Machine Design
Unlimited Air Racers: The ultimate hot rods

Unlimited Air Racers: The ultimate hot rods

Mechanics and ground crews use skill and ingenuity to turn old warbirds into modern-day racers.

Dick Medvick
Spencer Medvick
Mike Wilton

Contributing Editors

Edited by Stephen Mraz

Last year at the 37th annual Reno Air Races, over 180,000 spectators watched as more than 30 aircraft battled for the gold in the Unlimited aircraft division. The planes, mostly P-51 Mustangs and Hawker Sea Furies, hit speeds of nearly 500 mph and pulled almost 7 g's as they flew between 50 and 150 ft off the ground around the 8.2-mile course. These planes weren't originally designed to crank out that kind of performance or withstand the punishment the race demands. It's only through the good graces of extremely talented ground crews that any of the planes make it across the finish line.

Here's a little peek inside the bag of tricks these aviation hot rodders use to get the most out of their aging warbirds.

Piston-packin' power
Two of the fastest aircraft on the circuit are Critical Mass and Dago Red. Critical Mass is a TMK-20 Sea Fury, a British fighter carrying an air-cooled radial engine with two offset rows of nine cylinders for a total displacement of 3,350 cu in. Dago Red, a P-51 Mustang, carries a water-cooled engine, a 60°, 1,650-cu-in. V12 Rolls-Royce Merlin engine. This is stock displacement for both engines. With all the other modifications, Dago Red claims 3,600 hp, a significant increase over its original 2,200 hp, and Critical Mass ups the ante to 4,000 hp.

They burn 180-octane aviation fuel, a special air-race brew that contains manganese. During a race, Dago Red carries 110 gallons of fuel.

Dago Red, a souped-up P-51 Mustang piloted by Skip Holms, won last year's Reno air race for Unlimited-class aircraft. His average speed was 462 mph.

Both planes use pressurized fuel injection, not the electrical injection used on cars, and both run mechanical superchargers. Dago Red, for example, gets a 75-psi boost with manifold pressure between 140 and 150 in. of mercury. Its engine uses a two-stage two-speed centrifugal supercharger that is always kept in the low range — it runs at 23,000 rpm. The high manifold pressures, however, produces blow-by, and some of the air-fuel mixture is forced past the piston rings. To take care of the problem, Dago Red has an oil separator to remove oil from the crankcase breather system. The pump in this system pressurizes the tank to remove entrained air from the oil.

The Dago Red team does not use turbochargers because the engine compartment and structure can't dispose of the extra heat. Turbocharging would also mean extra cooling for the lubrication oil.

Antidetonation injection (ADI) prevents premature detonation due to the high manifold pressures. Dago Red carries 60 gallons of an ADI mixture, 50% water, and 50% methanol, that is injected into the cylinders during the race.

Magic for Merlins
Almost all of the Unlimited mechanics taking care of Merlin engines, such as those in Dago Red, use Allison connecting rods. They have a much larger cross section, and a better bearing design than the stock Merlin rods. But Allison rods are 0.10 in. longer than the Merlin's. Therefore, aircrews have to reposition the wrist pins to maintain the 6:1 static compression ratio necessary to decrease the chances of detonation.

Racing versions of the Merlin also do away with stock pistons. Instead, pistons are custom made from Alcoa aluminum forgings, and race teams have a variety of ring combinations to choose from, including headland, wedge, and barrel-faced compression rings, along with numerous oil-control ring designs.

Mechanics also replace original cam followers with carbide-faced versions. The originals are hard chromed which flakes away from the substrate. These abrasive little particles can wear down cam lobes after less than one heat's worth of racing. Contamination from worn cams is another problem attributed to chrome followers. Racing requirements mandate that engines use filters no finer than 100-micron, not small enough to catch the small pieces of chrome. Attempts are being made to develop camshaft grinds and cams with steel roller followers, but they are still in development.

Like hot rodders of the 50s, race teams modify their aircraft with parts from other planes. Voodoo, for example, is a P-51 with a prop from a Douglas A-1 Skyraider and a spinner from a Bell P-63 King Cobra.

Mustang mechanics try to use as many late-model Merlin engine parts as possible. They like to use Merlin engines from the 620-Series which were built for DC-4 transport planes in Canada around 1946, and often supplement them with a sprinkling of World War II parts. Engine technicians routinely install automotive-style spark plugs because they have greater heat ranges and higher pressure ratings than aviation-style shielded spark plugs.

To keep the overworked engine cool and functioning, a spray-bar throws water on the front of Dago Red's radiator. The plane carries 45 gallons of spray-bar water. It not only cools the engine by converting water into steam, it also contributes a little added getup-and-go. It seems the plane's forward speed forces the expanding steam through a nozzle in the fuselage, generating thrust in what is called the Meredith effect. You can see the trail of steam Dago Red leaves as it flies through the course. Her crew is considering injecting water into the exhaust pipes to increase the effect.

Mechanics also have to make sure the oil stays relatively cool. Dago Red, for instance, carries a modified liquid-to-liquid oil cooler. The cooler uses a radiator that juts into the airstream, making the plane less than perfect aerodynamically, and it adds weight. But the team is willing to sacrifice some weight and drag to keep the oil temperature down and increase the plane's chances of crossing the finish line.

The V12, 1,660 cu-in. Merlin engine leaves room for little else under Dago Red's cowling.

The perfect prop
Pylons are so close together along the race track that Unlimited aircraft never quite level off. The course consists of only left turns, therefore their right wings are always higher than the left. Critical Mass, a British plane, originally had its prop spinning counterclockwise (from the pilot's point of view), which tends to lift the left wing due to engine torque. The aircrew reversed the prop rotation so that torque now lifts the right, or outside wing and sets up the aircraft for the race's left turns.

Using a totally different approach, Dago Red's mechanics let the prop on their Merlin engine rotate the way the British intended, counterclockwise. They offset the torque and make their plane neutral by adjusting the rudder and horizontal stabilizers. They keep the plane in balance during the race by adjusting the amounts of fuel in the left wing and water for the spray bar and ADI in the right wing. This balance becomes more important near the end of the race, when the fuel and water have been consumed, and the lighter plane is less stable.

All of the racing teams want their prop tips just below transonic speeds or about 80% of the speed of sound. Therefore, everyone who does the calculations correctly has their prop tips at about the same speed. The calculations have to take into account the fact that these planes skid around the tight turns, so their 500-mph forward speeds contribute to the circumferential velocity of the props.

Teams use variable-pitch props to keep engine rpm and prop-tip speed constant. To get more power out of the engine, the pitch increases when the pilot pushes the throttle forward, taking bigger bites out of the air. Race teams want the engines to run at an optimum rpm, which is about 3,400 for Dago Red.

Maintaining a specific velocity at the prop tips leads to interesting trade-offs. For Mustangs, the stock gear ratio is 0.479:1, and the race ratio is 0.420:1. Teams might want lower gear ratios, but space constraints inside the gear-housing castings are so tight that lower gear ratios would mean dangerously thin castings. Therefore, mechanics shorten the props to maintain higher engine rpm.

Mechanics do a few last-minute repairs to Dago Red.

Body work
There are two basic ways to make planes fly faster: increase thrust or decrease drag, and mechanics will go to fairly extreme lengths to do either of these. For example, to reduce drag, they clip the wings. Critical Mass, for example, has 38.5 in. removed from each wing, giving it a 32-ft wingspan. And Dago Red has about 30 in. cut off each wing, taking the wingspan from 37 ft down to 32 ft. While shortening the wings reduces drag, excessively short wings make the planes unstable. As one mechanic says, "You get a real squirrely airplane." Larger planes like P 38's, which had wingspans of 52 ft, sometimes had additional length taken off the inside wing, but that was primarily to avoid hitting the ground during tight turns around the pylons. To reduce drag even more, mechanics wax their planes to reduce drag from skin effect, a common practice even back in World War II. And each team has its own "secret recipe" for finishing wax.

Braces and cases
While more could be done to increase horsepower in Unlimited planes, reliability would suffer. Instead, aircrews concentrate on making sure the pilot can sustain all the power available without destroying the engine or airframe. For example, Dago Red mechanics have added several braces to handle bending and fatigue stresses on the airframe created during 6.5-g turns around the pylons. They've also added braces to resist the torque generated by the engine, super-charger, and prop.

Mechanics have reinforced the accessory drive case with various gussets and bearing-locator plates, as well. This case is a complex casting located between the crankcase and the super-charger. It houses drives for the coolant, oil, hydraulic, and fuel pumps. It also contains drives for the magnetos, camshafts, generator, supercharger, and starter. The original casting suffered cracking and bearing-bore distortion due to stresses from the power needed to run the supercharger.

Another tough problem to over-come for mechanics is crankcase distortion at race power. The propeller shaft is higher than the centerline of the crank, and this imposes a severe bending and twisting moment that cracks the front of the case. Adding large diagonal braces contains this bending moment. But distortions of the inside of the case also create problems: fractured main bearing caps and web fractures in the case. This was solved by putting metal bars on the sides of the case, and an aluminum bar across the top of the engine.

At last year's race, Dago Red's team kept their plane alive until the finish and held off a determined challenge to win the gold. Shortly after the race, they tried for a course record: a 500-mph lap. Dago Red was barely off the ground before its pilot was forced to call "Mayday." The plane threw a connecting rod through the side of the crankcase.

But Dago Red will be back. Her mechanics will find better bearings and stronger piston rods. And chances are they will eventually break the 500-mph barrier. Like the Wright brothers in their Dayton bicycle shop and the hot rodders in their Detroit garages, mechanics and aircrews at Reno will continue to push past each new limit that stands in their way.

The aircrew for Voodoo brought more than one engine to the Reno Air Race.

The Reno Air Races, held at Stead Airport, an old Air Force airfield north of Reno, hosts races for more than just aircraft in the Unlimited Class. There are also competitions for AT-6, Formula 1, Sports and Biplane class planes. The AT-6 class includes the T-6 Texan, Harvard, and SNJ trainer aircraft that fly about 220 mph. Formula 1 Class planes are all powered by the 100-hp engine found in Cessna 150s, the Continental O-200. They peak at about 250 mph during a race. The Sports Class contains production-model kit-built aircraft with engines smaller than 650 cu in. And the Biplane Class is comprised mostly of Pitts S-1S and Mong Sport planes that reach 200 mph.

Each class has a distinct course, but they are arranged in concentric circles, with the Unlimited course being the longest at 8.27 miles. Strategies for all planes are roughly the same: fly fast and turn left at the pylons.

Planes must fly above the pylons (about 50 ft) and below 1,500 ft, an FAA restriction. "But for the most part, pilots don't fly higher than a few hundred feet," says Brian Fitzgerald, event coordinator for the races. "That's because it's hard to see and judge the pylons if they fly much higher."

To start the race, pilots once used to run to their planes, hop in the cockpits, start the engines and take off, all 24 planes aiming for the quickest line around the first pylon. Needless to say, many aircraft never made it past that first turn. Today, planes gather overhead on a lead plane prior to the start of the race. Planes with the fastest qualifying times form up closest to the lead, much like Indy racers lining up on the pole position. To begin the race, the lead or pace plane takes the 24 planes down the start corridor and onto the course. If no one jumps the gun or passes the pace plane, the pace pilot declares "Race start," and pulls up and out of the way. The first plane to complete eight laps wins the race.

What are the limits on Unlimited?

Although the racing class is called "Unlimited," there are still a few restrictions. They must be powered by piston engines, prop-driven, and capable of pulling 6 g's. As long as the plane meets these rules, it's no-holds barred. They can be home-built kits, assembled from scratch, found in a junk yard, or bought new off a lot. They can use anything from exotic fuel blends to nitrous injection, composite materials or salvaged aluminum, and carry any piston engine they can cram onto their airframe.


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