A VERY DIFFERENT ULTRALIGHT PREROTATOR

Ever since the Gyrobee first took to the air in June of 1990, Don and I have been completely happy with its performance and flight characteristics. It has been essentially a perfect for the flying we like to do, with one major drawback - the lack of a prerotator. The reason we don't have one is simple - the Part 103 254-pound weight limit. With a set of light blades (Brocks or Dragon Wings), we would have enough of a weight margin to add almost any kind of prerotator, but, with our Rotordynes, there is no easy option we can add and still stay legal!

Of course, the Gyrobee has hundreds of flights to her credit, so you can obviously fly without a prerotator. All that's required is to hand-start or "pat up" the blades. On your typical Bensen/Brock style airframe this provides useful exercise, ranging from mild to strenuous, depending on the weight of your blades and how fast you have to get them spinning to be sure they will catch when you start to taxi. With light, responsive blades (such as Brock's), you can even do it from the seat! Not so with the Gyrobee, which has quite a tall mast! You can't reach the blades at all from the seat and it is a long, full-arm stretch when standing on the ground, which is not conducive to putting any power behind your attempt to get the blades in motion.

Most of the time Don and I fly together and this presents no problem. The guy who will be flying sits calmly, strapped into the seat, while the human prerotator partner gets the blades going and starts the engine, leaving pilot partner free to concentrate on flying. Solo, its another story! Don is stronger than I am and usually he can get the blades going, start the engine, get strapped in, and still get moving before the blades have decayed past their critical speed. I am not athletic and don't have a prayer when starting the blades solo without a breeze to keep them kited up until I take care of all those other details. Even when successful, things are far too rushed and I'm half shot at the very time I should be it top form at the start of a flight!

Needless to say, we have always had our eyes open for something we could use. Engine driven prerotators (flexible shaft, as in the Wunderlich; hydraulic, used in Ernie Boyette's Dominators; and belt/shaft systems, such as Ken Brock's KB-3) involve varying amounts of hardware and almost all need brakes (another item we don't have) to hold the gyro as the engine is throttled up during prerotation. Electric prerotators, using a small starter motor mounted on the head, don't require brakes, but we never saw one that would be easy to implement on our particular machine, not to mention the weight of the starter motor and the battery. There are several hand-cranked systems as well (SportCopter has a nice one on their lightning and there was another interesting option using a modified recoil starter/Sprague clutch system at Brookville a few years ago), but either of these would have required considerable work to implement on our machine.

An Unconventional Idea

The summer of 1996 started out with a tremendous amount of rainy weather and very little flying. My personal prerotator crisis came when Don was out of town for over a week and the weather broke. Four perfect days for flying - which I couldn't use because there was no wind to help me spin up the blades! When Don got back, I informed him that we would solve the prerotator problem before he left town again - not matter what!

When you are that desperate, you'll try almost anything. In this case, almost anything translated to trying a wild idea that kept popping into my head periodically since we first started flying the Gyrobee. Usually the idea would surface after a long fruitless discussion about prerotators and one too many beers. In total frustration I would say something like:

"Why don't we just put a couple of model airplane engines out on the end of the hub bar and let them do the spin-up?"

This was definitely an off-the-wall idea, but in my agitated state-of-mind, I actually decided to check it out a bit - at least on paper. The disadvantages seemed obvious:

Fueling and starting the engines would be a major hassle. Actually, this probably wouldn't be so. Modern model aircraft engines are a work of art and, together with conveniences like clip-on glow plug batteries and electric starters, getting them running is pretty routine. It might add 10 minutes (probably less) to the preflight routine, but what's that to being able to fly when no one was around to help?
Having all that "stuff" out there will mess up the operation of the rotor. Well, maybe not! If each prop/engine/fuel tank module is identical, drag will pretty much balance out. On the ends of our five foot hub bar, the modules would only be moving at 54 mph at our flight rotor speed (305 rpm), so increased drag will be pretty trivial compared to the rotor system as a whole. If the units were carefully balanced, both chord-wise and in terms of total weight, there wouldn't necessarily be any major dynamic balance problem.
Having a pair of screaming engines on your hub bar is an invitation to amputate your face! Yes, on a standard gyro with a low mast that would be a problem, but the Gyrobee has a very tall mast. The only point the engines would get into food processing range would be directly behind the prop with the stick full back and that's the last place I spend any time! The normal precautions you take to avoid getting clobbered by the blades will let you avoid the little engines. Besides, by that point I already have my helmet and goggles on, so the only real precaution is to resist the urge to reach straight overhead and jump up and down!

The potential problems with such a system can be pretty easily disposed of, so what are the advantages? Well, it turns out there are quite a few!

This rotor drive system would be torque-free, so it wouldn't matter is it was still running when the gyro lifted off.
The system is very simple, requiring no additional hardware or modifications to the head or mast.
The system would impart no twisting stress to the rotor, head, or mast.
Both weight and cost should be significantly less that virtually any of the commercial options.

At this point I may have been on the edge of being certifiable, but the wild idea was definitely looking viable. The big question that remained was how much power would be required to get a decent pre-spin? Its important to realize that we weren't trying to get a jump-takeoff and would be perfectly willing to settle for the equivalent of a strong manual pre-spin! I won't go into the math, but the figures indicated that a total of 1.5-1.8 h.p. should be enough to equal or exceed the best manual spin-up we could manage (50-60 rpm), without all that nasty physical effort.

Fish or Cut Bait

In every project, there comes a time when you have to either go on to another idea or start spending money and cutting metal. The moment of truth came at the engine counter at the hobby shop. Don and I looked at each other and those cute little engines and we went for it! By the time we left, we had a very large shopping bag filled with the following items:

A pair of OS Max 40 SF ABC engines
Two Master Airscrew 10-7 composite props
A pair of Du-Bro prop spinners
Two Fox standard long glow plugs
A clip-on, rechargeable glow-plug igniter
Two 8-ounce Du-Bro fuel tanks
A Hangar-9 12V hand starter

The total cost for the bag of goodies was just over $400, $320 of which was for the pair of engines. The O.S. Max 40 SF ABC engines (0.40 cubic inch or 6.47 cc displacement) are not cheap, but they are a work of art, complete with ball-bearings on the crank, with an excellent reputation for power, reliability, and easy-starting. A pair of these engines puts out just about 2.5 horsepower, which might, according to my trusty computer, be enough power to hit 100 rpm on the pre-spin! Perfectly serviceable 40-sized engines are available for as little as $60 via mail-order suppliers, but I was far enough into the twilight zone on this project that I didn't want to cut corners the first time out! The spinners would dress up the engine modules and permit the use of an electric starter.

The final step was to design a pair of identical modules, machined out of 6061-T6 aluminum, that would handle the engine and tank mounting. The upper piece is constructed of 3/8 inch-thick stock and is machined-out to serve as the engine mount at the front end. The tanks ride directly behind the engines and are restrained by aluminum shrouds wrapped across the tank, secured by 6-32 stainless machine screws secured into holes tapped on the sides of the 3/8 inch base plates. These units clamp to the end of the hub bar using AN-3 bolts and a 1/8 inch bottom plate of 6061-T6. No drilling or other modification of the hub bar was required and the modules butt up against the base of the blade straps, providing additional insurance that they will stay where you want them! Provisions were made to add some lead ballast at the rear of the mount, to maintain the chord-wise balance of the hub bar but this proved unnecessary. The two modules were definitely over-built (1.6 pounds each, complete), but I tend to be conservative when I'm the one in the seat! The bottom-line was that we were still legal and neither of the units would shed any parts with the rotor up to flight speed!

Does It Work?

That's a silly question - if it didn't, this entire little project would have been one of the best-kept secrets in the PRA! The engines are easy to fuel and start and, by the time you've fired up the 447 and strapped in, the blades are up to a solid 100 rpm. If circumstances require, you can start the engines, tie down the rotor, and taxi out to a remote spin-up area. The engines run for almost 20 minutes on less than 8 ounces of fuel, so there is no hurry. I had expected a sedate rate of acceleration, but the blades get up and go as fast as any prerotator you will see, all without the slightest stress to the mast and head! The muffled engines make a wild sound that certainly gets attention as Doppler effects mount with blade speed.

A conventional prerotator will usually top out at a specific blade speed, depending on the total gear-down ration in the system and total blade drag. Beyond that point, the wind takes over and the prerotator will disengage. In contrast, the engines provide a beneficial power assist at all stages in your taxi and take-off run, which makes blade handling noticeably less critical and provides a very rapid transition to flight speed. As expected, there are no torque effects on lift-off. I had expected a slight stick-shake due to unequal fuel loads until both engines had burned up their fuel supply. We were pleasantly surprised to discover that we couldn't tell the engines were there, irrespective of relative fuel load and which engines were running. The first engine started will usually quit first, but you can't tell from the handling. The "clunk"-type fuel pickups drain virtually all of the fuel from each tank but there is not way to tell when the second engine quits either. The whole system works so smoothly and quickly, that the 8 ounce tanks are really over-kill. Just a few ounces of fuel will do for normal take-offs in light traffic situations, but it is nice to have the capacity to use longer runs for fly-ins and dawn patrols where the traffic density may force you to hold in line with the wind at your back.

By lining up the props with the hub bar, you can still do manual spin-ups, but we almost never do, even when we are both around, simply because the whole system works so well. Its certainly a bit more fuss than a conventional prerotator, but it has its own advantages in terms of a lack of stress to critical components, and it did fit our problems with the high mast and the need to say legal. Once we knew the system worked, we added a few touches to improve operating convenience. The first was a nifty spring-type starter tool, that stows neatly in the gyro and eliminates the need for the 12V engine starter. We also wired the glow-plugs of each engine to a plug-type fitting at the rear of each module, allowing us to power the glow plug without plugging the igniter unit into the top of the engine.

The starting procedure is extremely simple. Plug in the glow plug battery for engine #1 and start it with the spring starter. The engine will start and run at full throttle with no need to adjust the needle valve and the glow plug battery is unplugged without getting your fingers anywhere close to the prop. At this point you release the hub bar and the running engine will rapidly haul the blades around (a single engine will get the blades up to over 60 rpm!) so you can repeat the process with the second engine. Assuming both engines have been fueled in advance, a process that takes only 2-3 minutes, both engines can be running in 30 seconds or so with no effort!

One thing we didn't expect was that we have the ability to stop the engines very easily. You can do this by letting the relative wind accelerate the blades and then flatten the rotor disc for a moment to allow the blades to slow slightly due to drag. The fuel pick-up is directed to the rear of the tank and, as the blades decelerate slightly, the fuel is thrown forward in the tank, shutting down both engines in a few seconds! By using this technique, we can shut the engines down with most of the fuel still remaining. Since we carry the glow plug battery and starter on the gyro, we can fly cross-country to another airport, refuel the "big engine", and still have most of the fuel available in the rotor tanks for another half-dozen prerotations!

On the other hand, if you simply hold the rotor disc horizontal, the engines will hold the blades at 100 rpm for as long as 20 minutes, irrespective of wind strength, direction, or the taxi speed of the gyro - very handy in high-traffic situations. When you are ready to fly, the additional "boost" provided by the engines will bring up the blades to flight speed very quickly as you bring the stick back to feed air into the rotor disc. You can stop the engines with a slight pause in the spin-up at that point, or simply press on and let them shut down in the air when their fuel runs out! All in all, it works out so well I wonder why we didn't try it years ago. In the meantime, we are easy to spot at fly-ins - just look for the tri-motor gyro!

Figure 1. A front view of one of the engine modules, showing the engine, muffler, prop and spinner. The aluminum shroud to the rear encloses and retains the 8 ounce fuel tank.

Figure 2. Rear view of one of the engine modules, showing the lower plate that clamps the module to the hub bar using AN-3 hardware. The mufflers supplied with the engines are very effective and help direct the exhaust away from the modules.

Figure 3. A view of the hub bar showing both engine/tank modules. The two 40-sized engines chase each other around in a circle, bringing the 25 foot Rotordyne rotor system up to over 100 rpm as fast as any mechanical prerotator, without putting any stress on the mast or head!

Figure 4. Backing off just a bit, you can hardly notice the unconventional prerotor set-up.

UPDATES

Since the original system was installed over a year ago, I have made a few basic changes which I will document in detail both in Rotorcraft magazine and on this site. For the moment, here is the summary.

Diesel Engine Conversion

Glow plugs on conventional model aircraft engines are a real pain. First, you have to have a 1.5V battery pack to start the engines and, second, you can waste quite a bit of time trying to start an engine if you have burned out a plug and don't realize it. To get around these problems, I have converted the OS40 engines to diesel using diesel conversion heads ($55 each) available from Davis Diesel Development in Connecticut. The engines now start and run without glowplugs. In addition, there is considerably more power available. The glow engines swung a 10-6 prop while the dieselized versions swing an 11-9! Paradoxically, the greater power is achieved with a 50% reduction in fuel burn, primarily because the kerosene base used in the model diesel fuels has a better BTU rating than the nitromethane used in glow engine fuels. The engines also run cooler with better lubrication.

Dragon Wing Modifications

With the old Rotordyne blades, the engine mounts clamped directly to the hub bar, but the hub bar of the Dragon Wings blades we are now flight testing is too short for this type of mounting. The engine bearers are now mounted atop 3 inch high pylons which bolt to the inboard two bolts (out of the total of six) used to tie the blades to the blade straps on the hub bar.

With this set-up, the motors are only two feet apart instead of five, with the result that acceleration is a bit slower but still equal to or faster than a conventional prerotator. The Dragon Wings are a real pain to start by hand, but the VDU system brings them up with no hassle.

Ralph E. Taggart (gyrobee@aol.com)