Flying the Model:

The A-7 was my second attempt at a ducted fan model. The plane flew fantastically on a TD051 powered RK-049 fan. It climbed out smoothly and with authority. I used the central tank and a 2 oz. auxiliary fuel tank mounted above the RK-049 duct. The A-7 could stay airborne for over 15 minutes. No throttle was used. The plane is very stable in part to the swept wing, and scale anhedral. Dead-stick landings are simple and without drama. It comes in on a steady decent path, rock solid with no tendency to drop a wing tip.

Construction Highlights:

Here are some close up pictures: 1) Fan unit installed in fuselage, 2) Back of fuselage showing exhaust duct, 3) hatch removed showing servo installation.

The general outline is scale. The only deviations are the wing airfoil was thickened, and the fuselage cross section was simplified for easy building (fast!) The exhaust region was increased in size to get the fan-air out. The full flying stabilator is not used, instead a fixed stabilizer and a movable elevator was used. The plan of the stabilator was not changed. The ailerons are scale and work great. The wing is integral to the fuselage. The canopy is removable. The airfoil is semi-symmetrical from the root to the leading edge extension, and flat bottom from the leading-edge extension to the tip.

The model is made from Contest-Grade balsa and 1/64" plywood. The Main bulkhead, used to mount the RK-049 fan, is a wood/Carbon-Fiber (CF) laminate. This could also be made from plywood, if the CF plate is not available. The main bulkhead includes integral stub spars for the wing. The fuselage is built on a flat surface that is elevated about 3" from the table (use two 2x4's stacked on top of each other) All of the formers can be glued to the fuselage sides in one step.

The wing spars are 0.007" Carbon-Fiber strips. The stabilizer and fin have 0.007" Carbon-Fiber strips that are glued to the surface and act as external spars.

There is a hatch on the bottom of the fuselage that is removed for starting. The hatch is held in place with two simple nylon latches. With the hatch removed the whole RK fan unit can be accessed. A standard Sullivan 12 volt starter fits inside!

An extra air inlet is provided on the bottom of the hatch. DO NOT delete this hole. While some might call it a "cheater" hole, this is really a misconception. At the low airspeeds of model aircraft (.05-.15 Mach), the inlet needs to be much larger than the swept area of the fan rotor. Full-size jets suffer significant thrust loss at low airspeed due to their small inlets. Those inlets are optimized for cruise speeds of .75-.85 Mach. Look at early jets and you will see many doors on the inlets to increase airflow to the fan. No one called those "cheaters", you shouldn't either.

Nozzle exit area is crucial to a successful ducted fan. Nozzle area controls the thrust created; too much and there is no thrust, too little and high back pressure will cause the fan to stall. It is best described as a balancing act. I have shown the area that worked for the RK-049 fan unit. It is about 80% of the duct area. You will have to tweak this for each unique fan unit. I used cardboard at the field to trim the tailpipe area!

The only real downside of this plane was, of course, the TD051. A great engine when it starts, but a general pain most of the time. I ran 65% Nitro with two head gaskets. The problem is you could never get the small 5-40 screw tight enough on the rotor. I was forever spinning the rotor off the front of the engine. I ran a pressure system that easily flooded the engine. I ended up starting the TD051 with the glow-head loose. Sort or like a diesel engine. Once the engine started I cranked the head tight and adjusted the needle valve to get the right mixture.

Great candidate for a MGM-709 Fan or EDF

The MGM-709 fan will drop right in to this fuselage and is a direct upgrade to the RK-049 fan. The only modification required is to revise the bulkhead to match the rear profile of the MGM-709 duct. The prototype MGM-709 fan unit ran great. Starting the TD-09 is much easier.

The inside fuselage width is 3.5" so there are some choices for EDF. The model flew at ~26-30 oz. with the estimated 16 oz of thrust. I would recommend one of the larger 3" diameter EDF units. The interior volume of the A-7 is substantial. There should be no problem with battery placement.

About the Chance-Vought A-7 Corsair II

The A-7 was an attack aircraft that resembled the much sleeker F-8 Crusader. It was, in fact, an entirely new design. With its non-afterburning turbofan it was a new subsonic aircraft in the super-sonic age. Its purpose was to carry a large load and drop it accurately, which it did for many years. It also holds the honor of being one of a few Navy aircraft adopted by the Air Force for use in Vietnam. Because of the dual-service, there are a multitude of color schemes from Navy Dove Grey/White to full on jungle camouflage. The Navy aircraft carried wonderful tail logos and stripes that really show up on that huge vertical fin.

Now obsolete and retired the aircraft remains as an excellent candidate for modeling. High, shoulder mounted wing, cavernous fuselage, it is surprising more models of it have not been made.