"Table Talk" - How I make KF4 airfoils for park jets

Hi Everyone -

I am asked often about how I do KFs for my park jets, so I thought it time to do a more detailed video rather than trying to answer the same question numerous times.  Some of the information in the video below is obviously opinion and suited to how I like my planes to fly, but it is also backed up by a lot of field testing of different size and shapes of KF airfoils across a wide variety of planes.

Here is the link to the blog post I wrote as part of my blog series called "Which Park Jet for me?".

A close up picture of my drawing for the F-18 wing plate mock up I used in the video above.

The terms "fuselage notch" and "engine notch" are unique to RC Powers V5 plans, so when drawing the top KF on other plans, I just ensure I make the inner line along where the fuselage will be.  Then again for the bottom KF where the inner edge is indicated in red (sorry that it is a bit blurred😕), it should go along the outer edge of the engine/nacelle/intake part of the plane along the bottom.

Here is the close up picture of a Mig/Su style swept wing where the trailing edge sweeps forward.  The measurements and calculations are essentially the same as shown above.  Once I make the calculations for the wing area, I also draw out the top KF using the LERX which you can see in orange at the top of the picture.

For F-22/T50 style wings, it can be a little bit trickier as the wing has such a unique shape, so as mentioned in the video, I don't use the normal trailing edge to calculate the 40% as this would cause the KF to be too large.  In the case of this style plane, not only could this introduce more drag and the negative behaviors mentioned in the video, since the horizontal stab and elevon are so close to the wing, the "vortex" caused by the KF could interfere with these if the KF was too big.  Once making the calculations for the wing, I also include the area along the intake marked in orange at the top of the picture for the top KF.

In the video, I realize I might not have done a very good job of explaining how to trace out the bottom KF if you don't use the method of recycling the plans to cut them out.  In the picture below, I have highlighted the outer edge of the top KF in orange and drawn a line in red where the inner edge of the bottom KF would be.

Imagining that you have the top KF cut out, what I do then is lay it on another piece of foam and trace the portion to the left of the red line, making marks or using pins to mark on the foam where the inner edge of the bottom KF will be.  Then I join all the lines and cut out the bottom KF.

KF airfoils are certainly fun to experiment with and with a little foam and testing it is an inexpensive way to enhance the performance of your park jet or any other foam plane for that matter.😊  

Park Jet noise...the "other" sound of freedom.😎

Cheers,

Scott