wingman
Posts: 2792
Joined: Thu May 27, 1999 4:25 am

A Couple Of Questions

Sun Nov 12, 2000 2:39 am

Just curious:

1. Why do airplanes have different tail-end designs and what effect, if any, does the shape have? I ask this because I notice the 777 has an MD "style" squared-off tail like the 717 whereas the 757 and 767 have rounded cone-style ends.

2. On a 3 or 4 engine aircraft, would it be possible to have engines of different thrust levels or sizes? In other words, would it be economical to have a 747 or 340 with two 777 size engines inboard and two 737/757 engines outboard? What about a trijet with a smaller thrust engine at the rear?

Thanks for your answers.
 
gkirk
Posts: 23346
Joined: Thu Jun 15, 2000 3:29 am

RE: A Couple Of Questions

Sun Nov 12, 2000 3:05 am

Just imagine a 747 with 4 777 size engines on, that would be great, just think of the power that would cause, I think you would probably have to strengthen the wings a wee bit though.
I cant really answer those questions though, so see what others say.
When you hear the noise of the Tartan Army Boys, we'll be coming down the road!
 
airmale
Posts: 7125
Joined: Fri Sep 17, 2004 4:48 pm

RE: A Couple Of Questions

Sun Nov 12, 2000 3:27 am

this has been a week of revelations, i have been wondering myself about those 'pinched butts' MD a/c style, the best news was that the winglets make no difference on fuel consumption, according to a test carried out by united airlines on a 747-400 sans one winglet, why have we had to put up with those uglies for all these years especially the worst examples on the A310/320 family and the MD-11,YUKH! 
.....up there with the best!
 
Guest

RE: A Couple Of Questions

Sun Nov 12, 2000 3:41 am

It may seem more economical to have a 747 with two huge engines and 2 smaller ones, but the extra costs associated with maintenance of two engine types on a single airframe would make it unfeasible. Plus, you would have to consider a situation involving the loss of a GE90/Trent on takeoff. Would the remaining big engine plus the two smaller ones be enough?
 
Starship
Posts: 1038
Joined: Wed Nov 24, 1999 3:26 am

RE: A Couple Of Questions

Sun Nov 12, 2000 3:41 am

With regard to question # 1, I could only guess and say that tail cone design is dictated by trying different shapes in a wind-tunnel and seeing which shape offers the least drag.

In terms of question # 2, different engines have been used on a single aircraft, but from a servicing and maintenance point of view, I believe that this configuration is not entirely practical. I recall that the very early Shackleton reconnaissance aircraft had 2450HP Rolls Royce 57A Griffon engines inboard and Rolls Royce 56A Griffon engines outboard. In the second production batch, the outer nacelles were enlarged to take the 57A Griffon engines as well.


Avro Shackleton MR3 (sorry - couldn't find an image of the early model)

The Trident 3 of course had a small 4th engine in the tail to assist take-off, as the other three engines weren't quite powerful enough. It was shut down during cruise.


Click for large version
Click here for full size photo!

Photo © Propfreak


Trident 3B showing small 4th engine in the tail above the # 2

I can't think of any others off-hand.

Behind every "no" is a "yes"
 
wingman
Posts: 2792
Joined: Thu May 27, 1999 4:25 am

RE: A Couple Of Questions

Sun Nov 12, 2000 5:12 am

Thanks guys. That is amazing about the Trident. I never knew or noticed before. I assume the #4 shared the air intake with #3.
 
Guest

RE: A Couple Of Questions

Sun Nov 12, 2000 1:01 pm

I may be wrong, but I think that the Tridents's 4th engine was also the APU. Neat idea.
 
Starship
Posts: 1038
Joined: Wed Nov 24, 1999 3:26 am

RE: A Couple Of Questions

Sun Nov 12, 2000 10:39 pm

Now that I come to think of it, some of the Royal Air Force Shackleton MR3s had their outer engines replaced with jets, while retaining the RR 12-cylinder super-charged piston engines inboard. I have the details on video, so will check it out and report back later. The additional stress shortened the wing main-spar life considerably and they were retired more than a decade before their South African equivalents.
Behind every "no" is a "yes"
 
SAA-SAL
Posts: 345
Joined: Sun Nov 05, 2000 11:41 pm

RE: A Couple Of Questions

Mon Nov 13, 2000 12:16 am

The Il-18 (four engined propliner) has its inboard engines larger than its outboard.
SAA B747 SP, Luxavia B747 SP
 
Starship
Posts: 1038
Joined: Wed Nov 24, 1999 3:26 am

RE: A Couple Of Questions

Mon Nov 13, 2000 6:30 am

I got my story about the Shackleton jet engines mildly incorrect.

The RAF Shackletons went through a number of upgrades during their 40 years of service and at some point during 1961 the MR.Mk3 had a Phase 3 upgrade which added a 2,500-lb Bristol Siddeley Viper 203 turbojet in the extended rear of each outboard engine nacelle, fed by a retractable ventral inlet. The jets were used only on take-off and climb-out, to ease the strain on the Rolls Royce Griffons reducing wear by reducing take-off RPM.

This in turn put an axial strain on the wings - something that had not been taken into account during the upgrade - and with fatigue mounting dangerously, the entire MR.Mk3 force was withdrawn from service in September 1971.
Behind every "no" is a "yes"