When I first saw the Boomerang, Burt Rutan’s twin-engine, asymmetric, five-place, long-range oddity, my reaction, like that of a number of Rutan fans who witnessed the airplane’s debut at the 1996 Oshkosh, Wisconsin AirVenture, was Why? Why is there a small boom to the left of the fuselage? Why is the second engine, in the nose of the boom, five feet behind the engine on the fuselage nose? Why is the right wing almost five feet shorter than the left? Why are the wings swept forward? Why does the horizontal stabilizer, which joins the fins on the twin tails, extend past the right fin, but not past the left?
From This Story
In March, when Rutan received the National Air and Space Museum trophy for lifetime achievement, he gave an interviewer a short answer to those questions: “Self preservation.” The longer answer concerns how a conventional twin behaves when one of the engines quits. Last fall, with the help of Boomerang custodian Tres Clements and California flight instructor Chuck Coleman, I compared the engine-out performance of the Boomerang with that of a conventional twin.
Coleman and I flew his Beechcraft Baron around his home airport in Mojave. The Baron is one of the best selling twins on the market, a pretty, lightweight, six-seat aircraft that cruises at about 200 mph. Like most twins, it has an engine on each wing. If one engine fails, the asymmetry of the thrust will forcefully turn the airplane in the direction of the inoperative engine, which is producing drag.
To maintain straight and level flight, the pilot must add rudder and aileron in a coordinated fashion in the direction of the good engine. For example, if the left engine is out, the pilot adds right rudder and banks to the right. To earn a multi-engine rating, every pilot must perform this maneuver to the satisfaction of a flight instructor.
While controlling the aircraft in this way, a pilot must keep the airspeed above the minimum controllable speed (Vmc). For every twin-engine aircraft, this speed, the minimum at which the airplane can be flown with one engine inoperative, is established during flight testing. “In most twins, the [minimum controllable speed] is above the stall speed,” says Coleman. “So if you were to lose one engine abruptly and you’re below that Vmc/stall speed, it’s going to flip over on its back instantly.” Pilots train to avoid that situation.
During our flight, Coleman brought the left engine to idle and feathered the propeller (angling the blades to create the least drag). Thinking of airspeed, the first thing I did was add full power to the right engine. To keep from turning, I rolled in plenty of right aileron and pushed the right rudder pedal. I had to push it all the way to the floor just to keep the airplane flying straight ahead. In seconds, holding the pressure needed on the rudder pedal was making my leg shake uncontrollably. The yoke was turned almost as far as it could go. All the while I was keeping my eye on the airspeed indicator to make sure we stayed above the 80-knot blue line, indicating the Vmc.
Even though I was able to maintain my altitude and heading, it took immense effort. With that demonstration, I understood how difficult and dangerous it is to fly a traditional twin with only one engine, and what Rutan was trying to achieve with the Boomerang.
“My goal was to have the minimum control speed well below the stall,” Rutan says. In the Boomerang, the wing stalls at its root, near the fuselage, long before the outboard wing. The pilot experiences all the indications of stall, including buffeting, even though part of the airplane is still producing lift. Rutan wanted an airplane that could be controlled even after the onset of stall.
“The first time I feathered [a Boomerang prop] and was slowing down with the other engine at full power, it wasn’t real obvious what I should do with the rudder pedals,” says Rutan. “It’s real obvious on a Baron—you better be putting rudder in and quite a bit.” So I had found part of the answer to my question Why? But I was curious about How? And even more curious to know What does it feel like to fly the Boomerang on one engine? I got the answer to that question because Burt Rutan decided to retire.
Safety First, Range Next
The Boomerang had its roots in two of Rutan’s earlier designs: the twin-engine Defiant and the high-performance, single-engine Catbird. In fact, he used the Catbird nose gear and engine on the Boomerang.