Moderators: richierich, ua900, PanAm_DC10, hOMSaR

 
User avatar
SEPilot
Topic Author
Posts: 5739
Joined: Sat Dec 30, 2006 10:21 pm

MD-11 vs 77W wing loading

Wed Mar 10, 2021 12:41 pm

The MD-11 suffered from excessive wing loading, having a wing loading of 884 kg/m2. This led to several problems, the most important one being that it struggled to meet efficiency goals. In otder to try and reach those goals the CG envelope was moved aft, necessitating computer enhanced stability, and tail surface area was reduced to reduce drag. These resulted in higher approach speeds and tricky handling characteristics at landing, leading to several accidents. But the 77W had nearly as high wing loading at 804 kg/m2, and suffered none of these problems. Not only does it have an unblemished safety record but it is the unquestioned long range efficiency champion for its generation. In comparison the A330 has a wing loading of 645 kg/m2. Why doesn’t the 77W have any of the problems that have plagued the MD-11 due to high wing loading?
 
mxaxai
Posts: 2770
Joined: Sat Jun 18, 2016 7:29 am

Re: MD-11 vs 77W wing loading

Wed Mar 10, 2021 1:58 pm

Wing loading does not necessarily hurt efficiency. The only measure that directly influences induced drag is the aspect ratio, which is given as 7.91 for the MD-11, 8.67 on the 777-300 and 9.61 on the 777-300ER. (different calculation methods for wing area may yield different aspect ratio values).

To the contrary, the most efficient designs often come with high wing loading since this means a smaller wing, therefore less wetted area and less parasitic drag.

What wing loading does affect is runway and climb performance. In its size range, the MD-11 has one of the highest thrust-to-weight ratios in a one-engine-out scenario. (MD-11 at 0.194, 777-300ER at 0.149, A340-300 at 0.170, A330-300 (217t MTOW) at 0.141). Even with all engines operating, it sits above the others (MD-11 at 0.295, 777-300ER at 0.292, A340-300 at 0.227, A330-300 at 0.282). In other words, it's carrying too much engine for its mission, which impacts weight, cost and efficiency.

It also requires more attention to high lift devices to provide the necessary lift at low speeds, where the 777 has advantages thanks to more modern CFD. It doesn't help that the MD-11 used an evolution of the ancient DC-10 wing. The required compromises lead to the mentioned high approach speeds and tricky handling.
 
User avatar
SEPilot
Topic Author
Posts: 5739
Joined: Sat Dec 30, 2006 10:21 pm

Re: MD-11 vs 77W wing loading

Wed Mar 10, 2021 3:06 pm

mxaxai wrote:
Wing loading does not necessarily hurt efficiency. The only measure that directly influences induced drag is the aspect ratio, which is given as 7.91 for the MD-11, 8.67 on the 777-300 and 9.61 on the 777-300ER. (different calculation methods for wing area may yield different aspect ratio values).

To the contrary, the most efficient designs often come with high wing loading since this means a smaller wing, therefore less wetted area and less parasitic drag.

What wing loading does affect is runway and climb performance. In its size range, the MD-11 has one of the highest thrust-to-weight ratios in a one-engine-out scenario. (MD-11 at 0.194, 777-300ER at 0.149, A340-300 at 0.170, A330-300 (217t MTOW) at 0.141). Even with all engines operating, it sits above the others (MD-11 at 0.295, 777-300ER at 0.292, A340-300 at 0.227, A330-300 at 0.282). In other words, it's carrying too much engine for its mission, which impacts weight, cost and efficiency.

It also requires more attention to high lift devices to provide the necessary lift at low speeds, where the 777 has advantages thanks to more modern CFD. It doesn't help that the MD-11 used an evolution of the ancient DC-10 wing. The required compromises lead to the mentioned high approach speeds and tricky handling.

Thank you for this insight. I knew that the old DC-10 wing was part of the problem. I did not realize that there was such a big difference in aspect ratio. I was also aware that Boeing was very good at high lift devices.
 
Max Q
Posts: 9066
Joined: Wed May 09, 2001 12:40 pm

Re: MD-11 vs 77W wing loading

Thu Mar 11, 2021 12:43 pm

Not just about wing loading though, the horizontal stab on the MD11 was really undersized in an attempt to reduce drag, that mandated a higher approach speed and numerous instances with controllability in some cases resulting in tragic accidents
 
WIederling
Posts: 10043
Joined: Sun Sep 13, 2015 2:15 pm

Re: MD-11 vs 77W wing loading

Thu Mar 11, 2021 3:09 pm

SEPilot wrote:
mxaxai wrote:
Wing loading does not necessarily hurt efficiency. The only measure that directly influences induced drag is the aspect ratio, which is given as 7.91 for the MD-11, 8.67 on the 777-300 and 9.61 on the 777-300ER. (different calculation methods for wing area may yield different aspect ratio values).

To the contrary, the most efficient designs often come with high wing loading since this means a smaller wing, therefore less wetted area and less parasitic drag.

What wing loading does affect is runway and climb performance. In its size range, the MD-11 has one of the highest thrust-to-weight ratios in a one-engine-out scenario. (MD-11 at 0.194, 777-300ER at 0.149, A340-300 at 0.170, A330-300 (217t MTOW) at 0.141). Even with all engines operating, it sits above the others (MD-11 at 0.295, 777-300ER at 0.292, A340-300 at 0.227, A330-300 at 0.282). In other words, it's carrying too much engine for its mission, which impacts weight, cost and efficiency.

It also requires more attention to high lift devices to provide the necessary lift at low speeds, where the 777 has advantages thanks to more modern CFD. It doesn't help that the MD-11 used an evolution of the ancient DC-10 wing. The required compromises lead to the mentioned high approach speeds and tricky handling.

Thank you for this insight. I knew that the old DC-10 wing was part of the problem. I did not realize that there was such a big difference in aspect ratio. I was also aware that Boeing was very good at high lift devices.


very efficient so that tire speeds are a strong limitation in take off performance. :-)
i.e. the bird has to run for its life to take off.

too small tail is what turns the MD11 problematic.

T/W ratios : your compare does not work if you cross from 2 to 3 to 4 engines.
The A330 vs 77W comparison is informative though: A330 : more wing, better/cleaner high lift devices -> less T/W required.

Who is online

Users browsing this forum: No registered users and 12 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos