LERX vortex asymetric airflow?

Always wondered why the F-16 has a tailhook, or how big a bigmouth F-16's mouth really is ? Find it out here !
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by johnwill » 17 Apr 2015, 00:32

Patriot wrote:Only the first photo feels convincing to me. Fact is vast majority of photos revealing F-16s while turning (sudden pitch increase) show that right lerx vortex is quite longer and more pronounced than the left one is..
I'm guessing... Maybe it may have something to do with sidestick in the Viper. Maybe while making aft input on the stick the pilot's forearm axis is quite a bit off neutral center for the joystick axis which translates/results to/in adding a little roll input (to the right) at every pitch. Which then causes such an effect as asymetric, right lerx vortex being longer (stronger) on most occasions. Therefore, It may be a tendency. What do you think ??


You might be interested in a little ancient history concerning your point about the sidestick. If you already know this, sorry for wasting your time.

When the YF-16 was first flight tested in 1974, test maneuvers were flown that required "pure" stick inputs, either pitch or roll. It was very difficult to do that, first because the stick was rigid and provided no sensory feedback to the pilot, and second because of the offset stick as you mentioned. When trying to input a pure pitch command (no roll) almost every time some left roll was applied. Why? Because the pilot was not pulling straight back along the airplane x axis, but was pulling along his strongest path, toward his right shoulder. When pure roll commands were tried, usually some pitch command was accidentally applied. When the Full Scale Development flight tests were started in 1977, the stick command axes were rotated 15 degrees clockwise (looking down on the stick). I don't remember if the stick sensors were rotated or if the rotation was done with sines and cosines in the flight control computer, and it doesn't matter. In addition, a small amount of stick motion was allowed for better feedback (+/- 1/4 inch I believe). The result was that to input pure pitch command, the stick force was back and to the left, along the rotated axis. The test pilots found it much easier to provide the pure commands needed for flight test maneuvers. In normal operational maneuvers, where pitch and roll commands are almost always mixed together, the pilots had no trouble quickly becoming accustomed to either the original axes or the rotated ones.


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by Patriot » 17 Apr 2015, 11:01

Firstly.. Never heard of it.
Secondly.. I feel that might be and probably IS the answer to this subject.
Third. Thank you!
Fourth. I've seen here some time ago smoewhere a drawing of the relative mutual position of the stick in the cockpit.
I don't know but if I'm recalling it correctly the stick's x axis was 12deg (or 12,5deg) clockwise off plane's x axis. Can't say the Block though.


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by stratosfear » 02 Oct 2015, 11:22

Definately, a left roll input added a little bitty..

T/n #4056
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by 35_aoa » 03 Oct 2015, 02:33

I don't know if this is a real phenomenon, or why it happens, but as for the point about the sidestick, I can at least attest that much is true. It is a challenge, at least initially, to make pure aft stick movements……I probably put in a little right wing down/right roll unintentionally on at least my first 30 takeoffs in the thing.


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by johnwill » 03 Oct 2015, 15:19

Sounds like for your personal physiology and feedback, tbe stick axis rotation was too much. Ten degrees might have worked better for you. Like many things, the axis rotation was.a compromise.


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by stratosfear » 03 Oct 2015, 15:53

What else was a compromise John?


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by johnwill » 03 Oct 2015, 19:45

I was speaking of things like wing area, sweep, tail area, and so on. Take wing area for example. Everyone wants more area for better low speed maneuverability, but more wing means more weight, more drag, more cost. Many trade studies are run to try to get the most benefit for the least negative effect.


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by stratosfear » 04 Oct 2015, 22:51

johnwill wrote:Many trade studies are run to try to get the most benefit for the least negative effect.

Sometimes it's take 400 attempts to make the #401st the one! ;)
Btw, did you know John that the Viper actually is the only flying thing on the Planet Earth that has the afterburner length equal with the length of itself.. I doubt if GD engeeners at the verge of 60s ever planned it this way but end result is spectacular, agile and frankly speaking sexy - frpm every damn angle! These gentle, subtle yet aggressive lines and curves.. It's made like a glove, it's cosy. You not get into it, you wear it! Is there anything that could have been done better with the Sixteen at all ? Ps. No other fighter tears up the air with those Vortice as well... even at the smallest minimum aoa generated they comes off easily.. (photo) Personally what I love about the Viper the most is this bubble frameless single piece canopy and a beautiful line of it - credits goes to the one of it's kind Mafia ;)

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Last edited by stratosfear on 05 Oct 2015, 09:19, edited 1 time in total.


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by johnwill » 05 Oct 2015, 02:00

Glad you are pleased with the Viper. As one of the 60's GD engineers, I can tell you we were not in the least concerned with the appearance of the airplane. our total focus was the make it work, sell it, and build it so we could keep our jobs and keep buying shoes for the kids.


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by stratosfear » 05 Oct 2015, 09:11

Just out of curious.. always been wonder..
Who draw this beautiful genius outline of the actual Viper first? Harry H. somebody else? Or you maybe just were blowed the concepts in air tunnel one by one and someone finally said: This is it gentlemen, We've got it! ?....


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by sprstdlyscottsmn » 05 Oct 2015, 13:55

I'm sure every single aspect of the shape was determined by design parameters. And much the same as with the Mk.1 Spitfire, when your design parameters are to build a Sierra Hotel gunfighter and nothing else (Okay the Viper was allowed wingtip 'Winders) the result is eye-watering.
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by basher54321 » 10 Oct 2015, 18:55

John - taking the fun out of aircraft design since the 60s :P


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by johnwill » 10 Oct 2015, 20:17

:lmao:


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