FSWs, etc.

New and old developments in aviation technology.
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by Kryptid » 13 Nov 2008, 10:57

The technology forum sure seems to be my favorite place to post topics...

This is going to be a thread about a few different things that may or may not be related to each other.

1) I've seen a lot of negative criticism on forward-swept wings. Does that apply mostly to fighter aircraft, though? Could forward-swept wings be used more effectively on other aircraft types such as transports, bombers, recon, electronic warfare aircraft, etc.? What are the biggest problems with FSW?

2) The oblique wing seems to be an interesting technology currently in development. I've never read about such wings being proposed for fighter aircraft. I assume there is good reason, but would it be better than standard variable geometry like in the F-14 and F-111? I've read that the center of lift stays more in place in oblique wings than standard VG wings. Since it also has the wings joined into effectively a single lifting surface above the aircraft, it may also be lighter and less maintainence-prone than standard VG. When the wings are fully swept, however, they look as if the turn performance would be unequal in a left turn versus a right turn. That sounds like a disadvantage that enemy fighters might be able to exploit.

3) I believe that all of the fans, compressors and turbines spin in the same direction within a jet engine, right? Wouldn't this create a large amount of torque? Is such torque a significant consideration for single-engined aircraft like the F-16? Do control surfaces have to be used to keep the jet from rolling or is the engine torque unnoticeable?

4) As said above, the fans, compressors and turbines spin in the same direction. Have any "counter-rotating jet engines" ever been developed, akin to "counter-rotating propellors"? That is, there are an even number of fans, compressors and turbines, with equal numbers rotating in opposite directions. In propellors, this is supposed to eliminate the spin of the air generated by the propellors and make them more efficient. Would such a jet engine enjoy similarly improved efficiency? The biggest problem I can see with this is a significant increase in complexity and maintainence hours. Perhaps cost would go up, too.

5) The material tantalum hafnium carbide (TaHfC5) has a melting point of 7,619 degrees Fahrenheit. This is the highest melting point of any currently known compound. Do any jet engines incorporate this compound into their turbine sections? I know that having a high melting point alone isn't enough to make a good turbine material. I know that they need to be able to withstand thermal expansion and centrifugal force and whatever, but I do not know such properties of this substance. It might also be too expensive. However, if this material were able to be successfully incorporated into a jet engine, how much of a performance increase might we observe (turbine inlet temperature can now be much higher)?

6) Since ramjets don't have turbine sections, how much hotter can they operate than standard turbojets? Maybe I'm mistaken and they can't be hotter? I guess the nozzle would have temperature limits as well.


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by asiatrails » 15 Nov 2008, 06:00

Yes its used, biggest issue is weight. Go here for some information http://www.ultramet.com/ceramic_protect ... tings.html


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by That_Engine_Guy » 16 Nov 2008, 00:38

Kryptid wrote:1) I've seen a lot of negative criticism on forward-swept wings. Does that apply mostly to fighter aircraft, though? Could forward-swept wings be used more effectively on other aircraft types such as transports, bombers, recon, electronic warfare aircraft, etc.? What are the biggest problems with FSW?


:2c: I believe the FSW could be used anywhere a swept wing is used but, the FSW has more bending/twisting than a conventional swept wing and as such must be more rigid/stiffer. With that it must be made of exotic materials or it becomes heavier than it's conventional counterparts. Cost, weight, and possible limited internal volume will most likely keep the FSW from mainstream aircraft for some time. Don't forget the Germans used FSW on aircraft in WWII, and there has been others since, but none to enter production.

Kryptid wrote:2) The oblique wing seems to be an interesting technology currently in development. I've never read about such wings being proposed for fighter aircraft. I assume there is good reason, but would it be better than standard variable geometry like in the F-14 and F-111? I've read that the center of lift stays more in place in oblique wings than standard VG wings. Since it also has the wings joined into effectively a single lifting surface above the aircraft, it may also be lighter and less maintainence-prone than standard VG. When the wings are fully swept, however, they look as if the turn performance would be unequal in a left turn versus a right turn. That sounds like a disadvantage that enemy fighters might be able to exploit.


:2c: VG is heavy, expensive and complicated. I was the flavor of the day in the 70s, but has since fallen from favor of the fighter world. I doubt we will see another VG, or oblique wing on a fighter for some time. Again the oblique wing will suffer weight/cost/complexity penalties versus a conventional wing.

Kryptid wrote:3) I believe that all of the fans, compressors and turbines spin in the same direction within a jet engine, right? Wouldn't this create a large amount of torque? Is such torque a significant consideration for single-engined aircraft like the F-16? Do control surfaces have to be used to keep the jet from rolling or is the engine torque unnoticeable?


Jet engines (gas-turbine) do not make "torque" in the conventional use of the term. Jets are reaction engines, the "torque" of a jet would be forward movement. Remember in an engine HP spins one direction, while Torque tries to move the engine in the opposite direction. Jet engines propel exhaust in one direction, while the "reaction" is launching it's self forward. Wile all the turning blades in an engine may spin one way, and do exert some force in the opposite direction, the stators in the engine all point in the opposite direction, and counter most of the forces. Couple this with the fact the engine's main shafts are not actually driving a propeller, and there is no true "torque" being applied to the airframe.

BUT! Compressor stall a jet engine and watch it move around, the "torque" in this situation is caused by the sudden RPM and airflow change in the engine. It will move the engine around like "torque". Another form of "torque" in a jet is when the bearings fail and seize; in this case the rotational mass of the N1 and/or N2 rotor is rapidly transferred to the engine's body; sometime causing aircraft to "torque roll" or making a nacelle mounted engine tear free of it's mounts. (I've never seen such an example, but have heard stories of such things happening - anyone that can confirm?)

So to answer your question about the F-16s engine "torque", there is none, the aircraft makes no correction to attitude based on throttle setting or engine output.

Kryptid wrote:4) As said above, the fans, compressors and turbines spin in the same direction. Have any "counter-rotating jet engines" ever been developed, akin to "counter-rotating propellors"? That is, there are an even number of fans, compressors and turbines, with equal numbers rotating in opposite directions. In propellors, this is supposed to eliminate the spin of the air generated by the propellors and make them more efficient. Would such a jet engine enjoy similarly improved efficiency? The biggest problem I can see with this is a significant increase in complexity and maintainence hours. Perhaps cost would go up, too.


The F119 and F135 are "counter-rotating" designs. the N2 Rotor spins in the opposite direction of the N1 Rotor. This reduces the number of turbine stators, and turbine blades. By reducing the number of turbine stators, the combustion velocity is higher (reduced interference/drag) and allows the turbine blades to extract more horsepower per stage. This in turn allows fewer turbine stages to drive the compressors. Overall the turbine section of a multi-spool, counter-rotating gas-turbine engine is more efficient than others.

Note: the whole rotor spins in the same direction. (Not every other stage)

Spin in a gas-turbine engine is reduced/controlled by the opposite direction of the stators in relation to the blades. Other vanes inside the engine perform similar functions. (see graphic below) But like I mentioned above, by "counter rotating" the turbine section, stators there can be reduced.

There are also commercial aircraft engines that use counter-rotating designs. I think the Trent series from RR is one, and GE is using them now too I believe.

Kryptid wrote:5) The material tantalum hafnium carbide (TaHfC5) has a melting point of 7,619 degrees Fahrenheit. This is the highest melting point of any currently known compound. Do any jet engines incorporate this compound into their turbine sections? I know that having a high melting point alone isn't enough to make a good turbine material. I know that they need to be able to withstand thermal expansion and centrifugal force and whatever, but I do not know such properties of this substance. It might also be too expensive. However, if this material were able to be successfully incorporated into a jet engine, how much of a performance increase might we observe (turbine inlet temperature can now be much higher)?


Many different materials are used in turbines, and it is not only the melting point of the material but the strength of the material at the maximum temperature of the engine. Sure TaHfC5 may "melt" at 7,619*, but how much does it stretch or bend at 3500*, will it survive 14000 RPM? Is it strong enough to resist damage from foreign objects? (Resists cracking?)

IMO the key to turbine technology is a combination of these factors. Materials - what material best suits and survives in a turbine environment, Coatings, what coatings will offer the best enhancement to the basic material with the lowest weight and best durability, Cooling - film or effusion cooling that allows the blades to operate at a fraction of the temperature of the gasses passing by them. Airfoil design - blades of proper design to extract more power with less drag and keeping the blades and vanes stout to last longer with high FOD resistance.

Kryptid wrote:6) Since ramjets don't have turbine sections, how much hotter can they operate than standard turbojets? Maybe I'm mistaken and they can't be hotter? I guess the nozzle would have temperature limits as well.


They can, but high-temperatures get to them as well. Sure there are no rotating parts, but as you said the combustion section and nozzle will still heat and must be designed in a similar manner as turbine sections.

Well have to run for now...

Keep 'em flyin' :thumb:
TEG

(EDIT - Hit submit and not "attach" :doh: )
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Axial-flow-compressor.jpg
Film_Cooling.jpg


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by Kryptid » 16 Nov 2008, 01:40

Thanks for all of the information you guys, especially you TEG.

I just keep learning more and more from this board.
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by asiatrails » 16 Nov 2008, 06:14

TEG, great post.

Yes, all the Rolls-Royce three spool engines contra-rotate, some of the two spools do as well e.g. the Pegasus in the Harrier. For the F-35 the reason that both engines have contra-rotating spools is to reduce the precession couple in the hover.


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by johnwill » 16 Nov 2008, 06:41

TEG, great post, very informative. However, there is one small error concerning forward swept wings. There was one production FSW airplane, the Hansa Jet HFB 320. It was a German biz jet, produced from 1964 to 1973. The reason for the forward swept wing was structural, not aerodynamic. It was a mid-wing airplane, and the wing was swept forward to allow the spars to pass behind the cabin. Forty-five were built.
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by asiatrails » 16 Nov 2008, 22:30

John, forgot about the Hansa beast, Hans Wolke who designed the airframe is the same person who was responsible for the forward swept wings of the JU287 forward swept wing bomber. He was taken to the soviet Union after the war where he completed the second prototype and was also responsible for the design of the EF140.

Of interest is that the HFB320 is one of the few airframes with a hard service life. A current FAA AD based on a 2002 German AD has the following:

Airworthiness Limitations Revision

(f) Within 30 days after the effective date of this AD: Revise the Limitations section of the HFB 320 Hansa Airplane Flight Manual
(AFM) to state the following (or insert a copy of this AD into the Limitations section):

‘‘Do not operate the airplane beyond 15,000 total flight cycles, or 15,000 total flight hours, whichever occurs first.’’


Related Information

(i) German airworthiness directive 2002–158, effective October 3, 2002, also addresses the subject of this AD.


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by Kryptid » 17 Nov 2008, 15:32

What about forward-swept canards or stabilators? Since they are smaller than the main wing, wouldn't the twisting forces on them be less extreme?

Or what about wings that are mostly rearward-swept but are forward-swept only at the very tips?
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by That_Engine_Guy » 23 Nov 2008, 00:38

Sorry...

No "fighter aircraft" with FSW has entered production since WWII.... :wink:

With that AD limiting the HFB320 to only 15K cycles, seems it doesn't work real well even in a civil application. At least that design?
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by gtg947h » 24 Nov 2008, 02:58

Kryptid wrote:What about forward-swept canards or stabilators? Since they are smaller than the main wing, wouldn't the twisting forces on them be less extreme?


The forces would be less, but then again, you'd have less structure there to resist them, too. And besides, it probably wouldn't even be worth the effort.


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by Kryptid » 04 Dec 2008, 19:26

Once again on the oblique wing; I found this concept for an oblique-winged military aircraft:

http://www.aircraftdesign.com/ri-columbus%20vfmx%20obliquewing.jpg

The link title read as "Rockwell VFMX strike fighter (A-6 replacement study~1985)". I haven't been able to find much information on this design (admittedly, I haven't been looking for very long).


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by asiatrails » 05 Dec 2008, 03:23

Kryptid wrote:Once again on the oblique wing; I found this concept for an oblique-winged military aircraft:

http://www.aircraftdesign.com/ri-columbus%20vfmx%20obliquewing.jpg

The link title read as "Rockwell VFMX strike fighter (A-6 replacement study~1985)". I haven't been able to find much information on this design (admittedly, I haven't been looking for very long).



I believe that aerodynamically it is related to the Meleagris Gallopavo



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