A
powered aircraft counters its
weight through
aerodynamic lift and counters its
aerodynamic drag with
thrust. The aircraft's maximum
range is determined by the level of
efficiency with which
thrust can be applied to overcome the
aerodynamic drag.
Aerodynamics A subfield of
fluid dynamics,
aerodynamics studies the physics of a body moving through the air. As
lift and
drag are functions of air speed, their relationships are major determinants of an aircraft's design efficiency. Aircraft efficiency is augmented by maximizing
lift-to-drag ratio, which is attained by minimizing
parasitic drag, and lift-generated
induced drag, the two components of aerodynamic drag. As parasitic drag increases and induced drag decreases with speed, there is an optimum speed where the sum of both is minimal; this is the best
glide ratio. For powered aircraft, the optimum glide ratio has to be balanced with thrust efficiency. Parasitic drag is constituted by
form drag and
skin-friction drag, and grows with the square of the speed in the
drag equation. The form drag is minimized by having the smallest
frontal area and by streamlining the aircraft for a low
drag coefficient, while skin friction is proportional to the body's surface area, and can be reduced by maximizing
laminar flow. Induced drag can be reduced by decreasing the size of the
airframe, fuel and
payload weight, and by increasing the
wing aspect ratio or by using
wingtip devices at the cost of increased structure weight.
Design speed By increasing efficiency, a lower cruise-speed augments the range and reduces the
environmental impact of aviation. According to a research project completed in 2024 and focusing on short to medium range passenger aircraft, design for subsonic instead of transonic speed (about 15% less speed) with turboprop instead of turbofan propulsion would save 21% of fuel compared to an aircraft of conventional design speed and similar characteristics in terms of size, range and expected general technology improvements. Another analysis from 2014 compared the Airbus 320 from 2009 with a hypothetical turboprop successor flying at a 33% lower Mach number, concluding that the slower aircraft would have 36% less fuel consumption. Both state that the decrease of fuel costs enabled by lower design speed would overcompensate the increase of time-related costs resp. the decrease in
revenue passenger miles flown per day. In other words, subsonic turboprop aircraft would be more profitable than transonic turbofan aircraft even at current energy prices without additional costs related to climate action like emission fees, aviation fuel taxation or higher prices for sustainable aviation fuels compared to fossile kerosene. For
supersonic flight, drag increases at Mach 1.0 but decreases again after the transition. With a specifically designed aircraft, such as the (discontinued)
Aerion AS2, the Mach 1.1 range at 3,700 nmi is % of the maximum range of 5,300 nmi at Mach 0.95, but increases to 4,750 nmi at Mach 1.4 for % before falling again.
Wingtip devices Wingtip devices increase the effective
wing aspect ratio, lowering
lift-induced drag caused by
wingtip vortices and improving the lift-to-drag ratio without increasing the wingspan. (Wingspan is limited by the available width in the
ICAO Aerodrome Reference Code.)
Airbus installed wingtip fences on its planes since the
A310-300 in 1985, and Sharklet blended-winglets for the
A320 were launched during the November 2009
Dubai Airshow. They add but offer a 3.5% fuel burn reduction on flights over . On average, among large commercial jets,
Boeing 737-800s benefit the most from winglets. They average a 6.69% increase in efficiency but depending on the route have a fuel savings distribution spanning from 4.6% to 10.5%.
Airbus A319s see the most consistent fuel and emissions savings from winglets.
Airbus A321s average a 4.8% improvement in fuel consumption, but have the widest swing based on routes and individual aircraft, recognizing anywhere from 0.2% improvement to 10.75%.
Weight As the weight indirectly generates lift-induced drag, its minimization leads to better aircraft efficiency. For a given payload, a lighter
airframe generates a lower drag. Minimizing weight can be achieved through the airframe's configuration,
materials science and construction methods. To obtain a longer range, a larger
fuel fraction of the
maximum takeoff weight is needed, adversely affecting efficiency. The deadweight of the airframe and fuel is non-payload that must be lifted to altitude and kept aloft, contributing to fuel consumption. A reduction in airframe weight enables the use of smaller, lighter engines. The weight savings in both allow for a lighter fuel load for a given range and payload. A rule-of-thumb is that a reduction in fuel consumption of about 0.75% results from each 1% reduction in weight. The
payload fraction of modern
twin-aisle aircraft is 18.4% to 20.8% of their maximum take-off weight, while
single-aisle airliners are between 24.9% and 27.7%. An aircraft weight can be reduced with lightweight materials such as
titanium,
carbon fiber and other composite plastics if the expense can be recouped over the aircraft's lifetime.
Fuel efficiency gains reduce the fuel carried, reducing the take-off weight for a
positive feedback. For example, the
Airbus A350 design includes a majority of lightweight composite materials. The
Boeing 787 Dreamliner was the first airliner with a mostly composite
airframe.
Flight distance For
long-haul flights, the airplane needs to carry additional fuel, leading to higher fuel consumption. Above a certain distance it becomes more fuel-efficient to make a halfway stop to refuel, despite the energy losses in
descent and
climb. For example, a
Boeing 777-300 reaches that point at . It is more fuel-efficient to make a
non-stop flight at less than this distance and to make a stop when covering a greater total distance. -200 per distance Very long non-stop passenger flights suffer from the weight penalty of the extra fuel required, which means limiting the number of available seats to compensate. For such flights, the critical fiscal factor is the quantity of fuel burnt per seat-nautical mile. For these reasons, the world's longest commercial flights were cancelled . An example is Singapore Airlines' former New York to Singapore flight, which could carry only 100 passengers (all business class) on the flight. According to an industry analyst, "It [was] pretty much a fuel tanker in the air."
Singapore Airlines Flights 21 and 22 were re-launched in 2018 with more seats in an
A350-900ULR. In the late 2000s/early 2010s, rising fuel prices coupled with the
2008 financial crisis and the
Great Recession caused the cancellation of many ultra-long haul, non-stop flights. This included the services provided by Singapore Airlines from Singapore to both Newark and Los Angeles that was ended in late 2013. But as fuel prices decreased and more fuel-efficient aircraft have come into service, many ultra-long-haul routes were reinstated or newly scheduled (see
Longest flights).
Propulsive efficiency The efficiency can be defined as the amount of energy imparted to the plane per unit of energy in the fuel. The
rate at which energy is imparted equals thrust multiplied by airspeed. To get thrust, an
aircraft engine is either a shaft engine –
piston engine or
turboprop, with its efficiency inversely proportional to its
brake-specific fuel consumption – coupled with a
propeller having its own
propulsive efficiency; or a
jet engine with its efficiency given by its airspeed divided by the
thrust-specific fuel consumption and the
specific energy of the fuel. Turboprops have an optimum speed below . This is less than jets used by major airlines today, however propeller planes are much more efficient. The
Bombardier Dash 8 Q400 turboprop is used for this reason as a regional airliner.
Jet fuel cost and emissions reduction have renewed interest in the
propfan concept for jetliners with an emphasis on engine/airframe efficiency that might come into service beyond the
Boeing 787 and
Airbus A350XWB. For instance, Airbus has patented aircraft designs with twin rear-mounted counter-rotating propfans. Propfans bridge the gap between turboprops, losing efficiency beyond Mach 0.5-0.6, and high-bypass turbofans, more efficient beyond Mach 0.8. NASA has conducted an Advanced Turboprop Project (ATP), where they researched a variable-pitch propfan that produced less noise and achieved high speeds. == Operations ==