Why Is The Shape Of Helicopter Blades Important?

Helicopters fly with the help of rotor blades. The shape of these helicopter blades or rotor blades is not symmetric as it seems in the movies and pictures. These blades have a specific asymmetric shape known as Airfoil. More details about airfoil are discussed in this article later.

Helicopters are a part of the Rotary Wing family of aircraft. This is a simplistic explanation, but hundreds of books explain the answer to this question. The macro view of “How do helicopters fly?” looks quite easy, but tons of spare parts and their synchronous relationship make it into a sophisticated machine. Because of this, helicopter manufacturing becomes a sophisticated process. This is because different types of engineers work in the production of a single helicopter. Electrical engineers, electronic engineers, aviation engineers, computer science programmers, etc. are cardinal.

Helicopter With 3 wings

Bernoulli’s Principle :

To fly, an object must have a “lift”, which is nothing but an upward force. Wings in airplanes and blades in helicopters help in creating this lift. This lift is created due to the famous Physics theorem known as Bernoulli’s Theorem or Bernoulli’s Principle.

The Bernoulli Principle describes the relationship between the speed of air and the pressure in the air. When the speed of air increases, the pressure decreases, and when the speed of air decreases the pressure increases. You can read about Bernoulli’s Principle in detail here.

Now let’s answer, How Bernoulli’s principle and the shape of helicopter blades are interrelated. The wings of an airplane and the blades of a helicopter are curved on top and a bit flat on the bottom. This shape is called an Airfoil. This shape makes the speed of air over the top of the wing/blade faster than the speed of air under the wing/blade. As a result, the air pressure over the wings/blades is lesser as compared to the bottom of the wing/blade. this causes a movement from the high-pressure area to the low-pressure area and this makes the airplane/ helicopter move up.

The shape of helicopter rotor blades is similar to the wings of an airplane which helps to create the lift. An airplane has to move fast on the ground first to create the lift by letting sufficient air over its wings. Whereas, in a helicopter, this lift is created by the spinning blades. This is the reason why airplanes need a long runaway for flight and helicopters do not.

Shape of helicopter blades
Fig. 2 : Upward Lift

Helicopter Blades and Airfoil :

As discussed earlier, Airfoil is a shape (of aero plane wings and helicopter blades) that generates lift when an airplane moves forward through the air. Because of this shape (airfoil), the air flowing over the top of the wing accelerates and Bernoulli’s Principle that states any fluid exert less pressure as it moves faster, the object starts moving upwards.

Combining Bernoulli’s Principle with the pitch or deflection of the wing/blade to the oncoming air (angle of attack), we find a significant pressure difference, above and below the wing which causes lift.

Angle of Attack, Drag and Lift
Fig. 3 : Angle of attack

How Helicopter Moves Forward/Backward :

Just like an airplane faces up to get more lift from its wings, similarly, a helicopter uses a mechanism called a Swashplate to change the pitch/ deflection of its rotors to control the lift of its blades and direction of movement.

By changing pitch on all of the blades together, we can make the helicopter ascend, or descend and changing the pitch as they rotate to a specific side of the disc will cause the whole disc to tilt which helps in directional control. But when there is lift, there is drag and rotors are large enough to require a lot of force to turn.

This means that the engine must produce a lot of torque (rotational force) to turn them. But this also has a side effect of spinning the rest of the helicopter opposite to the direction the rotors are turning.

So to counter effect it, a Tail Rotor is used to push (or pull) on the tail to keep the helicopter straight and spin as per the requirement. When the thrust of the tail rotor is increased the helicopter will turn left and when thrust decreases the natural torque of the helicopter will turn right.

Lift Thrust Drag and Weight
Fig. 4 : Movement Of Helicopter.

Examples :

1. One can illustrate how wings work by doing this simple experiment. While riding in a car, with someone else driving, keep your arm straight out of a window (perpendicular to the car’s window) with your fingers together and your hand flat and palm parallel to the ground. You will feel the pressure of the wind pushing your arm back but keep your arm straight out. With your arm straight out and hand flat, rotate your hand so that your thumb moves upward. By doing this, you will quickly feel the force of the wind pushing your arm upward.

The more you rotate your hand upward, the more force will push your hand upward. If you rotate your hand down, the wind will push it down. This is what we are calling lift. Wings are designed in such a way to create lift effectively for the particular aircraft they are designed for.

2. One more example can be illustrated simply by blowing over the surface of a piece of paper. If we hold the paper up and blow horizontally over the top of it, it rises a little as shown in the figure below.

Illustration of Bernoulli's Theorem
Fig. 5 : Bernoulli’s Theorem Illustration

To get more info about this topic, and possibly with more detail in specific aspects of helicopter flying that you might be interested in, I’d recommend your reading some reference books in detail and, going to the root directory of the FAA’s Helicopter Flying Handbook, or exploring more pages of the Flying Mack.


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