Cord aerobatic model aircraft. Drawing, description

MODELING
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Cord aerobatic model aircraft. Tips for a modeler

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When designing a cord flight model of an aircraft, one has to solve a whole range of problems in search of a reasonable compromise between such parameters as stability and controllability. The fact is that the main requirement for aerobatics is high maneuverability when performing figures with a large margin of stability in level flight. As a rule, these conflicting requirements can only be "joined" if the individual qualities of the athlete and his inherent piloting technique are taken into account.

A simple example. If you have learned to fly models that have a larger gear ratio from the control stick to the elevator, then there is no point in switching to less sensitive control, borrowing a successful, but "foreign" aerodynamic scheme.

For the model of the generally accepted scheme (with flaps and elevator), the ratio of deviations of the control stick and the corresponding rudder can be considered as a criterion for control sensitivity. For example, if moving a stick to a certain angle causes the steering surface to rotate twice the angle, then the control sensitivity is said to be two. For current models, these values range from 1 to 2,5.

The optimal flap control sensitivity is 88% of the same elevator control setting.

Issues of aerodynamic layout. Each athlete, as a rule, develops a whole system of knowledge about the influence of the geometric parameters of the model on its flight performance. When developing the next model, the control sensitivity of the previous one is taken into account and, based on this, the power, weight of the model, specific load are selected, the wing and horizontal tail area is selected. After that, you can proceed directly to the layout.

Rice. 1. Cord flight model A. Kolesnikov (click to enlarge)

Fuselage layout. When performing a complex of aerobatic maneuvers, the fuselage of the model is constantly in the field of view of the judges. This makes it necessary to carefully select its contours, giving the model not too rapid forms. It is also important that the cockpit, propeller spinner and chassis wheel dimensions are commensurate with the dimensions of the pilot's model.

The layout of the wing and horizontal tail. Maneuverability and flight stability mainly depend on the correct selection of wing parameters - its elongation, narrowing, sweep, relative profile thickness, as well as on the location of the stabilizer relative to the aerodynamic focus of the wing.

As practice has shown, there are several ways to solve the optimal selection of these parameters. So, for example, to increase maneuverability, a wing with a small sweep is selected, the removal of the engine and horizontal tail is reduced (that is, the mass spacing is reduced), wings of large elongation are used.

Greater maneuverability is also provided by shifting the center of gravity back with a simultaneous increase in the stabilizer area. This aerodynamic layout is inherent in the models of American athletes.

Rice. 2. Kinematic diagram of the control system (click to enlarge)

Since stability is the inverse of controllability, the increase in the former is facilitated by measures opposite to those listed above and aimed at increasing maneuverability. The optimal ratio of stability and controllability is selected by the athlete during the design of the model and test flights.

The choice of specific load. The load, related to the total area of the wing and horizontal tail, usually ranges from 27-32 g/dm2. If the model is operated with a cord length of 17-18 m, then the load should not exceed 28-29 g/dm2, and with a cord length of 20-21,5 m - 30-32 g/dm2.

Aerodynamic and static balancing. The quality of the flight of the model largely depends on them. For balancing, the model is provided with fastening points for cargo, located on the wing consoles and in the rear fuselage. Structurally, they are bushings with internal thread M4 or M5. For the same purpose, a node is provided on the inner console that allows you to displace the openings for the output of the cord from the wing.

Model construction. Of all the models I have previously built, the drawings of which are now in front of you, seem to me the most successful. Its characteristic feature is the location of the engine axis, wing chords and horizontal tail unit on the fuselage construction horizontal. This ensures the same slant angles of the stream entering the stabilizer in both normal and reverse flight.

The wing is single-spar. The spar has a variable section: 3 x 7 mm at the root and 2,5 x 4 mm at the end. The necessary strength of the wing is provided by the walls glued between the buttocks of the spar. The ribs are cut from balsa plates 2,5 mm thick, lightened and reinforced along the perimeter with a shelf. The leading edge of the wing is covered with veneer 2 mm thick - this allows you to strictly maintain the profile between the ribs.

Along the trailing edge, along the entire span of the wing, flaps are differentially deflected at angles of +30° and +34°. They are carved from balsa plates and edged with a 2 mm thick pine lath.

The stabilizer, elevators and keel are also made of balsa with a density of 0,09 g/cm3. The area of the elevators is 65% of the area of the horizontal tail. The maximum deflection angles of the rudders +45°.

The model has a two-wheeled landing gear with a tail wheel. For ease of transportation, the main landing gear is made easily removable.

When finishing the model, synthetic and polyurethane paints were used.

The power plant is a home-made engine with a three-channel purge. Working volume - 7,5 cm3. The fuel system of the engine has an automatic stop, triggered by a jerk with a control knob with a force of 15-17 kgf.

I hope that my development will be useful for aerobatics modellers and will help them in designing models of their own design.

Author: A.Kolesnikov

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