ADAPTING BICYCLE DYNAMOS By Alien L. Bird, EAA 101370 290 N. Smith Ave. Corona, CA91720

It's getting more and more difficult to operate an aircraft (any kind of aircraft) without using a radio. In 1979, I completed a Bowers Fly Baby. I chose that design because I wanted a basic airplane and the Fly Baby is about as basic as you can get. It was powered by a Continental A-65 with no electrical system and was equipped with only the required instrumentation. It was just a great little airplane and in the 10 years I owned it, I flew it all over the U. S., including two trips to Oshkosh. The problem with having such a basic airplane is that it was difficult to install a radio. It was almost necessary to use a battery and a heavy wind driven generator which affects the performance of the airplane. For these reasons, I resisted putting one in my airplane. This meant that on my many cross country flights I was really forced to use uncontrolled airports. Most uncontrolled fields don't have services such as restaurants, etc. Then, to add to the frustration, I began flying with other guys with similar airplanes and the only way to communicate with them in the air was with sign language, most of which no one understood. But then came what appeared to be the answer to all of our problems, the handheld radio. These things really work great. They even have their own batteries. I bought a Communication Specialist TR 720 and put it in the old Fly Baby. It worked like a charm. Now I could go into controlled fields and everything. Plus now I could talk to the other guys I flew with who had also installed handhelds in their birds. Not long after starting to use the handheld, a couple of things became obvious. If you talk very much on the radio the battery doesn't last long. 24 AUGUST 1991

Secondly, you have to remember to recharge the battery after each flight. This can be a problem if you're on a long cross country because you must remember to bring along the charger and then take the radio with you to the motel to recharge the battery. When you get up in years you contract a disease known as CRS (Can't Remember Stuff), so I found that I would always forget to do one of the above and therefore the battery would be dead. And, of course, allowing for Murphy's Law, this would always happen at the most inopportune time. After a few such occurrences, I began to look for ways of charging the battery while flying without adding one of those big heavy, bulky wind generators. I remembered seeing articles in SPORT AVIATION where guys had converted bicycle generators to provide power for just such things. I called Ben Owen at EAA and he sent me all the info he had in his files on the subject. Everything I read lead me to believe that this approach offered the ideal solution to the problem. Bicycle generators are small, lightweight and have no brushes to wear out or replace. Furthermore, they can be driven by small diameter propellers which would not affect the already modest performance of the airplane. Little did I know that this would lead to an intensive research project that would take more than two years to complete and put about another 100 hours on the Fly Baby to do the testing of the

various iterations of the system. I began by using the same techniques as the guys whose info I got from EAA. I found a bicycle "generator" at my local bicycle store. The first thing I discovered is that it's not a generator at all but a dynamo. The difference being that a generator produces direct current and a dynamo produces alternating current. To provide charging current for a battery you must use direct current. This means that the alternating current from the dynamo must be converted to direct current. This is easy enough to do by simply rectifying the output of the dynamo. If all you needed to do was to use the output to charge a battery you could probably do that by connecting directly to the output of the rectifier, assuming that the voltage was correct. However, in addition to charging the battery the output from the rectifier also provides operating power for the circuitry in the radio. The output from the rectifier is not pure direct current. It actually pulsates at the speed of the dynamo. This pulsation is now heard in the radio receiver as a loud high pitched squealing noise. The only way to eliminate the noise is to filter the output of the rectifier to produce a purer direct current. With a bicycle dynamo this is not easy because the dynamo itself produces a lot of harmonic frequencies. To remove the primary frequency and all of the audible harmonics requires a filter system. A lot of time was spent on circuit design to

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OUTPUT TO RADIO RADII BATT OR CHARGING CIRCUIT Cli

AI-63D DYNAMO

PARTS LIST DESIG CR-I C-1.2 C-3.4 U-l R-l R-2 R-3 F-l

DESCRIPTION BRIDGE RECT. 1 AMP 25V CAPACITOR - 4700 WFD 25 VOLT CAPACITOR - 10 MFD 20 VOLT REGULATOR - LM317K RESISTOR - 5-OHM 2 WATT RESISTOR - 240-OHM 1/4 WATT RESISTOR - VARI. - 2 5K OHM 1/2 WATT FUSE - 1 AMP

T8-1

TERMINAL STRIP - 4 POSITION

PC8-1

CIRCUIT BOARD - VEC'OR BOARD OR PRINTED CIRCUIT BD

AIR POWER MODEL A P - 1 REGULATOR - CIRCUIT DIGRAM SHEET 3

DRAWN BY ALB

arrive at a satisfactory and economical filter. The next thing I found out about these little dynamos, any dynamo, is that the output voltage is directly proportional to the speed of the rotor, e.g., the faster you turn the thing the higher the voltage at the terminals. In fact, some of the dynamos I tested produced more than 70 volts. This in spite of the fact that they are advertised as 12 volts. I learned that when the manufacturers rate their units they assume that the bicycle will travel at a certain speed and will have a wheel of some average diameter. If you assume that the battery you're trying to charge is 12 volts, you actually need an optimum float voltage of 13.8 volts. There is a rotor speed that will produce the desired voltage but in practice it's not possible to keep a constant speed on the rotor due to varying aircraft speeds, etc. To provide a constant voltage output we need some sort of regulator which compensates for variations. Fortunately, there are simple, relatively inexpensive devices which work well in this application. With these devices the output voltage can be adjusted to match the required charging voltage of most handheld radio batteries. So, with the combination of rectifier, filter and regulator we can take the output of the dynamo and use it to charge the battery in our radio. I tested at least 10 different dynamos from different manufacturers and found

one that, even though rated at half an amp, will provide .5 amps continuously and reliably. Most handheld radios use approximately .1 amps while in the receive mode and about .75 amps while transmitting. Since we only transmit a small portion of the time, there's plenty of excess current at .5 amps to ensure that the battery remains charged or to recharge the battery should it be completely dead. As for the dynamo itself, to find the right combination to get it to work satisfactorily was by far the most challenging

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part of this effort. At first I tried to modify an off-theshelf unit by simply adding a model airplane propeller as the other guys had done who had provided data to EAA. I obviously was doing something wrong because I couldn't get more than 30 minutes of continuous operation out of the unit before the bushings would overheat and freeze up. In the beginning, the bushing failure was certainly exaggerated by the types of propellers I was using. I don't really know why but I had it in my thick head that the lower the pitch of the propeller,

SPORT AVIATION 25

the slower it would turn. Well, naturally just the opposite is true. I burned up several perfectly good dynamos before I figured that out. Finally, I borrowed a battery powered frequency counter that I carried along in the Fly Baby. What I found was that with the propellers I had been using I was turning the dynamo almost 20,000 rpm at 100 mph indicated. I then calculated that I needed to turn 7,500 rpm in order to produce enough voltage to overcome the losses introduced by the rectifier, filter and regulator and still have 14 volts available at the terminals which is the amount required to charge a 12 volt battery. To slow the unit down to that speed I needed a very coarse pitch propeller. I searched all of the hobby shops in southern California and talked to all of the model experts. I even called the propeller manufacturers and talked to them. They were all very helpful but all told me the same thing. The coarsest pitch I could get on a propeller the size I needed was 8.5 inches. I got a couple of these props and tried them. They did slow the rpm to just over 12,000 at 100 mph. So now how do I get the bushings to hold up under these kinds of loads? It

soon became obvious that I could not. The thing about the bushings is that they were designed for relatively short duration side loads as would be encountered in bicycle use. In this application they were being subjected to sustained high speed operation with end loads. Finally, they were lubricated with grease that was not held captive by seals and would be thrown out at the high speeds leaving the bushings dry in a very short time. I realized at this point that if this system was to work the bushings would have to be replaced with high speed ball bearings. The big question was, could I find a dynamo that could be modified in this manner and, next, could I find bearings that would fit the application? After 6 months of intensive effort I finally found a dynamo that could be modified and I found some bearings that would fit the application. Fortunately, this particular dynamo was of very good quality both mechanically and electrically. It will produce .5 amps reliably. The bearings were designed to sustain 20,000 rpm continuously and, as luck would have it, they pressfit into the barrel of the dynamo and fit the shaft diameter.

Nevertheless, the dynamo must be extensively modified to accommodate the bearings as can be seen in Diagram 2. 18,000 rpm calculates to a maximum sustained airspeed of 125 mph. If higher speeds are required then it would be necessary to make a propeller with a correspondingly higher pitch. I didn't try that because I don't know how to make propellers. I would have to say that all the work and research was worthwhile as the system worked even better than I had hoped. It always kept the radio battery charged and I never had to remember to plug it in again. In fact, because of CRS there were times when I had forgotten to turn off the radio but as soon as I started my takeoff run, the radio came back to life and the battery would re-charge very quickly. An added bonus is that the system did not noticeably affect the performance of the airplane. The entire package weighs less than 2 pounds. The system should work well on any homebuilt or ultralight aircraft with airspeeds less than 125 mph. If you're interested in learning more about this system, send $2 for postage and handling to Airpower, Inc., 290 N. Smith Ave., Corona, CA

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