F 16 AB applying at the slow speed increase AOA.

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 post25 Jun 2017, 09:03

In my Sim when I'm slow speed flying (under 200kt's)shallow dive and apply AB to compensate decrease in speed , my F16 A blk10.will pitch up and increase momentarily the AOA further more regardless speed increase up. It's this the case in real F16?
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boilermaker

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Unread post25 Jun 2017, 23:31

Power is meant to keep altitude

Pitch is what controls speed.

Apparently the software is translating power inputs into vertical rates of climb and higher AOA

Not sure if the F16 does that. But the F16 being fly by wire can be programmed to do any kind of things with power input or control inputs.

You could use the stick backward to create AOA without any gain of altitude while the software adjusts power to maintain flight path. Or a stick pull back could translate directly in increased elevation without any increase in AOA. Side stick inputs could be programmed by the pilot to translate into sideway slips without change in heading etc.

This was the experiment done on the AFTI. 3rd year Aero engineering students learn to simulate (ie. design simulators) to accomplish this stuff on a regular basis.
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35_aoa

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Unread post26 Jun 2017, 00:40

boilermaker wrote:Power is meant to keep altitude

Pitch is what controls speed.


Kind of, when we are talking about approach/landing gains of the flight controls. Obviously you need enough thrust to maintain level flight or you begin a descent in any regime, but in a really adaptive digital FBW/CBW aircraft (such as the F-16 or F/A-18), I can chop the throttle(s) to idle, and while I decelerate gradually, the FCS/FLCS is going to adjust the control surfaces accordingly, so that I maintain roughly the same altitude in 1G unaccelerated flight.......ie they will dig in the LEF/TEFs in order to increase CL so that I don't actually descend. At some point, the flight control surfaces reach their limit of effectiveness, and a descent will begin. That is a really long way of explaining that in a jet, outside of the approach to land configuration, power controls airspeed, and pitch controls altitude. In a BFM engagement, right up against the lift limit of the airframe (25 AoA for the Viper), one could correctly argue that once again, power controls altitude, pitch controls airspeed. But that is really the only other regime where this holds true. If you fly a true on speed AoA approach in the Viper, you are still influencing the nose a lot more for up/down glideslope corrections than you are in a purpose built AoA approach aircraft such as the Hornet. And even the Hornet, you are influencing the nose a lot with up/down power corrections. It is really only the Super Hornet, where you can get away with pure power on/power off for glideslope corrections without an additive longitudinal stick movement to stabilize the change. That's probably really deep down in the weeds, but if anyone cares, that is my analysis of how the 3 different aircraft compare in this sense.
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rheonomic

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Unread post26 Jun 2017, 06:32

One comment is that a statically speed stable aircraft will pitch up in response to a perturbation in speed. As the nose raises, this will tend to make the aircraft want to slow down and return to its trimmed state. (This is of course a simplistic criterion and in practice aircraft flying qualities will be designed to MIL-STD-1797B.) There's also a phenomenon called "Mach tuck" where aircraft tend to nose down when going supersonic, but that doesn't apply on an approach.

Since flight control laws were brought up, I'll add some comments. (Can't resist.) Digital flight control systems ("fly-by-wire") offer a lot of possibilities, as now I can make a Command Augmentation System (CAS) where control inceptor inputs command arbitrary states. E.g. on the right hand inceptor (i.e. the "stick") I can have the FLCS interpret longitudinal stick movements as pitch rate commands at low speeds and AOA/Nz commands at high speeds. For lateral stick movements, body axis roll rate is commanded at low speed and stability axis roll rate is commanded at high speed. (This is because stability axis roll rate rolls around the velocity vector, so the AOA is preserved; at low speeds, AOA is poorly defined so we roll about the body axis, where 90 deg roll converts AOA to sideslip). (In the absence of rudder input the FLCS generally regulates the sideslip angle to maintain coordinated flight.) These allow the pilot to precisely control the aircraft attitude. Generally, the pilot will control the Left-Hand Inceptor (i.e. "throttle") but there are autothrottle modes where the FLCS controls the throttle, e.g. for a speed or Mach hold. Based of these "inner-loop" CLAWs autopilot modes such as speed/Mach holds, altitude holds, waypoint following, etc. can be created.

These kind of rate command systems are good for general flying, but we can do better for certain flight conditions, e.g. the FLCS on the F-35B for STOVL modes and DFP/MAGIC CARPET for carrier approach. Let's say we want to make a CAS that makes it easy to capture a glideslope and align with the runway. We can design a system where longitudinal stick commands the flight path angle (FPA ~= Pitch - AOA) to capture a desired glideslope and lateral stick commands the crosstrack velocity. Then, the pilot can precisely position for an approach. (Might want to transition to an AOA command system prior to flare...I'm being general here and not going into too much detail...) For speed control, an autothrottle hold can be used to maintain the desired approach speed.

Generally speaking, a control system can track the same number of commanded states as there are independent control effectors.
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