I remember that article with the A-10 and the two cut props. They suggested stacking them together. I thought that would have been kinda hard on the motors if you didn't run a bushing through them to make them a set pair, and/or key them together and also reducing the hub thickness for better balance. Machining out a key into each hub to lock them together, bushed, reamed and balanced. Then you'r left with a possibly a weak blade after all anyway. But, it might have a place if things don't fan out.

Here's the specs on the kit I'm tracing:

Berkeley Models Inc.
Douglas F4D-1 Skyray
Manufactured: 1957
Scale: 3/4" = 1'
Wingspan: 24"
Free-Flight Wt: 10-1/2 oz.
Effective Wing Area: 250 sq. in.
OK Cub .049S or Cox .049 Thermal Hopper (shown)

Box reads: "First kit model with internal ducted fan to produce jet power. Henry Struck has been experimenting and testing this type of propulsion for the past five years, - resulting in this highly refined ducted fan design, developing six ounces of static thrust, - ample power for free-flight or controline. Out-of-sight flights were common on original. Landing gear optional."

The kit came with full-sized plans, die-cut and shaped balsa parts (nice cutting), die-cut white cardboard forms for the inner and outer thrust tubes, waterslide decals, vacu-formed canopy, nose and inner thrust tube cap, covering (00 Silkspan), brass screw package and ready-to-install metal propulsion fan.

It is scaled down very well, with lots of .020" sheeting (fuselage and leading edges) The thrust tube build up is interesting also, with an inner thrust tube supported by six curved straightening vanes by the impeller, and four at the outlet. I believe there are three other D/F's in Berkeley's series, an .049 F8U Crusader, .020 Tigershark, and an .049 Tenco.

Not a bad kit, and now that servo weights are under 0.3 ounces it could be easily converted, from the designed free-flight or Stanzel's Monoline control unit, into a two microservo elevon mix no problem, or maybe even up to four with rudder and throttle, or the new lightweight parkflyer retracts...