Help! Experimental wing design KFm-5A or KFm-5B which works best ?

[TheFlyingBrit]

Most of you already know about the Kline & Foggleman (KF) Step design for emulating wing chords.

I was wondering about the impact of a step combines with a curve and came across the KFm-5A and 5B, designs. Basically 5A has the step on the lower, underside of the curve, where 5B has the step on the top surface of the curve.

I have been considering a wing design for a prototype Scout, which reduces weight of the wing by using the KF step design. However, I also considered that if I was to add a curve to the wing in combination with the KF step I would increase lift.

Then I discovered the design already exists, great , but there are two variations .

I am prepared if necessary to trawl through the internet and look for information regarding which design gives the optimum lift. Or I could just build the two designs and test them.
However, my first port of call was the forum, to see if anyone has built or tested these experimental wing designs ? and if so what are there conclusions. Any information on this subject would be appreciated.
(Note - I suspect the KFm-5A will have the best characteristics for lift but, that's just my theory at the moment).

 

[Monte.C]

How do you want to build the curved wing? What material?

 

[Monte.C]

I searched images. It leads to loads of sites with pictures like this.

It shouldn't be hard to find the info you need.

 

[BATTLEAXE]

I think the undercamber of the wing will leave the difference between the two negligible. I did the Scout with a symmetrical KFm wing and it flew awesome so you are going in the right direction

 

[TheFlyingBrit]

I searched images. It leads to loads of sites with pictures like this.
View attachment 187089

It shouldn't be hard to find the info you need.

There is plenty of info but was hoping for some practical experience. The thing with KF steps is they normally work fine on slower models, because the air pocket behind the step doesn't tend to get disrupted thus the design sustains a chord shape and provides lift. However, when a model travels faster the turbulance disrupts the air pocket thus impacting the chord shape and has thge opposite effect.
I was thinking that a curved design with a step, would give a reasonable slow performance. Yet when the model speeds up the curvature of the wing would still help to maintain some lift, even if some lift is lost at the step.
Question is which curve is the best option for a sport version of the Scout ?

 

[TheFlyingBrit]

How do you want to build the curved wing? What material?

Foam board will probably be my choice of building material, although with the paper removed from the surface with the step ?

 

[TheFlyingBrit]

I was planning to try out my new acrylic filler to spread over the wing and provide a nice smooth aerofoil surface. The only snag is how to retain the curve in the foam without putting score marks and ending up with creases ?
The acrylic would only be used on the surface where the paper was removed.

 

[BATTLEAXE]

Question is which curve is the best option for a sport version of the Scout ?

No curve or a two step version. 3 layers of FB if you want to go slower, adds drag so the plane slows down well. All layers on top with the Steps at 1/3rd and 2/3rd of the total chord.

I put the Kfm on some fast planes and never seen a drop in lift at speed, if anything they are just like any other wing and balloon under throttle. For what you are looking to do, if there is a reduction in lift at speed, will be a speed w you wont reach anyway. I havent found that speed yet and on a Scout you will be fine. No need for a curve for that purpose. However you will give up the under camber at the wing tips the original design has so you will tip stall more easily

 

[TheFlyingBrit]

Think I can feel an experiment coming on

No curve or a two step version. 3 layers of FB if you want to go slower, adds drag so the plane slows down well. All layers on top with the Steps at 1/3rd and 2/3rd of the total chord.

I put the Kfm on some fast planes and never seen a drop in lift at speed, if anything they are just like any other wing and balloon under throttle. For what you are looking to do, if there is a reduction in lift at speed, will be a speed w you wont reach anyway. I havent found that speed yet and on a Scout you will be fine. No need for a curve for that purpose. However you will give up the under camber at the wing tips the original design has so you will tip stall more easily

That was another reason for the curve reduction in tip stall.

 

[Monte.C]

I was planning to try out my new acrylic filler to spread over the wing and provide a nice smooth aerofoil surface. The only snag is how to retain the curve in the foam without putting score marks and ending up with creases ?
The acrylic would only be used on the surface where the paper was removed.

There's the question. How to hold the curve shape without doubling the weight of the wing.

 

[TheFlyingBrit]

Wing formers springs to mind as a method of retaining the curve. However they would only work with effectively with the KFm-5B design, as there is no step on the underside of the wing to interfere with the strategically placed formers.
I could also run a CF rod through the formers across the length of the wing, that would increase wing strength but add extra weight.

 

[Monte.C]

Wing formers springs to mind as a method of retaining the curve. However they would only work with effectively with the KFm-5B design, as there is no step on the underside of the wing to interfere with the strategically placed formers.
I could also run a CF rod through the formers across the length of the wing, that would increase wing strength but add extra weight.

Ah, I didn't think you were considering that route.

 

[Monte.C]

I would be more comfortable with the shorter bit on the bottom, but that's just by eye and common sense, which may not be completely valid in this case. I wouldn't know until I found somebody's test results.

I don't see why you couldn't have on the bottom but you'd have to get the notch right. It would help if you were drawing it up on the computer.

 

[BATTLEAXE]

Wing formers springs to mind as a method of retaining the curve. However they would only work with effectively with the KFm-5B design, as there is no step on the underside of the wing to interfere with the strategically placed formers.
I could also run a CF rod through the formers across the length of the wing, that would increase wing strength but add extra weight.

The Scout if built light with the original design should fly decently slow, and takes care of all the points your looking to hit. You might be trying to reinvent the wheel here but i am interested to see where this goes.

 

[Merv]

You can heat form depron, not sure if it will work with fb. I’ve seen it done with a mold in an oven, this looks much easier.