Star Cobra (oz5878)

About this Plan

Star Cobra. RC sport canard / double delta model. For IC ducted fan, although the article also detailsalternate setup with a more conventional open propellor setup.

Quote: "A futuristic styled aircraft developed by three senior citizens. By Dave Bridges, Ken Willard and Bert Williamsen

Take a retired Pan American Airlines Captain, a retired Lockheed chief test pilot for RPV's, and a retired United Airlines Captain, put them all together and let them shoot the bull about airplane design concepts, and what do you get? A lot of bull shooting, naturally. But you also get a design which is intended to meet certain requirements. And that's exactly what the Star Cobra does.

What are the requirements? We set them up like this - not necessarily in order of importance, but just something to shoot for. Bull shoot, that is:

(1) It should look futuristic. A real eye catcher.
(2) It should be easy to build.
(3) It should fly essentially as easy as a normal sport model. Not a trainer, but a good sport job.
(4) It should be efficient.

Take one look at the photos of the model. and you'll agree that it meets the first requirement. Take a look at the plans and the construction photos and you can readily see that it's easy to build. As for the third requirement, you'll have to take our word for it - it is easy to fly. There are a couple of things to watch out for, which we'll point out later, but they really are of secondary concern.

As for the efficiency of the design, we decided to reach a compromise between top efficiency and simplicity of construction. The Fome-Core 'flat plate' wings are not as efficient as would be a built-up symmetrical section, but they certainly are a lot easier to make. Also, when it comes to slow flight, the high angle of attack of the double delta configuration results in just about the same low landing speed as would be the case were the wings to have symmetrical airfoils, In any event, the efficiency shows up very well when you consider the speed range; most sport models fly at around 35-45 mph, and land at speeds around 18-20 mph. The Star Cobra will exceed 80 mph, and will land (with high idle speed on engine) at less than l5 mph. So the efficiency is there.

One more item about efficiency and simplicity. You will note that two versions are shown for powering the Star Cobra. The design was originally conceived specifically for installation of the Midwest RK-20 ducted fan, using the K & B 3.5cc engine. One day, following some flights with the RK-20, we took the engine out for inspection. As we did so, the thought occurred to us: Hey, wonder how it would go if we made an alternate mount for the K & B 3.5cc engine, put a 7 in diameter 6 in pitch prop on it, and made a propeller driven version that still had all the appearance of a jet job?

So we devised a simple mount, stuck it in the opening where the RK-20 normally fits, and gave it a try. Terrific! It flew just as well - or maybe a tad better because the 7 in prop created a bigger thrust disc than the fan does, and made the rear control surfaces a bit more responsive at high angles of attack.

So, if you are on a limited budget, you can start flying the Star Cobra with the propeller driven installation, then add the RK-20 unit later on. Remember the old saying: Put enough horsepower on a barn door and you can fly it? Well, the wings on the Star Cobra may look like barn doors, but there's a difference, the double delta set-up provides for interacting forces between the two flat plates, and the slight differential settings in the control surfaces of the fore and aft planes offsets the parallel arrangement of the main surfaces..."

Supplementary file notes

Article pages, text and pics.

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