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Ing. L. Koutný: Racer Horkey Mustang

On last Openscale model of pylon racer Horkey got a lot of attention. We asked the designer and builder rolled into one to write a bit more about the model for server. So now you can enjoy information given by Ing. Lubomír Koutný.

Most of pylon races in Unlimited class are won by modified WWII fighters. Similar situation is with the airplanes breaking world speed record for propeller-driven airplanes with piston engine. In almost all cases those were modified American fighters Mustang or Bearcat. Big problem these days is to find veteran in a good shape. Even bigger problem is to find suitable engine and tune it to reach necessary 3000HP. The development of piston airplane engines was on top after the war. Since then the highest speeds are reached by airplanes with jet engines. These days the development in piston engines is on top in F1 race cars.

So the new interested people in race competition try to develop small airplanes equipped with F1 engine. So far these types (DESIGNE N, TSUNAMI, Rutan’s POND racer and others) had no success. Even famous designing offices (e.g. Sukhoi and his STRELKA) develop their projects. And last but not least is the former head designer of Noth American company and his projects. Probably the most interesting is twin HORKEY.

The main designer of North American company says that he comes from his fighter P-81 Twin Mustang of similar concept, but new racer is much lighter (75%), has no armament and designed for high speed in low altitudes. These requirements are respected in the design. The airplane has arrow wing, it respects area rule and uses the most powerful engines – turbocharged engines originally designed for F1 race cars. That’s projects author quotation.

The airplane is exceptionally interesting not just because of the arrow wing, but it has different fuselage nose lengths, so the fuselages axis can be close and propellers discs intersect in front view. This solution is interesting for rubber powered miniscale as it is desirable that propeller diameter is as high as possible.

However, the low quality 3-view shows exhaust pipe on the nose, which is suspicious in turbocompressor. The question is where is the turbocompressor located, possibly just behind the engine, where mechanical compressor would be. But when studying the 3-view and estimating the weights and moments we find that arrow wing is too far back and real airplane would be nose heavy. Nose heavy was the model and I was saving every milligram on propellers.

Color scheme was another challenge. I asked the biggest expert in US – Bill Hannan, but not much help from there. We agreed that there is no substantial cooler on the airplane. Airplane would have surface cooling then with corresponding metallic surface – no composite. That’s what Bill concluded: the plane should be in metal color (alcad), with black anti-glare strip, black propeller blades and black registration. The plane should reach over 1000km/h so the thousand should be there preceded by N. As it is a racer, there should be starting number on the wing, in different color, e.g. red. As it probably was number one in the competition, it is there…

Horkey has large control surfaces and rubber band hinges far back, but the combination of arrow wing and long nose lead to nose heavy plane. I had to balance it behind controls with plasticine. Both rubber bands are the same as P-30 Maruska ones (4 threads of TAN SS 2005 rubber, 750mm long). Maximum number of rolls at 20 degrees Celsius is about 2100. The trimming was done in three stages. The first one ended with the finding that airplane has too small incidence angle. Home modification (back outer parts of elevator were incised) and another trial. The first flights were with about 600 rolls, plane flew flatly on the horizon with no turn, finally stall and fall. I removed some plasticine from the back, but that lead to direct fall with the rubber motor running. Third stage started with incision of skewed rudder trailing edges and bending it inside, which further increased incidence angle. That helped, the plane started to behave more normally, only the propeller axes suppression had to be decreased (which is unusual, normally it is the other way round). The plane is still very sensitive for any change in control surfaces and those slowly return into their original positions, so before each flight it has to be carefully tuned. After tuning I had some great flights with 1200 rolls with height of about 20 m and rubber motor about one minute plus nice gliding, everybody would take such a flight on competition. One flight with 1400 roles was a real beauty with two large 360 degrees turns.

The building is not different from common miniscale. Model is not suitable for beginners. I do not know the exact bones weight, maybe there will be something on the plan, I usually write it there (I expect about 40 g), two bands 9.5g each, covering + paint about 8 g, 3 g of ballast on the tail. If I build this plane again, I would put the band hinges 5 mm in front of tail leading edge – no balancing, I would move the wing 10mm front and increase incidence angle by moving leading edge 3mm up compared to original plan. I thing that even the real plan could not have the COG in correct position with the shape according the 3-view I had.