Courtesy of Red Bull Newsroom
As the Red Bull Air Race World Championship evolves, teams are investing more time and money into their raceplanes to gain a competitive edge. Two main factors affect a pilot’s time through the racetrack (aside from pilot skill, of course): engine power and airframe drag. With teams relatively restricted in what they can do to the stock engine, the need for speed has focused on reducing drag.
Although hard to believe, the drag caused by the air entering the small cooling inlets on either side of the propeller accounts for approximately 10% of the total drag of the airplane – equivalent to nearly 30HP. That’s power that could be converted into speed if engineers can figure out how to cool the engine more efficiently.
Cooling can be increased in two ways: make the radiator (or more technically speaking, heat exchanger) larger, or increase the velocity of air passing through it. Fluid dynamics tells us that if we double the size of the heat exchanger, we also double its drag. However, if we double the velocity of the cooling air instead, we increase the drag not by a factor of two but by a factor of eight (heat exchanger drag increases proportionally with size, but as the cube of flow velocity). It follows that large, thin radiators in a stream of slow moving air are vastly more efficient at shedding heat than small, thick radiators in a stream of high velocity air. This phenomenon governs teams’ attempts to reduce cooling drag.
As fans of Formula 1 motorsports will know, this is accomplished by actively managing airflow with a variety of diffusers, ducts and vanes. However, compared to the water-cooled F1 engines, the Lycoming Thunderbolt Red Bull Air Race engine uses air-cooled cylinder heads. In addition, oil is cooled by heat exchanger and provides about one-third of total engine cooling.
Teams apply heat exchanger theory to their aircraft to reduce cooling drag. For cylinder head cooling, some teams use a plenum, a sealed airspace above the engine, which in combination with the air inlets, acts as a diffuser to slow down and pressurize air above the cylinders. This high pressure air is then routed directly against the cooling fins as it flows down past the cylinder heads and exhaust pipes. Finally, the cowl cooling exit is shaped to reaccelerate the heated air. Although not all teams use a plenum, most carefully fabricate the baffling which guides the cooling air past the engine to avoid air leaks that would otherwise compromise cooling efficiency.
Traditional Lycoming aircraft oil coolers are quite compact, which makes them less efficient but simplifies installation in the tight confines of the cowl. Mounted directly behind the rear cylinders, they also see a degree of prewarming of the cooling air by the engine. Race teams are increasingly replacing and relocating their oil coolers, utilizing larger cores to reduce drag and dedicated intakes to optimize diffuser geometry and avoid preheating. The main challenge of course is finding a place to package all this.
The final piece of the cooling puzzle is modifying the size of inlets and outlets to optimize cooling for race conditions. Varying the size of the cowl inlet during flight is not practicable, so finding the optimum inlet size is something most teams try to do in the off-season, or between races. On the other hand, cowl flaps, which increase or decrease the cooling flow exit area in flight, allow real-time fine tuning of cooling system air flow. It is claimed that the famous P-51 fighter, with its diffuser-fed liquid cooled radiators, had zero cooling drag since the expansion of heated air in the cooling duct actually created thrust. True or not, optimizing cooling can definitely make a big difference.
The international nature of the Red Bull Air Race means that cooling remains a significant challenge for teams, no matter how optimized the setup. A system that functions perfectly on a cool day at a European stop for example may seriously overheat the engine in Abu Dhabi, United Arabic Emirates reducing horsepower. In contrast, a system that can keep up with Abu Dhabi’s 50C temps will generate excessive cooling drag in a race where temperatures are cooler. So even if cooling can be mastered, there remains a compromise to be drawn: if cooling is too aggressively minimized, more speed will be lost to reduced power than gained from reduced drag.
What to look for in the hangars and in the racetrack: notice the relative sizes and shapes of the air intakes between different airplanes. Also notice the size and shape of the cooling air exit (typically coupled with the engine exhaust exit). Smaller sizes indicate a more efficient (or possibly just hotter running) system. Note too that some airplanes, such as Juan Velarde’s Edge V2, have separate oil cooler exhausts, indicating a highly optimized system.
About Red Bull Air Race:
Created in 2003, the Red Bull Air Race World Championship will celebrate its landmark 75th race at the 2017 season opener in Abu Dhabi. The Red Bull Air Race World Championship features the world’s best race pilots in a pure motorsport competition that combines speed, precision and skill. Using the fastest, most agile, lightweight racing planes, pilots hit speeds of 370kmh while enduring forces of up to 10G as they navigate a low-level slalom track marked by 25-meter-high, air-filled pylons. In 2014, the Challenger Cup was conceived to help the next generation of pilots develop the skills needed for potential advancement to the Master Class that vies for the World Championship.