Engine configurations
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Engine configurations
I've been meaning to ask this question for years. Apologies if it's initially posted on the wrong board: no doubt it will rapidly be relocated somewhere more appropriate...
What is it that determines engine positioning for twin-engined transport aircraft design in relation to size? It seems that in current practise the preferred location for engines - in the case of smaller aircraft, executive jets & RJs for example - is invariably at the rear of the fuselage whereas most larger aircraft nowadays place the engines on wing mounted pylons - which looks structurally more critical to my naive eyes, as well as noisier!
I have no reason for asking this except that it strikes me periodically when I have nothing better to think about!
And by the way PPRUNE, there seems to a bug in the entry/preview screen in relation to hard returns inserted into the text-entry window - it's stripping them unpredictably.
What is it that determines engine positioning for twin-engined transport aircraft design in relation to size? It seems that in current practise the preferred location for engines - in the case of smaller aircraft, executive jets & RJs for example - is invariably at the rear of the fuselage whereas most larger aircraft nowadays place the engines on wing mounted pylons - which looks structurally more critical to my naive eyes, as well as noisier!
I have no reason for asking this except that it strikes me periodically when I have nothing better to think about!
And by the way PPRUNE, there seems to a bug in the entry/preview screen in relation to hard returns inserted into the text-entry window - it's stripping them unpredictably.
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One of the main criteria that determines where an engine will go is the size of the engine! You would have a hard time squeezing an engine of suitable size under the wing of a Citation or Learjet. Consequently, bolting them on the back end is the next best option. Problem with bolting engines on the back though, is that you then have to beef up the rear end sufficiently to hold said engine (and its resultant thrust) in place. The underwing area is an easier area to build in the required strength to support a big fan engine.
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Here's an earlier thread 
AFAIK:
Reduced wing root bending moment therefore reduced wing mass reqd.
Provides anti wing flutter mass balance.
Easier maintenance and engine change.
Safer in event of fire or catastrophic failure.
Even read somewhere that interference drag was demonstrably reduced
cf fuselage mounting I guess but defo leave that to the wind tunnel chaps.
Don't forget range of cost considerations in construction and operation.
OTOH have you looked at the little waterspout under B737 fans on a wet taxiway?
Reduced wing root bending moment therefore reduced wing mass reqd.
Provides anti wing flutter mass balance.
Easier maintenance and engine change.
Safer in event of fire or catastrophic failure.
Even read somewhere that interference drag was demonstrably reduced
cf fuselage mounting I guess but defo leave that to the wind tunnel chaps.Don't forget range of cost considerations in construction and operation.
OTOH have you looked at the little waterspout under B737 fans on a wet taxiway?
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Maybe there's something here that I don't understand (& I just read the other thread). I can follow the pro/con arguments about the differences in the locations but what does the size factor contribute? I assumed that the relationship betwen engine size and airframe (& wing?) size is reasonably linear. From the above I gather that it isn't? At what size does the switch-over occur?
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It's not a hard-and-fast switch; the McDonnell 119/220 was a prototype for a 4-engine bizjet with underwing podded J34's, a small straight turbojet. I'm sure they were vulnerable to FOD & erosive wear.
Sometimes there's an aesthetic reason; a bizjet w/aft engines can pull up to the GA terminal, shut down #1 and discharge pax with #2 still running, then taxi away to park without restarting #1. (Saves another operating cycle on an engine too, a quantifiable $$$ amount)
And an MD80 flew some years ago with GE36 Unducted Fans on the tail - way too big for underwing mounting.
Sometimes there's an aesthetic reason; a bizjet w/aft engines can pull up to the GA terminal, shut down #1 and discharge pax with #2 still running, then taxi away to park without restarting #1. (Saves another operating cycle on an engine too, a quantifiable $$$ amount)
And an MD80 flew some years ago with GE36 Unducted Fans on the tail - way too big for underwing mounting.
Many early twin jets tended to go for tail mounting as you got the thrust lines nearer to the centre line which improved engine-out handling in those days of somewhat poorer reserves of power on the engines available. The 737 in 1968 was the first to break away from this.
Engines under the wing mean you need more room under there. Notable how high a 737 is compared to say an F100.
Engines under the wing mean you need more room under there. Notable how high a 737 is compared to say an F100.
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Engines above the wings?
Simplifying the "structural" consideration: All of the lift comes from the wings. A lot of the overall weight is the fuel, some more of the weight is the engines. Put fuel and engines where the lift is means that you don't have to work out great bits of metal to transfer the lift to some of the heavy bits!
But why have the engines slung under the wings, when putting them on top would presumably reduce FOD problems as well as increase ground clearance? (B wouldn't have had to have that odd oval engine casing on the 737). Only reason that springs to my mind is that TOGA power would result in a pitch-down moment, as opposed to pitch-up when the engines are under the C of G - probably not desirable!
But why have the engines slung under the wings, when putting them on top would presumably reduce FOD problems as well as increase ground clearance? (B wouldn't have had to have that odd oval engine casing on the 737). Only reason that springs to my mind is that TOGA power would result in a pitch-down moment, as opposed to pitch-up when the engines are under the C of G - probably not desirable!
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There are plenty of other angles from which the Honda-Jet looks pretty weird, though.
One big drawback of the overwing engine location is that the pylon is on the most sensitive part of the wing - if it's near the front it's right amongst the suction peak, and getting a pylon shape/location that works well for all conditions is tricky. If it's aftwards then the peak is out of the way, but now there's an adverse pressure gradient, so you're running the risk of causing some kind of aft seperation.
Underwing, the flow gradients are far more forgiving, but even there the engine can mess up the flow if it's far forward. Aerodynamicists hate hanging anything on the wing. We'd even get rid of the fuselage for that matter.
One big drawback of the overwing engine location is that the pylon is on the most sensitive part of the wing - if it's near the front it's right amongst the suction peak, and getting a pylon shape/location that works well for all conditions is tricky. If it's aftwards then the peak is out of the way, but now there's an adverse pressure gradient, so you're running the risk of causing some kind of aft seperation.
Underwing, the flow gradients are far more forgiving, but even there the engine can mess up the flow if it's far forward. Aerodynamicists hate hanging anything on the wing. We'd even get rid of the fuselage for that matter.
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thanks for posting the picture of the honda jet...wondering about that for awhile!.
instead of talking about the engineering thing about where the engines go, consider:
if the 737 put the engines on the tail, like a DC9 or BAC1-11, the vertical fin/rudder would have been much smaller...and the rudder problems might not have brought down those planes ( PIT and Colorado springs). Boeing wanted something that looked like a boeing and not a douglas.
With engines out on the wings you do get some advantages...but there is a bigger distance from the centerline of the craft and then you need a bigger rudder to handle the engine out scenario.
If an engine breaksup on the wing, hot bits might hit the wing and the fuel (british air tours b737-200). If it breaks up on the tail, it might cut control cables to rudder and elevator ( dc9 MKE?)
If a tire (tyre) blows on a DC9 the rubber might go into the engine. (by the way, the dc9/md80 has tail mounted engines)
Put the engines on the tail and you have to balance it with something in front.
While I understand why planes like the 777 and 747 have engines on the wings, planes down around the 737 and MD80 could go either way...and given the choice I like the engines in the tail.
Quieter for one! Can't suck a ground worker into the engines on a DC9/MD80 (not without a ladder!) unlike the 737.
But IF I designed a plane today, I think I might like the engines like the BAC lightning. One on top of the other ( with fantastic shielding between)...mounted on the tail, if you lost an engine you wouldn't need RUDDER. Maybe a little elevator trim. I think there is a euro fighter being designed that way now.
regards
jon
instead of talking about the engineering thing about where the engines go, consider:
if the 737 put the engines on the tail, like a DC9 or BAC1-11, the vertical fin/rudder would have been much smaller...and the rudder problems might not have brought down those planes ( PIT and Colorado springs). Boeing wanted something that looked like a boeing and not a douglas.
With engines out on the wings you do get some advantages...but there is a bigger distance from the centerline of the craft and then you need a bigger rudder to handle the engine out scenario.
If an engine breaksup on the wing, hot bits might hit the wing and the fuel (british air tours b737-200). If it breaks up on the tail, it might cut control cables to rudder and elevator ( dc9 MKE?)
If a tire (tyre) blows on a DC9 the rubber might go into the engine. (by the way, the dc9/md80 has tail mounted engines)
Put the engines on the tail and you have to balance it with something in front.
While I understand why planes like the 777 and 747 have engines on the wings, planes down around the 737 and MD80 could go either way...and given the choice I like the engines in the tail.
Quieter for one! Can't suck a ground worker into the engines on a DC9/MD80 (not without a ladder!) unlike the 737.
But IF I designed a plane today, I think I might like the engines like the BAC lightning. One on top of the other ( with fantastic shielding between)...mounted on the tail, if you lost an engine you wouldn't need RUDDER. Maybe a little elevator trim. I think there is a euro fighter being designed that way now.
regards
jon
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I would say that it has a lot to do with the design of the aircraft, CG locations, wing stability etc.
But as I learned in my ATPL studies the most important factor for placing the engines on the back of the fuselage instead of the wings would be that you got rid of the pitch up moment during acceleration in the air... however if this is true or not I don't know...
I've never flown an aircraft with fuselage mounted engines so I've never felt the difference...
Kind Regards
Tim
But as I learned in my ATPL studies the most important factor for placing the engines on the back of the fuselage instead of the wings would be that you got rid of the pitch up moment during acceleration in the air... however if this is true or not I don't know...
I've never flown an aircraft with fuselage mounted engines so I've never felt the difference...
Kind Regards
Tim
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Founder
You remind us of a good point about the change in pitch with adding power with underwing engines vs. the lack of change with tail mounted engines.
I have flown both the 737 and the DC9. There is a pitch change on the 737 when you add power or reduce power. This is one of the reasons I can't stand the 737. It is also one of the reasons I love the DC9. It is a much purer form of flying with the tail mounted engines. To borrow a phrase / title, you more closely "FLY THE WING" on a plane like the DC9.
You remind us of a good point about the change in pitch with adding power with underwing engines vs. the lack of change with tail mounted engines.
I have flown both the 737 and the DC9. There is a pitch change on the 737 when you add power or reduce power. This is one of the reasons I can't stand the 737. It is also one of the reasons I love the DC9. It is a much purer form of flying with the tail mounted engines. To borrow a phrase / title, you more closely "FLY THE WING" on a plane like the DC9.
No one has mentioned the deep stall issue yet (unless I have missed it).
Also didn't the Germans build the first wing-mounted podded-jet aircraft? The Me262.
They also put an overwing config aircraft into production. (MBB?)
Also didn't the Germans build the first wing-mounted podded-jet aircraft? The Me262.
They also put an overwing config aircraft into production. (MBB?)
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Originally Posted by TURIN
No one has mentioned the deep stall issue yet (unless I have missed it).
Also didn't the Germans build the first wing-mounted podded-jet aircraft? The Me262.
They also put an overwing config aircraft into production. (MBB?)
Also didn't the Germans build the first wing-mounted podded-jet aircraft? The Me262.
They also put an overwing config aircraft into production. (MBB?)
I wonder why the +-tail is not more popular? Conventional tailplane cannot work with tail engines, but Caravelle, the original tailmounted jet, seems to fly fine with +-tail. BAC 1-11 had the first T-tail and lost the prototype and all aboard to deep stall. B-727 and most other regional and private jets have had T-tails... but Dassault Falcons, both twins and trijets, do fine with +-tails.
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Originally Posted by TURIN
No one has mentioned the deep stall issue yet (unless I have missed it).
Also didn't the Germans build the first wing-mounted podded-jet aircraft? The Me262.
They also put an overwing config aircraft into production. (MBB?)
Also didn't the Germans build the first wing-mounted podded-jet aircraft? The Me262.
They also put an overwing config aircraft into production. (MBB?)
Deep stall will happen weather you have the engines mounted on the wings or the fuselage...
Kind Regards
Tim
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yes, T tail is more of the problem causer for deep stall than engine configuration, but
most tail mounted engines require T tail. DC9 had a special fix for the deep stall which seemed to work fine.
There are many planes with less than T tail that have rear mounted engines...
sabreliner and early citations, and the caravelle didn't have a t tail... perhaps we would call it a cruciform tail.
if you are right on the edge of a full stall in a wing mounted engine plane, adding full power might increase AOA for a moment and stall you.
add power in wing mounted engine, push forward on yoke, reduce power, pull back ( just to keep current pitch)
Heck, even the Piper LANCE one engine 6 seater had a T tail ( arrow IV too). But that is horse of a different color.
most tail mounted engines require T tail. DC9 had a special fix for the deep stall which seemed to work fine.
There are many planes with less than T tail that have rear mounted engines...
sabreliner and early citations, and the caravelle didn't have a t tail... perhaps we would call it a cruciform tail.
if you are right on the edge of a full stall in a wing mounted engine plane, adding full power might increase AOA for a moment and stall you.
add power in wing mounted engine, push forward on yoke, reduce power, pull back ( just to keep current pitch)
Heck, even the Piper LANCE one engine 6 seater had a T tail ( arrow IV too). But that is horse of a different color.
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Originally Posted by jondc9
yes, T tail is more of the problem causer for deep stall than engine configuration, but
most tail mounted engines require T tail. DC9 had a special fix for the deep stall which seemed to work fine.
There are many planes with less than T tail that have rear mounted engines...
sabreliner and early citations, and the caravelle didn't have a t tail... perhaps we would call it a cruciform tail.
if you are right on the edge of a full stall in a wing mounted engine plane, adding full power might increase AOA for a moment and stall you.
add power in wing mounted engine, push forward on yoke, reduce power, pull back ( just to keep current pitch)
Heck, even the Piper LANCE one engine 6 seater had a T tail ( arrow IV too). But that is horse of a different color.
most tail mounted engines require T tail. DC9 had a special fix for the deep stall which seemed to work fine.
There are many planes with less than T tail that have rear mounted engines...
sabreliner and early citations, and the caravelle didn't have a t tail... perhaps we would call it a cruciform tail.
if you are right on the edge of a full stall in a wing mounted engine plane, adding full power might increase AOA for a moment and stall you.
add power in wing mounted engine, push forward on yoke, reduce power, pull back ( just to keep current pitch)
Heck, even the Piper LANCE one engine 6 seater had a T tail ( arrow IV too). But that is horse of a different color.
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The DC9 does not have a STICK PUSHER.
A really fine piece on T tail and deep stall is located at:
http://oea.larc.nasa.gov/PAIS/Concep...eep_stall.html
and might prove an interesting read.
I also recall in the early days of understanding T tail and deep stall etc, that if presented with a high nose attitude, one could consider rolling by use of ailerons to recover.
The DC9 has aerodynamically powered elevator...at extreme aoa this was insufficient to move elevator for recovery, so sort of a hydraulic ram was installed and by moving yoke full forward would activate to move elevators...
those of you who taxi behind a DC9 or MD80 may notice the elevators are NOT always symetrical (especially with tail wind), just prior to takeoff you are supposed to push yoke forward to "ram" the elevator down...once rolling for takeoff the air flow is sufficent to control the elevators. Even if you didn't "RAM " the elevators down, it would all sort itself out by about 40-50knots or so.
A really fine piece on T tail and deep stall is located at:
http://oea.larc.nasa.gov/PAIS/Concep...eep_stall.html
and might prove an interesting read.
I also recall in the early days of understanding T tail and deep stall etc, that if presented with a high nose attitude, one could consider rolling by use of ailerons to recover.
The DC9 has aerodynamically powered elevator...at extreme aoa this was insufficient to move elevator for recovery, so sort of a hydraulic ram was installed and by moving yoke full forward would activate to move elevators...
those of you who taxi behind a DC9 or MD80 may notice the elevators are NOT always symetrical (especially with tail wind), just prior to takeoff you are supposed to push yoke forward to "ram" the elevator down...once rolling for takeoff the air flow is sufficent to control the elevators. Even if you didn't "RAM " the elevators down, it would all sort itself out by about 40-50knots or so.