bigphilnyc
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Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 5:32 am

I see how popular 717s are becoming, all in a day and age where hte twin seems to dominate.

Why would an airline want to choose a rear engine like a 717 over a more common twin like a small or middle 737?



Phil Derner Jr.
 
FredT
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 5:57 am

Now, there's a very complex answer to this. They would choose the 717...

...if the 717 meets their demands and requirements at a lower cost than the 737.

If the aircraft does the job cheaper, or is more suited for the airline at the same cost - it can have the engines installed in the john for all the airline cares.  Big grin

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Goldenshield
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 6:00 am

Flexibility. The 717 is based on a very successful product line (the DC-9), and also, it makes a great short hop aircraft. Based on data Ive seen (I cant quote the source, as it was a year or so ago), the dc-9 series has a better airframe record than the 737, in terms of safety.

But, it may also be for commonality. Take for instance Hawaiian. They have had dc-9-30's in their fleet for a long while. They have been wanting to upgrade them for a while as well. Well, with the 717, they get a newer aircraft, same size, but more fuel efficient.
Two all beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
 
Rick767
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 6:39 am

Like FredT said, airlines couldn't give a you know what. What they look at when selecting a new aircraft type is how well the capacity and performance will work for them, cost, flexibility, commonality with the exisitng birds (being replaced), commonality with other aircraft in the fleet, etc....

The airline I work for is probably going to select an Airbus order for longhaul ops this year, replacing the 767-300 aircraft. This isn't because the Airbus is tecnhically better than the Boeing equivalent, etc... (that is arguable...) but it boils down to what is going to be cheapest in the long run!
I used to love the smell of Jet-A in the morning...
 
Ikarus
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 6:52 am

First of all, the 717 is a twin - it has 2 engines

Secondly, the engine placement is subject to a lot of factors. For instance, engines at the rear allow for a cleaner wing design, a more quiet cabin, less risk of dirt ingestion, less ground clearance and a shorter landing gear, which saves weight and space in the belly of the aircraft, .... There are advantages. The main disadvantage is the maintenance (less accessibility) and the requirements of a T-tail (deep stall risk) and the issue of uncontained engine failures near the bulkhead & hydraulics.

Underwing engines have their own advantages - easy accessibility, they reduce the required strength of the wing if well placed (by acting as weights, they counteract some of the lift, and so the wing needs less strength), ....


Sometimes one design is more suitable, sometimes the other. (For small aircraft, tailmounted engines are generally better). Placing the engines under the wing is by no means the optimum, most modern decision - it all depends on the role of the aircraft, its size, and other factors.

Regards

Ikarus
 
m717
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 7:12 am

"First of all, the 717 is a twin - it has 2 engines,"

I'm surprised it took 4 posts for someone to point this out.
 
POSITIVE RATE
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 10:29 am

Also having rear-mounted engines reduces asymmetric problems in the event of an engine failure or shutdown.
 
LSTC
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 10:51 am

the dc-9 series has a better airframe record than the 737, in terms of safety.


Huh? I'll have to disagree with that statement. There have been over 4000 737s of all variants delivered. As far as I know the grand total for DC-9s and variants is in the 1000 range.

If you do a detailed search of the safety records, considering the fleet sizes and flight hours, I think you will quickly discover that the 737 wins hands down.
 
POSITIVE RATE
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 1:00 pm

"Huh? I'll have to disagree with that statement. There have been over 4000 737s of all variants delivered. As far as I know the grand total for DC-9s and variants is in the 1000 range.

If you do a detailed search of the safety records, considering the fleet sizes and flight hours, I think you will quickly discover that the 737 wins hands down".




Let's not forget the 737 did have it's share of problems- rudder problems/reversals and whatnot. The DC-9 never suffered from these types of problems or accidents. Also look at the number of DC-9's still flying in the world today- goes to show how robust a plane it is.

I bet there are more classic DC-9's flying today than there are 737-200's. The total number of DC-9's built was 976, production ending in 1982. The reason production stopped was because Douglas went on to build the MD-80 series- a more fuel efficient and quiet airplane than the DC-9.
 
XFSUgimpLB41X
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 9:08 pm

Definitely quite amusing... I kept reading the first posts in great detail trying to find the first person to point out that at "rear engine" plane is also a twin....
Chicks dig winglets.
 
L-188
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 9:09 pm

The makers of the VC-10, IL-62, 727, Yak 40, Yak 42, Falcon 50, Falcon 900, Trident might disagree with that.
OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
 
Ikarus
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 10:38 pm

L-188: You forgot the DC-10, MD-11, L-1011


and any plane having an APU in the rear...  Big grin


If we're gonna be anal, let's do it properly Big grin
 
Guest

RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 11:22 pm

Dear friends -
Why an airline operator selects a certain type of aircraft, there are many reasons, such as financial (cost of airplane and its operation), commonality with the existing fleet (maintenance and parts) and more...
xxx
We have wars about Airbus vs. Boeing, used to be Boeing vs. Douglas, I feel it is ridiculuous and childish to compare airplanes on the basis of some of the statements that are sometimes made in the forum. There are no "better" airplane vs. "other airplane"... there are "better airplanes" for such an operation, such an airline and... economic considerations...
xxx
I am a pilot, therefore merely involved with training and operations... my point of view, as pilot, of an airplane with engines in the tail (727) or engines located under the wing is strictly operations considerations...
xxx
Generally speaking, engines located under the wing are easier to maintain as they are close to the ground, sometimes no stand required to check oil... In the design also (engineering) airplanes having tail engine require fuel lines and shrouds to protect these lines, since tanks are far away... in the wings... some of these features make airplanes with tail engines somewhat heavier...
xxx
By the pilot standpoint, engines in the tail make the VMC speeds lower, therefore somewhat easier to fly in case of the failure of an engine. Another consideration, is when flying in icing conditions, with tail engines, ice accumulated on the wing (or fuselage) may end up to be damaging an engine.
xxx
In the 727 when we were to operate in icing conditions, we did place the engine anti-ice ON, as well as the wing leading edge anti-ice systems. In a wing mounted engine airplane, engine anti-ice is sufficient, the wing anti-ice is rarely (if ever) used... I recall of my days with PanAm, there were several engine failures in 727 because of ice ingestion... even the fuselage top VHF antenna in the 727 had to be heated, that was "critical"...
xxx
Another consideration, most if not all twins with tail mounted engines as well as the 727 and the Tridents, all have T-Tail design... the behavior of the T-Tail surfaces may often be a problem in flight stall and/or stall recoveries. Some airplanes in addition of a "stall shaker" require a "stick pusher"...
xxx
I sometimes fly as passenger in MD-80 series airplanes, lovely cabin, in the Y class section, the 3+2 configuration is much nicer than 3+3 of the 737/757s, and the engines are far in the back... very little noise... yet I am known to be a "Boeing man"...
xxx
Happy contrails  Smile
(s) Skipper
 
L-188
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RE: Rear Engine Vs. Twin Engine

Fri Jun 20, 2003 11:49 pm

Ikarus, we also forgot the TU-154!
OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
 
FredT
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RE: Rear Engine Vs. Twin Engine

Mon Jun 23, 2003 5:07 pm

One of the major drawbacks of rear mounted engines is the structural beefing up (read: added weight) required of the wing attachments and rear fuselage to carry the weight of the engines. Remember, the aircraft is resting on its wings when flying. The load path has to go from the wings to the engine mounts, and the engines are among the heaviest components. Ikarus hinted at this, but it deserves a bit more attention IMO.

You can also get interesting effects when reversing a rear mounted engine, blanking out the vertical stabiliser reducing your directional control. You also get that rearward wing placement, leaving you with the airflow around the loooong nose to cope with when the direction of travel through the air is not aligned with the fuselage.

I once wrote a small (five days work or so) paper, investigating and summing up the pro's and cons of different engine mounting (and starting) methods. Left me wondering how on earth they ever decided that rear engines were a good idea on an airliner!

BTW... in my very unresearched experience, rear mounted engines means a nosier cabin overall, and very much noisier in the rear. It seems there is at least one person who disagrees. Based on fact or opinion? Interested in hearing your take on it more in detail.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Ikarus
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RE: Rear Engine Vs. Twin Engine

Mon Jun 23, 2003 7:04 pm

FredT: I have never flown in a rear-mounted engined aircraft. I'm quoting our lectures on aircraft design here when stating that the cabin is more quiet - in general (except the rear, which is much noisier).

I think the idea is that much of the noise originates from the engine itself, and from its exhaust. So seats far ahead of the engine suffer mostly the noise of the air passing by the fuselage, with only a little engine noise. Those roughly parallel to the engine will suffer the noise of the engine much more noticeably, and those behind the engine endure the noise generated by the shear layers of air in the engine exhaust. So, in a plane, the most quiet seats would be in the front and the noisiest towards the rear. With rear-mounted engines, more seats are ahead of the engines, so more seats suffer less noise. However, that ignores how much of the noise is actually passed through the fuselage skin as vibrations - perhaps rear-mounted engines are often too rigidly attached, with too much vibration transmission into the rest of the plane?

Regards

Ikarus
 
FredT
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RE: Rear Engine Vs. Twin Engine

Mon Jun 23, 2003 7:21 pm

Ikarus,
own experience paxing and aero design literature/classes here. Engine noise coming through the structure being the likely culprit. Don't ask for references, don't have them. It would appear the subject is open to debate. Let's hope noone has investigated it properly, nothing spoils a good debate as much as someone knowing the facts! Think we could get a few mil euros from somewhere for a joint venture investigation? In other words, to go have a great time flying around and listening? Er... I mean, perform long and tedious research...  Big grin

Spent lots of time in old F28s as a youngster, living where flying is the way to travel. I recall the noise as being close to blinding, on top of deafening. Big grin

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
POSITIVE RATE
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RE: Rear Engine Vs. Twin Engine

Mon Jun 23, 2003 11:41 pm

FredT i would disagree with you about rear mounted engines meaning a noiser cabin overall. Perhaps the F-28- being an "old" generation jet was like that with it's very low bypass engines. But in the case of planes like the 717, DC-9, MD80 and the 727 it's proven fact that the cabin is overall quiter, except if you are sitting very close to the engines.
 
FredT
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RE: Rear Engine Vs. Twin Engine

Tue Jun 24, 2003 12:02 am

Positive rate,
proven fact? Proven where? Not disagreeing nor saying you are wrong, I am merely curious. As I was saying, I'm mainly going on my own opinion from paxing experience. Yes, read and heard the same as my take on it, but can't even recall where so it's not really valid information.  Smile

And yes, the F28 engines probably make more noise inside the cabin than anything regardless of where you mount the engines. Including on another aircraft! Big grin

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Guest

RE: Rear Engine Vs. Twin Engine

Tue Jun 24, 2003 12:23 am

FredT and Positive rate...
xxx
Interesting little detail I remember when I was a teen, when the first 707s and DC8s were starting to operate, I was living with my parents in Paris, and watching French TV - it was start of operations of Caravelles SE-210 with Air France... among other things, they were proud how "silent" the cabin was, to compare to the wing mounted JT-4 engines of the 707...
xxx
I guess, sponsored by Sud-Est Aviation, Air France and the music industry, they made a little publicity on TV, by making a recording by, the then pop music singer Sacha Distel, of the song "Oh, lonesome me" (french lyrics are "Oh, quelle nuit") - recording the master tape aboard the Caravelle while in flight... Just thought the trivia would interest some of you...
xxx
Happy contrails  Smile
(s) Skipper

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