cmartin
Jan 03, 2005, 05:39 PM
I have zero exerience with AG's but decided to see what might be involved in designing a micro-sized version. After reviewing the posted and printed information it looks like the rotor design might be the greatest challenge. So I worked up some hub and blade designs on Cad and woke up the laser. After the laser finished singing for its supper I have several variations in which the blades can be switched, pitch can be tweaked, the droop stop and coning angle stops can be changed. I wanted, in particular, to see how 3 or 4 (or maybe 2) blade configurations compared and how scaling down larger system hardware might affect performance. The picture shows some of the different pinned hinge and elastic hub and blade types that I've been seen running around the front yard in the wind and cold (the neighbors think I'm nuts!).
I started with built-up 3-rib airfoils covered top and bottom. Although very lightweight and functional, they are time consuming, delicate, difficult to maintain airfoil integrity and essentially impossible to shrink covering film without warping. For testing, I needed something easier to work up and experiment with. I made solid blades which are also quite acceptable but they are a bit heavier than I wished. After some head scratching I tried the laser cut geodedic rib pattern blanks and most of the problems disappeared. After a light sanding they can be covered without other finishing steps. I use both RA Microlite and Solite and shrink with an iron with no warping. The very lightest built-up 1-1/2" x 6" blades weigh .9 grams each, an equivalent geodedic weighs 1.3 grams and a solid blade 2.2 grams.
The Clark Y blade sets shown are different widths, thickness, aaspect ratios, 0 and 15 degree Delta hinge angles and 13" to 17" rotor diameters. I am beginning to sort out the relative advantages and disadvantages of these variables by hand testing outdoors but I have more combinations I want to try. It does appear that the lower aspect ratio Delta rotors are considerably more inclined to autorotate and establish usable lift. A 3-blade 1-1/2" x 6" (4:1 AR) rotor, for example, comes up to a reasonable rotational speed and lift at about 3 mph and at 5 mph it easily provides more lift than the weight of my model. By comparison, a set of 1-1/8" x 8" (7:1 AR) blades on the same hub with the same airfoil, pitch, hub and rake angle doesn't come up to autorotation below 5 mph. I use a hand-held Dwyer wind meter while testing. I don't understand this relationship with AR but it does help ballpark the pitch speed for the tractor drive and propeller. Adding a fourth blade retains the same autorotation capability as the three blade but with a discernable increase in lift. I will start flying with 3 blade and use actual flying comparisons to bet a better feel for this. I have fixtures and gages for asembling the hubs, shaping the airfoils and for setting/resetting the pitch. I find that a 14% Clark Y with 4 degrees negative riggers' angle seems to be a good starting place. Experience with a free-flapping hinged blade with 10 degrees up stops presents a problem. Only the slightest wind anomoly will pop the blade to the stop anywhere in its rotation and disrupt lift. I suspect there is so little mass to the blades that centripetal force isn't there to keep the blades in plane. I am gong to further limit the flap angle and try adding some rubber pads under the stop to provide some resistance to blade lifting...adding weight to the blades is a last resort. There are some other things I need to try before I can begin condensing my observations on these little rotors.
I just finished a flying test bed I call the GyroMite. It is designed for the JMP or RFFS and an M20 motor/gearbox. The AUW of the G.M. as pictured is 20.5 grams with the M20 and 145 mahr cell. Going to just rudder-motor and simplifying the landing gear (I got carried away a bit there) would drop the weight to 17 or 18 grams which may be needed for the M20. Flight attempts will await some more non-flying rotor testing and some better weather before I add my own piloting limitations to the list of variables. I can post more information and flight results (read attempts) but I would welcome any input from others who have chased these little rascals down the same path.
I started with built-up 3-rib airfoils covered top and bottom. Although very lightweight and functional, they are time consuming, delicate, difficult to maintain airfoil integrity and essentially impossible to shrink covering film without warping. For testing, I needed something easier to work up and experiment with. I made solid blades which are also quite acceptable but they are a bit heavier than I wished. After some head scratching I tried the laser cut geodedic rib pattern blanks and most of the problems disappeared. After a light sanding they can be covered without other finishing steps. I use both RA Microlite and Solite and shrink with an iron with no warping. The very lightest built-up 1-1/2" x 6" blades weigh .9 grams each, an equivalent geodedic weighs 1.3 grams and a solid blade 2.2 grams.
The Clark Y blade sets shown are different widths, thickness, aaspect ratios, 0 and 15 degree Delta hinge angles and 13" to 17" rotor diameters. I am beginning to sort out the relative advantages and disadvantages of these variables by hand testing outdoors but I have more combinations I want to try. It does appear that the lower aspect ratio Delta rotors are considerably more inclined to autorotate and establish usable lift. A 3-blade 1-1/2" x 6" (4:1 AR) rotor, for example, comes up to a reasonable rotational speed and lift at about 3 mph and at 5 mph it easily provides more lift than the weight of my model. By comparison, a set of 1-1/8" x 8" (7:1 AR) blades on the same hub with the same airfoil, pitch, hub and rake angle doesn't come up to autorotation below 5 mph. I use a hand-held Dwyer wind meter while testing. I don't understand this relationship with AR but it does help ballpark the pitch speed for the tractor drive and propeller. Adding a fourth blade retains the same autorotation capability as the three blade but with a discernable increase in lift. I will start flying with 3 blade and use actual flying comparisons to bet a better feel for this. I have fixtures and gages for asembling the hubs, shaping the airfoils and for setting/resetting the pitch. I find that a 14% Clark Y with 4 degrees negative riggers' angle seems to be a good starting place. Experience with a free-flapping hinged blade with 10 degrees up stops presents a problem. Only the slightest wind anomoly will pop the blade to the stop anywhere in its rotation and disrupt lift. I suspect there is so little mass to the blades that centripetal force isn't there to keep the blades in plane. I am gong to further limit the flap angle and try adding some rubber pads under the stop to provide some resistance to blade lifting...adding weight to the blades is a last resort. There are some other things I need to try before I can begin condensing my observations on these little rotors.
I just finished a flying test bed I call the GyroMite. It is designed for the JMP or RFFS and an M20 motor/gearbox. The AUW of the G.M. as pictured is 20.5 grams with the M20 and 145 mahr cell. Going to just rudder-motor and simplifying the landing gear (I got carried away a bit there) would drop the weight to 17 or 18 grams which may be needed for the M20. Flight attempts will await some more non-flying rotor testing and some better weather before I add my own piloting limitations to the list of variables. I can post more information and flight results (read attempts) but I would welcome any input from others who have chased these little rascals down the same path.