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dana62448

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Jul 29, 2008
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I'm working on a part 103 design and have a question about a double taper planform,
I'm using a high wing with a carbon D cell and drag spar supported by two points at the root
and a single strut attached at the lower longeron and 30-40% of the halfspan.
I would like to use a 20% Airfoil at the strut attach point, tapering to 12% at the tip and 16% at the root.
The goal is to optimize the spar depth and build a very light wing.
What are the airflow implications for this Stinson Reliant type planform,
particularly at the root/cabane attach point.
I am shooting for a 25mph stall.
I'm planning to use Junkers flaps for lateral control a la Backyard Flier,
which also accomplished this low stall speed with a very thick tapered wing.
 

WonderousMountain

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I fully approve of this.
Outboard taper is a very effective means good performance.
With your stall requirement, fully flapped is going to help.
 

dana62448

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Lozenge wing, big flaperons... now you have me picturing a little UL Lysander...
Yes! The wings have no internal openings or controls. Torque rods to Junkers Flaps. Wings unbolt with three fittings and hang from the cabanes with two rope loops and rest on the elevator.
 

pictsidhe

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If the inboard thickness variation is to keep the spar a constant height in that region, then that's what I'd do.

I'm another fan of thick foils for 103s and other slow planes. I'm going to use a ~20% inboard rectangular panel on my project, tapering to about 15% at the tips of the tapered outer panels.
 

Lendo

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I been advised that reducing Airfoil thickness as it transitions to the Tip, causes the Normal tendency of Root stalling first, to Tip stalling first.
George
 

cluttonfred

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Yes, typically there is a trade off between structural efficiency and aerodynamic handling characteristics when you taper a wing.

Take two similar airfoils (overall shape, degree of camber) and the thinner one will usually stall at a lower angle of attack. Here's NACA 23012 (12% thickness in yellow ) compared with NACA 23015 (15% thickness in green), you can see that the thicker airfoil keeps flying for a couple of degrees more after the thinner one starts to stall. Yes, you can correct that with twist at the tip, but then you're basically unloading that part of the wing and losing overall efficiency.

For a Part 103 design, if you really want tapered wings then I would suggest tapering the thickness a little more slowly than the chord so the relative thickness of the airfoil actually increases as you move out to the tip. In the words, a 12% thickness airfoil at the root that tapers 50% in chord at the tip but only 33% in thickness would end up with a 16% thickness airfoil at the tip.

23012 vs 23015.png
 

dana62448

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Thanks! I am suspecting that's what the Carbon Dragon designers did, when you look at the progression, root to tip, of the leading edge. Speaking of that airfoil, I have been curious about what appears to be a more sharp leading edge than many other popular choices. and, it seems to start low. By that I mean there seems to be early camber. What are the effects of these characteristics?
 

pictsidhe

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Yes, typically there is a trade off between structural efficiency and aerodynamic handling characteristics when you taper a wing.

Take two similar airfoils (overall shape, degree of camber) and the thinner one will usually stall at a lower angle of attack. Here's NACA 23012 (12% thickness in yellow ) compared with NACA 23015 (15% thickness in green), you can see that the thicker airfoil keeps flying for a couple of degrees more after the thinner one starts to stall. Yes, you can correct that with twist at the tip, but then you're basically unloading that part of the wing and losing overall efficiency.

For a Part 103 design, if you really want tapered wings then I would suggest tapering the thickness a little more slowly than the chord so the relative thickness of the airfoil actually increases as you move out to the tip. In the words, a 12% thickness airfoil at the root that tapers 50% in chord at the tip but only 33% in thickness would end up with a 16% thickness airfoil at the tip.

View attachment 99955
Depends on the airfoil family. Each family has an optimum thickness. This also varies a bit with Re and mach.
 

pictsidhe

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To get the stall progression I want, I'm using airfoils with more -ve Cm at the tip. I can get more lift with -ve Cm foils. The root foil suffers a small lift penalty with its more +ve Cm. This made more sense to me than crippling a good root airfoil so the tip stalls later.
 

cluttonfred

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I do have to ask since this is a Part 103 design...why bother with a complex wing? Unless the goal is soaring with a self-launching motorglider, I don't see that a complex wing is really necessary and it may even be counterproductive if the design is too clean and comes out too fast for Part 103. Personally, I'd make it a constant-chord wing for simplicity and docile handling and then taper the spars caps internally as needed for light weight while retaining a constant external profile.
 

dana62448

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I anticipated this question would show up early.
The simple answer is aesthetics I am a heavy pilot and I want to float and alight softly.
So my idea was to take whatever weight savings I could create with carbon and convert them to wing area.
The double taper at the spar is going to make a 13 inch deep spar
and I couldn't imagine a Hersey bar wing that would be elegant.
I have my weight, the two struts, the Verner 3V and some big tires on some Wilga-like gear to slow me down.
I'm already stuck with no two ribs alike, so why not make a little Reliant, Lysander, American Moth?
Am I missing a beautiful tapered Hersey bar wing somewhere?
 

cluttonfred

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To paraphrase Forrest Gump, "beautiful is as beautiful does." Given the speed limitations of Part 103, I actually can't imagine how it would be any easier or more effective to build a complex wing rather an a simple Hershey bar wing optimized for low weight. I am also surprised at the choice of a Verner 3V for a Part 103 ultralight design for a hefty pilot. You are sacrificing quite of bit of empty weight compared to using a light paramotor two-stroke.
 
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dana62448

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I'm surprised that the Verner is, 4 lbs lighter than a Casler and swings a huge prop.
The prop disc is the key.
I refuse to fly a two stroke and was always concerned that the 1/2 VW at 40 hp was minimal for the Mature Pilot. Watching Les Homan fly the Legal Eagle XL with the Verner has inspired me to try this carbon wing.
 

Aesquire

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I don't see the point in going with a rectolinear wing.
The two main advantages are fairly benign stall, ( roots first ) and ease of construction. A lot of pt103 craft are "ultralight" tubular ladder frame wing design. Light big panels, and the material limitations mean it's far easier to use a rectangle or a straight taper.

The latter part doesn't apply to this project, though, and I think you'd save weight with composite tapered outer panels. A compound tapered wing as described is pretty close to an elliptical for lift distribution, ( per Wainfan's latest Kitplanes article ) and if you can handle the compound curves with the materials, it's a win.

and, this is only partly tongue in cheek, if you are too fast, just add wires. A streamlined kingpost and single fore & aft & landing wires should do as a start, and you can then make the kingpost a simple tube if you need more drag. If you find you are both too fast, and too heavy, then consider lower wires as well instead of struts. Pure, light parasitic drag. :)
 

Lendo

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Dana Sharp leading edge (LE) for higher speed aircraft like jets.
Early Camber is a Turbulent Airfoil.
George
 
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