Eric (and others)

[IforB]

If everyone is waiting on the 787, then that's knackered then. That's one project that seems to be turning into a complete disaster. I hope they pull it off though.

[Eric Mc]

The era of the all composite aircraft has been promised for so long now - and we are still waiting.

[navier_stokes]

IforB said:

If everyone is waiting on the 787, then that's knackered then. That's one project that seems to be turning into a complete disaster. I hope they pull it off though.

True, but someone had to start somewhere

[dr_gn]

navier_stokes said:

IforB said:

Would having the engines in that configuration on the BWB's actually induce lift by increasing airflow over the body?

Possibly, the suction (upper) side of an aerofoil benefits (increases lift) from larger velocities (compared to the pressure side), as a simplification.

Also it may aid seperation at higher angles of attack because it will always force the flow towards the trailing edge - and therefore create less drag.

This may be offset by any turbulence generated by the engine intakes.

NASA also have a concept for replacing 3 or 4 conventional turbines with many mini turbines on the trailing egdes of conventional wings with the assumption of the above and the fact they should be significantly less draggy - I can't find a picture of them at the moment though.

Edited:

Found an example, see page 27:
http://www.rolls-royce.com/Images/whittle_tcm92-54...

The reason why this isn't implemented at the moment is because the turbines just aren't efficient at that size (yet).

Edited by navier_stokes on Sunday 25th October 15:29

According to most articles on the sublect, the main reason for mounting the engines there is to reduce cabin noise, not to give lift. The effect isn’t really mentioned anywhere from what I’ve seen, but from what I’ve read, I’d assume the following reasons would make the ‘lift’ concept fairly insignificant:

Firstly, for the engines to give much lift effect through air being ingested into the intakes, they would have to be mounted very close to (or within) where the boundary layer is, and on an efficient lifting surface (ie over the “wing” part of the aircraft like in your multiple fan trailing edge thing, rather than the “fuselage”). This gives all sorts of problems with turbulent air being ingested into the engines (especialy at high angles of attack) drastically reducing their efficiency.

The X-48B flying demonstrator has the engines pod mounted on pylons specifically located to be above the boundary layer, after the NASA simulations of low or fuselage mounted engines resulted in the aforementioned engine inlet problems (which are still being addressed). If you look at the location of the X-48B engine intakes, they would be sucking air away from the trailing edge rather than towards it. The reason NASA were investigating having the engines partially or fully located within the fuselage in the first place was to reduce or eliminate the drag of the nacelles, to reduce external noise, and to reduce overall drag through extracting the turbulent boundary layer in this region (in certain situations), rather than to accelerate flow over the fuselage.

Most effective ‘blown’ lift devices use high energy air from the engine exhausts being blown over the top of the wing, or under the wing onto extended flaps, or into the boundary layer at some point from the engine compressor stages. I’d question whether at cruise, the air being ‘sucked’ into the engines was travelling any faster than the aircraft was travelling through the air, which is perhaps why this ‘lifting’ concept hasn’t been pursued?

[dr_gn]

IforB said:

If everyone is waiting on the 787, then that's knackered then. That's one project that seems to be turning into a complete disaster. I hope they pull it off though.

So do I (for a few reasons!).

Even so, I can't help thinking of the all-composite "Team Philips" catamaran from a few years ago. Look what happened to that despite all the design, technology, analysis and cash:

[navier_stokes]

dr_gn said:

navier_stokes said:

IforB said:

Would having the engines in that configuration on the BWB's actually induce lift by increasing airflow over the body?

Possibly, the suction (upper) side of an aerofoil benefits (increases lift) from larger velocities (compared to the pressure side), as a simplification.

Also it may aid seperation at higher angles of attack because it will always force the flow towards the trailing edge - and therefore create less drag.

This may be offset by any turbulence generated by the engine intakes.

NASA also have a concept for replacing 3 or 4 conventional turbines with many mini turbines on the trailing egdes of conventional wings with the assumption of the above and the fact they should be significantly less draggy - I can't find a picture of them at the moment though.

Edited:

Found an example, see page 27:
http://www.rolls-royce.com/Images/whittle_tcm92-54...

The reason why this isn't implemented at the moment is because the turbines just aren't efficient at that size (yet).

Edited by navier_stokes on Sunday 25th October 15:29

According to most articles on the sublect, the main reason for mounting the engines there is to reduce cabin noise, not to give lift. The effect isn’t really mentioned anywhere from what I’ve seen, but from what I’ve read, I’d assume the following reasons would make the ‘lift’ concept fairly insignificant:

Firstly, for the engines to give much lift effect through air being ingested into the intakes, they would have to be mounted very close to (or within) where the boundary layer is, and on an efficient lifting surface (ie over the “wing” part of the aircraft like in your multiple fan trailing edge thing, rather than the “fuselage”). This gives all sorts of problems with turbulent air being ingested into the engines (especialy at high angles of attack) drastically reducing their efficiency.

The X-48B flying demonstrator has the engines pod mounted on pylons specifically located to be above the boundary layer, after the NASA simulations of low or fuselage mounted engines resulted in the aforementioned engine inlet problems (which are still being addressed). If you look at the location of the X-48B engine intakes, they would be sucking air away from the trailing edge rather than towards it. The reason NASA were investigating having the engines partially or fully located within the fuselage in the first place was to reduce or eliminate the drag of the nacelles, to reduce external noise, and to reduce overall drag through extracting the turbulent boundary layer in this region (in certain situations), rather than to accelerate flow over the fuselage.

Most effective ‘blown’ lift devices use high energy air from the engine exhausts being blown over the top of the wing, or under the wing onto extended flaps, or into the boundary layer at some point from the engine compressor stages. I’d question whether at cruise, the air being ‘sucked’ into the engines was travelling any faster than the aircraft was travelling through the air, which is perhaps why this ‘lifting’ concept hasn’t been pursued?

True, I haven't looked to much into BWB to be honest, but I should imagine there's quite a few problems involved with mounting engines that far back. Having said that conventional nacelles already have to deal with reduced/messy air flows for example from the fusalage when the plane is turning/in sideslip.

There is boundary layer suction technology out there at the moment and there is a lot of fundamental (read university) level research on employing MEMS devices on surfaces of a wings to aid separation. And this is a more likely future area for radical drag reduction/aerodynamic performance in say 20+ years due to the size of boundary layers (~mm) - the only problem with this is the energy used by a wing covered in these devices compared to the drag they reduce!

[dr_gn]

navier_stokes said:

dr_gn said:

navier_stokes said:

IforB said:

Would having the engines in that configuration on the BWB's actually induce lift by increasing airflow over the body?

Possibly, the suction (upper) side of an aerofoil benefits (increases lift) from larger velocities (compared to the pressure side), as a simplification.

Also it may aid seperation at higher angles of attack because it will always force the flow towards the trailing edge - and therefore create less drag.

This may be offset by any turbulence generated by the engine intakes.

NASA also have a concept for replacing 3 or 4 conventional turbines with many mini turbines on the trailing egdes of conventional wings with the assumption of the above and the fact they should be significantly less draggy - I can't find a picture of them at the moment though.

Edited:

Found an example, see page 27:
http://www.rolls-royce.com/Images/whittle_tcm92-54...

The reason why this isn't implemented at the moment is because the turbines just aren't efficient at that size (yet).

Edited by navier_stokes on Sunday 25th October 15:29

According to most articles on the sublect, the main reason for mounting the engines there is to reduce cabin noise, not to give lift. The effect isn’t really mentioned anywhere from what I’ve seen, but from what I’ve read, I’d assume the following reasons would make the ‘lift’ concept fairly insignificant:

Firstly, for the engines to give much lift effect through air being ingested into the intakes, they would have to be mounted very close to (or within) where the boundary layer is, and on an efficient lifting surface (ie over the “wing” part of the aircraft like in your multiple fan trailing edge thing, rather than the “fuselage”). This gives all sorts of problems with turbulent air being ingested into the engines (especialy at high angles of attack) drastically reducing their efficiency.

The X-48B flying demonstrator has the engines pod mounted on pylons specifically located to be above the boundary layer, after the NASA simulations of low or fuselage mounted engines resulted in the aforementioned engine inlet problems (which are still being addressed). If you look at the location of the X-48B engine intakes, they would be sucking air away from the trailing edge rather than towards it. The reason NASA were investigating having the engines partially or fully located within the fuselage in the first place was to reduce or eliminate the drag of the nacelles, to reduce external noise, and to reduce overall drag through extracting the turbulent boundary layer in this region (in certain situations), rather than to accelerate flow over the fuselage.

Most effective ‘blown’ lift devices use high energy air from the engine exhausts being blown over the top of the wing, or under the wing onto extended flaps, or into the boundary layer at some point from the engine compressor stages. I’d question whether at cruise, the air being ‘sucked’ into the engines was travelling any faster than the aircraft was travelling through the air, which is perhaps why this ‘lifting’ concept hasn’t been pursued?

True, I haven't looked to much into BWB to be honest, but I should imagine there's quite a few problems involved with mounting engines that far back. Having said that conventional nacelles already have to deal with reduced/messy air flows for example from the fusalage when the plane is turning/in sideslip.

There is boundary layer suction technology out there at the moment and there is a lot of fundamental (read university) level research on employing MEMS devices on surfaces of a wings to aid separation. And this is a more likely future area for radical drag reduction/aerodynamic performance in say 20+ years due to the size of boundary layers (~mm) - the only problem with this is the energy used by a wing covered in these devices compared to the drag they reduce!

I was looking at the X-48 as a D/F model and read a bit about it - I do remember something about if the nacelles were mounted further forward the airflow between the nacelle and the fuselage went supersonic at some point, causing all sorts of issues.

EDIT: Plus I'm guessing the top mounted engines would give some down pitch unless angled somehow?

Edited by dr_gn on Sunday 25th October 23:33

[navier_stokes]

Good point although I'm guessing you should be able to control the aero centre much more easily when your plane is one wing, hence no need for a tail either?

The flow over conventional airliners wings is already supersonic. I'm not sure how with a BWB you would be able to get up to current speed levels without a shock forming upstream of the engines. Looks like they are planning quite large sweep angles (in comparison to current commercial 'craft) which will help considerably, but I guess this is another reason why they're mounting the engines significantly above the body.

[dr_gn]

el stovey said:

That's such a great picture.



Love it!

It was Popular science who comissioned it, not Popular Mechanics as I posted previously. You can buy a print and put it on your bedroom wall. Here you go:

http://www.cafepress.co.uk/popsci.21529858

[FourWheelDrift]

My dad has a book from the 1950s somewhere with soemthing that looks like that concept in it, along with lots of other "future" things, such as a interstellar space travel to new worlds, electric cars everywhere and household robots doing the chores. All due to have happened 29 years ago.

[dr_gn]

FourWheelDrift said:

My dad has a book from the 1950s somewhere with soemthing that looks like that concept in it, along with lots of other "future" things, such as a interstellar space travel to new worlds, electric cars everywhere and household robots doing the chores. All due to have happened 29 years ago.

You mean to say you haven't got your own personal hovercraft?

[FourWheelDrift]

No

But then I wanted the personal Jet pack instead.

[dr_gn]

navier_stokes said:

Good point although I'm guessing you should be able to control the aero centre much more easily when your plane is one wing, hence no need for a tail either?

The flow over conventional airliners wings is already supersonic. I'm not sure how with a BWB you would be able to get up to current speed levels without a shock forming upstream of the engines. Looks like they are planning quite large sweep angles (in comparison to current commercial 'craft) which will help considerably, but I guess this is another reason why they're mounting the engines significantly above the body.

I keep forgetting all the details, but basically if you have a 'flying wing' like I was designing, you can get a tail down moment by sweeping the wing back, and transitioning the tip sections downwards (wash-out). The sweep effectively gives a very short tail moment arm. This counters the pitch-down tendancy of having the cg ahead of the aerodynamic centre, making the whole thing stable. It's not ideal because you never get the best lift:drag since the incidence of the wing can never be optimised for the entire span: some bits will always be giving more drag than they should. You can also have reflexed sections which achieve a similar result, but there are disadvantages there too which I cant remember. Sweep also gives some effective roll stability. From memory, something like 10 degrees of sweep is equivalent to one degree of dihedral (which is why many modern swept wing military aircraft have anhedral - in order to counter any excessive stability caused by sweep).

So what Im trying to say is that sweep isn't just relevant for high speeds. My glider flies very well slowly and is very stable too.