PIO at high alt. and supersonic speed

Operating an F-16 on the ground or in the air - from the engine start sequence, over replacing a wing, to aerial refueling procedures
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saberrider

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Unread post01 Sep 2016, 15:53

How Pilot induced Oscillation at high altitude and supersonic speed are cancelled?
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dragracingmaniac

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Unread post02 Sep 2016, 06:18

Fly lower and slow down? :devil:
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odattner

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Unread post03 Sep 2016, 14:33

I am not aware of PIO at these conditions but in general the best way to cancel PIO is to release controls or stop providing inputs... once you do that you are out of the loop and therefore PIO cannot happen.
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vilters

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Unread post04 Sep 2016, 00:32

Release stick: problem gone.

if you need to : Use manual trim on the pannel to change direction or FL.
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saberrider

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Unread post04 Sep 2016, 05:50

Thank you .
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neurotech

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Unread post04 Sep 2016, 22:43

dragracingmaniac wrote:Fly lower and slow down? :devil:

Don't laugh, the F-16 can experience instability at higher Mach, So slowing down a little might help.

Here is the video of a F-16 at M2.0
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jbgator

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Unread post06 Sep 2016, 02:33

I don't know where you get the info about instability at high alt, high mach and it certainly isn't evident in this video. During Functional Check Flights (FCF), of which I performed hundreds, a test point was to pulse the jet during the mach run (which was done around 40K out to about Mach 2). You would pulse the jet in pitch roll and yaw to insure the FLCS dampened the input quickly as the input was released. The jet was far more stable in those conditions and was actually sluggish as a result in that regime compared to subsonic. In fact the F-16 was the most stable jet I ever flew in that regime, way better than the F-4. To the original question of the thread, the F-16 FLCS is a performance command system. If you have it in neutral trim and make no control input it maintains zero pitch, roll, and yaw rate and will actively move the controls to counter any uncommanded deviation. The pilot is not required to do anything and, as stated, if he is causing the jet to PIO he merely needs to release the controls and it will stop. If he has it out of neutral trim in an axis it will stabilize to the appropriate rate in that axis (Pitch up or down at a fixed rate, roll left or right at a fixed rate, or yaw left or right at a fixed rate which will also produce a slight amount of yaw induced roll). You set the trim to neutral prior to takeoff and do not touch it during flight except to make minor adjustments to a jet that may be slightly out of rig or due to asymmetric stores. It must be trimmed with the gear down in Takeoff and landing gains above about 11 degrees AOA and when the Air refueling door is open but automatically returns to neutral trim setting when the gear are raised or the AR door is closed.

There is a big difference between stability and PIO. While reduced stability increases the chances of PIO it is not required. The only place I saw PIO issues in the F-16 was under the tanker. When the air refueling door is opened the FLCS gains change and the jet becomes far more sensitive to airspeed and trim change is required. Students would get the jet out of trim and get tense under the boom and start pitching up and down. I would take the jet (obviously in a 2-seater) and show them how stable it was if trimmed to AR airspeed by moving into the contact position on the tanker, telling them to turn around and look at me, and put my right hand on top of my helmet. Didn't leave it there long but it got the point across. PIO is not a problem at high altitude and high airspeed unless the pilot has put the airplane way out of trim or there are asymmetric stores loading and even then you have to work at getting it out of shape. Usually with a lot of stores on the airplane it won't go that fast anyway.
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saberrider

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Unread post06 Sep 2016, 06:28

Thank you ,it is clear to me now how hard it is to put F-16 in trouble at high alt.

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