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vahancrazy wrote:Hei all,
This is maybe a noob question for many of you but I am not an engineers.... and I failed to find the reply by research function.
The engine are normallly located near the centre or gravity due to structural reason + (if I am not wrong) it's easier for fuel intake system and some how better air resistence compared to rear mounted option.
Consider landing gear, fuel intake, air resistence and any other possible factors, can you, please, explain why engine over the wing or over the fuselage (still in central position) are not an efficient option as below wing?
Only such layout example I recall is the Be-200.
Thanks for your piece of knowledge!
vahancrazy wrote:why engine over the wing or over the fuselage (still in central position) are not an efficient option as below wing?
vahancrazy wrote:Only such layout example I recall is the Be-200.
kitplane01 wrote:The AN-72 is another. They did it for STOL reasons and accepted the aero penalty in cruise.
seb146 wrote:If I am not mistaken, the AN-71 and AN-74 have the same engine configuration.
trpmb6 wrote:As is the case for anything engineering related it all comes back to forces and moments and equilibrium. It's all about tradeoffs of where your forces are being applied and the moments they generate versus how much structure it takes to react said forces and moments. I'm glad Starlionblue posted the MD-80 because it, next to the 737, demonstrates this quite well.
There are many airplane force diagrams to look at out there. But in very simple terms, Wing has a force vector up, the horizontal tail a force vector (up or down depending on trim) and your engine a force vector forward. You balance all these about the center of gravity and tweak the angles and what not til you get an optimal design.
This is why I mentioned the MD-80. On the MD-80 the engine nacelles are actually not straight forward, they are at a positive angle of attack with respect to the fuselage horizontal. But this is due to downwash from the wing which causes the streamlines to not be horizontal with the fuselage but angled slightly downward, so in cruise the engine nacelle will be parallel with the streamline. However, the nozzle itself is angled to redirect this flow back towards the horizontal to reduce pitching moments. Same is true for the DC-9. So what you get is a kind of banana shaped "engine" (really nacelle) on the aft fuselage mounted engines.
One thing I've seen discussed about over-wing mounted engines is their ability to accelerate the flow over the wing for short take off capabilities. You see this a lot in remote control airplanes because they get a big boost. I'm not sure how efficient it is for a large commercial aircraft though. Must not be that efficient since we don't see many configurations like that.
Starlionblue wrote:BTW ignoring all the other arguments for wing mounted engines, mounting well behind the center of gravity, e.g. MD-80, is just fine, as long as you also move the center of lift back. Which is why the MD-80 wings are mounted relatively much further back compared to, say, the 737 wings.
kitplane01 wrote:Starlionblue wrote:BTW ignoring all the other arguments for wing mounted engines, mounting well behind the center of gravity, e.g. MD-80, is just fine, as long as you also move the center of lift back. Which is why the MD-80 wings are mounted relatively much further back compared to, say, the 737 wings.
That's mostly true, but not completely. Engines to the rear moves the center of gravity rearward, which reduces tail effectiveness, which requires a larger tail, which adds weight and draw, which is poopy.
fdxtulmech wrote:Could be also, the air gets "squeezed" between the engine and fuselage if they are on top of the wing and can cause some drag and design consideration. I remember when the EMB-145 was being developed, one of the original designs had the engines on top of the wing. Basically, converting a EMB-120 into a jet. ExpressJet (the launch customer for the 145) didn't like the idea mainly because of visibility of the passengers. The nacelles would be larger then the 120 and those already impeded visibility. They thought costumers would feel better in a plane that felt more like its bigger counterparts not just an engine swapped 120.
trpmb6 wrote:As is the case for anything engineering related it all comes back to forces and moments and equilibrium.
DocLightning wrote:trpmb6 wrote:As is the case for anything engineering related it all comes back to forces and moments and equilibrium.
Not quite. There are some practical considerations. Under-wing engines are by far the most favorable from an aerodynamic, structural, systems, and maintenance point of view. The nacelles can act as anti-shock bodies to soften up the cross-section change at the wing root. Their weight unloads the bending moment on the wing root. Placing them directly under the fuel tanks eliminates the need for long fuel lines running through the fuselage. Their position near the ground makes it easier to access them for maintenance.
However, there are two major disadvantages: 1) The aircraft must be lifted high enough off the ground to allow for engine clearance. This means longer, heavier, more complex landing gear and a more difficult-to-access fuselage for loading/unloading and maintenance of fuselage systems. 2) Placing the engines that close to the ground invites FOD ingestion (including ingestion of rampers).
Most of the initial RJ designs currently in service, along with the MD-80 and 727 were designed at a time when there were still some airports that didn't have belt loaders and some even had semi-prepared airfields. Today, most commercial airports in the world have paved runways (with the exception of some in northern Canada) and belt loaders. So the advantages to rear-mounted engines or engines mounted above the plane of the wings, in general, have mostly been obviated. That's why the new RJs have under-wing engines.
trpmb6 wrote:
Not to be pedantic, but everything you mentioned there can be broken down to forces and moments
Starlionblue wrote:Or you can ignore all that and end up with the dreaded VFW-614...
rjsampson wrote:Starlionblue wrote:Or you can ignore all that and end up with the dreaded VFW-614...
What was specificallly "dreaded" about the aircraft?
Starlionblue wrote:BTW ignoring all the other arguments for wing mounted engines, mounting well behind the center of gravity, e.g. MD-80, is just fine, as long as you also move the center of lift back. Which is why the MD-80 wings are mounted relatively much further back compared to, say, the 737 wings.
dlednicer wrote:> The Hondajet is an example of over the wing. No other business jet does this.
The Scaled Composites Triumph also had an over-the-wing engine installation. It was also the first airplane to fly with FJ44 engines. I did the aero design of the nacelle installation and will say that it wasn't easy.
kitplane01 wrote:dlednicer wrote:> The Hondajet is an example of over the wing. No other business jet does this.
The Scaled Composites Triumph also had an over-the-wing engine installation. It was also the first airplane to fly with FJ44 engines. I did the aero design of the nacelle installation and will say that it wasn't easy.
I'd be interested to here more.
Starlionblue wrote:Or you can ignore all that and end up with the dreaded VFW-614...
MHG wrote:Starlionblue wrote:Or you can ignore all that and end up with the dreaded VFW-614...
Makes me wonder what´s dreaded about it.
The fact that it was not successful had very little to do with the engine position.
The VFW-614 was specifically designed to be operated from unpaved runways in underdevelopped environments.
So, FOD avoidance was one of the top priorities (in which it definitely excelled)
Its design was ahead of its time and just too early before regional jets were in demand. Development took to long and was overtaken by Fokker´s F28 (not the least because Fokker took over VFW during the period - and obviously had no interest in a competitive design in house ...)
OldAeroGuy wrote:Can the Triumph's success be judged by the fact that the picture shows it on pylons rather than in operation?
Wikipedia wrote:....
The flight test program was completed and confirmed the targeted performance. The financial situation of Beech at the time, and competing projects, prevented consideration of commercial production. In February 1991, Rutan stated, "it had the potential for enormous improvements in efficiency compared to the King Air. It was as fast as the Citation II, but had 60% better fuel economy." [3] After the test program was completed, the airframe was mounted on a pedestal at Scaled's Mojave facility for several years. The Triumph is currently on display in the Joe Davies Heritage Airpark [4] at Palmdale Plant 42.
....
Starlionblue wrote:MHG wrote:Starlionblue wrote:Or you can ignore all that and end up with the dreaded VFW-614...
Makes me wonder what´s dreaded about it.
The fact that it was not successful had very little to do with the engine position.
The VFW-614 was specifically designed to be operated from unpaved runways in underdevelopped environments.
So, FOD avoidance was one of the top priorities (in which it definitely excelled)
Its design was ahead of its time and just too early before regional jets were in demand.
Development took to long and was overtaken by Fokker´s F28 (not the least because Fokker took over VFW during the period - and obviously had no interest in a competitive design in house ...)
As I said above...
I call it the "dreaded" VFW-614 because I find it hideous. It's a joke and I've been making the same joke for years on this site.
Starlionblue wrote:I call it the "dreaded" VFW-614 because I find it hideous. It's a joke and I've been making the same joke for years on this site.
Starlionblue wrote:Agreed with the above posters.
If aesthetics was the defining factor, the Super VC-10 would still be king.
WIederling wrote:Starlionblue wrote:Agreed with the above posters.
If aesthetics was the defining factor, the Super VC-10 would still be king.
How aesthetically satisfying was servicing the inner engines?
( probably a major win versus the Comet, right?
WIederling wrote:OldAeroGuy wrote:Can the Triumph's success be judged by the fact that the picture shows it on pylons rather than in operation?
I am always surprised when engineers delve into the misattribution box.Wikipedia wrote:....
The flight test program was completed and confirmed the targeted performance. The financial situation of Beech at the time, and competing projects, prevented consideration of commercial production. In February 1991, Rutan stated, "it had the potential for enormous improvements in efficiency compared to the King Air. It was as fast as the Citation II, but had 60% better fuel economy." [3] After the test program was completed, the airframe was mounted on a pedestal at Scaled's Mojave facility for several years. The Triumph is currently on display in the Joe Davies Heritage Airpark [4] at Palmdale Plant 42.
....