GYROBEE ELECTRIC VDU
Don (left) and Ralph (right) pose with the Gyrobee in front of a fully-restored B17 at Mason Aviation Days, 19 July 1998. This was the first operational day of flying with the electric VDU prerotator on the Gyrobee.
Back in 1996 I "invented" the Very Different Ultralight (VDU) prerotator concept by putting a pair of OS Max 40 (0.40 cubic inch) model aircraft glow engines on the hub bar. The two engines chased each other around in a circle, quickly bringing the blades up to a solid 100+ rpm and essentially solving our problem of a lack of a prerotator. The system worked very well, but did have several disadvantages:
These weren't trivial problems, but they couldn't compare to the sheer convenience of finally having a prerotator that would let us stay under the Part 103 weight limit.
In 1997 I converted the OS Max 40's to diesel operation, gaining about a 50% increase in power. This was the season we used the VDU with Terry O'brien's early model Dragon Wings blades with the short hub bar. On our Rotordyne blades, the VDU pods were mounted two feet out from the hub bar, providing a nice mechanical advantage to permit the engine thrust to accelerate the blades. With the Dragon Wings, the pods had to be mounted on short pylons to clear the head control bar and the pylons could only be set out about a foot from the head. This resulted in a significant reduction in acceleration and top-end blade speed, but the diesel 40's provided the power to get the Dragon Wings up over 100 RPM and well past the point where they might flap. The diesels simplified starting the engines, because we didn't have to use a glow plug battery. We seemed to go through glow plugs at a pretty rapid rate with the old system but the diesels, with no glow plugs, didn't have that problem. They started easily and ran well, but they did have one very significant drawback - their oily exhaust that was laden with black soot! The stuff got all over the airframe (not to mention the occasional spectator who stood too close) and was very difficult to clean up. By the close of the 1997 flying season I figured the engine investment in the initial system had essentially paid off, so it was time to try the electric alternative.
The advantages of an electric system are pretty obvious:
The disadvantages include:
I could get pretty long-winded on all these subjects, but I will confine myself to a simple description of what we used.
Pod Development - Phase 1
The initial electric VDU pods in clam-shell cases built by Jim Struble, a fine machinist and future gyro pilot. The pods are 13 inches long and have a 3 x 2.5 inch cross-section. Jim built the cases so they open quickly for battery replacement yet lock shut securely to make sure everything stays where it belongs. Each of the two pods weighs about 4.5 pounds. components.
Why Pods and Not a Commutator?
Commutators seem an easy way to route power from batteries on the airframe to the motors on the blades or hub bar. Unfortunately, there are some real design problems that have to be solved:
The commutator concept can certainly be realized, as Ernie Boyette demonstrated at Bensen Days, but I decided to try self-contained pods with no electrical connection between the hub bar and frame.
Batteries
A self-contained pod must use the same high efficiency (low internal resistance, large plate area) NiCad cells used by RC'ers flying electric aircraft or racing electric cars. The most widely available and economical battery packs are the six cell (7.2V) packs used by the electric car crowd. These typically have a capacity of 1500-1600 mAH and cost about $20 at most hobby shops, thanks to sales volume. Each pod uses a pair of such packs (see the right end of the pod illustrated above), connected in series to deliver 14.4V. A set of battery packs can be replaced in just a minute and the system is designed to get three full prerotation cycles on a charge.
Motors
Each pod contains an Astro Flight 625 samarium/cobalt motor. At 14.4V (25 amps), this motor will swing a 9x6 prop at 14,000 RPM, generating about 50 ounces of thrust. The Astro 640 motor, easily equaling the 40-sized glow engines in thrust (70 oz. on a 10x6 prop at 20V) would seem a logical choice, but would require three 7.2V packs for reasonable efficiency. It seemed worthwhile to try the 625 to save on both weight (the 625 is 15% lighter and there is a net weight savings of 33% for batteries) and cost ($100/motor internet price compared to $120 for the 640 and a total of $40 saved in batteries) compared with the 640. The 625 motor can be seen at the left end of the pod illustrated above.
Control
Turning the motors on and off is not trivial when you are talking about 25-30 amps/motor. I looked at a variety of solid-state controllers but, in the end, settled for a low-tech solution. Each motor is controlled by an inexpensive (about $6.00) 12V automotive relay rated at 40 amps. The relay can be seen right behind the motor in the picture above.
To control the relays, I designed a simple but sophisticated timer circuit that is mounted on the inside of the case of the "master" pod. When the controller circuits are on and armed (more on that later), there is a 64 second delay to permit the pilot to get strapped in. At arming +64 seconds the motors will both come on, running for a total of 64 more seconds before they shut down. The motors cannot be restarted without repeating the arming sequence, which serves two purposes - conserving battery capacity as well as providing a safety fail-safe once the motor run is complete. The control board in the "master" pod (illustrated above) controls both pods, with the second "slaved" to the master".
Charging
For the electric VDU system to be practical, there has to be a way to rapidly recharge the battery packs. Astro Flight makes a microprocessor-controlled digital peak charger (Model 110D) that can recharge the 14.4V battery pair in under 20 minutes when plugged into the cigarette lighter socket of your car. Total recharge time is thus 40 minutes for both packs. Initial bench tests of the system provided two strong 64 second runs on a fully-charged battery set, followed by a weaker third run. As the packs were taken through several charge/discharge cycles, their capacity improved to the point where three essentially full-power 64 second runs could be obtained from a single charge. Since that represents three flights, with only 40 minutes for a complete recharge, a single set of batteries (four 7.2V packs) may be adequate. Just to be sure, we have two complete sets of battery packs, assuring that flying will not be interrupted.
Phase 1 Flight Tests
By the time all the details of pod construction and bench testing were complete, we were down to one week until our mandatory air show appearance and just two weeks until Mentone. Since the Dragon Wings blades were still on the Gyrobee, I decided to test the pods without changing blades, even though I had misgivings about the position of the pods less than a foot from the hub. The 40-size diesels did the job in that position, but there was not a lot of power to spare, so the 25-equivalent electric system might have a problem.
That pessimism turns out to have been justified. At the 1-foot point (shown above), the pods would only get the blades up to 70-80 RPM during their 64 second run. This would have been adequate with most other blades, but you really have to get the Dragon Wings up to 90 RPM to hope to catch them during spin-up. I had higher expectations for the pods on the Rotordynes, where they would be located at the 2-foot point on the bar, but we decided to forge ahead and check flight characteristics while we still had the Dragon Wings installed.
The blades were patted up to 30-40 RPM by hand, at which point the pods would kick in and take them up to around 100 RPM. Don took her up for a test flight and there didn't seem to be any problems with stick shake or rotor stability, despite the long electric pods. What we did notice however was that the climb rate seemed pretty anemic. The Dragon Wings typically have excellent performance, so I began to worry that perhaps the long pods were creating turbulence that was degrading lift along the inboard blade section!
We switched the Rotordynes for the Dragon Wings, mounting the pods two feet out from the hub, just inboard of the blade straps. At that position, the pods could be bolted directly to the hub bar, without the use of the pylons shown above on the Dragon Wings blades. When the pods were fired up, they smoothly took the blades up past 100 RPM from a dead stop. Performance was even better if you gave the blades a modest push to overcome starting inertia prior to strapping in. I have commented in earlier articles about the importance of the position of the VDU pods on overall performance and this confirmed those observations. To get reasonable performance, the engines must be at the two-foot point. If you are using Dragon Wings or other blades with a really short hub bar, you will have to design mounting extensions to permit the pods to ride above the blades at approximately two feet out from the hub. If they are mounted close to the hub, as shown above, you are wasting much of the performance potential of the system.
With the Rotordynes in place, it was my turn to do flight testing. On the positive side, the rotor was smooth and spin-up was a breeze. The Gyrobee rotated normally and it looked like all was going well. Unfortunately, 15-20 feet above the runway it was obvious that there wasn't enough lift to safely fly out of ground effect. I converted the takeoff to a high crow-hop and put it down with runway to spare. Several more hops by both Don and I confirmed that the pods (probably the forward section) were creating turbulence that was spiraling out and degrading lift. Performance had been marginal with the otherwise excellent Dragon Wings. With the Rotordyne blades the Gyrobee had no useful climb margin! The differential between the two sets of blades was probably due to two factors:
Phase 2 Pod Modifications
We already knew that the smaller glow/diesel power pods caused no problems with flight performance, but we were running out of time to re-engineer the system. As a "proof of concept" exercise, Jim took the pods back to the shop and cut each of them just behind the motor. The mini-pods, containing just the motors were remounted at the two-foot point, while the remaining pod sections, now containing the batteries and control relays, were mounted inboard, near the hub, since that area was already cluttered with pitch blocks, the head, and control hardware. It was hard to think that they would cause any problem tucked in with all that other drag-producing hardware. In contrast, the motor enclosures were actually smaller than the earlier glow/diesel power pods, so everything should work!
When all the cutting and rewiring was complete, this was what the hub bar looked like! It certainly wasn't pretty, but, if it worked, we could always repackage it later! Given the way it was necessary to cut the pods, there was no longer any internal room for the controller board, so it has to be added in a small die-cast housing on top of the master battery/relay box. A second identical box, without a control board, was added to the second/slave battery box to maintain balance.
Here is a close-up of the read of the control box showing the two switches and a very bright LED. The left switch turns the controller on and off, with power supplied from the battery packs in the master battery/relay box below. The controller draws about 15 mA and would drain the battery pack in about four days if the batteries were left connected. With power OFF via the power switch, the batteries can remain connected with no battery drain or any chance that the motors will activate. Obviously, the POWER switch needs to be ON (up) for the controller to function.
The switch in the middle is the arming control. With the switch SAFE (down), the timing circuits are disabled and the motors cannot run, even with the POWER switch ON. When the arming switch is thrown to the ARM position (up), a very bright red LED (far right) comes on to confirm that the system is armed and "hot". The 64-second delay begins when the switch is thrown to the ARM position and the LED comes on. At any point the ARM switch can be returned to SAFE and the LED will go out and the system will be disabled.
Assuming the switch stays in the ARMed position (LED ON), the motors will kick in at 64 seconds and run for an additional 64 seconds. At the end of the timed run the motors will shut down and the system cannot be activated again until the arming switch is cycled to SAFE and then back to ARM.
Phase 2 Flight Tests
All the modification work was completed late Saturday, the day before Mason Aviation Days. Flight tests showed the system worked perfectly to spin up the Rotordynes with no adverse impact on flight performance.
The next morning (Sunday) I had the pleasure of flying the Gyrobee over to Mason's Jewitt Field just after dawn. As I slid into the pattern, down below me was a scene from 50 years in the past - a B17 Flying Fortress, a B25 Mitchell attack bomber, and a C47 Dakota. These fully-restored aircraft had been flown in from Willow Run Airport, the headquarters of the Yankee Air Force. It was a sight I won't forget! I landed, hooked up with Don and we had a pleasant breakfast and then kept the Gyrobee on display until the afternoon air show. Just prior to Don flying a demo in the air show, the B17 crew invited us to wheel our little craft over to where the B17 was parked, where a friend took the picture that leads off this little piece. Quite a morning, to say the least, and to top it off, we won a prize for the oldest homebuilt to fly in that morning. That was a bit of a shock, since we still look on the Gyrobee as a "new" aircraft!
The electric VDU system worked perfectly, greatly easing the apprehension we had always felt when the Gyrobee was performing before a huge crowd and we had to prerotate by hand. Don flew through his demo routine with me doing the narration and then headed back to Bergeon Field for a perfect ending to a fine day.
The electric VDU system wasn't pretty yet, but it was certainly working very well with no fuss or mess! The next weekend we brought the Gyrobee down to Mentone 98, where many of you may have seen the system and watched it work. It is so easy to use that it seems like cheating, but you won't hear us complain!
Costs
Here is the basic cost breakdown for the system:
Total cost for the system less enclosures, is thus about $437.00 - well below what you would pay for a commercial prerotator. Ordering information for the motors and charger can be obtained from the Astro Flight, Inc. web-site at http://www.astroflight.com.
Ralph E. Taggart (gyrobee@aol.com)