Deciding on an airframe

There are several key factors when deciding on an airframe, but as I see it, these are the three main factors:

• Weight
• Strength
• Cost

There are other factors, but many of them are not very flexible once you have set other elements of your design. For instance, if you are going to be lifting a heavy load, your motor size and prop radius are decided by the lifting power you have calculated. At that point the size of your airframe is more or less determined by the spar length, which is determined by the prop radius (and a few other factors).

A almost complete Arducopter kit, photo by Ole Bendik Kvisberg

It should come as no surprise that weight, strength and cost are all directly related. Choose components that alter one factor and you have surely changed one or both of the other factors. Using solid aluminium spars for the motors instead of hollow tubes increases all three factors, using carbon fiber decreases weight, increases strength and cost.

It is possible to get the most efficient design if you are some crazy engineer that can calculate the strength needed for operation or crashes. But from what I'm reading on the web, people just guess. Plus one needs to normally use easily available parts, rather than custom ordering the exact thickness and material that an engineer might determine is the most efficient.

But then, you also have to build it.

Photo by Kevin Vertucio

A kit is far easier than a handmade frame. There cost of the kit is certainly more expensive because the value of the design and identically produced parts increases the price vendors can ask for the kit. Sometime this cost is, er..., rather high ($1095.00 Droidworx AD-8 HL Airframe Kit).

Hand-making your frame does save you money, but takes more time since you will need to custom manufacture some parts. Alternately you can buy some of the frame and make the rest yourself (example: a frame plate sold individually by Mikrokopter US).

Here are there resources I am using to make my decision. A couple of handmade frames and a couple of kits:

• Cheapest kit on the planet. As used in the Instructables quad (see link on right). $15
• 1000mm Quad Copter Design (by "Old Man Mike"). About $35.
• Step-by-step frame build. Price not listed, but carbon fiber plates suggests $60+.
• AeroFPV Frame. Open source maker of the Aeroquad, costing $200
• Mikrokopter Okto XK Frame Set. Commercial frame costing $280
• Carbon SCARAB Quad/Octa. All carbon fiber frame by MultiWiiCopter, for a bling price of $350

Decision

For the prototype, which will be small anyway, no reason not to go with the cheap plywood frame from HobbyKing for $15. I can get two and thereby have spares arms for the inevitable crashes. This kit even comes with a camera mount tilting camera mount!

For the heavy lifter I believe I am going to go with a custom frame using carbon fiber. Keeping the frame as light as possible enables more flight time or more lifting power for the camera. Hand-building will keep the costs down.