Forum: F-16 Design & Construction

[Link pending approval] i am new to this site and was curious abt this question. Hope someone can answer my Q's.

Was wondering how is the AOA related to the LEF and how it works. I have a rough idea on the basics flow of information from the AOA. From AOA to CADC to DFLCC to PDU down to your Torque Shafts.

Im new in [Link pending approval] Am a crew chief in the 425th AMU (Black Widows) stationed in LUKE AFB,AZ. Which is a Singapore run squadron. Am doing a small project on the LEF system. Was hoping someone could give me a very detailed breakdown on how it works.

bezulba Idea

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the system is actually quite simple, You aoa's provide info to the dflcc/cadc of what angle the jet is in in terms of positve or negative attack angle and the dflcc computes what inputs are need to maintain control of the jet. The dflcc sends a signal to the command servo assy mounted on the pdu. the command servo is the electronic brain of the lef system. It recieves the signal from the dflcc and transmitts that signal in the form of a control arm connected to the pdu. for high aoa the control arm will cycle to the right and the pdu in turn will provde the hyd power to drive the lef down to create more lift, this power comes from the hyd motors spinning the #1 tourque shafts which feed to the angle drives and then down in to the lef. the lef has 4 rotary actuators which are connected by 4 tourqe tubes. They provide balenced power to drive the lefs up or down. When a low aoa signal is recieved the command servo arm will drive back to the left and the pdu motors will drive the lefs back to the 0 degree or streamlined position or 2 degrees up if the wow's are engaged. the entire purpose of this system is to provide more lift for the jet when flying at slow speed, and when manuvering at any speed. hope that helps you

Both of you guys better take a look at the 27GS.

Best Regards,

Joe, with the new ITODS T.O.'s a lot of the info on LEF operation has been taken out of the GS. I mean A LOT of info. It really doesn't help with theory of operation anymore. I was looking through the LEF part a couple months ago and was amazed how little info there is. I called some people about it and all they said was "You still got 7-levels out there? They can teach everybody." Are you kidding me?!?!?!?!?! What happens when those red-xers are gone?

Quote:

drive the lefs up or down.

The LEFs can actually go up?

They are positioned 2 degrees up on the ground and can be positioned full up by screwing with the system and pulling certain CBs. It happened to one of our jets a few months back, destroyed the flap seals and messed the torque shafts and A-brakes up. I'm not exactly sure what they did to cause this but there were some people standing in front of the MXG CC in blues explaining it to him.

Thanks for the [Link pending approval] really appreciate it. I did look at the 27GS but is does not relly tell you how the LEF works.

So what really happens to the LEF in these situations.

1) A/C going to take off.
2) When the nose wheel is off the ground
3) When all three landing gears of the grond.
4) Normal flight and supersonic flight.

Is it true that the LEF moves 2 degree up during supersonic flight.
whats the reason for this.

bezulba Idea

There is more involved with positioning the LEF than Angle of Attack. Mach number is also a player. There are equations in the DFLCC to take input of mach and AOA to calculate required LEF angle. If you plotted a graph of the equations (AoA horizontal axis, LEF position vertical axis) you would see that at low AoA, the angle is a constant -2 degrees (minus is up). As the AoA increases, there is a break point for different mach numbers, and the LEF moves down at a certain number of degrees for each degree of AoA. The LEF is not commanded to move above mach 1. In the analog airplanes (anything older than block 40), the LEF scheduling is not controlled by the FLCC, but by something called the Electronic Component Assembly (ECA).

With gear down, the LEF moves down to a fixed position, 30 degrees I think.

The LEF provides more lift by allowing the wing to go to higher AoA without flow separation and increased drag. The LEF moving down also reduces drag at a given AoA and moves the center of lift aft, which reduces the possibility of pitch-up and loss of control.

LEFs are positioned 15 degrees down for take off and landing. Roll commands are also dampered while the gear is down to prevent over compensation. It's been a while since I've looked at schematics but I think the LEFs are scheduled to 15 degrees when wheel speed hits 60knots (or the nose wheel leaves the ground, it could be the mains too I can't remember which) at which point nose wheel steering is disengaged.

Raptor_DCTR wrote:

They are positioned 2 degrees up on the ground and can be positioned full up by screwing with the system and pulling certain CBs. It happened to one of our jets a few months back, destroyed the flap seals and messed the torque shafts and A-brakes up. I'm not exactly sure what they did to cause this but there were some people standing in front of the MXG CC in blues explaining it to him.

This happens when people don't follow tech data when rigging the command servo to the PDU! Never seen it myself but I have heard some first hand accounts! They described it as "sounding expensive." LoL

Raptor_DCTR wrote:

Joe, with the new ITODS T.O.'s a lot of the info on LEF operation has been taken out of the GS. I mean A LOT of info. It really doesn't help with theory of operation anymore. I was looking through the LEF part a couple months ago and was amazed how little info there is. I called some people about it and all they said was "You still got 7-levels out there? They can teach everybody." Are you kidding me?!?!?!?!?! What happens when those red-xers are gone?

he's right the only good fi and gs we still have is the 80, there is so little left on the 27 that applies to Crew chiefs that it really isent worth spending the time on it. And yes I understand alot more goes into the lef scheduling besides aoa I just dident wnat to really confuse the kid

The shortage of info applies to all AFSCs, not just CCs. It's especially troublesome for specs. BTW I see you handle is a1378, I take that as a/c 84-1378 a block 25 from the 309th right?

negative 91-1378 block 50 55th fs

Roger

bezulba,

The LEF is at -2 degrees at low angle of attack at all mach numbers. When sub-sonic, the LEF will start down at some AoA depending on mach number. When super-sonic, the LEF stays at -2 degrees for all AoA. To understand why, you need to know that on the F-16 the center of lift is forward of the center of gravity when subsonic. This would cause the nose to pitch up, so the horizontal tail is positioned trailing edge down, creating an up load on the tail to balance the airplane. That up load on the tail reduces the lift required from the wing and fuselage, thus reducing the drag on the wing and fuselage. When super-sonic the wing lift moves aft, behind the center of gravity, which requires a down load on the tail to balance the airplane, requiring more lift from the wing and fuselage, and more drag. By keeping the LEF at 2 degrees up supersonic, the wing lift moves forward to reduce the amount of download on the tail, and reducing drag. In addition, the trailing edge flap is raised 2 degrees up, moving the wing center of lift further forward, to further reduce the download on the tail and reducing drag.

Bottom line, the LEF (and TEF) is at 2 degrees up super-sonic to reduce drag.

Sorry I can't explain it more clearly, but it is a complex system, operating with complex aerodynamics.

Forum: F-16 Design & Construction

BLK 52 LEF operation