How can an airplane's design make it more fuel efficient?

Question

787 v. 757, 747 v. A380. What changes to the airplane's aerodynamics make it more fuel efficient. (Aside from the engines.)

Answer 1

Improved aeroelastic / aerodynamics prediction.

Contersunk rivets are nice, but they have been around for decades, they are not recent development. Lighter materials (composite) are nice, but they do not change the aerodynamics, at least directly.

When an aircraft flies, the wing flexes and twists. This in turn changes the aerodynamics of the wing. With modern CFD techniques and advanced material deformation models, the internal structure of the aircraft can be tuned to optimize the shape of the wing and get a good load distribution on the overall span.

Another thing that can be done is to move the center of gravity further back. This makes the airplane less stable, but advanced computers in a fly by wire system are able to cope with it. The advantage is that the horizontal tail downforce is reduced (or even turned into lift) which in turn reduces the lift that the main wing has to provide, and with less lift comes a reduced induced drag.

Finally, supercomputers are put to use to evaluate the results of full Navier-Stokes equations on a more detailed model and finer "grid" representaion of the airflow. This permits to fine tune the outer shape of the wing so as to minimize the sonic shock on the upper surface of the wing of transonic airplane (so called supercritical wing profile attempt to have as much isentropic compression as possible).

Finally, there are other techniques being looked at, like laminar flow control, and ridgelets surfaces, but those are still experimental and are not yet ready to be put on aircraft currently being developped.

Answer 2

There are five things that affect fuel efficiency:

1. aerodynamic drag - non-lifting surfaces such as the fuselage and antennae, and lifting surfaces - wings
2. efficiency of powerplants
3. weight
4. atmospheric conditions
5. speed

Drag increases as the square of the speed (eg, an airplane flying at 600 mph has 4x the drag as it does at 300 mph, and 16x the drag as it would at 150 mph. Therefore an airplane flying slower will consume less fuel per hour. But we all want to get where we are going quickly, and the faster we get there the less time the engines are consuming fuel. Also we really can't control atmospheric conditions except to increase the altitude capability of an aircraft (which reduces drag) so aerospace engineers focus most of their attention on the aerodynamics and engines.

Like some of the above answers mentioned, reducing parasitic drag such as rivets is important. With composite materials coming into widespread usage, rivets are slowly being eliminated. But other things like fuselage shape, antennnas, window mounts, door fittings, strobe lights, etc. can be "smoothed" as part of the design process using a complex aerodynamic principle known as the Reynolds number.

For wings, engineers have a big problem coming up with a shape that is efficient at high speed yet allows the airplane to fly slow enough for a safe landing. Some military types like the FB-111 and F-14 had swing-wings, but they are expensive and trouble prone. The best we have available now are slats and flaps, but in the future we may see variable geometry wings that thin with increased speed.

Jet engines burn less than half the fuel that they did only 40 years ago for the same thrust, and advances are being made all the time. Like wings, an engine needs to produce large amounts of thrust at low airspeed for take-off, yet burn efficiently for extended cruise settings - those are two design criteria that don't coincide and trade-offs need to be made.

The less an aircraft weighs (basic empty weight, fuel load, and cargo/passengers) the less fuel is required to move it. Composite materials are showing tremendous potential to reduce aircraft weight, and the 787 will have a higher proportion of composites than any other aircraft.

Its not really possible to compare the 787 with the 757 or the 747 with the A380 as the size difference is too great.

There are some studies being done on lamellar flow wings and boundary layers, but I don't think we'll see those things in the commercial world for at least 20 years.

Some small corrections to another answer:

- the 757 was phased out in 2005 after 1050 were built. It was never really a replacement for the 727 which ended production in the early 1980's with more advanced 737's becoming more efficient

- the 767 is definitely not a replacement for the 727, as its about twice as big (412,000 lb TOW for the 767-300ER versus 210,000 lb for the largest variant of the 727).

- the 787 is a replacement for the 767, not the 727

Here's some more reading in reasonably simple terms on aerodynamics:

http://www.aerospaceweb.org/question/aerodynamics/

Answer 3

The primary factor for A380 is not the aerodynamics but simply the use of composites as part of the airframe to make it lighter. A380 looks quite good against the 35 year old 747. However, the new 747-8 which is a stretched version of the 747-ER actually has lower fuel burn per seat mile than A380. The 747-8 has only minor modifications to the airfoil shape, wingtips, flaps, and fairings.

The 757 and 767 are being phased out. They were replacements for the older 727. The primary advantage of the 757 and 767 over the 727 however was the use of high bypass fanjet engines. 787 is the replacement for the much older 707 and is an all new design. 777 shows some of its advantages as a new design by having longer range than A380 or 747-8. The new 787 should have similar advantages in range.

As some have mentioned the use of rivets is decreasing which reduces surface drag. This appears to be being replaced by welding and composite materials. It is also easier today to design lower drag fairings without thousands of hours of wind tunnel tests. There are some minor improvements due to airfoil, wing, and wingtip shape.

Answer 4

Very good answers above.

Another thing that they can do is increase the "aspect ratio" of various components, such as wings, and the fuselage. Aspect ratio is the ratio of length divided by width. The higher the aspect ratio in virtually all fluid mechanical models, the greater the efficiency.

If you have two identical sailboats each with a six square foot sail, one who's sail is 1 foot wide by 6 feet tall and one who's sail is 2 feet wide by 3 feet tall the one with the 1 foot wide by 6 foot tall will go faster.

If you have two ships that are equal abeam (have the same width), the same engines, and same weight, but one is longer than the other, the longer one will go faster.

Aspect ratio plays an important role in deciding how to dimension something that employs fluid mechanics.

Also, there is a reason why golf balls are dimpled. The dimples reduce the drag. To do this to an entire airplane would be extremely expensive and currently, we don't have a way of doing it inexpensively, however, in future construction methods, we might, and this would make planes more aerodynamic as well.

Answer 5

The 787 is using composite material which makes the aircraft lighter. Therefore less fuel is used. Another person mentioned making the aircraft "sleeker". Definitely aircraft shape has to do with how aerodynamic the airplane is. I am thinking the sleeker the airplane, the less drag is put on the aircraft and therefore less fuel is used.

Answer 6

Yes the A320 is more efficient because of its engine which the 737 NG's have now been using. The B737 classic has an HPTACC or High Pressure Turbine Active Clearance Control while the A320 & NG's have RACSB or Rotor Active Clearance & Start Bleed valves & LPTACC or Low Pressure Turbine Active Clearance Control, suffice to say they maximize turbine efficiency during cruise causing less fuel or in short fuel efficient.

Answer 7

have you ever held a big piece of cardboard on a windy day and tried to cross a parking lot with it only to find yourself in a total wrestling match? You then found that by aligning the piece of cardboard to the wind so that there was less resistance, your struggle became less? You just found out the meaning of efficiency in aerodynamics by reducing drag. Every time engineers find a way to reduce drag, improve and manage air flow, the aircraft become more efficient.

Answer 8

I have a possibly novel suggestion:
After the wings are connected to the fuselage, the whole wing surface should be coated with teflon.

If teflon is non-stick in your frying pan, it's also non-stick in the airstream, lets air flow over/under it more smoothly.
(And the bugs it collects at low altitude won't stick to it, either.)

HOWEVER: The wing is made up of very many panels, not all of them control surfaces.
And there are HUNDREDS OF SQUARE YARDS of wing surface.

To replace a panel, or a control surface, you'd have to cut the teflon around said panel; then, after replacing the panel, you'd have to re-coat the area again.

Not impossible, of course, but quite costly.

Hey, AIRLINES: ARE YOU LISTENING? My idea is free for your use.

Answer 9

An interesting aspect to this question is that Howard Hughes developed the principal of flat rivets for aircraft which dramatically reduced drag and thereby reducing fuel consumption............

Answer 10

Many aircraft are simply designed “slicker” than others by using countersunk (flat) fasteners and fairing in the surfaces along with preventing “Burbling” or turbulent air between control surfaces they make it easier for the aircraft to move through the air