Can anyone help with the 2013 corse work research and questions on helicopters?

  • 0 votes

Question 1 is how the turning rotor makes a helicopter move upwards

Question 2 is how the rotor is made to turn in a helicopter and in an autogyro

Question 3 is how autorotation is used to help land a helicopter safely if the engine fails

Question 4 is why a helicopter, falling during autorotation, could reach a terminal speed without the pilot changing any controls

Question 5 is the effect of the weight of the helicopter on the terminal speed.

Posted Mon 7th January, 2013 @ 12:57 by Jessica Eayres

2 Answers

  • 6 votes

For the last question:

The helicopter has a force (gravity)  pulling it down  and a force (air resistance) pushing up.

Gravity causes a  constant acceleration regardless of mass.

Air resistance increases as velocity increases, As the helicopter goes down the air resistance is low . As the  velocity increases due to acceleration from gravity , the air resistance increases.

The force on the helicopter due to the air resistance at some point equals the force on the helicopter due to gravity. this point is known as the terminal velocity. then the net forces on the helicopter are zero and the helicopter experiences no further accelerations.

The force on the helicopter  due to gravity increases as the mass of the helicopter increases. Therefore  a large force due to air resistance is needed to counteract the force due to the extra mass.

Since air resistance increases as velocity increases, helicopters with a heavier mass will have a higher terminal velocity and hit the ground faster.

Increasing the mass of the helicopter, at any given speed as it falls, the air produces the same force upwards due to the drag produced on the wing at that speed. Since the mass of the helicopter is greater, from F = ma  The same force up (due to the air) will produce a smaller acceleration up (or a smaller deceleration down) due to the increased mass. Therefore, because there is a smaller decelerative force, the helicopter will accelerate downwards a little bit faster.

Answered Thu 24th January, 2013 @ 23:08 by Adam Boni
  • 0 votes

I am in the same situton as you, here are my first 3 answers so far, with sources, unfortunately I am struggling with question 4 myself and will not be able to offer you any help on that matter, anyway, without further ado;

How the turning rotor makes a helicopter move upwards?

To get the helicopter off the ground you need a lifting force. This is produced by the blades or rotors. As they go round they cut into the air and produce lift. Each blade needs to provide a share of the lifting force. If there are two blades they will provide half each. Five blades provide a fifth each. If you spin the blade at an angle against the air it will cause lift, and in still air, the helicopter will hover. If the rotor is tilted and allowed to spin, it will fly in the direction that you tilt it. To go straight up or hover, the blades have to be at the same angle. This is known as collective pitch. To produce a lifting force, air must be made to flow over each blade. This is why the blades rotate. The shape of the blade is very important and it is designed so that the air will move faster over the upper surface as the angle changes. The shape of the blade is known as the aerofoil. It is the difference in the pressure between the upper and lower surfaces of the aerofoil that creates the upward force known as lift [1]. A typical helicopter has a main rotor and a tail rotor. These are similar to aeroplane propellers in that they create a force by moving the air. The main rotor sits above the vehicle and lifts it. Control is achieved by varying the pitch (or angle) of the blades of the rotor relative to the air flow and thus altering the balance of forces on the aircraft. 'Collective pitch' is the term given to changing the pitch of all the blades of a rotor by the same amount, increasing or decreasing the total force on the helicopter. 'Cyclic pitch' is a change of pitch that varies through the 360° of the main blades' rotation [2].

How the rotor is made to turn in a helicopter and in an autogyro?

The helicopter blades are rotated by  motor fitted under the blades, the rotary motion produced leads to the rotation of the helicopter blades, this rotation provides the upwards lift needed to carry the helicopter off the ground an into the air. To get the blades to move fast enough to lift off the ground, a motor is used to either drive the autogyro along the ground until it reaches a great enough speed or to jump-start the rotor. This is accomplished by attaching the rotor to the engine until the blades are going fast enough that the craft rises. Then the pilot uses a clutch like mechanism to switch the engine power from turning the rotor to turning the propeller that pushes the craft forward.
Forward motion is necessary for the autogyro to gain altitude. Movement creates airflow, which turns the rotor blades [4]. 

How autorotation is used to help land a helicopter safely if the engine fails?

A landing via autorotation is also sometimes necessary if the rear rotor blades stop functioning properly, no longer countering for the torque of the main rotor blades, so the helicopter will spin if the engine isn’t turned off.  Whether this happens and the pilot shuts off the engine or in the case of actual engine failure, once the engine drops below a certain number of revolutions per minute, relative to the rotor RPM rate, a special clutch mechanism, called a freewheeling unit, disengages the engine from the main rotor automatically.  This allows the main rotor to spin without resistance from the engine [5].  Autorotation is a condition where the main rotor is allowed to spin faster than the engine driving it.  All helicopters are fitted with a free wheeling unit between the engine and the main rotor, usually in the transmission. This free wheeling unit can come in different forms but one of the most popular is the sprag clutch. The free wheeling unit will allow the engine to drive the rotors but not allow the rotors to turn the engine. [6]

Bibliography:

[1] http://archive.excellencegateway.org.uk/VLSP22/030/005/010/005/e_030_005_010_005.htm

[2] http://www.bbc.co.uk/dna/place-london/plain/A245611

[3] http://science.howstuffworks.com./transport/flight/modern/helicopter6.htm 

[4] http://www.unc.edu/~franco/autogyro/physics.html 

[5] http://www.todayifoundout.com/index.php/2012/05/helicopters-wont-just-fall-like-a-rock-if-the-engine-dies-they-usually-can-be-landed-safely-this-way/ 

[6] http://www.helis.com/howflies/autorotation.php 

[7] http://2.bp.blogspot.com/-9zJmWRqW4FU/TuWpjnOdwOI/AAAAAAAAAtA/gw6aIQk8cAE/s320/300px-Airflow_in_auto-2+%25281%2529.jpg  

Hope this helps.

Answered Tue 14th January, 2014 @ 18:45 by Jacques13