Leezyjet
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Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 8:20 am

Could anyone advise me as to why no other manufacturer chose to use the Comet style of having the engines buried in the wingroots ?.

When you look at the Comet, it looks so much more aerodynamic compared to other a/c that have their engines hanging from the wings.

I can understand that on modern By-pass engines it would not be practical to do this, due to the size but why didn't any other manufacturers at the time choose to follow DHc and bury the engines in the wingroots ??.



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LY744
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 8:23 am

I don't know who followed who, but the Tu-16/104 has its engines (only two in that case) located similarly.

LY744.
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aloges
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 8:30 am

Who said...?  Smile


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The most probably reasons are cabin noise and vibrations. If you put the engine on pylons, it will be easier to reduce those effects than it is with wing root-mounted engines.

Besides that, I think it might have to do with the risk of an engine "flying apart", as witnessed on the UA DC-10 that crashed at SUX. If a blade separated from the engine, it would probably do more harm to a Comet/Tu-104 than to an A340/Il-96.
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aloges
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 8:36 am

As for military aircraft, there are at least the Avro Vulcan and the Tu-16 (as LY744 said):


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BR715-A1-30
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 10:22 am

I think Boeing decided to put them in separate pods, in case of an engine fire. That way the fire would not spread to the other engine.
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IMissPiedmont
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 10:41 am

BR715 is correct. The pod arrangement provides an extra safety margin. It's also a hell of lot quieter for passengers.
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 11:09 am

-Maintenance(the engines are harder to get to in the wing roots)
-Safety(if a Comet engine suffered an uncontained engine failure/fire the fragments would have a good chance of penetrating the fuselage and/or the other engine because it is located so close to it)
-Noise(having the engine so close to the fuselage makes more vibration and noise)
 
L-188
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 12:14 pm

Do you really want an uncontained engine fire or failure that close to the wing spar?
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411A
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 12:53 pm

Many reasons already mentioned are (more or less) correct,
however the primary reason for having the engines on the wings suspended from pylons is for...wing bending relief, and the related structural weight savings therefrom.
 
jwenting
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 1:05 pm

And don't forget that the size factor of todays high bypass turbofans doesn't really lend itself to wingroot mounting either.
They have a larger diameter (combined with a generally thinner wing) and are often shorter (a bit) too.
The latter means a longer intake duct, which causes a less optimal airflow into the engine and thus worse performance.
The space in the wingroot is also lost for fuel storage, costing you in range.
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cancidas
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 1:44 pm

Do you really want an uncontained engine fire or failure that close to the wing spar?

not to mention the center fuel tank. also, they are problematic when it comes to mx. vibration and noise in the cabin are reduced by putting them in pods. it's also a lot easier to change the engine if there's a problem when it's hanging in a pod, as opposed to being burried in the fuselage.
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FredT
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 3:42 pm

Didn’t see one of the main issues mentioned. Wing root mounted engines have to have long inlet/outlet ducts, meaning a significant loss of efficiency.

Fan sizes, fire protection, maintainability, noise, uncontained disc failuers, to a degree load path issues (though not as bad as with tail mounted engines)... I think that is about it. I wrote a paper on engine installations once, I’ll see if I can dig it up later and find more.

Cheers,
Fred
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Leezyjet
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 8:10 pm

Thanks for all the answers.

So if there are so many reasons not to have them there, then why did DH choose to have them in that location ?.

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VC-10
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RE: Comet Engine Location - Why No Longer Used?

Mon Aug 25, 2003 11:58 pm

The early Comets had engines with Centrifugal compressors so I guess to keep the engine profile within the wing shape, to reduce the drag, that was the only place where you could put the engines.

The advantage of that location was less assymetric thrust problems in the case of an engine out, bearing in mind the engines weren't that powerful so they could do without the bootful of rudder that would have been required to keep the a/c flying straight if the engines were placed any further outboard

[Edited 2003-08-25 17:29:11]
 
aloges
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RE: Comet Engine Location - Why No Longer Used?

Tue Aug 26, 2003 12:09 am

I think embedding the engines in the wing root didn't cause many airflow problems, either. Those old turbojet engines were longer than most current engines are, and this means that the intakes didn't have to be excessively long. They also didn't have to be very wide because of the lack of bypass fans, as stated before.
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citationjet
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RE: Comet Engine Location - Why No Longer Used?

Tue Aug 26, 2003 12:13 am

Airplane design is a compromise of many different criteria, with many choices/decisions to consider. If all criteria were treated the same, aircraft would look basically the same.
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VC-10
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RE: Comet Engine Location - Why No Longer Used?

Tue Aug 26, 2003 6:26 am

Having looked in a few Comet books now I can say the engine postion was chosen on the basis of minimising drag due to the low power to weight ratio of the early Ghost engines
 
prebennorholm
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RE: Comet Engine Location - Why No Longer Used?

Thu Aug 28, 2003 7:48 am

The wing root mounting was first seen on German bomber projects in the early forties - with up to six engines. Those projects later progressed in Russia and became the Tu-16 Badger bomber and the Tu-104 and Tu-124 airliners. In America I think that the Northrop YB-49 bomber was the only attempt (excluding early fighter planes).

With limited engine diametres this mounting method had obvious aerodynamic and structural advantages, but also a vast number of disadvantages as already mentioned.

One further disadvantage is that the engines take up much needed space for fuel tanks and landing gear.

Tupolev solved that problem by putting the landing gear into separate pods.
DeHavilland put separate fuel tank pods on the wings of later longer range Comets.
The Avro Vulcan bomber had a very thick and wide cord wing with room for everything (except passengers of course).

Another disadvantage is that as jet engines evolved into much more efficient turbofans, then they also became more sensitive to disturbed airflow into the intake, or you would risk a compressor stall. A side slip (e.g. caused by turbulence) at high altitude and high Mach number on a Comet or Tu-104 style plane would not be healthy if equipped with most modern and highly efficient engines.

You see a little of the same principle on trijets like B-727, HS Trident and L-1011. They all had slight performance loss on their #2 engines and higher probability of compressor stalls. The DC-10 and MD-11 went to quite some structural disadvantage to eliminate that slight problem.

It will be interesting to see how the BAe Nimrod Mk4 will perform with new BR715 engines, when they are ready for flight in a couple of years. But then, as the military planes they are, they won't scare the hell out of 200 pax if one day they should suffer a compressor stall.

Kinnd regards, Preben Norholm
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Leezyjet
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RE: Comet Engine Location - Why No Longer Used?

Thu Aug 28, 2003 8:21 am

Thanks again for all the informative answers.

I knew I could count on this forum to answer my questions.

Cheers guys  Big thumbs up

( hey I can now bore my colleauges with my newfound wealth of info - we sometimes quiz each other in the office about aviation trivia such as how many fan blades in an RB-211 - I now have some new material to challenge them with Big grin )

Once again big big thanks to all who took the time to reply. it is very much appreciated.

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bsergonomics
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RE: Comet Engine Location - Why No Longer Used?

Fri Aug 29, 2003 8:36 am

Sorry - got into this discussion a touch on the late side.

The use of engine pairs near to the wingroot gives less asymmetric thrust in the event of losing an engine - see next point.

For the safety hazard analyses for the NIMROD, it is assumed that, if one engine fails, then its pair will also fail. In other words, the analyses assumed that the aircraft effectively had two pairs of engines, rather than four separate ones. As you can imagine, this has a significant impact on the probability of loss of aircraft. However, since the NIMROD is designed to fly on two engines during normal service operations, this is not too much of a problem.

On a side note, the use of pylon-mounted engines helped significantly in fixing a major problem - wing flutter. It was found (by Boeing, I believe), that by mounting a weight (the engine) in the middle of the wing, flutter was drastically reduced.
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RE: Comet Engine Location - Why No Longer Used?

Fri Aug 29, 2003 12:25 pm

The engine pylons are designed to burn away in the event of an uncontained engine fire. There is a good picture in Air Disaster Vol 1 of a BOAC 707 with the number 2 engine on fire. After a few mins the pylon burned through and the engine seperated from the wing and fell to the ground. I don't know if the 747 is designed to do this too, or was it only a feature of the 707??
 
L-188
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RE: Comet Engine Location - Why No Longer Used?

Fri Aug 29, 2003 12:30 pm

Putting them out on pylons also makes it a hell of a lot easier for maintaince to get to the engines.


But knowing the way that designers work, there is no way that they where planning for that when they went to the design.

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RayChuang
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RE: Comet Engine Location - Why No Longer Used?

Fri Aug 29, 2003 3:57 pm

I think the reason why Boeing went to putting each engine on a separate pylon comes down to this:

1. Safety margins in case of fan blade failure--less likely chance of serious wing damage.

2. Easier to access the entire engine for maintainance purposes.

3. Easier to upgrade to newer engine designs when available. That's why the 707 design went from the original J57 engines to JT3D, JT4D, R-R Conway to the CFM56 installation on military versions and now even the JT8D-219 engine!

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