F35B / F32B lift system question?

Unread postPosted: 12 Feb 2010, 13:43
by imacca
I know this is all ancient history now, but im curious as to the differences between the lift systems on the two contenders for JSF. If i'm barking up a wrong tree here im sure someone will let me know.

On the 35B there is a lift fan that is shaft driven that lifts the front of the aircraft and the engine exhast lifts the back half. My undestanding is that the lift fan is driven off the engines fan.

On the 32B they used some kind of "direct lift". On that system did they use the airflow from the fan via ducts to lift the front half of the aircraft, and the engine exhast to lift the rear?

I'd think that sort of direct lift system would be more efficient as surely the ducting on the 32B would be lighter than the lift fan, gearbox and shafting on the 35B and there would be none of the mechanical losses of transfering power from one fan to another. Also, there would be less of the dead wieght in normal flight that is associated with the lift fan in the 35B.

I realise that there were probably lots of other reasons the X35 won out over the X32, but wondered if the lift mechanisms were one of those and why?

RE: F35B / F32B lift system question?

Unread postPosted: 12 Feb 2010, 14:48
by fiskerwad
imacca, here is one of the best programs I've seen on this:
http://www.pbs.org/wgbh/nova/xplanes/
Very informative and entertaining as well, painless learning!!
fisk :-)

Unread postPosted: 12 Feb 2010, 15:10
by Pecker
You pretty much hit the nail on the head with the F35B lift system, just missing the two roll posts (one under each wing) that provide lateral control and provide a little extra thrust.

The X-32 utilised a "3-poster" lift system design; a proportion of the fan bypass air was diverted to a forward 'screen', a spanwise slot nozzle that contributed thrust and, perhaps more importantly, created a cold air screen between the hot rear exhaust and the aircraft intake. All the exhaust air (the hot stuff;-) ) was diverted through two nozzles at the rear of the engine.

http://www.hitechweb.genezis.eu/xjsf3.htm

Both systems have(had) dead weight when in conventional flight. The F35B has a LiftFan, it's nozzle, driveshaft and all the associated plumbing and doors.

The X32 system was arguably lighter but still had to carry the direct lift rear nozzles, the forward nozzle and associated ductwork, plus some hefty diverter valves (both to block off the conventional exhust nozzle and open the direct lift nozzles). On top of that, the X32 engine was mounted some way forward in the airframe to permit the rear nozzles to be close to the aircraft CoG, thus the exhaust duct connecting the main engine to the reheat/augmentor and rear nozzle was a lot longer than usual.

Who knows what effect that had on the propulsion system CoG or how it compared to the F35B liftsystem......

Unread postPosted: 12 Feb 2010, 15:27
by imacca
Thanks for those links. can see from that the long connection between the engine and the main thrust nozzle. The lift nozzles appear to be pretty much in the middle ofthe aircraft. Any ideas on which system actually provides more lift?

Unread postPosted: 12 Feb 2010, 23:08
by bjr1028
They had a very hard time getting the x-32b off the ground.

Unread postPosted: 13 Feb 2010, 04:59
by kingalbert
The X-35 system provided much more lift. The reason why is that having the engine drive the lift fan is like increasing the bypass ratio of the engine, which increases thrust efficiency, especially at lower speeds.

Unread postPosted: 13 Feb 2010, 10:53
by poop_deck_popeyes_chicken
the whole program on PBS about the two planes.. first time i saw it pretty good show..

http://www.youtube.com/watch?v=1kNszWU7hTw

have to say Lockheed is kinda the king of fighters..

Unread postPosted: 14 Feb 2010, 01:53
by That_Engine_Guy
Most of the details have been covered.

The X-32B's JSF119-614 had a larger fan and created more thrust, but it alone had to support the aircraft.

(The JSF119-614) had a significantly larger fan than the Dash-611, raising the bypass ratio to over 0.6, making it arguably the most powerful fighter engine of the late 20th century, with a dry (without augmentation) sea-level rating of at least 180 kN (40,500 lb st).

The X-35B's JSF119-611 had the LiftFan to add thrust in hover, but the extra turbine power extracted did reduce the 'hover thrust' of the main engine compared to the non-STOVL JSF119 engines.

Added together (main engine/LiftFan) the thrust of the -611 was still a higher T/W ratio in the X-35. The X-35 hovered more easily and reliably than the X-32.

To hover, the X-32 needed to have parts removed (to lighten it) and suffered from exhaust gas ingestion. The 'jet screen' was designed to keep exhaust gasses from reaching the inlet, but it appears to have been inadequate.

Note: Gas turbine engines react poorly (and quite violently) to sudden/radical air density changes to the inlet while running at/near MIL power.

REF: graphics below.

Keep 'em flyin' :thumb:
TEG

Unread postPosted: 15 Feb 2010, 16:41
by butters
What do you think of having a transmission light and compact enough to fit in a fighter that has to transfer 20,000+ HP thru 90 degrees, a few feet from your head? We're talking about the same kind of power as a light frigate engine puts out. And does anyone have the specs on the RPM rate of the fan at full thrust? Or how far away you have to land from anything that isn't nailed down. Things like people and HueyCobras...?

To be blunt, I don't get the logic behind basing 90% percent of your national tactical airpower on a platform whose design is predicated on a STOVL dream toy for for the US Navy's army's airforce. Esp when the DOD's own analyses have shown that STOVL tac air offers no real world combat advantage whatsoever. Talk about the tail wagging the dog...

Unread postPosted: 15 Feb 2010, 17:42
by bjr1028
In theory it offers the ability for aircraft to safely operate from ships and paved airfields smaller than what a traditional aircraft could. The forward basing thing is a wash because of FOD. No ground based Harrier bas been deployed to a place where a conventional aircraft could not. This capability also comes at the price of performance and complexity. It doesn't have the range or payload of the CTOL or CATOBAR variants and as history has shown the more complex an aircraft the more things that can break and usually reliability goes down and maintenance requirements go up.

Unread postPosted: 15 Feb 2010, 17:44
by kingalbert
butters wrote:What do you think of having a transmission light and compact enough to fit in a fighter that has to transfer 20,000+ HP thru 90 degrees, a few feet from your head?


Sounds pretty neat.


butters wrote:And does anyone have the specs on the RPM rate of the fan at full thrust?


I suspect the designers do.

butters wrote:Or how far away you have to land from anything that isn't nailed down. Things like people and HueyCobras...?


The max takeoff weight of the CH-47D is 22,680 kg, which is more than the MTW of an F-35B. So the total amount of thrust generated by a F-35B should be tolerable.


butters wrote:To be blunt, I don't get the logic behind basing 90% percent of your national tactical airpower on a platform whose design is predicated on a STOVL dream toy for for the US Navy's army's airforce.


The airforce & navy versions don't have the lift fan.

Unread postPosted: 15 Feb 2010, 23:21
by Pecker
butters wrote: What do you think of having a transmission light and compact enough to fit in a fighter that has to transfer 20,000+ HP thru 90 degrees, a few feet from your head?


Well, it's more like in line with your butt...

Either way, so long as the engineers have done their job and the software prevents operation beyond the transmissions capabilities then i would be fine. It's no different than asking a Harrier pilot (or any pilot of a single engine fighter where the engine sits behind the cockpit) how he feels sitting in front of a fan spinning a xxxxrpm.


butters wrote:And does anyone have the specs on the RPM rate of the fan at full thrust?


Yes,people do have such information but please bear in mind that it is in all likelihood considered proprietary to the manufacturer and the operator/customer. The reasons are two-fold; not giving away technical information prevents potentional competitors/adversaries from gaining insight into capability (technologically) and from gaining information that might permit identification of the aircraft. Granted this is a LO design, so the engine face probably isn't easily visible from a radar perspective, but who knows what advancements may occur in the future....

Unread postPosted: 16 Feb 2010, 04:32
by That_Engine_Guy
butters wrote:What do you think of having a transmission light and compact enough to fit in a fighter that has to transfer 20,000+ HP thru 90 degrees, a few feet from your head?

The transmission is the last thing I'd worry about. How about blades (actually hollow 'blisks') spinning in a horizontal manner just aft of the cockpit? (see graphic below) They'll have much more rotating momentum than a few bearings/gears and a clutch-pack.
See also: http://www.f-16.net/f-16_forum_viewtopi ... r-asc.html

Nothing new with gearboxes, every engine has one getting power from the engine core via a tower-shaft. 90* down from the main shaft, then 90* again into the accessories' drive pads. They're much less likely to ever 'fail' compared to fan or turbine blades. (Plane of rotation anyone?)

What is new is the clutch that will absorb that much HP/torque in a couple seconds to spin up that LiftFan. :cool:

It isn't very big (compared to the engine or LiftFan) so it shouldn't have a whole lot of rotational inertia in a failure. I bet it is still tucked away nicely in a spot where it can't do much damage, just-in-case. The shaft will also need a 'fail' containment of some sort, wouldn't want it tying up the engine's turbine or a serious loss of thrust would occur.

butters wrote:does anyone have the specs on the RPM rate of the fan at full thrust? Or how far away you have to land from anything that isn't nailed down. Things like people and HueyCobras...?


It has been said in the X-35 project that the clutch takes 8500 RPM from the main engine. Not sure what the LiftFan actually turns. I'd guess it's near 1:1 drive, but that's only a guess.

http://www.rolls-royce.com/Images/LiftS ... 2-6697.pdf

RR says that the LiftFan, is a 50-inch, two-stage counter-rotating fan, with hollow blisks, capable of generating more than 20,000lbf of thrust.

The Fan Pressure ratio should be quite high in a counter-rotating design, and the vanes/nozzles used should also aid with thrust and stall margin.

Now for 'not being nailed down'? As a professional jet engine mechanic, you wouldn't catch me standing (or parking anything) closer than about 200' from one performing a vertical landing. The F-35B will have more down-draft than a chopper as it's jets are concentrated into 2 columns about 4' in diameter, both of which are 18K-20K of thrust. (not a 60'-80' rotor disk)

Then again this is why you don't see Harriers (or choppers) taking off from the ramp, but actually taxi out to a pad or the runway well away from 'loose' objects.

Nothing new there, basic vertical flight safety...

Keep 'em flyin' :thumb:
TEG

Unread postPosted: 16 Feb 2010, 15:28
by underhill
I should hope that there's a lot of Kevlar between the fan and the machinery operator.

As for tying up the turbine - almost any serious problem in powered-lift flight, unless you're really lucky and the thrust decays equally at both ends, is going to have the airplane swapping ends in pitch. The authority doesn't exist to deal with a significant out-of-balance situation. At that point it's so much for Messrs. Pratt, Whitney, Rolls and Royce, and hello Mr Martin and Capt Baker.

Unread postPosted: 16 Feb 2010, 19:17
by spazsinbad
From what I have read (no real details provided) there will be a 'sensitivity to this situation' switch but overall an automatic ejection system - during vertical flight - that will get pilot out faster than he can by any manual 'uhoh' seat of the pants mode. Sensors in aircraft/engine will determine when he needs to go and if.

Unread postPosted: 16 Feb 2010, 20:25
by Pecker
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

Unread postPosted: 16 Feb 2010, 20:54
by r2d2
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.

Unread postPosted: 16 Feb 2010, 21:32
by spazsinbad
Pecker, thanks for great info. What a BIG ‘WHEEL of Fortune’ that is. :-)

Unread postPosted: 17 Feb 2010, 14:38
by Pecker
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]

Unread postPosted: 17 Feb 2010, 14:47
by Pecker
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.

Unread postPosted: 17 Feb 2010, 19:31
by r2d2
Correct. If rpm is 2300 it makes about Mach 1.2 at the tips at sea level and + at higher altitudes.

Unread postPosted: 17 Feb 2010, 19:39
by Pecker
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.

Unread postPosted: 02 Mar 2010, 00:50
by r2d2
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???

Unread postPosted: 02 Mar 2010, 02:40
by That_Engine_Guy
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

Unread postPosted: 02 Mar 2010, 19:48
by Pecker
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.

Unread postPosted: 03 Mar 2010, 00:11
by JetTest
Much more advanced engine than the Pegasus counter-rotates.

Unread postPosted: 03 Mar 2010, 04:08
by Pecker
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.....

Unread postPosted: 03 Mar 2010, 05:20
by johnwill
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.

Unread postPosted: 03 Mar 2010, 05:36
by JetTest
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.

Unread postPosted: 03 Mar 2010, 16:54
by Pecker
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.

Unread postPosted: 03 Mar 2010, 18:14
by JetTest
F119

Unread postPosted: 03 Mar 2010, 19:01
by Pecker
JetTest wrote:F119


Oh, you mean the most advanced military, low-bypass turbofan in the 35,000lbf thrust class.....

....and in production.

Yeah....i'll give you that one. :wink: