Solved A few questions about under camber airfoils

Vimana89

Legendary member
So for my next build, I want to do a custom/scratch plane in a basic monoplane/tractor configuration, with a floaty, big chord/ somewhat lower aspect style wing, and I want to use the simple type of under camber air foil method you'd find on FT planes like my DR. I'll be using 3ch V tails do everything on this one. I'm using it as an opportunity to work with different types of air foils besides flat plate/no airfoil and simplified KFM styles, starting with the easiest types to construct. I also want this plane to be nice and slow and floaty.

So as somebody with no real experience employing this type of air foil on custom builds, I have a few questions I'll be looking into, but in the mean time, figured somebody here on the forums would have the answers. So what I'm wondering about is:

1.) How much camber? what's the minimum/maximum degree of camber one would want, and what might be ideal amount for a plane like the one I want to build?

2.) How far back to crack the wing? With KFM1, the top layer is 40% of the wing's total chord. How far back along the wing's chord should I crack it?

3.)How should the wing be angled when mounted on the fuselage? I suppose I could look at my DR1 for a rough approximation, but this may differ with the degree of camber, so I figured I'd ask.

Thanks in advance(y)
 

synjin

Elite member
Download the plans for the Mini Scout. The angle of attack and the airfoil brake are pretty obvious on that model. I modified one to use a KFM2 wing and used the same AOA and cord length.
 

Vimana89

Legendary member
Download the plans for the Mini Scout. The angle of attack and the airfoil brake are pretty obvious on that model. I modified one to use a KFM2 wing and used the same AOA and cord length.
Good idea, I think that would be a good starting point. Thanks for the suggestion.
 

BATTLEAXE

Legendary member
Ideally i think the wing should be cracked on the CG, so 25-33% of the total chord. Like it was mentioned the Mini Scout would be a great reference. And the TE should be lower then the LE for AOA on a slow floaty unit. And Vtail is almost your signature now huh
 

Tench745

Master member
I think the Mini-Scout and the FT Speedster (not sportster) are good examples of a simple under-cambered wing and are a good starting point for your own experiments. Undercambered wings go quickly from lifting to not when you near 0° Angle Of Attack. Both the Scout and Speedster have a few degrees of AOA on the wing for this reason. 4° on the Speedster, not sure on the Scout.
 

Vimana89

Legendary member
Ideally i think the wing should be cracked on the CG, so 25-33% of the total chord. Like it was mentioned the Mini Scout would be a great reference. And the TE should be lower then the LE for AOA on a slow floaty unit. And Vtail is almost your signature now huh
I wouldn't necessarily say it's my signature...yet:cool:. I've tried it a couple times, and the one thing I made that was super successful with it was the four wing plane. It's certainly my favorite method for doing a 3ch plane, because I think it gives the best control response and flight envelope short of having a full 4ch. Plus...you know. It looks cool. @Crawford Bros. Aeroplanes 4ch V-tail version of my Arrowhead is also a real winner. Thanks for the good info.
 
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Vimana89

Legendary member
Thanks for the great responses everyone. I've downloaded the Mini Scout plans and it it's showing me pretty much everything I need to know. Tomorrow should be a good day to get building(y)
 

Vimana89

Legendary member
If you would like a more aerofoil shaped wing just let me know as it is a little more involved and gives a better look!

Have fun!
Thanks! For this one, it's a small light plane on an A pack motor, and I'm most likely going to go with the style as on the Mini Scout for lightness and ease of construction...however, I'd also be very interested to learn the way you are talking about, so yeah, please do share! If I don't use it for this one, I'll certainly end up using it for something else, and having options and learning multiple ways to do something is always a big plus.
 

Hai-Lee

Old and Bold RC PILOT
Thanks! For this one, it's a small light plane on an A pack motor, and I'm most likely going to go with the style as on the Mini Scout for lightness and ease of construction...however, I'd also be very interested to learn the way you are talking about, so yeah, please do share! If I don't use it for this one, I'll certainly end up using it for something else, and having options and learning multiple ways to do something is always a big plus.
Simply instead of a single fold line make a series of folds (to give the wing profile you require and make a some ribs for use under the wing. The ribs will remain exposed and actually trap the air under the wing stopping its sideways motion. With the creases being filled with glue and the ribs glued in place the wing can have any shape you require and when the glue is dry the wing will be rather rigid as well.

For a plane of the size of the simple scout you could do with a single rib on each wing as long as the wing root is held to shape by the fuselage slot.

I also build Balsa and love this type of construction. Look up Jedelsky wings.

Have fun!
 

Vimana89

Legendary member
Simply instead of a single fold line make a series of folds (to give the wing profile you require and make a some ribs for use under the wing. The ribs will remain exposed and actually trap the air under the wing stopping its sideways motion. With the creases being filled with glue and the ribs glued in place the wing can have any shape you require and when the glue is dry the wing will be rather rigid as well.

For a plane of the size of the simple scout you could do with a single rib on each wing as long as the wing root is held to shape by the fuselage slot.

I also build Balsa and love this type of construction. Look up Jedelsky wings.

Have fun!
Alright, I get what you are saying. That's not significantly more involved or heavier, and sounds like the better air flow characteristics would be desirable. Is this sort of what you were talking about?
1596691636110.png
 

Hai-Lee

Old and Bold RC PILOT
Alright, I get what you are saying. That's not significantly more involved or heavier, and sounds like the better air flow characteristics would be desirable. Is this sort of what you were talking about?
View attachment 176507
Yes it is though the original Jedelsky used balsa slabs and sanded them to profile it is equally valid to use the ribs to hold the wing profile shape.

As for Jedelsky designs the last one I built, (and gave away), was a Super Turkey converted to motor glider!
DSCF0043.JPG


Have fun!
 

Vimana89

Legendary member
Alright, thanks to all the great responses and suggestions, I'm pretty confident I know what I'm doing. I'll be getting to work soon here, and hopefully the result will be a lot of fun to fly.