Nose Down ing as well as the wide-stance gear on the four-hladed 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-biaded rotor system permitted two of the blades to be folded back alongside the third blade over the tail to make an ideal con figuration 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 120 mph. 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 antitorque 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 overspeed-ing 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 tw;sted 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 gound (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.
Informative brochures and advetisingof the autogiro in 1931 and 1932 by the Autogiro Company of America.
(Autogiro Company of America)
Kellett Autogiro advertising folders.
No Landing Fields Needed
For the Autogiro.
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