11 Oct 2017

Red Bull Air Race 2017: Weight saving and flying faster

Since 2014, Red Bull Air Race rules have specified a minimum aircraft race weight of 698 kg, including pilot and fuel. Previously, teams could shave weight however they saw fit. However, over-aggressive lightening of structure could compromise airframe strength so this rule has been revised. In addition, a previous rule used average pilot weight as a handicapping strategy, although that didn’t seem quite fair either. Currently, most races see 8-10 teams at the minimum weight. Some airframes, such as the Edge V3, are already engineered for maximum lightness, giving those teams an initial advantage but also making additional weight reduction more challenging.

Weight impacts racers in three distinct areas: linear accelerationcentripetal acceleration, and stability. First and most obviously, a lighter vehicle accelerates more rapidly than a heavy vehicle. Racers bleed speed with every loop and turn, so having the lightest aircraft possible helps the airplane regain speed.

More important than straight line acceleration is centripetal acceleration. Race planes in the Red Bull Air Race World Championship experience high G force, meaning that the wings generate huge amounts of lift (centripetal force) to pull the airplanes through all those tight curves and loops. While the wings are aerodynamically and structurally capable of producing such huge lift, the price paid is induced drag and trim drag.

Wings produce lift by creating a pressure differential between the top (low pressure) and bottom (high pressure) of the wing. At the tips however, air leaks from the high pressure to the low-pressure side, creating swirling vortices that represent wasted energy: this is called induced drag. The greater the lift, the stronger the vortices. We’ll discuss the use of tiplets to reduce vortices in a future article, but irrespective of wingtip design, reducing aircraft weight reduces the lift required for high-G maneuvers, which in turn reduces vortice size. Thus, lighter aircraft have less induced drag.

In addition, airplanes typically generate negative lift with the tail. This is necessary to achieve stability, since the wing generates a nose-down torque that must be counteracted by the tail. If the wing’s center of lift is located (as is normal) behind the center of gravity (CG) of the aircraft, increasing wing lift during a high-G maneuver requires increased down lift from the tail to maintain balance. This is called trim drag and again, lighter aircraft will have less trim drag than heavier aircraft.

The location of the CG has a huge impact on airplane performance. Moving the CG forward of the center of wing lift requires the tail to push down harder, creating extra drag. The advantage however is that the airplane is more stable and easier to fly in this configuration – and smooth flying is less ‘draggy’ than ‘twitchy’ flying. In contrast, moving the CG aft reduces the load on the tail and the racer is potentially faster if the decreased stability doesn’t lead to excessive pilot control inputs (which cause drag). Modern fighter planes use this approach, called “relaxed stability”, to improve speed and maneuverability, and would be unflyable without multiple computers maintaining artificial stability.

Interestingly, even though the minimum raceplane weight is 698 kg, teams strive to reduce weight below the minimum. Why? Well, if you reduce the weight to 688 kg, for example, you must place 10 kg of dead weight back into the airframe in order to avoid disqualification. However, you can now replace that 10 kg wherever you’d like – and that can be a huge advantage. Although teams hold their aircraft CG location a closely guarded secret, those teams with dead-weight capacity have an advantage in being able to experiment with fore/aft CG location to achieve the best compromise between speed and stability.

Teams employ various methods to achieve weight loss. Probably the easiest is to put the pilot on a diet! But assuming that has already been taken care of, some of the low hanging fruit involves replacing steel bolts with titanium, replacing traditional aircraft batteries with lithium-iron batteries, utilizing lightweight components like Beringer wheels, and generally going over the aircraft with a fine tooth comb to eliminate any excess weight. That could entail drilling lightening holes, replacing fiberglass parts with molded/prepregged/autoclaved carbon fiber, and even ensuring that excess bolt threads are trimmed flush with the ends of nuts or cutting electrical wires to the absolute minimum length. While most of these tactics will only yield a few grams of weight loss, in aggregate the total can be many kilograms. In a race where only a few hundredths of a second separates winners from losers, weight loss is serious business for team in the Red Bull Air Race World Championship.

What to watch for: Notice whether aircraft seem to be smooth or twitchy about the pitch axis – this may indicate the CG set up. Wingtip vortex rings can sometimes be seen when aircraft fly through their own smoke.

Photo: Juan Velarde’s race place © Predrag Vuckovic/Red Bull Content Pool