RC Helicopter

radio controlled helicopter (RC) is a model aircraft whose characteristics are different from a radio controlled aircraft due to differences in construction, aerodynamics and flight experience.

The radio – controlled helicopter is a machine with complex mechanics, which unlike other flying machines, can perform flights in all directions (forward, backward, sideways) and remain perfectly stationary (hovering) above of a point.

The radio controlled helicopters in the hands of experienced pilots are able to achieve excellent results (3D flying and aerobatics ) that are not feasible with life-size helicopters (with exceptions like the Tiger of Eurocopter or Bo 105 , capable of real exploits). There is also a category “Model” where the goal is to resemble as much as possible the real model by dressing the mechanics of a fuselage and accessories (multi rotor head, navigation lights etc. ..)

History

Early models of helicopters reduced engine (clockwork or spring) have existed from the xviii th century, but they were obviously not radio controlled. After a long research phase, it was in 1907 that the helicopter was able to lift with a pilot, to really take off after 1945, eclipsing the models.

The first true radio controlled helicopter was designed in 1969 by Dieter Schlüter 1 . This model won the competition Harsewinkel ( Germany ) for which the inventor received 1,000 DM 2 . He managed to fly his helicopter on two flights of 5 seconds , 3 meters from the ground 2 . The first model sold was the Bell Huey cobra Schlüter , nicknamed the famous Bell 222 , released in 1977. It was the first radio-controlled helicopter to start without a belt. He set a new record (unofficial) with a flight of 10 min 36 2. The official record, from June 1970 , with this same model was 27 min 51 s of flight on a circuit of 11,5 km 2 . The Jet Twin of Graupner equipped with an engine of 10 cm 3and the collective was the first radio-controlled helicopter to cross the Channel 2 . Then, many models were marketed before arriving at the helicopters currently on the market.

Since then, many prototypes have been built and tested until 1980, when Graupner radio-controlled helicopters made their appearance. The use of these machines became accessible to a large audience from 2000 , where brands like Sylverlit designed relatively inexpensive and low power electric helicopters for flying indoors or in a garden.

Operation

Aerodynamic operation

Swashplate : Plunging view of a simple model plate :
1 fixed ring (blue)
2 movable ring (silver)
3 bearing
4 control rod (pitch)
5 control rod (roll)
6 pitch rod (towards the blade )

A radio-controlled helicopter operates like a normal sized helicopter : the main rotor is the main propeller that allows lift, it is controlled by the swashplate . The anti-torque rotor or counter-rotating rotors are devices intended to prevent the helicopter from rotating in the opposite direction of the main rotor.

Flight Controls

The various controls of the helicopter are ensured by small servo-controlled engines . The radio control must be able to manage:

  • the angular velocity of the main rotor (commonly called “throttle control”, it controls the rate of climb so the altitude);
  • the swashplate (control of pitch and roll , as well as the rate of climb when the helicopter is equipped with a collective pitch );
  • the anti-torque device ( yaw control ).

The control of the swashplate is done by means of the general pitch control (also called collective step ) held by the right hand of the pilot on the remote control in the case of “mode 1” (French standard) ‘or by the left hand in the case of mode 2 (American standard), there are 2 other modes of control in the world which are modes 3 and 4.

The throttle and pitch control is usually multiplexed on the same channel of the remote control. In the case of a helicopter with fixed pitch , only the gasses make it possible to control the altitude of the aircraft .

The control of the yaw is generally done using the anti-torque rotor by changing the pitch or the speed of rotation of the tail rotor . A gyroscope can be a steering aid in stabilizing the yaw of the helicopter.

These different commands make it possible to make the helicopter perform most of the maneuvers of an airplane and others that are not possible for it, such as hovering and in reverse, they are thus similar to real helicopters 3 .

Helicopter birotor contrarotative

Main article: Counterrotating rotor .

With two coaxial lift rotors (rotating around the same axis) like the Dragonfly 53 and the Micro 47G . These helicopters are particularly intended for the learning of the piloting of radio-controlled helicopter. They are more stable and easier to control than normal helicopters: they can be piloted by novices. They have two superimposed rotors rotating in the opposite direction and do not have a tail rotor: this avoids the use of an anti-torque rotor 4 .

Helicopter birotor in tandem

This model has two tandem lift rotors (one behind the other). This system in which the rotors rotate in opposite direction allows to cancel the reaction torque of the rotor on the cell. Its main promoter was the American Frank Piasecki .

Flight Types

Radio-controlled helicopters operate almost exactly like helicopters .

Physical appearance

As with aircraft, the relative speed of air and canopy generates a mechanical action that allows the craft to fly. There are two components of this aerodynamic action:

  • the drag that is the resistance to advancement. Its action on the rotor of the helicopter results in a couple tending to rotate the device around its axis, hence the need for an anti-torque device.
  • the lift that lifts the device.

The control of a device is then based on the management of this lift. While on the aircraft, control surfaces allow to modify the lift of the wings to turn, climb …, on the helicopter, as on wind turbines , we modify the pitch and inclination of the blades. However, there is a difference: if on the aircraft we act individually on each rudder , the helicopter controls the lift of a blade according to its position relative to the aircraft. This is the role of the swashplate, the main part of the flight control device.

Some types are more maneuverable than others (like the collective helicopters ). The most manoeuvrable models are often the most difficult to fly, but have better acrobatic abilities.

Movements

Hover

Main article: Hovering .

The rotor of the helicopter being driven at a constant speed, the vertical displacements of the helicopter are obtained by the only modification of the pitch of the blades. At this point in the flight, the lift of the blades remains the same on one revolution of the rotor.

There is a position where the overall lift is exactly the weight of the aircraft: the helicopter can remain motionless. If it is lower, the device goes down. If she is superior, he goes up.

Flight translation

For the helicopter to advance, a horizontal component force is needed. If we increase the lift of the blades when they are 90 ° before passing over the beam of the tail rotor (angular shift due to gyroscopic forces), their plane of rotation tilts forward, thanks to a beating articulation connecting each blade to the axis of rotation, and the inclination of the lift produces the necessary horizontal component. However, there remains a main vertical component that opposes the weight allowing the maintenance in the air, and the horizontal driving component generates the movement of advancement, so acceleration to a speed where the overall drag (resistance to progress of the helicopter) will balance with the driving component.

The principle is the same regardless of the direction of travel desired.

  • Hovering balance : liftand weight cancel each other out

  • Imbalance: the tail goes upwards, the helicopteris unbalanced

  • Steady movement: dragadds to lift and weight ; the helicopter advances

Autorotation

Main article: Autorotation .

This phase is an emergency procedure in the event of engine failure resulting in the main rotor. The principle is to go into gyroplane mode : in this mode, the blades turn spontaneously under the effect of the relative wind produced by the fall (from bottom to top, from the point of view of the helicopter) by producing a lift that slows significantly the fall of the aircraft when approaching the ground, and thus makes it possible to limit the breakage. For that, a command makes it possible to reverse the collective pitch. It remains a delicate maneuver, because the optimal orientation of the blades depends on the circumstances (speed of fall, current speed of the blades, …) and the success is not guaranteed, even if in theory the simple fact of leaving the blades in free mode is enough.

On a model helicopter, autorotation is not really one; it is rather the phenomenon of reel-brake (rotor driven by the air from bottom to top with the negative step) whereas on the life-size helicopters, the autorotation is a force of drive of the rotor (force autorotative) by the air threads from top to bottom on the part of the outer rotor disk and from the bottom up (anti-autorotative force) on the internal part of the disk, and this for a positive step: the “mini step” calibrated by the manufacturer . The speed of the rotor is self-regulating and determined by the drop rate, the mass and the dynamic parameters of the device (rotor and blades).

Manufacturing

Materials

  • The woods ( balsa , samba , plywood , pine …), are used for fuselages of thermal helicopters, light woods being preferred to woods such as pine used only as a stiffening structure;
  • The metals are found in tubes, control rods more or less rigid, even for the construction of the aircraft itself ( aluminum , and even titanium );
  • Resins, as glues, coatings, are used for the interlining of structures (such as nitrocellulose coating), or, with synthetic fiber fabrics, for the construction of fuselages or wings;
  • The polystyrene used for some heavier constructions but more rapid wing;
  • The cardboard and paper variety are also used;
  • The kevlar , carbon fiber or glass, are also commonly used for the fuselages and blades;
  • The depron is more and more widespread;
  • EPP ( Expanded Polypropylene ) is increasingly used for the production of small to medium-sized models that are highly impact resistant, for learning or slope flight .

The helicopters contain many moving parts, as the swash plate which allows the inclination of the rotor controls the direction.

The construction of a helicopter requires more precision than that of an aircraft because the helicopters, even the smallest ones, undergo important vibrations which can cause problems during the flight.

Motorization

Compressed air motor

The air motors are rarely used. They are mostly found in free flight.

Internal combustion engine

Internal combustion or “thermal” engines (usually 2-stroke single cylinders) are often used, some of them being unique models (star engines, multi-cylinders in line or V for aircraft); the OS brand engines are very popular due to their exceptional qualities (they rarely stall) 5 . They can be fed with methanol (self-ignition with incandescent candle), or even with gasoline (classic spark plug).

Methanol engines are often used with fuel containing nitromethane (CH 3 NO 2 ) from a few% to 20% or 25% in competition (significant increase in performance, increased power and cooling due to nitromethane). Methanol “thermal” helicopters are available in various sizes, 0.30, 0.50, 0.60 and 0.90 cubic inches (4.92, 8.19, 9.83 and 14.75 cm 3 , respectively) . The bigger the engine, the more it can drive a main rotor whose blades are long and therefore allow the liftan overall bigger and heavier camera. We are talking about various “classes” of helicopters: class 30 (engine 28 to 32, rotor diameter of 1 to 1.20 m , 3 to 4 kg ), class 50, class 60 and class 90 (rotor measuring up to to 1.80 m diameter for 5 or 6 kg ).

Helicopters with thermal engines are currently more common than those with electric motors whose development is more recent but whose success is increasing.

Electric motor

The electric motors (such as brushless motors or brushless ) are increasingly used, thanks to the progress of the accumulators. Batteries lithium polymer high performance are increasingly used.

The latest technological developments in the field of batteries have made it possible to increase the flight range of electric models. However, there is a risk of overheating and explosion of these new lithium polymer batteries 6 .

Electric RC helicopters were first used indoors, since they emit no smoke or toxic gas. Larger electric helicopters are now being manufactured for outdoor use, offering interesting layouts for acrobatic flying, they are becoming popular. Their low noise level makes them particularly suitable for flying in a residential environment where noise emissions can be limited, as in Germanyfor example. There are “thermal” helicopter conversion kits in electric models.

The smallest RC 7 helicopter is the Picoo Z , followed by the Micron FP helicopter , both equipped with a battery-operated electric motor.

Turbine

  • Gas turbine engine developed according to full-scale helicopter techniques. Initially fueled by propane gas, they are now powered by more traditional liquid fuels (oil, kerosene). They make it possible to obtain high onboard powers with respect to the mass of the engine (in exchange for a high consumption).

Electronics

  • The receivers, mounted in the helicopters, are the pendants of the remote controls , and distribute, according to the waves received, orders to the servomotors .
  • The actuators , often called “servo”, are the motors driving control surfaces using rods and cables. Their size, weight and power can vary greatly depending on their function and model size.
  • Accumulators are used to power all the electronics of a helicopter, even the engine.
  • Various circuits are built and used as needed. They can be used for the illumination of flying models, the launching of parachutes , sound effects …
  • The gyroscope, banché to the servo of antiouple allows to enslave the axis of antioupoup in a precise direction. In conservative heading mode, it allows to counter the action of the wind which would tend to put the helicopter in the axis of the wind. There are also gyroscopes to stabilize the pitch and roll axes (replacing a mechanical bell bar). We are talking about Flybarless.

Radio control

Main article: Model aircraft remote control .

Radiocommands

Graupner remote control .

Small fixed-pitch helicopters require only a four-channel radio ( gas , ailerons , control surfaces , flaps ) while for collective models it is necessary to have at least 5 channels, 6 being the most common. Because of the interaction between the various controls, the most sophisticated radios include mixed function settings such as throttle / collective and throttle / rudder.

Some are very sophisticated, allowing dual control, or having functions such as controlling the remaining flight time before the tank or battery is empty, or detecting any suspicious hardware behavior, and allow the user to do so. land your helicopter before the crash to avoid damage to the aircraft.

The main brands are Spektrum , Futaba , JR , Hitec , Airtronics , Sanwa , Multiplex .

Radio prices vary from fifty to several thousand euros .

Practices of the radio-controlled helicopter

Competitions

The acrobatic flight competitions for helicopters historically follow the rules of the International Aeronautical Federation (FAI), which for helicopters bear the name F3C. These include figures of flight and acrobatics.

An advanced form of RC Helicopter Piloting is called 3D. During a 3D flight, helicopters perform advanced acrobatic tricks, sometimes in freestyle or a series of predetermined figures by the organizers of the competition. There are many 3D competitions in the world, the two most famous are the 3D Masters in the United Kingdom and the eXtreme Flight Championship (XFC) in the United States .

Leisure

RC helicopters are usually driven by enthusiasts in the form of a hobby. The piloting of radio controlled helicopters is one of the hardest disciplines of model making. In order to gain complete control of such a device, it is necessary to train regularly. We must repeat the same gestures at each session to acquire good reflexes and of course always keep his cool.

In parallel, an excellent knowledge of the mechanics of his helicopter will allow the pilot to optimize the settings and thus to have a more stable and safe flight especially after a crash, something almost inevitable in the course of this learning.

It can be interesting to practice this leisure in a club where you can benefit from advice on investment in equipment (helicopter, radio control, battery, fuel, charger, etc. ), share his passion for helicopter with other enthusiasts and be able to train safely (thanks to the insurance associated with the registration to the club and the national federation, FFAM for example). It should not be forgotten that the blades of an RC helicopter with variable pitch can reach 200 km / h , which can be a danger if all the precautions are not taken.

Commercial and Military Applications

Radio-controlled helicopters are sometimes used for aerial photography, observation and surveillance. Some companies design RC helicopters for these specific applications.

Research also exist regarding drones , they can be accessible to students and individuals through competitions such as the International Aerial Robotics Competition  (en) (IARC) 8 or the Challenge minidrones ONERA 9 . One can see autonomous helicopter projects such as the MARVIN 10 project , or the DSAAV (MIT) 11 .

Economic Data

The price of RC helicopters can range from twenty to several thousand euros . Initial investment and maintenance usually have a higher cost than fixed wing machines. This can be explained by the presence of many machined parts, rotating parts subject to significant wear (mechanical transmission, gears, swashplate), more sophisticated electronics (mixing channels, gyroscope).

Chinese helicopters are very cheap but have a quality often perfectible. Many models RTF ( Ready To Fly  (in) , that is to say, ready to fly from their transmission output) on the market and allow for a patient and cautious beginning to taste the joys of piloting. In addition, in case of crash and breakage, often cheap plastic parts can be changed. The main problems of these helicopters are problems of design, materials (cheap plastic) and low electronic quality (tending to improve in recent years). It can therefore be very interesting to improve its rotary wing with metal parts of good quality, to use a better quality electronics (receiver, radio, servos) …

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