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

beardy 6th Mar 2020 14:38


Originally Posted by Just This Once... (Post 10704876)
Zero feet per minute.

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

Vessbot 6th Mar 2020 14:44


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.

"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)

ShyTorque 6th Mar 2020 14:45


Originally Posted by Vilters (Post 10704873)
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.


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

safetypee 6th Mar 2020 14:58

'…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 ?

Trumpet trousers 6th Mar 2020 15:08

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

T28B 6th Mar 2020 15:23

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 ...

beardy 6th Mar 2020 15:35


Originally Posted by ShyTorque (Post 10704886)
Yes, precisely. So the way to descend is to reduce power. From what Beardy wrote, it appears he thinks differently.

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.)

212man 6th Mar 2020 16:18


You say (correctly) that "gravity is an acceleration," which generates a force
I would say that is the wrong way round! Gravity is the force of attraction between two bodies

Mogwi 6th Mar 2020 16:40


Originally Posted by 212man (Post 10704971)
I would say that is the wrong way round! Gravity is the force of attraction between two bodies

I thought that was pheromones!

Mog

PaulH1 6th Mar 2020 17:17

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

jindabyne 6th Mar 2020 18:04

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

Finningley Boy 6th Mar 2020 18:51

So what about the F-35A?

FB

NoHoverstop 6th Mar 2020 19:38

<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..."


Originally Posted by stilton (Post 10704422)
The approach speed is so slow it brings up the obvious question though, I think itís about a 30 knot overtake, assuming 20 knots wind over the deck thatís around 50 knots IAS

At such a low speed I donít see how youíre generating any worthwhile, additional wing lift so whatís the point if the idea is to allow higher landing weights ?

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.


Originally Posted by safetypee (Post 10704899)
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 ?

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.

safetypee 6th Mar 2020 21:52

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.



Pauljw 6th Mar 2020 23:22

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

paul

beardy 7th Mar 2020 03:24


Originally Posted by 212man (Post 10704971)
I would say that is the wrong way round! Gravity is the force of attraction between two bodies

Gallileo put that idea to bed a long time ago.

beardy 7th Mar 2020 04:44

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.

Just This Once... 7th Mar 2020 10:32


Originally Posted by beardy (Post 10705511)
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.

Vessbot 7th Mar 2020 13:34


Originally Posted by Just This Once... (Post 10705710)
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?

Just This Once... 7th Mar 2020 14:10

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


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