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Structural Issues Of Tri-jets

Fri Jun 18, 2004 1:02 am

I read in my Boeing book that tri-jets had "structural issues." Yet they never went into specifics. Can anyone fill me in on the structural problems with tri-jets?
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Topic Author
Posts: 2587
Joined: Wed Sep 17, 2003 12:17 pm

RE: Structural Issues Of Tri-jets

Fri Jun 18, 2004 1:17 am

Maybe to be a bit more specific, I read that Boeing avoided the tri-jet concept when designing the 777 because of the tri-jet's structural issues...what structural issues bothered them in particular? And what are the complexities of designing tri-jets?
Fly one thing; Fly it well
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RE: Structural Issues Of Tri-jets

Fri Jun 18, 2004 1:23 am

Not sure, but I would guess having a big engine on a structure (vertical stabilizer) that is very stressed in-flight anyway, even without the engine.

Probably the lack of heavy duty spars or other support structures are an issue as well, when they are pretty easy to come by in the wing area where most engines are attached.
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RE: Structural Issues Of Tri-jets

Fri Jun 18, 2004 3:41 am

The entire rear fuselage of a tri has to be significantly enhanced to carry a heavy engine.

Its easier to enhance the wings, and much less heavy.

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RE: Structural Issues Of Tri-jets

Fri Jun 18, 2004 4:55 am

The wings carry the plane... the fuselage just "hangs" from the wings; And hanging something from the wings therefore requires much less structural strengthening than hanging something from the end of the fuselage (where it will exert intense stress especially during turbulence!).

The rear fuselage also needs to carry the thrust generated by the tail engine (which requires strengthened structures again).

In addition, you´d need fuel lines all the way back (and up!) to the tail engine (additional systems and fire protection are required!);

You´d have the risk of a severe engine failure taking out the tail fin or the rudder (and possibly damaging the horizontal stabilizer as well);

You´d have to route rudder control lines around the engine (which will require additional protection);

All this translates to heavier and more complex structures of a tri-jet compared to wing-mounted engines only.
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RE: Structural Issues Of Tri-jets

Fri Jul 16, 2004 8:41 pm

All structural issues are addressed, as they appear. Every airplane has its own can of worms. Engineers design and modify the structures as their service history evolves. Look at the Service Bulletins for ATA chapter 53. You'll find several dealing with fatigue inspections, modifications to certain locations, etc.

As for tail mounted engines vs. wing mounted engines...A tail mounted powerplant (727,DC-9, Caravelle) produces a clean wing. Everything in engineering is about trade-offs; just depends on what their design objectives are.
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