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Assymetrical Engines

Thu Apr 18, 2002 10:46 am

I have a couple of questions. Today I was reading about the MDXX (in Giant Jetliners) and it appeared that one of the stumbling blocks was the inabality of the MD11 "banjo fitting" to accomodate a larger engine (without major $$$$).

1) Would it be possible to have a tri-jet with two symetrical engines on the wings and a different (smaller) engine on the tail?

2)Has this idea ever been proposed on a 4 engine aircraft? An example would be a an A340-600 with two different variants of Trent engines, ie two TRENT variants of 53,000 lbs and 65,0000 lbs thrust on each wing.

Perhaps a silly question but answers or thoughts would be appreciated

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RE: Assymetrical Engines

Thu Apr 18, 2002 10:46 pm

I seem to remember that the classic Trident aircraft had a fourth smaller 'booster' jet engine installed just beneath the #2 engine. This was, I believe, was for 'hot & heavy' operations. And it was only an wasn't standard equipment. Also, the B-47, in the beginning, had the two outboard engines of lesser thrust than the the four inboard engines. And let not forget the B-36...'six a-turning & four a-burning'.  Wink/being sarcastic Regards.
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Topic: RE: Asymmetrical Engines

Fri Apr 19, 2002 1:46 am

When the DC-10 was in the planning stages, one proposed design had one engine under one wing and two under the other. Now that would have been asymmetrical.

I seem to recall that MD sounded out the airlines in the 90s about the possibility of putting bigger engines under the wings than in the tail, but they wouldn't hear of having to maintain two different engine types on one aircraft.

Boeing was considering putting a bigger APU in the 777 that would produce thrust for takeoff, but when GE determined that the GE90 had room to grow to 115,000 lb. and more, the idea was dropped.
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RE: Assymetrical Engines

Fri Apr 19, 2002 5:01 am

What have you been on??????
One engine under one wing, and two engines under the other..????? Don't think so...although german wartime engineers did come up with quite some assymetric aircraft, several of which have been flown quite succesfully.

Anyway. To answer your question Vinovalentino:
From an operational point of view, an airline would not want two different type of engines under one aircraft. For fleet commonality [sp?] reasons, airlines tend to stay with the least number of engine-types possible, even over different aircraft types [B767, B747]. If possible at all, you want to reduce the number of aircraft types and the number of engine types in your fleet. Introducing another engine type in one aircraft is an absolute maintenance nightmare! No go!

However, besides the maintenance issue, there is absolutely no reason at all which would exclude a different engine spec for the #2 position on a tri-jet. No problemo at all. Please note that this is NOT considered as being assymetrical engine configuration!
Even Boeing was [and maybe still is] looking at this option to increase the 777 potential. The idea was to install a big ass APU, which would deliver several thousands of pounds of thrust [only required at take-off and initial climb sector], thereby increasing MTOW without going to even larger engines [115000 lbs+] under the wing. During cruise APU power would not be required.

Current commercial aircraft tends toward twins...which is not the most natural platform for assymetric engine installation....

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RE: Assymetrical Engines

Fri Apr 19, 2002 5:09 am

The Valsan conversion for the 727 had JT8D-200 engines on the pods and a D-17A engine in the center position.

On the Pratt powered 777, it is possible to install a 4077 in place of a 4090 engine. But before the airplane could fly, maintenance would have to adjust the shorting cap to the FADEC on the remaining 4090 engine to derate it to the 4077 thrust level.
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RE: Assymetrical Engines

Fri Apr 19, 2002 6:08 am


Thanks for the responses. In my infinite wisdom I came to realize that the term Assymetrical Engines would refer to a plane with say a 35,000 lbt engine on one wing and a 45,000 on the other. The examples I gave in my original post were in fact symetrical in that the left and right sides were equal.

Now what about a 777 a pod mounted Spey on the left side of the fuselage...That would be assymetrical... and ugly.. and silly... and...

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