Forum: F-35 Engine

Last edited by megasun on Apr 06, 2012 - 10:26 PM; edited 2 times in total

How can Engine Hover Thrust be more than Military Thrust?
Is more fuel dumped into afterburner when hover?

Edit: Thanks for splitting this post. Allow me to add the numbers for this question here.

Maximum Thrust (in pounds): 43,000
Military Thrust (in pounds): 28,000

Hover Thrust (in pounds): 39,400
Main Engine: 15,700
Lift Fan: 20,000
Roll Post: 3,700

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The hover thrust includes the thrust produced by the lift fan which is not used while in forward flight.

a better, thermodynamic, question is how does an engine that can only make 28,000 lbs of thrust have 13,000 lbs of that removed through venting and gearing and turn the 13,000 lost into a 12,000 (or 25,000 depending on viewpoint) gain?

When the fan is engaged the combination essentially becomes a high bypass ratio turbofan, improving its efficiency. That's why it can produce about the same thrust as in AB with a much lower SFC. TEG is the expert though, so you should get his take.

From: http://f-16.net/f-16_forum_viewtopic-t- ... pw600.html

We've been down this road before.

That_Engine_Guy wrote:

So we'll split this up real quick; first the F135-PW-600 (STOVL sub-type) Figures (in bold) from PW's F135 site used for discussion:

Maximum Thrust (in pounds): 43,000 / (191.3 kN)
This is with nozzle straight in conventional flight mode, FULL augmentation available to the pilot, engine operates like the -100/-400

Intermediate Thrust (in pounds): 28,000 / (128.1 kN)
You'll notice this is the same for all variants in conventional flight, but will change when the LiftFan is engaged as some of the 'thrust' is converted to HP to drive the LiftFan. Some thrust is also lost by bending the exhaust; the flow is slowed in the bend, reducing it's velocity. This is why the 'Main Engine' thrust is lower in a hover or in STO mode. More details to come.

Short Takeoff Thrust: 38,100 (169.5 kN)
This is with nozzle swiveled down approximately 45% (or so) and LiftFan engaged. NO augmentation available as nozzle is 'bent'. Thrust from LiftFan directed down/rearward as commanded through vanes.

Hover Thrust: 39,400 / (175.3 kN)
(total vertical thrust of all components breaks down as follows)

Main Engine: 15,700 (thrust from rear nozzle, 90* bend, HP extracted for LiftFan, provides 'aft post' of thrust.)
Lift Fan: 20,000 (LiftFan pulls HP from gas-stream via the Low Pressure turbine, provides 'front post' of thrust)
Roll Post: 3,700 (Ducts on sides of engine, keep aircraft 'balanced' on front/rear posts)

Bypass Ratio
Conventional: 0.56
Powered Lift: 0.51

Overall Pressure Ratio
Conventional: 28
Powered Lift: 29
(You may notice BPR/OPR are different for the -600, but this is mostly done by the operating mode of the engine, all 3 variants are identical until the extra parts are installed on the -600. Drive shaft, 3 Bearing Swivel Nozzle, and Roll Posts.)

The F135-PW-600's FADECs will shift the motor's operating parameters, (vane positions, pressure ratio, fuel flow, nozzle ratio, etc) to provide extra 'power' to spin up the LiftFan during STO or VL situations. When you extract additional power from the engine's exhaust gasses for the LiftFan when engaged to the Low Pressure Turbine, the engine must compensate. You can't get rotational energy for the extra fan without giving up some thrust from the main engine. 'Bleeding' high pressure air from the core compressor will also lower the engine's power; not as much air is being moved through the combustion chamber, or out the rear nozzle.

Say you extracted 100% of the 'power' from the exhaust gasses of a turbine engine, you'd have a turboshaft engine that makes HP not Thrust. All the power from the hot gas is taken up by the turbine, the result is a hot breeze from the exhaust but no 'push'. The 'push' was internal against the turbine blades (sometimes called buckets) and converted into rotational HP to drive something else (like a prop, rotor-head, or transmission). You may notice in a high by-pass ratio engine, there are many low pressure turbines needed to extract LOTS of HP to turn that huge fan up front. As a result the fan of such engines give most the thrust, and the core's thrust is limited due to the turbine extracting so much power from the exhaust stream.

Now on to the F135-PW-100/-400

Maximum Thrust (in pounds): 43,000 / (191.3 kN)
You'll notice this is the same for all variants in conventional flight

Intermediate Thrust (in pounds): 28,000 / (128.1 kN)
You'll notice this is the same for all variants in conventional flight, but will change when the LiftFan is engaged as some of the 'thrust' is converted to HP to drive the LiftFan. Some thrust is also lost by bending the exhaust; the flow is slowed in the bend, reducing it's velocity. This is why the 'Main Engine' thrust is lower in a hover or in STO mode.

Bypass Ratio: 0.57

Overall Pressure Ratio: 28

Now the F135-PW-100/-400 are basic jet engines, augmented low by-pass ratio turbofans. Their thrust is 'conventional'. The only major difference would be the -400 having been 'Navalized' for salt water corrosion resistance. It may use different materials or have special coatings to prevent ocean going corrosion.

beepa wrote:

How will the change from free flow exhaust to a restriction (swivel outlet) affect the power output/back pressure?

To answer this directly, it's not the nozzle that's pulling power as much as a turbine that is extracting HP and reducing exhaust gas velocity from the core of the engine. The Roll Posts are also 'bleeding' air from the core. The nozzle bend will reduce velocity somewhat, and yes that will reduce thrust some too. All add up to change the 'dry thrust' of the 'main engine' as the -600 uses STO or VL mode.

beepa wrote:

In full dry thrust the nozzle is 'narrow', does the swivel/bend act in a similar way so the nozzle remains 'open' to produce similar power outputs???

Yes the engine can compensate some, but changing the back-pressure too much (more or less) will affect the stall margin of the engine's fan. The Roll Post operation also adds stress to the engine and would also affect stall margin. The engine will use the methods mentioned above to compensate for the additional loads during STOVL operation.

Clear as mud? Shocked

Follow on questions? Shrug

Keep 'em flyin' Thumb

TEG

Thanks for nice explanation.
I think I can take it this way, engaging the lift fan won't generate more horse power, but as the velocity of exhaust gas and lift fan are lower in this mode, the combined hover force is higher.

megasun wrote:

Thanks for nice explanation.
I think I can take it this way, engaging the lift fan won't generate more horse power, but as the velocity of exhaust gas and lift fan are lower in this mode, the combined hover force is higher.

Engaging the lift fan does make the engine work harder and the engine's controls are programmed to compensate and work for STOVL operation.

By removing energy from the exhaust with the added turbine draw (it's not free-wheeling anymore), you loose some thrust, by bleeding air from the compressor for the roll-posts you loose some thrust, by bending the exhaust you loose some thrust, so the FADEC changes parameters in the engine to compensate; it runs hotter, operates the main engine nozzle differently to maintain the pressure ratio, the compressor variable vanes will likely react as well to change the compression ratio in the high and/or low speed compressors. Not to mention additional inlets open to allow a greater air flow into the main engine at lower forward flight speeds.

Meanwhile the LiftFan takes it's consumed thrust in the form of horsepower and converts it back into thrust. The LiftFan has a very good pressure ratio (as fans go), and is counter rotating, making it very efficient.

So while the main engine makes a less thrust, that thrust is more than made up for by the LiftFan and the Roll-Posts bleeding high-pressure air from the compressor through their own nozzles under the wings.

Remember the two main 'thrust-posts' must balance out the aircraft. Too much thrust from the main engine nozzle, and the aircraft will pitch forward. To much LiftFan thrust and the aircraft will pitch back. This is why the claims of more power are mute in this mode of flight. Even IF the main engine could make additional thrust, without an equal amount increase from the LiftFan (which is state-of-the-art-efficiency already...) the aircraft would not be able to utilize the additional power.

So while the engine compensates for the power shifts internally, it also has to work in concert with the flight control computers to keep the aircraft balanced on 3 axis.

TEG

Put in terms of basic physics, thrust is force, while the limiting factor of the engine is mostly power. You can increase force while keeping power constant by increasing the reaction mass while reducing it's velocity, which is effectively what engaging the lift fan does.
Though I can't hope to match TEG on the specifics of exactly how it does that.

My favorite way to express it is that adding another fan is like making the main fan bigger, which means increasing the amount of air being pushed and the bypass ratio. That way, it's related to the choices made in engine types & sizes for different kinds of planes, and to a major limiting factor for Harriers, and to the tricks up the sleeves of the engines in Blackbirds and YF-23s.

As a non-engineer, I like to think of it as different gears in a transmission. Different ratios for different speeds. You can move a small amount of air quickly (low-bypass) for high speeds (F-22), or a large amount of of air slowly (high-bypass) for more efficient cruise speeds (F-35). Generally, moving larger masses of air gets you more mechanical advantage for a given amount of energy (like using 1st gear to pull something heavy). The most extreme aircraft example of this is found in helicopters. A loaded CH-53 weighs over 40,000lbs, but it's engines wouldn't produce anywhere near that amount of thrust if they were decoupled from the gearbox with their exhausts pointed at the ground. The F-35's lift-fan doesn't move air anywhere near as fast as a jet engine, but it moves a lot more of it, giving it the mechanical advantage to hold the plane up even though basic engine thrust is less than the plane's weight.

BTW I probably just made a fool of myself, so listen to the real engineers if they don't like what I said.