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STH5 - "The Kestrel" Something a little different.


Senior Member
As some of you may have seen I'm in the throws of building a couple of foam core composite planes (Thread Here), entirely inspired by reading the threads both on here and on RC Groups. Unfortunately being limited to only building in the weekends it leaves me with a great deal of time to dream up ideas for other planes. Using foam board as a medium means that getting these ideas in the air is amazingly easy.

After a few doodles and working on a wing planform that has been floating around in my head for a while now I came up with this:

I'm sure it's pretty obvious where I got the idea for the wing shape from. Birds of prey and many seabirds (actually most birds) have this shape to their wings. In no way does this imply that anything with this wingshape is going to fly well, in fact through many tests aircraft designers have come to the conclusion that it's more due to the need to be able to get their wings out of the way when they're not flying that birds have evolved into having bent wings. The best fliers out there, like the wandering albatross, have very long straight wings but I have always been taken by the classic bird of prey shape.

So the first step was to build a small throw plane to test the concept. To my great surprise it flew really well. Actually I shouldn't have been surprised, as they say, if you have the right COG and enough power, a toaster will fly. Thankfully my idea wasn't at all toaster shaped so power wasn't too much of an issue!

That evening I got to building and I applied some rules (not for any other reason than aesthetics) derived from photography. Namely the rules of thirds. The rules of thirds suggests that the most effective and attractive photos typically have the image split into thirds either horizontally or vertically (over simplified but this is the gist of it). I decided to leave the rudder for now and went with a bank and yank configuration with two servos on the wings so that I could at a later stage experiment with flaperons.

So I give you - The Kestrel

It flies really well once the CG is sorted and has a surprisingly benign stall characteristic. It will basically pancake from the stall and float all the way down. It flies really well fast or slow and is capable of pretty decent roll rates and loops.

Some specs:

A single sheet of 1000mmx800mm Foam Board

Wingspan: 39.5 inches or 1000mm
Length: 33 inches or 840mm
AUW: 30oz or 850g
Wing airfoil: KFM2

Motor - EMAX CF2822
Prop - 9x7 Slowfly
ESC - 18amp Hobbywing Flyfun
Battery - 2200mah 3cell 25C
Servos - 3xTurnigy 9g

If there is interest I will do what I can to work up some plans.

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New member
It's a medal winner! Good job.
The addition of the KF foil sure makes for slow stall speeds.


Senior Member
I wonder how that wing layout would work as a flying wing. Hmmm.... The W wing...
Hehe - I'm going to have to say sorry because I beat you to that idea a while ago :D

I even built a flying version of it. The flying version ended up with too much weight and not enough wing area. I will revisit it eventually but it proved to have a very narrow CG range. It flew really well though.

If you look in the background you will see it stripped of it's electronics - relegated to the pile of discarded test birds

I like that your working drawings are every bit as detailed as my working drawings! LOL
Haha yeah, I'm a bit of a 'bull in a china shop' guy when it comes to designing. Idea on paper and then straight into the building!
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I can see where the W wing would be cg sensitive. It basically averages out to a straight wing, in a sense. What did you build the motror nacelles from? They look very nice.
heh, you know you've done something right when Mr Dick Kline (the K in KF - i.e. one of the designers of the KFm airfoil) comments on your Youtube video :D!!! Very chuffed!
he said "Congratulations on a fine build. It must be pretty satisfying to see something that started in your mind become a reality. ~ Dick Kline The K in KF"

Nice to have one of the inventors piping up :D
I got through filling the other side and sanding it back, was doing a finishing fill and dropped the sodding thing dinging the trailing edge corner. I'm going to have to cut it out and reform it out of flox. So annoyed with myself.