The purpose of the part in front of the wing

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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yasirbhojani

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Unread post17 Feb 2009, 05:34

Hello,
I have been noticing since a long time that whenever F-16 lands, there is a part in front of the wing that faces down but as soon as the F-16 touches the ground, it faces back to the normal. For further explainaition of what I am talking about, I found a picture. I want to know that what is it called and what is its purpose and why it is mostly used on landing.
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Yasir Bhojani
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Raptor_DCTR

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Unread post17 Feb 2009, 05:58

Those are the LEF's (leading edge flaps). They are there to produce extra lift to the wings during high AOA and low airspeeds. The reason they are up on the ground is they are wired to the left and right main WOW (weight on wheels) switches. They are schedule to -2 degrees on the ground. The only time they move on the ground is during FLCS self test or during LEF ops checks. In flight they are scheduled as a function of AOA and airspeed. It's been a while since I've worked on the Viper but I believe what you are seeing in the picture is a +15 degree angle for take off and landing config. Conversely they will be scheduled at -2 degrees during take off until weight is removed from either main landing gear WOW switch at which point they drive to +15 until airspeed increases. Like I said it's been a while but I think the operation range of the LEFs is -2 to +20 degrees. On analog FLCS jets (block 30/32 and below) LEF scheduling is accomplished through the ECA (electronic component assembly). On digital FLCS jets (block 40/42 and above) it's done through the DFLCC (digital flight control computer). Hope that helps, if I missed anything help me out guys
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yasirbhojani

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Unread post17 Feb 2009, 06:10

Is there any situation in which it might be used at a degree of +20 ???
If yes, which one's ???
And i no, then what's the need of it ???
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Yasir Bhojani
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yasirbhojani

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Unread post17 Feb 2009, 06:46

Please, can any one of you people label this picture with all the control surfaces ???
And yah, if F-16 has stabilators and they completely move, then what is the need for a further control surface just on the stabilator ??
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Yasir Bhojani
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johnwill

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Unread post17 Feb 2009, 06:52

Raptor DCTR's explanation is right except the LEF is scheduled by AoA and Mach Number, not AoA and airspeed, and the operating range is -2 to +30 degrees.

For the LEF to have a deflection of 20 or 30 degrees, the airplane must be subsonic at a relatively high AoA, say 20 or 22 degrees. The speed must be subsonic since the LEF is always at -2 deg when supersonic.

The LEF helps to increase lift, and it also moves the center of lift aft. At low airspeeds, that increases departure resistance, which is a fancy way of saying it helps to keep the airplane from pitching up out of control.

If you are talking about the forward section of the stabilator (horizontal tail), it isn't a movable surface. The line which looks like a control surface hinge line is simply a structural break between the basic tail and a removable leading edge. The LE is very thin and sharp, so can be damaged easily. It needs to be removable to make replacement easy.
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johnwill

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Unread post17 Feb 2009, 07:54

The horizontal tails (stabilators), rudder, trailing edge flaps, and the leading edge flaps are outlined. However, the leading edge flaps are not control surfaces. The LEF responds to airplane motion, but does not control it.
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yasirbhojani

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Unread post17 Feb 2009, 15:56

Ok, thanks buddy. But I still have one query. You didn't outlined the part in front of the tail. Is it too a structural break ???
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Yasir Bhojani
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Guysmiley

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Unread post17 Feb 2009, 16:22

yasirbhojani wrote:Ok, thanks buddy. But I still have one query. You didn't outlined the part in front of the tail. Is it too a structural break ???


Re-read this:

johnwill wrote:If you are talking about the forward section of the stabilator (horizontal tail), it isn't a movable surface. The line which looks like a control surface hinge line is simply a structural break between the basic tail and a removable leading edge. The LE is very thin and sharp, so can be damaged easily. It needs to be removable to make replacement easy.


And johnwill, that's an awesome job outlining the surfaces, you should be getting a kickback from some kind of LM outreach program! :D
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johnwill

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Unread post17 Feb 2009, 18:47

Thanks, Guysmiley. I already get a kickback from LM every month in the form of a retirement pay check.

Yasir,
Glad to help. Here's another tip - sometimes you will hear the trailing edge flap called a "flaperon" because it combines the function of flaps and ailerons. Although the F-16 horizontal tail combines the function of an elevator and aileron, no one calls it an "elevon". However, "elevon" is the name used for trailing edge control surfaces on a delta wing airplane.
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ViperDude

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Unread post17 Feb 2009, 20:32

That minus 2 degrees on the LEF is about 1 3/4 to 2 inches when measured, as I would just put my index finger up there and measure from the tip of my finger to the 2nd joint...lol.

Here is one for you Johnwill, the first time I climbed in the F-16 XL I noticed these 4 letters on the flight control panel..L E A F, bring back any memories?

Cheers,

ViperDude
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johnwill

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Unread post17 Feb 2009, 21:22

Sorry, ViperDude, I never looked at the XL flight control panel. Can you give me a clue?

So you used your finger to check out LEF position? Maybe that was the first digital scale!
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r2d2

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Unread post17 Feb 2009, 22:11

I hope it was not to mean Leading Edge Actuator (or some other word) Fault :lol:
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MechFromHell

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Unread post18 Feb 2009, 00:09

johnwill wrote:Maybe that was the first digital scale!


:lmao:
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lamoey

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Unread post18 Feb 2009, 03:10

From working on Block 15's the LEF would normally stay at 0 degrees unless flight conditions or AOA required something else, except above mach 1 it would move up to -2 degrees.
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johnwill

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Unread post18 Feb 2009, 07:56

The YF-16 LEF never went above 0 deg at any time. However, I remember the FSD airplanes and all production airplanes as having -2 deg as the normal position at low AoA at all mach numbers. I checked through all the photos in the F-16.net photo gallery and found all airplanes, including block 15, showed the LEF up 2 deg while sitting on the ground. That may not be conclusive, but it indicates that's where the LEF is at low AoA.

The thing about moving up to -2 deg above mach 1 does describe the symmetric movement of the flaperon, not the LEF. The transition from 0 deg to -2 deg begins at 0.95 mach and is completed at 1.05 mach.

The purpose of moving both surfaces up in supersonic flight is to create a nose up pitch moment, which reduces the down trim load on the horizontal tail. Less down load on the tail means less lift required and less induced drag and trim drag. Everybody wins. The nose up pitch moment is created by a little more up load on the LEF (up 2 deg) and a little more down load on the flaperon (up 2 deg).
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