Kit 31: Ornithopter and Entomopter

Gakken Kit #31: The ornithopter and the entomopter, 2400 yen.
Ever since mankind has wished to fly, we’ve been using birds as the starting point, generally basing the designs on wings that flap rather than the more modern fixed-wing concept. These kinds of aircraft are called “ornithopters”, from the Greek “ornithos” (bird) and “pteron” (wing). If you’re under age 30, there’s a good chance that you may have been exposed to ornithopters either in school science classes or as a toy at home. The Ornithopter Zone is a good resource for building your own and entering them into competitions.

On the other hand, insects haven’t been as popular a research subject for artificial flight. The “entomopter” (which gets its name from the Greek “entomo” (insect) and “pteron” again) is, as defined by wikipedia, a “multimode (flying/crawling) insect-like robot developed by Prof. Robert C. Michelson and his design team from the Georgia Tech Research Institute (GTRI)”. It is classed as a subset of the ornithopter. According to Michelson’s own official Entomopter Project website, DARPA funding for a new kind of powered-flight craft was provided in 1998, and the first patent on one of the moving components of the craft was granted in 2002. While some of the applications are for the military, there’s interest in using the entomopter for Mars surface exploration.

(Ornithopter on the launcher stand.)

Because both the ornithopter and the entomopter are so well documented on wikipedia, there’s no point in my talking about the mechanics or physics here. The important thing is that kit #31 allows you to build rubber band-driven versions of both and compare them in terms of motion and flight effectiveness. (For motive power, the ornithopter front wings flap; the tail is just used for changing attack angles. For the entomopter, the front wing is one unit that wags up and down, and the stationary tail just acts as a stabilizer.)

#31 has roughly 40 parts. Gakken recommends 90 minutes for assembling both kits. It took me closer to 2.5 hours, partly because I was having trouble figuring out the instructions, and the fact that I kept using the entomopter parts for the ornithopter (which is the first one you build according to the instructions). If you buy kit #31 as an import, odds are that the English instructions will be posted to the Gakken site by the time you get it in your hands. This is very important, because there are a couple easy mistakes to make which can ruin the kit. First and foremost is the fact that the plastic sheet for the wing material is in fact two sheets that you start out cutting at the same time. You only need one sheet, so separate them before cutting the wings out, or you may end up mangling your back-up material. Second is that the entomopter front and back wings look very similar, but the front wing needs to be the wider of the two. Third, when you’re applying the wings to the double-sided tape, it helps to have someone assisting you. The film is drawn to the tape, making it hard to apply correctly. For the ornithopter, there’s a notch in the film that goes at the front of the front wing. You want to center this notch around the “shoulder” of the wing, and avoid having too much slack around the joint before the wing touches the tape. Unlike the mess that I created of things. Fourth, the shoulder action of the entomopter can be a little confusing when it comes to centering the carbon rod and affixing the wing material. It may be obvious to most people (apparently not to me, though) that the center point is the back spine. You want the arms to extend 10 cm either side of the spine, and the dotted line of the wing to sit atop the spine. The entomopter will still fly regardless, but it won’t look as cool otherwise.

(Launcher stand, cocked.)

Also, some of the thinner pieces can be very brittle and will snap in two while you’re removing the flash from the parts. This may be one good reason for not bothering trimming the flash off to pretty up the final assembly. I snapped the back spine of the entomopter, and gluing the pieces together added to my total assembly time. (I should mention that the pieces come still attached to the molding frame and need an x-acto knife to be cut apart. If you build the launcher you’ll also need a small Phillips driver for the 3 screws.)

As for flying the kits – there’s a little rubber band-driven launcher that holds the wing drive rod in place as you wind the main rubber band for either the entomopter or the ornithopter. The launcher will fire the kit into the air for those people too skittish to hold it in their hands before letting go. Again, it helps to have two people – one to hold the prop in place and the other to wind the rubber band using the crank in the back. Because of the torque involved, if you try to do it yourself holding only the front and back of the kit, you can damage the balsa wood body piece if you do it wrong. I found that I could wind the rubber band better without the launcher and I got better results just by throwing the kits into the air by hand. Even fully wound, the ornithopter only flapped for 5 seconds, and the entomopter for 3. If the wind is right, the ornithopter will get another 3-5 seconds of glide time. The entomopter isn’t designed for gliding and will land within a couple of seconds. Your results will vary depending on how far off the ground you throw the kits (obviously, starting at the top of an apartment building will get more glide time).

The mook includes articles that talk about how both birds and insects fly, as well as getting into the specifics of both plastic models. There’s a speculative piece on bio-mecha for exploring Mars in the year 2040, and a pictorial on the construction of a 787 jet aircraft. Additional articles include the inevitable history of failed man-powered craft; different kinds of home-made flying models; pictures of birds, insects and mammals in flight; mods to the two kits (basically different patterns for the wing materials); and some alternative power sources. The latter article ties in with the entomopter, because the original design includes an anaerobic chemical engine applicable for use in Mars’ atmosphere. The remaining articles are about the possible evolution of dinosaurs into birds; a paper punch-out zoetrope of a bird in flight; a manga on how the human tongue detects flavor; and home-made projects such as a cardboard box Gundam model, and a bicycle made mostly out of wood. I should also mention that there’s a Japanino project for syncing up a digicam with the entomopter as it comes off the launcher.

As for the zoetrope – it’s a small strip of magazine bond paper placed on a disk in a circle, with notches and pictures on either side, rotating on a toothpick. One side is a bird in flight, the other is the ornithopter. The effect doesn’t work well unless you tape the pieces together, making it more difficult to flip the strip over to see the second animation. And the slots in the sides are too narrow to let you see the pictures on the strips clearly. A nice idea but not very successful. However, it should give you some ideas if you choose to make your own zoetrope. One mod that would help would be to take a 3″ 6×32 bolt, a nut and some washers, and use that to make a more stable axis. This could then be mounted to a turntable to facilitate the rotation.

Links given in the mook:
Smart Bird
Butterfly Robot

More ornithopter designs
Mars drone plane
Sano Magic’s Wooden Bikes
Otto Lilienthal Museum

Note: Otto Lilienthal was a German inventor who pioneered glider flight. His research was used by the Wright Brothers for their own powered airplane designs. Otto was known as “the Glider King”.

In the last entry, I posted a rant on how the increased prices were threatening to make the Otona no Kagaku kits a bad deal if you buy them as imports in the U.S. at roughly a 2x mark-up. Kit #31 is cheaper, at 2400 yen ($28 USD). Is it worth buying as an import? Unless you’re a science teacher, probably not. This kit IS a great introduction to artificial flight, and it could be a good way to take up making your own craft, if you go to the Ornithopter Zone site for more blueprints and suggestions on getting into competitions. So, in that sense, money is probably not an obstacle. But, if you just want to have something to build and play with, you may be happier getting an RC kit from a local store for the same amount.

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