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Salute!

I Go with Vark-breath.

Found my GD manuals ( pre-Dash 1, pre any USAF dashes).

As Vark says, a spring "commands" the ISA to a "neutral" position when there's no hydraulic pressure. My diagrams don't clearly show how this works, but there's a few things on the ISA hydraulic diagram that might explain things.

The diagram shows two check valves - one from each active hydraulic system, and associated doofers for each check valve called a "restrictor, horizontal tail only". It also shows two power pistons that the pressure works with. Redundant, as either can move the "power ram" that is connected to the control surface.

Two anti-cav accumulators per hydraulic system are in the series.

So looks to me that what I was doing was moving the tail until both pressures were equal. The check valve on one hyd-sys kept fluid from moving that way, and when I finally compressed the anti-cav doofers enuf on the other hyd-system piping, the sucker wouldn't move anymore.

I would bet that the position was what GD thot was "neutral".

Great discussion, and if anyone can find the diagrams, might help.

What would really help would be to find a bird with the drooping slabs and/or flaperons and push them up to see if they ever lock.

Gums sends ...

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Would this help? (F-16A)

Thanks for the schematic Kaasjager. I see the spring in question that causes the liquid lock.

Good luck to you guys in EURO 2008 Applause

VarkVet wrote:

Thanks for the schematic Kaasjager. I see the spring in question that causes the liquid lock.

Good luck to you guys in EURO 2008 Applause

No problem dude. And thanks, we'll need allot of luck to win from France and Italy. *fingers crossed*

Dusting off some memory here... Each ISA has two little valves on it that can be moved by hand if the panel is removed and you have access to the ISA. Gravity eventually causes the flaperons to droop and the slabs to generally sag nose-down (leading edge of the slab up), even though all flight controls are commanded to neutral as the engine winds down.

You can easily push the surfaces back to neutral, but no further without actuating one of the two valves on the ISA. If you activate the proper valve, you can then push the surface into the "up" position (flaperon trailing edge up, slab leading edge down). Once in those position, you can easily push the surface back to the neutral position. However, you must actuate the other valve on the ISA to get the surface to go back to the down position.

Knowledge of this feature on the ISAs has saved at least one dumb airman's arm when it got caught between the flaperon and the fuselage as the engine wound down and his buddy grabbed a speed handle, uncovered the ISA, and actuated the valve to allow the flaperon to bleed back to the down position and freeing his arm.

Newbie Joined: Apr 10, 2008 Posts: 8 Status: Offline

Re: Locking ISAs

<<I don't know how to describe it with technical accuracy like you wrenchbenders. All I know is there were these "claws" around the actuators and they were normally held open if there was hydraulic pressure to the ISA.>>

Sorry, no "claws" involved. The ISA is made of two parts... the Actuator, and the Servovalve. Togther, they become an Integrated Servo Actuator, or ISA. With no hydraulic pressure applied, i.e. a parked jet, the hydraulic passages inside the Servovalve will allow the Actuator to be mechanically moved towards the commanded position (with zero volts present, the "commanded position" is approximately neutral. So long as there is fluid inside the ISA, but not pressurized, you can walk up to a jet and push up on a flaperon or push up on a horizontal tail trailing edge, and the surface will move towards neutral. Once neutral, however, you will be unable to move it back to where you found it. Sort of like a Chinese finger-trap. The locking at neutral is hydraulic in nature, and no "claws" are involved. The same thing will happen if you lose both hydraulic systems during flight Sad

You are gonna crash, but before the jet hits the ground, aero loads will have moved all five ISAs to their neutral positions. Notice I'm being careful about "neutral" versus "zero". The neutral position of the flaperon ISA is 1.5 degrees trailing edge up. The feedback linkage rigging is set this way during ISA installation. The neutral position of the horizontal tail ISA is 2 degrees trailing edge up, also set via feedback linkage rigging during ISA installation. The neutral position of the rudder ISA is zero rudder. When the jet is up and running, the FLCC sends the needed electrical commands to put the surfaces where it wants them; the mechanical rigging positions are transparent to the observer with FLCS online.

Newbie Joined: Feb 28, 2008 Posts: 13

I was reading about Gums' assymetric LEF condition and would like to bring up that for numerous F-16's delivered in the early 1980s, the LEF Asymmetry brakes were not installed due to reliabilty problems with the brakes locking. There was a jumper plug installed in it place which jumped the circuit around to make the FLCS think the LEF were symmetric. Once the improved assymetry brakes were available, they were retrofitted to all aircraft. So if the brakes were not installed and you broke a torque tube or rotary actuator, the LEF's had to be locked by the pilot, otherwise the good side LEF would continue to operate.

TNX, Viper,

I am not sure what all was disconnected, as the troops who left out the pin for the drive tube connect may have also left other things "undone".

All I know is that the left LEF was still moving all around while the right one was up 60 degrees or so.

I bunted over to get the left one up to minus 2 deg and hit the "lock" switch. At least I had one less doofer moving around.

To this day I can't believe the thing actually flew. Only after some analysis did they tell me I only had about a pound of control authority for roll. But you know the old saw, "so far, so good", and I got it back to the place I took it from my crewchief.

Gums

TJSmitty wrote:

Where does the broom handle and tie string come in to play???

(it's a "B-shop" thing..... Very Happy

)

Smitty

The string is hung on the AOA Probes so than when the broom handle is rotated, the AOA probes rotate, thus telling the jet its experiencing variations in AOA, and causes the LEF to rotate/move. I think the mechanical nature of the PDU and how the torque tubes, Rotary actuators are setup that they are somewhat "locked", but not with a mechanical lock short of the A-Brake (atleast on Block 25's that I'm aware of).

Enthusiast Joined: Dec 10, 2003 Posts: 99 Status: Offline

Purplehaze wrote:

Kaasjager. wrote:

VarkVet wrote:

Correct … the asymmetry brakes are an electrically actuated mechanical lock (fun to rig) designed to automatically shut the LEFs down if the flaps attempt to split.

Don't remind me, can take all afternoon to rig them. Laughing

How many folks here used a paper clip to rig them when they found out support didn't have the rigging tool?

Also Gums I worked on block 5's, 10's, 25's, 30's and 50's and I may be wrong but all the ISA's worked the same way. With that said I still think it is a liquid lock rather then mechanical, prove me wrong and next time I'm down south I'll buy you your favorite liquid......!!!!!

Purple

Purple, Don't remind me....
I always kept a paper clip just for that. I don't remember support actually having any rigging tools...
Vipers for me were Blocks 5 10 25 30 32 40 42... and yes I seem to recall all of the ISA's worked the same way. Might still have my FLCS CDC's here with the diagrams and descriptions too...

William G

Enthusiast Joined: Dec 10, 2003 Posts: 99 Status: Offline

TJSmitty wrote:

Where does the broom handle and tie string come in to play???

(it's a "B-shop" thing..... Very Happy

)

Smitty

Smitty.
Remember to pull the probe heat breakers ok...
Always fun when you watch your boss going on about safety, and they forget that little fact!!!

"Ok Will, watch as we rotate the probes, the LEF will move too... What's that smell???"

HAHAHA Always fun that was...

William G

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Forum: F-16 Design & Construction

Ok so LEFs, how are they actually operated?