Helicopters are considered marvels of modern engineering, and while they are some of the most graceful and maneuverable aircraft in the world, they are also complex and challenging to operate. Unlike airplanes, which generate lift by using their forward speed to move air over their airfoil-shaped wings, helicopters utilize their airfoil-like rotors to move the aircraft. More than that, the rotors are spun by the engine, and they point up towards the sky to produce lift, not thrust.
As previously mentioned, helicopters use airfoils to make lift, and the rotor blades that spin above the vessel are also known as airfoils. In general, an airfoil is a shape commonly found in the cross-section of an airplane’s wings, serving to control the flow of air around them. More specifically, the primary purpose of an airfoil is to accelerate the air that passes over the top of each wing. While airplanes and helicopters make their lift in varying ways, they both produce lift using the same principles.
The foundation by which most modern flying machines make lift is based on Bernoulli’s principle. Essentially, this principle outlines how airfoils work. According to Bernoulli’s principle, the pressure exerted by a fluid is reduced as the speed of that fluid increases. In this example, the scientific definition of fluid is any substance that takes the shape of its container; thus, this definition illustrates how the principle applies to air and aircraft.
On an aircraft, the shape of the vessel influences the air flowing around it. The curved upper surface makes the air move faster across it, therefore reducing its pressure. Consequently, the pressure difference created between the higher pressure below the airfoil and the lower pressure over it is equal to the amount of lift being generated. As the rotor blades on a helicopter spin, the air above them is reduced in pressure, resulting in the production of a lifting force.
It is important to note that not all lift comes from Bernoulli’s principle. With this in mind, another vital consideration to make is the component of lift that comes from the angle at which the airfoil meets the air. This is known as the angle of attack (AOA) and can be manipulated by the pilot. When the angle becomes steeper, a majority of the air strikes the lower surface of the airfoil and bounces downward.
Although most lift is produced according to Bernoulli’s principle, when the AOA increases, more of the lift comes from Newton’s Third Law. Based on Newton’s “Third Law of Motion,” there is an equal and opposite reaction for every action. For instance, if air is being deflected downward off of the lower surface of the rotor blade, a lifting force opposite it is being created. While each rotor blade produces lift individually, the result is that the entire span of the rotor’s disc generates lift.
If the lifting force is pointed straight up and its total force exceeds the total weight of the loaded aircraft, the vehicle will rise. Following the same principle, if the amount of lift is reduced to match the force of the vessel's total weight, the aircraft will hover. Finally, if the lift is further reduced, the aircraft will sink. To achieve such ends, the AOA on the rotor blades is changed. The blade’s pitch controls the AOA, while a lever in the cockpit allows the pilot to control everything else by simply moving it up or down.
One of the most important things that pilots must master is the ability to control the amount of lift that their aircraft is producing. To get the vehicle off the ground, the pilot must make their aircraft produce a lift force greater than the force of gravity, and the exact amount varies based on the density of the air and the weight of the plane. When a pilot keeps their aircraft at a steady altitude during flight, this means that the lift force is precisely equal to gravity.
In a helicopter, pilots control the amount of lift in one of two ways. First, the pilot can make the rotor blades spin faster. Second, the pilot can increase each rotor blade’s AOA. There are, of course, other considerations to take into account. For example, the pitch of the blades is closely linked to the speed of rotation. That being said, as the AOA is increased, more power is required to maintain the speed of the rotors at a constant RPM. Furthermore, to increase the AOA, pilots pull the collective control.
The additional drag on the rotors from the increased AOA usually slows down their rotation, so the throttle must be increased to maintain a constant rotor speed. As a result, newer choppers are designed with the collective and throttle controls tied together. Keep in mind that by adding power to the rotors, torque is increased. For this reason, pilots must utilize the anti-torque pedals to adjust the turning tendency introduced as the throttle increases. If lift must be reduced to hover or descend, the aforementioned steps must be done in reverse.
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