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May 18, 2009, 08:53 AM
Registered User

KF-airfoils on an F-20 foamy pusher prop

Hello to all,

I want to know if anyone could help me with the size KF-airfoils should be and the best placement of them also. I will tell you what I did and how the plane reacted.
I built an foamy pusher prop F-20 from a plan on and it originally flew well but I wanted more speed out of it for low passes and I also wanted more lift so I could land it at slower speeds.
When I added the KF-airfoils it was VERY pitch sensitive and on the turns it seemed that it was not responsive until I had the stick all the way to the right or left and then it was too much. It almost wanted too roll.

Now Iíll tell you how I set them up on the plane.

I used the airfoils on top and bottom of the wings. I over lapped the
leading edges of the KF-airfoils over the leading edge of the wing about
ĺ of an inch and on the trailing edge the airfoil was 1 inch from the ailerons.
Should I have put the leading edges of the airfoils even with the wing and further
from the control surfaces and then should I have also made the bottom one shorter than the top I donít know. I need help.

You can see my videos of the plane with and with out the airfoils at to see for your self.

Thank you for any help you can give.
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May 18, 2009, 11:38 PM
Registered User
If the KF step is on top of the wing it should only go back to 50% of the chord of the wing. If on the bottom, it should only go back to 40% of the chord. You can put the KFm on both the top and bottom, just end the top at 50% and the bottom at 40%.
May 19, 2009, 12:19 PM
Registered User
It seems that if the KFm strip is too close to the ailerons it causes turbulence. That could be your turning problem. I'm sure someone more knowledgeable will chip in with the technical answer. Did you move the centre of gravity back after adding the KF strip?

May 19, 2009, 01:01 PM
magic612's Avatar
Originally Posted by chris_jacques
I wanted more speed out of it for low passes and I also wanted more lift so I could land it at slower speeds.
I don't know an enormous amount about airfoils and aerodynamics, but I think these two points are at opposite ends of the airfoil spectrum. An airfoil that excels at high speed is not going to be ideal for low speeds, and vice versa.

There may be a practical solution in the KFm universe (or other airfoils, for that matter), but I think at some point it will have to be a compromise of some sort, especially as you increase speed and still want slow landings. The question is: Which aspect is more important to you? Perhaps using flaps might be a reasonable option for getting the slower speed landings you're looking for while maintaining a thin airfoil profile for high speed maneuvers?

Just a thought.
May 19, 2009, 02:17 PM
My plans are in my blog
Rusty-Gunn's Avatar
Yup, adding a K-F may reduce overall top speed (as wittnessed by other posters here), but does wondes for slower speeds.
May 19, 2009, 04:35 PM
Registered User
uhupor's Avatar

KF (Kentucky Fried) Airfoil on Pusher Jets

I have built the Shumate Saab Grippen and made a kind of KF-airfoil on it. Mainly so it does not look like a piece of flat cartonage with prop when flying slower. At least you have some lift and can fly more horizontal.

I hope, I did not Maiden the thing until now. It should have enough speed with the Mega 16/15/2 with 4'100 RPM we will see with the tiny airfoil what happens.... ;-)

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