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Pet Project: ULSRS E-Heli
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"Ultra Low-Speed Rotor System" Proof-of-Concept Demonstrator "How Slow can you Go?" I've been wondering just how slow one could spin a main rotor and produce a flyable heli. One reason for dropping the head speed is to help create a higher degree of realism when flying scale birds. Since there aren't too many full-scale heli's spinning their rotor systems at 1000rpm or higher, I was hoping a serious drop in speed to around 600rpm could be done successfully to generate a more realistic flight appearance. I was right, it works and has more lift that I imagined it would. Slower may be possible but I'll have to work on that a little as lift capability goes away as the blade velocity decreases. One of two interesting side effects of lowering the speed is the way the blades produce an extremely realistic "whop-whop" sound unlike any model heli I've ever heard. Playback of the first test flight video gave me the impression there was a "real" heli hovering out of view of the camera and providing the sound effects for my model. No, the only other outside sounds are those from the airliners flying directly overhead. This modified electric heli was the sole generator of the blade sounds. The other interesting side effect is the vastly improved motor efficiency as in the watts needed to fly the airframe. I'll not reveal my finding on this quite yet as I need to verify the numbers first. But let's just say I'm completely shocked (pun intended). Sept. 10, 2007: Further testing has shown a power expenditure of approximately 40W/lb while hovering. I'll refine this value as more data is collected. Below are preliminary pictures of my modified Miniature Aircraft Ion-X on the day of the first test flights, Sept. 8, 2007. There's only one still-image of flight (as of 9-10-07 there's three more flight shots) as I had no helpers to document this event and I had to take this picture myself while controlling the heli. One shot got what I wanted to see! Flying done on this day was performed without any electronic stability assistance to help with the handling of the flybarless head. As there was almost no wind, there was absolutely no issues with control. Adding wind or significant forward flight speed to the rotor system will produce an unwanted pitching and I'll be using a pair of CSM 'Heading Lock' gyros to handle stabilizing duties as I've done in the past. More details on the airframe and installed equipment as time allows and adjustments are finalized. **Sept 17, 2007 Update** Many flights have been performed while at the Dalton Scale event. No flight data other than power returned to LiPo packs was seen. The Helitronix V1 mixer originally installed was replaced by the much smaller and finer resolution V2 thanks to Joel of Helitronix. This change had no real effect on the performance of the model but did make flying more consistent. This evening a few tests were attempted to see the operational conditions of the heli with my BNB Products DPR-100 power recorder, a cool device that allows the collection of volts, current, temperature and RPM for display on a PC. Said 'attempted' because the first flight and a try at acquiring data gave me a "No Data Collected" message. Big waste of time. Lost some battery capacity from that test. An 'automatic' recording of motor RPM which, after manipulating the method used to detect motor pulse-per-rev count, almost gives me true main rotor RPM. I say 'almost' because I still have to take the displayed RPM readings and divide by 2 for the real value rotor head RPM value. Also 'almost' because my positioning of the Hall sensor needs a slight change. Pulses weren't consistently detected for some reason. What was recorded was the power and RPM under hover and max pitch at both 600RPM (75% throttle as governed) and a touch above 700RPM with the throttle curve at 100%. This is with power being provided by a 12S2P, 15C LiPo battery. A screen capture of the last flight is shown below. What I really wanted to accomplish with the data collection is to see how much payload this heli can lift and what pitch will be needed along with power to do the lift. I ran out of daylight and time so I'll have to try this again when all is ready. Here's the present equipment list and mods to the heli followed by the known operational conditions in flight: Here are some operational specs as best I can recall them: **Sept 19, 2007, Minor Update** Looking at the watts used with the 10S battery pack and the 100% throttle curve setting needed to provide the 600RPM head speed, I've decided to change the 15T motor pinion gear to 17T. This give an almost 30-1 ratio and some extra oomph when using the lower voltage power source, 10S vs 12S LiPo packs. System peak current will climb but now so should the heli when part way into the 10S pack charge. At the present 34.2-1 ratio, once the pack voltage falls to 37V under load, there's almost no lift left to carry the heli's weight, forget about lifting any sort of payload. Some time before next week I should have this change implemented and I'll add a note to this effect. **October 31, 2007, Minor Update** I took a little trip to Maryland for almost a month and brought the ULSRS heli with me to demonstrate and to try the 30-1 gear ratio. I recognized a serious problem with the CSM560 gyro used to handle elevator stabilization. Seems it is more sensitive to motor vibrations than I could ever have imagined and would cause an aft drift to the swashplate as the motor ramped up to the governor setting. This was a very consistent effect but it had me pulling my hair out troubleshooting the reason for it. Made for almost impossible lift off the ground without tapping the tail blades down. Didn't help in being able to relax and enjoy the flight neither. I've been fighting this aft drift since installing the gyros on cyclic and really didn't have the time to diagnose the handling issue. Now that I know what's going on I may remove both gyros for now and concentrate on some other non-forward flight tasks. Also, the limited space in the Bergen canopy is forcing contact between the stack of gyros and the upper, inside portion of the canopy. This is making for an even more severe transference of vibes to the gyros. If CSM would have followed through with providing me with a pair of SL720 gyros as they originally offered, I would be further along with that portion of this project. Also have concluded that the 10mm steel I chose for the main shaft is too soft for the loads imposed. It was a salvaged stainless steel bar from an old photocopier and even slightly undersized to boot. A new one needs to be fabricated from drill rod or from a Bergen hardened main shaft. I believe the current shaft is bent. **Dec 04, 2007, Minor Update** It was time to pull that huge rotorhead off the airframe and advance to the next stage of low RPM heli flight. The large rotorhead wasn't really designed for this heli but was the only means available of employing the 950mm Vario blades, the longest in my inventory. I've since received a set of MAH 950mm carbon blades (thanks Craig Burgess!) and since these blades are more standard in root thickness, a different design head could be addapted as opposed to being chained to the 22mm thick roots of the big Vario blades. I wanted a head with a design similar to what one would commonly see on one of the most popular of full scale heli's, the Bell 206 or 'Jet Ranger'. A close cousin to the Jet Ranger, the Bell AH1 Cobra was in the same class. Thinking what's in my spare parts box that's easy to modify and would have the qualities I'm in need of lead me to Hirobo's designs and the modular construction needed to simplify the changes. I did some digging and found my SSR-II head received on the first Eagle I purchased. Someone tried to remove the black anodizing with a buffing wheel and didn't do a very nice job. This isn't a head I'd use on anything intended for restoration so it is the perfect core assembly for this project. I didn't feel the SSR head hub would be worth sacrificing to the mods needed for the flapping stopper hardware so I searched for a worthy substitute. Ah, a crashed and badly bent GPM Pro rotorhead hub looked as if it would fit and with a little cutting, reboring and plugging, it's perfect. The polished side plates of the SSR head were sanded to give them a consistent appearance and the polished grips were rubbed with 3M Scotch-Brite to blend the surface texture. I could have just thrown the parts together but I like the bits to look nice too. The pictures can be seen in the title: "New ULSRS Rotorhead Design: Damperless Seesaw using modified Hirobo SSR-II and GMP Pro head parts"
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As promised, I brought this low RPM demonstrator to Dalton and let it cover much more sky than possible in my front yard. Except for some minor tuning issues with the cyclic gyros, it handled very well and showed no undesirable quirks. I did manage to tap the tail blades on the runway and dirt a few times but not enough damage occurred to prevent safe flight. Need to speed-up the tail rotor or increase the number of tail blades so I can install shorter ones and still allow sufficient counter-torque and safe blade-to-ground clearance.
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Video of Whirlybird in flight
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Video clip of 1/8 mile dragstrip run
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