Variable bypass?

[GravityHater]

Is there a turbofan out there that can mechanically alter the bypass ratio in flight? Especially is there one in regular civilian use?

 

[9GClub]

What would be the point?

 

[GravityHater]

of my question?
To learn things.

 

[PolarTwins55]

SR-71. http://www.sr-71.org/blackbird/j-58/index.htm
diagrams there show the operation of the inlet.

 

[9GClub]

of my question?
To learn things.

No...... what would be the point of a variable bypass setup? In the case of the SR-71 I guess it has to do with efficiency across the performance envelope and transonic shock wave issues.

 

[GravityHater]

what would be the point of a variable bypass setup?

"the turbojet is more efficient at low altitude;
the turbofan is more efficient at high altitude"

my thought was divert fan air into the core at low altitude. (and go as much 'fan' as possible at high altitude)
I'm sure there is some reasoning why not, it could be even that the quoted statement above is somehow invalid or incomplete.

Thanks

 

[9GClub]

I'd think that the -fan would be more efficient at low altitude (just speculating, I have to check my books) because of the greater air density. Seems like at higher altitudes you'd want everything going through the core. Low-bypass turbofans/ turbojets are certainly better in high-speed applications.

 

[hindsight2020]

whoa fellas, the SR-71 does not have a "variable bypass ratio" engine at all. Just like the MIG-21, they both have pure turbojet engines (read NO bypass) with variable GEOMETRY inlets.


The reason behind the design is to allow the formation of an oblique shockwave at the tip of the cone that would feed the inlet with air at subsonic speed (necessary for the engine operation), while the aircraft flies at supersonic speeds. The cone retracts and extends depending on the speed of the aircraft as to obtain the optimal angle of that shockwave to feed subsonic air to the inlet with the least stagnation pressure loss as possible.

 

[Bluto]

"the turbojet is more efficient at low altitude;
the turbofan is more efficient at high altitude"

I agree, I'm pretty sure 9G is right. I think you've got it backwards.

 

[fletch717]

But can the mig do a negative 4G inverted dive?

 

[mzaharis]

2 things:

1. I used to work on the J58 used on the SR-71 as a manufacturing engineer at P&W. It DID have a bypass of sorts, in the form of 6 gigantic tubes from the compressor to the afterburner, bypassing the combustion chamber and turbines. Not TRUE bypass of the type one would see in a turbofan, but some air did go from the compressor straight to the afterburner. The bypass amount could be changed based on engine operating needs. This is independent of the moving cone mentioned earlier that manages inlet shocks and keeps the oblique shock from the cone properly placed at the lip of the inlet.

http://aerostories2.free.fr/acrobat/technique/J58/J58A_genesis_eng.pdf

2. The GE F-120 turbofan, their contender for the Advanced Tactical Fighter (now known as the F-22 Raptor), had a variable bypass, their solution to efficiently adjust the engine between the needs of lower-speed flight and supercruise (Mach 1+ flight without afterburner). They never released drawings of the bypass valves, afaik. I was working on its competitor, the Pratt and Whitney F-119, which was selected, presumably due to its simpler, easier to maintain design (although, like any other military contract selection, the reasons are sometimes murky).

 

[9GClub]

whoa fellas, the SR-71 does not have a "variable bypass ratio" engine at all. Just like the MIG-21, they both have pure turbojet engines (read NO bypass) with variable GEOMETRY inlets.

Yep, that's what I thought. Quite a few jets have variable-geometry inlets... the Tomcat and Concorde come to mind... or am I thinking bleed-air doors.... I'm over my head here without doing research, but suffice it to say that intake engineering can get pretty complex. It's also interesting to note how most intakes are offset a couple inches from the fuselage (perhaps most notably on the F-4/F-16) in order to avoid sucking from the boundary layer. If I'm not mistaken, the Raptor does NOT incorporate this design (i.e. the intakes are blended to the fuselage)..... wonder what the rationale is in that case....

 

[mzaharis]

It's also interesting to note how most intakes are offset a couple inches from the fuselage (perhaps most notably on the F-4/F-16) in order to avoid sucking from the boundary layer. If I'm not mistaken, the Raptor does NOT incorporate this design (i.e. the intakes are blended to the fuselage)..... wonder what the rationale is in that case....

If you look at this picture of the F-22 Raptor:

http://www.globalsecurity.org/military/systems/aircraft/images/f-22-1999043-f-0000l-001.jpg

Here's an even better photo:

http://www.globalsecurity.org/military/systems/aircraft/images/f-22-990167a.jpg

You'll see that the inlets actually do have some gap from the side of the fuselage that is used to split off bloundary layer air.

You may be thinking of Lockheed-Martin's F-35 (the JSF aircraft). They claim to have built a new inlet design with a "bump" on the wall of the inlet that somehow excludes the boundary layer without the use of a splitter (sorry, I don't know how). It simplifies the stealth design, apparently, eliminating a cavity which is hard to design without radar reflections, and helps to shield the fan face of the engine. You can see it best in this artist's conception.

http://www.globalsecurity.org/military/systems/aircraft/images/jsf200a.jpg

You can also see it in this photo of the X-35C photo:

http://www.globalsecurity.org/military/systems/aircraft/images/x-35c-0606_5.jpg

 

[GravityHater]

ok I reversed it
the question is not invalidated about variable amounts of air sent through vs fanned outside.
i am guessing from the responses that its not been done and that the reason is that 'it's just not needed' or the expense to benefit ratio is not there.
thanks

 

[9GClub]

You may be thinking of Lockheed-Martin's F-35

I was thinking of the JSF, my bad. Thanks for those pics man, they helped.

 

[fletch717]

O.K dungeons & Dragon players, it's time get out of the basement and go look for some girls.

 

[dseagrav]

O.K dungeons & Dragon players, it's time get out of the basement and go look for some girls.

Sure, we plan to be out and about while you and the rest of your swingin' jock friends are down at the clinic getting swabs pushed up your urethra to see what fun new STD you got this week from chasin all those women...

 

[fletch717]

Don't forget your clearasil