kfm4 question

AMMO193

New member
Good day!
There is a famous design called the DS wing or poor man's wing known for it's simplicity and not requiring alot of gizmos. As you might know it's KFM2 airfoil, I decided to make mine Kfm4 as shown below.

IMG_20241221_231108.jpg
IMG_20241221_231139_edit_80214360992967.jpg

My main question: do the steps always have to be as thick as the main wing? (5mm + 5mm+ 5mm = 15mm total thickness)... My kfm4 steps are 3mm each and the main foam wing is 5mm....so 11mm total as shown:

Screenshot_20241221_220842.jpg

What are the pros and cons of keeping the steps slightly less? Is it going to sacrifice stability for less drag due to less total thickness, will it be the best of both worlds? And yes, I'm definitely going to bevel the leading edges for smoothie airflow. So I just need to know how it will affect by having kfm steps in general be less thick than the main wing.
Many thanks 😁
 
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Michael9865

Elite member
I found this chart on RCGroups in a a thread called “Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion.“ Hopefully this helps you, if not the thread is full of valuable theory and practical data.


1734816731239.jpeg
 

Merv

Moderator
Moderator
...Is it going to sacrifice stability for less drag due to less total thickness, ...
...So I just need to know how it will affect by having kfm steps in general be less thick than the main wing...
The thickness of a wing is a topic of great debate. There will not be less stability, BUT there will be less lift if you make the wing super thin.

Most wings are going to be 12-15% thick. That is, if the wing has an 8 inch cord, it should be about an inch thick. Thinner wings produce less lift and are not nearly as strong. Thinner wings (up to about 9% or so) will have less drag, go faster and are better suited for gliders and racers. If you go much thinner, drag starts to increase.

There are a lot of planes that skip the KF altogether. They just use a single sheet of foam for a wing. This ultra thin wing produces a lot of drag. The KF reduces drag over the flat wing. The FT faceted has less drag than a KF. And a completely smooth airfoil has the lowest drag. At least that has been my experience.

For me, the FT faceted wing has the best time & cost to build vs performance.
 
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AMMO193

New member
The thickness of a wing is a topic of great debate. There will not be less stability, BUT there will be less lift if you make the wing super thin.

Most wings are going to be 12-15% thick. That is, if the wing has an 8 inch cord, it should be about an inch thick. Thinner wings produce less lift and are not nearly as strong. Thinner wings (up to about 9% or so) will have less drag, go faster and are better suited for gliders and racers. If you go much thinner, drag starts to increase.

There are a lot of planes that skip the KF altogether. They just use a single sheet of foam for a wing. This ultra thin wing produces a lot of drag. The KF reduces drag over the flat wing. The FT faceted has less drag than a KF. And a completely smooth airfoil has the lowest drag. At least that has been my experience.

For me, the FT faceted wing has the best time & cost to build vs performance.nks for the input, what's FT Faceted, can you show me pictures or a certain design that uses this?
 

AMMO193

New member
Thanks for the input, may I ask what's ft faceted?? I've searched Google and all I got was a bunch of christmas lights 😆
Can you show me an example or a certain plane that uses this?
I found this chart on RCGroups in a a thread called “Kline-Fogleman (KFm) Airfoils - Building/Flying Discussion.“ Hopefully this helps you, if not the thread is full of valuable theory and practical data.


View attachment 247181

Thanks I'll look into it
 

Merv

Moderator
Moderator
...what's ft faceted??...
It's a term I made up.
FT wings don't have a smooth airfoil, they are an approximation of an airfoil. Rather than a smooth airfoil, the FT wing goes straight for a bit. Then changes angles, then straight, then another angle change. It forms a facet approximating an airfoil.

The FT wing has a good glide slope, better than a KF wing. Much better than a flat wing. But not as good as a smoothly curved airfoil.
 
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AMMO193

New member
It's a term I made up.
FT wings don't have a smooth airfoil, they are an approximation of an airfoil. Rather than a smooth airfoil, the FT wing goes straight for a bit. Then changes angles, then straight, then another angle change. It form a facet approximating an airfoil.

The FT wing has a good glide slope, better than a KF wing. Much better than a flat wing. But not as good as a smoothly curved airfoil.

Gotcha! Although nothing beats flat surfaces for me... I always use 10mm foamboard and leave it flat. Much less draggy for me.
 

Mr NCT

VP of SPAM killing
Moderator
The FT wing has a good glide slope, better than a KF wing. Much better than a flat wing. But not as good as a smoothly curved airfoil.
Here's some good real world testing of different airfoils:
Honest, it really exists and is about airfoils :LOL:
 

Merv

Moderator
Moderator
Basically a kfm4 airfoil with less drag by having slightly smaller steps reducing overall thickness (11mm instead of 15mm)
If the wing cord is around 100 mm, reducing the thickness will help reduce drag.
If you wing cord is around 160 mm or more reducing the thickness may increase drag.
It's not about the absolute value of the thickness. What matters is the thickness compared to the wing cord.

If you are going for speed, you want the thickness 9-12% of the wing cord, if endurance 11-15%, if heavy lifting 13-16%
 
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