The Tiger's Tale

"Did you just say there's an RV-4 at 36,000 feet?"

August 1, 2003

"We were climbing at 6,000 feet a minute and I thought, 'If this works we're going to rip the record up!'" said Bruce Bohannon after he streaked skyward during a record attempt in his Exxon Flyin' Tiger. Then a piston broke. There was nothing Bohannon could do except pull the mixture out and dead-stick the Tiger back onto the ground. Loss of the piston top had unbalanced the finely tuned high-performance engine and it self-destructed. That was in the spring of 2000, or before turbo (BT) as Flyin' Tiger crew chief Gary Hunter marks time.

That flight at the Sun 'n Fun EAA Fly-In in Lakeland, Florida, was the last time Bohannon attempted a record flight using an engine that was fortified with a nitrous oxide system.

In November 2002, concurrent with AOPA Expo in Palm Springs, California, Bohannon, Hunter, and the Flyin' Tiger were much more successful. They captured three world records for piston-power aircraft of any weight (Class C-1): altitude of sustained horizontal flight (40,604 feet), time to climb to 9,000 meters (16 minutes, 3 seconds), and time to climb to 12,000 meters (31 minutes, 3 seconds). On that same flight Bohannon added four records for airplanes that weigh between 1,102 and 2,205 pounds: the two time-to-climb records, a record for absolute altitude (41,300 feet), and a record for horizontal flight at altitude. In recognition of his record-setting flight, the National Aeronautic Association presented Bohannon with its Blériot Medal.

Bruce Bohannon got his first airplane ride in a Piper J-3 Cub and was flying crop dusters at age 18. Since then he's taught aerobatics and helped design, build, and race Pushy Galore, a pusher-style race airplane.

He had a job flying a Hawker HS125-600 corporate jet for about a year, but that was when he was younger. He's logged more than 9,000 hours and most of it would qualify as "high risk" flying. He's tall, lanky, and low-key enough to enjoy setting records, flying test hops, and messing about in crop dusters. He's also made more than 150 parachute jumps. He even proposed to Donah, his wife, as they were free-falling together. It may sound like Bohannon likes to flirt with risk, but, if the design of his airplane is any reflection of his psyche, he's surprisingly conservative.

The airframe

The airframe of the Flyin' Tiger is very low-tech — built utilizing traditional riveted aluminum construction techniques. The forward half of the airframe is from the Harmon Rocket kitplane while the aft part of the fuselage, the wings, and the tail are parts from Dick VanGrunsven's RV series of homebuilt airplanes.

The empennage uses out-of-the-box RV-8 parts and the wings are out of an RV-4 kit, down to the 32-gallon-capacity fuel tanks. Aren't long wings, like the ones on the U.S. military's extreme-altitude Lockheed U-2 spy plane, required for high-altitude flight? Bohannon doesn't think so — the Flyin' Tiger's wingspan is a meager 23 feet — and he has the world records to prove he's right.

A tale of two engines

Nitrous oxide is a gas (dentists use it to calm anxious patients — it's been called laughing gas) that can wring tremendous power increases out of engines when injected into the induction system. But the required pressures and temperatures that develop within the engine are so extreme that fuel mixture settings are critical.

"We were climbing at 6,000 feet a minute, but the air density and the nitrous gas pressures were changing so rapidly I just couldn't keep the mixture where we needed it. We estimated that the horsepower output for the nitrous engine was 430. If the mixture is too rich we crack pistons; if it's too lean the engine would be damaged from detonation. I let it get too rich," said Bohannon.

The nitrous oxide system was removed, and the decision was made to increase the engine's power output by using hot-rodding techniques.

The hot-rod engine

"The [Lycoming IO-540] engine we're using now was modified by Phil Haponic and his experts at Mattituck [now Teledyne Mattituck Services Inc.], and it has plenty of power," said Bohannon.

A specially ground crankshaft increased the displacement to 555 cubic inches, and high-topped pistons raised the compression ratio to 12-to-1. (The compression ratio of an unmodified Lycoming IO-540 is 8.7-to-1.) Raising compression ratios increases engine efficiency but reduces detonation margins.

One way to reduce the possibility of destructive detonation is to use specially mixed higher-octane fuels that are more resistant to detonation. Hunter said all Exxon Flyin' Tiger flights use 100LL avgas.

The engine was mated to a turbonormalizer system to maintain power when climbing.

"Kelley Aerospace supplied us with a large turbocharger from a Piper Navajo that works with our modified Lycoming IO-540 engine," said Hunter. "It's the biggest turbocharger that's available."

"Since we have plenty of power at sea level, the turbocharger allows us to maintain manifold pressures as we climb," said Bohannon. Normally aspirated engines lose power with gains in altitude. Bohannon applies full power during the takeoff run — after passing 1,500 feet he reduces the power settings to 25 inches of manifold pressure and 2,500 rpm. As manifold pressure drops off during the climb, he closes the turbonormalizer wastegate to maintain 25 inches. It sounds easy, but it's just one of the many factors that require constant attention.

Since 100LL avgas is used, detonation margins are increased by lowering the temperature of the compressed air from the turbocharger before it enters the engine. After exiting the turbocharger the heated air is ducted through a custom-made (by Kelley Aerospace) 20-by-20-by-4-inch intercooler that is mounted within the fuselage forward of the cockpit.

"Phil Haponic told me that this engine would detonate if I let the inlet air temperatures go above 100 degrees [Fahrenheit]. The hardest part about flying these records is keeping control of all the temperatures," said Bohannon.

"The intercooler installation works so well I never saw inlet air temps get above 20 degrees C [68 degrees F] during my time-to-climb flight," added Bohannon. Hunter and Bohannon are mulling over a decision to reduce the size of the intercooler, and increase the size of the two oil coolers.

The wastegate

"I wanted a manual wastegate so that I have control of the turbo system in case of an emergency," said Bohannon. This primitive but dependable setup requires that Bohannon constantly monitor inlet air temperatures, cylinder head temperatures (CHTs), fuel mixtures, and oil temperatures while flying an IFR flight plan. An IFR flight plan is required for all flights above 18,000 feet, and Bohannon gets a lot of cooperation from air traffic controllers.

His record attempt at Lakeland last year was cut short because of routing into an existing cold front. "When I ran into the cold front I lost peripheral reference; I didn't want to go into the clouds," says Bohannon. "The controllers gave me two choices. I could climb to the north (and continue in IFR conditions) or descend to the south, so I turned around."

Although the attempt came up short during this flight, it was not without a lighter note. "Jacksonville Approach, this is airliner XYZ, did you just say there's an RV-4 at 36,000 feet?" asked one incredulous captain. Bohannon says he hears that call on every attempt.

The rigors of the flight

Airline passengers, nestled in cushy seats and surrounded by heated and pressurized air, aren't aware of the dangers that occur during each flight far up into the flight levels. Jet pilots who are familiar with high-altitude physiology are in awe of what Bohannon is doing.

Temperatures drop to 69 degrees F below zero at the tropopause (36,089 feet) and stay there. There's no heater in the Exxon Flyin' Tiger so Bohannon has to perform his duties in cold that few humans have ever experienced.

In preparation for each record attempt Bohannon must pre-breathe 100-percent oxygen for an hour and 20 minutes to purge potentially fatal nitrogen from his bloodstream.

Above 25,000 feet he must use an oxygen system that forces oxygen into his lungs because there's not enough pressure at that altitude to push the oxygen molecules into the bloodstream. If the oxygen system fails or malfunctions at 40,000 feet, Bohannon has less than 15 seconds before he passes out. Bohannon can't respond to the statement he often hears from innocents on the ground — "I'll bet it's beautiful from up there" — because the side windows are iced over, and he's busy monitoring and ministering to his engine. Heat extracted from the turbocharged air by the intercooler exits forward of the canopy. This primitive defrosting system gives Bohannon limited forward vision.

Temperature problems?

Listen to Bohannon describe leveling off at 40,000 feet. "I'm still getting about 16 inches of manifold pressure at that altitude, so the engine is still putting out a lot of power. When I level off I have to leave the throttle wide open or the engine will cool off too fast."

Hunter again, "We get excellent cooling in level flight, but we're still fighting cooling problems during climb." The crew has investigated fixes from louvers, to cooling lips, to side cowl air exits as they whittle away at the cooling problems. Hunter said, "We're now convinced the shape of the cowling is the primary reason for the problem."

"If I don't climb at 140 miles per hour the engine temperatures get too hot. I can get another 500-feet-a-minute climb at my best rate of 85, but I can't keep the engine cool at that speed," said Bohannon.

One trick Hunter and Bohannon use to keep CHTs within the engine operating range is to spray water (it's really a type of antifreeze) over the two oil coolers. As with the other systems on this airplane, the oil-cooling spray system is not automated so Bohannon must constantly keep his scan active as he switches between flight engineer, pilot, and navigator duties on the Exxon Flyin' Tiger.

It climbs and flies fast

During the level flight at 40,000 feet Bohannon says he checked the airspeed indicator and was surprised to see it indicating 165 miles per hour, which he later calculated to be a true airspeed of 322 miles per hour.

After 5 to 7 minutes of level flight the CHTs have fallen down to about 350 degrees and Bohannon can pull the power back slowly without setting off the shock cooling warning alert system on the Vision Microsystems VM-1000 engine management system. Then it's time to descend to 20,000 feet where it's a relatively balmy minus 12 degrees F. By this time, Bohannon's feet are numb. And he still has to land the Flyin' Tiger. But wait, there are more engine management factors Bohannon has to juggle that the rest of us never have to think about.

Propeller blade tip Mach numbers

Propellers begin to lose efficiency when tip speeds approach 70 to 80 percent of the speed of sound (Mach 1) because the drag coefficient off the tip increases rapidly above those speeds. To maintain the best propeller efficiency of the Hartzell three-blade prop, Bohannon has to constantly reduce the engine rpm as he climbs.

At sea level on a standard day, Mach 1 is 661.7 knots — at 35,000 feet on that same standard day it has dropped by 13 percent to 576.6 knots. Since engine rpm is directly related to turbocharger performance, any reduction in rpm also affects the engine power output. Again, another task on Bohannon's long climb checklist.

In spite of their successes Bohannon and Hunter know that they've barely scratched the surface of their airplane's potential. So they keep tinkering with the Exxon Flyin' Tiger.

So don't be surprised if you read that Bruce Bohannon and the Exxon Flyin' Tiger team are attempting to set some more speed and distance records in the future. Bohannon has proven that he can juggle the rapidly changing requirements of piston-powered ascension — why should level at altitude be difficult?


E-mail the author at steve.ells@aopa.org.