JTiger
So I was perusing the forum and glanced over the RCP F-18 v3 instructions in this thread: 
https://www.rcpowers.com/post/building-instructions-for-leagacy-plans-10099094?pid=1309544983

Anyhow, page 10 states, "Round All Leading Edges." Now, we know that a rounded leading edge is more aerodynamic than a squared off leading edge, but we also know that a tapered trailing edge is significantly more aerodynamic than a squared off trailing edge. If you look at any    airplane wing the trailing edge tapers down to a pretty sharp edge to keep the airflow from separating. Now foam can't really hold a sharp edge like this, but it seems that even a somewhat rounded edge would be better than just a square edge, right? 

The only theory I can think of supporting a square trailing edge is that it is more likely to create a relatively "even" turbulence behind the wing (fig. B, D, E), whereas a rounded trailing edge is more likely to create an oscillating turbulence (fig. C). 

flow_patterns.gif  

Another possibility would be that the rounded trailing edge would create a flow more like the smooth ball here, while the square trailing edge might be closer to the dimpled ball?

flow_patterns.gif

Any thoughts?
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Sidewayskiwi
When using DTF board i use bamboo skewers hotglued and recessed into leading edges for strength aerodynamic efficiency and longevity
Trailing edges i bevel and treat with foamtac style  glues as they become quite hard and durable then varnished (usually minwax or modpodge)
Depron i usually use light balsa sheet to make a reinforced leading edges glued then sanded after i have cut the profile on the KF leading edges 
i reckon its at least doubled the life of my planes 
Trailing edges on Depron cut and sanded taper treated with foamtac style glue then varnished for painting same as DTF 
Like every thing dont taper too sharp 
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DualDesertEagle
Well ironically it's a sharp edged step in the airfoil that improves the airflow, looking like this:

[kline-fogleman-airfoil-design-2] 
So I'd guess that something similar happens on a square trailing edge. So far I've never tampered with the trailing edge on any of my depron planes so I can't give u any test data on it, but since a KF step causes drag I'd say a square trailing edge also causes drag, but possibly with the same effect as on the KF step except double sided.
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Sidewayskiwi
Yes the sharp edge helps generate the KF bubble we want 
The bubble is stable because of the pocket it sits in and it borders the wing surface and vertical step surface to anchor too
At the trailing edges we have two intersecting airstream packages one more laminar than the other 
What we ideally want is a orderly merging of the the two airflows (never happens but theory is a fine thing)
A square trailing edge will attempt to give us two competing and counter rotating bubbles or pockets (like the ones we want to generate in the KF step) These will interfere with each other and increase drag and turbulence 
the wake turbulence generated can also be subject to wave theory where positive waves and negative waves cancel each other positive on positive and negitive on negitive waves will reinforce each other and have twice the effect 
Basically we have chaos theory at the trailing edges airflows going any which way and battling each other as one airstream is of lesser density and higher speed and the other slower denser airmass 
Aerodynamicists aim by design to to lessen this uncontrolled collision by using shapes and devices  to bring order to the chaos as Turbulence literally sucks and you want your plane to be thrust forward not sucked back
Aerodynamics is the black art of magic ,witchcraft and wizardry which confuses most and is talked about in hushed and muted tones
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JTiger
Since a KF step causes drag I'd say a square trailing edge also causes drag, but possibly with the same effect as on the KF step except double sided.


That seems possible. In this video from about 3:16 to 3:45 it look like the trailing edge step forms a continuation of his KF step air pocket.

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Sidewayskiwi
Yes you can see the turbulent voids 
smoke has mass so in a rotating turbulent mass smoke is thrown to the outside 
everything causes drag its just how much and the benefit vs loss ratio 
KF airfoils yes have drag but that drag also anchors the KF bubble ( which discounting particle friction) is basically frictionless meaning an overall advantage 
Also for dramatic benefit the KF step is waaaay bigger that practicable and what is used is 3-6mm step usually 
look up wave theory and bernoulli's theorem they treat airflow as a liquid and is easier to demonstrate and understand 
at the end of the day its magic everyone needs a little magic in their lives
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