Airfoil suggestion for a very slow flyer? - RC Groups
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Nov 18, 2003, 12:26 AM
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Derek_TX's Avatar

Airfoil suggestion for a very slow flyer?

I bought a GWS 300C with H gearing a while back and planned to put it on a very large, light, no-wind airplane.
Rough specs:
no-taper slow-stick style, 1/2" thick foam ribs spaced pretty far apart, CF rod spars (3 to give some torsional rigidity too), covered with stretched pallet wrap. CF fuselage. 15x7 prop, 700 mAh NiMH 8 cell or a 2s 1200 lipo. AUW should be under 16 oz, area obout 1300 sq in, W/L under 1.5 oz.sf. span 72", chord 18"

I dont have the stall speed of the motor, but my intuition says it should be pretty slow going, and otta work on this glossamer idea. I was looking at some airfoils and I was leaning toward something along the lines of the OT undercambered types. One that looked sorta close was the "FX60126" included with compufoil.

I am open to airfoil suggestions, and any other ideas. Already know I am a nut, so no help needed there!
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Nov 18, 2003, 05:52 AM
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The FX60-126 is a very good choice. You might also consider Dr. Drela's AG2- series of airfoils. See:

Of course if there is much covering sag between ribs it doesn't much matter what airfoil you choose because the chosen airfoil only exists at the rib itself. You might be better off using a thin curved plate of foam with imbedded carbon spars. A camber of 3 or 4 % should work well.
Nov 18, 2003, 12:10 PM
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Derek_TX's Avatar
Thanks Ollie. I will run some turbulator spars if sagging is a major issue. Will try to post pics when it is done.
Nov 18, 2003, 02:13 PM
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(Oops, wrong thread, sorry)


F-16 Falcon NACA 64A204 SAME
F-18 Hornet NACA 65A005 mod NACA 65A003.5 mod
F-22 Raptor NACA 64A?05.92 NACA 64A?04.29
Last edited by rcjetpilot; Nov 18, 2003 at 03:01 PM.
Nov 18, 2003, 04:11 PM
B for Bruce
BMatthews's Avatar
Actually for extremely low speeds it's been shown many times that you can't do much better than the basic simple single surfaced arc similar to what indoor models use. This same style shows up in that little Mickey Mouse ear shaped model that uses bent carbon rods for the outline braced with some a single post and kevlar thread bracing to bow the wings up for both the dihedral angle and the airfoil shape all at one go. The mouse ears being the simple bent carbon rod V tail surfaces.

Your stated choice of material, while imaginative, strikes me as being heavy by indoor RC standards let alone indoor free flight standards. You may want to do a bit more research. In particular the indoor RC end of things. Even though you will be using this model outdoors the no wind spec means that you are basically doing an indoor type model.

Good hunting.
Nov 23, 2003, 11:17 PM
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John Boren's Avatar
Way back when I was part of a team that won the SAE payload carrying contest we used the Wortman FX63137. It was designed for Human powered aircraft. It is a low drag high lift airfoil. When flying the cargo plane without the payload I was able to thermal the plane. Really a great airfoil.

John Boren
Nov 24, 2003, 10:54 AM
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Derek_TX's Avatar
DBM: A single sheet would be fine, except the weight will not be attainable. With bracing, this is exactly an indoor building method, except stretched to 72" span. It will be interesting to see how 'heavy' this turns out. 12-16 oz with 1300 sq inches doest seem heavy, works to a reasonable W/L... also with no indoor venue, some strength is needed. Remember, this is a plane being made to fit an H geared GWS S300C. I want something happy to fly at 10 mph or so ... intuitive guessing on the speed, as I dont know the stall speed of the H geared motor turning a 15x7 prop. Additionally, the only material I am working with foam-wise is half inch thick 4'x8' sheets. No FFF yet.

JB: I will check this one out, lots of undercamber seems apropriate.
Last edited by Derek_TX; Nov 24, 2003 at 11:11 AM.
Nov 24, 2003, 11:40 AM
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Bill Glover's Avatar
10 mph is jogging/running speed and not actually that slow ... I have a 28" aerobatic model of around 9 oz that stalls at not much more (11 mph according to MotoCalc)!

IMO the kind of wing loading you're talking about (1.7 oz/ft sq) will be almost unflyable outdoors. That's about the same as my Stubenfliege, which is thrown around (indoors) by air moving when doors open etc.!

If you do go for it, bear in mind that safely transporting and storing a plane that big and light won't be easy. You are talking about 9+ square feet of wing!

I have a 40" foam-winged f/f model for CO2, it's plenty strong enough for flight loads (white foam hot-wired into Gottingen curved-plate section - undercambered), but I've had the wings break a number of times while carrying it in anything other than flat calm conditions. One indoor venue I used had automatic double doors in the entrance lobby - the draft of air when they opened nearly always broke the wings as I carried it in! Yet the model could hit a wall and fall 15' to the ground without damage.
Nov 27, 2003, 06:27 AM
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HELModels's Avatar
18 inches is probably a good choice for average chord length since you are looking for super low wing loading. For camber, maybe 3-4 %, as Ollie recommended.
10 mph is jogging/running speed and not actually that slow
... but the wing loading is off the charts.

lots of undercamber seems apropriate
My reason for less camber would be the very light wing loading, if you are looking for efficient flight(less profile drag). A higher wing loading at that speed seems to warrant more camber like 6-8%.
Nov 27, 2003, 11:16 AM
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Derek_TX's Avatar
Thanks for the input gentlemen!

I will get pics posted when this come into existance. What may happen is a weight increase in favor of strength, but let's see how it goes
Nov 27, 2003, 07:18 PM
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Bill Glover's Avatar
Originally posted by BMatthews
Actually for extremely low speeds it's been shown many times that you can't do much better than the basic simple single surfaced arc similar to what indoor models use.
This shot of my Stubenfliege wing looking from tip to root shows the section nicely (the reflection of the chord-wise Kevlar cords in the Mylar upper surface). The carbon tube at the bottom of the pic is the centre section of the wing.
Nov 27, 2003, 07:19 PM
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Bill Glover's Avatar
And the whole plane, to put it in context.

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