How do helicopters tilt

This helicopter did not fly completely free due to its lack of stability;. He proposed the concept of cyclic pitch for rotor control;. Ellehammer designed a helicopter with coaxial rotors. The aircraft made several short hops but never made a properly flight;. He was the first specialist who described the helicopter autorotation;. He could be considered the most important person in the helicopter design. The helicopter is a complex aircraft that obtains both lift and thrust from blades rotating about a vertical axis.

The helicopter can have one or more engines, and it uses gear boxes connected to the engines by rotating shafts to transfer the power from engines to the rotors Figure 1. Typical helicopter drive train. The most common helicopter configuration consists of one main rotor as well as a tail rotor to the rear of the fuselage Figure 2a. A tandem rotor helicopter has two main rotors; one at the front of the fuselage and one at the back Figure 2b.

This type of configuration does not need a tail rotor because the main rotors are counter rotating. It was proposed by the Serbian man Dragoljub Ivanovich in The single main rotor a and the tandem rotor helicopter b. A variant of the tandem is the coaxial rotor helicopter Figure 3a which has the same principle of operation, but the two main rotors are mounted one above the other on coaxial rotor shafts.

This constructive solution was developed by Nicolai Ilich Kamov. Another helicopter type is the synchropter, which use intermeshing blades Figure 3b. This type of helicopter was proposed by Charles Kaman. The coaxial rotors a and the intermeshing blades b. If the two rotors are mounted either side of the fuselage, on pylons or wing tips, the configuration is referred to as side by side Figure 4. The side by side rotors. Another aircraft type that should be mentioned is the autogiro invented by Huan de la Cievra , which is a hybrid between a helicopter and a fixed wing airplane.

It uses a propeller for the forward propulsion and has freely spinning nonpowered main rotor that provides lift. The basic flight regimes of helicopter include hover, climb, descent, and forward flight, and the analysis and study of these flight regimes can be approached by the actuator disk theory, where an infinite number of zero thickness blades support the thrust force generated by the rotation of the blades [ 1 ].

The air is assumed to be incompressible and the flow remains in the same direction one-dimensional , which for most flight conditions is appropriate. Also, the main and tail rotors generate the forces and moments to control the attitude and position of the helicopter in three-dimensional space. At the plane of rotor, the velocity through the rotor disk is v i named the induced velocity and in the far wake the air velocity is w. The helicopter in hovering flight. For a steady flow, the above equation becomes.

This equation requires the condition that the total amount of mass entering a control volume equals the total amount of mass leaving it. The principle of conservation of fluid momentum gives the relationship between the rotor thrust and the time rate of change of fluid momentum out of the control volume. The left part of Eq. In projection on rotational axis, Eq. The induced velocity at the plane of the rotor disk is v hover ,.

The power required to hover is the product between thrust T and induced velocity v i ,. This power, called the ideal power, forms the majority of the power consumed in hover, which is itself a high power-consuming helicopter flight regime.

In assessing rotor performance and compare calculations for different rotors, nondimensional quantities are useful. The inclusion on the half in the denominator is consistent with the lift coefficient definition for a fixed-wing aircraft. The rotor power, C P , and rotor torque, C Q , are defined as. Considering the helicopter in climb, one can see that the flow enters the stream tube far upstream of the rotor and then passes through the rotor itself, finally passing away from the rotor forming the wake Figure 6.

When the helicopter leaves the hovering condition and moves in a vertical direction, the flow remains symmetrical about the thrust force line, which is normal to the rotor disk. The flow becomes very complex in a medium descent rate condition, but in climb, the mathematical approach is close to that used in the hover conditions.

The axial climbing flight. Applying the principles of conservation for mass, momentum, and energy like in the hover we get:. The left part of the above equation represents the square of induced velocity in hover, v h 2 , and replacing it, we get. The power consumed is given by the product of the thrust and the total velocity through the rotor disk, that is. The stream tube in descent. Even if the sign of thrust is negative, that does not mean that the thrust is negative, because the assumed sign convention consists of positive velocity w , in down direction.

According to the conservation energy principle, it follows that. The valid solution is. An approximation of the velocity in this region, called vortex ring state, could be [ 1 ]. Figure 8 shows the graphical results from this analysis, made in the Maple soft program.

Induced velocity variation. In the normal working state of the rotor, if the climb velocity increases, the induced velocity decreases and also, in the windmill brake state if the descent velocity increases the induced velocity decreases and asymptotes to zero at high descent rates.

In the vortex ring region, the induced velocity is approximated, because momentum theory cannot be applied. The flow in this region is unsteady and turbulent having upward and downward velocities.

During normal powered flight, the rotor generates an induced airflow going downward and there is a recirculation of air at the blade tips, having the form of vortices, which exist because higher pressure air from below the rotor blade escapes into the lower pressure area above the blade.

The rate of descent that is required to get into the vortex ring state varies with the speed of the induced airflow. Although vortices are always present around the edge of the rotor disk, under certain airflow conditions, they will intensify and, coupled with a stall spreading outward from the blade root, result in a sudden loss of rotor thrust.

Vortex ring can only occur when the following conditions are present: power on, giving an induced flow down through rotor disk; a rate of descent, producing an external airflow directly opposing the induced flow; low forward speed. Using Eqs. For the vortex ring state, we can use the approximation 31 for the induced velocity ration, therefore in this case, the power ratio is.

According to the power to power in hover ratio values, shown in Figure 9 , the power required to climb is always greater than the power required to hover, namely this ratio is greater than unity.

In descent flight, the rotor extracts power from the air and uses less power than to hover. Power required as a function of climb and descent velocity. Rotor in forward flight. Dividing Eq. The above equation can be very easy to be solved in Maple soft. The primary way to distinguish between different main rotor systems is represented by the movement of the blade relative to the main rotor hub.

The main categories are fully articulated, semi rigid, and rigid. In hovering flight, the blades flap up and lag back with respect to the hub and reach equilibrium position under the action of aerodynamic and centrifugal forces. In forward flight, the asymmetry of the dynamic pressure over the disk produces aerodynamic forces that are the functions of the blade azimuth position.

The hinges allow each blade to independently flap and lead or lag with respect to the hub plane. The lead-lag hinge allows in-plane motion of the blade due to the Coriolis and radius of gyration changing in flapping movement. Transition from hover to forward flight introduces additional aerodynamic forces and effects that are not found when the helicopter is in stationary hover. Due to the difference in relative airspeed between the advancing and retreating blades, the lift is constantly changing through each revolution of the rotor.

Figure 12 shows the flapping, lead-lag, and feathering motion of a rotor blade. Blade movement axis. In a fully articulated rotor, each main rotor blade is free to move up and down flapping , to move forth and back dragging , and to twist about the spanwise axis feathering. Semi rigid rotor has, normally, two blades attached rigidly to the main rotor hub and is free to tilt and rock independently of the main rotor mast, one blade flaps up and other flaps down.

The rigid rotor system cannot flap or drag, but it can be feathered. The natural frequency of the rigid rotor is high, so the stability is difficult to be achieved. The single rotor helicopters require a separate rotor to overcome the effect of torque reaction, namely the tendency for the helicopter to turn in the opposite direction to that of the main rotor.

It has the purpose to transmit cyclic and collective control movements to the main rotor blades and consists of a stationary plate and a rotating plate.

The stationary plate is attached to the main rotor mast and the rotating plate is attached to the stationary plate by a bearing surface and rotates at the same speed as the main rotor blades.

The neutral position of the cyclic stick changes as the helicopter moves off from to hover in forward flight. Trim control can adjust the mechanical feel in flight by changing the neutral position of the stick. Collective pitch lever controls the lift produced by the rotor, while the cyclic pitch controls the pitch angle of the rotor blades in their cyclic rotation. This tilts the main rotor tip-path plane to allow forward, backward, or lateral movement of the helicopter.

The power required for flight is the second work that must be transmitted to the shaft of the rotor. In general, for a helicopter in forward flight, the total power required at the rotor, P , can be expressed by the equation. Inductive power is consumed to produce lift equal to the weight of the helicopter. From the simple 1-D momentum theory the induced power of the rotor, P i , can be approximated as. The profile power required to overcome the profile drag of the blades of the blades of the rotor is.

The parasite power, P P , is a power loss as a result of viscous shear effects and flow separation pressure drag on the fuselage, rotor hub, and so on. Because helicopter fuselages are much less aerodynamic than their fixed-wing counterparts for the same weights , this source of drag can be very significant [ 1 ]. The parasite power can be written as.

In addition, when calculating the power required of the helicopter, the required power of the tail rotor must also be calculated. It is calculated in a similar way to the main rotor power, with the thrust required being set equal to the value necessary to balance the main rotor torque reaction on the fuselage. The use of vertical tail surfaces to produce a side force in forward flight can help to reduce the power fraction required for the tail rotor, albeit at the expense of some increase in parasitic and induced drag.

The power needed to rotate the main rotor transmits to the main rotor from the engine through the transmission Figure A helicopter that is flying forward can stop in mid-air and begin hovering very quickly. We'll cover this signature maneuver next. Sign up for our Newsletter! Mobile Newsletter banner close. Mobile Newsletter chat close. Mobile Newsletter chat dots. Mobile Newsletter chat avatar.

Mobile Newsletter chat subscribe. Prev NEXT. First, he or she nudges the cyclic lever forward. That input is transmitted to the lower swash plate and then to the upper swash plate. The opposite happens to turn left, and also forwards and backwards. As the rotor disk begins to tilt in the direction of the turn the aerodynamics change and the lift vectors move from being vertical towards horizontal and this begins the turn.

The more the pilot banks the helicopter, the more the lift vector tilts away from vertical, and the more the helicopter will descend. To counteract this, the pilot must raise the collective to increase engine power, increase the pitch of blades collectively and increase the lift. Once Lift and Weight are matched again, the aircraft will turn without climbing or descending.

This is a level turn. When rolling out from the turn, the pilot must remember to reduce power or the helicopter will have too much lift being produced and the helicopter will begin to climb once flying straight and level in forward flight. Flying all the controls in balance, at the right time and by the right amounts is what takes time to learn, hence the reason why students pilots look like they are all over the place in their early stages of helicopter lessons!

Turning in a helicopter seems simple but the aerodynamics and mechanical engineering to complete that task are complex. So next time you watch a helicopter turning in a hover or as it flies over you just have a think of the forces and mechanics involved for just doing something as simple as turning!

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