Four-Motor Scratch Build

Foam board and packing tape — cheap!

Why a Scratch Build?

I don’t want to have too much money at risk when I fly. And, since I like to have building projects underway to have something to do when the weather doesn’t permit flying, I like to build rather than buy some plug-and-play foamie. So, here is my latest foam board and packing tape creation.

What to Build?

In deciding what to build, I looked in my salvage and spare parts. Since I had a set of four inexpensive 20 amp quad-rotor speed controls that I got for my drone project last year, but didn’t use, it was natural that I employ them in a four-motor airplane. I also had some 6×4 props. For batteries, I had some 2S LiFe batteries which I could connect in series to 4S (which would be interchangeable with 3S LiPos that I also had).

I didn’t have the motors on hand, so I went to innov8tivedesigns.com to look at the motor propeller data sheets for different motors to see what would work. The Cobra 2213/12 with 6×4 props and 11.1 volts would run at 18.48 amps which would not burn out my 20 amp ESCs.

In addition to the motors, I needed to buy a BEC, since the multi rotor ESCs do not have BEC. The motors and BEC were all I needed to buy for this project. (Well, plus the $1 per sheet foamboard at Dollar Tree, and the packing tape.)

Design and Construction

With my power system already selected, I could now pick a general size for the airplane. 11.1 volts x 18.48 amps x 4 motors = 820 watts. Shooting for 100 watts per pound, eight pounds would be the max weight I would want.

The foam board comes in 20×30 inch sheets, so a 60 inch span with 9 inch chord Armin Wing would be easy. For general proportions of the fuselage, I looked at the C-130 four engine military transport. From that, I sketched out the design and started measuring and cutting the foam board.

I am not providing detailed design and construction instructions because I made several goofs in both the fuselage and wing. Also, you would have different salvage parts in your junk box. If I did it again, there would be improvements. Use this as inspiration for your own designs!

The End Result

As you see it in the first photo, it came in just over 5 lbs. With over 800 watts of power, there is plenty!

There is a lot of wire in the wing, with four motors and four speed controls. I was able to cut small holes in the foamboard and fish the wires to the center of the wing.

I programmed the transmitter for differential thrust, so the ESCs on each wing are connected with a Y, but the left and right wing ESCs each connect to a different channel on the RX.

The differential thrust allows me to dispense with a servo for nose wheel steering. The nose wheel is free castering.

The differential thrust also allows for a fast flat spin, which is exciting! Just remember to bring the throttle to idle to recover!

I hope this inspires you to do your own scratch building!

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