F35B / F32B lift system question?

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Pecker

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Unread post16 Feb 2010, 20:25

underhill wrote:...almost any serious problem in powered-lift flight, unless you're really lucky and the thrust decays equally at both ends.....


Yeah....that's something that has probably resulted in restless nights for the designers/engineers of every V/STOL, jet-borne aircraft ever conceived in the run-up to first flight/transition, whether engine related or due to some other mechanism.

Rotor-borne aircraft, at least those of the multi-engine variety, have the option/benefit of transferring power from the 'good' engine, but even they're not exempt from the single-point-of-failure issue.

http://www.aiaa.org/tc/vstol/wheel.html

Oh, and i just HAD to post this:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800015801_1980015801.pdf
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r2d2

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Unread post16 Feb 2010, 20:54

You can calculate the maximum rpm of the fan if you know its radius. I believe angular speed of the fan at its tips are below Mach 1. Without calculation I'm expecting some gear reduction ratio.

I have my vote on the F-35 concept because it provides ample thrust. More thrust gives it a reliability despite the complexity of the system. Of course this is my opinion.
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spazsinbad

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Unread post16 Feb 2010, 21:32

Pecker, thanks for great info. What a BIG ‘WHEEL of Fortune’ that is. :-)
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Pecker

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Unread post17 Feb 2010, 14:38

spazsinbad wrote:Pecker, thanks for great info. What a BIG ‘WHEEL of Fortune’ that is. :-)


Or "Wheel of Misfortune".....even with the F-35B in production, less than 10% of flying VSTOL protypes have ever made it into production.

r2d2 wrote:You can calculate the maximum rpm of the fan if you know its radius. I believe angular speed of the fan at its tips are below Mach 1


I'm pretty sure that Mach 1 no longer presents a limit for fan tip speed. If i could only find a notional 100% spool speed for something that isn't classsified then we could do the math.....[/b]
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Pecker

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Unread post17 Feb 2010, 14:47

r2d2 wrote:You can calculate the maximum rpm of the fan if you know its radius. I believe angular speed of the fan at its tips are below Mach 1.


I found some specs for the GE90-115 as an example. Fan dia 128in and N1 speed over 2300rpm. Of course, it all depends on where within the envelope you achieve over 2300rpm. If it's at sea-level, standard day, that puts the tip speed over 390m/s (i.e supersonic), but if it's at altitude then the tip speed will be lower.
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r2d2

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Unread post17 Feb 2010, 19:31

Correct. If rpm is 2300 it makes about Mach 1.2 at the tips at sea level and + at higher altitudes.
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Pecker

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Unread post17 Feb 2010, 19:39

r2d2 wrote:Correct. If rpm is 2300 it makes about Mach 1.2 at the tips at sea level and + at higher altitudes.


The latter depends on the exchange rate between the sqrt(Tamb) term in calculating the speed of sound and whether the 2300rpm (mechnical) fan speed is attenuated by the effect of a corrected speed limit on fan operation, which is a 1/sqrt(Tamb) term.

Either way, it's fair to say that todays modern fan designs are not encumbered by a sonic tip speed limit.
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r2d2

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Unread post02 Mar 2010, 00:50

Either way, it's fair to say that todays modern fan designs are not encumbered by a sonic tip speed limit.

Correct again. Blade tip speeds of Mach 1.1 are common for contemporary turbofans (although I still think this speed is also related with the speed of sound to some extent).
But on the other hand -pls. correct me if I'm wrong- this is a counter rotating fan(s) design. According to my judgment, blade tip speed will be lower than a single fan design. May be I'm mistaken???
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That_Engine_Guy

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Unread post02 Mar 2010, 02:40

r2d2 wrote:But on the other hand -pls. correct me if I'm wrong- this is a counter rotating fan(s) design. According to my judgment, blade tip speed will be lower than a single fan design. May be I'm mistaken???

Yes, the LiftFan design is counter rotating. I would agree with you on it being a subsonic design; the interaction between two fans spinning opposite directions at super-sonic speeds could be very tricky, if not counter productive - restricting flow due to shock waves.

:2c: TEG
[Airplanes are] near perfect, all they lack is the ability to forgive.
— Richard Collins
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Pecker

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Unread post02 Mar 2010, 19:48

That_Engine_Guy wrote:Yes, the LiftFan design is counter rotating. I would agree with you on it being a subsonic design; the interaction between two fans spinning opposite directions at super-sonic speeds could be very tricky, if not counter productive - restricting flow due to shock waves.

:2c: TEG


According to wiki the liftfan contains 'inter-stage vanes', a form of conditioning stage between the two fans. Given the vast range of speeds and airflows that the engine has to accommodate and, thus, the wide range of exit swirl that would be given off by the 1st stage fan, it's not surprising that some form of conditioning is required.

Counter-rotation in engines does have it's drawbacks (bearing speeds being one) but it does remove or at least lessen the issue of gyroscopic influence on the airframe. The Pegasus engine in the Harrier had counter-rotating HP and LP stages for similar reasons.
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JetTest

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Unread post03 Mar 2010, 00:11

Much more advanced engine than the Pegasus counter-rotates.
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Pecker

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Unread post03 Mar 2010, 04:08

JetTest wrote:Much more advanced engine than the Pegasus counter-rotates.


You are absolutely right, Trent 1000 and the GE/RR F136 being good examples, but Pegasus (IIRC) was the trend-setter in terms of reduced gyroscopic loading in a STOVL platform.

In conventional flight it's not so big a deal as aerodynamic controls are plenty powerful enough to overcome the effects (though still undesireable) of gyroscopic precession.

However, in a VTOL platform, conventional aero controls are not effective in the hover and roll/yaw/pitch authority is provided by engine bleed air ejected through suitably placed nozzles (that said, the F35B LiftSystem doesn't use this system as it vectors thrust from the main engine and Liftfan). Now, if every time the pilot inputs a yaw adjustment, he has to simultaneously counter the resulting pitching moment then not only is his workload increased but bleed air usage also increases.

Anything that can be done to reduce the amount of bleed air required during hover/slow speed flight is a bonus as it means more thrust and more bring-back capability.

And I would imagine that gyroscopic precession is a pretty uncomfortable feeling for the pilot too.....
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johnwill

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Unread post03 Mar 2010, 05:20

Pecker said, "(that said, the F35B LiftSystem doesn't use this system as it vectors thrust from the main engine and Liftfan)" and "Now, if every time the pilot inputs a yaw adjustment, he has to simultaneously counter the resulting pitching moment then not only is his workload increased but bleed air usage also increases."

I believe the F-35B does use bleed air for roll control. Any gyro moments (yaw causing pitch, pitch causing yaw) will automatically be countered by the flight control system, so the pilot will not have to be burdened by having to counter those effects him(or her)self.
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JetTest

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Unread post03 Mar 2010, 05:36

F35 does have roll-posts, and Rolls and GE are not the only counter-rotating engines. In fact, the most advanced operational engine in the world counter-rotates.
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Pecker

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Unread post03 Mar 2010, 16:54

johnwill wrote:I believe the F-35B does use bleed air for roll control.

You're absolutely right.....my bad. So far as i'm aware the roll posts are continuously providing a level of thrust to improve hover performance and, when roll control is needed, that level of thrust is modulated (plus or minus from some datum level) to provide a roll moment.


johnwill wrote:Any gyro moments (yaw causing pitch, pitch causing yaw) will automatically be countered by the flight control system, so the pilot will not have to be burdened by having to counter those effects him(or her)self.

Quite likely that is correct, but why make life more difficult for yourself by having gyro moments present in large amounts in the first place?


JetTest wrote:Rolls and GE are not the only counter-rotating engines

And it was never my intention to suggest that it was a technology limited to those two companies, i was merely using those two engines as examples.


JetTest wrote:the most advanced operational engine in the world counter-rotates

You know this conversation would go more quickly if you avoided vague statements and made a clear (albeit it, in all likelihood, contentious) point.
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