Tommycat (oz5677)

About this Plan

Tommycat. Sport model for IC ducted fan.

Quote: "Tommycat. Inspired by the F-14 Tomcat by Grumman and designed by Bob Kress, this ship is distinctly different in looks and performance. It uses the new Axiflow RK-20B ducted fan and K&B 3.5 cc engine.

The Tommycat is the outcome of an attempt to combine the best of the aerodynamics of its big brother, the Grumman F-14 Tomcat, with a near-optimum, yet simple, ducted fan installation.

I had the good fortune to serve as the F-14 Engineering Manager from its design inception until its first introduction into US Navy operations. In addition to having the sophisticated long range Phoenix missile system and Mach 2.4 speed, it is probably the best maneuvering dogfighter amongst today's crop of high performance aircraft due to its swing wing. The fine maneuvering aerodynamics of the F-14 make it an attractive model for scale. On the other hand, it is structurally complicated and has difficult lines to model plus the complexity of the variable sweep wing. In spite of the foregoing, I know of at least one 1/10 scale project underway using two customized (slimmed down outer shell) RK-40 Axiflo fans. Simplification then was the keynote.

My first objective was to simplify the F-14 structure, lines and controls but retain its good aerodynamics. The key to the solution was to do away with the twin engine nacelles yet retain the aerodynamic top view, which is the major factor in the F-14's good maneuvering performance. The swing wing was fixed in its most stable 55 degree leading edge sweep, allowing a rather far aft CG to be used with safety. The wing tips were further extended to allow an even more aft CG with the added advantage of increased aspect ratio. The 55 degree sweep is also a good compromise between too high a leading edge sweep angle (max is 68 degrees on the F-14), which interfere with induced drag (too low an aspect ratio due to low span) and leads to too hot landings vs a lower sweep which results in more high end drag due to a thicker wing section. The Tommycat wing sections which result are a thin 8 percent.

The horizontal stabilizer was then moved up to the wing plane and joined to the wing by filling in the remaining wing-tail gap, thus forming a modified delta-elevon configuration. The filled-in wing-tail gap does not significantly affect the aerodynamics. The wing tip is twisted 5 degrees leading edge down, just like the full scale article. The ventrals were somewhat enlarged in area for increased directional stability and modified to perform the dual functions of fins and skegs to prevent the tailpipe from hitting the ground. Stripped off the original nacelles, the model is about as clean as you can get and is capable of high maximum speeds. The bent-up wing tips were a final adjustment to yield positive spiral stability at high speeds.

From a propulsion point of view, a podded installation is best, having the best inlet and the shortest internal duct. Recognizing that the resulting delta-like F-14 is quite thin in the aft center area, it was possible to cut away a part of the wing and replace it with the ducted fan, which still acts as a sort of 'ring-wing' lifting surface. The airfoil sections forward of the fan were then streamlined to that no separated flow would then enter the fan.

You might think that the fan would be swallowing 'bad air' off the fuselage/wing in front of it and thus suffer performance losses. True, it swallows bad air, but with the highly streamlined body and airfoils, it is only a thin boundary layer. The momentum losses of this boundary layer are accounted as model drag. The layer passes through the fan without upsetting the fan airflow and thrust. Only at high angles of 'attack at or beyond stall do gross airflow distur-bances enter the fan, but in that regime, you don't really care about thrust loss.

With a flight weight of 5 lb and 4 lb of thrust from a standard (no pipe or pressure) K&B 3.5 cc on 25% Nitro, the available thrust-to-weight ratio is high at .8 to 1. Furthermore, the thrust does not fall off as fast with speed as a prop, so high end performance is hot.

The only thing different about the Tommycat is the landing attitude which is a bit nose high, typical of deltas. But its flying qualities, like its forebear, are excellent at these attitudes so you can 'drag it in' with confidence. With 430 square inches of wing area (excluding the duct) the landing speed is quite moderate at the 5 lb flight weight. We did not use retracts on the prototype, but with such clean basic aircraft, the high end performance gain due to their use should be very dramatic.

During the flight testing we varied the wing tips, fins, ventral fins, center of gravity and control system until we could get sucessful and predictabile flights. The final configuration is shown on the plans, but some of the construction photos show the original version..."

Later kitted by Kyosho. See Kyosho Tommycat (oz8640).

Direct submission to Outerzone.

Update 24/05/2016: article pages, text & pics added, thanks to RFJ.

Supplementary file notes

Article pages, text & pics.

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