F-35 and Airshows

Discuss the F-35 Lightning II
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fbw

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Unread post04 Jun 2019, 00:02

With a twin tail, buffeting may be unavoidable as AoA increases. I recall the 1979? Study of the effect of buffeting on the F-15 at high angles of attack, I’ll put it up if I can find it (I forget the severity, or conclusions). IIRC, they studied the fatigue issues with buffeting on the F-22 vertical tails, like the F-15 before (and F/A-18?).

Price to be paid for yaw control at high AoA, vortices are going to interact with tails (and bursting vortices as AoA increases is going to create strong buffeting). Wonder at what AoA the F-35 begins to buffet compared to the F/A-18E/E?
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steve2267

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Unread post04 Jun 2019, 03:20

viper12 wrote:Why not install a stall horn on the F-35 ? Would it be too distracting or not effective enough compared to buffeting ? There may also be some advantage to let the fly-by-wire suppress buffeting for extending the life of the wings, but better get the opinion of a structures engineer.


While I have not studied buffeting as it pertains to very high Angle-of-Attack flight, I am fairly certain buffeting is occuring due to flow separation at high alpha, and the vortices that are created and shed from, in the F-35's case, nose/fuselage chines and the "lip" at the top of the air intake as it blends back into the leading edge of the wing. On other aircraft, vortices will form, roll up from, and shed from the strakes or leading edge root extensions (LERX) such as exist on the F-16 and F/A-18.

I don't really think fly-by-wire or flight controls, despite the magic that the F-35 CLAW creates, can "suppress buffeting." The buffeting is a natural result of the air flow over and around the airframe at high angles of attack.

Also, the flow may separate or vortices shed from different locations along the airframe port vs starboard, and there is nothing that I am aware that can control that -- short of building in geometry that "trips" or causes the airflow to separate at the same place. BUT... I would imagine the geometric discontinuity required for such a "trip" would be bad ju-ju for stealth shaping.

I believe the transonic rolloff that appeared in flight testing and is a momentary event (though the sky was falling if you listened to the F-35 detractors at the time), was also related to an asymmetric flow separation phenomena that occurred at one point in the transonic flight regime.

ETA: I don't think this was a problem with the Viper, because the single tail did not afford the Viper enough lateral or yaw stability to operate at these really high angles-of-attack. I seem to recall the Viper was alpha limited to 25°, and these buffet / flow separation phenomena may not present themselves until 35-40°. But this is pure speculation on my part and I may be wrong. And the more I think about it as I type this... FBW may be more on the correct path than I... that vortices shed from the forward fuselage interacting with twin tails, especially canted outwards, as found on the F-35, F/A-18, Mig-29, SU-27 etc may be more responsible for buffeting than airflow separating off the LERX / fuselage chines. Or it may be a combination of both. But I still don't think flight controls can eliminate the buffet.
Take an F-16, stir in A-7, dollop of F-117, gob of F-22, dash of F/A-18, sprinkle with AV-8B, stir well + bake. Whaddya get? F-35.
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quicksilver

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Unread post04 Jun 2019, 07:31

In simple terms, buffet is a natural consequence of aerodynamic shaping and flow separation from lifting surfaces as aoa is increased. Good lay analysis above but a little too much emphasis here on one (vertical) tail versus two tail as a determinant of buffet.

The straight-wing Piper Cherokee (one vertical) I flew in my youth would buffet very noticeably and very predictably with increasing intensity as one flew slower and eventually stalled. Demonstration of the ‘seat of the pants’ feel for that buffet (and stall) was an important (vital!) part of pre-solo flight training in the Cherokee, as it was later (with much greater practice and emphasis) in Naval Aviation flight training in the T-34 and T-2C (also fairly ‘conventional’ cross-section, straight-winged aircraft with a ‘hard’ leading edge). In fact, there were features of sections of the leading edge designed to create flow separation (and the consequential buffet cueing) on an inboard section of the wing earlier (at a lower aoa) than the outboard section and thereby warn you before you stalled and potentially lost roll control (and in some cases rolled off and entered a spin). Stall recovery and/or roll-off avoidance and later spin recovery were life saving teaching points (that you were graded on repetitively).

Things changed when we got to the TA-4J; it had a swept wing and a ‘soft’ leading edge. At typical maneuvering speeds the TA-4 provided a really nice nibble of buffet of slightly increasing intensity just before the slats came out. When the slats came out, the jet got smooth again even with increasing aoa; when it started to buffet again, it was ‘telling’ you that it didn’t have anymore aoa/lift to give you. When you stalled it straight ahead, it didn’t roll off, it just developed a very high rate of descent (that might almost get you killed if you were distracted in the landing pattern on a pitch black night in south Texas).

The Harrier II and the Eagle (both McAir products of roughly similar vintage) had a hard leading edge with significant geometric twist. Although the aoa numbers at which it occurred were different, the buffet characteristics ‘felt’ very similar in both aircraft (I had the good fortune to backseat in a two-seat Eagle on a number of occasions). The first nibble of buffet told you where your best energy turn was without looking; from there on, more pull would get you more turn but with much more buffet (and energy loss) until you got to the point where the nose just stopped moving (and your back teeth were almost chipped).

The point I make here is that buffet isn’t necessarily a bad thing; it is simply a consequence of the aerodynamic features (design compromises :shock: ) of a given design. One vertical stab or two is (used to be anyway) a clue as to how much alpha you’re gonna get from the design — not (directly) how much buffet you will have. Lotsa design features gonna play in that equation.
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spazsinbad

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Unread post04 Jun 2019, 08:07

'quicksilver mentioned above: "...Things changed when we got to the TA-4J; it had a swept wing and a ‘soft’ leading edge. At typical maneuvering speeds the TA-4 provided a really nice nibble of buffet of slightly increasing intensity just before the slats came out. When the slats came out, the jet got smooth again even with increasing aoa; when it started to buffet again, it was ‘telling’ you that it didn’t have anymore aoa/lift to give you. When you stalled it straight ahead, it didn’t roll off, it just developed a very high rate of descent (that might almost get you killed if you were distracted in the landing pattern on a pitch black night in south Texas)…."

I was reluctant to comment because 'type of aircraft flown' but this comment by 'QS' hit the spot. We were advised to fly the A4G 'on the burble' (as a general rule). Woe betide 'getting the slats out' because of the problem of asymmetric slats (which would cause an instant role - remember the A-4 has a 720 degree per second roll rate at best role speed). The instructor demonstrated the 'straight ahead dirty configuration at low level' stall. Talk about fast DOWN ELEVATOR speed. Jeepers I was convinced. As I mentioned earlier flying at 1,000 feet on a BLACK night at sea was really difficult for the first time ever in the circuit for another CCA (GCA) for my second night carrier landing. :shock: Concentrate concentrate. :shock:
RAN FAA A4G Skyhawk 1970s: https://www.faaaa.asn.au/spazsinbad-a4g/ AND https://www.youtube.com/channel/UCwqC_s6gcCVvG7NOge3qfAQ/
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Unread post04 Jun 2019, 12:14

It was reported that the F-35 has more buffeting than the Super Hornet at high AoA. At one point some years ago, they were going to have the FCS limit the F-35 to 50 deg AoA. I don't know where that went with the current FCS. In any case, what else can pull 9g with 2x2,000lb bombs at 50deg AoA? With or without a bit of buffet.
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shania

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Unread post04 Jun 2019, 16:31

Cockpit video with commentary

Capt. Olson describe each maneuver and explain what goes into flying the new demo

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Unread post06 Jun 2019, 11:58

F-35 vs J-31 - USA vs China Fighter Competition- https://www.youtube.com/watch?v=po9y2MkMRc8

RAN FAA A4G Skyhawk 1970s: https://www.faaaa.asn.au/spazsinbad-a4g/ AND https://www.youtube.com/channel/UCwqC_s6gcCVvG7NOge3qfAQ/
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gc

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Unread post06 Jun 2019, 12:04

spazsinbad wrote:F-35 vs J-31 - USA vs China Fighter Competition- https://www.youtube.com/watch?v=po9y2MkMRc8



No comparison. Unable of high alpha low speed and no equivalent to pedal turn and dojo drift. And the F35 accelerates vertically out of most manoeuvres using its sheer power. And this is just talking about kinematic performance, not stealth, sensors and sensor fusion.
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Unread post06 Jun 2019, 13:20

J-31, easy to detect, just follow the smoke plume. :devil:
F-35 even the "no smoke" is stealth. :D
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sferrin

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Unread post06 Jun 2019, 13:30

That's the orginal with the engines of a Fulcrum. Think X-35. This is the current version, and it will (eventually) have better engines:

lLxNMLp.jpg
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Unread post06 Jun 2019, 16:02

It will get better, but really isn't in the same league.

Comparatively speaking, it'll be underpowered, its stealth won't be as good and the fuel/weapons load will be far inferior. They'll be no STOVL version, albeit I can see it on their carriers. Export potential? If they can get it to be even marginally stealthy, it should sell to countries like Pakistan. Which could be a very big problem for India...

I'd say the race now is really who (China or Russia) can produce a stealthy export first. This aircraft will be a far greater threat to the US (and our allies), if nothing else because it'll be produced in far greater numbers than the J-20.
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Unread post06 Jun 2019, 16:25

sferrin wrote:
lLxNMLp.jpg

I like the looks of this 下-35!
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Unread post06 Jun 2019, 16:57

FC-31 doesn't even have full support of PLAAF and PLAN. It's mostly funded by Shenyang themselves since they want to try to woo customers after they lost J-XX to Chengdu.
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sferrin

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Unread post06 Jun 2019, 19:07

disconnectedradical wrote:FC-31 doesn't even have full support of PLAAF and PLAN. It's mostly funded by Shenyang themselves since they want to try to woo customers after they lost J-XX to Chengdu.


That changed recently.

https://aviationweek.com/defense/avic-s ... -after-all
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ricnunes

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Unread post06 Jun 2019, 19:49

Well, in the end isn't Chengdu and Shenyang basically the same thing: "Companies" that belong to the same owner - The Chinese government? :wink:

Anyway, I can see potential for this FC-31:
- A very big threat to any 4th and 4.5th gen fighter aircraft.
- A cheaper aircraft than the J-20 and Su-57 which would give it a much bigger export potential and the potential of being manufactured in bigger numbers which thus would make it an even bigger threat.
A 4th/4.5th gen fighter aircraft stands about as much chance against a F-35 as a guns-only Sabre has against a Viper.
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