What is the optimal wing sweep angle for a passenger airliner?

Question

There are many different types passenger planes, but is there a optimal wing angle for these planes, that improves the plane's take off speed for example, or the fuel efficiency

The Boeing 747 has a wing seep angle of 37.5 degrees, the Airbus A380 has a wing sweep angle of 33.5 degrees. These are two of the largest planes in the world. So is the optimal range for a wing sweep angle between 33.5 and 37.5 degrees, or are there certain regulations stating different sweep back angles that a passenger airliner can use.

Answer 1

Generally, sweep makes the wing less efficient in creating lift and complicates structural design. Therefore, aircraft designers try to use as little sweep as they can get away with. However, sweep delays the onset of compressibility effects and allows jet aircraft to improve their maximum transport performance (the product of speed and payload).

Forward and backward sweep both help to fly faster, but backward sweep is preferred because it makes stalling less dangerous (if properly designed), improves directional stability and allows to make the wing lighter. The shortest take-off distance will be achieved with zero sweep - if it were not for the need to limit compressibility effects, sweep would only be used in tailless aircraft.

The main factor for choosing wing sweep is the desired cruise Mach number, and Mach 0.85 has become the de-facto standard. Other factors are:

• Relative wing thickness: A thicker wing needs more sweep, because thickness adds its own compressibility effects.
• Maximum local Mach number of the airfoil where recompression is shock-free: So-called supercritical airfoils help to reduce sweep, and older designs (before the A310) had generally more sweep because their critical Mach number would had been lower at equal sweep.

Regulations have nothing to do with it, only engineering determines the sweep angle. Since the design cruise Mach number is quasi-fixed at Mach 0.85 for long-range airliners, the sweep angle of different aircraft is very similar. Regional jets normally fly a little slower to improve fuel efficiency, and their sweep angle is less than that of their big, long-range cousins.

Answer 2

No, there is no specific sweep angle which serves best for improving the sweep of the plane wing. This is because the sweep is NOT introduced just for improving the fuel efficiency of the plane, but the following factors are also considered by commercial airliners while finalizing the design sweep of a wing:

1. Design Mach Number
2. What extent of longitudinal stability the airplane manufacturer expects out of the wing (This would depend upon the final usage of the plane)
3. How much part of the longitudinal stability the manufacturer expects from the Dihedral, and the rest from the sweep back.
4. Is the manufacturing of such a sweep back (and dihedral) wing feasible enough, and if not, what additional resources need to be invested to manufacture this?
5. What kind of control system (Fly by wire or not) is being used?

After optimizing on all the above said factors (and the fuel efficiency), the aircraft is modeled under CFD to get results, and then it is decided what sweep back to use

Answer 3

Jan and Peter are right, here is a little more background.

Long before airflow over an airframe and wing structure could be fully modeled with CFD, the relationship between wing sweep and mach number at cruise speed and altitude was known to an accuracy within a few degrees of sweep. This meant that the first jet liners with swept wings (Boeing 707 series and Douglas DC-8 series) used 35 degrees (or something close to it) for their wing sweep.

Why is that specific sweep angle still used in modern designs? Because the engine technology is still based on subsonic air-breathing gas turbines, and so the cruise speeds and happiest altitudes for turbine-powered cruise are just about the same today (550 MPH, 35-40,000 feet) as they were in 1958.