Thrust
Topic Author
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Aircraft Wingspan Effects

Mon Apr 18, 2005 10:39 pm

It surprises me to see that the 777, A330, and A340 all have greater wingspans than the 747 Classics...my question to you is why...the 747 classic is a wider, much heavier aircraft than all three of these...and with the exceptions of the A346 and 773 is longer than all three of these types. Why was decided that these three aircraft have a greater wingspan than the 747 Classics? I'm just not quite down to this concept yet...I always thought the smaller the plane, the less wingspan you needed. Yet the 772ER is 23 feet shorter than the 747 Classic and has a wingspan greater than the 747 Classic's by about 3 feet.
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FredT
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RE: Aircraft Wingspan Effects

Mon Apr 18, 2005 11:24 pm

A higher aspect ratio (wing span/mean chord) is beneficial for aerodynamic efficiency. On the other hand, a long, slender wing will require thicker beams meaning more weight per unit wing area. As always, it is a compromise. Progress within construction and materials make it possible to build slender wings without as much of a weight penalty and the wings get longer.

Have a look at gliders. They are very light and small and yet have very long wings.
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N60659
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RE: Aircraft Wingspan Effects

Tue Apr 19, 2005 1:51 am

Thrust,
I agree with FredT. Additionally, the third dimension to span and chord is the cross-section. Remember, the 747 classic was designed during the mid-late 60s when computational power was still in it's infancy and a predominant amount of aerodynamic data was obtained by testing in the wind-tunnel. With improvements in computing power, Computational Fluid Dynamics software has yeided much more sophisticated aerofoil sections tailored for different phases of flight. The trend on commercial airliner wing design has been to move toward slender, longer wings made possible by the wing cross-section. I know this is a rather simplistic answer to a very complex subject, but I hope it helps.

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Lemurs
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RE: Aircraft Wingspan Effects

Tue Apr 19, 2005 5:04 am

Also remember that "span" is from tip to tip in a straight line, not the line of the leading edge... and the 747 has a much more swept wing than the 777. This means from leading edge to tip is a longer distance on the 747, which along with the the other features of the wing would give it a greater area for the same "span".

747-400 = 37.5 degrees sweep
777-300 = 31.6 degrees sweep
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UAL747
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RE: Aircraft Wingspan Effects

Tue Apr 19, 2005 7:24 am

The 747 has a greater wing area than the 777. The MD-11 and DC-10 also feature a wing sweep greater than that of the 777. (Not sure about the L-1011).

Also, doesn't the greater sweep of the wing allow for a higher crusing speed? IIRC, the MD-11, DC-10, 747 and 727 have a greater cruising speed than later Boeings.

UAL
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N60659
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RE: Aircraft Wingspan Effects

Tue Apr 19, 2005 8:19 am

Quoting UAL747 (Reply 4):
Also, doesn't the greater sweep of the wing allow for a higher crusing speed?

That is my understanding as well. IIRC (this is from my foggy memory, so if I'm wrong, please correct me), the B-47, 367-80 and the 707 were built with wings swept back at 35 deg. Initially, for the 747, Boeing wanted the aircraft to cruise at M0.87 or so. Initial designs put the wing sweep angle at around 40 deg. However fears of Dutch roll and other aerodynamics related gremlins tempered the design and a compromise sweep angle (splitting the difference) of 37.5 deg was implemented. With this sweep back angle the 747, even today, has the highest cruising speed of all commercial airliners of M0.85. Unfortunately, higher cruising speeds also leads to higher fuel burn so for the sake of better trip economics, newer airliners cruise between M0.81 and M0.84.
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