FAQ for F-15 AFM Development

[SDsc0rch]

yes, things are improving

 

however, with asymmetrical loads (ie.. fire one missile - now the a/c is unbalanced) a slight roll develops - you don't notice it at first, but look away and do something then look back to the HUD and you've developed some bank towards the heavier side

 

a) shouldn't the CAS system deal with that automatically ?

 

and b) when i try to trim out the very slight roll moment, i am unable to get it "just right"

[HiJack]

yes, things are improving

 

however, with asymmetrical loads (ie.. fire one missile - now the a/c is unbalanced) a slight roll develops - you don't notice it at first, but look away and do something then look back to the HUD and you've developed some bank towards the heavier side

 

a) shouldn't the CAS system deal with that automatically ?

 

and b) when i try to trim out the very slight roll moment, i am unable to get it "just right"

Good question. Here are some info posted by strikeeagle earlier in another post. http://www.f15sim.com/operation/f15_flight_control_system.htm

 

I guess we have to wait for someone from ED or a real pilot to answer this.

[GGTharos]

It is already answered in the -1.

 

Roll trim is MANUAL. As in you're the trim.

[Svend_Dellepude]

The CAS should auto-trim asymmetry anyway.

 

The CAS will do a lot of things within its authority to keep the plane coordinated and flying straight for you - that's what I hear anyway. It should basically try to deal with any un-commanded flight situation, this includes smoothing out a lot of turbulence, and this should include the roll channel. I need to dig much deeper to say for sure though, or find someone who really does know :)

 

It is already answered in the -1.

 

Roll trim is MANUAL. As in you're the trim.

 

Are you talking about the same thing here?

[GGTharos]

Yes.

 

Like I said, the -1 clearly states that roll trim is MANUAL.

 

Are you talking about the same thing here?

[Svend_Dellepude]

No problem, just that i too was of the assumption that the CAS too care of unwanted roll induced due to asymmetry and apparently you were too.

 

Can you quote what the -1 says?

[GGTharos]

That was before I read the manual in detail, and it was a fair assumption - certainly the CAS might mitigate some roll resulting from yaw, though the main function is to remove un-commanded sideslip.

 

I don't have the -1 handy right now; if I don't forget I'll put up the quote - it might be in the GS too rather than in the -1, I don't recall.

 

In any case, it's a tiny snippet, it just says that roll trim is manual. The automation in the roll CAS I think has more to do with roll damping and ARI than trimming.

 

No problem, just that i too was of the assumption that the CAS too care of unwanted roll induced due to asymmetry and apparently you were too.

 

Can you quote what the -1 says?

[SDsc0rch]

yeah.. i thumbed through the dash-one this morning

 

it never directly says CAS compensates for asymmetries - that's not the purpose for CAS (at least i was unable to find it)

 

also, correct me if i'm wrong but it seems to me asymmetric loads are one thing - and trim is another ---- we are trying to put the two together and i wasn't able to find where the manual discussed the two together

[Svend_Dellepude]

Found it!

In the -1 section 6 "flight characteristics" it says pretty clearly that yaw CAS will take care of asymmetry and keep the plane coordinated within authority, but it also says that when you start pulling G's, lateral input is needed opposite the heavy wing.

Because according to the -1 drag is a minor thing compared to the increase in weight with G's.

But we don't know how draggy the missiles are in game.

It furthermore says that the Eagle is resistant to departure with 3 missiles asymmetry up to 30 Units AoA which is 10 units more than what we are experiencing now.

[GGTharos]

Found it!

In the -1 section 6 "flight characteristics" it says pretty clearly that yaw CAS will take care of asymmetry and keep the plane coordinated within authority, but it also says that when you start pulling G's, lateral input is needed opposite the heavy wing.

 

Yaw CAS takes out uncommanded sideslip, not roll. The -1 clearly states you can expect a slow roll with asymmetric weight. Roll trim is completely manual with the exception of T/O trim functions.

 

It furthermore says that the Eagle is resistant to departure with 3 missiles asymmetry up to 30 Units AoA which is 10 units more than what we are experiencing now.

The FM is still being worked on. While I'd love to give detailed reports, unfortunately that won't happen without permission.

[Svend_Dellepude]

Maybe I didn't make it too clear, but I meant just that.

CAS will not take care of roll caused by asymmetry, thats the pilots job.

The roll channel is just for fine tuning the rollrate so it reflects lateral stick input.

 

EDIT: Looking forward to news when it's time. :)

Edited April 20, 2015 by Svend_Dellepude

[SDsc0rch]

yeah.. checked that section (page 6-3)

 

unfortunately it doesn't talk about CAS "trimming out" asymmetric loading - it really just addresses spin recovery and departure resistance

 

in fact, to further emphasize the "manual" trimming out of weight imbalances, the dash one explains how to counteract it by moving fuel in the wing tanks - and they provide the formula for calculating "foot-pounds"

 

: /

 

good hustle though..

[Svend_Dellepude]

I think it was section 6-1 but not sure. i'll find it for you. It's there..

Anyway GG answered it in post #210

 

In the meantime there is a good and more technical read of the FCS here:

http://www.f15sim.com/?page_id=16

Maybe you already know it.

[Svend_Dellepude]

This is more of a power bug and it will be fixed. F-15 was accidentally tuned with 15C weather performance for 20C weather, IIRC.

 

So basicly a lot had to be redone?

 

Only a temporary issue. Don't expect your fight to get much better ... the F-15 will lose some things and gain others. A more realistic F-15 FM will enable a pilot who thinks and flies like a pilot (as opposed to a vpilot :) ) to do a lot more than they can right now.

It would even give some nice things to vpilots, like getting rid of the TRO.

 

Sounds good to me. So an update soon? :)

 

What is TRO?

[GGTharos]

I don't know if it is 'a lot of work', it could be as simple as changing the temperature for the given line on the graph - but since I don't know how the code describes performance, I can't answer that question.

 

As for an update, no idea when. TRO is transonic roll-off. You know that fun un-commanded roll of insanity you get when you pull hard (and you have ANY roll input at all) between 0.95-1.05M? Shouldn't be happening. You might get a wing dip in one direction or the other (In RL. I know a bunch of planes have this behavior, I don't know if the F-15 'experiences' it at all), but not insane uncontrollable rolling.

 

So basicly a lot had to be redone?

 

Sounds good to me. So an update soon? :)

 

What is TRO?

[SDsc0rch]

...

 

TRO is transonic roll-off. You know that fun un-commanded roll of insanity you get when you pull hard (and you have ANY roll input at all) between 0.95-1.05M? Shouldn't be happening ....

 

aka.. the BANE of my existence!!

[Ktulu2]

aka.. the BANE of my existence!!

 

copy that LOL

[Svend_Dellepude]

I know that! :)

Actually it seems to drop the wing up to around 1.3M.

Another thing i noticed recently is that when you step on the rudder, the advancing wing makes a drop while the nose is in transit.

Shouldn't the advancing wing rise?

[GGTharos]

I don't know, I'd imagine so just on general principle. I'm surprise because the wing inside the turn should drop - basically rudders give you yaw and roll.

[Svend_Dellepude]

Well, once the nose has settled the inside wing drops like it IMHO should. This happens only when you have yaw motion present.

And this is, again IMHO, is probably contributing to the uncontrolled roll you have at transonic speed.

[SinusoidDelta]

The F-15 had some serious issues with departure from controlled flight. The Bitburg Roll is one example. There are some great research papers on the topic. Does anyone know if the F-15 in DCS is modeled true to life with an inherent right wing heavy 1,700 ft*lb asymmetry?

[GGTharos]

No, the F-15 is very stable with the exception of some very specific conditions, including high AoA which will make any aircraft unstable.

 

As to the asymmetry, why don't you test it and find out? I don't know the answer right now but I can find out.

 

The F-15 had some serious issues with departure from controlled flight. The Bitburg Roll is one example. There are some great research papers on the topic. Does anyone know if the F-15 in DCS is modeled true to life with an inherent right wing heavy 1,700 ft*lb asymmetry?

[SinusoidDelta]

No, the F-15 is very stable with the exception of some very specific conditions, including high AoA which will make any aircraft unstable.

 

As to the asymmetry, why don't you test it and find out? I don't know the answer right now but I can find out.

 

I agree that the F-15 is a stable jet until it isn't. The Bitburg Roll is somewhat similar in nature to the TRO problem in DCS. I don't know what is considered "insane uncontrollable rolling" but 60 deg./sec IRL sounds pretty damn close.

 

Regardless, I have no idea what the true nature of departure/autoroll susceptibility in the transonic regime is with weight and aerodynamic asymmetries. If this information exists, please direct me towards it. I only know a lot of research was performed in an attempt to quantify what factors increase departure susceptibility and I doubt it was done for s's and g's.

 

Regarding the lateral weight asymmetry, I would gladly "test it out" but I don't know how with any degree of certainty. It's kind of a critical detail for a flight simulator, I really would appreciate if you could find out.

 

Is there any way to log advanced telemetry? It'd be invaluable to make excel plots of inputs vs control surfaces, CAS authority, yaw acceleration, sideslip, etc. especially with different loadouts. There are a considerable number of potential virtual "test pilots" here. The community could help quantify and analyze these flight model issues and take some of the burden off ED. Just my .02 cents. I'll get off my pedestal now. :music_whistling:

[GGTharos]

I agree that the F-15 is a stable jet until it isn't.

 

Sounds like every plane out there!

 

The Bitburg Roll is somewhat similar in nature to the TRO problem in DCS. I don't know what is considered "insane uncontrollable rolling" but 60 deg./sec IRL sounds pretty damn close.

It's really not. We're seeing 200dps at low AoA. Very NOT like the Bitburg roll.

 

Regardless, I have no idea what the true nature of departure/autoroll susceptibility in the transonic regime is with weight and aerodynamic asymmetries. If this information exists, please direct me towards it. I only know a lot of research was performed in an attempt to quantify what factors increase departure susceptibility and I doubt it was done for s's and g's.

Yes, a lot of research has been done, and every time it's done the CAS gets an upgrade to stop it from happening.

The Bitburg roll is an interesting example, but the end it is said to 'not be well understood', and in any case if you do read the research you'll find that the eagle doesn't enter what they call a 'true departure' too often, even when they're trying for it.

 

Regarding the lateral weight asymmetry, I would gladly "test it out" but I don't know how with any degree of certainty. It's kind of a critical detail for a flight simulator, I really would appreciate if you could find out.

My gut says yes, but I'm not sure it's really testable (or at least easily testable). You could turn the CAS off, but in theory the horizontal stabilizers can still be physically rigged to deal with it.

 

Is there any way to log advanced telemetry? It'd be invaluable to make excel plots of inputs vs control surfaces, CAS authority, yaw acceleration, sideslip, etc. especially with different loadouts. There are a considerable number of potential virtual "test pilots" here. The community could help quantify and analyze these flight model issues and take some of the burden off ED. Just my .02 cents. I'll get off my pedestal now. :music_whistling:

Some of these things you can log with lua. Frankly unless you're an F-15 pilot though or you're trying to match public empirical research, you're probably not doing anything that's very useful. That's not to say it can't happen, just that it's rare. There's also a lot of data that's simply not available.

 

I still see people coming around complaining that 'this FM sucks, the F-15 can't climb straight up!' Usually they don't even know what the -1 is :)

[Mohammad12000]

F15 in dcs clicklabel or not?