To try and change lift by changing fan blade speed would be too slow to modify the lift for "tiny" and "delicate" changes in how the plane hovers.
Like a helicopter, the F-35 is able to maneuver in very small movements and change very fast (in fact likely better than a helicopter).
So the pilot can "nod" to you, and move around the front nose with tiny, delicate moves. So tiny movements by the F-35 and the flight stick are rapidly accepted by the flight computers.
Now a helicopter uses variable pitch blades to make rapid changes, which allows helicopters to achieve tiny, delicate, and fast "little" moves. So tiny fast changes in lift are required for these delicate moves.
Change of engine throttle to change fan lift would be far too slow (for the F-35, or for a helicopter).
To save weight, and complexity, the F-35 lift fans are counter rotating, and they are light weight carbon fiber.
So, to rapid change lift and have rapid tiny changes in lift, they simply change the area/size of the exit exhaust area of the lift fan.
Think of those "variable" nozzles much like a window shutter to let more or less light into a room.
The shutter looks like this:
So, you can open a bit more, or close a bit more.
However, when you close the shutter more, then where does the air go? It has to go somewhere! (remember, up to 20,000 lbs is created by that fan).
Well, there is a second set of doors that open right behind the one large fan lid.
That second set of doors can be seen here:
Those 2nd set of doors behind the big lift fan lid are to allow venting of the lift fan.
So you need a fast and delicate means to change lift fan output. So amazing and so tiny are these changes, that the pilot can "nod" the nose down a bit. Very tiny changes are achieved.
So to change the fan downforce you thus close or open the lift fan exit (exhaust) area doors. This can be closed or opened rather fast, and also by tiny amounts. This gives delicate control.
However, when you squeeze close this exit area, then the lift fan air has to go some place else, so it is vented out those doors right behind the big lift fan lid door.
So, if you close down the exit hole on the bottom of the F-35 fan, then that air takes another path – right out those 2nd set of doors.
9 out of 10 diagrams on the internet call that second set of doors an auxiliary inlet. However, that is NOT 100% correct. It should be called an auxiliary exhaust.
Now, to be fair, because the cold inlet turbine compressor is also exposed to that vent area, then no doubt that area likely remains "negative" pressure most of the time.
However, this is quite a brilliant design, since that lift fan exit area in general has some amount of "close" and thus "some" amounts of fan exit air is going to be vented (by-passed) right into that "box" behind the lift fan.
And that box just so happens to be right where the hungry for cold air turbine inlets of the massive F135 engine is. (What a nice design!)
So likely that area remains negative due to the compressor inlets. However, that area will go positive with most of the lift fan exhaust closed.
In this case, then those two doors are in fact a lift fan exhaust vent – not an intake vent!
I should also point out that due to vast amounts of cold air hitting the deck with this lift fan system, Lockheed engineers state that the overall deck temperatures are less than a Harrier when landing on the deck of a ship.
So while both Harrier and F-35 tend to heat up the decks on a vertical ship landing, the F-35 actually is less of a heating problem then is the Harrier for such vertical landings.
References to my statements:
With this system, nozzle thrust may be directed in an arc of 41.75-104
degrees (fore/aft aircraft coordinate system), at a rate of 40 degrees
Ok, so far, above ONLY talks about directing the down thrust.
But, we have this:
Independent control of the three VAVBN actuators provides the
capability to vary the nozzle throat area independent of the vector
To save reading, we have:
VIGV = top intake part (variable )
VAVBN = bottom exit part (also variable).
So above states the VAVBN actuators (the EXIT PART OF THE FAN) can vary the nozzle throat area.
Lift fan blades are NOT variable pitch.
There no evidence, no diagram, or ANY information of ANY kind in existence that CHANGES this claim of mine. I stand by this claim.
Lift exit fan can open/close or "vary" the fan output.
Again, the above document 100% supports and verifies my claim.
My claim that when closing down the 20,000 lbs of fan vent flow, that flow HAS to go some other place, and I am claiming that flow goes to the air box behind. This is a SPECULATION on my part. I do believe this to be what occurs but I do NOT have a link or article that outright states this (some diagrams DO suggest this, but that would not be proof). So this claim of mine that fan air vents to the air box behind is NOT verified by me, and until I can provide solid evidence of this, then that claim should be taken with a grain of salt.
As for the speculation of air venting out of the fan box, this here is a photo.
It is "difficult" to tell if that is a vent door, but does seem to look like one. As noted - I am open to opinions on this - this right now thus is still a speculation on my part.