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Rubber Powered Model Autogyro ExperimentsVideo of my FAKIR autogyro in flight
I am experimenting with a FAKIR autogyro model from the plans in Bill Hannan's "Model Plans & 3-Views International" Volume 2. My best flights have been with the simple solid rotor hub built according to the plans. However, I believe this rotor is inefficient in flight due to it's lack of flexibility. Note that the plan does recommend using very flexible balsa for the blades. In a better design I am looking for higher efficiency, requiring less power (thinner rubber) to fly, which should allow flight duration to be increased. As can be seen from the video of one of my FAKIR's better flights (click on the link above), duration is short, averaging 14 seconds with a loop of 1/8" rubber with about 500 turns.
FAKIR autogyro with rigid rotor hub (side view)
FAKIR autogyro with rigid rotor hub(top view)
In the building and re-building of my FAKIR, it has deviated very much from the plans in order to accommodate experimentation. In the pictures below you will notice that the thrust angle adjustment is achieved with four nylon screws that press against the propeller bushing. The nose design also allows for very quick change of the propeller bushing assembly. The nose is made of 1/32" plywood pieces laminated together with CA, ground down with a dremel, and coated with CA to prevent splintering. I drilled and tapped the nose to take the nylon screws.
FAKIR nose modified with adjustable thrust angle (left rear view)
FAKIR nose modifiet with adjustable thrust angle (right front view)
The fuselage is actually hollow, being made out of two 1/32" sheets with thin balsa strips separating them on the inside (rubber slot) and outside edges. The rotor pylon wires are tied together at their lower ends with thread and embedded in a laminated beam of balsa that slides within the hollow fuselage. This allows adjustment of the CG under the rotor axis without adding weight. A close-up view of the pylon sliding slot is shown below. A piece of Scotch-tape at the front wire prevents the pylon from shifting in the slot during hard landings. The elevator has been hinged with tabs of thin copper sheet to allow easy adjustment.
FAKIR autogyro modified with sliding pylon for CG adjustment
My main area of experimentation is with different rotor designs. Shown below are pictures of a rotor hub with Flapping Hinges. The best flights were achieved with the blade shafts bent at a 70 degree angle to the hinge. The result of this is that as the blades flap upwards, the angle of attack is reduced, which reduces the lift, but increases the force that drives the blade forward in rotation. This causes the rotor to spin faster and the increased centrifugal force on the blades forces them downwards. At any speed of airflow up through the rotor, the forces are in balance such that almost a constant coning angle is maintained. Powered flights are a different story. With the autogyro under power, the blades were flapping up too high at the front, resulting in a dive into the ground. The pictures show a wire spring on each blade which was added to counteract this problem. The FAKIR with this flapping hinged rotor, performed very well in drop tests, with a definite glide forward that does not occur with rigid rotors. Powered flights were still a disaster, in a different way. The torque reaction from the propeller caused the fuselage to roll at the flapping hinge and turn the autogyro on it's back. To prevent the propeller torque reaction from being transmitted to the rotor, I added a horizontal hinge on the rotor pylon just below the rotor hub which I call the "Roll Hinge". (The Roll Hinge in the picture is shown threaded and glued solid, which I did when I was finished experimenting with this rotor type). This gave very promising results. In test flights with this arrangement, the FAKIR would fly in a straight line and gain altitude. However, the torque reaction of the propeller caused the fuselage to gradually tilt to the left until the hard stops were hit on the flapping hinges. At this moment the autogryo would flip over and crash. The longest recorded flight was 7 seconds, but some of that time was spent upside-down. The ?Roll Hinge? is obviously not a successful means of compensating for propeller torque reaction as it?s effect is so short lived. If propeller torque reaction could be eliminated, I am sure that a flapping hinged rotor would provide excellent flights. This suggests using two motors with counter rotating propellers, or contra-rotating propellers (future experiments). One thing I have come to realize is that if I built the flapping hinges farther out from the center of rotation, it would allow the rotor to better resist the torque reaction of the propeller. This will be worked into future experiments.
Rotor with spring loaded flapping hinges (bottom view)
Rotor with spring loaded flapping hinges (top view) I am now working with a rotor that has Lead/Lag hinges. I have flown a configuration similar to the one shown below, with a counter-clockwise turning rotor. I have not had much flying time with this to have conclusive results, though it is more stable than the rotor with flapping hinges. I have always suspected that the FAKIR may suffer from some roll instability because the counter-clockwise turning rotor has the retreating blades resisting the propeller reaction torque. The advancing blades are capable of generating more lift, so I built clockwise turning blades in hopes that it better resists the propeller reaction torque, and gives the FAKIR more roll stability. For blade pitch adjustment, I hold the FAKIR over a fan and determine which blades are tracking higher or lower. The colored blade tips help with this. High tracking blades are given a little more negative angle of attack, and low tracking blades are given a little more positive angle of attack, until all the blade tips are tracking in the same path. The autogyro will wobble in flight unless the blade tracking is good. The colored lines at different distances from the hub center are useful when holding the autogiro over a fan and illuminating with a stroboscope to observe the behavior of the Lead/Lag hinges.
FAKIR with clockwise turning blades on Lead/Lag hinged hub |
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发表于 2009-3-31 13:03
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