Air Speed 10 mph

Air Speed 10 mph

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 oveT 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 v.js 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 autogirc are flapped up in the front and flapped down ir the rear (fig. 9). The blades climb up from theii low position at the tail to the high position at the

Relative wind from blade flapping up

Resulti ng relative wind

J .ssss* ' Blade flaps up Relative wind — NOSE »

Resulti ng relative wind

Resulting relative wind

Relative wind <Jt NOSE

Relative wind <Jt NOSE

Resulting relative wind

Blade flaps down

Relative wind from blade flapping down

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-

Homemade Rotor Head BladeHomemade Rotor Head Blade

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 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 C.G. 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 systerr (shock strut and tire) are not designed proper!) or not serviced properly, the reaction to theii being compressed will try to push that side o the autogiro up at the same time the two blade; that are closer together are on the opposite sidi of the autogiro.

The combination of the weight and C.G. shif and the landing gear reaction put a strongei force into the autogiro (fig. 13-c). This car if the landing gear does not dampen it out, build up stronger and stronger with each revolution o the rotor until the autogiro rolls onto its side (fig 13-f). The craft will also shake in a fore and af direction, too, but because of the longer fore anc 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 happen; on touchdown, the damage has been done before the pilot can react, and there is little hi can do in any event. If it happens on runup, hi has one chance; de-clutch the engine from thi rotor drive and apply the rotor brake. Thi might cause all the blades to lag to the rear limi of their damper travel and they will be in ai even spacing from each other. There is n< record of this being successful. It is a theoret ical practicality.

Figure 13 is not meant to imply that after th number of oscillations shown that the autogin would overturn. Depending on a number o factors, it could happen in two or three or con tinue to rock without overturning. This unwant ed activity is called "ground resonance" o "ground instability." It is never a problem witl four-bladed autogiros because the four blade were wire braced to each other. It was difficul for the blades to get as close together as in th three-bladed systems. Removing the win^ from autogiros when three-bladed rotors wit control in the rotor came about, brought with narrow landing gears that did not resist the rod

Hub Tilted Aft

Nose Up

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