Helicopter

It was Leonardo Davinci's idea, to use a big, screw-like device (aerial screw, Figure 39) to “drill”

air and generate an airflow downwards, thus creating lift force oriented upwards.

Figure 39. Leonardo's “aerial screw”

This idea has grown in the first half of the XX century into the full scale and models/UAVs, but as helicopter construction is a pretty complex one (both natural and scale), it is not very common to be used as UAV. Helicopter's body mimics a dragonfly, but the nature of the generation of the lift and control is different than in case of insects.

A regular helicopter has a large rotor with at least two blades. Each blade can be rotated parallel to its length, this way changing the lift force generated by each blade. Moreover, each blade can be virtually rotated independently; thus, the main rotor can “vector” the lift, enabling the helicopter to roll and pitch.

To operate helicopter up and down (change total lift) one uses collective pitch, so changing all blades angle of attack (aforementioned rotation parallel to its length) increases or decreases total lift generated by the main rotor (Figure 40).

Figure 40. Collective pitch idea in helicopters (main rotor)

To roll and pitch, there is a way to change the position where each blade generates a larger lift, using so-called “cyclic pitch”. Angles of the blades are altered while rotating, dynamically (Figure 41), so the same blade generates different lift depending on its current position. That causes the lift to change its effective direction that no longer is perpendicular to the main rotor rotation surface.

Figure 41. Cyclic pitch idea in helicopters (main rotor)

The most notable part of the helicopter is a mechanism, that drives the main rotor and controls blades (rotor hub, Figure 42).

Figure 42. Rotor hub

In many micro drone constructions, the collective pitch is implemented with the change of the rotation speed of the motor driving the main rotor. This construction, however, excludes cyclic pitch thus rotating in pitch and roll axes is implemented other way (see below, Figure 44) and usually limits the number of rotation axes the device can use while in the air.

Additionally, base helicopter construction has a tail rotor (anti-torque), and its main responsibility is to compensate force generated by the main rotor. The tail rotor is perpendicular to the main rotor and pushes or pulls the tail, thus also enables the helicopter to yaw (Figure 43).

Figure 43. Anti torque tail rotor

In the full-scale helicopters and large UAVs, main rotor and tail rotor are usually driven parallel, as the rotation of one impacts another. Hence, the tail rotor has rotatable blades that can change the force generated even at the constant rotation speed. In case of the tail rotor, all its blades are controlled parallel, that is different than in case of the main rotor where each blade can be virtually controlled independently. In the case of smaller UAVs, the tail rotor motor is usually separate from the main rotor motor and controlled independently of the main rotor with an electronic controller. In large scale helicopters, the tail rotor can be exchanged with the air-jet outlet of the hot gases leaving turbine (or turbines) that drive the main rotor: this seems to be a more efficient solution, as it uses exhaust gases to implement anti-torque force, but also problematic to control thus not very common.

10.2.2.2.1. Dual main rotor helicopter

Torque compensation can be implemented using counterrotation. There are two known solutions:

▪ coaxial - counter-rotating, pretty common in small RC models (Figure 44), but is also present in full scale ones: Kamov Ka-50;

▪ tandem - as in well known Boeing CH-47 helicopter - popular Chinook (Figure 45).

Small scale RC helicopters are usually simplified on their construction, not to include tail rotor classically, while rather equipped with two rotors mounted on the coaxial shaft, driven by separate electric motors and counter-rotating. This way torque of one rotor is compensated with a torque of another one. This construction is additionally equipped with a tail rotor that is parallel to the main one, driven by separate, 3rd electric motor, that enables the helicopter to pitch.

This construction can only pitch and yaw, cannot roll. Yaw is implemented with a change of the relative rotation speed of two main rotors (Figure 44).

Figure 44. Miniature RC helicopter with coaxial, counterrotating two main rotors

In commercial helicopters, coaxial, counter-rotating main rotors are used in lightweight constructions.

Another solution is used in the heavy lifting helicopters with two main rotors mounted on their endings, so-called “tandem”, like i.e. CH-47, where there is no tail rotor at all. Still, both main rotors provide collective and cyclic pitch (Figure 45).

Figure 45. Two rotor helicopter (CH-47 Chinook) 10.2.2.2.2. Flybar

Many scale helicopters introduce the flybar: a coaxially mounted bar, usually with extra mass by its endings, mounted over the main rotor (sometimes parallel, sometimes perpendicular), to stabilise small models, where rotor blades present small inertia, due to their low mass thus causing instability in flight. Introducing a flybar increases helicopter stability but also lowers its manoeuvrability and response.

10.2.2.2.3. Pros and Cons

Each airframe has features and drawback. Here we discuss the most noticeable ones.

Pros:

▪ Helicopter can hover;

▪ Can even move backwards;

▪ There is no minimal turn ratio (as in fixed-wing) and it can pivot in place;

▪ Uses vertical take-off and landing (VTOL).

Cons:

▪ Active generation of the lift (far less efficient than a fixed-wing);

▪ Main rotor failure causes immediate fall, still, there is a rescue procedure called autorotation but so far hard to implement in scale models;

▪ Complex mechanics and servicing.