Weekend (mis)Adventures 2: RC Planes, part II

A couple weeks after our last adventure, D managed to tree his Gamma 370. However, it dawned on us that the entirety of the propulsion components of the 370 (Motor, ESC, and battery) were available from the local hobby shop. Given that the rest of the Gamma 370 was just some foam in the shape of an airplane, we decided it would be interesting to try to build a plane from “scratch”. Here’s what we had to work with:

The wing is from an ancient RC plane I had a long time ago, which I promptly crashed and irreparably damaged. Some of its electronics later wound up in our boat experiments. Aside from that, the core of the plane is a 3/8″ square stick of wood. We started with balsa, but after breaking it a whole bunch of times in test tosses we switched to basswood. Here’s the finished plane:

The vertical and horizontal control surfaces are balsa sheets connected to each other with some very thin cellophane tape (It’s significantly thinner than normal packaging tape — D and I refer to it as “China tape”). Control rods are made out of some mild steel wire and connect to some micro servos (light blue, middle of plane). The wing is held on by some rubber bands, as are all the non-servo electronics. The motor’s mounting is simply jammed on to the front of the stick, which we wrapped in some blue masking tape to make large enough to fit snugly. The “landing gear” consists of some thick steel wire bent into two curved whiskers and is also taped to the stick. While these whiskers tend to get plastically deformed during landings, we reasoned that that was a good way to absorb excess energy.

We fiddled about trying to figure out what the right dimensions were, and in the end we decided to just try to fly and see what happened. The plane actually turned out to fly quite well, but it also flew very fast so it was somewhat hard to control. We also neglected to take much video of the thing…

Flush with our success, we decided to make another larger plane from scratch, including the wing. We reasoned that the previous plane (“Ghetto 370”) flew too fast because the wing area was too small. So we built a giant wing out of foamcore, tilted it up at a similar angle to the wing on the Stick 75, which has excellent slow-flight characteristics. We also added a larger dihedral angle to the wings and raised them in the hopes of improving our roll stability (the Ghetto 370 tended to stay tilted until you took some action to correct it). Lastly, we decided to try a V-tail because we decided it would be a lot easier to build. Here’s the wing:

As you can see, the wing is simply some foamcore “ribs” with foamcore sheets covering them.

Unfortunately, we neglected to carefully consider the placement of the center of mass of the plane. In general, the center of mass of the plane should be placed forward of where the center of pressure of the plane is, as it helps level the plane out if it begins to tip up or down. If the center of mass of the plane is too close to the center of pressure, or is behind it, the plane becomes difficult or impossible to control. Thus our first attempts at flying the plane just ended with it flipping over suddenly, stalling, and smashing into the ground tail-first.

This weekend, we moved all the electronics as far forward on the plane as possible, which helped to move the center of gravity forward, but didn’t completely solve the problem. After a crash tore off the wing mount, we decided to move the wing mounting back by a couple of inches, effectively moving the center of pressure towards the back of the plane. After this last adjustment, as well as some learning on our part, we were able to fly the plane in the park near my house, which is a lot smaller than the one we usually go to (seen in the video accompanying the last plane post). Here’s the final product:

In an attempt to avoid losing this plane to a tree, this message is written on the side:

Here’s one more picture.

The half-black half-white nature of the plane is not because we ran out of foamcore, but because we wanted to be able to tell which way the plane was flying when it was far away. As it turned out, a much more likely source of crashing was me being confused as to which way left was when the plane was flying towards me.



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