The Sound Of Thunder

Airplanes don't have transmissions, so there's no way a flying machine will ever accelerate with a Corvette, but if you're sitting behind the prop of a Thunder Mustang, you very well may wonder if someone snuck in a Borg Warner when no one was looking.

Sitting in the back of Wayne Richards' immaculately built Thunder, it seemed the roar from up front was reminiscent of a AA fuel dragster. We were off the ground and accelerating toward the 150-knot best climb speed in about seven seconds. Wayne allowed the airplane to run to the end of Van Nuys Airport's runway 16R before pulling into a steep climb at almost 6,000 fpm. We turned crosswind and downwind, pushed over to level at 2,000 feet MSL to avoid interfering with airline traffic inbound to Burbank, and the Thunder exited the pattern to the north for a high-speed run through the local mountains.

If you fly most of your hours behind piston engines, you'll be hard-pressed to find a more enthusiastic airplane than this 3⁄4-scale P-51. Real Mustangs have always been iconic warbirds, generally considered the best piston military fighters ever conceived. And they had a sound like no other airplane above the planet.

Sadly, most of the P-51s were chopped into pieces after World War II. Today, there are just over 100 examples still flying or in the process of restoration, and such a limited supply has pushed prices well into seven figures. A properly rebuilt Mustang sells for at least $1.5 million, and a true jewel restoration can go for twice that.

Such realities have severely constricted the market except for the Tom Cruises of aviation, pilots for whom price isn't an object. It has also created a demand for something that can offer the same performance for a lower price of admission and reduced hourly operating cost.

Like its inspiration, the Thunder Mustang has built a near-legendary reputation for performance, matching or exceeding that of the real Mustang. Additionally, they've captured the sound of the Mustang right down to the last decibel.

The Thunder was the long-term dream of Dan Denney, a designer from Idaho who made his initial reputation by designing and building the experimental Kitfox. This small Rotax-powered fabric-covered folding-wing, store-it-in-your-garage two-seat taildragger was wildly successful, and Denney built and sold over 1,000 kits before selling the company and initiating his concept of a 3⁄4-scale all-composite P-51.

Denny began R&D on the Thunder Mustang project outside Nampa, Idaho, in the early 1990s and had a flying prototype by the mid-1990s. The resulting airplane was a near-perfect Mustang copy, but with a fuselage scaled down to 3⁄4 of the Mustang's, and wings reduced to 5⁄8 scale. Wingspan and area are reduced disproportionately, since the Thunder weighs only about a third of the original North American fighter. (Denny also told me the cockpit is two-and-a-half inches wider than scale. You can reduce the size of the airplane, but you can't shrink the pilots.)

The heart of the Thunder Mustang is Ryan Falconer's remarkable all-aluminum V12 engine. Falconer traces his engine expertise back to Roger Penske, Andy Granatelli and Galles Racing. His engines have won the Indianapolis 500 and have become standards for the Can-Am and Trans-Am racing series, and his company, RF Industries, has been building high-performance competition auto and boat engines for the last 30 years.

When Denney came to Ryan Falconer 20 years ago and proposed that the new Thunder Mustang would be the perfect vehicle for an aviation application of the Falconer V12, Falconer agreed and set to work redesigning the engine for aviation application.

In some respects, Falconer's 600-cubic-inch power plant is reminiscent of a dramatically scaled-down Rolls-Royce Merlin, though the new mill is much lighter, significantly modernized and fitted with dual FADEC systems. Unlike the original airplane's supercharged Merlin, however, the Falconer V12 is normally aspirated, typically configured to deliver optimum power at a density altitude of 5,000 feet rather than sea level. More than coincidentally, that's the elevation of Stead Field in Reno, Nev., site of the annual Reno Air Races.

The engine is redlined at 4,500 rpm, geared down 2.8 times to produce 1,607 revs at the prop. The stock propeller is a four-blade composite 94-inch diameter MT, though some builders, primarily race pilots, have switched to a three-blade Hartzell that forsakes the original Mustang look, but offers slightly less drag and better speed. Racer John Parker has turned laps of 330 knots at Reno in his airplane, Blue Thunder, and currently holds the world speed record for normally aspirated piston aircraft at better than 333 knots.

Pure speed isn't the Thunder's only outstanding qualification, however. It's also a remarkably quick machine coming off the ground and headed uphill. Unlike many other aircraft performance parameters, takeoff is almost directly related to power loading, and the Thunder Mustang has one of the lowest power-to-weight ratios in the business. Specifically, there's 640 hp to lift only 3,200 pounds of airplane, so power loading is an amazing 5.0 pounds/hp. That isn't the lowest ratio in the industry, but its close. Once established in a climb at 150 knots, Wayne Richards' Thunder will rocket uphill at better than 5,500 fpm, nearly double the rate of a P-51D.

Richards decided to buy a Thunder exactly because of the extreme cost of an original Mustang. Like many young men who grew up around high-performance autos (Richards currently owns 10 cars ranging from antiques, classics, daily drivers and performance machines), he was drawn to aviation as the next logical step. "I grew up with high performance machines, mostly cars and motorcycles, and I was involved in racing both types through my 20s and 30s," Richards explains.

Richards owns an auto performance shop in the San Fernando Valley north of Los Angeles, smack in the middle of the world's busiest aviation hub, so it was perhaps only normal that he'd eventually step up to flying.

He soloed in eight hours, went on to earn his license and buy a Pitts S2A, then purchased a new S2B in 1987 that he still owns. "The Pitts is loads of fun, and I still get a kick out of flying it, but what I really wanted was something faster---a lot faster," says Richards. "I flew several aircraft, including a T6 and a P-51, and of course, the Mustang was the ultimate, but even if you could afford the price of admission, operating costs and maintenance would be prohibitive."

Richards considered constructing his own Thunder Mustang early on, but the 8,000- to 10,000-hour build time was prohibitive. "I make a living working with machinery, so I wasn't intimidated by the build process, but the time required suggested it would be eight to 10 years before the airplane would be finished. I wasn't willing to wait that long." Dean Holt of Mt. Vernon, Wash., currently owns all rights to the Thunder Mustang and is in process of producing a quick(er) build kit. See "To Learn More" on page 36.

As a result, Richards went shopping in the used Thunder market, a very small group of about 17 airplanes. He found his current airplane in Florida, a recent product of unlimited air racer Gunther Balz. "I think Gunther has built at least five airplanes, using every type of construction material, and he'd already put 100 hours on his Thunder when I bought it. Skip Holm and I went to Florida and flew it back to California in one day. Like any new owner of a homebuilt, I made numerous changes and improvements to bring it up to my standards, but as far as I know, it's fairly representative of a well-built Thunder Mustang," Richards comments.

From the outside looking in, Richards' Thunder Mustang looks to be an extremely accurate copy, a very precise, scaled-down clone of a P-51. In fact, however, it turns out to be better than the beloved Mustang in many respects. I flew Lee Lauderbach's near-perfect Mustang, Crazy Horse, several years ago in Florida and noted a few interesting differences.

First, despite the Thunder's superior acceleration and climb, torque isn't nearly as much of a problem. Sudden full-throttle application on the ground for takeoff or on short final during a go-around doesn't result in a loss of directional control or dramatic torque roll as it might in the P-51. The real Mustang is a much heavier airplane, 11,600 pounds versus the Thunder's 3,200 pounds. Similarly, a related problem arises when the Mustang's airspeed builds past about 250 knots. All that inertia makes for extremely heavy pitch response, demanding both hands on the stick.

In contrast, the Thunder retains comparatively light stick forces right up through 300 knots. Neither airplane is especially difficult to land, but the Thunder may have a slight advantage, again, because it's lighter and easier to correct.

Flying from the rear is more than a little challenging, as there are no instruments in back, and the only controls are throttle, prop, stick and rudders. The front seat folds forward to allow entrance/egress, so by definition, the rear seater must climb in first and exit last. Room in back isn't exactly spacious, but it's typically wide enough to accommodate most big men.

Engine start is a little different than in other aircraft. The dual FADEC ECUs take care of everything, timing, prime, mixture, so start is mostly a process of turning on the master, ignition and boost pump and hitting the start button. There's no primer or alternate air, and mixture control is adjustable but preset for start. Compression ratio is a high 10.9 to one, and when the engine comes to life, it does so with enthusiasm. It might be a stretch to compare it to a Merlin, but the Falconer's angry snarl as it catches and settles down to idle seems distinctly similar.

The Thunder's deck angle during taxi is significantly nose up, so S-turns are mandatory. As with the full-size Mustang, pushing the stick forward unlocks the tailwheel for maneuvering on the ground. Holding the pole full back locks the tailwheel in the trail position with only six degrees of travel.

Just as with other high-performance aircraft, things become a little busy in the cockpit during takeoff. Once the throttle hits the forward stop, the pilot immediately lifts the tail and then has only a few seconds before it's time to rotate at 80 knots. After that, he needs to retract the wheels quickly to avoid exceeding the gear door limit speed of 130 knots. With wheels in the wells, speed leaps to a best rate of 150 knots, and it takes at least 30 degrees nose up to hold that speed with full throttle.

One anomaly noted by test pilot Dave Morss on the prototype is that the Thunder Mustang climbs so fast, the digital manifold pressure gauge is dropping by roughly five inches a minute, so it's hard to get a firm reading of mp during climb.

With all that power on tap, aerobatics thrill. Roll rate is about 90 degrees/second, so a full, max stick deflection roll demands only four seconds. Loops and vertical rolls are no problem, but it's important to keep all maneuvers positive, as there's no inverted fuel or oil system. Push the airplane into even a slight negative G, and that big 12-cylinder engine will stagger. With a 10.9 to one compression ratio, the blades will spool down to a complete stop if you hold outside Gs for more than a few seconds. (That's by no means a measure of the airplane's strength, however. Denny designed the Thunder to withstand +7.3/-4.9 Gs at gross. The airplane's structure is carbon fiber with a honeycomb core, probably stronger than an equivalent aluminum structure.)

If your mission is cruise, you'll see 280 knots at 10,500-11,500 feet, burning 22 gph. With 102 gallons aboard, the airplane has 3.5 hours endurance plus reserve for a range of nearly 1000 nm. The pilot needs only to monitor temperatures and adjust coolant, oil doors and fuel trim as necessary to keep the engine happy. With all those cylinders firing in sequence, the Falconer engine is almost as smooth as a turbine (though some owners have actually installed Walther turbine mills).

Wing loading is just under 30 pounds/square foot, and that translates directly to a fairly smooth ride in turbulence. Like the original fighter, it plows through rough air with little trouble.

Descents are as you like them with liquid cooling out front. Shock cooling isn't a concern, so you can descend as fast as your ears will tolerate with that big four-blade MT running interference. Redline is the same as the P-51's top number, 439 knots, so there's no risk of overspeeding the airplane.

At the bottom of the envelope, approaches work well at 100 knots with a wheel landing at 85 knots. Stall speed is below 70 knots, so you're not pushing the envelope at a final approach speed of 90.

Wayne Richards' airplane represents more than simply another immaculately constructed homebuilt work of art. It is, in some respects, an ultimate, the maximum performance you can wring from a normally aspirated, piston engine. WWII fighters needed superchargers to fly high and fast so they could outpace the bad guys. Today, jets have relegated those wonderful piston fighters to museums and occasional air shows, and the limited supply of 65-year-old Mustangs continues to diminish.

If pilots such as Wayne Richards and Dean Holt are successful at keeping the Thunder Mustang alive, however, we may continue to see a version of the airplane maintaining the legacy of the legendary P-51 Mustang forever. And no one will ever forget that sound.