Andy's DIY, my thoughts on KF

Photos from Andy below.

I think that the KF foil is somewhere in between a flat plate airfoil (any plate is an airfoil) and an airfoil.

A flat plate is thin whereas an airfoil is thicker, so a flat plate will have less drag. Improving the leading edge ought to render it less draggy, that's where KF comes in. An airfoil is airfoiled to reduce the drag that is inherently due to its thickness. It seems a KF (I haven't tried KF myself), introduces leading edge and thickness.

Thickness is good for lift capability because it introduces camber to the air, camber causes the air to flow downwards to create reactionary lift, as I termed it, and by the reduction in pressure on the upper surface which is also called lift (so by this definition, a flat plate has no lift since it is not cambered). A KF seems to be just one way of relying on the air to create a sort of boundary to guide the air flow, which in my opinion, will not be as effective as an airfoil because I would guess that despite the slow speed of our model, there wold still be turbulence in the case of a KF and turbulence is energy dissipated.

To add thickness, an airfoil could be positioned at an angle, this creates more downthrusting of the air and it modifies the effective airfoil shape.

At low to medium angles, the flat plate, with its flat leading edge, has about the same drag since turbulence at the leading edge was existing even at zero degrees of angle. When there is no difference, then the state is constant and we woudn't detect any changes.
At these angles, reactionary lift is produced. Reactionary lift increases fairly proportionally and constant with the angle since the downward vector element of the airflow increases by the same angle. Induced drag is correspondingly increased since the lift is offset to the direction of flight.
Come to an extreme angle, the reactionary lift is of no use since it is at such an angle that there is no vertical lift component.

At zero angle, the airfoil with its camber (symmetrical airfoil excluded here), has both reactionary lift as I termed it, and the more traditionally described lift element due to lesser pressure on the upper surface. No turbulence at the leading edge.
At low to medium angles, the camber changes, and the leading edge guides the air minimising the resulting drag. Reactionary lift increases fairly proportionally and constant with the angle since the downward vector element of the airflow increases by the same angle. Induced drag is correspondingly increased since the lift is offset to the direction of flight.
Come to an extreme angle, leading edge drag forms and we noticed instant changes to the model and there is no practical lift for the same reasons as the flat plate.

The sharper the leading edge, the less leading edge drag at zero to low angles, but the usable angle range is proportionally limited. With a blunt leading edge, you get a wider angle range that is usable, but you have more drag at zero angle to begin with.


Andy scratch-built this Mini-Popwing.

He and Hanif each built a mini ME109 from a plan available off the internet. I understand that Hanif arrange an internet company to do the cutting.

Hanif with his Spitfire in October. All foam board, he followed the plan and instructions off the internet. Too bad he followed the foam spar instruction because the wing was noticeably weak, a bit of G- manouvre and it became bendy.