- What is a Gyroplane
- Autorotation
- Gyroplane Rotor
- Dynamics
- Stability
- Advantages
- Disadvantages
- Conclusion
- Sources
Imagine your student comes to you with an idea for a new form of transportation. It’s an airplane and a helicopter combined, with a spinning rotor on the top and wings on the side. You look at their drawing and think, “There’s no way this thing could fly.” If you feel you can’t just smash two flying vehicles together like play-dough and expect it to work, then here is a 1926 photo of the one of the craziest-looking flying vehicles to change your mind!
This vehicle goes by many names, autogyro, gyroplane, and gyrocopter. These crazy contraptions have been around for almost 100 years and they are the ancestors of all modern helicopters. In this article, we will go into what they are, how they work, and why you don’t see them around anymore.
What is a Gyroplane?
Gyroplanes are rotorcraft that use an unpowered rotor to generate lift. They emerged in the 19-20s and 30s as some of the first rotary aircraft in the world. Their inventor was a self-taught Spanish engieer, Juan De la Cierva. The technical problems associated with rotary aircraft were first identified using these early gyroplane design. Gyroplanes first developed the cyclic pitch and roll control to adjust the angle of the force developed by the rotors. Modern gyroplanes consist of a keel, pod, mast, horizontal tail surface, vertical tail surface, rotor, and propeller.
Some Gyrocopters can be made in your garage from easy-obtainable materials like aluminum tubing. The Bensen B-8 gyrocopter was a homebuilt design that used a 72 Hp air-cooled engine, but could also be used as a towed glider with the engine removed. It took around 100-120 hr man-hours to construct and used plans published in magazines. The main structural components for this were a keel tube, an axle tube, and a mast tube, all built from 1/8 x 2 x 2 inch 6061-T6 aluminum tubing as shown in the image below.
One of the most famous gyroplane designs was used in the 1967 James Bond movie You Only Live Twice. The Wa116 was used in the James Bond Film and was a custom gyroplane designed and flown by Ken Wallis.
Gyroplanes were also used in WW2 German submarines to increase their observing range. The rotor blades are made from birch bonded to a steel strip running from the root of the blade to the tip. It uses a built-up D-section spar with birch skin covered in a doped cotton.
Here is a closer image that I took of this particular one at the Air and Space Museum.
Autorotation
Although these aircraft look like helicopters, they have one significant difference. The rotors on the top are unpowered and use the same principles as a falling maple leaf to keep the aircraft in the air or bring it into a soft, slow landing. This is called Autorotation, a state where the rotor is driven by only the aerodynamic forces acting on it.
The rotor blade cross-section is simple airfoil as shown below.
The velocity due to the spin of the rotor is different at the location on the rotor that it is measured. The rotor velocity, \(\Omega T\), is greatest at the rotor tip and zero at the axis of rotation, this also means that the relative velocity \(W\) is different at different places on the rotor.
When the rotor is spinning, the root of the blade is stalled, which produces only drag and not very much lift. At the tip of the rotor blade, the rotational velocity is the highest, and therefore, the drag component is high enough to pull the lift vector rearward. In the middle of the blade, the lift vector and angle of attack are just in the right places so that the lifting force acts ahead of the axis of rotation of the rotor. That means that the aerodynamic forces acting on these sections “pull” the rotor blade forward, maintaining its rotational speed.
This variation in the tilt of the aerodynamic force vector along the rotor span creates a balance between the accelerating forces caused by the forward-tilting component, which helps to spin the rotor forward, and the drag forces on the tip and root of the blade, which prevent it from spinning too fast. When these forces are in equilibrium, the rotor maintains a constant RPM. The image below shows another representation of the accelerating and decelerating forces.
The diagram below shows how the relative airflow must pass up from below the rotor disk to generate the forces required for autorotation. The rotor speed is controlled by the free-stream wind velocity and the Angle of Attack (AOA) of the rotor.
Gyroplane Rotor
With autorotation, if the rotor blades slow down, the lift is reduced. Helicopters require a tail rotor to counteract the torque on the fuselage that is generated from driving the rotor blades. As the gyrocopter rotor is unpowered, the blades do not generate torque opposite the rotation like helicopters. Therefore they do not need an anti-torque tail rotor like the helicopters do
With any type of rotorcraft, one side of the rotor is advancing into the oncoming air and one side is moving away from the oncoming air. The advancing side of the rotor is seeing more airflow across it than the retreating side. If a rotor was rigid, then this would cause a massive increase in force and bending moments of the rotor when it passed through the advancing region. One way to prevent that is to create hinge to let the rotor flap up and down. This reduces the forces on the rotor hub and blade as well as balancing out the pressure in the rotor disk.
Without flapping hinge:
With flapping hinge:
A rotor with more inertia reduces the gyroscopic precession moment generated by a flapping hinge. This moment is generated around the roll axis, but the hinge causes a 90-degree phase lag, which means that the effects occur in the pitch axis. During steady-state autorotation, the flapping angle of the blade varies periodically with constant amplitude, and the rotor speed is constant.
To increase lift, the rearward shaft angle is increased. This increases the area of the rotor with respect to the relative wind, which accelerates autorotation and increases rotor speed. To decrease lift, the shaft is tilted forward, which decreases the autorotative speed, thus decreasing lift. To start an unpowered gyroplane rotor, the pilot spins the rotor(either manually, or with a smaller motor) until it moves as fast as possible. At the start of the takeoff run, the pilot tilts the rotor shaft rearward to accelerate the rotor from 100 rpm to around 200-250 rpm. Then, the cyclic is increased to a full back position; the gyroplane takes off with a rotor speed ofaround 300 rpm.
Gyroplane rotors have a hump speed below which autorotation is not possible as the decelerating torques are greater than the accelerating torques for any rotor shaft angle. Near this hump speed, the rotor behavior is dependent on the section drag coefficient data of the rotor airfoil.
Rotor Teeter Angle
A more straightforward method for the flapping hinge is to let the rotor rock back and forth like a seesaw. This equalizes the pressures similarly to the articulated hinge but uses a simpler method. This is common in smaller gyroplanes. In the image below, a gyroplane is hung in a museum by the teetering hinge bolt. The rotor is then free to seesaw about the rest of the aircraft as it is not level with the disk below it.
The gyroplane rotor teeter angle is important because excessive teeter angles under specific flight phases can lead the rotor blade to strike part of the aircraft. This would result in catastrophic damage to the rotor blade, causing the aircraft to crash. The teeter angle is also critical because the upper limits of the RPM range are due to centrifugal forces, and the lower limits are due to excessive blade flapping. Excessively low RPMs create these hazardous flapping conditions.
The teeter angle can be described with the following equations
$$\beta_{1_c}=\gamma_t\cos\Psi$$
and
$$\beta_{1_s}=\gamma_t\sin\Psi$$
Where the teeter angle \(\gamma_t\) is the orientation of the hub bar relative to the shaft, and \(\beta\) is the disc coordinates. \(\Psi\) is the blade azimuth position, which is zero on the centerline in front of the pilot.
The strike plate that limits the teeter angle must also be able to prevent the blade tips from touching the ground during ground handling as they droop when the rotor is not spinning. Aggressive maneuvers can also cause the rotor teeter angle to exceed limits, causing a safety-of-flight situation.
Gyroplane Airframe Dynamics
The basic force balance for the entire gyroplane is shown below. The tilted rotor blade has a positive angle of attack \(\alpha_R\). At flight velocity \(V\) the airflow upward through the rotor keeps it spinning. \(F_R\) is the rotor force that acts perpendicular to the plane of rotation. The propeller force \(F_{prop}\) acts against the drag force \(D_p\) and the rotor drag force \(D_R\approx F_R\cdot\sin \alpha_R\). The rotor tilts around the roll and pitch pivot bolts to create roll and pitch movements. The yaw is controlled by the rudder.
Stability
Surprisingly, horizontal tailplanes on gyroplanes are less effective in improving dynamic pitch stability than on fixed-wing aircraft. Gyroplane static and dynamic stability is sensitive to the location of the center of mass in relation to the propeller thrust line. The rolling moment of the gyrocopter is caused by the reaction torque from the rotating rotor.
As the angle of attack on the gyroplane rotor increases, the lift increases. This causes a downward pitching moment due to the rearward placement of the rotor in relation to the CG giving a favorable speed stability condition as increases in AOA create an increasing downward pitching moment, and decreases in AOA create an increasing upward pitching moment. The yawing moment of a gyroplane exhibits linear behavior with rudder deflection, but There needs to be a gradient in yawing moment with respect to sideslip angle. Because of this, endplates on the vertical tail have a positive effect on the lateral stability as they provide more area for the aerodynamic force to act on.
Gyroplanes have a higher frequency lightly-damped, longitudinal phugoid(long-period) mode than fixed-wing aircraft. The angle of attack also affects the rotor speed, which affects the lift generated by the rotor. Therefore, an unstable phugoid affects the airworthiness and handling qualities of a gyroplane more than it would affect a fixed-wing aircraft, as an unstable phugoid in a gyroplane can cause unstable rotor speed responses to control inputs, therefore affecting the safety of flight. The short-period mode for the gyroplane also becomes faster as the speed increases and is is sensitive to altitude due to the true airspeed influence. The root-locus plot below shows how the short-period mode increases its frequency (rises up on the frequency axis) as the airspeed is increased.
The vertical CG also affects the short-period and phugoid modes. The vertical location of the CG should be within 2 in of the propeller thrust line. This ensures pitch stability as a pendulum effect can be encountered if the propeller thrust is too far out of line. Lowering the CG of the gyroplane reduces the frequency of the short-period mode in the same way that raising the metronome weight (inverted pendulum) increases the frequency. Raising the thrust line also makes the short-period mode slower, just like lowering the center of gravity.
Forebody shielding, like windscreens, offers more comfort for the pilot but creates higher drag at negative angles of incidence. This is caused by the tail moving into the wind shadow of the body of the gyroplane. Also, if the vertical tail moves into the propwash of the rotor, it interacts with the rotor swirl. The effects of forebody shielding are reduced at higher sideslip angles as the tailplane is able to move out of the wind shadow of the forebody.
Tailplane
Surprisingly the absence of a horizontal tailplane on a gyroplane does not impact the overall safety of the aircraft. The tailplane of the gyrocopter stabilizes the pitching moment, as it is located behind the center of gravity, while the body cowling destabilizes it. The tailplane affects the short-period modes, but it may not affect them to the point that they are no longer in compliance with certification regulations, although the pilots would feel the effects of a faster short period. The phugoid mode is also mostly unaffected by the absence of a horizontal tailplane.
Another aerodynamic phenomenon to be aware of is a tail stall. The gyroplane tail stall occurs at positive angles of incidence that are greater than around 20 degrees. This can cause the aircraft to become marginally unstable above these angles. In addition to lateral stability, endplates on the horizontal tail help to reduce induced drag but result in a sharper stall. Moving the horizontal tail further from the body reduces the tail stall angle of attack while also changing the gradient of the pitching moment curve as a longer lever arm gives the tail more pitch authority. The mass of the rotor blade affects the pitch-damping aerodynamic derivative of the gyrocopter.
Advantages
Gyroplanes do not stall like fixed-wing aircraft and, therefore, don’t have a minimum speed for safe flight. When the rotor is in autorotation, the aircraft will settle gently to the ground if the engine fails. Gyroplanes have better low-speed flying characteristics than fixed-wing aircraft. The simplicity of the design makes for a meager cost of production and maintenance for the gyroplanes. Gyroplane rotor systems are simpler to build and maintain than helicopter rotor systems.
Disadvantages
Gyroplanes are susceptible to roll-over on touchdown during incorrectly applied controls during landing. This is caused by the rotor force as well as the high center of gravity and tricycle landing gear. Taxiing at too high of a ground speed and severe crosswinds can lead to a dynamic roll-over scenario. Although they can fly slowly, they cannot hover like a helicopter which greatly limits their utility.
Gyroplanes in the UK have a fatality rate of 6 per 1000 flight hours. This is far higher than the general aviation rates. Just like any rotorcraft, they are not safe in zero-g flight with an unloaded rotor. is dangerous because, with no lift, the rotor speed would decrease rapidly. When the lift was again applied, the rotor would flap dangerously, leading to a rapid pitch down that causes the rotorcraft to tumble and invert into an unrecoverable flight situation. Some gyroplanes also lack responsiveness to nose-down inputs at specific rotor speeds.
They create more drag than aircraft of a comparable size and flight speed and are not good for long-distance or high-speed flights. In the event of a powerplant failure, the increased drag results in a much smaller area of possible landing sites. Depending on the terrain that the aircraft is flying over, this may become more hazardous than a conventional fixed-wing aircraft with its superior gliding distance.
Conclusion
Gyroplanes never seemed to gain widespread adoption once the helicopters gained popularity. They were worse at flying fast and far than fixed-wing aircraft, and worse at flying slow and hovering than the helicopters. They are in the Goldilocks zone of bad performance for the types of missions that aircraft and helicopters typically perform. Still, they were one of the most important stepping stones to the development of the helicopter and are still enjoyed by small communities today.
Sources
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