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

Vimana89

Legendary member
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

Legendary member
Pics
 

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Vimana89

Legendary member
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

Legendary member
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

Legendary member
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

Legendary member
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

Legendary member
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 :).
 

Vimana89

Legendary member
@Vimana89 - I took your measurements and drew plans up with a few tweaks like narrowing the fuse and tapering up. Going swappable with a C pack motor. I tweaked the tail to remove the rudder and Hstab. Do you think that will be ok? Also where is cg?

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First off, that looks really nice! Seeing that so slick and clean, and the color scheme...awesome work. That should still fly great as a 3ch. I liked having the rudder for wingover turns and barrel rolls, but it handles fine just "yank and bank" with the elevons, and axial rolls will be just as smooth.

CG: huge brainfart that I didn't add the CG to the list of stats:(...while I still had the planes. I am waiting to get some proper foam to rebuild this, and don't actually have it around right now. I flew the crap out of the old one and it wore down, and though I probably had it written down somewhere, I can't find it at the moment.

From what I remember, and running a quick search to find a similar wing shape, this example I just found in search looks about right. I'm not too proficient with these tools, and it would be an odd shape to calculate, but although the wing in the picture has minor differences, if memory serves me, the CG location in proportion to the wing looks spot on. The plane is actually very forgiving with CG. It has a sweet spot where the handling was great when I flew mine, but even if it was pretty significantly off in either direction(especially nose heavy), it would still fly without any major malfunctions besides poor handling. Hope that helps.

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jpot1

Elite member
Ran it through two online cg calculators. One came back at 8.25” and the other closer to 10”. Going to split it and start 9” back from the nose. Did you use any reflex?
 

Vimana89

Legendary member
Ran it through two online cg calculators. One came back at 8.25” and the other closer to 10”. Going to split it and start 9” back from the nose. Did you use any reflex?
I used just a tiny bit of positive reflex when I was flying it, between that and fiddling with the CG until it was dialed in, it definitely helped with the elevator punchout and smoothed out any little bit of dippy/nose down tendencies.
 

Vimana89

Legendary member
@Vimana89 - gotta maiden today. What a fun flying plane. Definitely a little twitchy on the roll but was like a rocket on a c pack motor. Let me know if you want me to throw the plans I drew up on this thread.
Yeah, feel free! I'd really appreciate that. I'm not the best at making plans, so usually people much better at it wind up making plans of my planes, which by their simplicity actually do translate well to plans without too much effort. I'm glad you had a lot of fun with it, I figured it would be really fast on a C pack. Just on the eMax B pack with 10x4.5 slow fly props, mine still got up to a pretty good top speed.