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Underwing Engines - Counteract Lift?  
User currently offlineZSOFN From United Kingdom, joined Jun 2005, 1413 posts, RR: 5
Posted (8 years 10 months 3 weeks 5 days 2 hours ago) and read 2483 times:

Just musing over a piece of toast earlier...

So we know from Bernoulli that air flowing faster on one side of a wing than the other will create a force in the direction of the surface with higher airflow velocity, due to lower pressure.

But surely if an engine in standard configuration (hanging below and in front of the wing) generates airflow underneath the wing, we would see a downward force created, thus working against the wing's intended design purpose.

So what if the engines were placed above and in front of the wing? Wouldn't we see an increase in overall lift as the air gets blown over the top of the wing's surface? (remember the experiment of blowing over the top of a piece of paper and seeing how it rises)

Of course there must be reasons why this is impractical, as we don't see this happening. But what are those reasons?

23 replies: All unread, jump to last
 
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 1, posted (8 years 10 months 3 weeks 5 days 2 hours ago) and read 2476 times:

Good thoughts, but:

- Modern underslung engines are not really "under" the wing. The objective of the pylon is twofold. First, to throw the engine out forward in order for it's weight to counteract wing twist. Secondly, to get the engine out of the airflow around the wing. This last also ensures it doesn't disturb airflow around the wing too much in it's turn. There is probably some effect, but the advantages of underslinging seem to be great.
- The reasons for underslinging being a good thing are manifold, including wing twist counteraction, gravity fuel flow, maintenance ease, cabin protection from wayward fan blades (sorta), construction simplicity, cabin noise prevention...
- Having said that, there are aircraft with over the wing engines for space reasons (the dreaded DFW-614) and for the very lift reasons you talk about (An-70/72/74).


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"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 2, posted (8 years 10 months 3 weeks 5 days 1 hour ago) and read 2469 times:

And then there's that new engine Honda were going on about recently which they claim is more efficient above the wing than below, rather similar in appearance to the "dreaded" VFW-614. I never got the full story on that.

Quoting ZSOFN (Thread starter):
So we know from Bernoulli that air flowing faster on one side of a wing than the other will create a force in the direction of the surface with higher airflow velocity, due to lower pressure.

Ooo... I think there are a lot of techie people here who think Bernouilli's over-rated. I remember some pretty serious debates between the Bernouilli fans and the Newton fans.  Smile


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 3, posted (8 years 10 months 3 weeks 5 days 1 hour ago) and read 2454 times:

Quoting David L (Reply 2):

Quoting ZSOFN (Thread starter):
So we know from Bernoulli that air flowing faster on one side of a wing than the other will create a force in the direction of the surface with higher airflow velocity, due to lower pressure.

Ooo... I think there are a lot of techie people here who think Bernouilli's over-rated. I remember some pretty serious debates between the Bernouilli fans and the Newton fans.

They're both right. They're both wrong. Aaaaah!

The simple answer is that it's not that simple. For a better explanation, read from here http://travel.howstuffworks.com/airplane4.htm to here http://travel.howstuffworks.com/airplane16.htm. Contains lots of pictures for the lazy reader.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 4, posted (8 years 10 months 3 weeks 5 days 1 hour ago) and read 2449 times:

Quoting Starlionblue (Reply 3):
They're both right. They're both wrong. Aaaaah!

That was my take but some wouldn't budge.

Quoting Starlionblue (Reply 3):
Contains lots of pictures for the lazy reader.

Cheers!  Smile


User currently offlineAirfoilsguy From , joined Dec 1969, posts, RR:
Reply 5, posted (8 years 10 months 3 weeks 5 days 1 hour ago) and read 2432 times:

I see one problem with over the wing engine design. If the engine becomes unbalanced and breaks off (modern under wing engines are designed to do that) it could take the wing with it or go through the cabin.

User currently offlineKl671 From Canada, joined Jul 2005, 141 posts, RR: 0
Reply 6, posted (8 years 10 months 3 weeks 4 days 23 hours ago) and read 2392 times:

Quoting Starlionblue (Reply 1):
the dreaded DFW-614

I know it was a commercial disaster, but why was the 614 dreaded?

Just Interested!


User currently offline727EMflyer From United States of America, joined Mar 2005, 547 posts, RR: 0
Reply 7, posted (8 years 10 months 3 weeks 4 days 13 hours ago) and read 2333 times:

Quoting ZSOFN (Thread starter):
But surely if an engine in standard configuration (hanging below and in front of the wing) generates airflow underneath the wing, we would see a downward force created, thus working against the wing's intended design purpose.

No.

At risk of flaming by the theoretical purists, I am here to tell you that there is an amount of upward lift being generated by the airflow beneath the wing. Lift via the classic bernouli theory requires two dimensional airflow, one dimension being above, and the other being below. Both bodies of air begin the lifting process at equal pressures and velocities. The wing's shape changes the pressure and velocity of both bodies of air and they work together to generate the lift. Now, if I only move air below the wing (the engine generated flow) I increase the pressure below the wing, relative to that of the air above the wing... same as is done by bernouli, and same as you can demonstrate by blowing below that same sheet of paper you were blowing above. This does assume, though, that there is at least a slight upward angle of attack to compress the airflow from the engine... although that is going to be necessary for bernouli anyhow, right?


User currently offlineDavid L From United Kingdom, joined May 1999, 9524 posts, RR: 42
Reply 8, posted (8 years 10 months 3 weeks 4 days 13 hours ago) and read 2328 times:

As Starlionblue suggested, wings seem to generate lift because of a phenomena known as "Lots of Different Stuff". Stop me if I'm getting too technical.  Smile

User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 9, posted (8 years 10 months 3 weeks 2 days 20 hours ago) and read 2215 times:

Quoting ZSOFN (Thread starter):
So we know from Bernoulli that air flowing faster on one side of a wing than the other will create a force in the direction of the surface with higher airflow velocity, due to lower pressure.

But surely if an engine in standard configuration (hanging below and in front of the wing) generates airflow underneath the wing, we would see a downward force created, thus working against the wing's intended design purpose.

727EMFlyer is on the right track, but for the wrong reasons.

All Bernoulli says is that the total pressure in an airflow is constant (as velocity increases static pressure reduces). Total pressure is changed only when you add or remove energy. The engine adds energy to the ambient flow, so the total pressure in the jet exhaust is higher than for the ambient air. It's not reducing pressure on the undersurface at all, it might be increasing it.

The C-17 gets a lot of its short field performance from that very high energy underwing airflow being deflected down when the flaps are extended. The An-72 uses the Coanda effect to generate a similar downward thrust deflection.

Quoting David L (Reply 2):
And then there's that new engine Honda were going on about recently which they claim is more efficient above the wing than below, rather similar in appearance to the "dreaded" VFW-614.

In the case of the Honda biz-jet design the engines are mounted above but behind the wings. In fact they are almost exactly where they would be if mounted on the fuselage like a conventional biz-jet, but without the fuselage mounting structure and with some wing bending relief to go with it. I think that's where the efficiency gains come from, not anything to do with overwing engines as such.

One curious marketing idea for the Fokker-VFW 614 was that in some way the wings would blanket the engine noise heard below, making it a good neighbour jet. They even produced adverts with diagrams showing the sound shadow. Big grin

BTW, Bernoulli and Newton are both right (I wouldn't presume to argue with either of them), it just depends where you draw your system boundaries. As with the wave and particle theories of matter you can use the most convenient theory for your purpose and get the same answer.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6484 posts, RR: 54
Reply 10, posted (8 years 10 months 3 weeks 1 day 23 hours ago) and read 2156 times:

Quoting Kl671 (Reply 6):
I know it was a commercial disaster, but why was the 614 dreaded?

The 614 wasn't dreaded at all. It was a fine plane. In some respects it was ahead of its time, but in other respects it quite soon became outdated. And they were probably too expensive compared to older turboprops.

Ahead of its time: It was the forerunner of the modern regional jet.

Outdated: It came out as a noisy fuel guzzler just as noise awareness really materialized and in the middle of the 1973 and 1980 fuel price jumps.

It performed sterling regional service in the hands on Cimber Air here in Denmark. For far too short time. And it was replaced prematurely by 10-15-20 years old Nord 262 and F-27.

New subject. During the test of the 614 they discovered a bad interferance between the pylons and the fuselage. With prototypes already flying they had to go back to the drawing boards and wind tunnels and design new profiles for the pylons in order to reduce a major drag buildup at high speed. That created a major delay of the whole project and harmed the overall economy of the project badly. Also the crash of one prototype followed by redesign of the elevator control system added badly to the problems and delays.

BTW, have you noticed several similarities between the East German Baade 152 jet airliner and the VFW-614? Especially tail design. At least one of the Baade 152 designers - Fritz Freytag if memory serves me well - fled west and joined the 614 design team after the USSR dictated close down of East German aircraft industry in 1961.

The Baade 152 has roots all way back to the Junkers Ju 287 jet bomber which made 16 test flights in 1944 - with landing gear from Convair B-24 Liberator. It evolved over the years in Moscow into further developments like the EF 131, EF 132, EF 140 and finally the OKB-1 150 bomber - and the Baade 152 airliner when the so called Baade team of German aircraft designers were allowed to go home.

It is hardly incidental that the Baade 152 shares the two last digits of the name with the Ju 52.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 11, posted (8 years 10 months 3 weeks 18 hours ago) and read 2094 times:

Quoting Airfoilsguy (Reply 5):
I see one problem with over the wing engine design. If the engine becomes unbalanced and breaks off (modern under wing engines are designed to do that) it could take the wing with it or go through the cabin.

Actually the dropping of engines method is out of favor since the Amsterdam Cargo 747 crash. The thinking is now that it's better to keep the engines with the plane since shearing them off usually creates more problems than it solves.

Quoting Kl671 (Reply 6):
Quoting Starlionblue (Reply 1):
the dreaded DFW-614

I know it was a commercial disaster, but why was the 614 dreaded?

Take a look at the picture again!  Wink I know it's a matter of taste, but I just find the thing monstrously ugly. Big grin



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 12, posted (8 years 10 months 3 weeks 12 hours ago) and read 2070 times:

Quoting Prebennorholm (Reply 10):
Outdated: It came out as a noisy fuel guzzler just as noise awareness really materialized and in the middle of the 1973 and 1980 fuel price jumps.

But as I pointed out, it was supposed to be quiet due to the engine placement, according to the marketing copy produced before the aircraft even flew. So they were aware of noise issues, just didn't address them properly. Compare with the BAe146, an aircraft of similar vintage, yet quiet enough to satisfy Orange County Airport in California.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineKaddyuk From Wallis and Futuna, joined Nov 2001, 4126 posts, RR: 26
Reply 13, posted (8 years 10 months 3 weeks 11 hours ago) and read 2065 times:

For the most part, its the TOP surface of the wing that is almost key for creating lift... thats why the flap tracks are hung underneath the surface and why the engines hang underneath etc etc...

Bernoulli explains the airflow, Newton explains the forces...  Wink



Whoever said "laughter is the best medicine" never had Gonorrhea
User currently offlineKeta From Germany, joined Mar 2005, 448 posts, RR: 0
Reply 14, posted (8 years 10 months 3 weeks 7 hours ago) and read 2036 times:

Quoting Kaddyuk (Reply 13):
For the most part, its the TOP surface of the wing that is almost key for creating lift... thats why the flap tracks are hung underneath the surface and why the engines hang underneath etc etc...

Bernoulli explains the airflow, Newton explains the forces...

Yeah, I usually don't think in Bernoulli anymore. His equations are right, but they're applicable only in certain conditions, and lead to incorrect conclusions of lift.



Where there's a will, there's a way
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6484 posts, RR: 54
Reply 15, posted (8 years 10 months 3 weeks 5 hours ago) and read 2006 times:

Quoting Starlionblue (Reply 11):
Take a look at the picture again! I know it's a matter of taste, but I just find the thing monstrously ugly.

I know what you mean, Stallionblue. First time I saw the VFW-614 30 years ago I thought the same.

But it is with airliners as with women. When we learn about their supperior non-visual qualities, then they become the prettiest thing on earth even if they never would have a chance at a Miss World contest.

During the last 29 years I have regarded the 614 as really cute.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17068 posts, RR: 66
Reply 16, posted (8 years 10 months 3 weeks ago) and read 1971 times:

Quoting Prebennorholm (Reply 15):
But it is with airliners as with women. When we learn about their supperior non-visual qualities, then they become the prettiest thing on earth even if they never would have a chance at a Miss World contest.

Especially when they are feeding your baby  Wink Ok had to get that in. I happen to find oddball planes strangely endearing. I even own a book about them which I highly recommend: http://www.amazon.com/gp/product/158...-4017743?s=books&v=glance&n=283155 . And I agree that the VFW-614 is kinda cute in a "dreaded" fashion Big grin



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3545 posts, RR: 67
Reply 17, posted (8 years 10 months 2 weeks 6 days 19 hours ago) and read 1950 times:

Quoting Keta (Reply 14):
Yeah, I usually don't think in Bernoulli anymore. His equations are right, but they're applicable only in certain conditions, and lead to incorrect conclusions of lift.

You mean all those years of integrating wing pressures were a waste of time?

Actually, Bernoulli does a pretty good job with lift but doesn't account for drag very well except with an extraordinary amount of effort.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineKaddyuk From Wallis and Futuna, joined Nov 2001, 4126 posts, RR: 26
Reply 18, posted (8 years 10 months 2 weeks 6 days 12 hours ago) and read 1935 times:

Quoting OldAeroGuy (Reply 17):
You mean all those years of integrating wing pressures were a waste of time?

Probably  Wink I studied Bernoulli's theorem in school, college and then in engineering ground school. I have never nor do i ever expect to use it in a working environment...

 Wink



Whoever said "laughter is the best medicine" never had Gonorrhea
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3545 posts, RR: 67
Reply 19, posted (8 years 10 months 2 weeks 5 days 22 hours ago) and read 1896 times:

Quoting Kaddyuk (Reply 18):
Probably I studied Bernoulli's theorem in school, college and then in engineering ground school. I have never nor do i ever expect to use it in a working environment...

But if you do airfoil design, much of what you do is explained by Bernoulli. You are using the theory in a working environment.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineKeta From Germany, joined Mar 2005, 448 posts, RR: 0
Reply 20, posted (8 years 10 months 2 weeks 2 days 14 hours ago) and read 1785 times:

Quoting OldAeroGuy (Reply 17):
You mean all those years of integrating wing pressures were a waste of time?

No, no. I guess I expressed myself wrong  Smile I know Bernoulli is useful, in fact I'm using it right now in thermodynamics. What I mean is that when these questions are placed, the intuitive answer is not accurate. You know, if it was for Bernoulli (basic), a supercritical airfoil wouldn't create lift, an increase in the angle of attack wouldn't add to lift... I know that's not true, nor what you get in a deep analysis, but it's the intuitive idea you get.



Where there's a will, there's a way
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 21, posted (8 years 10 months 2 weeks 1 day 20 hours ago) and read 1716 times:

Quoting Keta (Reply 20):
You know, if it was for Bernoulli (basic), a supercritical airfoil wouldn't create lift, an increase in the angle of attack wouldn't add to lift...

Bernoulli's equation does not say a supercritical wing won't work, or that angle of attack is unimportant. It is how people think it works which causes such confusion. If you look at the pressure plot for a supercritical wing you will see a net upward force. Similarly if you compare pressure plots for a wing at different angles of attack you will see different lift forces. Bernoulli in action.

ALL Bernoulli's equation does is tell you how static pressure will vary with airspeed. Local airspeed will vary according to the wing shape, but that is nothing to do with Bernoulli's equation.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineKeta From Germany, joined Mar 2005, 448 posts, RR: 0
Reply 22, posted (8 years 10 months 2 weeks 1 day 12 hours ago) and read 1698 times:

Quoting Jetlagged (Reply 21):
It is how people think it works which causes such confusion

That's what I meant. I'll quote my words: "I know that's not true, nor what you get in a deep analysis, but it's the intuitive idea you get"

Quoting Jetlagged (Reply 21):
ALL Bernoulli's equation does is tell you how static pressure will vary with airspeed. Local airspeed will vary according to the wing shape, but that is nothing to do with Bernoulli's equation.

I have read several times that it's the other way. In a fluid, and acceleration is the result of a pressure gradient, not the other way. The Bernoulli equation says all points in a steady state flow have the same energy, it makes a relationship between velocity and pressure, but it does not say that the one causes the other.



Where there's a will, there's a way
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2565 posts, RR: 25
Reply 23, posted (8 years 10 months 2 weeks 1 day 6 hours ago) and read 1677 times:

Take the flow through a venturi tube. The continuity equation

density * area * velocity = constant

gives the velocity at any point depending on the area ratio. Bernoulli's equation will then give the pressure gradients.

Flow around an aerofoil is not going to be accurately predicted using Bernoulli's equation, but pressure would be (and hence lift), given the local velocity.

I agree it can be looked at the other way around, but I was never taught to use Bernoulli's equation to predict velocity. Certainly, the equation is only another statement of the principle of conservation of energy. The basic equation deals only with incompressible fluids, so is not very applicable to aerodynamics, but there is a compressible flow equation.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
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