Forum: F-16 Design & Construction

just to clarify some stuff.... The viper only lands btwn 11 and 13 AOA...11 being the norm. 15 is the stall/scrape the speedbrake attitude. As far as the LEF are concerned they are a function of mach, aoa, AND altitude. They are always auto to maximize lift and/or decrease drag. Exceptions : WOW on both ldg gear, throttle at idle and MLG speed is greater than 60 knots, asymetry brakes r locked, or flcs is in stby gains. Smile

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Edit. Sorry.

it's been a bit....but, i could have sworn that LEF's actuated based on NOSE WOW...NOT main WOW.

Last edited by falconfixr1 on Feb 19, 2009 - 06:10 PM; edited 2 times in total

but this could be my imagination...i expedited the last six years of my career...everyone else did the hard work

The LEFs are stowed at 2deg UP whan weight is on the MAIN wheels. They then rotate down to optimal angle as soon as weight goes off the mains during takeoff. Their function is transparent to the pilot. They retract to the -2 position as soon as weight's on the wheels upon landing.

This function was changed from 0-deg when the first prototype made its first "flight" during a high-speed taxi test back in 1974 and they discovered the airplane liked to fly too much. They also changed the engine tail feather scheduling at idle thrust for the same reason.

It's been a long, long time since I've done a programmed gains check on the F-16, but I think that's where you'll find LEF movement as a function of AOA/mach number. I remember you take weight off wheels, hook up a TTU-205 to both the pitot tube and the alpha probe and set the AOAs according to the book. Oh yeah--you also raise the gear handle to UP with hydraulic power on the jet. Thank goodness for gear pins! Pretty interesting check, if you ask me.

i understand the function of LEF....i could've sworn, however, that it was tied into the NLG WOW. But I may be thinking that way because AOA is....(first to go is your memory!)

I hope this will clarify LEF scheduling during flight (WOW effects not shown). I did not put in the breakpoints for angle of attack because I can't remember them. To determine LEF position at any flight condition, just find the point where the angle of attack and the interpolated mach number meet. For example, mach 0.7 lies half way between mach 0.6 and 0.8.

You can see that cruise at low AOA has a LEF position of -2 deg for all mach numbers.

SixerViper said:

Quote:

This function was changed from 0-deg when the first prototype made its first "flight" during a high-speed taxi test back in 1974 and they discovered the airplane liked to fly too much. They also changed the engine tail feather scheduling at idle thrust for the same reason

That's not true. The YF airplanes always had 0 deg as max up position of the LEF. The -2 deg change was incorporated on the FSD airplanes. The engine nozzle changes on the YF were done to reduce idle thrust. The airplane simply taxied too fast and would over heat the brakes.

The only YF change made due to the flight 0 event was to reduce roll sensivity gain with gear down.

johnwill, Any chance you remember what T.O that information is in? Is it in a Job Guide or a F.I. Is it different in any block? I am really curious about this guys and appreciate your input.

i was speaking right outta the -1 for 40/42

deadseal,
As far as I know everything you said is OK, except altitude does not affect the scheduled position. I think there is also a "LEF Lock" switch which allows the pilot to lock the LEF at its current position.

gastonrivera78,
I have never seen the LEF schedule in any TO, but then the -1 is the only TO I'm familiar with. The schedule shows up in several different engineering design documents, both flight control and structures. If you need the schedule, I can probably get one for you.

As far as I know, that schedule applies to all blocks, with one minor exception. Back very early in the program, it was found the the power drive unit would "stall" under some circumstances (which can't be explained in a few words), thus putting the LEF in the wrong position and possibly over-loading it. The solution was to increase the torque capability of the drive so it would not stall. That took time to design, manufacture, and retro-fit. By modifying the schedule, we could prevent the stall, but that compromised the effectiveness of the LEF. So the modified schedule was installed in all airplanes until the new drives were installed, then the normal schedule was re-installed.

Thank you for the offer johnwill, but not necessary, I just go to work and look at the -1. Cool to know if I see some fault related to this in the future. Thank all of you for the new info, like I said before, learn something every day.

falconfixr1 wrote:

i understand the function of LEF....i could've sworn, however, that it was tied into the NLG WOW. But I may be thinking that way because AOA is....(first to go is your memory!)

Nah, its the mains. Every time we do an LEF ops check we always have to open MLG WOW circuit breakers so they will follow the AOA's when they are rotated.

http://www.youtube.com/watch?v=x9Yww1-tYmM

good video, you can actually watch the LEF's schedule down on T/O roll...

And in that video (time = 21 sec) it looks very much like the LEF goes down when the NLG lifts off.

Forum: F-16 Design & Construction

The purpose of the part in front of the wing