7
$\begingroup$

The Boeing 747-8 has a new airfoil design much different than a Boeing 737's airfoil.

Boeing 737 airfoil:

enter image description here
(source: How Airplanes Fly?)

Boeing 747-8 airfoil:

enter image description here
(source: Boeing)

I am a little confused of why it is shaped like this. What are the advantages of this new profile?

It is also more thin, wouldn't that reduce the amount of fuel stored in the wings?

$\endgroup$
  • 6
    $\begingroup$ Why don't you compare it to the old 747 airfoil? This is like asking why an apple isn't segmented like an orange. Different designs have different goals and efficiencies! $\endgroup$ – egid Sep 4 '15 at 0:11
  • 4
    $\begingroup$ Your picture shows some generic airfoil, but certainly not that of the 737. The 737-NG and 747-8 airfoils don't look very different. $\endgroup$ – Peter Kämpf Sep 4 '15 at 13:01
11
$\begingroup$

The airfoil profile you've shown is called a supercritical airfoil.

Typical airfoil sections are curved on the top and the bottom. The airflow over the top of the airfoil is accelerated i.e. the airspeed over the top of the airfoil is more compared to the free stream velocity. The associated reduced pressure helping to create lift.

Laminar flow over NACA airfoil
Source: www.symscape.com

From the figure, two things can be inferred:

  • The pressure over the upper surface is lower compared to the lower surface
  • The pressure gradient is favorable in the forward section of the airfoil (where most of the lift is produced), while the gradient is adverse in the rear half, which can cause boundary layer transition and possibly separation, if the gradient is too severe.

Adverse pressure gradient
Source: code7700.com

For low speeds (and low angles of attack), this causes no problem in lift generation. However, most of commercial airliners fly at in the transonic regime (with speeds ~ Mach 0.9), the flow over the top of the wing may reach supersonic speeds even when the aircraft itself is flying at subsonic speeds.

Transonic flow over an airfoil
Source:fluid.mech.ntua.gr

The speed at which this happens (flow over wings becomes supersonic) is called the critical Mach number. The flow decelerates through a normal shock wave, resulting in a severe adverse pressure gradient over the rear of the airfoil.

NACA 0012 in transonic flow- Pressure
Source: fluid.mech.ntua.gr

This causes serious problems like:

  • The adverse pressure gradient over the shock wave causes flow separation, decreasing lift and increasing drag. The speed above which the drag increases significantly is called the drag divergence Mach number

Schileren photo of flow seperation
Source: history.nasa.gov

  • As the speed increases, the shock wave moves to the rear, as does the lift producing region. This causes a nose down pitching moment and stability issues.

Transonic flow patterns
"Transonic flow patterns" by U.S. Federal Aviation Administration - Airplane Flying Handbook. U.S. Government Printing Office, Washington D.C.: U.S. Federal Aviation Administration, p. 15-7. FAA-8083-3A.. Licensed under Public Domain via Commons.

The supercritical airfoils are designed to overcome (or delay) these problems. Designed by Richard Whitcomb, the main characteristics of these airfoils are,

  • 'flattened' upper surface
  • highly cambered aft section
  • smaller leading edge radius compared to the traditional airfoils (that is why they are so thin)

Supercritical airfoil
Source: aerospaceweb.org

The main advantages of supercritical airfoils are,

  • It increases the drag divergence Mach number. For example, in the F-111, the drag divergence Mach number was increased from 0.76 to 0.88, a 16% increase.
  • The shock waves are developed further aft when compared to the normal airfoils
  • They greatly reduce the shock induced boundary layer separation, reducing drag

The figure below shows a supercritical airfoil in a transonic flow. Note the location of the normal shock compared to the 'normal' airfoil.

Supercritical airfoil
Source: space.bas.bg

The supercritical airfoils were not available when the Boeing 737 was first designed. However, the 737 NG airfoil incorporates elements of supercritical design,as does the Boeing 747-8 airfoil.

The supercritical airfoils reduce the fuel required, as they allow the aircraft to cruise at higher speeds with lesser drag. Additionally, the Boeing 747-8 has a wing span of ~68 m, which gives plenty of storage space for fuel. Also, improvements in engines and airfoils means that the 747-8 has nearly 50% more range compared to the original 747-100.

$\endgroup$
  • 2
    $\begingroup$ The leading edge radius of a supercritical airfoil is bigger than that of a conventional airfoil, which gives them a higher $c_{L_{max}}$ as a nice side benefit. $\endgroup$ – Peter Kämpf Sep 4 '15 at 12:54
  • $\begingroup$ So the (b747-8) airfoil is designed to reduce wave shock drag $\endgroup$ – Ethan Sep 4 '15 at 18:42
  • 1
    $\begingroup$ Yes, to delay the onset of wave drag. $\endgroup$ – aeroalias Sep 4 '15 at 18:54
4
$\begingroup$

Some things to keep in mind when comparing two very different planes:

The 737NG is the latest version of the 737, and it was designed in the 90's. The design changes included a new wing. The 737 is designed for short flights, with a top range of about 3,000 nmi. The 737 has a design cruise speed of about M0.78.

The 747-8 is the latest version of the 747, and it was designed in the 2000's. The design changes also included a new wing. The 747 is designed for long flights, with a typical max range of 7,700 nmi. The 747 has a design cruise speed of about M0.85, and is much larger than the 737.

So the two planes span at least a decade, and are optimized for very different profiles. The 747 will be optimized more for cruise, and at a higher speed. The 737 will be optimized less for cruise and more for shorter segments.

Another point is that the airfoil is not uniform along the wingspan. As the 737 link shows, the airfoil shape will change along the span.

However, the two flight profiles include cruise speeds of around M0.8, which means that both planes will be optimized with a supercritical airfoil, as is explained in another answer. Therefore both airfoils will have much more in common with each other than compared to, for example, an aerobatic plane. I'm not sure where you got the idea that your first image is a 737 airfoil. The source site just says it is typical for a "stunt plane", which the 737 is not.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.