There is no greater concern among pilots and airplane owners today than the cost of fuel. Prices vary widely from airport to airport, but $5 is often on the low end and $7 a gallon is not the top. And in many instances jet fuel costs more than avgas, a reversal of traditional pricing. Fuel costs for most airplane owners have doubled in the past year and nobody can predict future trends, but higher prices sure seem more likely than lower.
So now we all want to own and fly the most fuel-efficient airplane, but what is it? There is no single answer because as with all desirable characteristics in an airplane we always must trade one attribute for another. If fuel were really the only driving factor in finding the most efficient aircraft, powered parachutes and motorgliders would win hands down. Or everybody in a jet would cram into a piston-powered airplane because jets can’t match the fuel efficiency of a reciprocating engine driving a propeller. What we really want is not the most fuel-efficient airplane possible, but the most thrifty one that suits our mission.
The aviation industry uses a metric called specific range to measure fuel efficiency. Specific range is the number of miles — normally a fraction except for piston singles — that an airplane flies through the air per pound of fuel consumed. For example, a piston airplane with a true airspeed of 150 knots while burning 12 gallons per hour (72 pounds) would have a very good specific range of 2.08. A business jet cruising at 440 knots true burning 1,200 pounds per hour (pph) has a specific range of 0.37, good for a jet.
Specific range can be calculated for the cruise condition, as I have done above. But a more useful measure is specific range for the entire trip. When fuel consumed for start, taxi, climb, approach and landing are all measured against the block speed, the specific range will be worse than for cruise.
Though we buy fuel in volume — by the gallon or liter — and the tank’s capacity is determined by volume, engines burn fuel and air in mass, so measuring fuel consumption in pounds is more useful. At standard temperature of 15° C (59° F) a gallon of avgas weighs about 6 pounds. A gallon of jet-A fuel at standard temperature weighs in at about 6.7 pounds. Avgas density changes very little over the normal temperature range, but jet fuel density can vary by several percent, so a gallon of really cold jet-A is going to take up less space in the tanks than a warm gallon, but the useful work of the fuel will still be measured by the pound.
By using the specific range calculation it’s easy to compare airplanes to one another or to measure the efficiency of power settings. For example, let’s say that a powerful piston single can cruise at 140 knots while burning 72 pounds per hour (pph), which equals 12 gallons per hour. That yields a specific range of 1.94. If you increase the power and fuel flow to 132 pph and the true airspeed climbs to 180 knots, the specific range is down to 1.36. The airspeed increased by about 22 percent, but the fuel efficiency decreased by about 30 percent.
As you can see, the fuel efficiency of a piston airplane is largely in the hands of its pilot. More speed equals less efficiency. The greatest specific range airspeed for a piston airplane is so slow that few of us would ever contemplate it unless we absolutely had to stretch the fuel all the way back to shore. In general, an indicated airspeed at about the best rate of climb speed is also the most fuel-efficient speed in level cruise. The heavier the airplane, the higher its maximum efficiency speed, so to achieve true maximum range you must continuously slow down as fuel burns off to maintain the same angle of attack and thus drag.
This amazing gain in fuel efficiency at lower speeds was driven home to me flying to Oshkosh this past summer. Controllers told me to pull the indicated airspeed back to 120 knots to stay in trail of slower IFR traffic ahead. My Baron was burning about 30 gph to indicate 170 knots, but it took about 16 gph to hold the 120 indicated. Fuel flow was nearly halved to cut airspeed by less than a third.
Pilots of turbine airplanes actually have less control over the fuel efficiency of their flights because there are so many variables, first among them being air traffic control. Turbine engines are at their least efficient down low where the air is dense. As the airplane climbs and the air thins, the turbine produces less power and thus consumes less fuel, but the drag of the thinning air on the airplane decreases faster than the power from the engine drops, so the airplane speeds up and the fuel flow goes down. There is an optimum altitude for every turbine airplane at its present weight and any level lower than that optimum decreases fuel efficiency. In crowded airspace the pilots of turbine airplanes seldom are cleared for unrestricted climb to the optimum altitude, so the airplane doesn’t come close to matching its potential specific range. And on the ground at idle a turbine burns a surprisingly large percentage of its optimum cruise fuel flow, so takeoff delays really cut into fuel efficiency in a jet compared to a piston engine.
The other big variables for turbine airplane pilots are air temperature and weight. When air temperature aloft is warmer than the international standard atmosphere (ISA) the air is less dense so the engines have less mass — fewer pounds — of air to compress and burn. There is also a slight decrease in drag because of the lower air density, and engine fuel flow goes down some, but in this case engine power falls off faster than the drag benefit, so at warm temperatures a turbine-powered airplane specific range suffers.
The weight of a jet also greatly impacts its specific range. When heavy the highest altitude that the wing can support effectively is lower, so fuel flow is higher, drag greater, and the airplane travels a shorter distance on the same amount of fuel. Because the weight of necessary fuel and payload changes with almost every flight, the specific range of a jet, or less so a turboprop, changes, too.
Jets have long had large swings in fuel efficiency at various airspeeds with the longest range cruise speeds typically much slower than high-speed cruise, often 50 to 100 knots slower. Higher airspeeds in any airplane create more drag, but there is a particular issue at jet speeds that is linked to the speed of sound, or Mach 1. At speeds below Mach 1 the air behaves, well, like air. The molecules of air bounce around and get out of the way of the advancing airplane. But when airflow approaches or exceeds the speed of sound it begins to compress and behave more like a fluid and drag shoots way up. At the point where airflow hits Mach 1 a shock wave forms, separating the faster and slower moving air streams, and that wave is like sticking a speedbrake up into the air stream. Drag at the point of the wave skyrockets and gobs of power is required to overcome the increase.
No operational civilian jet can exceed Mach 1 in level flight or is approved to go that fast even in a dive over the United States, but the air flowing over the wing and fuselage of typical jets does reach the speed of sound. The reason is that as the jet moves through the air at say Mach .80, or 80 percent of the speed of sound, air flowing over the wing and fuselage must accelerate to pass over the airplane. This is called local airflow, and depending on the shape of the wing and fuselage, the local airflow can accelerate to Mach 1 at normal cruise speed and a shock wave forms even though the airplane is not supersonic. That speed is called the “critical Mach” of the wing because that is where the drag jumps up and fuel efficiency goes into rapid decline.
Aerodynamicists have made many advances in jet wing design to control the shock waves formed in the local airflow. Wing sweep makes the air believe the airfoil section of the wing is thinner so it accelerates less to pass over and under the wing. The “super critical” wing design has a thicker and flatter forward section with a concave cusp on the underside trailing edge of the wing. The flat section delays the formation of a shock wave, and the cusp helps move it back and minimize its intensity.
The newest wing designs, such as those on the Falcon 7X and Gulfstream G650 that is in development, control shock wave formation so well that the difference between maximum range airspeed and high-speed cruise has shrunk. Gulfstream predicts that its new G650 wing will be so aerodynamically efficient that maximum range will be achieved at a Mach .85 cruise speed, which is actually high-speed cruise for most jets. Gulfstream engineers say the G650 will fly 7,000 nm with IFR reserves in still air at Mach .85, and no further at any slower airspeed.
Another big factor in fuel efficiency is the size of the airplane. A bigger fuselage has to shove more air out of the way so it creates more drag and thus requires more power. As with the wing, the shape of the fuselage can reduce drag somewhat, and its length to diameter ratio — the fineness ratio — can also help at higher airspeeds, but there is no getting around the fact that more cabin space is going to require more fuel to push through the air. Passengers want the most room possible for comfort, but it must be paid for at the fuel pump.
As you can see, everything we all want in an airplane — largest possible cabin, fastest possible speed and longest range — all work against fuel efficiency. So even at today’s fuel prices we make trades. Higher fuel cost may force some to trade down, but it doesn’t alter the desirability of speed, comfort and range.
However, I think it’s useful to examine some airplanes that were designed with fuel efficiency high on the list, or perhaps, even at the top of the list. Among those are the earlier Mooneys, particularly the 201 M20J model, and the Piaggio P180 Avanti that is discussed fully on page 46. Among the jets that offer good efficiency and range are the Falcon 50/900, Learjet 30s and Gulfstreams.
By all accounts Al Mooney strove for efficiency even though he did his fundamental design work in the 1950s when fuel was relatively cheap and plentiful. His goal was to design an airplane that delivered the most speed with the least powerful engine, and he was successful. Mooney chose the then new to general aviation laminar flow airfoil to help control drag, and designed a fuselage and cabin big enough for people, but with no extra room, particularly in cabin height. The first wings were made from wood because the plywood skin could be formed into the very smooth shape needed to optimize laminar airflow. When the wing was changed to aluminum Mooney used flush rivets to retain the smoothness.
The Mooney series reached its height of fuel efficiency versus performance in the 1970s with development of the 201. The goal was to fly at least one mile per hour for each horsepower available from the engine. The 201 has a 200 hp Lycoming four-cylinder IO-360, and it managed 201 mph true airspeed with everything just right.
The country had suffered two oil shocks brought on by supply disruptions from the Middle East, and the topic of fuel availability and price was paramount in aviation. Mooney owners had always bragged about how little fuel they used, but the quirks of the airplane kept it in a niche. The 201 with its new sloped windshield, greatly improved cowling and, most importantly, its vastly better cabin furnishings broke out of the niche and thousands were sold. The airplane cruises at about 160 knots on 60 pph (10 gallons) fuel flow for an excellent specific range of 2.66.
As memory of the oil crisis faded, and some excellent and very powerful engines became available from Continental, Mooney introduced models with ever more power and higher cruise speed. Top airspeed is what pilots valued most over the past 10 to 15 years, and Mooney delivered with its Acclaim at the top of the piston single lineup for max cruise speed. If you pull the power back in an Acclaim or Ovation to 60 pph, you probably won’t go as fast as the 201 because the new airplanes are heavier. And it would be silly to cruise that slowly in the powerful Mooneys because you paid upfront for the speed, and you are wearing out the airplane while traveling fewer miles. We pilots tend to think in terms of hours, but it’s miles flown over the ground that count and cost.
We are also seeing steady improvement in the fuel efficiency of jets. The improvements are coming from both the engines and aerodynamics. And jets are now available in a greater range of sizes and performance categories, so you can match the mission and its required fuel to the airplane more efficiently. The aerodynamics at piston airplane airspeeds are so mature that we won’t see the same degree of efficiency gains in those airplanes, but small improvements are constant.
The drive for fuel efficiency in jets was more a desire for longer range and there are two very important ways to increase range — lighter weight and lower drag, and the ability to fly higher.
The Falcon 50 is an example of a jet where emphasis was placed on weight and drag and it achieved very long range in the 1970s when it was designed. The three engines, supercritical airfoil and very sophisticated structural design controlled empty weight and drag, yielding intercontinental range. The Learjet 35 and 36, and the Gulfstreams, all emphasized climb capability and could go directly to 41,000 feet or higher at maximum weight. At those flight levels fuel flows and drag were reduced and range increased.
Now nearly every recently designed business jet can reach 41,000 feet and above, so that has leveled the playing field and put the emphasis back on the lightest possible structure, most efficient wing and lowest drag shape, and a number of new jets are capable of astonishing nonstop trips.
So, to answer the question, what is the most fuel-efficient airplane? It’s the one that delivers the most speed, comfort and range that you demand with the highest specific range. A pilot’s life remains one of tradeoffs, and fuel efficiency is now a big item on the list, but the other choices are still there, too.
Reference : http://www.flyingmag.com/what-most-fuel-efficient-airplane