F-15C Technical Discussion

[Curly]

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

 

Is the PTC really modeled then? It seems likes it's more of a conventional trim system. Ie hitting the trim causes the elevator pitch to change. In the F 15, the trim signals alter zero force position reference. For example: If the pilot trims into a 2g turn at zero stick pitch force. The 2g becomes the reference for zero stick force instead of the normal 1g.

 

To me it seems like a very basic trim model is implented. I'm not even sure how well the CAS is modeled. Maybe it's because I fly with some non linearity, but it seems like G per stick defection stays does not stay constant. Which is what CAS does.

[GGTharos]

Yes, the PTC is really modeled. You can see it while operating the F-15. Just accelerate from 300kts level to 500kts for example, you'll see the force remains at 1g. That's the PTC at work.

 

G per stick deflection is fairly consistent, but shouldn't be perfect (it isn't in the real deal).

[strikeeagle]

Just for gee-wiz, this is the PRCA. It weighs about 50-60lbs and is a pain in the ass. The PTC, RRC, and any other "C" is connected to it...

 

 

 

[Curly]

Yes, the PTC is really modeled. You can see it while operating the F-15. Just accelerate from 300kts level to 500kts for example, you'll see the force remains at 1g. That's the PTC at work.

 

G per stick deflection is fairly consistent, but shouldn't be perfect (it isn't in the real deal).

 

I see that the net effect of the PTC is modeled. What I meant was, is the system as a whole modeled? You could cook up some tabular data and LERP between points based on air speed and acceleration to determine Stab position to achieve a constant 1 g trim.

 

When I ask is PTC really modeled, I mean PTC as an integer which is computed along with other factors to determine the position of flight surfaces. The position of the PTC arm is fundamentally an integer (it's part of hydro-mechanical computer) which is summed with the position of the Pitch Ratio Changer (PRC) to drive a PRCA hydraulic boost actuator which moves all the downstream linkage and also drives a linkage input to the PRCA roll axis and to the Aileron Rudder Interconnect (ARI). With both the ARI and PRCA feeding data across to each other. With PRCA feed into Lateral Longitudinal Mixer and the ARI directly into the rudders. Resulting in a complex dynamic in which both the ARI and PRCA are determining the position of the flight control surfaces along with CAS compensating.

 

 

If you use the Mechanical lateral control authority schedule alone to determine aileron, stabilator and rudder position of a given input you would get results which tend to be on the oversteer side. The missing piece is the roll and pitch rate changers. Which, modify the amount of control surface deflection based on stick input and air speed data.

 

The scheduling issues seem to be a result of the lack of the the pitch and roll rate ratio controller and changer. The reason I asked if it was really modeled was because A. the the Pitch ratio gauge in the F-15 is static and B. disengaging the pitch CAS results in unexpected behavior. The hydraulic Pitch system should still keep the PTC functionality, unless turning it off put's it in emergency mode. Where even there it should lock to more a stick neutral position.

 

The force to G ratio should stay fairly consistent through most of the flight envelope. Except at a high angles and high and low speeds. When the pitch ratio controller is near zero and you can't get enough elevator to pull the desired load factor. Other wise CAS should be driving the stabilator surfaces to match the load factor based on stick position. It's one of the prime functions of that system.

Edited December 8, 2014 by Curly

[GGTharos]

Yep, unfortunately I can't answer your question; really, the 'net effect' is all you need, as long as it is correct. :)

 

Cofcorpse could tell you exactly what's modeled if he feels like it :)

[Not_G]

I didn't read through all of the posts so if this was already said, disregard. What I'm pretty sure the TO is talking about is using the JFS to motor the engines which will give you hydraulic pressure, not 3000 but enough to make a difference. The JFS is supposed to be able to run until 3A is dry... its not smart to do because they break enough as it is so its not something you would do unless it's an emergency. But motoring the engines will spin your hydraulic pumps and in turn create pressure. What I think strikeeagle is talking about (and this may be what the '86 tech data is talking about) is the limited duty mode which was removed from 15's long ago, it allowed the JFS/CGB to act as an APU of sorts and give you hyro/elec power with out operating the engines.

Edited December 8, 2014 by Not_G