F-15C Technical Discussion

[strikeeagle]

[strikeeagle]

[strikeeagle]

Great PRCA explanation

 

http://www.f15sim.com/operation/f15_flight_control_system.htm

[strikeeagle]

Come on man, you know what I was saying and yes I changed it.

 

I know what he is saying and the book does say "The JFS, WHEN ENGAGED, provides sufficient hydraulic power for flight control and emergency generator operation..."

 

Like I said, I'd have to read the procedure myself and what he was saying IS correct. It is important to realize that if the airplane is at 210kts, spinning the JFS would motor the engine therefore turning the hydro pumps. So, the pilot can pitch for 210.

 

The book also says eject before losing flight controls. I think the JFS would crap out before it got back on the ground w/it spinning at 100% trying to keep the hydro power going.

Edited December 3, 2014 by strikeeagle

[strikeeagle]

at any regards the JFS even if it could provide enough power to spool an engine to 12% it's not due to no fuel. Therefor it's safe to assume fuel exhaustion mid air and below 350kts will result in loss of control unless it happens within a few feet of touchdown and only brake pressure is needed would be my synopsis.

 

True statement. Yeah, back to the OP and what he said:pilotfly:. For this sim, the flight controls move regardless if there is hydro power "available" or not.

Edited December 3, 2014 by strikeeagle

[GGTharos]

It sounds strange that they would reccomend it if it was to fail quickly. The reason I'm thinking this is that they're mentioning 210kts, which is best glide speed, possibly implying many minutes of flying (about 1nm/1000' altitude, and at this speed you're doing about 3.5nm/min).

[strikeeagle]

It sounds strange that they would reccomend it if it was to fail quickly. The reason I'm thinking this is that they're mentioning 210kts, which is best glide speed, possibly implying many minutes of flying (about 1nm/1000' altitude, and at this speed you're doing about 3.5nm/min).

 

I'd agree with that. I asked a couple other CC's and the JFS can run for 150 seconds. That's the limit on the ground, but the JFS/CGB are very temperamental. I changed them often...especially the JFS.

[strikeeagle]

Tharos, the recommended max useage of the JFS is ~60second, that doesn't mean it will fail at 60second.

 

Just like anything you can exceed this but if you do it must be removed from service and refurbished ect. ect. if those are built like those sunsolris apu's they will take one hell of a beating before they crap.

 

I'd say it'd last a good 10-15 minutes atleast. hell 5 minutes is better then 0 minutes in an emergancy and when feces hits the fan who cares what something isn't rated for if it does the job and could save your life or save millions in loosing a jet loosing a 30,000 dollar JFS is the least of your concern.

 

334th crashed a jet in the 90's because the pilot shut down the good engine after the other caught on fire...they punched. Didn't have airspeed and altitude to figure what was what.

[GGTharos]

Would it be fair to say that in flight, additional cooling due to airflow may allow it to operate longer?

 

And yes you're right, equipment can fail, pilot might not have speed/altitude to deal with it etc. It's an emergency, and emergencies suck :)

 

 

Technical question for you: How fast does the PTC move, and does it have any limits?

 

It if assumes the pitch CAS zone limits (+/- 10 deg), how is this mechanized since it's supposed to 'center' the pitch CAS zone?

 

The reason I'm asking is because the PTC doesn't have enough authority in-game to trim for level flight at 40000'.

 

I'd agree with that. I asked a couple other CC's and the JFS can run for 150 seconds. That's the limit on the ground, but the JFS/CGB are very temperamental. I changed them often...especially the JFS.

[strikeeagle]

Would it be fair to say that in flight, additional cooling due to airflow may allow it to operate longer?

 

Maybe

And yes you're right, equipment can fail, pilot might not have speed/altitude to deal with it etc. It's an emergency, and emergencies suck :)

 

 

Technical question for you: How fast does the PTC move, and does it have any limits?

 

HA! I have no clue. I was a CC, not a flight control specialist. I've changed a couple PTC's that are connected to the PRCA (which is THE most important item in the system). I'd have to pull out the 27GS to tell you that, but I don't have one.

It if assumes the pitch CAS zone limits (+/- 10 deg), how is this mechanized since it's supposed to 'center' the pitch CAS zone?

 

The reason I'm asking is because the PTC doesn't have enough authority in-game to trim for level flight at 40000'.

 

My question is, how do sim designers take a physical and tangible device and replicate is for a game? To use a system in actual flight in one thing, but to say that the system is duplicated in a game is another.

 

[GGTharos]

You can simulate the entire system if necessary, but it's not always necessary or desireable :)

 

You take the logical diagram of the system, if it exists, or any mathematical representation of it, and you model that. So you model inputs into a component, and the outputs based on those inputs. It's a mathematical relationship, or a list (if input A is this, then output is that), or just a straight forward logic gate. It depends on what you're modeling :) And then you connect a bunch of components together - outputs go to inputs etc.

 

cofcorpse coded it up, so he can tell you details, I don't know what the code looks like :)

Edited December 3, 2014 by GGTharos

[strikeeagle]

You can simulate the entire system if necessary, but it's not always necessary or desireable :)

 

You take the logical diagram of the system, if it exists, or any mathematical representation of it, and you model that. So you model inputs into a component, and the outputs based on those inputs. It's a mathematical relationship, or a list (if input A is this, then output is that), or just a straight forward logic gate. It depends on what you're modeling :) And then you connect a bunch of components together - outputs go to inputs etc.

 

cofcorpse coded it up, so he can tell you details, I don't know what the code looks like :)

 

FREAKING COOL!!!

[strikeeagle]

Tharos...click here http://www.airlinepilotforums.com/military/85249-f-15-jfs.html

 

I asked the question on a airline pilot forum where there are plenty of F-15 drivers.

[strikeeagle]

You know the flight control system, especially the CAS, relies on inputs to the stick force sensor. This box is at the base of the stick grip where the paddle switch is located. With CAS on, you can strap the stick itself down to prevent it from moving in any direction however comma if you apply pressure to the stick grip, the flight control surfaces will deflect.

 

Is this in the sim model? Can the pitch issues folks have been experiencing be because of this?

[GGTharos]

In the sim it's effectively based on stick deflection because that's all the input you get. In effect it's the same idea - in the real plane the stick always 'feels' like it's in the same place, based on force, for the virtual pilot it always 'feels' to be in the same place because you deflected it by the same amount :)

[strikeeagle]

The JFS of the 15 runs at 50% (idle) RPM until a finger lift is raised to start a motor. The JFS fuel control then "adds" more fuel and the clutch brake on the CGB releases allowing the JFS to accelerate to 100%. So for our airplane, it doesn't continuously run at 100%.

Edited December 3, 2014 by strikeeagle

[Svend_Dellepude]

Would it be fair to say that in flight, additional cooling due to airflow may allow it to operate longer?

 

And yes you're right, equipment can fail, pilot might not have speed/altitude to deal with it etc. It's an emergency, and emergencies suck :)

 

 

Technical question for you: How fast does the PTC move, and does it have any limits?

 

It if assumes the pitch CAS zone limits (+/- 10 deg), how is this mechanized since it's supposed to 'center' the pitch CAS zone?

 

The reason I'm asking is because the PTC doesn't have enough authority in-game to trim for level flight at 40000'.

 

I have written several posts about this issue and i don't think it's a problem with the PTC but more with the drag of the FM.

In this thread in post #1 and #3:

http://forums.eagle.ru/showthread.php?t=134250

I have provided some tracks and written some of my thoughts about this issue.

Moreover i have thought about the idea that one or more of the lift surfaces accidentally have been turned upside down, thus resulting in downforce and not lift.

This issue is most explicit around M1.0 where you have the most drag. Below M0.8 and above M1.x-2.0 the PTC have enough travel to keep the aircraft level.

 

I have no proof for this so feel free to ignore this post as have been done with the others.

[GGTharos]

The FM is being looked into.

 

The PTC does have a problem, it's not drag vs PTC. The PTC just doesn't have enough authority (IMHO) to deal with the transients the plane experiences AFAIK.

 

The issue you're reporting may also be quite real. They're separate issues though.

 

I have written several posts about this issue and i don't think it's a problem with the PTC but more with the drag of the FM.

In this thread in post #1 and #3:

http://forums.eagle.ru/showthread.php?t=134250

I have provided some tracks and written some of my thoughts about this issue.

Moreover i have thought about the idea that one or more of the lift surfaces accidentally have been turned upside down, thus resulting in downforce and not lift.

This issue is most explicit around M1.0 where you have the most drag. Below M0.8 and above M1.x-2.0 the PTC have enough travel to keep the aircraft level.

 

I have no proof for this so feel free to ignore this post as have been done with the others.

[Svend_Dellepude]

The FM is being looked into.

 

The PTC does have a problem, it's not drag vs PTC. The PTC just doesn't have enough authority (IMHO) to deal with the transients the plane experiences AFAIK.

 

The issue you're reporting may also be quite real. They're separate issues though.

 

Thank you for responding.

Although I can't help seeing them as connected. The PTC is now compensating for something it shouldn't. So it's reaching it's authority limit in flight regimes That it normally wouldn't (IRL).

Not saying you are not right. Because I don't know that, just saying that they might be separate issues, but in this case one problem (drag) seems to be a contributing factor to PTC running out of authority.

 

I don't fly any of the RU birds, does the su27 have the same issues in the fl25+ range? it maynot be the FM but rather the the engine?

 

Sorry I haven't taken the a10c up that high due to it takes to frigen long.

 

AFAIK it seems to be a unique issue of the F-15 FM.

Edited December 3, 2014 by Svend_Dellepude

[GGTharos]

Personally I don't understand what you're demonstrating exactly.

 

You're talking about a nose pitch-down tendency. I'm talking about PTC operation, and I'm fairly certain it's too limited in-game. Aerodynamics do cause things to become related.

Maybe the PTC is issue demonstrates what you're talking about, but it also flat out doesn't have the authority it ought to IMHO.

[Svend_Dellepude]

Personally I don't understand what you're demonstrating exactly.

 

You're talking about a nose pitch-down tendency. I'm talking about PTC operation, and I'm fairly certain it's too limited in-game. Aerodynamics do cause things to become related.

Maybe the PTC is issue demonstrates what you're talking about, but it also flat out doesn't have the authority it ought to IMHO.

 

It's apparent difficult for me to explain. :)

I'm talking about the PTC now working to compensate for the constant negative G's that the airframe now want's to pull and thus reaching its limit because the airframe is doing something it shouldn't.

 

The F-15 can stay level in on it's own in inverted flight, but can't in normal flight, which causes the PTC to only operate in about 25% (estimation) of it's total regime of authority.

To me to PTC authority is the symptom of a problem with the FM not generating the lift it's supposed to.

[GGTharos]

The PTC is working in half of the range of authority that should be available to it.

 

In any case, you're saying the F-15 will fly level upside down with CAS off at around M1.0?

[Svend_Dellepude]

I believe you. :)

 

Yes Sir. It will stay level up to around 20K feet at M1.0 inverted and CAS off.

Altitudes higher that that it starts gently dipping the nose towards the ground.

50% fuel.

[Curly]

Would it be fair to say that in flight, additional cooling due to airflow may allow it to operate longer?

 

And yes you're right, equipment can fail, pilot might not have speed/altitude to deal with it etc. It's an emergency, and emergencies suck :)

 

 

Technical question for you: How fast does the PTC move, and does it have any limits?

 

It if assumes the pitch CAS zone limits (+/- 10 deg), how is this mechanized since it's supposed to 'center' the pitch CAS zone?

 

The reason I'm asking is because the PTC doesn't have enough authority in-game to trim for level flight at 40000'.

Would it be fair to say that in flight, additional cooling due to airflow may allow it to operate longer?

 

 

 

Technical question for you: How fast does the PTC move, and does it have any limits?

 

It if assumes the pitch CAS zone limits (+/- 10 deg), how is this mechanized since it's supposed to 'center' the pitch CAS zone?

 

 

 

The reason I'm asking is because the PTC doesn't have enough authority in-game to trim for level flight at 40000'.

 

 

CAS matches performance to for stick force. The stick force is set at 4.25 lb/g, if the pilot pulls 8.5 lbs of for force on the stick the and the craft does not pull 2 g's the CAS system will power servo motor to drive the stabilators up to +-10 degrees to match the required input. The CAS servos connect to the mixer linkage which will drive the stabilator hydraulics directly.

 

The mechanical inputs are transmitted to flight control surface actuators by way of the control stick boost pitch compensator (CSBPC). The CSBPC is a hydromechanical analog computer made up of two units: a pitch and roll channel assembly (PRCA) and an aileron rudder interconnect.

 

The pitch trim controller (PTC) is part of the PRCA, it's travel is limited by the pitch ratio changer.The PTC is driven by a control valve which operates as a function of normal acceleration (load factor) and control stick position. The PTC is damped to prevent disturbances from affecting the mechanical control system. PTC travel is limited as the PRC nears minimum ratio. When pitch CAS is engaged, the CAS interconnect (CASI) servo controls the PTC.

 

Limited pitch commands are applied to the PRCA CAS interconnect (CASI) servo. The CASI servo drives the PRCA pitch trim compensator and forces the mechanical system to track the pitch CAS. The tracking function minimizes differences between CAS and mechanical commands so that if pitch CAS fails, the mechanical system can take control at the point of failure.

 

The function prevents CAS from back driving the control stick and keeps the neutral point the same when CAS engaguges. When they talk of the PTC driving CAS It's descbing the hydro-mechanical analog computer taking input and compenstaing for CAS.

 

http://www.f15sim.com/images/F-15_Longitudinal_Control-2.jpg

 

Perhaps the trim probelm at 40,000 ft has to do with with PTC only having +-10 degrees of stabilator deflection? As they figured CAS and PTC were the same. PTC really should have almost unlimited stabolitor deflection unless the mechanical advantage (number of degrees of collective stabilator deflection per degree of longitudinal stick deflection)

via the PRC is low.

 

http://www.f15sim.com/images/F-15_Pitch_Control-1.jpg

http://books.google.com/books?id=NUnlAwAAQBAJ&dq=f+15+cas+servo&source=gbs_navlinks_s

[GGTharos]

The PTC right now has less authority in-game than CAS.