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!

If I am not mistaken, the AN-71 and AN-74 have the same engine configuration.

I am not an engineer, but I would think it has to do with transferring fuel to the engines. Because the tanks on a 737 are in the center, it is "level" to get the fuel to the engines laterally (side to side) and for rear mounted engines, the fuel would go back instead of up. Probably also makes the pumps work less?

This is a good question. Thank you!

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!

Over the wing typically interferes with the aerodynamics of the most crucial part of the plane. Under the wing aerodynamics is less critical.

Attached to the wing is typically better because the more weight further out on the wing the less bending stress on the wing.

Basically, everyone puts the engine below the wing because (1) less aero problems than above and (2) less bending stress than off the wing. Engines are somewhere else typically because the plane is so small the engines will not fit between the wing and the ground on landing.

The Hondajet is an example of over the wing. No other business jet does this.

The AN-72 is another. They did it for STOL reasos and accepted the aero penalty in cruise.

The Be-200 put the engines where they are to keep them out of water spray (Amphibious aircraft)

vahancrazy wrote:

why engine over the wing or over the fuselage (still in central position) are not an efficient option as below wing?

You will find a couple of explanation, and a couple of reasons. The most important reason however is probably the air flow (air distribution) over the wing. The air flow over the wing is faster than below the wing. So basically any structure over the wing will be affecting pressurized air flow, while below the wing due to slower air flow speed (and pressure) it's less influencing.
When talking about how it affects the wing, if you moved engine from below the wing to over the wing, with pylons etc, you'll loose wing efficiency. It will be definitely have more drag, or less lift. You'll end up with longer, heavier wing than reference one (with engine below).

Cheers,
Adam

kitplane01 explains it well. I'll add that you get both bending and twisting relief on the wing with an underslung engine in front of the wing. Massive structural advantages in addition to gravity feed, easier servicing etc...

Or you can ignore all that and end up with the dreaded VFW-614...

vahancrazy wrote:

Only such layout example I recall is the Be-200.

VFW 614, Honda Jet ( both over the wing )
and obviously a range of sea planes, like the WWII BV138, Do26, ..

Some more:
https://en.wikipedia.org/wiki/Category: ... g_aircraft

Engines over the wings can be useful on unprepared runways to prevent foreing objects ingestion.

And because it looks cool.

Wow! So cool and clear info. Thanks a lot folks! That's how this forum rocks!

kitplane01 wrote:

The AN-72 is another. They did it for STOL reasons and accepted the aero penalty in cruise.

The An-72/74 use a blown wing ( Coanda Effect )
Boeing's YC-14 was a similar design. ( and ~~ the same creation time frame )

seb146 wrote:

If I am not mistaken, the AN-71 and AN-74 have the same engine configuration.

AN-74 is the "Arctic transport" version of the AN-72
( started life as AN-72A )

AN-71 is the AWACS version of the AN-72

Essentially versions of the same type all around.

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.

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.

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.

As kitplane01 mentions, blowing air over the wing is great for STOL take-off, but it is inefficient in the cruise. So unless you're doing STOL take-offs...

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.

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.

Yep. It's all connected. The tailplane on MDs is way back on the end of the fin, increasing moment arm. Of course, all that moving stuff on top of the fin make things heavier by itself.

On the plus side, fuselage mounted engines give much less of a yaw moment with an engine out, so the fin and rudder can be much smaller, as you can clearly see in the pictures.

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.

The original attraction for underslung engines on pylons ahead of the wing
was their effect as anti shock bodies
https://en.wikipedia.org/wiki/Anti-shock_body
( Research data from Germany that found its way to Boeing at the end of WWII.)

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.

Even worse, you can end up with little shocks forming if the geometry isn't quite right. This leads to noise issues, buffeting and, as you noted, additional drag.

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.

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.

Not to be pedantic, but everything you mentioned there can be broken down to forces and moments

Another down side to having the engine mounted below the wing is that the moment generated by forward thrust increases the wing moment about the inboard-outboard axis that is generated by lift about the quarter-chord. That increases wing torsion.

Again, all about forces and moments.

trpmb6 wrote:

Not to be pedantic, but everything you mentioned there can be broken down to forces and moments

Fundamentally, all macroscopic phenomena can. But I think of baggage loading and reaching the engine for maintenance as being a different kind of consideration.

Hi:
(poor mechanics, nobody think on them)
Low able to be maintained, having to climb into wing even to service oil, them was considered a big minus.

rgds

> 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.

Starlionblue wrote:

Or you can ignore all that and end up with the dreaded VFW-614...

What was specificallly "dreaded" about the aircraft?

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?

Just the aesthetics. I mean, have you seen it?

Side note: I found some really interesting pictures of a later model VFW-614 used as a testbed. https://www.dlr.de/dlr/en/desktopdefault.aspx/tabid-10203/1127_read-266#/gallery/112

Note the Airbus FCU, and on the left the screens and sidestick. In another pic it has Airbus sticks on both sides.

DLR ATTAS:
see https://www.google.com/search?q=dlr+attas

now decomissioned and followed up with the
A320 based ATRA platform:
https://www.dlr.de/dlr/en/desktopdefaul ... lery/19368

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.

But rear mounted engines add weight as the thrust and anti-icing must be brought forward to the location of drag and icing. Advantage for FOD avoidance.

Below the wing is easier to service too. However, Douglas put features in the 717 nacelle (at a weight penalty) to minimize the advantage of working with feet on the ground next to your tool box.

Tools are dropped. I saw three dropped today. All onto concrete and not dentable material.

It is all weight and cost optimization. Overwing engines do put the cargo hold closer to the ground. Which doesn't matter if gear height is set for a tailstrike. Not an issue for the short and light Hondajet nor MD-80 due to aft gear being.. very aft!

Lightsaber

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.

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.

Can the Triumph's success be judged by the fact that the picture shows it on pylons rather than in operation?

Another interesting potential future use of overwing engines is to place multiple engines in a row so they ingest the boundary layer. The faster inlet stream means higher propulsive efficiency plus an ingested boundary layer is one that needn't be decelerated from supersonic, thus less overall shock wave and compressibility drag. This concept works best with a BWB design:

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)
VFW-614 design was ahead of its time and just too early before regional jets were in demand.
Btw. "614" stands for (19)61 design number 4 that was taken ...
Development took to long and was eventually overtaken by Fokker´s F28 (not the least because Fokker took over VFW during the period - and obviously had no interest in a competing design in house ...)

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.

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.
....

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.

"Hideous" is in the eye of the beholder ...
But I get what you mean.
Still, some pretty ugly designs in aviation fared much better ...

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.

It is "form follows function" esthetics. You lack in taste ))))

Agreed with the above posters.

If aesthetics was the defining factor, the Super VC-10 would still be king.

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:

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?

You're making my arguments for me.

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.
....

Financial considerations are always part of an airplane's market place success. The fact remains that the Triumph is on a pylon, not in production.