Servo Pushrod Model Airplane Stand


I'd been interested in how pushrods in sleeves could be used to animate small sculptures since seeing a small robot convincingly articulated this way at Disneyworld. I surveyed the raw material at the hobby shop and came home with nylon tubbing and music wire.

I also came home with a pre-built model airplane. My original idea had been to flex the nylon sleeve itself but though I needed more experience with simple pushrod mechanics before attempting this. My fall back plan was two axis control of some platform, perhaps a disk holding a laser or another flag.

The pre-built would be the perfect target for two-axis remote control. It could be said to be flying with many opportunities to create convincing if not realistic maneuvers in pitch and roll. The price and time commitments were right too: ten dollars and zero hours.

    
    1:72 scale p-40b/c "Warhawk"

First Build

My first thought was to build in a plastic box that would become the base of the stand. Unfortunately the sleeve and pushrods needed to exit vertically which means that there wouldn't be enough room in the box for the pushrods to gather as they enter the sleeve unless the box were also vertical, standing on its end. This simply wasn't stable.

I chose instead to build with FoamCore using three pieces, a large base for stability and then two crossed pieces on top of that with one cross piece running high and holding the nylon tube. This worked well.

     

I've only been moderately successful getting the range of motion. I used two L-bends to hook into fuselage holes I drilled into the airplane's wheel wells. This holds ok because the splayed wire has torsion holding it in place. However, this arrangement doesn't allow much side-to-side (roll) of the aircraft. I want more dramatic motion.

The pitch control comes from a third wire running into a third hole drilled center and behind the wheel well wires. I tried a Z-bend for this wire which doesn't do anything other than keep the wire end trapped inside the fuselage. I need to think about how that Z could possibly work.

The hobby shop has other parts I haven't tried using. I will consult model building advice on the web looking for simplifying technique. Here is some that is promising.

For computers I just duplicated the setup I initially used for the ScribblerProject. This gives me two knobs, one for pitch, and one for roll.

Automatic Control

I wanted to have the model do something when left alone. This desire finally got me to implement the long promised VariableSineGenerator whos input controls the frequency of the output. I've ended up with this circuit:

    Knob --> Sine --> Motor (roll)

Knob -----------> Motor (pitch)

This setup satisfies the first order requirement of offering some obvious controls that influence ongoing behavior.

Change List

I'd like to fly this with a stick. I can imagine that a good interface would be to let you fly it when you move the stick and then have the computers riff on whatever moves you made once you let go. I'll start programming with a variation on the FIFO part. Maybe having two, with drifting clocks, will be enough riffing to be interesting.

I'd like to build a two-piece stand by just omitting the base board. This is the first time I cut slots in the foam to allow pieces to intersect. This worked great. I will do more of this.

I wish I hadn't used scotch tape to hold the nylon pushrod sleeve in place. Yuck.

The pushrod configuration at the servos would look more graceful if I ran the pitch pushrod off-axis so as to match the splay of the wide roll pushrod.

I wish I hadn't ruined my wire strippers using them to cut the steel music wire. Airfieldmodels.com suggests grinding through the wire with an emory wheel in a dremel tool.

 

Last edited May 4, 2008
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