The F-35 thread, Mk II
 

Saw this demonstrated for the first time on a documentary


Very impressive, the demonstrated level of stability and almost rock steady flight path precision on a very slow landing and extremely short roll out to a braked stop


The approach speed is so slow it brings up the obvious question though, I think it’s about a 30 knot overtake, assuming 20 knots wind over the deck that’s around 50 knots IAS


At such a low speed I don’t see how you’re generating any worthwhile, additional wing lift so what’s the point if the idea is to allow higher landing weights ?


At the moment it looks like an interesting demonstration of slow flight enabled by the aircraft’s vertical lift system


Unless they’re planning higher speed RVL’ s ?

If they are moving forward, does that not make it a short landing, rather than a vertical one?

Why do you think the wing would not be generating any lift at 50 knots?

Yes, plus we don't know the ship and windspeed, so the actual airspeed is probably significantly higher than the groundspeed visible.

My first thoughts too, but searching online this does seem to be the accepted terminology. Mind you, all landings have a vertical component or else you'd never touch down....

I don’t know about allowing a significantly higher landing weight but the main advantages of an “RVL” on any aircraft designed for vertical landings are that the pilot has a far better view of the touchdown point and as the OP noted, the aircraft is easier to fly in the lateral sense because there are more visual cues. It may also retain some natural aerodynamic stability until touchdown.

Interesting to consider the balance of thrust vectors vertical and horizontal vs weight, lift and drag. Could you end up using higher power settings if the wing is not producing sufficient lift?

No, because the aircraft is in a constant descent. However, as the IAS decreases and aerodynamic lift reduces, power will be increased to compensate.

Another factor is that in a fully vertical descent, the engine may ingest hot air from its own exhaust and possibly dirt. Neither are good.

Why no?
Constant descent is a red herring, if it is constant there is no acceleration so vertical lift vector = weight. Vertical lift vector is the sum of vertical thrust vector and vertical lift vector. As you say as speed decreases lift from wing decreases and engine thrust must compensate although the airframe drag will decrease which will require less engines thrust to overcome and may compensate. Unless the Rod in the approach is sufficient for landing then that too must be slowed by a further increase in thrust.
It is all speculation on my part, the truth is out there!

Gravity is an acceleration. If your vertical lift vector = weight then you are not descending towards the herrings.

RVLs on land were done on the Harrier, they let you use shorter strips amongst other things. What the F35B brings to the game is the ability to do them on a ship reliably. An example of use is bringing back weapons in hot conditions when you don't have enough thrust for the vertical landing (It's expensive jettisoning £100K wepaons so you can land)

Whilst Dave Morgan probably did the first SBRVL in 1982, the Indians have done them in SHARs too.

They must be practising them at Marham, 'cos they are very NOISY!

Constant descent is not an acceleration. Watch the 'g' meter.

An aircraft does not stand still when thrust =drag, now rotate that concept through 90°

So lowering the nose at a given level of thrust does not cause the IAS to increase or that to maintain a given speed whilst lowering the nose I don't need to reduce thrust?

Last time I rotated your concept and pointed the nose at the nadir my G meter seemed pegged at zero. Maybe it was broken.

OK, I'll write slowly.
When thrust =drag vectors in the horizontal plane one can have a constant speed in the horizontal plane, if you want you can express it in feet per minute.
Rotate the concept through 90°
When weight=lift vectors in the vertical plane one can have a constant speed in the vertical plane, if you want you can express it in feet per minute (rate of descent)

​​​​​​If you have access to O level physics notes or AP 3456 it may be worth having a look at them.

Effectively it was broken for the task at hand, as in that attitude it no longer shows vertical acceleration. If you swivelled it 90 degrees so it shows acceleration along nose-tail (i.e., vertically) and flew a constant TAS downward, it would show 1G.

Same as if you stand on a bathroom scale in an elevator (or, lift) travelling up or down at a constant rate, it will show the same as when you're standing on it in the bathroom.

You say (correctly) that "gravity is an acceleration," which generates a force, and if we were subject to only that force and no other, then we would therefore be accelerating. But the part you're missing is that we are also subject to other forces at the same time, such as the floor of the bathroom (or the lift of an aircraft in steady flight) pushing up on us. Because that force balances the gravitational force, the sum is zero and we do not accelerate.

My big skool stuff is in there somewhere but still need your teaching on why a constant rate of descent requires the same thrust as for level flight of how a g-meter can be used in establishing a rate of descent before you expand your teachings further. Heck, until you offered to help I thought I could set and maintain a constant RoD and speed with no thrust at all.

I edited my above post to add more, as you were typing this. I won't do that anymore to avoid cross-wiring.

Beardy, So from a constant IAS in straight and level flight, please explain how you begin a descent while maintaining that same IAS?

shy torque Take this example.
Take off and go to cruise altitude.
Trim the aircraft to cruise speed, and let go of the yoke. (Keep heading with rudder only).

Reduce power. What happens? Speed stays at the trimmed speed but you descent.
Increase power. What happens? Speed stays at trimmed speed but the aircraft climbs.

Trim sets the speed.
Power controls climb or descent.

Zero feet per minute.

Oh dear, you seem to be confusing speed and acceleration.

"why a constant rate of descent requires the same thrust as for level flight" can't be explained because it's false.

In a climb or descent you're adding a component of weight (weight times the sine of the climb/descent angle) to drag (for a climb) or to thrust (for a descent). If you want to maintain the same speed, thrust will have to be adjusted to compensate. The steeper the descent, the more forward weight component added to thrust, the less thrust you will need, all the way until this forward weight component equals drag. Then you cannot do a constant-speed descent any steeper, unless you add more drag to absorb even more forward weight component.

In all of these cases, once the angle is established and we've seen the speed not to move, (or, everything is settled down) all the forces are balanced and there is no acceleration.

"how a g-meter can be used in establishing a rate of descent" It can't. At all steady rates of descent (even a million feet per minute) the zenith-nadir value of G is 1, and the aircraft Z-axis value of G is the cosine of the descent angle. (That would make it zero in your previous example of a 90 degree descent)

Yes, precisely. So the way to descend is to reduce power. From what Beardy wrote, it appears he thinks differently.

'…so what’s the point if the idea is to allow higher landing weights?' op #1
An alternative is that RVLs reduce stress on the ground surface, temp and erosion. This could reduce the cost of shipborne operations, not requiring expensive surface coatings or enable greater flexibility in the choice of a landing spot. The latter point could be applied to other landing sites, not requiring significant preparation, but probably not rough strips as per Harrier.

Re lift / pitch during F35 landing, it might not be practical to use higher pitch angles because the proximity of engine exhaust to the ground - erosion, etc. There might be a small beneficial compromise identified from flight test, but how this would be integrated into the specific control system is not clear.

If a simplistic view of F35 VL control is that lateral stick controls left-right motion and the thrust lever fwd-aft; the conventional stick input for pitch, controls vertical rate (thrust), then there is no direct pitch change control.
I suspect that there is some automatic integration within the control law, but again not very much pitch change would be available if the nozzle height is critical. Similarly that wing lift with forward motion / WOD may not decrease the thrust required for landing / nozzle clearance to have any benefit.

Are F35 VLs essentially made at constant attitude ?

What this needs is a current, or ex, CFS QFI, I’m sure one will be along shortly....

The F-35 thread, Mk II
 

I have closed the venerable F-35 Cancelled, Then What ? thread.
It has served its purpose and is getting a bit unwieldy.
I wish to extend a sincere "Thank You!" to all of you who have, over the past decade, contributed to the lively discussion therein.
This is the new F-35 general purpose thread.
Thank you @stilton for getting it started ...

Not at all. I refer to the horizontal frame of reference, not the aircraft, which changes with pitch. When you decrease the power a speed stable aircraft will descend in order to maintain its speed. The initial part of the descent is an acceleration in the vertical plane until a new stable equilibrium is achieved and then the RoD will be constant. The aircraft frame of reference may or may not pitch, swept wings and blown wings tend not to pitch, just sink in the same attitude.
At a constant rate of descent (vertical speed) the vertical component of lift equals weight.
I refer you to Newton's first and second laws of motion. They've been around for a while and are close enough to true to be used in aerodynamics.


Interestingly current CFS basic teaching is not that power controls descent, but that pitch does. ie Point at the numbers and control speed with power. It wasn't always thus (the teaching.)

I would say that is the wrong way round! Gravity is the force of attraction between two bodies

I thought that was pheromones!

Mog

Or it could be argued that gravity is the effect of the curvature of the space/time continuum and not a force at all!!

I used to do rolling vertical landings every day in my four years at Valley.

And sent quite a few chaps on to do just the vertical bit

So what about the F-35A?

FB

<meanwhile in a distant/nearby underground/subsea/suburban/London/volcano-based lair/log-cabin/tent/basement-flat-under-a-pillar-box-outside 221B Baker Street>
"What's that Lassie/Penfold/Oddjob? Some children are stuck down a well? No? There's an SRVL discussion on PPRuNe? Golly! Something must be done! Alert the Peoples Liberation Army Navy (and coincidentally anyone else who reads PPRuNe) for incoming rumours..."

50 KIAS (other numbers are available, but it's not my place to repeat them here) on a wing as big as the F-35B's generates significant lift when compared to the mass of things like (expensive) stores rather than in comparison to the mass of the whole aircraft. I.e. at F-35B/QEC SRVL airspeeds (look them up, or just watch the TV footage carefully), the extra lift is worth having.

At SRVL speeds, waggling the stick fore/aft pitches the nose down/up. The control law (with support from PW/RR) ensures that the flightpath follows, given that the aircraft is mostly jet-borne and hence the change in wing-lift isn't going to give the required flightpath change by itself. If radically changing pitch attitude is seen as a problem close to the deck, then one might design the manoeuvre such that the required flightpath doesn't waggle around much, and back that up with a (not-just-fortuitously) suitable flight control system and appropriate visual landing aids and pilot procedures/training. However, what of actual VLs? Many moons ago, the (at the time) response to longitudinal stick inputs of a simulation of an aircraft involved in the development of control strategies for what (eventually) became JSF, elicited expressions of concern and even criticism from some test pilots who flew it, including TPs from the USA. It was felt that not properly exploiting the wing lift available at modest airspeeds was, well, wrong, and hence having the aircraft pitch in response to stick inputs intended to change flightpath, even at speeds where pitch changes themselves did not do much to flightpath, would be right. A decision was required as to what constituted the minimum speed below which having the nose pitch up and down in response to longitudinal stick inputs was silly ("Chaps, am I doing a VL or not?"). Subtleties were raised. Ships were mentioned. Anxious glances were made at pocket watches as yet more subtleties were raised and the faint sound of covers being removed from beer pumps in nearby villages could be discerned. Faced with this potentially awkward situation, a decision was taken, stuff was implemented, the aircraft flown, positive reviews recieved and the issue was never spoken of again. Well, OK, obviously "Team JSF" reviewed this stuff and designed an appropriate control law including stuff to address lessons learned from the research programme, but you get the gist of it anyway. Probably.

NoHoverstop, thanks for the explanation.

I doubt that our paths crossed. My very limited understanding came from two guest flights in '175' before the new control modes were incorporated; this was for 'experience' of VL as a baseline for simulator assessment of new concepts - "40+ years ago". At that time, and during coffee room discussions with the Harrier project pilot (PB), there was mutual interest in new control laws and pitch (vertical) inceptors.

The Bedford 1-11 had on-board programmable computation for various pitch laws which became the basis of Airbus development, and with reduced stability margins. Also, the aircraft had integrated spoiler - pitch control for DLC where the vertical flight path was adjusted for quick action (stick input), before pitching so that control response would mimic conventional aircraft. The Harrier (VL) team, successfully went their own way.

In a similar time frame there was considerable work on helicopter agility, but I don't recall that this programme overlapped with jet-borne flight.

I am not a pilot , but the F35 is landing just like a helicopter apart from the flair then rolling along and braking .

paul

Gallileo put that idea to bed a long time ago.

If a rolling vertical landing is required because of weight and thrust limitations, how is a go around handled? Is there a minimum height below which a go around cannot be executed without jettisoning stores? ie one is committed to land.

Well if your theory regarding thrust required for a constant descent then clearly the aircraft will just crash. If the other theory is correct then the aircraft will add power and perform a go-around. Jettisoning bombs or firing missiles on the go-around is an unlikely plan of action.

Just This Once, when you're standing on a bathroom scale in your house, what does it read (be it in lb, kg, or stone)?

Say you put that scale in an elevator at the top floor of a tall building, push the button for the ground floor, wait 10-15 seconds for the speed to steady out, and then get on it? What do you suppose it would then read?

Would the readings be a) the same, or b) different?

Vessbot - I didn't introduce the topic of G into the equilibrium of flight forces, so you will need to adjust your aim.