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# The F-35 thread, Mk II

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# The F-35 thread, Mk II

7th Mar 2020, 14:35

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Originally Posted by Just This Once...
Vessbot - I didn't introduce the topic of G into the equilibrium of flight forces, so you will need to adjust your aim.
My last question does not refer to G, but it depends exactly on the equilibrium of forces in downward motion.

Also, you very much did use G in an example of downward flight, as an argument that concludes from the G indicating zero, to that the forces must be unbalanced in order to descend. This was in support of your incorrect statement that "If your vertical lift vector = weight then you are not descending." I pointed out the flaw in it, namely that the G read in that case is not from an instrument that shows the G in the axis under question, and that the actual G is still 1 (but into your straps, not your seat).

Third, even if you yourself didn't introduce G, it is still very much there in any examination of flight forces: it is merely the force in question divided by weight, so we can talk about a generalized aircraft and start with a number of 1 as a matter of convenience.

But no matter, we can use an example weight instead of G, and examine what happens just the same. To that end, I already sensed that you're not comfortable in your position and will use any tactic available to misdirect/avoid the discussion, which is why I tried to cut out the levels of abstraction as much as possible and demonstrate the concept in the most simple and direct way I can, and asked you about your actual weight in my last question.

So, again, are the forces on your body the same or different in steady downward motion as compared to sitting still?
7th Mar 2020, 15:58

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Again, it was suggested to me to watch the g-meter, which was irrelevant and hence my flippant answer.

I'm very comfy with the simple stuff and the difficult. But this is simple, transitioning from equilibrium of 4 forces by lowering the nose with constant power will produce an acceleration in 2 axis - vertical and horizontal. If the desire is for a constant airspeed then either thrust has to be reduced (less power) or more drag added. Your lift vector is now pointing away from the vertical due to your descent angle (or glide angle) but weight remains the same. Down you go, with the lift vector sum less than the force of gravity acting on your weight, initially in in non-equilibrium / accelerated flight. If/when equilibrium is achieved in the descent with both the vertical and horizontal acceleration at zero the descent will continue as the weight vector remains in the same vertical plane but the total lift vector sum is reduced as it is angled towards the flight path, working (partly) in opposition to drag.
7th Mar 2020, 16:41

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Originally Posted by Just This Once...
Again, it was suggested to me to watch the g-meter, which was irrelevant and hence my flippant answer.

I'm very comfy with the simple stuff and the difficult. But this is simple, transitioning from equilibrium of 4 forces by lowering the nose with constant power will produce an acceleration in 2 axis - vertical and horizontal. If the desire is for a constant airspeed then either thrust has to be reduced (less power) or more drag added. Your lift vector is now pointing away from the vertical due to your descent angle (or glide angle) but weight remains the same. Down you go, with the lift vector sum less than the force of gravity acting on your weight, initially in in non-equilibrium / accelerated flight. If/when equilibrium is achieved in the descent with both the vertical and horizontal acceleration at zero the descent will continue as the weight vector remains in the same vertical plane but the total lift vector sum is reduced as it is angled towards the flight path, working (partly) in opposition to drag.
The vectors you talk of are forces which when acting on a mass produce acceleration (Newton's 2nd law of motion.) The situation you describe would produce an ever increasing vertical speed as the mass accelerates vertically. To maintain a vertical speed there has to be no acceleration, so there no force imbalance in the vertical plane.
The lift from the wing is perpendicular to the flightpath and the drag vector, thus has no effect on it.
Next time you set yourself up in the descent at the same speed as level flight, note which way you trim and think why. (hint: you trim nose up to maintain a greater AoA that is needed to provide more lift perpendicular to the wing chord to provide the same vertical vector that you had before you commenced the descent.)

It is a lot simpler just to read a simple guide to aerodynamics, they usually cover Newton's laws and have pictures.

Last edited by beardy; 7th Mar 2020 at 18:02.
7th Mar 2020, 17:50

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Has anyone mentioned translational lift yet? RVLs must benefit from that, not just avoiding hot gas ingestion, pilot's view on approach..
7th Mar 2020, 17:55

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Just This Once, I wrote the below on a commute before I read your last post, which is mostly right with the trig and it seems like you’ve been playing coy on us with a better understanding of equilibrium and acceleration that you had let on. I think in the last sentence you meant to say that weight (not lift) is partly working in opposition to drag. Lift and drag are always perpendicular to each other by definition, so no component of either can add or subtract to the other.

Bottom line, you must agree that weight, no matter the descent rate, remains the same and continues to equal the sum of the upward forces (from jet thrust and wings) in a VTOL vertical descent.

——

A little while ago I identified the exact confusion that’s happening, and there’s a grain of truth in what you’re saying, but not valid in the way that you think. I hesitate to clarify it, because it’s likely that you’ll miss the subtlety and go “aha, I was right all along and here you finally admit it!”

But anyway,

You connected the discussion about forces in vertical flight to the more typical (and ingrained in us) scenario of reducing thrust to introduce a descent in normal airplane flight. It’s of course true that more thrust is required to climb, and less to descend. As I showed earlier, for a descent, that’s a consequence of drag. In an airplane-centered frame of reference, the more descent, the more weight is aligned with your thrust, the less thrust required to offset the drag. Put another way, the more descent, the more drag resists your vertical motion, the less thrust you need.

Your reasoning is: well since descent=less thrust is true in mostly-forward flight with a descent, why not in vertical flight? Let me remind you here that even though thrust is less, all the forces are still in equilibrium!

(The missing thrust now is balanced by drag that wasn’t there before. This is further complicated by that the total drag for the same IAS is still the same. So how can drag be the same yet more at the same time? The total vertical force is now Lift times the cosine of the descent angle, plus Drag times the sine of the descent angle. Or, the same reason that in a quartering crosswind, the headwind and crosswind components don’t add up to the total wind)

If we take it to an extreme case of a vertical descent (still in a normal airplane, 90 degrees pitch into the ground) all this talk of trig and components simplifies out. Total downward force = thrust + weight. Total upward force = drag. For a constant downward speed, since we must be in equilibrium, Thrust+Weight = Drag. Simple as that. Still in equilibrium, still at 1G (into your straps, not indicated on the G meter).

Now we move into a VTOL in purely vertical descent. Being that it obeys the same laws of physics, as long as it’s descending at a constant rate, the forces must still be in equilibrium, meaning that the total upward force must still equal the total downward force. Now there is only one downward force, weight, which is unchanging. The upward force is of course there, and part of it is thrust, with the remainder being drag (drag now acting upward). (It’s the same 3 forces as the last scenario, but with thrust acting upward instead of downward.) Weight = Thrust+Drag.

What this means is that the faster the descent rate, the higher the drag, the less thrust is needed. But the total upward force (thrust+drag) is still the same! The subtlety is that at descent rates near landing, the descent rate is miniscule compared to the other scenarios. The associated drag is correspondingly miniscule, and essentially all the weight is balanced by thrust. But keep in mind that even if you take an exact accounting, exactly all the weight is balanced by the total upward force.
7th Mar 2020, 18:01

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Originally Posted by beardy
Next time you set yourself up in the descent at the same speed as level flight, note which way you trim and think why. (hint: you trim nose up to maintain a greater AoA that is needed to provide more lift perpendicular to the wing chord to provide the same vertical vector that you had before you commenced the descent.)
This is not true. If speed stays the same, AOA will stay the same (actually, a very slight reduction as explained in the next paragraph), and there is no trim due to speed change or anything to do with the vertical motion. There is trim for the thrust-pitch couple, which maybe up or down, or nothing. In my current plane for example, I have to trim down in this scenario.

The missing vertical vector that you (correctly) identify a need for, is provided by a drag component (which is now inclined upward) and not lift. And Just This Once is correct in saying that lift (not the sum of the vertical forces, just lift itself) is now reduced, since its opposing component of weight is reduced. Both are multiplied by the cosine of the descent angle. (They’re both reduced the same way, by the way, for a climb angle.) This reduction is extremely slight (.998 G for a steady 3 degree descent) so is rarely talked about in pilot training materials. Easier to ignore, without significantly affecting anything.
7th Mar 2020, 18:08

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7th Mar 2020, 20:28

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PrOOne - thank you - I was losing the will to live on this one. And I am now really glad that I didn't get sent to CFS.

Mog - nice to see a sensible post from you yesterday!! Pheromones!! New word for me!! When we shared a cabin on (OK, in) HERMES, I had no idea that you were excreting pheromones. I thought it was methane related, but you are never too old to learn!!

RVL's - In the GR5/7/9 era, an RVL did make a serious comeback weight increase. With the 67 deg barn door flaps, even 50 knots forward speed made some serious lift on the wing - plus the proximity of the flap to the rear nozzles generated some real induced lift - Bucc guys will understand the benefits of this - but the Fowler flaps with the induced airflow worked really well. I seem to remember about a 4000lb increase above hover weight.

No doubt the QFI's who have taken this thread over will have an opinion - or several!! And will also argue about the benefits or otherwise of Fowler flaps, with or without blown air!!

I have absolutely no idea if the F-35 benefits the same way as the Harrier 5/7/9/AV8B did.

As to all the other stuff in recent posts about vertical/horizontal and other forces - I used to think that I could fly aeroplanes - I still think that I can - but all this has now made me doubt it all, so maybe I should give up recreational flying and worry more about avoiding Coronavirus!
7th Mar 2020, 21:16

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H - just a tad concerned about the attraction between two bodies bit, 'pheromones', you, Mog and a cabin on Hermes....................
8th Mar 2020, 07:33

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Sorry for the digression.

Was/is there a point in Harrier operations when a go around from RVL was/is not an option? ie insufficient thrust available to arrest descent and accelerate horizontally to gain more wing lift? Is there a similar point for F35 ops?

BTW CFS was fun, but in a different way.
​​

Last edited by beardy; 8th Mar 2020 at 09:04.
8th Mar 2020, 12:40

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While watching the RN getting its F-35s up to speed is interesting, I found the ongoing Finnish fighter competition more informative.
According to Aviation Week, the F-35 sales effort was with a 4 aircraft detachment, but did not shine. ( https://aviationweek.com/defense-spa...er-trials-data )
Only two F-35s arrived, but one went unserviceable and the remaining unit was unable to complete all the flights.
It seems that even with 500 aircraft produced, the type is still well short of operational reliability.
8th Mar 2020, 12:58

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Originally Posted by 42go
H - just a tad concerned about the attraction between two bodies bit, 'pheromones', you, Mog and a cabin on Hermes....................

At least I didn't nick the Captain's bath!!
8th Mar 2020, 17:19

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I only borrowed it!!
9th Mar 2020, 13:06

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Originally Posted by beardy
Gallileo put that idea to bed a long time ago.
Why do you say that? Nothing about what that experiment aimed to proved (and which may or may not have physically taken place) contradicts what I said, and entirely agree with Newton's laws of universal gravitation and his second law of motion.
9th Mar 2020, 15:25
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Came past Culdrose this morning on the road to Gweek and was rather interested to see an F-35 lurking in a hangar - disappointed to learn it's likely to be a mock up to assist with ground handling training!
9th Mar 2020, 17:53

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As an ex-A2 and long-lapsed BSc(Aero Eng), the main point which springs to my mind is that with an aeroplane such as the F-35B which has significant thrust vectoring, variable wing borne and jet borne lift, classic Thrust / Lift / Weight / Drag relations aren't wholly relevant once it starts to get into RVL territory.

Amazingly clever and seamless transition from normal speeds to RVL speeds - all done by FM, I guess. (F*****g Magic!).

There's a really good article in the Feb 2020 edition of Aeroplane concerning the VAAC Harrier and it role in perfecting the control method for the F-35B. Written in language which even those who haven't had the benefit of the secret handshake and other CFS rituals might just understand! Also an article on the BAC 221 and another on the absurd SNECMA Coleoptere.

10th Mar 2020, 08:22

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Originally Posted by etudiant
While watching the RN getting its F-35s up to speed is interesting, I found the ongoing Finnish fighter competition more informative.
According to Aviation Week, the F-35 sales effort was with a 4 aircraft detachment, but did not shine. ( https://aviationweek.com/defense-spa...er-trials-data )
Only two F-35s arrived, but one went unserviceable and the remaining unit was unable to complete all the flights.
It seems that even with 500 aircraft produced, the type is still well short of operational reliability.

Only one aircraft doing what it can. Where others had many and even AEW. Does anyone really think the F-35 won't be chosen as the best aircraft, when everything is assessed?

It had 90% availability at red flag
https://www.janes.com/article/94680/...very-milestone

Also 90% on deployment.
https://www.airforcemag.com/deployed...istics-system/
F-35s from Hill’s 4th Fighter Squadron deployed to Al Dhafra Air Base, United Arab Emirates, for six months last year. The jets almost instantly began conducting airstrikes while 70 percent of the fleet was able to conduct its mission, said Brig. Gen. David Abba, director of the Air Force’s F-35 Integration Office. By the end of the deployment, that rate had climbed to more than 90 percent.

Last edited by golder; 10th Mar 2020 at 08:47.
10th Mar 2020, 08:40

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Originally Posted by golder
Only one aircraft doing what it can. Where others had many and even AEW. Does anyone really think the F-35 won't be chosen as the best aircraft?

Be great to see their configuration control matrix for all these "at a clip" software updates
10th Mar 2020, 09:17

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Came past Culdrose this morning on the road to Gweek and was rather interested to see an F-35 lurking in a hangar - disappointed to learn it's likely to be a mock up to assist with ground handling training!
RN School of Flight Deck Ops got four mock-ups a couple of years back.

http://www.naval-technology.com/unca...ining-5856094/

10th Mar 2020, 10:01

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Originally Posted by weemonkey
Be great to see their configuration control matrix for all these "at a clip" software updates
I have unbounded confidence that LM and JPO, can think up lot's of ways for the partners and FMS buyers to be billed.
Though it does bode well for open software architecture, that will allow this. There won't be a tranche 1.