Helicopters - How do they fly?

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Helicopters – How do they fly?

With the ability to take – off and land vertically anywhere, from acting as a means of civilian transport to performing rescue missions in the deadliest of all war zones, these machines are one of a kind in the aviation industry. The concept of such a flying machine dates back to the era of Leonardo da Vinci. He came up with the concept of vertical flight with is design of the ‘aerial screw’ but was not able to prevent the rotor from making the whole body rotate. Major advancements in the following centuries were made with rotor crafts being used in small numbers by the Nazi Germany for transportation and medical observation.

With the use of turbine engines in the modern era, many shortcomings of the piston (weight and moving components) engines used previously were overcome. Below is the Boeing CH-47 Chinook with a max take-off weight of 22,680 kg or 50,000 pounds.

How do they fly?

A helicopter or a chopper works with similar principles to that of a fixed wing aircraft, where the wings are the rotor blades in this case.

These rotor blades contain the appropriate airfoil shape and the rotor rotates at the required rpm to generate the necessary lift.

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Now that gets the chopper to move vertically up or down, but what gets the chopper to move horizontally?

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Now the question arises about how to get the rotor to rotate at an angle to generate the required horizontal thrust? This is done by changing the angle of attack of each rotor blade with respect to its position. This causes a decrease in vertical thrust and an increase in horizontal ground airspeed.

The rotor blade pointing towards the rear of the chopper would be at a higher angle of attack compared to the rest of the blades and the one opposite to this would have a shallow or lower angle of attack to reduce the lift. This higher angle of attack increases the thrust on the rear-side of the chopper thereby pushing the chopper forward.

Similar type of movements are done by the rotor blades on all the other sides of the chopper by changing the angle of attack of the rotor blades to generate the required horizontal thrust.

If the chopper wants to roll right, when the rotor blades while rotating arrive by the left of the chopper, the angle of attack of that blade at that position increases, which increases the thrust generated by the left side of the chopper causing the chopper to roll right. The thrust on the opposite side or on the right side of the chopper decreases due to the shallow angle of attack.

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The change in angle of attack of the rotor blades are done by a mechanism called swash plate mechanism. It works with and interconnected disc attached to the axel of the rotor blades.

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Now what does the chopper do if it wants to yaw or rotate itself?

This is where Newton’s third law comes into picture. When the helicopter is in the air, the blades of the helicopter which generates the thrust for lift also generates an opposite force on the body of the helicopter. This causes the body/cabin of the helicopter to rotate in the opposite direction due to the torque generated by the blades, similar to what was encountered by da Vinci with his ‘aerial screw’ model.

This torque which causes the rotation is countered by the tail rotor. There are various other ways to counter this problem, but the most widely used is the tail rotor method.

FAA Rotor Flying Handbook.

Other methods of countering these are by using two rotors one above the other spinning in opposite directions called as coaxial rotors or by having another main rotor called as tandem rotors. Below is an image of the Kamov KA-52 with coaxial rotors and the CH-47 Chinook with tandem rotors.