Autogyros
An introduction, the physics behind them, and how they compare to other aircraft

This picture of an autogyro thanks to www.avnet.co.uk/gyro1.htm
By LaMont Council, Hazel Gray and Veronica Franco
for instructor Duane Deardorff, who teaches Physics 24 at UNC-CH
Link to project description here
Last updated 12/3/00

THE INTRODUCTION

To see what the 1929 edition of the Encyclopedia Britannica says about the autogyro and the helicoptor, click here.

  The most interesting aspect of autogyro flight is that it depends entirely on autorotation.  Autorotation is exactly what it sounds like—the rotor blades revolve automatically due to the movement of air.  And how does that work?  Well, the blades are normal rotor blades, the same as those found on helicopters.  They have an airfoil shape, also found on airplane wings, which can be seen in the picture to the right (Thanks to the NASA Ames Home Page at http://george.arc.nasa.gov/dx/basket/storiesetc/foilshok.html).  In a helicopter, however,  these rotor blades are attached to a motor.
    Autogyro blades work like the wings on a maple seed or the sails in a windmill.  Take the maple seed; when it falls, the wings spin the seed and so slow its descent.  This is nature’s example of autorotation.  There is no motor causing the seed to turn.  Instead, the air creates resistance in the wings, which makes the seed turn.  A windmill uses this principle of autorotation to harness energy.  Here, the sails are at a flat angle relative to the wind.  In this manner, the wind pulls them around and they rotate against the airflow.
     Like the windmill, the autogyro uses an angle to the wind of about two degrees relative to the plane in which they rotate.  Since the blades are in an airfoil shape, they turn into the airstream fairly easily.  To generate lift, the blades must be turning pretty fast.  This creates a lot of resistance to upward airflow, and this resistance provides lift.
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 and so on, as we have just discussed.  Since the rotor is not connected to an engine, if the pilot wants to land, all he has to do is switch off the motor pushing the craft forward.  Since the autogyro is still moving forward (even though it is decelerating) it still has autorotation and so still produces lift.  Once the craft slows down to about 15 mph, the lift generated is not enough to keep the craft in the air, and it slowly and gently perches itself on the ground.
    Now that sounds all fine and dandy, but I know you’re saying “show me how it works”!  Here is a vector diagram which came from www.engr.umd.edu/~jeffl/autogyros.html.

You can see that the blade, the small airfoil in the center of the larger diagram, is about two degrees off from the relative wind.  Let me state again that the airfoil used in the autogyro is the same shape as those used for other flying craft.  The relative wind is created by the forward motion of the craft and by the wind created by the rotor.  This is shown in the smaller diagram to the right.  To orient yourself, pretend the cockpit faces 180 degrees from the line labeled “relative wind due to aircraft movement”.  Obviously the craft is moving forward, and so creates that vector.  The spinning rotor blades also create some wind at about a 90-degree angle to the wind produced by the forward motion of the craft (remember that this is on an x-y-z coordinate system).  By adding these vectors, you get the resultant relative wind, which is marked “relative wind” in the larger diagram.
    The vector marked “lift” is at a 90-degree angle to the relative wind vector.  This makes sense, because as we discussed earlier, a resistance of the turning rotor blades to upward airflow creates lift.  Notice that the blade also creates drag.  The drag is parallel to the airflow.  By adding the drag and lift vectors, shown by the dashed line, you can see that the resultant force lifts the aircraft.

    The autogyro has a very unique way of flying, setting it apart from both the airplane and the helicopter.  The differences mainly come from the utilization of different basic principles to provide lift for the aircraft, and to propel them forward.
    The airplane uses two wings connected to its body to provide the lift needed during a flight.  The wings are designed so that when air passes over them, the speed of the air above the wings is greater than the speed of the air below the wings.  This causes the air above the wings to have a lower pressure than the air below the wings; this difference in pressure causes a net force upward, towards the air with the lower pressure, and lifts the plane.  An autogyro, on the other hand, uses a rotor similar to that on a helicopter to provide lift for the aircraft.  As explained earlier, when the blades turn they provide resistance to upward airflow creating a lift.  This is very different from the airplane’s flight mechanism.  There are several consequences to this difference.
    First, because airplanes depend on their wings to provide lift, in order for the airplane to increase its lift the wings must move faster.  Since the wings are connected to the plane, this means that the whole aircraft must move faster.  An autogyro, on the other hand, uses its rotor to provide lift, so it must only increase the speed of the rotors in order to increase its lift, not the speed of the body of the craft.  The autogyro can also fly at lower speeds than the airplane.  This is because the airplane has to increase its angle of flight in order to fly slowly.  At a slow enough speed, the airflow over the wings would not be sufficient to continue providing lift for the airplane.  At this point, the airplane would stall, and come crashing to the ground.  The autogyro, however, is unable to stall.  If the relative wind going through the rotor was not enough to maintain a lift, the autogyro would not come crashing to the ground but would slowly descend.  This is because as it fell, the rotor would slow down gradually; even a small amount of spin would maintain enough lift to keep the autogyro from crashing to the ground.
    The airplane has certain advantages as well.  While the autogyro can fly more slowly than most airplanes, it cannot fly at the same high speeds as an airplane.  At high speeds, the rotor of the autogyro produces a great deal of drag, making it unsuitable for such high speed flights.  Airplanes, however, can be modified to have small wings which produce less drag and allow them to make such high speed flights.
    There are also several fundamental differences between the autogyro and the helicopter seen to the left.  These differences again come largely from differences in the aircrafts’ basic flight mechanisms.
The autogyro and the helicopter seem very similar in their design.  Both have the rotor to provide lift, and both use a motor to propel them forward.  However, the biggest difference between the helicopter and the autogyro is the way the lift is powered in each aircraft.  In the autogyro, the lift is powered by forward propulsion.  This can be seen clearly in the vector diagram provided above.  When the plane moves forward using the propeller, a relative wind is created that goes through the rotors and lifts the autogyro.  The propeller is powered by a motor, but the rotor is powered only by the wind.  A helicopter, on the other hand, uses a motor to power both the movement forward and the lift; one motor-powered propeller is used for both functions.
    There are several consequences of this difference in powering.  First, less fuel is required for the autogyro; this is because the motor used only has to power propulsion and not lift for the aircraft.  However, this also means that the autogyro cannot hover; the autogyro must be traveling forward in order to maintain lift.  If it attempted to hover, the autogyro (below right) would gradually descend.  The helicopter does not have this problem because no forward motion is needed to maintain lift, only a functioning engine. 
    Another difference between the helicopter and the autogyro (this picture from www.avnet.co.uk/gyro1.htm) is the way that the air flows over the blades of the rotor.  In the autogyro, the air flows up through the blades, providing a lift and a small amount of propulsion forward; this can again be seen in the force diagram.  In helicopters, the air flows down over the blades instead of through them.  As a result, the airflow holds the blades back instead of pushing them forward.  In a helicopter, the pilot must angle the blades forward in order to provide propulsion forward.
    There are also other small differences between the airplane, helicopter, and autogyro.  Flying an autogyro is generally cheaper than flying an airplane or helicopter because less fuel is needed.  Also, if the engine fails, it is much safer to be flying in an autogyro than an airplane or a helicopter.  The airplane would not only be unable to fall slowly, it would require a large, clear area to land safely.  The helicopter has the potential to initiate autorotation, but it would be difficult and valuable height could be lost in the process; this too would not be very safe.  The autogyro, however, would not require a large landing area, and would be able to gradually descend; this makes it much safer.