Fuel Cells, Part 3

KITPLANES Magazine, April 2003

Electric Powered Aircraft – Part 3
Motor and propeller selection are the topics this time.
By James Dunn

 
This article is the third in a special multipart series. Part 1 (April ’02) outlined the background and challenges involved in developing an electric-powered airplane and the overall significance of this challenging project. Part 2 (September ’02) explained the approach used to select the ideal aircraft to electrify and the specifications of the candidate airframe selected for this project.
    This month’s article covers the selection of the electric motor and propeller and explains the performance benefits and power available from a high-efficiency electric drive system.

Project Objectives

    This project focuses on designing, building and testing a safe, practical two-place general aviation airplane powered by DC electricity from fuel cells and advanced rechargeable batteries. The airplane can take off, fly more than 250 miles on a singe charge, and land safely. This breakthrough project is being developed with special funding from NASA and the Foundation for Advancing Science and Technology Education (FASTec), a not-for-profit 501-c3 program. The charter is to explore new frontiers in science while educating our population on the benefits of advanced energy storage and transportation systems. If successful, this unique aircraft could create a new paradigm for future transportation technology, paving the way for the next century of flight.
    To demonstrate the feasibility and usefulness of electrically propelled aircraft, an existing light-weight, low-drag, carbon composite aircraft is being converted to electric propulsion, replacing the typical gasoline-powered internal-combustion (IC) engine with a special high-efficiency electric drive system with advanced controls and instruments. The electricity to power the aircraft will be provided by a bank of advanced high-energy rechargeable batteries, augmented in Phase II by a hydrogen-powered fuel cell to extend the range.

 Why Electric Aircraft?

     Electric-powered aircraft offer many benefits including dramatic improvements in reliability and safety, lower maintenance and total lifecycle costs, significant improvements in environmental compatibility (noise, emissions and fuel), improved performance, and improvements in ease of operation and passenger comfort.
    The biggest benefits are reliability and safety. With only one moving part (motor armature plus propeller), electrically powered aircraft should be far less susceptible to failure; there’s not much to fail.
    Electric drive also offers significant improvements in performance. Initially, performance of conventional GA planes-particularly overall range-will be difficult to match. But in terms of total available peak power per pound, electric motors have a huge benefit over gasoline-fueled engines. For a similar power rating, the electric motor can weigh significantly less than a comparable gasoline engine and produce significantly more peak power. This is due to the much higher operation efficiency and short term over-power potential of electric motors, allowing them to produce up to 300% of rated power for short durations, critical for takeoff, rapid climb and missed approaches. Electric motors will also offer dramatically better performance at altitude because they do not breathe air and don’t suffer from loss of power at high altitude. The are also immune to carburetor icing and fuel contamination.
    The aircraft selected fro conversion is a high-performance, all-carbon, two-place French DynAero Lafayette III provided by American Ghiles Aircraft in Dijon, France. The basic diagram of the electric drive system is shown in Figure 1.

Motor Selection

    To begin the propulsion system selection process, we established several basic objectives:
    1. The primary goal was to match the performance of the standard gasoline-powered version of the Lafayette III Bushplane…still using a tractor-type drive configuration.

    2.The second objective was to provide the highest level of total energy efficiency, providing the best performance, range and operating time, along with the least amount of electrical energy. This includes the combined efficiency of all components from the energy source (batteries) to the motor and propeller.

    3.The third objective was to minimize the total weight of the propulsion system components because the energy source components will be significantly heavier than the gasoline and tanks it will be replacing. Total propulsion system and energy source weight is critical to range and payload.

    4.The final objective was to select components that had already proven their reliability with a minimum of 50 units being successfully deployed in field applications. This is particularly important because the overall reliability and safety are the most important overall considerations in pioneering aviation projects like this.
   
Target Power Level. The normal engine of the target aircraft is a Rotax 912S, which produces 100 hp at 6500 rpm. The Lafayette III, however, is an extremely efficient design, providing exceptional performance with only 80 hp (130-knot cruise and 180-knot top speed). Because electric motors can typically produce significantly more power for short periods, we targeted the motor power level selection toward the optimum cruise power levers, knowing the motor has the ability to produce similar peak power levels to the Rotax 912 for takeoff and climbing flight.
    We also decided to look for a motor that would produce peak torque at low revs (1200-2700 rpm), thereby eliminating the need for reduction gearing that is typical of most Rotax engine installations. Eliminating the need for gears or belts and pulleys not only reduces the weight but also increases the overall reliability and efficiency of the electric propulsion system.
    Electric motors have a huge power density benefit over gasoline-fueled IC engines, particularly in terms of total peak power. Thus, for a similar power rating, an electric motor can weigh as little as 50% of the weight of a comparable gasoline engine, yet still produce significantly more peak power. This is due to the much higher operating efficiency and short-term over-power potential of electric motors. Electric motors are usually rated in kilowatts (kW); 1 hp= 746 W (or 1 kW =1.33 hp). Our target electric motor power level is 40-60kW (53-80 hp).

 Efficiency. Electric motor technology is quite mature, with numerous high-efficiency brushless DC and AC induction motors currently available that should be suitable for aircraft applications. Most modern electric motors provide efficiencies of 80-95%, based upon the basic motor design approach.
    This is a dramatic contrast to the efficiency of most typical IC engines of only 18-23% (diesel engines provide much higher efficiencies of 27-36%, but still significantly lower than electric motors). Our target was to find a motor and controller combination that would provide at least 90% total combined efficiency over the target operating band of 1500-2700rpm (prop speed). Several basic motor design approaches were considered including both AC and DC designs.
    Weight. Although the Lafayette III has an extremely high empty-to-gross-weight range, we need to reserve as much weight capacity as possible for the energy source (batteries, fuel cell, and other system components) as time is the critical factor that will determine our useful range and payload. The total firewall-forward weight of the Rotax engine-complete with muffler and all peripheral equipment and gasoline tank-was estimated to be 165 pounds. As a rough rule of thumb, DC motors can produce approximately 1 hp per pound in the size range we were seeking (50-100 hp). After talking with a number of motor suppliers, we set a target weight goal of 110 pounds for the combined motor and controller (leaving 55 pounds for the heat-exchange system and instruments). The propeller was assumed to weigh approximately the same as similar propellers for gasoline-powered aircraft.

Picking the Best

     Several different motor technologies and design methods are used in industry including AC- and DC-powered design approaches. The most efficient designs typically use permanent magnets (instead of coils) in the stators. These require less energy to create the basic magnetic field, with permanent-magnet brushless DC motors usually being much more efficient than older brush-equipped designs. They also offer much higher reliability and virtually no maintenance.
    After reviewing several candidate motor designs, we decided that a brushless DC permanent-magnet (BDC-PM) design would be best suited for our application, rather than an induction, switched-reluctance, or other motor design configuration.
    Although brushless DC motors require a more sophisticated controller unit (typically a three-phase, pulse width-modulated [PWM]), total efficiency is typically 3-7% higher than brush-style motor. (Brushless motors replace the brushes and commutators with a controller that electronically switched the power to the coils, eliminating the brush and commutator wear and arcing).
    Brush-type motors were also eliminated from selection due to the use of hydrogen on board for the fuel cells and the risk of ignition from sparks produced by the brushes. After significant analysis, we created the following requirements for the electric motor:

The Gang of 12

    A study was done of available motors that met the target requirements, and a total of 12 motor candidates were analyzed with the help of Solectria Corp. in Woburn, Massachusetts. Nine motor manufacturers considered were AC Propulsion, Fisher Electrical, GE, Kollmorgen, Lynx Motion Technology, Solectria, Technologies M4, UQM Technologies, and Zytek. Of the 12 candidate motors, only three suppliers appeared to come close to meeting the target requirements. They were Technologies M4 of Toronto, Zytek of the U.K., and UQM Technologies of Golden, Colorado.
    The best candidates from the three key suppliers were the Zytek PM4.2 60-kW BDC liquid-cooled motor with an MC6.2 controller; Technologies M4-B2R-670 Drive system using a 75-kW induction type; and the UQM Technologies Corp. Caliber EV53 53-kW BDC motor with a CD40- 400 controller.
    The final motor selected was the EV218 53-kW (71 hp) brushless DC motor from UQM Technologies Corp. This motor offered the best overall fit with our requirements. It has been used in a wide range of electric vehicles and other demanding applications. The only drawback of high-performance motors is the cost, due primarily to the relatively low-volume production. If produced in high volumes, motors like this should cost no more than $3000, versus about $15,000 currently. 

Propeller Selection

    Objectives. The propeller for the electric drive should meet or exceed the performance of the normal recommended propeller for our AGA airframe: an MT three-blade prop made in Germany.
    1. The main objective was to optimize the propeller efficiency and suitability for use with direct electric motor drive. Because the electric motor selected produced optimum torque at low rpm (in the 1000-2500 rpm range), direct drive was possible, eliminating the need for speed reduction.
    2. The ideal application with the most operating flexibility should provide the means to operate as a variable-pitch prop, or in constant-speed mode, with a control system that allows the pilot the option of setting the prop pitch to a specific setting or using the constant-speed option (maintaining a fixed propeller speed).
    3. An electrically actuated propeller was required as our aircraft has no hydraulic or vacuum system.
    4. Propeller system weight should be kept to a minimum, ideally under 20 pounds.
    The candidates included MT, Airmaster, Warp Drive and Ivoprop. (Other manufacturers may also offer suitable props, but these were the candidates suggested by the aircraft manufacturer.)
    Although all of the propeller choices offered benefits, the Airmaster AP332, a high-quality, electrically operated constant-speed propeller system, was selected for several reasons.
    1. The Airmaster AP332 propeller has demonstrated good performance on a wide range of aircraft with engines in a similar power range to ours. These aircraft included several high-performance aircraft in the same class as the Lafayette, most notably the Europa. (See the applications part of the Airmaster web-site http://www.propeller.com/.)
    2. The Airmaster propeller is also fully feathering with a consequent large pitch range. This feature seemed of interest as it could allow for future investigation of using the windmilling propeller to provide regenerative power generation on descent.
    3. The Airmaster AP332 propeller control system has extremely low power consumption; only about 1A current is drawn while the propeller is changing pitch (a couple of seconds occasionally), which is significantly less than most other electric hubs.
    4. The Airmaster AC200 electronic constant-speed controller is completely configurable by the operator. This means that with simple programming, you have complete control over the preset speeds programmed into the controller. Using the Auto/Manual mode, the pilot also has the option of selecting specific pitch settings to quickly match the electric motor output and propeller performance for specific predefined flight modes.
    5. Airmaster uses Warp Drive composite blades providing a high-aspect-ratio blade platform. Warp Drive propellers are built using an all-carbon-fiber matrix. No foam, fiberglass or gelcoat is used in these blades. The structural, performance and practical advantages of a carbon propeller over any fiberglass, wood or metal prop are many, including superior strength, light weight, and (we hope) a longer useful life. The construction of Warp Drive’s blades allows for simple repair of basic nicks and gouges, and the blades are individually replaceable. We determined this to be the best blade platform for the aircraft, motor power, prop diameter (68 inches) and target airspeed range.
    Airmaster Propellers Ltd. has supported our project greatly, even providing a propeller at no cost. The company sees the advanced technology of its propeller systems as a good match to the ground-breaking technology focus of our project.

Prop Adaptor and Mounting

    To minimize mechanical losses, it was determined that the propeller should ideally be connected directly to the motor shaft, eliminating the complexity of gears, belts and pulleys. This was accomplished by designing a special propeller adaptor assembly. (See Figure 2.) The propeller adaptor assembly includes 10 machined parts plus mounting bolts.
    The total combined propulsion system efficiency was optimized with the UQM Technologies motor directly driving the Airmaster propeller, with no losses to gearing or belt reduction systems. The total aircraft reliability is also enhanced with fewer parts to fail. Although this specific propulsion system offers an extremely high total overall operating efficiency, there are several other configurations that could also provide suitable performance for this unique application.

What’s Next?

    Picking the energy source to power the electric drive will be the topic next time. The airplane has begun first taxi tests using batteries not intended for flight. First flights with airworthy batteries are expected in a few months.