How do helicopters hover?

Question

I know that helicopters blades are like rotating wings that generate lift in the same way, but how does this make a helicopter hover?

Other question; how does the tail rotor help with the balance of the helicopter and why does it have to be on the left side of the vertical stabilizer instead of in the center?

Answer 1

To understand how a helicopter works (and in fact the mechanics of other aircraft too), it's helpful for you to be familiar with a key concept in physics, called Newton's Third Law of Motion. It says, in general terms, "for every action, there is an equal and opposite reaction".

Here's a simple thought experiment. Picture yourself wearing a pair of rollerskates, standing up, holding a basketball. Now, throw the basketball out in front of you using a two-hand pass (bring it to your chest and push it out in front of you). You'll find yourself moving backwards on the skates. You won't be moving as fast backwards as the ball will be moving forwards, because you have more mass than the ball, but you'll move, because the force of you pushing the ball out forward created an equal force moving you in the opposite direction.

A helicopter, and in fact most powered aircraft, work the same way. when a helicopter's rotor moves through the air, it pushes air downwards through a combination of its airfoil design, which moves air downwards as it passes over the top of the rotor, and an upward angle to the blade relative to the oncoming air that pushes the air with the underside of the rotor. By Newton's Third Law, the force pushing the air downward creates an equal and opposing force pulling the rotor, and the helicopter attached to it, up into the air.

To hover, all that's required is for this force pulling the helicopter up (you can consider it as either "lift" or "thrust") to equal the force of gravity on the helicopter, which is its weight. If these forces balance, the helicopter will remain the same distance off the ground. If lift is greater then weight, it will rise into the air, while if weight it greater, it will sink or fall.

Now, normal aircraft have to move forward through the air in order for their wing to get the airflow needed to generate lift. A helicopter, by rotating its wing, can produce the airflow over the lift surface without the entire helicopter having to move forward (or backward or sideways for that matter). So, it can rise straight up into the air and truly hover over a single spot on the ground.

The tail rotor of a helicopter is necessary to counter another force produced by the helicopter; torque. Torque is similar to force - really it is a force - but it's being produced in a circular direction instead of a straight line. The helicopter's engine uses torque to spin its rotor. Again, by Newton's Third Law, if the rotor of the helicopter is being spun clockwise by the engine, the helicopter itself will begin to spin counter-clockwise.

To counteract this torque, most helicopters have a second, smaller rotor which works the same way as the big one, but directs the force of its thrust sideways in the opposite direction to the way the helicopter's body wants to spin.

This sideways rotor doesn't necessarily have to be on the tail, but it works best if it's on the end of a long lever arm, such as the tail boom of the helicopter. The closer it is to the main rotor's shaft, the more thrust it has to produce in order to counteract the torque of the main rotor.

It doesn't have to be on the side of the tail either; many modern helicopter designs have the tail rotor spinning inside an aerodynamic duct in the middle of the tail:

Helicopters also don't necessarily have to have a tail rotor at all; all that's required is equal, opposing torque to that of the main rotor. A common substitute for a tail rotor is a second, counter-rotating main rotor:

Answer 2

The major forces acting on the helicopter are the same as that of the aircraft.

• The helicopter wings generate lift which is perpendicular to the rotor plane (the planes in which the rotor rotates).
• This lift vector is rotated to the forward to get the thrust.
• The weight of the helicopter acts vertically downwards.
• The drag is opposite to the direction of movement of the helicopter.

^{Source:www.danubewings.com}

At hover, this lift is simply equal to the weight of the helicopter. There is no forward movement and thrust is equal to zero.

In aircraft, the forward motion of the aircraft creates airflow over the wings, which in turn generate lift. In helicopters, the blades are rotated to generate airflow over the blades. However, the rotation of the blades causes a torque to act on the helicopter, which has to be balanced to prevent the rotation of the helicopter.

There are various ways to counteract this torque. In one of the most widely used methods, introduced successfully by Igor Sikorsky in VS-300, this balancing moment is provided by the tail rotor.

^{"Heli tail rotor dia" by FAA - FAA - H808321 Rotorcraft Flying Handbook. Licensed under Public Domain via Wikimedia Commons.}

There is no need for the tail rotor to be in any particular side of the vertical fin. For example, the Mi-8 had it on the starboard (right) side.

^{Source: www.military-today.com}

Mi-17, which was developed from Mi-8, had the tail rotor shifted to the port side of the vertical fin.

^{Source:www.forceindia.net}

Some helicopters like the EC-145 has the tail rotor in the middle of the vertical fin, an arrangement called the fenestron.

^{Source: blogs.star-telegram.com}

However, the helicopters need not even have a tail rotor for hover, The necessary anti torque can be generated by multiple ways, like:

Tandem rotors in case of CH 47

^{Source: www.helis.com}

Coaxial rotors in case of Ka 50

^{"Russian Air Force Kamov Ka-50" by Dmitriy Pichugin - http://www.airliners.net/photo/Russia---Air/Kamov-Ka-50/0920728/L/. Licensed under GFDL 1.2 via Commons.}

Intermeshing rotors like K-Max

^{Source: www.ainonline.com}

NOTAR system like MD 520N

^{"Md500n.g-smac.arp" by Adrian Pingstone - Own work. Licensed under Public Domain via Commons.}

Hiller HOE-1 used a tipjet

^{Source: www.aerospaceweb.org}