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Vimana89 Hawk Moth (a simple, highly swept, semi-tailless 4ch design that flies and scales great!)

Vimana89

Well-known member
#1
Some of you are already familiar with this design. I built basically the first version of it (before having a name) for the Build-Ruary challenge with a three step(KFM 3 I believe?) air foil on an F pack radial motor, and while it showed nice handling, was underpowered and horrendously inefficient on battery duration. I built the second version(which became the standard successful one) a couple months back as part of the quarantine build-off, using a 2 step KFM1 style air foil and "old" B pack motor, and it's been my go-to all-around 4ch flyer ever since.

I recently built a micro sub-250g version on a radial A pack, and while I'm still tweaking props and CG a bit, it flies really nice for its size and gives a full 4ch flying experience in a small package, while showing that the design scales well. I figured I'd give the design its own thread and share my design and build process, as well as my experiences flying both the original and micro versions so far.

So what is the Hawk Moth all about? Well, It's a really simple 4ch tractor-prop design with a highly swept wing similar to that of an English Electric Lightning using a KFM1 style air foil (in my case I used a simplified version with no fold/just glued). The fuselage is just a basic B fold "noob tube", and although there is a full fuselage, rudder, and small horizontal tail surface above the VS/rudder, it's effectively a tailless (or at least semi-tailless) design in that the tail section does not extend past the wing tips, and elevons (rather than ailerons) on the wings are utilized.

The inspiration came from a plane on Outerzone called the "Blitz", which itself is basically a shortened, semi-tailless version of a Lightning with a tractor prop. I simplified this design further for easy foam board construction, and changed up the rudder section a bit, but the concept is basically the same.

Building is a no-brainer, and flying is easy, although I would definitely not say first plane/trainer level easy. The handling qualities are somewhat like I'd expect a swept-wing cold war jet to behave, crossed with a hint of flying wing (although the high sweep gives it a bit more "pendulum stability" than your average wing, and the position of the rudder makes it a lot more effective). It yanks and banks well just with the elevons, but also really responds great to rudder, which gives it a ton of maneuverability.

Both axial and barrel rolls are super clean and crisp, and the plane banks extremely smooth(and can hold a really steep bank almost like a knife edge very gracefully). Wingover turns are great, and it can loop decent too, but with loops on mid size or larger versions some altitude and technique are required because it loops wide and carries a lot of downward momentum. This plane has no nasty stall characteristics, and any stalls will be gentle.

This is definitely no dedicated slow flyer, but generally handles well at lower speeds, especially with the right setup. Higher speed handling is good too, and it could be very fast if set up for speed. The design is flexible and well-rounded for a lot of situations and a great all around flyer. The micro version is a little more touchy/squirrely, has a faster roll rate, and loops a bit better.
 
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Vimana89

Well-known member
#4
Specifications/build info

Original
wing span: 31"
wing length: 26"
wing chord: 6.5"
top KFM layer: 2.75"(40%)
elevons: 3"
fuselage length: 24.25"
fuselage width: 2"
fuselage height: 2"
VS/rudder length: 6.25"
rudder: 2"
VS/rudder height: 4.75"
horizontal stabilizer length: 4"
horizontal stabilizer span: 6"
note: original has a "zero tip chord" which I've been told is less than optimal. Adding a bit of squared-off wingtip area just before the elevons may improve performance very slightly.

Micro
wing span: 20"
wing length: 20"
wing chord: 4.5"
top KFM layer: 2"(40%)
wing tip chord: 5.25"
wing tip length: 1.25"
elevons: 2.75"
fuselage length: 18.25"
fuselage width: 1.25"
fuselage height: 1.25"
VS/rudder length: 5.75"
VS/rudder height: 4"
rudder: 1.75"
horizontal stabilizer length: 3.75"
horizontal stabilizer span: 4"
note: fuselage can be widened to accommodate a battery compartment and space for electronics

Electronics
Builder's choice, but for the original, I'm running the "old" B pack motor on 10x4.5" props with a 3s 1000mAh batter, and the micro uses an A pack motor with 5x4" or 6x3" props, or anything in between on a 3s 650mAh battery.

Notes
Most of the electronics can be hidden in the fuselage if desired rather than taped across the top like I did, but the one thing that will be difficult to conceal is the servo cables from the elevons. These can be taped along either the tops or bottoms of the wings, although they may be able to be mostly hidden by running them under the KFM folds, although a small portion of them would probably still be visible where through-wired into the fuselage.

Overall, the build is extremely simple and the exact measurements can be tweaked to preference.
 
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Vimana89

Well-known member
#7
For speed I think it would be worth trying a C pack on the larger 30" version or an F pack on the 20". I played it safe and went B and A respectively, but I think anyone who flies faster planes like goblins, fast war birds, and powerful EDF jets would appreciate this platform with the right motor. Unlike some of those planes, it has really gentle stall characteristics and lands easier, especially with an overpowered motor.
 

mayan

Well-known member
#8
For speed I think it would be worth trying a C pack on the larger 30" version or an F pack on the 20". I played it safe and went B and A respectively, but I think anyone who flies faster planes like goblins, fast war birds, and powerful EDF jets would appreciate this platform with the right motor. Unlike some of those planes, it has really gentle stall characteristics and lands easier, especially with an overpowered motor.
Sounds to me like you were doing your home work :p.
 

Vimana89

Well-known member
#9
Sounds to me like you were doing your home work :p.
A bit of homework, a bit of trial and error with the first build-ruary version. After that, it was mostly just flight testing to see where it liked the CG best and testing different props. For the air foil I really just went lazy and used glue and no folds, but I did some research on KFM and the general rule of thumb for the most basic two-step(KFM1) is that the top layer should be 40% of your wing chord, which is good to know.
 

mayan

Well-known member
#10
A bit of homework, a bit of trial and error with the first build-ruary version. After that, it was mostly just flight testing to see where it liked the CG best and testing different props. For the air foil I really just went lazy and used glue and no folds, but I did some research on KFM and the general rule of thumb for the most basic two-step(KFM1) is that the top layer should be 40% of your wing chord, which is good to know.
You see now I learned something :).