightly more flange material. (The same is true of the practices for bending the rib flanges - my rib flanges are all slightly larger than called for, but when I calculate the additional weight penalty, we're talking something like a pound or two for all of the ribs, so I'll probably leave them as-is.)
    Attached Files
    4-Place Model 'B' Serial 1529B (with many years to go...)

  • #2
    I commend you on your note taking. Your example is usually what I strive for, but by the end of the page the organization loses all hope.

    That 1/16" term in the Bob/Newton method threw me off too. I think if anything it should be subtracted not added, because the metal takes a shortcut around the bend vs the theoretical 0 radius corner.

    Anyway, I went the practice strip method. I made reference lines at known locations on the practice strips, then I adjusted the total blank width and bend locations as required until I got what I wanted, then took those numbers to the real thing.

    Scratch building Patrol #275
    Hood River, OR


    • #3
      I did the same as Chewie.

      I bent multiple test strips on my small leaf break at home, then used the dimensions determined to get the blanks sheared at a local HVAC shop.

      Then I again bent up some test pieces on the big brake(because its setback and radius was a bit different than my small home brake) to tweak the final measurements before "Bending for the money".

      Separately, I made a small Excel sheet to calculate based on standard formulas used it a few times for getting "in the ballpark" for other bending that needed to be done.


      • #4
        Thanks all. I definitely intend to try a couple of test strips first no matter what. I just get a little leery when I see nearly a 1/4" difference in assumptions. I'm just trying to make sure I'm not crazy by balking at _adding_ length to the blank - physics says otherwise. I have a cheapo 18" HF brake I've used for the rib stiffeners, and will try it for a few 6" test pieces. When I do get the main ones to the brake, I'll try a few more test pieces first before trying on the full-size blanks.

        And Chewie, I recopied my notes so they'd make sense for this forum on a screenshot... my original notes are far, far more messy and scattered. "I aim for the stars, but sometimes I hit London."
        4-Place Model 'B' Serial 1529B (with many years to go...)


        • #5
          Story time!

          When I was a green apprentice, I had to do my 6 months in our hangar's sheet metal section (loved it, I wish I was a specialist sheety).
          They were shutting down our contract at the time, and one of the old-timer sheeties came over to me and told me he'd been offered a job in a technical college, teaching sheetmetal work.

          "That's good news Al - you're a great mentor, it's the perfect job."
          "Yeah, but I'm not sure... there's a lot to learn..."

          It turns out, after 40 years repairing aircraft, he didn't know how to calculate a setback for a sheetmetal bend. None of our sheeties could, and none of them felt confident teaching it at the college, even though it's a simple calculation. They all knew I'd just done my trade training, and I knew how to put it on paper, and that's why Al asked me to show him.

          Afterwards I asked Al - "So all those parts you make up for the Chinook - how do you know where to bend it?"

          They used test strips every time, because the brakes are so old and worn, that the calculations never work.

          I've forgotten myself how to calculate the bends, so I'll have to ask the next apprentice :-)

          The Barrows Bearhawk: Who knew my wife could get jealous of a plane?


          • Bcone1381
            Bcone1381 commented
            Editing a comment
            Sounds like Educational Theory vs Real World Practicality!!!

            My first go at it was the same. Do the math, go test it, then make an adjustment. I learned that things like the for-aft placement of the nose bar as it clamps the sheet to the stationary bed in relation to the bending leaf on the Leaf Brake is never accounted for in the math, effects the bend radius.  I bet there is a standardized call out for this.  I'm a novice...  A press brake may have its advantages.
            Last edited by Bcone1381; 11-17-2020, 07:34 AM.

          • James
            James commented
            Editing a comment
            Yeah Brooks - the way the nose bar slides forward was allowed for with Al's method by putting a square up against the bar, and marking that reference line on the test piece. When you make up the good piece, you might have to clamp it six times to find that point again.

            Also, you need to mark exactly where along the width of the folder you did the test bend, because we can't set up the folder square ever since that guy on night shift used it to bend up some steel for his boat trailer and bent the casting, etc, etc...

        • #6
          I liked this video for the math and method.

          Make brake shoes for the bend.
          Spars are bent to match already made ribs. Do not pre cut center ribs to length in an attempt to account for capstrips. Any notches can be made after the rib is selected for the best fit location along the spar. When you bend the spar, no matter how much you measure, there will be differences along the length of the spar.

          The center ribs also will have a range of sizes. This might be a random effect; possibly the center ribs were cut at different angles relative to the grain of the sheet of 2024. They will have some variation at the flange bend. Sort your ribs into a range of tighter and looser fits as compared to your test bend. Mark the ribs with the grade of fit as compared to spar test bend. After ribs are graded for their fit they will be matched up along the length of the spar for the best "average" fit locaton along the spar. If necessary Desert bearhawk has a youtube video on making a rib joggle tool.

          Make a nose radius shoes in various sizes out of steel strapping material.

          Cover shoes with a roll of galvanized roof flashing.

          Tape shoes and flashing to brake foot. and bend using an inclinometer.
          Attached Files
          Last edited by sjt; 11-17-2020, 04:02 PM.
          Austin Tx


          • #7
            I've been working on my wing drawing in CAD...the three resources that have been the best help that I've used are:

            1. MMPDS - Metallic Materials Properties Development and Standardization Manual. This is the structural engineering repository document that is used by the US military and civilian stress engineers alike.

            2. The Aviation Maintenance Technician Handbook (various copies out there). This is a "shop guide" designed for mechanics in the field as a primer. This handbook is a treasure trove of all kinds of reference data from sheet metal to weight and balance and touches on everything in between. Specifically for sheet metal, all the formulas etc. are "streamlined" that are in the MMPDS so it is a ready reference to be used in a shop without have a stress nerd standing in the shop.

            3. AC 43.13 - When in doubt, most reference the FAA Advisory Circular for direction.

            All That being said...the MMPDS drives the Tech Handbook...and the sheet metal stuff in the AC43.13 but depending on what you're doing or trying to research is what resource you would use. MMPDS can be found online, the Tech Handbooks can be purchased or if you have a buddy in aircraft maintenance these can be acquired easily. The AC43.13 is online on the FAAs website.

            A simple diagram in the Handbook shows the basic way to calculate a flat layout to a bent layout...and vice versa. Laying out sheet metal has to take into account the thickness of the material. The thickness determines where an imaginary line in the center of the sheet metal is...this line is called the "neutral line". If you know where that line is...and you are making a specific bend, that neutral line will have a specific length. By adding the flats and the neutral line length, you get the total width of a flat sheet part.

            Making coupons and bending them to your flat pattern layout takes the tolerance slop out of the specific brake you are using...but you can accurately do a flat layout of the part using the above references.

            Attached Files


            • #8
              Originally posted by sjt View Post
              Tape shoes and flashing to brake foot. and bend using an inclinometer.
              An inclinometer would be nice....

              But, The brake I use at my EAA chapter workshop has a counter weight attached to the leaf that extends toward the back of the brake. As the leaf is raised the counter weight rotates from 12 o'clock aft towards nine o'clock. I was taught to tape a of scrap rod to the counterweight to act as a pointer, and place a yard stick against the wall. As the leaf rotates, the pointer on its counterweight arcs around and approaches the hard stick. I can index the angle that way and as I try my test pieces, I can get repeatable accurate angle to my bends.

              Its common for scratch builders to make the ribs first, then fabricate the spars. If I were to do it again, I would fabricate the spars first. Making precise placed bends with a few thousandths is tough, and It seems like it would be easier to adjust my rib form block slightly to match the spar than to do it the way I and others have done it.....match the spar to the ribs.

              Brooks Cone
              Southeast Michigan
              Patrol #303, Kit build


              • #9
                Interesting idea! In retrospect, I agree with Brooks. I think it might be easier to match the as yet unbuilt ribs to the as-built spars.
                Bearhawk Bravo #1508B - Scratch Build (wings)
                Northern Idaho


                • #10
                  Originally posted by Bcone1381 View Post

                  Its common for scratch builders to make the ribs first, then fabricate the spars. If I were to do it again, I would fabricate the spars first. Making precise placed bends with a few thousandths is tough, and It seems like it would be easier to adjust my rib form block slightly to match the spar than to do it the way I and others have done it.....match the spar to the ribs.
                  Hello everyone! I’m just about to start my project and this suggestion really intrigues me. What would be the possible pitfalls doing it this way that I’m not thinking about?


                  • #11
                    It could be 50/50.

                    If you are trying to be economical, then you will end up cutting the rib blanks at various angles. No one would waist the material. The end result from cutting the ribs from a sheet at different angles relative to the 2024 grain of the sheet will produce ribs that are slightly different in their bending characteristics.

                    No mater which method is used the ribs will vary at the flange, and the spar c channel will also vary along its length. That is why I am a proponent of not cutting the notches for the capstrips early. I wait and find out what ribs would best fit along the spar at each location.

                    For me it is just a matter of delaying making any alterations to the ribs until I have selected that rib for that location on that spar.

                    If you have a machinist background you will have to forget measuring things to a few thousandths. We make these airplanes with plastic mallets.
                    The only concession I made was dedicating a single tape measure to be my airplane tape measure.

                    b.jpg cc.jpg

                    I hide my tape measure from my boys. My wife is not a problem. She is not allowed in the garage.
                    Attached Files
                    Last edited by sjt; 11-19-2020, 02:42 PM.
                    Austin Tx