Why does helicopter fly

Because helicopter fuselages are much less aerodynamic than their fixed-wing counterparts for the same weights , this source of drag can be very significant [ 1 ]. The parasite power can be written as. In addition, when calculating the power required of the helicopter, the required power of the tail rotor must also be calculated.

It is calculated in a similar way to the main rotor power, with the thrust required being set equal to the value necessary to balance the main rotor torque reaction on the fuselage. The use of vertical tail surfaces to produce a side force in forward flight can help to reduce the power fraction required for the tail rotor, albeit at the expense of some increase in parasitic and induced drag.

The power needed to rotate the main rotor transmits to the main rotor from the engine through the transmission Figure But the main rotor cannot get all the power, which is developed from the engine, as part of it is spent for other purposes and does not go to the main rotor.

This part of the power of the motor that is transmitted to the main rotor is called available power. It is defined as the difference between effective power and total loss. Excess power—this is the difference between the available and the power required. The rotor downwash is unable to escape as readily as it can when flying higher and creates a ground effect.

When the rotor downwash reaches the surface, the induced flow downwash stops its vertical velocity, which reduces the induced flow at the rotor disk Figure Influence of ground effect on the induced flow. Figure 15 shows the effects of this on the power required to hover.

If the hover height in ground effect must be maintained, the aircraft can only be kept at this height by reducing the angle of attack AoA so that the total reaction produces a rotor lift exactly equal and opposite to weight. It shows that the angle of attack is slightly less, the amount of total rotor thrust is the same as the gross weight, the blade angle is smaller, the power required to overcome the reduced rotor drag or torque is less and the collective control lever is lower than when hovering out of ground effect.

Influence of ground effect on the rotor drag. These conclusions are also true to flight in ground effect other than the hover, but the effect is smaller. Autorotation is an emergency mode. In the case of vertical autorotative descent without forward speed without wind, the forces that cause a rotation of the blades are similar for all blades, regardless of their azimuth position [ 2 ]. During vertical autorotation, the rotor disk is divided into three regions as illustrated in Figure 16a : driven region, driving region, and stall region.

Figure 17 shows the blade sections that illustrate force vectors. Force vectors are different in each region, as the relative air velocity is lower near the root of the blade and increases continually toward its tip. The combination of the inflow up through the rotor with the relative air velocity creates different aerodynamic forces in each section along the blade [ 2 ]. Autorotation regions in a vertical descend and b forward autorotation descend. Force vectors in vertical autorotation.

In the driven region, illustrated in Figure 17 , the section aerodynamic force T acts behind the axis of rotation. This force has two projections: the drag force D and lift force L. In this region, the lift is offset by drag, and the result is a deceleration of the blade rotation. There are two sections of equilibrium on the blade—the first is between the driven area and the driving region, and the second is between the driving region and the stall region. At the equilibrium sections, the aerodynamic force T coincides with the axis of rotation.

There are lift and drag forces, but neither acceleration nor deceleration is induced [ 2 ]. In the driving region, the blade produces the forces needed to rotate the blades during the autorotation.

The aerodynamic force in the driving region is inclined slightly forward with respect to the axis of rotation. This inclination provides thrust that leads to an acceleration of the blade rotation. By controlling the length of the driving region, the pilot can adjust the autorotative rpm [ 2 ]. In the stall region, the rotor blade operates above its stall angle maximum angle of attack , causing drag, which tends to slow rotation of the blade.

Autorotative force in forward flight is produced in exactly the same scheme as when the helicopter is descending vertically in still air. However, because of the forward flight velocity there is a loss of axial symmetry in the induced velocity and angles of attack over the rotor disk. This tends to move the distribution of parts of the rotor disk that consume power and absorb power, as shown in Figure 16b. A small section near the root experiences a reversed flow; therefore, the size of the driven region on the retreating side is reduced [ 1 ].

Helicopter stability means its ability in the conditions of external disturbances to keep the specified flight regime without pilot management [ 3 , 5 ]. Let us consider the longitudinal motion of a helicopter on the hovering regime Figure Longitudinal motion of the helicopter in hover. Recall that a helicopter, like any aircraft, is considered statically stable, if it after a deviation from the steady flight regime tends to return to its original position.

Suppose, for example, that as a result of the action of a wind gust U the thrust T is deflected backward see Figure 18b.

Under the action of the horizontal component, the helicopter will start to move back with a speed V x , and under the action of the moment M it will start to rotate relative to the roll axis, increasing the pitch angle with the angular velocity q see Figure 18c. Both effects: both the translational velocity and the rotation of the fuselage, and hence the axis of the rotor, will cause the resultant forces T on the rotor to tilt to the same side, opposite to the original inclination.

This will cause the appearance of a horizontal component and a longitudinal moment, already oppositely directed, due to which the helicopter will tend to return to the initial pitch angle and to zero forward speed. This means that the helicopter is statically stable in pitch angle and hover speed. Its static stability is due to the properties mentioned above: speed stability and damping. Consider, however, the further movement of the helicopter.

The inclination of the resultant in the direction of parrying disturbance is too great because of the presence of velocity stability. It leads to the fact that the helicopter in its movement to the initial position skips the equilibrium position and deviates in the opposite direction, but already by a large magnitude. The motion of the helicopter takes the character of oscillation with increasing amplitude.

The aircraft, which in the free disturbed motion ultimately leave the initial equilibrium state, is called dynamically unstable. Thus, a helicopter on a hovering regime is dynamically unstable. The roll motion on the hover has a similar character. The difference here is manifested only in the period and the degree of growth of oscillation, which depend on the moments of inertia of the helicopter, different in pitch and roll.

The helicopter is neutral in the yaw angle and the altitude on the hover. This means that the helicopter does not tend to keep a given course angle or a given flight altitude. At the corresponding disturbances these parameters will change. But their change will continue only as long as the perturbation is working. At the end of the disturbance, the course angle and altitude will not change.

It can be said that the helicopter is stable with respect to the yaw rate and the vertical speed. This stability is explained by the fact that the main rotor at an increase of the airspeed in a direction opposite to the thrust reduces its thrust, and conversely, when this speed decreases—increases the thrust, thus creating a damping force in the direction of the axis of rotation. Therefore, the tail rotor creates a large damping yaw moment on the helicopter, and the main rotor—a damping force for vertical helicopter movements.

In forward flight, the efficiency of helicopter control and the derivatives of the damping moments and moments of stability with respect to the main rotor speed vary insignificantly. However, the moment derivative with respect to the angle of attack, which for the main rotor corresponds to the instability, begins to play an important role. This instability can be compensated if the fuselage of the helicopter has a stabilizer, which improves the desired degree of stability in the angle of attack.

But it is difficult to provide satisfactory longitudinal stability even with well-designed stabilizer. In the forward flight, the roll movement is strongly connected with the yaw movement, just as it does on the airplane. The own lateral motion of a single-rotor helicopter during a forward flight, as a rule, is periodically stable. In the low-speed modes, while the relationship between the roll and yaw movements is still small, and the roll motion, like the hovering, is unstable, the lateral motion of a single-rotor helicopter is unstable.

Static stability of helicopters with two main rotors differs slightly from the stability of the helicopter with one main rotor. The tandem main rotor helicopter has a significantly greater longitudinal static stability, and the coaxial main rotor helicopter has a greater lateral stability.

This is explained by the change of main rotors thrust at a disruption of the equilibrium. So, the helicopter, essentially, cannot maintain a steady flight regime.

There are four basic controls used during flight. They are the collective pitch control, the throttle, the cyclic pitch control, and the antitorque pedals Figure Basic helicopter controls. The collective pitch control changes the pitch angle of all main rotor blades. The collective is controlled by the left hand Figure As the pitch of the blades is increased, lift is created causing the helicopter to rise from the ground, hover or climb, as long as sufficient power is available.

The variation of the pitch angle of the blades changes the angle of attack on each blade. The change in the angle of attack causes a change in the drag, which reflects the speed or rpm of the main rotor. When the pitch angle increases, the angle of attack increases too, therefore the drag increases, and the rotor rpm decreases. When the pitch angle decreases, the angle of attack and the drag decrease too, but the rotor rpm increases.

To maintain a constant rotor rpm, which is specific to helicopters, a proportional alteration in power is required to compensate for the drag change. The purpose of the throttle is to regulate engine rpm if the system with a correlator or governor does not maintain the necessary rpm when the collective is raised or lowered, or if those devices are not installed, the throttle has to be moved manually with the twist grip to maintain desired rpm.

Twisting the throttle outboard increases rpm; twisting it inboard decreases rpm [ 2 ]. The correlator is a device that connects the collective lever and the engine throttle. When the collective lever raises, the power automatically increases and when lowers, the power decreases. The correlator maintains rpm close to the desired value, but still requires an additional fine tuning of the throttle.

The governor is a sensing device that recognizes the rotor and engine rpm and makes the necessary settings to keep rotor rpm constant. Under normal operation, once the rotor rpm is set, the governor keeps the rpm constant, and there is no need to make any throttle settings.

The governor is typical device used in turbine helicopters and is also used in some helicopters with piston engines [ 2 ]. The rotor control is performed by the cyclic pitch control, which tilts the main rotor disk by changing the pitch angle of the rotor blades. The tilting rotor disk produces a cyclic variation of the blade pitch angle. When the main rotor disk is tilted, the horizontal component of thrust moves the helicopter in the tilt direction.

Figure 20 shows the conventional main rotor collective and cyclic controls. Let's not even worry about getting back down for the moment -- up is all that matters. If you are going to provide the upward force with a wing, then the wing has to be in motion in order to create lift. Wings create lift by deflecting air downward and benefiting from the equal and opposite reaction that results see How Airplanes Work for details -- the article contains a complete explanation of how wings produce lift.

A rotary motion is the easiest way to keep a wing continuously moving. You can mount two or more wings on a central shaft and spin the shaft, much like the blades on a ceiling fan. The rotating wings of a helicopter function just like the airfoils of an airplane wing, but generally helicopter airfoils are symmetrical, not asymmetrical as they are on fixed-wing aircraft.

The helicopter's rotating wing assembly is normally called the main rotor. If you give the main rotor wings a slight angle of attack on the shaft and spin the shaft, the wings start to develop lift. In order to spin the shaft with enough force to lift a human being and the vehicle, you need an engine, typically a gas turbine engine these days. The engine's driveshaft can connect through a transmission to the main rotor shaft.

We agree, Jauquin! We bet it took a great deal of hard work, planning and determination to build that helicopter! It's pretty awesome to see it flying with the help of the pilots!

We really liked today's Wonder, too, Azhir! We think the students and the pilots worked together like a team to reach their goal! We learned so much from today's Wonder and we're glad to hear that you did too! We Wonder if you will create something like a human-powered helicopter in the future!?

We certainly agree, Pablo! We bet it takes a great deal of determination to succeed- we hope those students and the pilots win! These students and pilots have really tried their hardest, great point, Kamaria! We hope they are successful and win the prize for their awesome invention! We bet they are working hard to create a safe, soft landing for the helicopter and the pilot! Wasn't that an amazing Wonder video, Henry!? Collin and Henry must be very powerful to get the helicopter so high off the ground!

Great point, Carla! We hope that Collin is okay, but we bet he jumped right back in the helicopter after they repaired it!

Very cool! You've got a great idea of how a helicopter works, Aniyah! Thanks for sharing your comment with us! We bet you'd LOVE the Wonder video today-- it shows a group of engineers who are working on a human-powered helicopter!

Thanks for stopping by Wonderopolis today! Great Wonder, Mrs. Reasor's Class! We bet you'll enjoy checking out this site that explains the original helicopter designed by Igor Sikorsky!

We bet you'll enjoy learning about clay animation, but we're so excited that you're WONDERing about cartoon animation, too!

WOW, thanks so much, Wonder Friend curiosity! We're so glad that today's Wonder made you feel like you were floating in thin air! We hope you'll join us for more fun very soon! Great question, Jack T! We love applesauce here at Wonderopolis! We believe applesauce is somewhere in between, as it's made of a solid but pulsed, mashed and blended into a liquid substance. Check out all the new information you've learned today, Wondergirl!

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Follow Twitter Instagram Facebook. How do helicopters work? What can helicopters do that airplanes cannot? What are some of the special jobs helicopters can do? Wonder What's Next? Tomorrow's Wonder of the Day has ears, but it can't hear a thing! Be sure to grab a friend or family member to help you explore the following activities: Want to make your own helicopter? First, you'll need some basic things to get started.

A powerful engine would be a good place to start. Then you'll need several hundred pounds of high-strength steel…What? Don't have those things around the garage? Not to worry! Here's a simpler version you can try instead. With just a few common items, you can make a paper toy that behaves just like a mini-helicopter! If you could fly anywhere in the world, where would it be?

The North Pole? The South Pole? A Caribbean island? Once you've settled on a destination, give some thought to HOW you'd like to fly there. Would you rather fly on an airplane or a helicopter? Make a list of pros and cons of both airplanes and helicopters. Share your list with a friend or family member.

Do they agree with you? Why or why not? What is the largest helicopter? How about the fastest helicopter? Do your own independent Internet research about helicopters. Try to find the answers to these and any other interesting helicopter-related questions you can think of.

Share what you learn with a friend. Did you get it? Test your knowledge. What are you wondering? Wonder Words aircraft sleek incite blade rotor hover capability military ambulance mobility aeronautical engineer amaze bulky lift troops runway prototype Take the Wonder Word Challenge.

Join the Discussion. Dec 2, Sounds like you need to dig a little deeper by taking a Wonder Journey, simon! May 29, That's a great question. We don't know the answer. And my dad was in the army in a Chinook helicopter. He did it for 14 years. Apr 25, That's super cool. Thank your dad for his service! My dad works for sikorsky global helicopters in coatesville pennsylvania and he is a helicopter mechanic and I love to visit his job. That's awesome, william. What is your favorite part of a helicopter?

Jacob Cook Apr 24, If you say it enough times, the word helicopter sounds kind of weird. Kensington Jones Feb 22, Sep 4, Hi, darian! We encourage you to take a Wonder Journey to find out!! Nov 4, Wonderopolis Apr 11, Higgins' Class Jan 3, We are learning about force and motion. How does a helicopter use force and motion?

We really liked the picture of the first helicopter. Is it difficult to peddle the human helicopter? How does the helicopter go up? Higgins' class. Wonderopolis Jan 3, DeJanae R Nov 14, Wonderopolis Nov 14, We hope he was okay, Seto K!