Moderators: richierich, ua900, PanAm_DC10, hOMSaR

 
uta999
Topic Author
Posts: 942
Joined: Mon Jun 14, 2010 11:10 am

Future twin engines over the wing

Thu Feb 14, 2019 9:52 am

With the latest planned aircraft engines seemingly getting ever bigger, is it time to position future aircraft engines above the wing?

The 787 and 737 are real ground huggers, but even future A350s might struggle to fit the latest hairdryers under the wing. Does it involve a complete redesign of both the engine, and the inner wing box? could it lead to easier maintenance and engine changes? Lowering an engine onto the pylon is far easier than lifting one. It would also greatly reduce FOD, and jet bridge / vehicle accidents and be safer for ground staff.
Your computer just got better
 
User avatar
MalcolmInTheMoM
Posts: 29
Joined: Tue Jul 31, 2018 6:32 pm

Re: Future twin engines over the wing

Thu Feb 14, 2019 11:13 am

They could also just raise the wing like on the ATR, for example.
 
User avatar
Starlionblue
Posts: 20349
Joined: Fri Feb 27, 2004 9:54 pm

Re: Future twin engines over the wing

Thu Feb 21, 2019 2:27 pm

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.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
GalaxyFlyer
Posts: 6702
Joined: Fri Jan 01, 2016 4:44 am

Re: Future twin engines over the wing

Thu Feb 21, 2019 3:22 pm

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.
 
User avatar
Starlionblue
Posts: 20349
Joined: Fri Feb 27, 2004 9:54 pm

Re: Future twin engines over the wing

Fri Feb 22, 2019 1:20 am

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.


I dread the VFW-614 because to my eyes it just looks "wrong". :)
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
LH707330
Posts: 2402
Joined: Fri Jun 15, 2012 11:27 pm

Re: Future twin engines over the wing

Fri Feb 22, 2019 6:48 pm

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.

To add to that:

- The compression loads of an overwing pylon would result in a heavier pylon
 
stephanwintner
Posts: 79
Joined: Thu Jan 03, 2019 5:04 pm

Re: Future twin engines over the wing

Fri Feb 22, 2019 7:45 pm

LH707330 wrote:
To add to that:

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


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.

I do agree that handling is probably easier if the engine is under the wing. On-wing servicing is probably also easier.

Stephan
 
ODwyerPW
Posts: 1624
Joined: Thu Dec 09, 2004 6:30 am

Re: Future twin engines over the wing

Fri Feb 22, 2019 8:40 pm

Have a look at the HondaJet HA-420

http://www.hondajet.com/hondajet/innovations/

they talk specifically about this.
learning never stops.
 
ODwyerPW
Posts: 1624
Joined: Thu Dec 09, 2004 6:30 am

Re: Future twin engines over the wing

Fri Feb 22, 2019 9:17 pm

There are also some tech papes that talk about flight characteristics of this engine arrangement

http://www.hondajet.com/gallery-and-dow ... cal-papers
learning never stops.
 
WPvsMW
Posts: 2252
Joined: Thu Mar 23, 2017 7:30 pm

Re: Future twin engines over the wing

Sat Feb 23, 2019 2:41 am

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).
 
WIederling
Posts: 9580
Joined: Sun Sep 13, 2015 2:15 pm

Re: Future twin engines over the wing

Sat Feb 23, 2019 1:31 pm

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... ;)


It did what it was supposed to do.
But the idea behind what it was supposed to do was wrong. Jet replacement for DC-3 :-)
and after the merger Fokker was more interested in pushing their own types ( F-27, F-28 )

Actual performance can't have been bad for the time ( BPR 8.7:1, 0.32 lb/hr/lbf )
https://en.wikipedia.org/wiki/Rolls-Roy ... 45SD-02%29
Murphy is an optimist
 
WIederling
Posts: 9580
Joined: Sun Sep 13, 2015 2:15 pm

Re: Future twin engines over the wing

Sat Feb 23, 2019 6:44 pm

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).


One of those cheap "7 year" time machines would have been sufficient. ( and probably most entertaining.)
The last flight worthy VFW 614 was decommissioned in 2012 due do no further engine spare parts available.
https://www.dlr.de/dlr/en/desktopdefaul ... allery/112
Murphy is an optimist
 
User avatar
kitplane01
Posts: 1620
Joined: Thu Jun 16, 2016 5:58 am

Re: Future twin engines over the wing

Sun Feb 24, 2019 12:52 am

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).
 
GalaxyFlyer
Posts: 6702
Joined: Fri Jan 01, 2016 4:44 am

Re: Future twin engines over the wing

Sun Feb 24, 2019 3:33 am

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
 
User avatar
Starlionblue
Posts: 20349
Joined: Fri Feb 27, 2004 9:54 pm

Re: Future twin engines over the wing

Sun Feb 24, 2019 3:40 am

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


Image
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
User avatar
Faro
Posts: 2013
Joined: Sun Aug 12, 2007 1:08 am

Re: Future twin engines over the wing

Sun Feb 24, 2019 9:17 am

And with CFRP frames now there's whopping wing flex to deal with...at least on 787-type designs...admittedly it's lesser in the inboard sections but it's still there...


Faro
The chalice not my son
 
stratclub
Posts: 1373
Joined: Fri Jan 05, 2018 10:38 pm

Re: Future twin engines over the wing

Sun Feb 24, 2019 10:36 am

How is wing flex a bad thing? How big of a problem is wing flex on the 787?
 
User avatar
Faro
Posts: 2013
Joined: Sun Aug 12, 2007 1:08 am

Re: Future twin engines over the wing

Sun Feb 24, 2019 11:21 am

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
The chalice not my son
 
United857
Posts: 116
Joined: Fri Dec 18, 2015 12:37 am

Re: Future twin engines over the wing

Sun Feb 24, 2019 10:38 pm

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.
A319 A320 A321 A333 A343 A346 A388 B712 B733 B737 B738 B739 B744 B748 B752 B764 B772 B77L B77W B788 B789 CRJ2 E145 E75S E75L E190 MD88 MD90
AA AC B6 CA CX CZ DL EK FM HU JL KA LH LX MU NH NK TK UA US
 
User avatar
Faro
Posts: 2013
Joined: Sun Aug 12, 2007 1:08 am

Re: Future twin engines over the wing

Mon Feb 25, 2019 3:12 pm

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.




Yes...essentially my impression but stated in a more pointed, convincing manner...thanx...


Faro
The chalice not my son
 
stephanwintner
Posts: 79
Joined: Thu Jan 03, 2019 5:04 pm

Re: Future twin engines over the wing

Mon Feb 25, 2019 5:17 pm

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.


Again, I think (as an engine design engineer, not an aerostructures design engineer) that the pylon isn't driven by the dynamics, but rather by overload and crash considerations. Blade off, etc. Perhaps fatigue under vibe loads. So I'm not convinced that the pylon, or adjacent wing, end up being heavier. (Yes, there are fuse pins. That means the pylon is supposed to be stronger than them - including the scatter in min vs. max yield strength.). Achieving a slightly more rigid pylon to counteract the over-centering effect probably doesn't increase the weight much.

Stephan
 
stratclub
Posts: 1373
Joined: Fri Jan 05, 2018 10:38 pm

Re: Future twin engines over the wing

Mon Feb 25, 2019 5:56 pm

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.....................
 
LH707330
Posts: 2402
Joined: Fri Jun 15, 2012 11:27 pm

Re: Future twin engines over the wing

Mon Feb 25, 2019 11:12 pm

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.
 
stratclub
Posts: 1373
Joined: Fri Jan 05, 2018 10:38 pm

Re: Future twin engines over the wing

Mon Feb 25, 2019 11:41 pm

Thank you.......
 
stephanwintner
Posts: 79
Joined: Thu Jan 03, 2019 5:04 pm

Re: Future twin engines over the wing

Thu Feb 28, 2019 7:27 pm

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.
 
kalvado
Posts: 2970
Joined: Wed Mar 01, 2006 4:29 am

Re: Future twin engines over the wing

Thu Feb 28, 2019 10:23 pm

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.

Well, from simple geometry it should be cos(angle to plane)
FOr 3 degree bend, cos(3 deg)=0.9986, that is 0.14% loss. While non zero, I fail to see that as a huge concern. if there is any benefit from the flex, however small it is, probably it is enough to offset the loss.
 
User avatar
kitplane01
Posts: 1620
Joined: Thu Jun 16, 2016 5:58 am

Re: Future twin engines over the wing

Fri Mar 01, 2019 5:02 am

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
Posts: 79
Joined: Thu Jan 03, 2019 5:04 pm

Re: Future twin engines over the wing

Fri Mar 01, 2019 4:49 pm

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


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 ?
 
User avatar
kitplane01
Posts: 1620
Joined: Thu Jun 16, 2016 5:58 am

Re: Future twin engines over the wing

Fri Mar 01, 2019 6:08 pm

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 ?


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

I'm looking at page 622, figure CS4.4. For mach higher than 0.77 (which I should have said) over the wing has less drag than no engines at all.

I gotta say I don't understand this. It seems unlikely that Honda has found some new aerodynamics that Boeing and Airbus have missed. And it seems unlikely that B and A would ignore any advantage they could find. And it seems unlikely that Honda is lying. And it seems unlikely that the Hondajet is the unique combination of factors that give advantage to over the wing engines. So I'm kinda stumped.
 
stephanwintner
Posts: 79
Joined: Thu Jan 03, 2019 5:04 pm

Re: Future twin engines over the wing

Fri Mar 01, 2019 6:51 pm

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.
 
WPvsMW
Posts: 2252
Joined: Thu Mar 23, 2017 7:30 pm

Re: Future twin engines over the wing

Fri Mar 01, 2019 7:06 pm

kitplane01 wrote:
Sorry. Didn't mean to shout. Just wanted to draw contrast. Sometimes communicating is hard via text.


Suggestion: Use italics or bold, in the full editor.

Back on topic: Honda seems to prioritize low overall height (short landing gear, less weight), and identified an overwing engine configuration that outperforms rear engine configurations (but not underwing configurations).
 
User avatar
trpmb6
Posts: 3018
Joined: Thu Apr 19, 2018 5:45 pm

Re: Future twin engines over the wing

Fri Mar 01, 2019 7:41 pm

Structurally there are many reasons for hanging the engines under the wing when you have the ground clearance to allow for it. But ultimately any configuration has multiple positives and negatives that get weighed. Starlion mentioned the more prevalent ones. There are other stability and control considerations as well. The thrust vector and its offset from CG will change your horizontal stabilizer configuration for instance. We iterate on many minute changes early in design that can result in drastic changes to flight characteristics if not fully examined on the macro side of things.
 
User avatar
trpmb6
Posts: 3018
Joined: Thu Apr 19, 2018 5:45 pm

Re: Future twin engines over the wing

Fri Mar 01, 2019 8:17 pm

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
Posts: 79
Joined: Thu Jan 03, 2019 5:04 pm

Re: Future twin engines over the wing

Fri Mar 01, 2019 9:11 pm

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.


Thanks. I understand that all. It seems to me we agree that thrust is a major element, and thrust would be a shear or tensile load whether above or below the wing (depending on which part of the structure we speak of). Buckling can be designed for by increasing the 2nd moment, rather than the weight. Clearly, there is a limit to the desirable wing volume (drag), so rather than making the wing thicker one chooses to make the skins & spar caps thicker on the compressive side. Weight vs. drag trade.

In the case of a pylon, a fatter or longer pylon for an above vs. an under-wing pylon adds some bh^3 buckling stiffness. Aero might be unhappy with a fatter pylon, so maybe you trade bh^3 for gage, but that's a trade study. It's a small fraction of overall drag. I still suspect an overwing mounted engine structure wouldn't necessarily be much heavier than under-wing, as long as it's more or less in the same spot relative to the wing spars. If it gets further from them in whatever direction, it would be longer hence heavier - then again, if it gets closer to the spars, it might even be lighter....
 
User avatar
kitplane01
Posts: 1620
Joined: Thu Jun 16, 2016 5:58 am

Re: Future twin engines over the wing

Sat Mar 02, 2019 5:23 am

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.


Everything you wrote makes sense. I've never heard of anyone claiming that an underwing engine has less drag than no engine (though I've not read everything ever written). To be as good as the Honda installation, it would have to be.

Alternatively, can anyone think of a reason why the aero for a Honda style installation would perform less well when scaled to 737 size?

Lastly, can you imagine how much airliners.net would blow up the day someone creates line drawing with a Boeing stamp showing over-the-wing Pratt Geared Turbo Fans on a 737.
 
ELBOB
Posts: 347
Joined: Sun Jun 21, 2015 6:56 am

Re: Future twin engines over the wing

Sat Mar 02, 2019 11:22 am

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...
 
WIederling
Posts: 9580
Joined: Sun Sep 13, 2015 2:15 pm

Re: Future twin engines over the wing

Sat Mar 02, 2019 12:12 pm

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...


The bendy wing ( using a slimmer profile ) is heavier than the one with the fuller profile.
I beam: increasing web height reduced material required in the flanges for the same load.
increasing web height increases stiffness to the third power. ( reduced by the lighter flanges )

Reason why the A350 with a substantial increase in wing area and span
isn't significantly heavier ( if at all ) than the 7810.
The same design selection causes the massive OEW increase on the 777X.
Murphy is an optimist

Who is online

Users browsing this forum: No registered users and 11 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos