Air BP, the official fuel and carbon reduction partner of the Red Bull Air Race, knows just what fuel a raceplane needs to perform at the top level of motorsport. And as it turns out, the demands of a cutting-edge raceplane are a lot different to those of racing cars.
"Those differences are intrinsic in the engine design," says Dr. Alisdair Clark, Air BP's Aviation Fuels Research and Development Manager. "In auto racing, teams will use the lightest engine they can manage as long as it will last the race, and they'll push it to the limits. In aviation, however, the engine can't be stripped right back due to the risk associated with failure. A greater margin of safety must be built in because you need to stay in the sky!"
In the Red Bull Air Race, the engines are standardised not only for safety but to ensure a level playing field. A high-performance Lycoming Thunderbolt AEIO-540-EXP – six cylinders, air cooled and about 320 horsepower at 2,950rpm – that was specially designed for the sport is the trusted powerplant for every team. Air BP produces fuel aimed to help such aviation engines perform reliably and to their optimum.
"Once on the racetrack, our engines run full throttle throughout," says Red Bull Air Race Technical Director Jim "Jimbo" Reed. "When you're flying at 200 knots, 20 metres above the ground, reliability is a very good friend to have."
Among the factors that must be considered in fuel development:
Aviation engines are different to those of racing cars – for example, they are slower revving. For aviation, propeller speed is very important. "It's all about dependability, and the role of an aviation engine is to turn the propeller at 2,000 to 3,000rpm" says Clark. "Meanwhile, in a racing car the prerequisite is to generate maximum heat energy, which can be achieved by different types of fuel, with high-revving engines burning a lot of fuel very quickly in small amounts – it's a simpler way to get a lot of power out of an engine with the advantage of a gearbox."
Formula One engines, for instance, are limited to 15,000rpm, while the Red Bull Air Race rules specify a maximum of 2,950rpm.
Engine combustion chamber size differs as well. Aviation engines typically use much larger combustion chambers that burn the fuel/air mixture relatively slowly, compared to hot-running racecar engines. At top dead center on the 10-to-1 compression ratio engine type used in the Red Bull Air Race, the clearance volume in the head is about 164ccs, while in ground-based F1 motorsport the total combustion volume for the whole engine is 1600ccs.
Additionally, an obvious but key difference is that the engines powering racing cars like those in F1 must cope with high longitudinal and lateral loads, while Air Racing is a sport in three dimensions, so raceplane engines cope with motion in all three senses: roll (motion around the longitudinal axis), pitch (motion around the lateral axis) and yaw (motion around the vertical axis).
Turbochargers and superchargers
Some, though not all, auto racing motorsports allow teams to turbocharge their engines, enabling greater power output for the given engine size. And of course many of our daily drivers have turbocharged engines, too. But – again to ensure competitiveness as well as safety – Red Bull Air Race engines are not turbocharged or supercharged, making fuel quality all the more important. Outside the racing circuit, use of turbochargers in aviation is relatively common, not so much as to increase power, but to maintain performance at altitude where the air density is lower.
In many racing cars, the use of substances such as ethanol and methanol is permitted, which contain oxygen and can increase power output. But they can also reduce range, and with reliability being paramount, ethanol and methanol aren't used in aviation fuels, aside from specialist additives such as MethMix.
However, in the Red Bull Air Race, pilots can ensure they are running their engine at its optimum by adjusting the fuel mixture manually. For peak performance and perfect, efficient combustion, the engine needs a ratio of 14.7 parts air to one part fuel. The pilots have a hand control to adjust the fuel mixture, but it's a delicate balancing act. If there's too much fuel in the mixture, it will be too "rich," meaning the engine will lose power. Similarly, if there is too little fuel in the mixture it will run "lean," again losing power. In other words, there's a lot more to flying a raceplane than just pulling on the stick!
Racing cars use multiple gears to optimise engine speed and road holding. Although some aircraft do have geared engines, the raceplanes of the Red Bull Air Race do not, which saves weight. However, the sport's standardised Hartzell propeller does provide a function with similarities to gearing. The three blades – which are essentially small wings pulling the aircraft through the air – have an adjustable pitch, or angle of attack. For low speeds, a flat pitch is best, like the low gears of a car, but for race speeds, a much steeper pitch is needed to efficiently transfer engine power to the air, like a car in a high gear. The pilot can control the angle of the propeller blades from the cockpit. (Fun fact: On some aircraft, the tips of propeller blades actually go supersonic, but that would be a disadvantage in Air Racing, because there is a large increase in drag when prop tips get close to the speed of sound. Red Bull Air Race propeller tips reach about 0.89 Mach at race rpms.)
The difference in aviation and auto engine design means a difference in the octane of the fuel, too. "Octane is the fuel's ability to wait for a spark, how much compression and heat it can stand before igniting," Clark describes. "The higher the octane rating, the less likely it will be that the fuel will ignite unexpectedly at higher pressures and temperatures, which potentially could cause damage to the engine."
That knock you sometimes hear when a car drives by on the street? That's the sound of detonation, the fuel exploding. A smooth-running engine is crucial for aviation, and in the Red Bull Air Race the raceplanes fly on 100LL (low lead) octane, while Clark says that the best fuel commonly available for racing cars on the ground features a Motor octane of about 88. "In everyday life, everyone takes fuels for granted, but racing experts are always interested in that little bit of difference that can make them go faster, and that goes for Air Racing as well as motorsports on the ground," Clark notes. "At Air BP, it's great to be involved with fuels research, looking to improve performance together with safety and reliability."