Model Engine Prerotators

One of the OS Max 40 model aircraft engines on the Gyrobee, converted to diesel operation. The Gyrobee flew on Dragon Wings blades at Mentone 97 and, due to the very short hub bar, the engines had to be mounted on short pylons as shown here.

Last year (1996) I introduced the VDU (Very Different Ultralight) prerotator concept in an article in Rotorcraft. The idea is a simple one - use a pair of model aircraft engines to provide a light but effective prerotator for ultralight gyros. Since then, a lot of people are looking at the idea (and even implementing it), but there is also some mis-information circulating that can lead to unrealistic expectations for this system, not to mention money spent on hardware that doesn't work as expected. I want to start with a realistic assessment of what you can expect using model aircraft engines for prerotation, and then get into the various ways you can implement such a system.

What to Expect

Prior to getting into the details of what kind of performance you can expect, let me start with the premise that the whole reason for "inventing" the VDU approach was to avoid the hassle of manual blade spin-up. Although we had flown the Gyrobee for years in the manual start mode, starting the blades by hand is less than ideal for a number of reasons:

• Reaching the blades to do the spin-up is a real hassle if you have a tall mast and it is hard to put a real push into the blades.
• Assuming you can get the blades up to 50-60 rpm without cardiac arrest, you still have to start the engine, get strapped in, and get the gyro moving. This can take enough time that the blade speed can decay to the point where they can't be picked up.
• You then have to spent time taxiing into the wind, slowing increasing the blade speed, until you are past the point where the blades might flap.
• If you get caught in runway traffic, especially with an unfavorable wind, you can lose all that precious blade speed and have to start all over again.

My hope was that the model engines would at least produce the equivalent of a strong manual start (50-60 rpm). Since the blades couldn't decay with the engines running, most of the problems cited above would disappear. We might still have to coax them up to speed to begin the final take-off run, but that would be a vast improvement over a manual start!

What we got, using a pair of O.S. Max 40 engines (with 10-6 props) was a solid 100 rpm on our Rotordyne blades, with the engines located about two feet out from the rotor hub. This speed was verified with both the computer tachometer in the Digipod and timing of the blade rotation with a stop-watch. This was fast enough to eliminate all the problems in the list shown above and allowed us to taxi straight out to the active, ignoring the wind, and immediately begin a take-off run. Since the engines continued to provide thrust all through the spin-up sequence during take-off, we ceased to worry about high-speed flap (over-running the blades) and we had significantly shorter runs to boot!

In order to have a realistic appreciation of the results you can achieve with available engines, you have to understand what limits the system. As blade speed is increased, drag increases. If we assume the engines generate a specific amount of thrust at any given power level, the blades will stop accelerating when the blade drag is equal to the available thrust. The weight of the blades has relatively little impact on top speed, although lighter blades will accelerate faster during the initial stages of spin-up.

Shown below are the calculated speeds that can be achieved with a pair of engines of commonly available displacement. The table is based on the following assumptions:

• Calculations are referenced to the measured performance of a pair of OS Max 40 (0.40 cubic inch) glow engines, equipped with a well-matched 10-6 prop and running a "sport fuel" mix of 5% nitromethane.
• The blades are 10 foot Rotordynes on a 5 foot hub bar.
• The engine modules are placed approximately two feet out from the center of the hub bar.

The Module MPH column shows the actual airspeed of the engine module at the rotor speed indicated. Over the speed range shown here, this has essentially no impact on performance with a direct-drive glow engine, but it will be significant when we discuss geared electric engines at a later point. In looking at these numbers, the following cautions are in order:

• The OS Max 40 engines are near the top of their class with respect to power output. Performance will be reduced with less expensive engines, although the impact will be small.
• The calculations assume that each engine size is properly propped. The manufacturers recommendations should be followed here.
• Increasing the "nitro" content of the fuel will increase performance, but it is doubtful that the small increase in blade speed is worth the added stress on the engines.
• The engines will be operated at full throttle with the needle-valve adjusted just to the rich side of maximum power output.
• The position of the engines is very important. As the mounting distance is shortened, relative to the two feet of our initial installation, maximum speed will decrease. Increasing the distance beyond two feet will improve spin-up to a point, but the limiting factors will then be drag generated by the engine modules, loss of efficiency with a faster engine velocity, structural limits with respect to mounting, and increased centrifugal loading of the modules, including mounting hardware, module components, and stresses imposed on the engines as they run.
• The calculations assume that power output/thrust is proportional to engine displacement. This is not strictly true but the errors, with respect to maximum rotor rpm, will be modest.

Based on our experience with the prototype VDU unit, these calculations would appear to be quite reliable. For example, we have prerotated on a single engine on a number of occasions. Calculated rotor speed with a single 0.40 engine is 71 rpm and this is very close to the measured value. This year we converted the 0.40 glow engines to diesel and the diesel head manufacturer claims a 50% power increase - a value supported by numerous reviews in the various model aircraft magazines. This would make the pair of 0.40 engines essentially equivalent to a pair of 0.60 engines, which the table suggests should top out at 122 rpm. In our case, we measured 120 rpm, which is too close to argue with.

Is It Worth It?

It is obvious that it is fairly easy to obtain prerotation in the 90-110 rpm range using readily available model aircraft engines. This may seem modest compared to the 200-300 rpm that is possible with hydraulic and other very powerful engine-driven systems, but it is comparable to what many pilots actually obtain using friction-driven, flexible shaft systems and is a major improvement over spinning up the blades by hand! The advantages are significant:

• Cost: We spent approximately $400 for the prototype system, using top-of-the-line engines ($150 each). There are very reliable 40-size engines available in the $50 range that would reduce the total cost to about $200. This is very inexpensive compared to conventional systems.
• Weight: Total weight for a typical system will be 4-5 pounds or less.
• Stress: This approach puts no differential stress on the head and mast. Watch these components on a gyro with a conventional prerotator and you will see the difference!
• Torque-free: This is a torque-free drive system, so you can (and will) lift off with the system running. The only effect is a slight, but observable, increase in climb performance while the engines are running.
• Sustained input: When a conventional system tops out, the relative wind must supply all the energy to bring the blades up to a faster speed. Since a conventional system is usually disengaged for reasons having to do with torque, the wind has to also sustain the blades at the top speed achieved by the prerotator. The VDU system provides useful thrust through the entire spin-up sequence. If both a conventional and VDU system pre-spin to 100 rpm, the VDU-equipped gyro will be off the ground faster with less chance of over-running the blades.
• No brakes: A conventional engine-driven system will require effective brakes to hold the gyro during the prespin. No brakes are required for the VDU system and the main engine doesn't even have to be running! Not having to install brakes is an important issue for some machines if they are to stay Part 103 legal.

The positive aspects of the system are very persuasive, but there are some negatives that you have to consider, most of which apply to glow/gas/diesel engines, but not to electrics:

• Fueling and Starting. You have to fuel the engine modules and get them started. This will require various accessories, which are included in the description of each engine option. All of this is additional work, compared to just starting your engine and engaging a prerotator.
• Safety. When starting the engines, you will be up close and personal with a prop turning 10,000 rpm or more! You should have the engine set up so you do not have to tweak the needle valve, even if this means operating with a bit less power. Your routine needs to be developed to keep your face and hands away from the prop. You should wear your helmet and goggles and leather gloves are not a bad idea. You cannot let yourself get distracted or you will get hurt!
• Mess. All the internal combustion engines have an oily exhaust residue that will be picked up by the blades, hub bar, and head. Diesels are the worst in this respect, glow engines produce a moderate mess, and gas engines probably the least. It will have to be cleaned regularly and that's extra work. On the plus side, bug trash will not tend to stick to your blades, so they will run smoother, longer.

Is the hassle worth the benefit? That's something only you can say, but the answer is definitely yes for Don and I. We flew our Rotordynes for years by hand-starting them, but never did so again after installing the VDU system. Even prerotating with a single engine is infinitely easier and more relaxing than a hand-start and it's a positive pleasure when both are running. This year (1997) we have been test-flying an older set of Dragon Wings with the original linear twist. We proved to ourselves that you can hand-start them, but it is extremely difficult and nerve-wracking. With the VDU prerotator they are real pussy cats. If you think you might like to try a VDU system, it's worth looking at the various options.

Glow Engines

Glow-plug model aircraft engines are the standard and they are universally available, both at local hobby shops and mail-order outlets. These engines are highly refined, easy to start, and very reliable. In addition to the engine, the recommended prop, and a spinner (so you can use an electric starter), you will need the following:

• Fuel. Model aircraft glow fuel is available from the same outlets that provide your engines and other supplies and is best purchased in gallon containers. Use a "sport" blend (lower nitromethane), as there is no reason to over-stress the engines and the lower nitro fuel will cost a bit less.
• Fuel tanks and line. These are readily available from hobby outlets and the tanks come in a variety of sizes. We use 8 ounce tanks.
• Fuel bulb or pump. This is necessary to transfer fuel from the jug to the individual modules.
• Electric starter and battery. Hand-held electric starters make the job of starting even a stubborn engine quite easy. These are normally powered by 12V gel-cell or sealed lead-acid batteries.
• Ni-Cad Glow-plug module. These clip on to the glow-plug while the engine is started, after which they are removed. These are small enough to fit in your pocket and they come with a charger so you can keep them fully charged until your next flying session.

These supplies will add about $100 to the total price of your system, over and above the cost of the engines.

The glow engines work quite well and parts and supplies are easy to get. They do produce a moderate amount of exhaust residue, but it is clear and cleans up easily. The biggest hassle with these engines is the glow plugs. We seemed to go through them regularly and, if an engine wouldn't start, a bad plug was almost always the reason. This is where 40-size engines are ideal. If one engine will not start, you can always prerotate on the other. If you will be flying to a spot where it is not convenient to haul your supplies, you can fuel both modules, using one to leave and the other to come back! All-in-all, glow engines are your least expensive option.

Diesel Conversions

Almost any standard glow engine can be converted to diesel operation using after-market heads produced by Davis Model Products (Ph. 203-877-1670). Heads for our OS Max 40 cost $55 each, or $110 for the set. The conversion is simple - just replace the glow head with the diesel head! Set-up will take a little time - there is a master compression screw to be adjusted and optimized and new needle-valve settings, but once adjusted properly, the engines start easily and run very strongly. Advantages of the diesel option include:

• Power. Because kerosene, the base for the model diesel fuel, has more BTU's than the methanol in glow fuel, power output will be up by a solid 50%. As glow engines, our 40's swung a 10-6 prop. As diesels, they are swinging an 11-9 prop and act like a pair of 60's.
• Cooling and lubrication are superior to that seen with glow engines, so the diesels will actually run cooler while producing more power.
• No plugs. There is no glow plug, so it can't fail, and you don't need the Ni-Cad igniter for the glow plug. There is no ignition circuitry, so a diesel is about as simple as it gets.

Using the diesels is pure simplicity. Put fuel in the tanks, prime the engines, apply the starter, and they are running with lots of power! It many ways these are clearly the best option, but there are two problems. The minor one is that diesel model fuel is not as widely available as glow fuel, although most hobby shops can order it as you need it. You can also order diesel concentrate direct from Davis, mixing it with your own K-1 kerosene, which you can probably get locally. Diesel fuel, like gasoline, will rot the silicon rubber fuel lines and tank stoppers used for glow fuel, but most hobby outlets stock or can order gas conversion kits ($3-$4) that will convert your tanks for use with either diesel fuel or gasoline.

The real problem with diesels is that the produce large quantities of black, sooty oil residue that makes a real mess and it is not really easy to clean up, compared with the residue from glow fuel. In all other respects the engines are marvelous, so I am looking at mechanical approaches to scavenging the oil from the exhaust stream. If that can be made to work, the diesel option is at the top of the list.

Ignition Engines

A number of companies are starting to market modules to convert glow engines to ignition service. The conversion modules are still relatively expensive ($150-$200), but they eliminate the glow plug hassle and let you burn the same pre-mix you are using in your main engine (assuming you are using a two-stroke). I will keep on top of developments in this area, for the gas engines probably leave less exhaust residue than any of the other options and they would be attractive if the price were to come down.

Electric Motors

Ever since the mid-70's there has been a steady evolution of electric model aircraft motors. A wide range of motors is available and, at first glance, they seem ideal in the VDU mode.

• The motors can be turned on and off via a switch, which means that you don't have to get up there to start them and you can shut them down just as easily. This is not only convenient, it is the ultimate in safety since you don't have to be anywhere near the motors and props when they are operating.
• They are clean, with no oily exhaust residue that has to be cleaned up every few flights. Assuming you have the airframe wired to supply the needed power from an on-board battery supply, they don't require any extra support equipment such as fuel and starters. This is very good when you fly to other airports, since you don't have to haul this stuff, either on the gyro or via ground support.
• Finally, they are quiet, so we aren't producing any more noise than you are at present with your gyro.

In short, they look ideal and this, coupled with some optimistic, even fantastic reports of their performance as prerotators, has led to some considerable enthusiasm. Let me start with the painful business of debunking at least one of these reports. Ray, in Sri Lanka, reported that a pair of Astro Flight cobalt 60 motors, with a direct drive prop, got his Sky Wheels blades up to about 200 rpm using a 12V power source. I don't know how the rotor speed was measured in this case, but the reported results are simply not possible! On 36V a cobalt 60 motor is roughly equivalent, in direct drive, to a good 0.6 cubic inch engine. If you look at the earlier table, we can expect perhaps 120-125 rpm from a strong 60-size glow engine - never 200! To add to the problems, Ray's 60 motors were operated from 12V, which means that the available power was way down from the 36V (it's a function of both voltage and current) where the cobalt motor is equivalent to the 60-size glow engine! I won't go through the calculations, but there is no way the blades could have reached even the 100 rpm that we get with a pair of 40-sized glow engines! That said, it is possible to make electrics work as well as glow engines, or even a bit better, but it isn't easy or cheap. Let's establish a target and then see what it will take to get there.

We have been quite happy with the performance of our 40-size engines, so let's use the 100 rpm achieved by the glow version of that system (a bit less ambitious than the diesel performance figures) as the target. Each of these engines turns a 10-6 prop at roughly 12,000 rpm to achieve that performance, so let's see what we can do with with electrics.

Since the glow engine system uses a pair of 0.4 cubic inch engines, a good starting point is a pair of Astro Flight Cobalt 40 engines (Model 640), directly driving the prop. Astro Flight suggests a 10-6 prop for this engine, so we are directly comparable to the glow-engine situation. Assume we place them 2 feet out from the hub, just like the original VDU. Obviously, since prop and location are comparable, we can tell the story entirely on the basis of prop rpm, which, in direct-drive, equals motor rpm. If we can achieve 12,000 rpm on the prop, we will have comparable performance and we can expect the blades to come up to 100 rpm - our target. Less than 12,000 and the blades will be slower, higher than 12,000 and we can expect that we will exceed our target speed.

Astro Flight rates the 40 Cobalt engine as performing best on 16-21 "cells", where a cell is a 1.2V NiCad. Here is where we run into our first problem! 16 NiCad cells = 19.2V while 21 of them produce 25.2V! This is considerably higher than the 12V we typically have with an on-board storage battery. In the table below, I will use four different voltages - a 12V battery source, 16.8V or 14 cells (because your local hobby shop sells 7 cell NiCad packs for electric race cars and we can use two of these in series), 19.2V or 16 cells, and the maximum of 25.2V or 21 cells. The data sheet for the Astro Flight 640 motor (the sport Cobalt 40) says that in will produce 682 rpm/V on the recommended 10-6 prop, so here is what we would get:

The hard fact here is that the system simply cannot make our target of 100 rpm on the blades at 12V and it will miss it by a considerable margin. If we run it at 16.8 (14 cells) to 19.2V (16 cells) we are in the ballpark. Given the nature of the power/drag curve, the differences in this range will have little practical significance and I would go with the 14 cells to take advantage of the availability of the 7 cell packs available for the electric cars. If you were to series three of these packs for 25.2V, we significantly exceed our glow engine target and the results might come close to what the diesel engines deliver.

A hot topic in electric flight circles these days is gearing. By gearing down the motor, you can swing a bigger more efficient prop. Astro Flight does make a geared version of the 640 motor (the 640G) that swings a 12-8 prop with a 1.6:1 gear reduction. Since our gyros fly much better with reduction drives, why wouldn't the same idea work here? Well, unfortunately, there is a problem and it has to do with the propeller pitch speed and it's relationship to the speed of the engine through the air. At our target speed of 100 rpm, VDU modules are traveling at a bit over 14 mph (which is why we can ignore their drag) when located 2 feet out from the hub, but this is a significant percentage of the pitch speed of the geared prop and it only gets worse as you lengthen the moment arm. If you put the engines three feet out, they are traveling at a bit over 21 mph and the problem gets worse, not better.

The best analogy I can give is the transmission in your car. First gear will give you a very strong start, and the geared version of the 40 motor will have a lot more thrust than the direct-drive version and will initially accelerate faster. You don't drive anywhere in first gear however, for the car will "top out" at too low a speed to be practical. The same thing happens with the geared motors. We don't consider this problem with our gyros, simply because the much higher pitch of our props produces a pitch speed that is quite high compared with the maximum speed at which our machines fly. I won't repeat the calculations here, but you could only come close to our 100 rpm target on 25.2V (with the module located two feet from the hub bar), and in all cases, the top speed will be lower than the same motor in direct drive. Performance would be degraded still further by moving the modules out to 3 feet!

Since these are all paper calculations, you may be tempted to ignore this advice and try the geared motors anyway. Be my guest! The cost will be higher and they won't work as well, but go ahead and try.

If we assume that we can have a practical VDU system built around two Astro Flight 640 direct-drive motors and a pair of 10-6 props, where do we get the voltage and how do we deliver it to the motors? That turns out to be a not-so-easy question.

Commutator on the Head

We can get our 25V by using a pair of small motorcyles batteries, gel-cells, or sealed lead-acid cells. With a little clever switching, we can charge them in parallel from the lighting coil in flight, but switch them to series connection to drive the motors when prerotating. The problem is, to get the voltage from the batteries on the airframe to the motors on the hub bar. One direct approach is to use a split-ring commutator to deliver the voltage to the rotating head. We need to handle both the positive and ground leads, since we don't want to draw high current through the frame, since this would mean the various bearings in the system would be carrying the voltage and they could arc over! The current demands of the system present some real design problems. The 640 motors will draw about 35 amps each at 25.2V or a total of 70 amps! With this kind of current, we cannot tolerate any significant resistance anywhere in the drive circuit, for this will significantly reduce the operating voltage and hence performance. Since 25.2V is a bit more than we really need, we do have a small margin for error that we would not have it we ran the system at a lower voltage. You would need to use heavy gauge wire for the ground and power leads and the design of the commutator to deliver the power without significant losses would be demanding. All the switching circuits should be low resistance as well, suggesting the use of starter solenoids to do the actual power switching, controlled by a smaller set of switches at your seat.

With a little design ingenuity and care to avoid resistive losses, a pair of the Astro Flight 640 motors can make a prerotator that is very effective, has none of the drawbacks but all of the advantages of the fuel-operated engines, and is as easy or easier to use than a conventional system.

At 70 amps total current draw, the running time of the system is directly related to battery capacity. Conservatively, you can expect a running time of 0.8 minutes for every ampere/hour of battery capacity. If you are using a pair of batteries rated at 10 AH, you could thus expect about 8 minutes total running time. Since getting the blades up to speed requires no more than about a minute, you should be good for 6-8 prerotations. If you set up the wiring to let you charge the batteries when the engine is running and the prerotator is off, you will get significantly more total time, so that even smaller batteries are practical.

There are a number of manufacturers making electric model aircraft motors, but most are significantly smaller than what is needed in this application. Astro Flight engines work very well and, properly selected, will deliver the power you need. Information on the Astro Flight motor line can be obtained from the AstroFlight website. Shopping this way is particularly attractive since they have special internet pricing. The list price for the 640 motor is $179.95 but is priced at $120.00 for internet customers. The website also includes useful links to sources of information on electric aircraft motors and their use as well as complete specifications on their entire line of products.

Summary

This ended up being a lot longer than I intended, but I thought it was important to provide some sound design data for those thinking of giving it a try. The VDU concept is a very viable approach to prerotation, but misinformation, either positive or negative, can lead to unnecessary expense and frustration.

Ralph E. Taggart (Gyrobee@aol.com)