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Old Oct 21, 2010, 11:12 AM
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Originally Posted by viking60 View Post

KF stepped discontinuities are a type / class of turbulating structure which is very easily implemented when building with layers of flat foam sheet, FFF, etc..... but we may need to do some re-thinking, and be careful so that we don't fall into the trap of letting the commonly used material dimmensions dictate what shape / depth these structures assume.

KF steps are 'innies' - they are structures where the wing surface drops away from the contour of wing surface in front of it, forming a 'pocket' behind the KF step. This is a surface discontinuity where, in some cases, a circulating vortex may be captured while the wing is in flight.

Effective air boundary layer turbulating structures on a wing's surface can be either 'innies', or 'outies', and in some cases they can be merely more angular changes in the surface of a wing.

Old-timer & sailplane designers have known for a looooong time that using turbulator spar structures and 'trip strips' ('outies') on an airfoil's surface improves the performance of a wing. And the key to this is that even very minimal height / depth structures- (only 1/32" height for a trip strip) very effectively turbulate the boundary layer air flow, reducing surface layer separation bubbles across a wider range of angles of attack, and across a wider air speed range [for a given wing loading.]

How deep of a KF step / pocket we might want to use on various aircraft types for various desired flight performance envelopes is still a question that bears a lot of closer scrutiny and experimentation.

My recent testing on 'KF3" variant wing prototypes has been evaluating whether shallow depth KF variant stepped structures produce the same benefits (trapped circulating vortexes & turbulated boundary layer air flow) with less drag when compared to deeper stepped discontinuities. The subjective results of the KF3P prototype flight tests indicate that they do.

For making modifiable test wings where I can test deeper vs. shallower stepped structures, working with the various thicknesses of Depron & FFF is working well for me for now; I can add or change thicknesses by 1mm of foam at a time if I want, tape it on place, remove it again, and still keep a very clean and structurally sound wing structure throughout all of the various configurations. I can temporarily add other turbulator structures for some tests, too.

VIKING... I totally agree with your assessment. The steps do not have to be that deep. Below is my Condor with a rather small step. This is the plane I used to challenge the distance record for the first manned flight down at Kill Devil Hills, NC. If the step is too deep it creates a bigger vortex which then produces excess drag. The small step allows the vortex to remain attached and helps prevent separation. Your work on the step heights has greatly increased our knowledge about this phenomenon.

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