`` NEWTON Autorotation
 
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Name: Cynthia
Status: student
Grade: 6-8
Location: NY
Country: USA
Date: Winter 2013-14


Question:
Paper helicopter/ whirlygig/ whirlybirds: How does autorotation work and how does the body affect this?

I have recently become very curious with toys called whirlygigs/whirlybirds/paper helicopters. These toys are not often used and so I cannot find much information about how they work. Here is how a paper helicopter looks like: http://www2.mae.ufl.edu/haftka/eoed/Project_files/image002.gif or http://www.stephanieblakey.me.uk/models/Simple-Helicopter/Helicopter-thumb.jpg

So far, I have managed to learn that paper helicopters are more similar to gyroplanes/autogyros/gyrocopters rather than helicopters and that the paper helicopter spins to the ground using a process called auto-rotation.

What I am confused about is what auto-rotation is and how it works. It does not make sense that because of air resistance the blades spin. After all, should not air resistance simply lift the blades up? Why does air resistance turn the blades? Also, I have found that paper helicopters generally have a square body. However, in some models, the body is angled (the first picture of a paper helicopter has a square body whereas the second picture has a more trapezoid body). I have tried making both models and found the more angular model to fall more slowly, but I am uncertain as to why this happens. How and why is it that the body affected how slowly I saw the paper helicopter fall to the ground?

Replies:
Cynthia

Essentially, auto-rotation is a means of rushing air through the rotor blades (like a windmill) while falling to keep the rotor blades spinning and then converting the energy in the spinning rotor blades to lift at the bottom of the fall to soften the impact with the ground. Air resistance of the rotor blades to the wind flowing through them keeps the rotor blades spinning, building their momentum to be converted to energy at the bottom of the fall to soften the impact with the ground. Auto rotation in a helicopter or gyro copter is the equivalent of a fixed wing aircraft gliding to a landing with the engine at idle or off.

Gyro copters have free spinning rotor blades that are not connected to an engine. The gyro copter relies on air blowing through the rotors to make them spin to develop lift, so they need a means of developing that wind velocity to spin their rotor blades to develop their lift. Gyro copters without engines have to rely on a tow from an airplane with an engine to get up to altitude so they can free fall through the wind to develop sufficient energy to soften their impact with the ground.

Some modern gyro copters of today come equipped with engines. The engines provide the forward thrust necessary for the gyro copter rotor blades to spin and create lift. Please see the video at this URL:

http://www.youtube.com/watch?v=AptB9l-ajSA

In engine powered helicopters, where the rotor blades are connected to an engine, there are mechanical links that break the connection of the rotor blades to the engine when the engine seizes (fails). Once free of the seized engine the rotor blades keep spinning while falling through the air. About 100 to 50 feet above the ground, the pilot changes the pitch of the rotor blades to full "up" pitch to change the rotor blade momentum to maximum lift to soften the impact with the ground. This is a routine technique practiced over and over again by helicopter pilots to perfect their auto-rotation technique. But if a helicopter loses a rotor blade at altitude, it impact the ground at full speed.

After looking at the photo of the paper copter, the body part that offers the least resistance to the air (drag) as it spins would least drag the spinning velocity of the craft and provide a longer flight while the rotor part that provided the most drag would provide the longer flight. Compare this to the description of a classical parachute.

Sincere regards, Michael B. Stewart


Hi Cynthia,

This is certainly a complex subject, but I will try to explain best I can.

A helicopter will generate lift because the blades are at an angle of attack to the oncoming air. That is, the relative wind hits the blade at an angle from below. It is similar to how a fan is designed.

In autorotation, the helicopter is descended vertically, and instead of air being forces down by the blades, air is now traveling upward through the blades. Because there is some angle between the blades and the oncoming air, a force is applied on the blades in the plane of rotation as well. This is the force that causes the rotation.

If you look at the blades such as in your picture, you can divide the length of a blade into two regions. You can call the middle of the blade the "driving region" and the tip of the blade the "driven region". In the "driving" region, the upward airflow is primarily devoted to causing rotation of the blades. In the "driven region", the paper blades tends to bend a little more in response to the drag force, increasing the angle of attack with the oncoming air. This part of the blade generates most of the lift.

The FAA (www.faa.gov) published the Helicopter Flying Handbook which has a discussion on autorotation with plenty of diagrams, and it is free for download from the FAA site. I would really recommend checking that out. Best of luck.

Regards, John C. Strong


Hi, Cynthia

Thank you for your excellent question. I am an electrical engineer who has some interest in aircraft. I cannot completely answer your questions but I believe that I can partially answer, and help get you pointed in the correct direction.

I agree that "air resistance" is too simple an explanation. I can tell you this much:

The rotor of a helicopter is actually a rotating wing. I would like to point you to Wikipedia for some explanation of that: http://en.wikipedia.org/wiki/Wing

I believe that the helicopter rotor also functions (at least partially) as a rotating air screw. Helicopters are actually quite complex. In normal flight while moving forward, the "pitch" of the rotor blades changes back and forth. Think about it: As the rotor rotates during forward flight, on one side of the aircraft the blades are moving into the (wind caused by aircraft forward motion.) On the other side, they are moving away. This means that the rotor air speed on the left side of the helicopter is different from that on the right side. The helicopter needs to compensate for this to avoid an imbalance of lift. It does so by changing the pitch of each blade back and forth with each rotation of the rotor.

When a helicopter autorotates, it is very much like a glider gliding. I understand that landing safely without engine power (gliding in other words) is one of the first things that a helicopter pilot learns. I myself have still have some questions as to how an autogyro (or gyroplane) works, and I think you would like to understand those questions as well. I hope that I have successfully pointed you in the direction of answering your questions.

Best regards, Bob Zwicker


Hi Cynthia,

Thanks for the question. For a great introduction on autorotation, I would like to refer you to the Wikipedia webpage ( http://en.wikipedia.org/wiki/Autorotation_%28helicopter%29). I would also recommend looking at the references at the end of the article.

The more angled model will fall slower to the ground since the larger angle of attack provides greater lift.

I hope this helps. Thanks Jeff Grell


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