F35B / F32B lift system question?

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imacca

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Unread post12 Feb 2010, 13:43

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?
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fiskerwad

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Unread post12 Feb 2010, 14:48

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 :-)
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Pecker

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Unread post12 Feb 2010, 15:10

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......
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imacca

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Unread post12 Feb 2010, 15:27

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?
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Unread post12 Feb 2010, 23:08

They had a very hard time getting the x-32b off the ground.
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kingalbert

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Unread post13 Feb 2010, 04:59

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.
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Unread post13 Feb 2010, 10:53

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..
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That_Engine_Guy

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Unread post14 Feb 2010, 01:53

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:
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Attachments
DirectLift.jpg
x32b.jpg
x32b-2.jpg
LiftFan.jpg
x-35b.jpg
x35b landing.jpg
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Unread post15 Feb 2010, 16:41

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...
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Unread post15 Feb 2010, 17:42

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.
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kingalbert

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Unread post15 Feb 2010, 17:44

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.
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Pecker

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Unread post15 Feb 2010, 23:21

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....
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That_Engine_Guy

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Unread post16 Feb 2010, 04:32

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:
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Unread post16 Feb 2010, 15:28

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.
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Unread post16 Feb 2010, 19:17

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.
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