The Theory of the Autogiro

All the World's Rotorcraft

The AUTOGIRO literally flashed across the sky in the early twenties with Sr. Juan de la Cierva's craft making a successful flight in Spain on January 9, 1923. The rotor was turned by a phenomenon called "autorotation."

In about a twenty year flurry of activity, it seemed as though the autogiro was sent to make the helicopter, with which many inventors had labored for years and, years, a success. The helicopter interests had been trying since the latter part of the nineteenth century. The problems had been many, but two outstanding ones were power for flight and control in flight. With the arrival of the gasoline engine, the problem of
power dissolved. Adequate control for such a machine that was intended to lift straight up and come straight down and fly with a great range of speed was not so easily solved.

By the early thirties the autogiro had a control system that used the rotating blades for control for vertical flight and at very low speeds.

About this time the clouds of World War II could be seen and the United States Military released a request for bids from aircraft manufacturers to design and build a helicopter.

Contracts were given to Sikorsky Aircraft in Bridgeport, Connecticut and Platt-LePage in the Philadelphia area.

In 1940, Sikorsky had a helicopter that could fly but the control system was so complicated that it was an impractical helicopter system to market.

Col. Frank Gregory, who was in charge of rotary wing design and procurement for the U.S. Air Corps, urged Sikorsky to enter into an agreement with the Autogiro Company of America as a licensee and thus have the use of all Autogiro Company's patents and designs. Sikorsky did this and with the autogiro rotor system added, had a very successful and relatively simple helicopter. Sikorsky began delivering helicopters to the United States Military in the early forties and with these deliveries, the autogiro activity effectively ceased. Without the work that the licensees of Cierva's principals in Europe and the work of licensees in the United States, success would not have come to the helicopter so quickly.

Autorotation was not invented by the helicopter engineers as a way to lower their crafts safely to the ground when their powerplant fails. Nor was it invented by Juan de la Cierva.

The force that makes autorotation possible was known to aeronautical inventors at least as far back as 1909. Nature has produced millions of tiny autorotating "craft" that deliver the maple seed to earth each year.

From a book titled Practical Aeronautics by Charles Hayward, copyright 1912; "-constants used by Lielienthal show the arched surface (of an airfoil) still possess supporting powers when the angle of incidence becomes negative, i.e. below the horizontal. The air pressure becomes a propelling force at angles exceeding 3 degrees up to 30 degrees." So it can be seen that the scientists of those days knew of a forward propelling force.

A quote of an unknown author from the same book "-by this construction, the air was thrust upward on the outer surface while the air rushed in to fill the partial vacuum thus formed, exerting a powerful lift at the same time was pushed forward, thus tending to diminish head resistance."

Later in the book, while discussing some soaring experiments by Octave Canute as early as 1909: "-at certain angles, the total air pressure acting on the plane (wing) cease to act in a line normal to the plane (wing) or its chord, instead, the line of action of this force takes a position well in front. The pressure thus materially acting in the dual role of supporting and propelling force."

A "force diagram"
from Practical Aeronautics (fig. 1) shows the action of an airfoil in flight showing one inventor's understanding of the forces: one vertical arrow, showing "lift" and another arrow pointing foward to illustrate the propelling force.

Messrs. Jackman and Russel, in the same book, referred to experiments with stuffed birds used as "flying models." "-Thus we have a bird weighing 4.25 pounds, not only thoroughly supported, but propelled forward by a force of 0.359 pounds at 17 miles per hour. Other experiments discovered similar actions.

Figures 2, 3, and 4 try to explain the "rules" for lift, drag (resistance) and the forward propelling force.

Figure 2
shows the "rules":

1. Lift acts at 90 degrees from the relative wind.

2. Drag acts parallel to the relative wind.

With knowledge of this forward propelling phenomena it is easy to see that Cierva would decide to anchor one end of his airfoil(s) and when they were propelled forward, they would rotate.

In figure 3 it is assumed that the values of lift
and drag, (measured in pounds) are drawn to the same scale. It can be seen that the "lift" line crosses the vertical axis of the airfoil. The value of the lift line ahead of the axis is a propulsive force. In figure 4, the angle of attack of the airfoil is increased which increases the drag. Lift is increased also, but not at the same rate as the drag. In this figure you will see that the "lift" line does not cross the vertical axis and no autorotative force is produced. In an actual situation, very soon after the rotor was put in this angle of attack and if no power was being applied to the rotor, the rotation would stop.

Persons can usually understand that as long
as the autogiro is in level flight, that the angle of attack will remain within the limits for producing an autorotative force. Many never could, however, imagine how the angle of attack of the rotor blades could be kept at a low value when the autogiro was descending vertically, and the air is flowing straight up at the rotor.

Figure 5 shows that while the autogiro and the rotor system are descending vertically, usually at about 10 miles per hour, the rotor tip speed is about 200 miles an hour, and because it is in vertical descent, the air speed at the tip in any point around the circumference of the rotor is the same and there is no "advancing" blade which meets a greater air speed for part of its circumference
or a "retreating" blade which meets a wind blowing towards its trailing edge, because it is not in forward flight. It will be seen from figure 5 that a resolution of all the winds will show a resulting wind from a low enough angle to permit autorotation to continue.

Early in Cierva's experiments he discovered to his dismay that his rotocraft rolled over on their sides as they began to leave the ground.

At this time, Cierva was solving another problem. He was concerned with the bending stresses on the rotor blades as the lift increased on one side of the rotating circle and decreased on the other.

Juan de la Cierva was a structural engineer
and had in the past designed trusses for bridges that were pinned at the ends to permit motion at the ends to relieve bending.

Cierva never said that he "invented something" or that he "discovered something." He said, "God permitted (him) to know something."

He applied the bridge design principle to the autogiro rotor blade attachment at the hub. The pin at the hub permitted the blade to "flap" or rise and fall as it rotated (figure 6). When the blades were permitted to flap they not only relieved the bending. but allowed the additional lift on the "advancing" blade to cause the blade to rise, rather than roll the autogiro over. In this case, de la Cierva said "God permitted him to
know two things." Cierva also put a vertical hinge on the blade to permit it to move fore and aft to relieve the bending as the drag increased on the "advancing" side and decreased on the "retreating" side.

In normal flight, the forward speed of the autogiro adds to the air speed passing over the advancing blades and subtracts from the air speed that the retreating blades move in. As the blades advance, the increased air speed causes the blade to climb or "flap." As it does, it decreases its angle of attack (fig. 7). This action effectively equalizes the lift on each side of the rotor disc and permits the autogiro to fly level in forward flight instead of rolling because of the "unbalance of lift" across the rotor disc.

On the autogiros produced by Pitcairn and Kellett, the rotating mast of the rotor was inclined toward the retreating side and also inclined toward the rear, in that way "encouraging" the blades to flap (fig. 8-a & 8-b). To take care of the differences in lift that might be caused by this offset when the autogiro was descending vertically, a lead weight was bolted inside the tip of the right wing.

In forward flight the blades of an autogiro are flapped up in the front and flapped down in the rear (fig. 9). The blades climb up from their low position at the tail to the high position at the nose and descend back to the tail position. An imaginary line drawn from the tip of the most-forward blade to the tip of the rear-most blade would describe the blade tip position at any point in its rotation. This is called the "tip path plane" (fig. 9). It must be further understood that the air passes up through the rotor disc, unlike the downwash from a helicopter rotor.

Although the rotor could not stall, even when the autogiro is flying at very low airspeed
or even zero airspeed, the airplane-type control surfaces that were used on early autogiros could stall. When brought in for a landing, and the nose pulled up to reduce the contact speed, all control was lost. If the autogiro was too high when the flare was performed and the nose was not directly into the wind, the autogiro might begin to drift away from the wind. If this did happen and the autogiro contacted the ground in this altitude, the down-wind wheel would strike the ground sideways and the lift from the rotor, high above the wheel would cause the craft to roll over (fig. 10).

This was seen as a serious problem and the early pilots, who were professionals for the most part learned to avoid this condition. As more autogiros were manufactured and sportsmen pilots who might have had the same piloting experience, bought them an increase of these crosswind accidents occurred.

Cierva had begun his experiments with the rotor providing lateral control, but for some reason he abandoned it in favor of the airplane-type control surfaces. All the autogiros delivered in the United States from 1931 through 1934 had airplane type control surfaces.

Soon Cierva's Autogiros again were equipped with lateral control provided by the rotor and at the same time the rotor also controlled the autogiro longitudinally. All this was accomplished with a simple principle; tilting the rotating axis of the rotor in the direction that control was wanted. "Something for nothing" was too much to ask of the rotor; although the system was startingly simple: -the control stick moved the rotor hub directly (fig. 11), through only one or two belcranks to provide the mechanical advantage so that the loads in the control stick were only a few pounds (fig. 12). The rotating part of the rotor system weighed about 300 pounds. With this mass spinning at 200 or more rpm, a powerful gyroscope was attached to the hand of the pilot. Any out-of-balance of the rotor, was fed back to the pilot's hand.

Any inflight disturbance was usually quickly dampened out by the rotor lead lag/dampers. But on the ground, during runup for takeoff or just after touchdown, it was an entirely different experience (fig. 13-a through f).

In figure 13-a, the rotor is stable because all three blades are equally spaced around the hub, 120 degrees apart.

In figure 13-b, the rotor pattern has been disturbed and two blades are closer to each other than they are to the remaining blade. Here you will see that although the C.G. of each blade has not changed, the collective C.G.'s of the two blades act against the remaining blade, with the collective C.G.'s at a new location and with their weights added together.

This tries to pull the rotor head toward the CG. of the two blades, and because the landing gear tries to resist the autogiro rolling over, the tire compresses and the landing gear is compressed (fig. 13-b).

If the landing gear shock absorbing system (shock strut and tire) are not designed properly or not serviced properly, the reaction to their being compressed will try to push that side of the autogiro up at the same time the two blades that are closer together are on the opposite side of the autogiro.

The combination of the weight and C.G. shift and the landing gear reaction put a stronger force into the autogiro (fig. 13-c). This can if the landing gear does not dampen it out, build up stronger and stronger with each revolution of the rotor until the autogiro rolls onto its side (fig 13-f). The craft will also shake in a fore and aft direction, too, but because of the longer fore and aft stance the overturn will be to one side or the other. There are no certain number of oscillations until the autogiro upsets. If this happens on touchdown, the damage has been done before the pilot can react, and there is little he can do in any event. If it happens on runup, he has one chance; de-clutch the engine from the rotor drive and apply the rotor brake. This might cause all the blades to lag to the rear limit of their damper travel and they will be in an even spacing from each other. There is no record of this being successful. It is a theoretical practicality.

Figure 13 is not meant to imply that after the number of oscillations shown that the autogiro would overturn. Depending on a number of factors, it could happen in two or three or continue to rock without overturning. This unwanted activity is called "ground resonance" or "ground instability." It is never a problem with four-bladed autogiros because the four blade were wire braced to each other. It was difficult for the blades to get as close together as in the three-bladed systems. Removing the wing from autogiros when three-bladed rotors with control in the rotor came about, brought with narrow landing gears that did not resist the rocking as well as the wide-stance gear on the four-bladed autogiros with wings. Neither does ground resonance usually occur with two-bladed craft because one blade always opposes only one blade.

Observers of the autogiro noticed that the propeller thrust line was tilted down (about five degrees) (fig. 14). Many surmised that this was done "to blow air on the rotor in order to keep it turning." The tilted engine caused the thrust line to pass through the C.G. of the autogiro which was unusually high because of the extra weight of the rotor system high above.

The three-bladed rotor system permitted two of the blades to be folded back alongside the third blade over the tail to make an ideal configuration for storage. The autogiro could then be towed behind a vehicle when the need arose. At times it might be more convenient or more economical to tow it.

To this point in its development the autogiro could cruise at about 100 to 110 miles per hour, when fixed-wing craft with the same horsepower were cruising at 120mph. They could fly very slowly, as slow as 20 miles per hour and could land vertically with adequate control. It was still necessary, however, to make a short run of 20 to 50 feet to become airborne. To most persons it would have seemed logical to continue driving the rotor with the engine and take off vertically.

One of the main reasons this could not be done was that while the rotor was being driven a torque was being imparted to the fuselage, with the weight of the autogiro on its wheel, the brakes could be used to prevent the rotor torque
of the rotor from rotating the fuselage. If the autogiro rose into the air without some anti-torque device that would be effective in flight, the fuselage would rotate in the opposite direction as the rotor.

The second reason is that the incidence angle, or blade pitch, remained at about four degrees for all the autogiro's flight modes. This angle was not great enough for an efficient vertical flight even if the torque problem could be taken care of.

A solution was at hand, called "jump takeoff." The autogiro could lift itself into the air without power in the rotor and could temporarily have an increase in the rotor blade which would be reduced once in the air.

The system was relatively simple. The blades were all set by the pilot at zero pitch or "no lift position" while the engine turned the rotor at about 150 percent of normal rpm. The rotor drive was quickly disconnected from the engine and simultaneously the rotor blade angle was increased to about 9 degrees. With the energy that was put into the rotor by overspeeding it, it continued to turn at a higher speed than normal with the blade angle at 9 degrees and the autogiro rose straight up to 5 or perhaps 20 feet, depending on the atmospheric conditions. As the rotor rpm slowed, the blade angle automatically returned to its 4 degree autorotative angle.

If one has ever played with one of those little toy propellers that were pushed up to a twisted flat metal strip by hand, you can see that the hand pushing the little propeller up the strip was the "power." As the propeller left the twisted strip, the power ceased and it continued to rise. This toy was not intended to autorotate and the steep angle of the blades soon spent all the energy in the propeller and it fell to the ground (unlike the autogiro).

The development of the autogiro came to a halt with the accomplishment of vertical takeoff, slow flight and vertical landing. Several "jump takeoff" autogiros were delivered to the military, but by this time the helicopter was an everyday flying machine.

The autogiro is still an excellent type of aircraft and perhaps some courageous developer will take it on.

George Townson "Autogiro. The Story of the Windmill Plane", 1985