Twins: The Other Advantage

In the earliest days of aviation, multi-engine airplanes were introduced primarily for the ability to carry more weight. The world’s first twin, Sikorsky’s biplane S-21 Russky Vityaz (Russian Knight), was proposed in 1911 “when no known aircraft could lift more than 600 kilograms [1,323 pounds].” At the time, the world’s record for aircraft payload was held by the French pilot Ducis, who had flown 600 kilograms to a height of 800 meters (2,600 feet). When it first flew in 1913, the huge-for-the-time S-21 was capable of taking off with more than twice that payload, 1,300 kilograms (2,866 pounds). Like many twins, the Russky Vityaz’s operational life was cut short by the loss of an engine at low altitude, but with a twist: the big biplane was crushed by the engine of a Morane-Saulner that fell off the single-engine French airplane during landing. History doesn’t record the fate of the Morane-Sauler’s pilot. The S-21 was not rebuilt, but the basic layout was developed into the first four-engine airplane for even more payload capability as an airliner and a First World War bomber.¹

Early twin-engine airplanes—very high-drag designs with fixed landing gear and non-featherable propellers—could not fly on a single engine. So, redundancy was not a design consideration as the multiengine concept arose. Even some WWII training twins and early postwar designs like the Twin Stinson, which morphed into the Piper Apache, could not maintain altitude on one engine under most conditions, the second engine considered to be more of a “glide extender.” By the time the Cessna 310, the Piper Aztec, and the Beech Travel Air were introduced, though, the science of aeronautics had advanced to where most consider the ability to fly on one engine (albeit marginally) to be the greatest advantage of flying a light twin. In touting multiengine redundancy, we sometimes overlook the other great advantage of flying a twin compared to its single-engine brethren: the ability to carry more weight, in extra fuel if we need it or in the seats and baggage areas if our flight allows trading fuel for payload. Let’s see what real-world payload advantage exists for the pilot of a twin.

Excess power
You can discuss the theory of lift all day (multiple theories, actually), but ultimately, to go up, you need power. Before the sailplane pilots chime in, they need power to climb, too, just externally applied energy in the form of a tow, thermals, or other vertical airflow. For the powerplane crowd, in general, the more power you have, the faster you can climb, or alternately, the more weight you can carry aloft.

What’s the minimum amount of power needed to maintain altitude at a given weight? Assume you slow to the airspeed at which drag is least, best lift over drag (L/D), essentially Best Glide speed adjusted for the airplane’s current weight. Establish level flight and slowly, gradually reduce power until the airplane is flying level and precise at this speed. It’ll take somewhere in the neighborhood of 35% maximum power at maximum weight, less power at lighter weights. Let’s use that to illustrate my points to follow, although the logic remains the same even if that 35% figure isn’t spot on.
At 6,000 feet above sea level, leaned for maximum horsepower, a normally aspirated engine will develop about 80% its rated power at full throttle and full RPM. Manifold pressure drops at about one inch per 1,000 feet in the lower atmosphere, and standard air pressure at sea level is very close to 30 inches, so at 6,000 feet, 24 inches is available and 24/30 = 80%. If 35% of that power is needed just to hold level flight at least drag speed, then the airplane has 45% excess power remaining to fly faster or climb higher, or some compromise between the two. Add weight (such as with a gross weight increase that does not add power), and the airplane will not be able to fly as fast or climb as rapidly. Sure, other factors play a part, for example, the added drag of the extra engine, longer wing, and larger tail surfaces of a typical twin versus a comparable single. You can see, however, that in general, more power means more capability with more weight.

Civil war
Sorry to pit brother against brother, but let’s do some comparisons between airplanes offered as essentially identical fuselages in single- and twin-engine versions. We’ll look at Beech Bonanzas, Travel Airs, and Barons of similar vintages, and the turbocharged Piper Turbo Lance and Seneca II.

First, let’s look at the late 1950s Beech Bonanza and Travel Air.

The 1959 K35 Bonanza came equipped with an IO-470C engine rated at 250 horsepower maximum. Maximum gross weight is 2,950 pounds. This results in a power loading (pounds per horsepower) of 11.8 – each horsepower carries 11.8 pounds at maximum. The Model 95 Travel Air of the same model year has a pair of O-360-A1A engines rated at 180 horsepower each, 360 horsepower total. The 95’s maximum gross weight is 4,000 pounds, for a power loading of 11.1 pounds…not much different from the K35. But at roughly the same power loading, the twin-engine Travel Air can carry 1,050 more pounds aloft. That’s a 36% increase in weight, although it takes 44% more power to do it.

Manufacturers’ useful load figures are often optimistic. But given they follow the same assumptions, they may provide interesting comparisons. Beech says the (typical) useful load of the K35 is 1,118 pounds. That’s the maximum combined fuel, occupant, and baggage weight. Beech’s useful load figure for the 95 Travel Air is 1,430 pounds. The contemporaneous twin, then, can carry 28% more payload than the single.

Let’s do the same exercise with an A36 Bonanza and a Baron 58, both of the 1984 model year. Maximum gross weight for the A36 is 3,650 pounds on a 300 horsepower IO-550 engine. That’s 12.2 pounds per horsepower. The same model year Baron has a pair of IO-550s, 600 horsepower total, and a maximum gross weight of 5,500 pounds, 18.3 pounds per horsepower. The Baron can carry 1,850 pounds more into the air, although it needs twice the horsepower to do so. Playing the useful load game, the A36 has 1,416 usable pounds (optimistic, but as published) while the Baron’s useful load is 2,083 pounds, a 47% increase.

On the turbocharged side, let’s compare the single-engine Piper PA32RT-300 Turbo Lance II and the multi-engine Seneca II. Maximum gross weight of the single is 3,600 pounds—12 pounds per horsepower. The Seneca II’s maximum takeoff weight is 4,570 pounds, carried by 400 total horsepower at sea level and 430 total horsepower at 12,000 feet. That gives the Seneca II a power loading of 11.4 pounds per horsepower at sea level and 10.6 pounds per horsepower at altitude (ignoring, for simplicity, the weight reduction of fuel burned in the climb). Completing our comparison, the twin can carry 970 pounds more, a 27% increase over the single, on 33% more power (at sea level).

Other factors—Normal category certification for the Beech twins versus Utility for the Beech singles (both the Lance and Seneca are certified in the Normal category), single-engine climb considerations for the twins—also play a part in determining maximum gross weight. But it’s easy to see the power of power when it comes to carrying weight. And more engines, moving the engines to the wings, adds baggage space and improves center of gravity flexibility in the twins. Many of a flying life’s problems may be solved by a 300-pound capacity nose baggage compartment.

The point
There is no 600-horsepower engine available to hang on an A36 airframe, no 360-horsepower engine approved to bolt onto a K35 airframe…though it’s theoretically possible someone could earn approval to hang a 400-horsepower Lycoming IO-720 on the Lance. If the pilot of a late-1950s K35 wanted a Bonanza-sized and quality cabin but wanted to carry over one-third again the weight, he or she had no choice but to add a second engine. The same holds true for the owners of 1980s-vintage Beechcraft® wanting to carry almost 50% more weight. All else being equal, power equals payload.
Don’t minimize the added redundancy and potential safety of the second engine for the well-trained and prepared pilot. But when enumerating the advantages of flying a twin versus a single, don’t forget the significant increase in payload the second engine affords.

¹Sergei Sikorsky, The Sikorsky Legacy (Arcadia Publishing, 2007)