Helicopters

It’s been a while since I last wrote something about planes. My new job had occupied me so much that I could not write about these wonderful machines J Since you all know pretty much about planes, I decided to write something related to Helicopters this time.

The word 'helicopter' is adapted from the French hélicoptère, which originates from the Greek helix = 'spiral' or 'turning' and pteron = 'wing’.

Since its been a few months after I updated this blog, I would suggest you to read my earlier post about how an aircraft flies and all that because -the basic concepts of flight no matter fixed wing or rotary wing, remains the same.

It’s said that flying a Helicopter is far more tough than flying a plane !! This is because of the numerous Stability Controls that the pilot has to take care of while flying it.

Helicopters have rotating blades and a tail rotor as the rotating parts on it. The Elevator is another important part. The rest of the anatomy of the craft is as shown. The Helicopter's spinning blades not only lift it into the air, they are also a means of propulsion-more of a combined wing and propeller. Each of the long, thin blades-the number which varies from 2 to 8, has the aerofoil shape of an aircraft fixed wing and the leading edge of the wing is angled upwards at a small angle.

To lift the helicopter off the ground the blades are rotated and their pitch-the angle at which they meet the airstream- is gradually increased. As a result, air pressure decreases above each blade and increases underneath it, providing the upward force. When the lift beneath the blades is greater than the weight of the helicopter, it rises into the air. This is how it lifts off from the ground.

Once it lifts off the ground, how does it gain height?? Applying more engine power increases the collective pitch of all the blades and lifts the craft higher.

Once the helicopter is airborne, flight is controlled by adjusting the amount and direction of lift created by the blades. The pitch can be altered by two controls - collectively by the Collective Pitch Lever or separately by the Cyclic Pitch Column.

The Collective Pitch Lever angles all the blades at the same pitch. It is used by the pilot for vertical ascents and descents. As the pitch is increased and more lift is generated, the throttle is usually opened up automatically to provide the necessary extra power.

The Cyclic Pitch Column adjusts the pitch of each blade individually as it sweeps around. If the column is moved forwards, each blade’s pitch increases as it passes over the tail and decreases as it passes over the front of the helicopter.

This provides more lift over the tail than the front of the helicopter which makes the whole rotor tilt forwards. The helicopter is propelled in whichever direction the rotor is tilting. So, if the rotor is tilting forward, the craft flies forward. To change the direction while hovering, the pilot can use two foot pedals to change the thrust of the tail rotor and swing the craft’s tail around.

The Tail Rotor is a very small part of the whole craft, but one of the most important parts!! The tail rotor provides the directional control at low speeds and in Auto rotation[ A condition where the engine has lost the power and the blades can be kept spinning to glide the machine down to the ground]. Also when the helicopter is hovering, it counteracts the tendency of the fuselage to move in the opposite direction to the blades- called as Torque Reaction.

For single engine helicopters, the tail rotor is essentially a torque eliminator. However some helicopters eliminate torque by using two rotors or by having two sets of blades spin in opposite direction along the same rotary axis.

So, now that controlling the pitch of the blades is known, how does the helicopter do all that maneuvers??

Flying Forward

Easing the Cyclic pitch column forwards gives more lift over the tail of the helicopter. This causes the rotor to tilt and propel the craft forward at a constant altitude.

Flying Sideways

The pilot just moves the Cyclic Pitch Column to the direction he wants to go, causing the blades to give more lift on one side than the other. The rotor tilts and propels the helicopter in the chosen direction.

Hovering

Now this is an interesting and entertaining show to see a helicopter hover. To hover, the pilot adjusts the pitch of all the blades with the Collective Pitch Control so that the amount of lift generated b the rotary wings just exceeds the weight of the helicopter.

Flying Backward

By easing back the Cyclic Pitch Column, the blades are given more lift as the pass over the front of the craft than over the back, so the rotor tilts backwards enabling the helicopter to fly backwards.

Versaitily but not Speed !!

The maximum speed capable by helicopters is around 400 Kmph. This is because around that speed, the blades are travelling at the speed of sound at which all the normal aircraft encounter drag and lift problems. The retreating blades are however travelling at a lesser speed than the forward speed of the copter and can scarcely generate any lift.

Although the engines provide abundant power, most of it is used for Lift. An Aero plane with a similar engine would fly much farther and much faster for the same fuel consumption. This is the tradeoff between Versatility and Speed and Fuel Economy.

Future

The problems with Stability and other design aspects led to the abandoning of helicopters for nearly 30 or more years in favor of fixed wing designs. However, with the invention of jet engines and other developments in fixed wing designs, the situation has changed. Though mot much can be speculated, the future of helicopter design seems to lie in a Hybrid Aircraft capable of acting as a part helicopter and part plane- called as a Tilt-Rotor Aircraft.