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Pitch CAS in F-15C is implemented wrong


KlarSnow

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To any ED Flight model peoples. I will put a track up tomorrow if needed, I believe your pitch CAS implementation in the F-15C is incorrect and is making the aircraft harder to control than it should be at low speeds and high AOA.

The Stick should get heavier (require more aft movement) to generate higher AOA as you get slower with the pitch CAS on currently you have the opposite. With Pitch CAS on the stabs are fully dug in at half stick travel as the jet decelerates below 250 knots. This results in a loss of pitch control authority and results in the jet pitching up heavily uncommanded as you slow down with a frozen stick.

The jet is easier to control in the high AOA regime below 250 knots (25-30 and above AOA) with the pitch CAS off than with the pitch CAS on. This is incorrect. You should not be able to sustain greater than 35 AOA without holding the stick fully aft. If you slow the jet in game below 250 knots with the stick half aft the AOA will increase until you are stalled and pegged at nearly 40 AOA without moving the stick. This is commensurate with an uncommanded increase in the stabilators that is putting them at or near the full pitch up limit. This is in effect reducing the effective movement of the stick by half or more. This makes it very very difficult to maintain or sustain any AOA between 25 and a full stall, and makes unloading from a fully loaded maneuver require you to nearly center the stick since the stabs are barely moving as you move the stick.

I am happy to take this to a PM to discuss this further and why specifically this is wrong. I do not think a track will help since I believe the issue is a confusion about how the pitch CAS system works, and thus how it is implemented.

This is from Eagle Talk, a magazine published in 1974 about the incoming F-15 Eagle. Can be found online easily. This is opposite how Pitch CAS currently works in the game.

Screen Shot 2022-06-13 at 1.47.14 AM.png

Here is further on in Eagle Talk Vol 1 where it talks about high AOA characteristics and stalls, the key thing here is it says stalls are stabilized at full aft stick both CAS ON and CAS OFF, This is not how it works in DCS currently, you can be fully stalled at half aft stick with the CAS ON

Screen Shot 2022-06-13 at 1.52.29 AM.png

Screen Shot 2022-06-13 at 1.53.18 AM.png

here is a quote from the Preliminary F-15A-1 from 1975, this line or something similar to it is in section 6 of every single F-15-1

Screen Shot 2022-06-13 at 1.02.06 AM.png

Screen Shot 2022-06-13 at 12.58.11 AM.png

This is saying that with CAS on or off, the nose should get heavier as you increase AOA regardless of speed, this is what the CAS washout means, the CAS is providing extra authority for stick movement, so it washes out as you slow down so that the stick is heavier as you slow down. This is explained better in newer  -1's that are just after rule 1.16 but are available on the internet, including an F-15A-1 from 1986 that is easily found but I will not link here.

Currently the FC3 F-15C works exactly the opposite to this with CAS on, as you slow down the Nose gets lighter and you barely need half aft stick to hold a stabilized stall.

here is ED's line about the pitch CAS on the F-15C modules store page, This is directly contradictory to what is said above. The damper is also washed out in authority, and the stick gets heavier which increases your pitch control precision at higher angles of attack.

Screen Shot 2022-06-12 at 11.15.28 PM.png

I added a track of the incorrect behavior, I limited my stick saturation so that I could not unintentionally pull more than half aft stick, and did a max AB turn with the stick at half aft. With the pitch CAS on the aircraft immediately pitches all the way up to 40 plus units of AOA and continues to stall. With the Pitch CAS off It stabilizes at roughly 35 units of AOA and does not stall. After a couple of turns demonstrating this I did a turn where I turn the pitch CAS on and off in the turn and you can see the jet swap between dug in and not as the Pitch CAS goes on and off. Again the Pitch CAS on should be washing out its inputs as we increase AOA, not increasing them. 

F15CPitchCasBug.trk


Edited by KlarSnow
added track
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4 hours ago, KlarSnow said:

To any ED Flight model peoples. I will put a track up tomorrow if needed, I believe your pitch CAS implementation in the F-15C is incorrect and is making the aircraft harder to control than it should be at low speeds and high AOA.

The Stick should get heavier (require more aft movement) to generate higher AOA as you get slower with the pitch CAS on currently you have the opposite. With Pitch CAS on the stabs are fully dug in at half stick travel as the jet decelerates below 250 knots. This results in a loss of pitch control authority and results in the jet pitching up heavily uncommanded as you slow down with a frozen stick.

The jet is easier to control in the high AOA regime below 250 knots (25-30 and above AOA) with the pitch CAS off than with the pitch CAS on. This is incorrect. You should not be able to sustain greater than 35 AOA without holding the stick fully aft. If you slow the jet in game below 250 knots with the stick half aft the AOA will increase until you are stalled and pegged at nearly 40 AOA without moving the stick. This is commensurate with an uncommanded increase in the stabilators that is putting them at or near the full pitch up limit. This is in effect reducing the effective movement of the stick by half or more. This makes it very very difficult to maintain or sustain any AOA between 25 and a full stall, and makes unloading from a fully loaded maneuver require you to nearly center the stick since the stabs are barely moving as you move the stick.

I am happy to take this to a PM to discuss this further and why specifically this is wrong. I do not think a track will help since I believe the issue is a confusion about how the pitch CAS system works, and thus how it is implemented.

This is from Eagle Talk, a magazine published in 1974 about the incoming F-15 Eagle. Can be found online easily. This is opposite how Pitch CAS currently works in the game.

Screen Shot 2022-06-13 at 1.47.14 AM.png

Here is further on in Eagle Talk Vol 1 where it talks about high AOA characteristics and stalls, the key thing here is it says stalls are stabilized at full aft stick both CAS ON and CAS OFF, This is not how it works in DCS currently, you can be fully stalled at half aft stick with the CAS ON

Screen Shot 2022-06-13 at 1.52.29 AM.png

Screen Shot 2022-06-13 at 1.53.18 AM.png

here is a quote from the Preliminary F-15A-1 from 1975, this line or something similar to it is in section 6 of every single F-15-1

Screen Shot 2022-06-13 at 1.02.06 AM.png

Screen Shot 2022-06-13 at 12.58.11 AM.png

This is saying that with CAS on or off, the nose should get heavier as you increase AOA regardless of speed, this is what the CAS washout means, the CAS is providing extra authority for stick movement, so it washes out as you slow down so that the stick is heavier as you slow down. This is explained better in newer  -1's that are just after rule 1.16 but are available on the internet, including an F-15A-1 from 1986 that is easily found but I will not link here.

Currently the FC3 F-15C works exactly the opposite to this with CAS on, as you slow down the Nose gets lighter and you barely need half aft stick to hold a stabilized stall.

here is ED's line about the pitch CAS on the F-15C modules store page, This is directly contradictory to what is said above. The damper is also washed out in authority, and the stick gets heavier which increases your pitch control precision at higher angles of attack.

Screen Shot 2022-06-12 at 11.15.28 PM.png

 

😉

https://www.nasa.gov/centers/dryden/pdf/87873main_H-914.pdf

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  • 2 weeks later...

If you want to go deeper into the F-15 control inaccuracies, the jet will pitch up or down based on whether the AOA is increasing or decreasing. That is to say, the velocity vector and the aircraft datum (nose) will gravitate towards each other under acceleration, and away from each other under deceleration. The F-15's CAS should hold the datum steady under those conditions. It has been this way since LOMAC iirc.

 

Spoiler

 

 

f15pitchcasbug01.trk


Edited by henshao
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1 hour ago, henshao said:

If you want to go deeper into the F-15 control inaccuracies, the jet will pitch up or down based on whether the AOA is increasing or decreasing. That is to say, the velocity vector and the aircraft datum (nose) will gravitate towards each other under acceleration, and away from each other under deceleration. The F-15's CAS should hold the datum steady under those conditions. It has been this way since LOMAC iirc.

 

  Hide contents

 

 

f15pitchcasbug01.trk 308.94 kB · 0 downloads

 

 

Not sure if I am understanding exactly what you are discussing, but that is the function of the PTC.  PTC will drive to maintain 1G using the stabs, so the nose will move as a result.  That's how the pitch trim works.  The system is G commanded to 1 G with no stick inputs.  In addition, there is 'anti-stall' features that are AoA dependent, which is what Klar was discussing above, which change the amount of available pitch travel.


Edited by Rainmaker
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1 minute ago, Rainmaker said:

Not sure if I am understanding exactly what you are discussing, but that is the function of the PTC.  PTC will drive to maintain 1G using the stabs, so the nose will move as a result.  That's how the pitch trim works.  The system is G commanded to 1 go with no stick inputs.

Let me know if you watched the video I added. If the jet is holding 1g the flight path should not change as it does

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12 minutes ago, henshao said:

Let me know if you watched the video I added. If the jet is holding 1g the flight path should not change as it does

I watched the video.  The PTC has travel limits.  And, as above, the jet has an anti-stall inhibiter that is AoA based.  When it gets to certain limits, the nose up pitch authority is reduced, so yes, the flight path marker will drop as airspeed continues to reduce as there is no longer as much pitch-up authority given to the stabs.  The jet will not keep driving the nose up to a stall, that is the purpose of that system.  That's why, as Klar pointed out above, the stick force for AoA is increased as speed drops below the commanded G capability of the airplane.


Edited by Rainmaker
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16 hours ago, Rainmaker said:

I watched the video.  The PTC has travel limits.  And, as above, the jet has an anti-stall inhibiter that is AoA based.  When it gets to certain limits, the nose up pitch authority is reduced, so yes, the flight path marker will drop as airspeed continues to reduce as there is no longer as much pitch-up authority given to the stabs.  The jet will not keep driving the nose up to a stall, that is the purpose of that system.  That's why, as Klar pointed out above, the stick force for AoA is increased as speed drops below the commanded G capability of the airplane.

 

Which is the PTC trying to hold steady, the aircraft nose or the velocity vector? because it is doing neither

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2 hours ago, henshao said:

Which is the PTC trying to hold steady, the aircraft nose or the velocity vector? because it is doing neither

PTC moves stabs. No stick input equals 1G. The system is designed to hold the velocity vector over transient changes, but not across all regimes. It alleviates trimming constantly, but it doesnt operate like a att hold mode. Over certain alpha, nose gets heavy (you can read the nasa paper I posted).  Above about 23 alpha, the system actually stops commanding 1 G. There are things about the current PTC actions that I dont particularly think are quite right, but that’s different from me saying its a bug. 

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2 minutes ago, Rainmaker said:

but it doesnt operate like a att hold mode.

 

Now this is contrary to what I've heard from F-15 pilots, at least for something as simple as increasing or decreasing level speed, but I can't prove it for sure

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2 minutes ago, henshao said:

 

Now this is contrary to what I've heard from F-15 pilots, at least for something as simple as increasing or decreasing level speed, but I can't prove it for sure

Again, transient maneuvers. That’s not to mean 200-600knts and back again. 

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5 hours ago, henshao said:

 

Now this is contrary to what I've heard from F-15 pilots, at least for something as simple as increasing or decreasing level speed, but I can't prove it for sure

Looking at your video again, what it really shouldn't be doing IMO is climbing during initial accel. The water line holds steady and the VV raises and the jet climbs. It should be taking stab trim out there at that point and relaxing the nose down. 

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Klar, to add to your original post, I’m almost under the assumption that the pitch CAS is working backwards. The jet AoA limits with CAS off, but the stall inhibitor, is pitch CAS related as its nulling the stabs through CAS. Shouldnt even be possible with CAS off, right?  Almost like, when its off it’s on. 

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  • 3 weeks later...

@KlarSnow

After the lengthy discussion on discord, I went and did some more playing around with this.  I have some hypothetical thoughts on this I want your opinion on.

To the first point.  Take a look at the awful depiction that I attached.  I think this is where you are seeing the CAS on/CAS off differences between the two.  As I tried to convey on discord, I think we are faced with some shortcomings due to everything being measured by force in the real jet vs us using stick position.  Same stick position equals different stick force between CAS on/off due to that anti-stall inhibitor.  We hold the same stick, but it's seen as you were providing more force to hold the same stick pos, even though we are not when sitting at our PCs.  The PRCA is going to see this as you wanting more pitch/pitch rate with CAS on vs CAS off due to that.  It's not related to CAS and it's pitch authority, it's do to the mech side seeing more force because of the inhibitor putting more force on the stick to stay in the same place.  "Heavier stick", but we can't feel heavier.  When I went full aft stick CAS on/off, I didn't see any difference between the two...but that kind makes sense as you are max out, CAS can't give you more, etc.  When I was partial stick, I saw the alpha spikes you saw but I believe that can be explained by the attached photo and what it's interpreting.

To the second point.  I think if this is an issue, it's a PTC/mech scheduling issue, maybe based on alpha.  Even though it might appear to be better on the mech only side, I'm wondering if that is masking itself.  I'm going to look around to see if I can find some papers, but I'm thinking that the PTC may make adjustments to the stab movement based on alpha rates that perhaps we don't have.  That perhaps might be something, that even know you deliver more force to the stick, the PTC may ignore the command to drive the stabs for more pitch/pitch rate since you are beyond the point of achievable G anyway.  This is where some input regarding what happens relative to stick pos in the real jet is probably invaluable, but that's info/experience I can't provide.  If X stick pos is required to create X low speed flying quality, or stall, or not stall...I'm useless in that regard as far as what should and shouldn't be right.

Edit: Just to get ahead of someone thinking I posted something not allowed, that stick force graph is from the NASA doc I posted above.

 

New Bitmap Image.png


Edited by Rainmaker
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