The polars listed above are 2D estimates assuming the wing is infinitely long with no spanwise flow, and give a Cl Max from 1.6 to 1.8 at a Reynolds number of 3x10^6. Wind tunnel data from NACA report 824 indicates the Cl Max for the 4415 to be somewhat lower than that, about 1.3 for similar Re. It has never been clear to me whether I should treat the numbers from the NACA report as 2-D or 3-D. That the data is from an actual airfoil and the values are considerably lower than computer model estimates suggest to me that it is 3D. However, the report does say the data is from the Langley 2-D tunnel.
To be useful, we need the lift coefficient to be corrected for 3D effects. For that, it is again easiest to consult Mr. Wilford's spreadsheets. S405-WindDesign.xls specifically. This uses the Schrenk method to approximate the lift coefficient distribution over the wing, and then the wing coefficient of lift. Using that spreadsheet and the lower computer model 2D Cl from XFoil of 1.6, I arrived at a corrected Cl in the clean condition of 1.38.
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( 2.8 / 63 )Thus far, the wings on my RW-11 have several notable changes from Roger's plans to support the higher gross weight that I am designing the airplane to handle. The first order of business is to determine what the member stresses will be based on the performance envelope and loading.
To do that we need to know what that performance envelope looks like. The easiest way to go about that is to use Neal Wilford's S306-Airloads-FabricWings.xls spreadsheet. The calculations are not in any way difficult, but Neal has gone through all the trouble of sorting through the various load cases in compliance with FAR 23 Appendix A and CAM 04. For the sake of education, we'll work through the calcs here.
Let's outline what operating conditions my RW-11 will be designed for:
Gross Weight (W): 1200 lbs
Maximum Operating Speed: 100 knots
Positive Load Factor (n1): 3.8 (Normal)
Negative Load Factor: -1.9 (Normal)
We also have some values based on the geometry of the wing:
Wing Area (S): 135 S.F.
Flaps up CL Max: 1.38
This gives us the information required to find points A, C, B, E, F, and G on the flight envelope diagram. With the gross weight, load factor, wing area, and CL Max we can calculate the minimum design maneuvering speed (A,G):
or 
...and the minimum design dive speed (D,E):
...based on the Simplified Design Load Criteria in Part 23, Appendix A. The minimum design cruise speed:
...is greater or equal to what I expect my RW-11 to actually be capable of, so n3 and n4 are 1.0 and -0.5, hence points C and F have 3.8 and -1.9 as a load factor, respectively. These are the same as the max positive and negative load limits specified above. Using the equations and data above, my V speeds look like this:
Minimum Design Flap Speed: 64 knots
Minimum Design Maneuvering Speed (A,G): 85 knots
Minimum Design Cruising Speed (C,F): 99 knots
Minimum Design Dive Speed (D,E): 139 knots
I've also created a PDF of my calculations.
Up next, we will see how those V speeds translate into flight loads.
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( 3 / 37 )The old method of logging progress on a web page was getting very tedious. It got to the point where I didn't even bother updating the site with my progress.
A little over a year ago I installed some blogging software (Simple PHP Blog) on my site, and am just now moving all the previous information into this new format. I'll be adding entries for the progress made over the last year as well.
I didn't do the greatest job taking pictures all the time, so over the last couple of days, I tried to catch up on that as well. Most of the images are posted with the related entries. However, here are a couple more pictures showing the current status.
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( 3.1 / 40 )I scarfed together 4 strips of 1/16" plywood to form the gusset strip covering the top of the rear spar. This gusset attaches the center and rear section of ribs to the spar. It isn't a structural necessity to scarf this part, it just makes a nicer joint.
I had a little extra epoxy, so I also whipped up a couple of gussets that hold the wing tip bow on.
2 hr
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( 2.9 / 29 )I made a jig to help shape the top of the wing tip bow to match the ribs. It is simply a straight 1"x6" that is long enough to bridge several ribs. The end is fashioned to attach a Dremel tool. I have a 1/4" collet in the Dremel with a 3/4" straight cut router bit. The bit is set flush with the bottom of the 1x6. By laying the jig perpendicularly across the inner ribs, I can follow the rib profile with the cutter.
I set the bit high at first and profiled the tip bow in multiple passes. A little follow-up with the sanding block, and the tip bow looks better than I had hoped. I profiled the bow from the front spar back to the trailing edge. I only had one spot where I wasn't careful enough with the profiling jig, and the router cut a little deep on the inside edge of the bow. It's a small nick that will eventually be beneath a gusset, so no worries.
I also made the final glue joint where the bow attaches to the front of the front spar. The bow had a fair amount of spring in it to force it into the proper shape, so I added a small filler block to reinforce the joint. Eventually the leading edge skin will act as a gusset at this joint, but it will probably take some abuse from sanding and shaping before the L.E. skin is attached.
2 hr
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( 2.7 / 36 )When I first laminated the wing tip bow, I was disappointed that it didn't look like it would fit correctly. I thought I had layed it out in CAD correctly, but when I put the bow on the wing, it looked like it most of it would be trimmed away in the profiling. I was going to fix this by adding additional laminations to the inside of the bow so that there was sufficient material remaining for the desired stiffness after shaping.
A second look at the situation determined that the additional laminations would be unneeded. I trimmed the ends of the spars to match the 3/4" thickness of the bow. With minimal forcing, I could get the bow to fit the way I wanted, except possibly for the leading edge area. I trimmed the bow and glued it in place. The resulting cross section of the bow should be roughly trapezoidal in shape - 3/4" wide with the height tapering from 3/4" high on the inside to 3/8 of an inch at the outside.
1 hr
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( 2.9 / 20 )I trimmed the root end of the spars and trailing edge. Then I glued on the root rib.
In hindsight, this was a stupid way to make the root rib. I should have simply left off the ply and glued in the root rib with all the other ribs. Doing it the way I did requires more fiddling around getting things to fit properly. Same for the nose rib. The ply could have been attached afterwards. Since I already made the port side root rib, I don't even get the benefit of learning from this mistake.
1 hr
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( 3 / 15 )I cut and glued in the remaining top diagonals for the ailerons. The aileron end rib was also glued to the trailing edge.
1 hr
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( 3 / 20 )I cut and glued in about half of the top diagonal braces for the ailerons. I also made an aileron end rib that will form the outboard end of the aileron. The full rib at that location will remain intact as part of the wing tip, so a separate rib for the aileron was required.
2 hr
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( 3 / 20 )With the wing flat on the table and the trailing edge along the fence, I glued the trailing edge to the aileron ribs after trimming the ribs to length.
1 hr
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