# How does the F-35B not flip over during transition?

As the engine nozzle rotates, the lift-fan cannot. Wouldn't this imbalance of moments around the center of gravity result in the F-35 pitching up onto its back?

As can be seen at 5:14 in this video:

the F-35B is capable of transitioning from vertical hover to horizontal flight.

It accomplishes this in part by gradually rotating the engine nozzle from the vertical position to the horizontal.
Let us assume that the throttle input stays constant in the following scenario: As the nozzle rotates, the vertical component of its thrust is reduced (as the horizontal component increases) as

$$thrust_{horizontal} = thrust \cdot \cos(\theta)$$

$$\theta$$ being the angle between the jet exhaust and the vertical axis.
The problem is, the lift fan does not rotate. And since it is attached by shaft to the engine, we assume it always rotates at the same RPM as the engine itself.
Therefore, the vertical component of lift of the lift fan does not change for a given throttle input, while the vertical component of the jet engine nozzle does!

So as the nozzle transition occurs, the thrust of the lift fan (ahead of the center of gravity) stays constant, but the vertical thrust behind the center of gravity (rear engine nozzle) reduces: shouldn’t this cause the aircraft to pitch upwards and flip onto its back? Yet in the video the aircraft seems to pitch up only ten or so degrees (I assume due to the increased coefficient of lift on the wings) but otherwise stays flat in pitch.

The thrust of the lift fan must be somehow modulated to accomplish this, right? If so how is this done if its shaft speed is tied to that of the engine?

Possible solutions:
1. As the aircraft’s forward velocity increases, the lift of the wings compensates for the reduced vertical lift of the rear engine nozzle.
2. The aircraft pitches up so as to deflect the angle of the lift fan from the vertical to match the angle between the engine nozzle and vertical.

• "Let us assume that the throttle input stays constant in the following scenario:" That is a really bad assumption to make on such an advanced aircraft. Obviously, the nozzle thrust is adjusted to compensate for the changing nozzle angle. Commented Feb 5, 2019 at 15:02
• That assumption was just made to simplify the formulation of the question. Of course I actual flight the throttle would be adjusted. Commented Feb 5, 2019 at 16:06
• Please consider reviewing which answer you accepted. Commented Feb 6, 2019 at 6:35

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:

From Lockheed:

https://www.lockheedmartin.com/content/dam/lockheed-martin/eo/documents/webt/F-35_Air_Vehicle_Technology_Overview.pdf

page 25:

quote:

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 per second.

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 angle

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.

My claims: 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.

NOT verified: 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.

# Edit 2

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.

• You are correct that the LiftFan blade pitch can be adjusted to vary the thrust of the fan. However, the latter half of your answer regarding the Aux Inlet doors is inaccurate. These doors open to increase airflow to the engine during hovering operations because the side-mounted fixed inlets don't provide enough airflow if the aircraft is not moving forward. Lift fan exhaust is directed out the bottom of the aircraft through a third set of doors, and is controlled by a set of movable guide vanes. LiftFan exhaust is not supposed to interact with engine intake -and in fact this can be dangerous! Commented Feb 5, 2019 at 22:42
• The fan blades are NOT variable pitch. the amount of down thrust is controlled by the sent of "vanes or open/close doors". To reduce fan down thrust, you CLOSE the hole on the bottom. When you close that hole, then WHERE does the air go? (answer: it goes out into the area exposed by the second set of doors behind the lift fan. The lift fan is NOT variable pitch. To reduce down force, you close the hole on the bottom. If you close that hole, then the air HAS to go some other place - has to take some other path. That other path is right into the area with the two doors that open behind the fan Commented Feb 5, 2019 at 22:46
• If you could provide a source for your information that would greatly strengthen your answer. Commented Feb 5, 2019 at 23:02
• @MikeB is correct, no air is redirected to the engine, or an exhaust (you've mislabeled it I'm afraid). The other two answers are correct (it's a function of variable inlet vanes, and nozzle vanes — the latter, does not fully close, the vanes have a range of only 41.75–104°, and they move in 3 pairs of 2... independently). Source: lockheedmartin.com (PDF)
– user14897
Commented Feb 6, 2019 at 5:56
• One correction - the dorsal intake behind the lift fan intake is not the lift fan exhaust vent. It is an auxiliary air intake for the engine to provide extra air during high power setting at low airspeeds where the intakes cannot do so. Commented Feb 6, 2019 at 8:18

Your assumptions (constant thrust for both lift fan and exhaust) are wrong. From wikipedia:

"For pitch control, the areas of exhaust nozzle and LiftFan inlet are varied conversely to change the balance between them while maintaining their sum, and with constant turbine speed"

So yes, the thrust is modulated to accomplish this.

From what is available on the internet, the F35B's lift fan is more than just a simple shrouded fan. There is at least some variable guide vanes functioning as nozzle. So to answer your question, no, the lift from front and back seem to be completely (lift fan by variable guide vanes, engine nozzle by, well, a variable nozzle) adjustable.

• Appricate the diagram. As noted, this shows the bottom box is "variable". Looking at the box, it does "hint" at when those vanes are closed, then vents on the shaft side would be where such air flow goes. As I stated in my post, this is the direction of the air when the vane box is closed that I "assume" the air goes, but it not 100% clear from the above. However, the above at least does verify my claim that the exit box is "variable" in output. Commented Feb 7, 2019 at 2:42
• @AlbertD.Kallal no, the "vent" behind the inlet of the lift fan is a secondary inlet for the main engine because when the F35B hovers close to the ground the air on its side may be disturbed by the downwash reflecting off the ground thus it cannot breathe this 'dirty' air and need a secondary inlet on its dorsal side. Commented Feb 7, 2019 at 8:40
• Not disputing that vent claim. However, like blow-out doors on the cowlings of the 707, or the Harrier, the air pressue goes negative and those spring loaded passive doors open to increase intake area for the cold compresser side to pull in more air. However, as the plane gathers speeds, then the air around the intakes goes positive, and those blow-in doors shut. I specific labelled and used the term "lift fan exhaust area". If I am correct about the fan exhaust venting to that area, then I stand by my claim that that area will and can see positive air pressure. Commented Feb 7, 2019 at 17:39
• The first diagram here shows what looks like "vents" on the bottom of the variable nozzles box - and they POINT in the direction of the air box behind the lift fan. There are no such vent like structures on the side of that nozzle box, but it SURE looks like vents to the rear (or towards the air box). As I stated, if you close the output of 20,000 lbs of air, it has to go some place else. If you stop venting of that thrust down, then it goes rearward to the box behind. This means that area will (can) go positive air pressure, and at that point those two open doors are in fact venting air. Commented Feb 7, 2019 at 17:44
• I am VERY open to that air does not vent from the fan area. However, when f35 is flying say at 80 knots with that big lid open, it going to catch a LOT of air, and force it down onto the fan, if the fan exits are closed, then it makes sense that the air now can flow to the air box behind. It is not 100% given that this occurs, and I am open to that the vanes on the lift fan system simply "close down" the air flow but it would make sense to vent the air back to the box area behind as you "close down" airflow though the fan system. Commented Feb 7, 2019 at 19:14

My understanding is that the lift fan (two sets of contra-rotating blades) is driven from the main engine via a permanently rotating drive shaft, but that the drive from the shaft to the lift fan is connected/disconnected from the lift fan via carbon clutch plates. In conventional flight it makes little sense to waste engine power driving the lift fan when not needed. With regards to the moveable vanes beneath the lift fan - they do not ‘close’ like a set of shutters as has been suggested, but move through a limited arc to direct the lift fan thrust from vertically straight down for hovering to 45 degrees rearwards to add a proportion of thrust when transitioning to conventional flight. It is these same vanes that can also rotate to 5 degrees forward to provide thrust to cause the aircraft to move backwards when hovering. As the lift fan is driven at engine speed when hovering and transitioning, the lift fan inlet guide vanes on top of the lift fan vary the volume of air ingested by the lift fan and hence the thrust of the lift fan itself. The balance between main engine exhaust thrust and lift fan thrust is co-ordinated by the digital engine control system. After transitioning to conventional flight, the lift fan clutch disengages the lift fan from the engine-driven drive shaft and the lift fan ceases to rotate. Beneath the fuselage, the lift fan exhaust doors close (not the lift fan exhaust vanes). The pair of doors on top of the fuselage are not vents, these are the AAIDs (auxiliary air intake doors) to allow increased air flow to the main engine during the hover (as noted in previous posts). This is covered in the 2014 Air International F-35 special.

Those 2nd set of doors behind the big lift fan lid are to allow venting of the lift fan.

Incorrect. AAIDs - Auxiliary Air Inlet Doors - provide additional air to the F135 intake, not "venting of the lift fan".

"and an auxiliary inlet." F-35 Air Vehicle Technology Overview, page 14

"The exhaust is discharged through a thrust vectoring nozzle on the underside of the aircraft to provide balanced lift with the 3BSM." F-35 Air Vehicle Technology Overview, page 14

• This is a useful comment, but it doesn't provide a full answer to the question. Commented Mar 11, 2022 at 11:01
• Read my comments close. As a general rule, yes those doors on top are intakes. However, as the plane moves and gains forward speed, then both the lift fan, and the standard front air intakes are receiving very good airflow. At that point, the air flow from the front intakes and possible the lift fan lid intake are now moving sufficient amounts of air into that intake area - my position is at that point, air pressure will go positive in that intake box area - hence at that point, it becomes a venting system, not a intake system. That 20,000 lbs thrust with close bottom has to go some place. Commented Mar 12, 2022 at 0:57