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-   -   The F-35 thread, Mk II (https://www.pprune.org/military-aviation/630295-f-35-thread-mk-ii.html)

stilton 6th Mar 2020 02:47

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 ?



Traffic_Is_Er_Was 6th Mar 2020 03:04

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

BrogulT 6th Mar 2020 05:11

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

212man 6th Mar 2020 07:57


Originally Posted by BrogulT (Post 10704467)
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.


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

ShyTorque 6th Mar 2020 08:15

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.

beardy 6th Mar 2020 08:22

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?

ShyTorque 6th Mar 2020 08:44


Originally Posted by beardy (Post 10704580)
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.

beardy 6th Mar 2020 10:33


Originally Posted by ShyTorque (Post 10704599)
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!

Just This Once... 6th Mar 2020 11:07

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

Davef68 6th Mar 2020 11:09

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.



57mm 6th Mar 2020 11:20

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

beardy 6th Mar 2020 11:29


Originally Posted by Just This Once... (Post 10704713)
Gravity is an acceleration. If your vertical lift vector = weight then you are not descending towards the herrings.

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°

Just This Once... 6th Mar 2020 11:55

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.

beardy 6th Mar 2020 12:28


Originally Posted by Just This Once... (Post 10704754)
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.

Vessbot 6th Mar 2020 14:06


Originally Posted by Just This Once... (Post 10704754)
Last time I rotated your concept and pointed the nose at the nadir my G meter seemed pegged at zero. Maybe it was broken.

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.

Just This Once... 6th Mar 2020 14:12

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.

Vessbot 6th Mar 2020 14:18


Originally Posted by Just This Once... (Post 10704854)
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.

ShyTorque 6th Mar 2020 14:21

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

Vilters 6th Mar 2020 14:28

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.


Just This Once... 6th Mar 2020 14:29


Originally Posted by beardy (Post 10704779)
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)

Zero feet per minute.


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