GalaxyFlyer wrote:

Ok, I’ll bite, I’m old enough to remember the dreaded VFW-614, but not why it’s dreaded. I also remember the dreaded Hansa Jet, which had the performance of a brick as I understand it.

Starlionblue wrote:

Putting the engine above the wing creates more problems than it solves.

- Airflow above the wing is more critical for lift than below, so you don't want to mess with it.
- Engines slung ahead and below mitigate wing twisting moment induced by lift.
- Engines below the wing mean a bit less noise in the cabin.
- I don't see how lowering an engine onto the pylon is easier than lifting it. You just winch it up to the attachment points.

Finally, engines above the wing result in stuff like the dreaded VFW-614. And nobody wants a repeat of the dreaded VFW-614...

The reason the ATR and many other turboprops have high wings is because they are small and propellers are large.

LH707330 wrote:

To add to that:

- The compression loads of an overwing pylon would result in a heavier pylon

Starlionblue wrote:

Finally, engines above the wing result in stuff like the dreaded VFW-614.
And nobody wants a repeat of the dreaded VFW-614...

WPvsMW wrote:

Put me in the Wayback Machine, and I'd rather fly in a VFW-614 (row 1, of course) than a Baade 152 (navigator's seat in the glazed nose preferred).

GalaxyFlyer wrote:

Designs are optimized mostly for the mission, not some purely aerodynamic outcome. The mission is defined by what marketing people and buyers want, pure aero efficiency is rarely the goal—cost, delivery date, comfort, long-term value, range, etc are the goals. In design, the C-Series had a very sculpted, low-drag pylon for the GTF engine. So efficient, it would hsve required lots of custom plumbing and valves, cost dictated a blocky pylon that allowed off-the-shelf parts at a small cost in efficiency.

Any design can work depending on the brief.

GF

stratclub wrote:

How is wing flex a bad thing? How big of a problem is wing flex on the 787?

Faro wrote:

stratclub wrote:

How is wing flex a bad thing? How big of a problem is wing flex on the 787?

With an over-wing engine installation, wing flex will I believe increase the (compressive) dynamic stresses on the pylon attachment point to the upper wing surface as compared with the (tensile) dynamic stresses on a classic attachment point to the lower wing surface...an engine dangling below a wing has an added structural stability factor automatically provided by gravity...above the surface that stability factor is I believe lesser in magnitude because gravity is now acting in the opposite direction and not contributing to stability...

Admittedly an intuitive impression, perhaps someone more technically minded or qualified in airframe structures can add to this point?...


Faro

United857 wrote:

Faro wrote:

stratclub wrote:

How is wing flex a bad thing? How big of a problem is wing flex on the 787?

With an over-wing engine installation, wing flex will I believe increase the (compressive) dynamic stresses on the pylon attachment point to the upper wing surface as compared with the (tensile) dynamic stresses on a classic attachment point to the lower wing surface...an engine dangling below a wing has an added structural stability factor automatically provided by gravity...above the surface that stability factor is I believe lesser in magnitude because gravity is now acting in the opposite direction and not contributing to stability...

Admittedly an intuitive impression, perhaps someone more technically minded or qualified in airframe structures can add to this point?...


Faro

Basically an underwing engine is like a pendulum. Any lateral forces acted upon it is naturally countered by gravity and the engine will naturally want to center itself under the pylon.

On an over wing engine, it’s like balancing a pencil on your finger. The moment the pencil moves ever so slightly off center, gravity wants to move it further off center. Therefore, the pylon needs to be much stronger and thus heavier to provide a “centering” force on an over wing engine, a force that gravity took care of in the underwing design.

United857 wrote:

Basically an underwing engine is like a pendulum. Any lateral forces acted upon it is naturally countered by gravity and the engine will naturally want to center itself under the pylon.

On an over wing engine, it’s like balancing a pencil on your finger. The moment the pencil moves ever so slightly off center, gravity wants to move it further off center. Therefore, the pylon needs to be much stronger and thus heavier to provide a “centering” force on an over wing engine, a force that gravity took care of in the underwing design.

stratclub wrote:

How is wing flex a bad thing? How big of a problem is wing flex on the 787?

stratclub wrote:

stratclub wrote:

How is wing flex a bad thing? How big of a problem is wing flex on the 787?

I would still like to know the answer to this.....................

kitplane01 wrote:

Putting the engine above the wing would allow slightly shorter landing gear, and might reduce noise transmitted to the ground a little bit.

But everyone thinks that it also reduces lift/drag ratio a slight bit (1%-2%???). Except Honda. Honda says it improved aerodynamics for them. No one is calling them a liar, but no one quite believes them either.

It would be interesting to see these options put in aerodynamic order. Anyone have a guess???

- Engine over wing (Hondajet)
- High Wing (BAE-146, An-158)
- Engines mounted aft (see DC-9, Gulfstream 650, etc)

It's fair to assume that Boeing/Airbus have explored every option one can think of, and they seem to be moving in a different direction (see SUGAR, blended wing-body, etc).

LH707330 wrote:

stratclub wrote:

stratclub wrote:

How is wing flex a bad thing? How big of a problem is wing flex on the 787?

I would still like to know the answer to this.....................

Your lift vector bends inward, thus making it less useful. Boeing seems to think that the reduction in form drag makes up for it.

stephanwintner wrote:

kitplane01 wrote:

Putting the engine above the wing would allow slightly shorter landing gear, and might reduce noise transmitted to the ground a little bit.

But everyone thinks that it also reduces lift/drag ratio a slight bit (1%-2%???). Except Honda. Honda says it improved aerodynamics for them. No one is calling them a liar, but no one quite believes them either.

It would be interesting to see these options put in aerodynamic order. Anyone have a guess???

- Engine over wing (Hondajet)
- High Wing (BAE-146, An-158)
- Engines mounted aft (see DC-9, Gulfstream 650, etc)

It's fair to assume that Boeing/Airbus have explored every option one can think of, and they seem to be moving in a different direction (see SUGAR, blended wing-body, etc).

Keep in mind, Honda is not saying they got a better L/D with the over-wing location, compared to an under-wing location. They are comparing to a totally clean aircraft with no engines (nice theory, slightly impractical. Limited range, as it were.) and a fuse side mounted engine. An under-wing design is impractical for their size (landing gear heights etc). I wonder how it would do on an aerodynamics-only basis though.

It only works for modest bypass ratios, but it occurs to me to wonder how an under-wing layout does compared to a fully optimized, buried engine design (ala Comet) and an over-wing layout. Forget about the gear, etc.

The problem with an overwing engine is that interference between the nacelle and wing can accelerate the airflow and cause a strong shockwave, reducing the drag rise Mach number – the airspeed at which shockwaves form and drag begins to rise rapidly. Fujino says computational analysis and windtunnel testing enabled Honda to find a “sweet spot”, an overwing engine location that minimises the shockwave. The result, he says, is a 5% better lift-to- drag ratio – aerodynamic efficiency – in the cruise than a rear-engined aircraft.

kitplane01 wrote:

stephanwintner wrote:

kitplane01 wrote:

Putting the engine above the wing would allow slightly shorter landing gear, and might reduce noise transmitted to the ground a little bit.

But everyone thinks that it also reduces lift/drag ratio a slight bit (1%-2%???). Except Honda. Honda says it improved aerodynamics for them. No one is calling them a liar, but no one quite believes them either.

It would be interesting to see these options put in aerodynamic order. Anyone have a guess???

- Engine over wing (Hondajet)
- High Wing (BAE-146, An-158)
- Engines mounted aft (see DC-9, Gulfstream 650, etc)

It's fair to assume that Boeing/Airbus have explored every option one can think of, and they seem to be moving in a different direction (see SUGAR, blended wing-body, etc).

Keep in mind, Honda is not saying they got a better L/D with the over-wing location, compared to an under-wing location. They are comparing to a totally clean aircraft with no engines (nice theory, slightly impractical. Limited range, as it were.) and a fuse side mounted engine. An under-wing design is impractical for their size (landing gear heights etc). I wonder how it would do on an aerodynamics-only basis though.

It only works for modest bypass ratios, but it occurs to me to wonder how an under-wing layout does compared to a fully optimized, buried engine design (ala Comet) and an over-wing layout. Forget about the gear, etc.

The claim less drag than the rear engine case, AND THEY CLAIM LESS DRAG THAN THE NO ENGINE CASE! They also claim high angle of attack before stall than either case.

Rear engine case:

The problem with an overwing engine is that interference between the nacelle and wing can accelerate the airflow and cause a strong shockwave, reducing the drag rise Mach number – the airspeed at which shockwaves form and drag begins to rise rapidly. Fujino says computational analysis and windtunnel testing enabled Honda to find a “sweet spot”, an overwing engine location that minimises the shockwave. The result, he says, is a 5% better lift-to- drag ratio – aerodynamic efficiency – in the cruise than a rear-engined aircraft.

https://aviation.stackexchange.com/ques ... t-position

No engine case: See page 622 of https://www.hondajet.com/Content/pdf/te ... m15May.pdf

stephanwintner wrote:

kitplane01 wrote:

stephanwintner wrote:

Keep in mind, Honda is not saying they got a better L/D with the over-wing location, compared to an under-wing location. They are comparing to a totally clean aircraft with no engines (nice theory, slightly impractical. Limited range, as it were.) and a fuse side mounted engine. An under-wing design is impractical for their size (landing gear heights etc). I wonder how it would do on an aerodynamics-only basis though.

It only works for modest bypass ratios, but it occurs to me to wonder how an under-wing layout does compared to a fully optimized, buried engine design (ala Comet) and an over-wing layout. Forget about the gear, etc.

The claim less drag than the rear engine case, AND THEY CLAIM LESS DRAG THAN THE NO ENGINE CASE! They also claim high angle of attack before stall than either case.

Rear engine case:

The problem with an overwing engine is that interference between the nacelle and wing can accelerate the airflow and cause a strong shockwave, reducing the drag rise Mach number – the airspeed at which shockwaves form and drag begins to rise rapidly. Fujino says computational analysis and windtunnel testing enabled Honda to find a “sweet spot”, an overwing engine location that minimises the shockwave. The result, he says, is a 5% better lift-to- drag ratio – aerodynamic efficiency – in the cruise than a rear-engined aircraft.

https://aviation.stackexchange.com/ques ... t-position

No engine case: See page 622 of https://www.hondajet.com/Content/pdf/te ... m15May.pdf

Yes, I read their ASME paper.

Did you read my post - the one where I said, "They are comparing to a totally clean aircraft with no engines" ? Or did you just start shouting ?

kitplane01 wrote:

Sorry. Didn't mean to shout. Just wanted to draw contrast. Sometimes communicating is hard via text.

stephanwintner wrote:

How do you figure that? Compressive failure stresses in metals are generally to equal or higher than tensile. The pylon needs to handle crash and hard landing overloads, which I expect are limited by yielding in the shear, tensile, or compressive directions. I can imagine a slight difference in modal response due to pretensioning, but I doubt that drives the design.

trpmb6 wrote:

Bah, too late to edit my last post. Meant to add this:

stephanwintner wrote:

How do you figure that? Compressive failure stresses in metals are generally to equal or higher than tensile. The pylon needs to handle crash and hard landing overloads, which I expect are limited by yielding in the shear, tensile, or compressive directions. I can imagine a slight difference in modal response due to pretensioning, but I doubt that drives the design.

Compression loads tend to size structure for crippling and column failures which have allowables that are much much lower than a simple material compressive stress allowable. Its geometry based. Its why the skin on the upper surface of a wing will be two or three times the thickness of the lower skin. (Same with the spar caps). Tension is always preferred where possible.

That being said, we're talking about an engine here. The loads coming from the engine have more to do with thrust than anything else (I can't believe I just wrote that actually). Orders of magnitudes more important.

stephanwintner wrote:

Again - they aren't claiming an overwing design is better than an under wing design.

They claim that, if they adjust positions carefully, for their particular nacelle, wing, and fuselage shapes, they can get some beneficial effect compared to a no engine configuration, and that they can also get a better design than a rear-fuse mount. I'm not an aerodynamicist, but as a mechanical engineer, that does not seem totally unreasonable. The flow into the intake and around the nacelle is interacting with, and apparently guiding the flow over the wing and perhaps along the fuse, and keeping it smoothly attached to the wing.

I imagine B and A have carefully explored position, nacelle shape, pylon shape, etc to optimize the flow around their wing, I would not be surprised if they also get some beneficial interaction between flows. In fact, I'd be surprised if they did not.

The comparison between an under and over-wing configuration isn't answered by Honda's study, as far as I can see, doubly so since they used a modest bypass engine.

LH707330 wrote:

Your lift vector bends inward, thus making it less useful. Boeing seems to think that the reduction in form drag makes up for it.

ELBOB wrote:

Re: wing flex

LH707330 wrote:

Your lift vector bends inward, thus making it less useful. Boeing seems to think that the reduction in form drag makes up for it.

They also seem to reckon that the reduction in structural weight is advantageous, versus a more rigid wing. They can add additional bendy wing to compensate for the loss of lift faster than the mass curve increases.

Airbus tend the other way. Douglas tended reaaaaaally far to rigid wings, the DC-10 and MD-11 feel like they ride on iron plates...