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Hornet stick feeling in the aircraft...


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Hey, Thanks all fr the great inputs!!

 

Aaron, thanks a lot for the intel, it correlates what Curly posted about a center thunk. You should break from it at 3lbs up to 7,4lbs.

Curly, those values you posted are indeed in the document you linked but it also mention a different (lighter) lateral breakaway and pull force.

 

So stick force are lighter in roll then pitch, right? To the values indicated in that table?

 

BR

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Controlers TM f/a 18 stick on Virpil warbrd base, TM cougar f16 stick on cougar base, Cougar F16 throttle on TUSBA, ch pedals, TM cougar MFD

 

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Hey, Thanks all fr the great inputs!!

 

Aaron, thanks a lot for the intel, it correlates what Curly posted about a center thunk. You should break from it at 3lbs up to 7,4lbs.

Curly, those values you posted are indeed in the document you linked but it also mention a different (lighter) lateral breakaway and pull force.

 

So stick force are lighter in roll then pitch, right? To the values indicated in that table?

 

BR

 

TLDR: If you’re looking to model your stick forces via a cam profile. Probably best to model the F-15’s and skip the breakout force dead zone. See Pages 40-43.

https://www.nasa.gov/centers/dryden/...ain_H-1073.pdf

If your stick doesn’t have an adjustable force profile, just leave it be It’s probably fine.

 

 

Yes stick forces are lighter in the roll, that pretty typical for an FCS.

If you’re looking to shape the cam profile here are a couple things to keep in mind. This gets complicated because your trying to model something that may or may not be modeled.

 

Unless the breakout forces are modeled. It will be hard to replicate the effects of the breakout forces (dead-band) without a force feedback stick. The break out forces in the real jet function as a movable neutral zone. In DCS, setting the dead zone just applies area of zero input around center of stick. Once your stick is outside of that dead zone, the input jumps to the next variable up. EG if you set a dead zone of 25. Once Your stick position hits 26 the input to the jet is 26. So you can lose areas of fine command with the dead zone.

 

2. In the real jet the stick moves further aft than forward, 2.5 inches to 5 inches. This also changes depending on the flap setting. 2.5 inches forward and 3.5 aft in half and full.

 

I don’t know how they handle this in game, possibly scaling the forward inputs down. More forward joystick commands less in game stick movement than aft joystick? IE moving your joystick forward 1 inch moves the hornet’s stick forward a half inch. Vs moving your joystick aft 1 inch then moves the Hornets stick Aft 1 inch. You end up with more definition in the forward throws this way BTW.

 

3. The force gradient might not be perfectly linear on the real Hornet and moves with the trim position.

This is better illustrated with some force gradients charts from the F-15. Again nice and open sources NASA stuff.

 

https://www.nasa.gov/centers/dryden/...ain_H-1073.pdf

 

Fig 17 on page 40 illustrates this. It takes more force per centimeter to move stick within 50 centimeters of deadzone than it does outside this zone. And the G command per unit of force as seen in Fig 18 follow suit, Stiffer around the center.

 

What’s interesting on NASA doc I posted earlier is Function 20.

 

https://ntrs.nasa.gov/citations/19920024293

 

Which shows volts (fcs input) to stick displacement. We can see it’s linear. However, I think this is an oversight by the author of the NASA doc. In the older work I also posted https://apps.dtic.mil/sti/pdfs/ADA176333.pdf.

One page 68 the author notes he linearized the longitudinal, lateral, rudder and RSRI gradients. For Function 20 he converts the non-linear equation to y = x (7.0 + .02|x|). Looking at image of Gradient on page 87 of the DTIC doc, we can see the stick gradient is very non-linear.

 

20 years later, NASA uses the linearized gradient, y = x (7.0 + .02|x|) for function 20 in the FCS. Which we know from the older docs is an approximation. However in the Feed forward integrator section of the FCS they use the non linear gradient, y = x (0.06+.0156|x|), for function 115, which is called Nonlinear Stick Gradient. The NASA model also incorporates some of the non linear dynamics left out the older DTIC model (Func 116, 117).

 

Without spending more time than I care to, I can’t tell if they are adding non-linearity to stick gradient there. Which is my hunch because ,The older Function 20 and Function 115 both look like a non linear stick gradient with a small breakout force.

 

Either way, You’re probably best just trying to model those F-15 force to CM displacements. That would get you close enough without force feedback. Which I don’t think is implemented on the Hornet yet.

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3. The force gradient might not be perfectly linear on the real Hornet and moves with the trim position.

 

The stick in the Hornet has linear mechanical feedback after that light breakout force, so for the purposes of making a flight sim device he only needs to consider whether he wants to build something with a 25lb pull to put in his basement. Either way, for the purpose of discussion, the papers you're referencing are just different individual models designed to replicate the real jet's performance, I wouldn't look into the voltage output of the NASA simulator stick too much. Something as simple as using different a different sensor for stick travel would account for the difference. The stick itself is obviously physically linear either way, the difference is in the software-driven stick travel required as defined by the various feedbacks. It creates the impression of changing stick force per g/alpha.

 

Only FBW jets with this control scheme can be fairly represented with a non-FFB stick, so it's good news.

 

Aaron, thanks a lot for the intel, it correlates what Curly posted about a center thunk. You should break from it at 3lbs up to 7,4lbs.

 

Not sure about it being that heavy at the actual stick grip, it's more of a slight nuisance in my experience.

 

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Not sure about it being that heavy at the actual stick grip, it's more of a slight nuisance in my experience.

 

 

Yeah the Cams with center notch that Virpil offers are still ok, you dont feel it that much but for very fine and precise movement like close by formation or AAR, you feel it somehow. On my other base I installed the "thunk" less cams which are great too but have a bit more resistance towards the edges. Virpil also sels a linear cam (with or without the notch) and extra strength springs so I was wondering if I could replicate the feeling of the stick better then what it actually is in my setup. Love that base, it's so precise..

 

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Controlers TM f/a 18 stick on Virpil warbrd base, TM cougar f16 stick on cougar base, Cougar F16 throttle on TUSBA, ch pedals, TM cougar MFD

 

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Modules F18, F16, F86, Mig15, FW 190D9, Nellis range map, Aggr campaign, Middle East map

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the papers you're referencing are just different individual models designed to replicate the real jet's performance, I wouldn't look into the voltage output of the NASA simulator stick too much. Something as simple as using different a different sensor for stick travel would account for the difference.

 

 

You don't understand the content of those papers. They are not referring to a physical simulator with a joystick. So there isn't a "different sensor for stick travel". They're making a computer program to replicate the FCS and the aerodynamics. The inputs are data typed into program, then iterated and output as set of variables, Matrices in the case of DTIC paper. Again, no joystick.

 

The NASA simulation was designed to produce trim shots based on stick position, thrust, ect. For the results to be valid they had to use the actual FCS and it’s governing functions. Which is why they went to such great lengths to publish those.

 

I was wrong about one thing y = x (7.0 + 2|x|) is non linear. It’s Just the graph in the DTIC (MDC) is more exaggerated. That is literally the stick function gradient as defined by the manufacturer.

**Stick Gradient Function**

y = x (7.0 + .02|x|)

 

For half Stick Aft, 2.5 inches

 

18.75 = 2.5 (7.0 + .2|2.5|)

Y/X = 7.5 = Slope of Gradient

 

For Full Stick Aft. (5 inches)

40 = 5 (7.0 + .2|5|)

Y/X = 8

 

 

The slope increases as inputs increases. There is a reason why this done that you don't understand.

 

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You don't understand the content of those papers.

The slope increases as inputs increases. There is a reason why this done that you don't understand.

Quick temper? Not much fun to discuss with...

They are not referring to a physical simulator with a joystick. So there isn't a "different sensor for stick travel". They're making a computer program to replicate the FCS and the aerodynamics. The inputs are data typed into program, then iterated and output as set of variables, Matrices in the case of DTIC paper. Again, no joystick.

Point taken.

 

You're pulling a lot of interesting information from those documents but I think you're missing the point. OP is asking about the physical design of the stick in the Hornet and whether or not a tailored breakout force exists in the physical axis, not software deadbands or modeled nonlinear sensor outputs. Bottom line, the Hornet's stick has a noticeable physical breakout force followed by increasing linear resistance. If he wants his stick to feel roughly like a Hornet he should opt for a cam that requires a slight breakout force.

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...Bottom line, the Hornet's stick has a noticeable physical breakout force followed by increasing linear resistance. If he wants his stick to feel roughly like a Hornet he should opt for a cam that requires a slight breakout force.

 

You guys are breaking out equations and I'm still trying to define 'thunk' :D

 

 

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  • 4 weeks later...
On 11/19/2020 at 7:06 AM, Curly said:

2. In the real jet the stick moves further aft than forward, 2.5 inches to 5 inches. This also changes depending on the flap setting. 2.5 inches forward and 3.5 aft in half and full.

 

I don’t know how they handle this in game, possibly scaling the forward inputs down. More forward joystick commands less in game stick movement than aft joystick? IE moving your joystick forward 1 inch moves the hornet’s stick forward a half inch. Vs moving your joystick aft 1 inch then moves the Hornets stick Aft 1 inch. You end up with more definition in the forward throws this way BTW.

 

Found this thread today and super interested (as I'm at this stage of planning a build as well)

Ran a couple of rough tests in the sim, there's no difference in stick displacement (and minimal difference in FCS displayed data) in any of the flap positions.

 


My question is - if there is a 3lb breakout force, but the stick is force sensing, does that mean you can still make fine control inputs without any actual movement in the stick?

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10 hours ago, HammerUK9 said:

 

Found this thread today and super interested (as I'm at this stage of planning a build as well)

Ran a couple of rough tests in the sim, there's no difference in stick displacement (and minimal difference in FCS displayed data) in any of the flap positions.

 


My question is - if there is a 3lb breakout force, but the stick is force sensing, does that mean you can still make fine control inputs without any actual movement in the stick?

In short the FCS won’t accept input unless it’s greater than the breakout force.
However, there are a few thing to unpackage  and explain here.
First, the stick in Hornet isn’t force sensing. A series of LVDTs sense the movement of the stick and communicate this to FCS computer in the terms of voltage. So the input to the FCS is based on the position of the stick. Not the force exerted on the stick.  There is also a series of springs which provide resistance to the stick's movement. This is called the feel system.

 

When you use the trim buttons, The feel system moves the stick. The FCS and feel system are setup in a way so that the displacement of the stick results in a consistent force irregardless of it’s trim position.  Meaning, no matter the trim position of the stick,  the same amount of force is required to displace the stick a given distance. EG if the stick is trimmed full forward, moving the stick 3 inches aft requires the same amount of force as needed to move the stick 3 inches aft, if no trim was applied. Here is a visualization of that from the F-15’s FCS.

   Force to Stick Postion H-1073 pdf.png

 

The Breakout force is a deadband that moves with the trim position. It’s meant to replicate the slack that exists in many conventional flight controls and eliminate any accidental inputs to the FCS. For a lack of better term the stick is stuck in the trim position until you overcome the breakout force. The FCS isn’t responding to inputs until the pilot overcomes the breakout force and displaces the stick. As is illustrated in this in this document about the F-15’s FCS from NASA.

 

Stick Force to Load Factor H-1073 pdf.png

 

 So the best way to make fine adjustments is with the trim system, Trim inputs bias the FCS and move the stick. Zero trim input means zero bias in the FCS. In the case of the Hornet, zero trim commands 1 g in most conditions. Pressing the trim switch aft will causes the FCS to seek a G greater than one by pitching the aircraft nose up. If you wanted you could trim the aircraft to hold a 2 g turn. And moving the stick out of the breakout position would command the FCS to seek the bias + the input.  So if we trimmed the F-15 to 2 g’s and applied 45 Newtons of force, The FCS will seek close to 6 g. As the input to the CAS is the trim bias (2) + stick command ( ~4g) .

 

However, there are pilots who don’t like the feel of breakout and will trim the aircraft nose down and hold the stick aft beyond the breakout force. Thus any slight force applied to stick moves it and results in an immediate input to the FCS. This way you can make many fine inputs. This technique coupled when coupled with a beefier center spring allows groups to put on impressive displays. 


Edited by Curly
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2 hours ago, Curly said:

In short the FCS won’t accept input unless it’s greater than the breakout force.
However, there are a few thing to unpackage  and explain here.
First, the stick in Hornet isn’t force sensing. A series of LVDTs sense the movement of the stick and communicate this to FCS computer in the terms of voltage. So the input to the FCS is based on the position of the stick. Not the force exerted on the stick.  There is also a series of springs which provide resistance to the stick's movement. This is called the feel system.

 

When you use the trim buttons, The feel system moves the stick. The FCS and feel system are setup in a way so that the displacement of the stick results in a consistent force irregardless of it’s trim position.  Meaning, no matter the trim position of the stick,  the same amount of force is required to displace the stick a given distance. EG if the stick is trimmed full forward, moving the stick 3 inches aft requires the same amount of force as needed to move the stick 3 inches aft, if no trim was applied. Here is a visualization of that from the F-15’s FCS.

   Force to Stick Postion H-1073 pdf.png

 

The Breakout force is a deadband that moves with the trim position. It’s meant to replicate the slack that exists in many conventional flight controls and eliminate any accidental inputs to the FCS. For a lack of better term the stick is stuck in the trim position until you overcome the breakout force. The FCS isn’t responding to inputs until the pilot overcomes the breakout force and displaces the stick. As is illustrated in this in this document about the F-15’s FCS from NASA.

 

Stick Force to Load Factor H-1073 pdf.png

 

 So the best way to make fine adjustments is with the trim system, Trim inputs bias the FCS and move the stick. Zero trim input means zero bias in the FCS. In the case of the Hornet, zero trim commands 1 g in most conditions. Pressing the trim switch aft will causes the FCS to seek a G greater than one by pitching the aircraft nose up. If you wanted you could trim the aircraft to hold a 2 g turn. And moving the stick out of the breakout position would command the FCS to seek the bias + the input.  So if we trimmed the F-15 to 2 g’s and applied 45 Newtons of force, The FCS will seek close to 6 g. As the input to the CAS is the trim bias (2) + stick command ( ~4g) .

 

However, there are pilots who don’t like the feel of breakout and will trim the aircraft nose down and hold the stick aft beyond the breakout force. Thus any slight force applied to stick moves it and results in an immediate input to the FCS. This way you can make many fine inputs. This technique coupled when coupled with a beefier center spring allows groups to put on impressive displays. 

 


 

While I can’t speak directly on the -18, the comparisons made to the -15 in that context are incorrect. The F-15 doesn’t need displacement to react. It literally has a ‘stick force sensor’ located at the base of the stick. It does what the name would insinuate that it does, it senses force pressure put on the stick to drive the CAS inputs to the flight controls. You can freeze that stick entirely and still have flight control movement.  Stick displacement drives cables which run the actuators (CAS on or off), CAS inputs are driven by stick force in the -15. The two are not really comparable.  
 

The F-18, although I do not know exactly how it is integrated, also has a SFS at the stick grip base. Why?  No idea if it uses positional sensors. 


Edited by Rainmaker
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1 hour ago, Rainmaker said:

edit:  Here lies the answer. https://apps.dtic.mil/dtic/tr/fulltext/u2/p002709.pdf

First page, second paragraph. 

-18 was apparently changed during development. 

 

 

Ah thanks for that! I wonder if they kept it then simply because they'd have to redesign the stick or some other mount for the paddle switch.

 

 

3 hours ago, Curly said:

In short the FCS won’t accept input unless it’s greater than the breakout force.
However, there are a few thing to unpackage  and explain here.
First, the stick in Hornet isn’t force sensing. A series of LVDTs sense the movement of the stick and communicate this to FCS computer in the terms of voltage. So the input to the FCS is based on the position of the stick. Not the force exerted on the stick.  There is also a series of springs which provide resistance to the stick's movement. This is called the feel system.

 

When you use the trim buttons, The feel system moves the stick. The FCS and feel system are setup in a way so that the displacement of the stick results in a consistent force irregardless of it’s trim position.  Meaning, no matter the trim position of the stick,  the same amount of force is required to displace the stick a given distance. EG if the stick is trimmed full forward, moving the stick 3 inches aft requires the same amount of force as needed to move the stick 3 inches aft, if no trim was applied. Here is a visualization of that from the F-15’s FCS.

   

The Breakout force is a deadband that moves with the trim position. It’s meant to replicate the slack that exists in many conventional flight controls and eliminate any accidental inputs to the FCS. For a lack of better term the stick is stuck in the trim position until you overcome the breakout force. The FCS isn’t responding to inputs until the pilot overcomes the breakout force and displaces the stick. As is illustrated in this in this document about the F-15’s FCS from NASA.

 

 So the best way to make fine adjustments is with the trim system, Trim inputs bias the FCS and move the stick. Zero trim input means zero bias in the FCS. In the case of the Hornet, zero trim commands 1 g in most conditions. Pressing the trim switch aft will causes the FCS to seek a G greater than one by pitching the aircraft nose up. If you wanted you could trim the aircraft to hold a 2 g turn. And moving the stick out of the breakout position would command the FCS to seek the bias + the input.  So if we trimmed the F-15 to 2 g’s and applied 45 Newtons of force, The FCS will seek close to 6 g. As the input to the CAS is the trim bias (2) + stick command ( ~4g) .

 

However, there are pilots who don’t like the feel of breakout and will trim the aircraft nose down and hold the stick aft beyond the breakout force. Thus any slight force applied to stick moves it and results in an immediate input to the FCS. This way you can make many fine inputs. This technique coupled when coupled with a beefier center spring allows groups to put on impressive displays. 

 

 

Cheers, much appreciated and would make any construction much simpler if there's no force measuring component.

 

NATOPS says:
"2.8.2.2.5 Pitch and Roll Trim Switch. Normally, movement of the pitch and roll trim switch electrically biases the flight control computers and the stick does not move. Little if any pitch trim is required in the auto flap up mode due to the automatic trimming functions within the flight control computers. In MECH, pitch trim moves the control stick fore and aft, changing the stick neutral point. There is no mechanical lateral trim."

 

So I'm assuming your comments regarding deadband moving with trim apply to MECH mode, but in normal/DEL modes the stick will behave much like a home joystick.

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8 hours ago, Rainmaker said:

5F83BBB2-F623-4EFE-AC97-3C7CB71F0FC2.jpeg

182D25B0-2CF8-4C1B-9CDF-44F80165E0C9.jpeg

 

Hi all, really enjoying the conversation in this thread-thanks to those contributing. 
 

On a side note, can anyone tell me what model (A, C, etc) and lot this Hornet shown is? It’s quite different in layout from our DCS recreation. 

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25 minutes ago, Brass2-1 said:

Hi all, really enjoying the conversation in this thread-thanks to those contributing. 
 

On a side note, can anyone tell me what model (A, C, etc) and lot this Hornet shown is? It’s quite different in layout from our DCS recreation. 


Sorry. That was related to my previous post. Thats a -15E cockpit. 

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9 hours ago, HammerUK9 said:

 

Ah thanks for that! I wonder if they kept it then simply because they'd have to redesign the stick or some other mount for the paddle switch.

 

 

 

Cheers, much appreciated and would make any construction much simpler if there's no force measuring component.

 

NATOPS says:
"2.8.2.2.5 Pitch and Roll Trim Switch. Normally, movement of the pitch and roll trim switch electrically biases the flight control computers and the stick does not move. Little if any pitch trim is required in the auto flap up mode due to the automatic trimming functions within the flight control computers. In MECH, pitch trim moves the control stick fore and aft, changing the stick neutral point. There is no mechanical lateral trim."

 

So I'm assuming your comments regarding deadband moving with trim apply to MECH mode, but in normal/DEL modes the stick will behave much like a home joystick.

There are/were😉 a few Hornet pilots around here or some other boards or voice channels they visit. They still remember how the 'thunk' feels, hehe.

I did try a real Hornet (early C) simulator once, decades ago but I can't remember how the 'breakout' felt. 

On a related note, I remember reading some article on spin testing in early Hornets and on top of all this FCS stuff, springs, etc. there are also counter weights that were part of the dampening mechanism, especially with hands off recovery. 

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18 hours ago, Rainmaker said:


 

While I can’t speak directly on the -18, the comparisons made to the -15 in that context are incorrect. The F-15 doesn’t need displacement to react. It literally has a ‘stick force sensor’ located at the base of the stick. It does what the name would insinuate that it does, it senses force pressure put on the stick to drive the CAS inputs to the flight controls. You can freeze that stick entirely and still have flight control movement.  Stick displacement drives cables which run the actuators (CAS on or off), CAS inputs are driven by stick force in the -15. The two are not really comparable.  
 

The F-18, although I do not know exactly how it is integrated, also has a SFS at the stick grip base. Why?  No idea if it uses positional sensors. 

 

The Stick force sensor is integrated in the F-15 FCS as a way to detect command error. That is the difference between the command G (stick position) and the actual G. The hydro mech system of the F-15 is geared to deliver a consistent G per stick displacement based on the air data (dynamic and static pressure). There however areas of transient response where the hydro mech system alone will not deliver the command G.  This where the CAS system comes in play.

 

The stick force sensor in a way double checks what the pilot is asking for and moves the stabs via the CAS servos to meet the G command schedule.  However the CAS system can only move the stabs +- 10 degrees. So what happens when the pilot deflects the stick full aft at 250 knots? The hydro mech system moves the stabs a fixed amount based on the air data. Let just say the basic gearing of the hydro mech system can only deliver 7 g in these conditions.  Well, then the CAS reads the force inputs from the stick sensor and goes, oh the pilot is commanding 8g, here's another 10 degrees of stab. The CAS then further bias the hydo mech system through the CAS interconnect servo. Causing the hydro mech system to move the stabs further than basic air data limits would normally allow. The CAS then goes, well there is still full force on the stick sensor, so we're still commanding 8g, but now were at 7.5 G. So now I'm reducing that +10 stab I gave to 5, and I'm going to bias the hydro mech system a little bit more. This loop continues biasing the hydro mech system until the CAS servos are centered and no longer command further stab movement. The CAS provides immediate error correction and biases the basic hydro-mech system to meet the designed G schedule.  The the stick sensor and load factor are the error signal.

 

But yes you can actually fly the F-15 FBW with the hydro mech system disconnected, using the force sensor alone. However your are limited to +-10 degrees of Stab movement. So the entire flight envelope is not available. 

 

The hydro mech system also incorporates a means to correct for command errors based on stick position too. It's done through the Pitch Trim Compensator. Which is what the CAS is biasing as it makes corrections.

 

Stick inputs are fed into a high-class, accelerometer-controlled servo loop known as the load factor error sensor (LOFES), a part of the PTC. This stick input establishes the neutral or zero point it works around. For example, let us assume that a pilot, or the trim actuator, is holding the stick in a position commanding a load factor of one g. Any subsequent deviation from that setting will be sensed by the PTC accelerometer which will valve hydraulic pressure to the pitch trim compensator piston, repositioning the piston and commanding the required amount of collective stabilator to keep the aircraft at one g. This series trimming capability is true for disturbances created by flap, speedbrake, and landing gear extensions. Acceleration and deceleration are also compensated for, producing an essentially neutral speed stable airframe. Since the trim change we've described is "series," no stick movement is noted.

 

With the CAS off, the aircraft will still try and hold 1 g with the stick in neutral. It will also maintain The stick force to G schedule when the stick is displaced via the air data scheduling of the stabs. However there are areas in flight envelope where it will not be able to do so.

 

The system requires both stick movement and force sensing to deliver the entire flight envelope.

It is not like the F-16's where the force sensor is the sole input to the FCS.

 

F-15_Longitudinal_Control-2.jpg

 

 

The Hornet has the force sensor in the base because the original FCS was supposed to use only the force as FCS input. There were a lot of problems with this implementation though.  You can read about them here. http://aviationarchives.blogspot.com/2015/01/f-18-flying-qualities-development-report.html

 

So the force sensor was replaced with position sensors. In many ways the F-15's FCS is the fore-bearer of the Hornet's. Except the Hornet digitized the functions of hydro mech system. Many of which where simply analog computers. The mechanical ratio changers on the F-15, that geared stabliator deflection based on air data. We're replaced with a computer.

 

Forward Loop.png

 

The CAS hydro-mech interconnect,  CAS Servos, and pitch trim trim controller and pitch trim compensator replaced by a feed forward integrator.


Edited by Curly
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2 hours ago, Curly said:

The Stick force sensor is integrated in the F-15 FCS...

 

 

this is probably a topic better served in a different thread but while I have your ear, here's something I've been wondering about the F-15 and it's pseudo-canard intakes and you seem to know a lot about ol' rodan...do you think that if the intake ramp schedule were shifted slightly, so that they maintain a slight AoA, with the resultant lift forward of the CG and the unloading of the tail, that the Eagle CAS is capable of keeping the plane flying with effectively relaxed stability? or would it need a "real" FBW and perhaps modification of the intake cowl system

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6 hours ago, Curly said:

The Stick force sensor is integrated in the F-15 FCS as a way to detect command error. That is the difference between the command G (stick position) and the actual G. The hydro mech system of the F-15 is geared to deliver a consistent G per stick displacement based on the air data (dynamic and static pressure). There however areas of transient response where the hydro mech system alone will not deliver the command G.  This where the CAS system comes in play.

 

The stick force sensor in a way double checks what the pilot is asking for and moves the stabs via the CAS servos to meet the G command schedule.  However the CAS system can only move the stabs +- 10 degrees. So what happens when the pilot deflects the stick full aft at 250 knots? The hydro mech system moves the stabs a fixed amount based on the air data. Let just say the basic gearing of the hydro mech system can only deliver 7 g in these conditions.  Well, then the CAS reads the force inputs from the stick sensor and goes, oh the pilot is commanding 8g, here's another 10 degrees of stab. The CAS then further bias the hydo mech system through the CAS interconnect servo. Causing the hydro mech system to move the stabs further than basic air data limits would normally allow. The CAS then goes, well there is still full force on the stick sensor, so we're still commanding 8g, but now were at 7.5 G. So now I'm reducing that +10 stab I gave to 5, and I'm going to bias the hydro mech system a little bit more. This loop continues biasing the hydro mech system until the CAS servos are centered and no longer command further stab movement. The CAS provides immediate error correction and biases the basic hydro-mech system to meet the designed G schedule.  The the stick sensor and load factor are the error signal.

 

 

 

The intent of that was to not go down a rabbit hole on the -15s system (this is the -18 forum after all), merely to clarify your comparison to it as far as stick positional movement with that of the -18 isn't comparable.  For the -15's CAS, it's force related, not related to movement.  Is CAS limited in control surface movement without mech input? Yes.  Is stab deflection limited with CAS off?  Only in some instances, so CAS is not always needed for full movement either.  But, the -15s stick doesn't require movement to sense input and make schtuf happen, the -18s stick apparently...does. 

 

See the following

"The FCS isn’t responding to inputs until the pilot overcomes the breakout force and displaces the stick. As is illustrated in this in this document about the F-15’s FCS from NASA."

 

That is the part that's untrue.  I'm not sure what the -15 dissertation that followed was for or attempting to achieve.

 

FWIW, at 250 knots in your example, he/she/it should be at a 1.0 pitch ratio or very close to, so putting the stick in the seat pan expecting more out of the AFCS is pretty futile at that point.  At that speed and thinking you will see 7, 8 ,9 G...well, good luck  The only real thing that CAS is going to increase at that point is rudders (15* CAS off/30* CAS on).  But no sense going down that road either, because again, it's an -18 forum.

 

Manual trim is another topic in itself as it manually operates stab positions through mech cable/pully, as well as resetting the center position.  The -15 also does the same laterally.  The -18 again, non comparable, because it's not using that side of the system under normal operation. 

 

I'm a -15 guy by trade, spent far too many hours crawling over/under/inside parts of her than I can no longer contort my body to get anywhere close to the places I once used to have to reach, so my understanding of the -15s operation is pretty descent.  I have pretty good books available to me when it comes to the rest.


Edited by Rainmaker
clarification
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2 hours ago, Rainmaker said:

 

The intent of that was to not go down a rabbit hole on the -15s system (this is the -18 forum after all), merely to clarify your comparison to it as far as stick positional movement with that of the -18 isn't comparable.  For the -15's CAS, it's force related, not related to movement.  Is CAS limited in control surface movement without mech input? Yes.  Is stab deflection limited with CAS off?  Only in some instances, so CAS is not always needed for full movement either.  But, the -15s stick doesn't require movement to sense input and make schtuf happen, the -18s stick apparently...does. 

 

See the following

"The FCS isn’t responding to inputs until the pilot overcomes the breakout force and displaces the stick. As is illustrated in this in this document about the F-15’s FCS from NASA."

 

That is the part that's untrue.  I'm not sure what the -15 dissertation that followed was for or attempting to achieve.

 

FWIW, at 250 knots in your example, he/she/it should be at a 1.0 pitch ratio or very close to, so putting the stick in the seat pan expecting more out of the AFCS is pretty futile at that point.  At that speed and thinking you will see 7, 8 ,9 G...well, good luck  The only real thing that CAS is going to increase at that point is rudders (15* CAS off/30* CAS on).  But no sense going down that road either, because again, it's an -18 forum.

 

Manual trim is another topic in itself as it manually operates stab positions through mech cable/pully, as well as resetting the center position.  The -15 also does the same laterally.  The -18 again, non comparable, because it's not using that side of the system under normal operation. 

 

I'm a -15 guy by trade, spent far too many hours crawling over/under/inside parts of her than I can no longer contort my body to get anywhere close to the places I once used to have to reach, so my understanding of the -15s operation is pretty descent.  I have pretty good books available to me when it comes to the rest.

 

I felt the need to describe the entire flight contronl system in detail because, they way you talked about it made it seem like that the force sensor was the singular input. It is not. Nor is it the largest input.  The CAS and therefor the stick force transducer, can command at most +-10 degrees of stab.

 

The example was meant to be an illustrative overview of the FCS operation not a literal account of the air data schedule. 

 

Re "The FCS isn’t responding to inputs until the pilot overcomes the breakout force and displaces the stick. As is illustrated in this in this document about the F-15’s FCS from NASA."

 

What is the pitch force deadband network in the CAS and why do you think it exists? If the stick is centered in the 1 G trim position, how much force do you have to apply to overcome the dead band filter and cause the stick transducers to apply voltage to the CAS?   What is the mechanical breakout force required to actuate the hydro mech system, how far does the stick move after that force is applied?

 

The desired result of both systems F-15/F-18 is to provide a constant G per stick force. The sticks are geared, for lack of a better term, to require a specific amount of force to move them a specific distance.  In the F-15 the stick physically actuates  the stab servos to attempt deliver that response. Moving the stick an inch aft takes ~10 lbs of force and is designed to result in a load factor of ~4g, given the right conditions. When the stick force transducers outputs 10 lbs of force, if the G is less than 4, Then previously described Feedback loop begins. So the command load factor is dependent on the stick position the same way it's dependent on the stick force, since they are derivatives of each other. The Hornet operates the same way. It just has a digital position sensor commanding the stab servo and is only physically linked to servo in degrade modes. Moving either stick a fixed distance / force commands a predetermined load factor. Force and distance are coupled.

 

 


Edited by Curly
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2 hours ago, Curly said:

I felt the need to describe the entire flight contronl system in detail because, they way you talked about it made it seem like that the force sensor was the singular input. It is not. Nor is it the largest input.  The CAS and therefor the stick force transducer, can command at most +-10 degrees of stab.

 

The example was meant to be an illustrative overview of the FCS operation not a literal account of the air data schedule. 

 

Re "The FCS isn’t responding to inputs until the pilot overcomes the breakout force and displaces the stick. As is illustrated in this in this document about the F-15’s FCS from NASA."

 

What is the pitch force deadband network in the CAS and why do you think it exists? If the stick is centered in the 1 G trim position, how much force do you have to apply to overcome the dead band filter and cause the stick transducers to apply voltage to the CAS?   What is the mechanical breakout force required to actuate the hydro mech system, how far does the stick move after that force is applied?

 

The desired result of both systems F-15/F-18 is to provide a constant G per stick force. The sticks are geared, for lack of a better term, to require a specific amount of force to move them a specific distance.  In the F-15 the stick physically actuates  the stab servos to attempt deliver that response. Moving the stick an inch aft takes ~10 lbs of force and is designed to result in a load factor of ~4g, given the right conditions. When the stick force transducers outputs 10 lbs of force, if the G is less than 4, Then previously described Feedback loop begins. So the command load factor is dependent on the stick position the same way it's dependent on the stick force, since they are derivatives of each other. The Hornet operates the same way. It just has a digital position sensor commanding the stab servo and is only physically linked to servo in degrade modes. Moving either stick a fixed distance / force commands a predetermined load factor. Force and distance are coupled.

 

 

 


Well, You are pretty far from describing the whole system. But point still standing, they are more dissimilar than similar In the example you used.  To what degree?  It’s the difference between a jet that can still fly and one that doesn’t.  

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