Which Flight Model Does the F15 Use?

[Rex]

Can someone who has a little free time do a little test for me?

 

1) Hop in the F-15, map is irrelevant, for ease of the experiment, a clear, low-wind day is probably best

2) Climb to 25,000 feet, give or take a couple of grand

3) Set Altitude hold

4) Set the throttles to full burner

5) Keep them at 100% while you listen to this video in its entirety (optional - but I think it does make you go faster)

6) When you get to 700+ IAS, press either rudder pedal to the floor

7) Observe what happens

 

Questions:

 

1) Is what happens, what you expected to happen?

2) Is it actually possible to get full deflection on the rudders at that speed without them ripping off and losing control?

3) What drug, or possibly, combination of drugs, made Kenny Loggins think that haircut was fashionable?  Talk about taking a ride into the danger zone.

 

If you repeat this exercise in the Hornet (although probably at less than 700kts), you'll get the reaction I would personally expect, which is to say ... not much of one. The rudders don't deflect much at that speed. Ever-so-slightly, if at all.

 

That said, I thought the F15 and F18 use the same PFM flight model, so I'm a little confused by the disparity in rudder effect.  Is it possible that they didn't model the rudder at all for the 15 besides throwing full yaw one way or the other?

 

Maybe I just don't understand how the F15 works, and maybe it's completely normal?

 

 

P.S.  After doing some testing, it appears that all FC3 aircraft exhibit this "omnipotent-rudder" effect.  I'm a little confused as to what reality should be here.

 

[KlarSnow]

The F-15 has a conventional flight control system, it does not limit your inputs through a computer like the hornet does. You can in most situations pull or apply enough force depending on your airspeed to break the aircraft and it will let you. It and the hornets flight controls are completely different in how they work. This has nothing to do with the flight model, and how you control the aircraft.

 

The hornet and F-15 have very different flight control systems, do not expect how you fly the hornet to transfer over to any other aircraft, it takes care of you and prevents you from doing stupid things that can break or depart the aircraft like slamming full rudder in while supersonic.

 

The hornet has a computer between your stick inputs and the control surfaces, this is the Fly by wire aspect of it. If you move the stick or push the rudder pedals, the computer takes that input and outputs something within the controllability of the aircraft.

 

The F-15 ( for the most part) is a direct connection to the control surfaces, there is a pitch ratio for the stabs (elevators) that automatically changes how much throw you get out of them as you get faster, and in the real jet there is a mechanical limiter for the rudder pedals above Mach 1.5, The manual just says it limits it to half of rudder travel, and does not say whether that will keep the aircraft controllable or not, so you have to be careful with your inputs. IRL you know this and don’t use, want, or need rudder inputs at those speeds so it’s not an issue.

 

I am simplifying this, the F-15 does have a CAS system that is sort of fly by wire that has limited authority over the rudders, however you still have more than enough authority to break something if you decide to do something stupid like that.

 

It sounds like you might need to do some research to see how these very different aircraft actually work IRL, they are not very similar at all in how they fly.

 

The Russian aircraft are all completely different as well.

 

 

 

 

Edited June 21, 2021 by KlarSnow

[draconus]

11 hours ago, Rex said:

1) Is what happens, what you expected to happen?

2) Is it actually possible to get full deflection on the rudders at that speed without them ripping off and losing control?

...

That said, I thought the F15 and F18 use the same PFM flight model, so I'm a little confused by the disparity in rudder effect.  Is it possible that they didn't model the rudder at all for the 15 besides throwing full yaw one way or the other?

1. Pure half rudder, starts with yaw then makes ac roll - yeah, expected but wouldn't call it normal since nobody will do that IRL.

2. Full deflection is not possible over M1.5 with the functional limiter as @KlarSnow explained. Mind that no damage is modeled for FM here, so you can't rip off any flight surfaces, stabs or wings just by stupid flying in F-15C. You can have battle or collision damage and it is affecting FM though.

3. What do you mean they didn't model rudder at all? Mind you there's also Aileron Rudder Interconnect, and it's modeled too, just automatically disengaged over M1.0. The Eagle and Hornet both use PFM - meaning similar level of modeling. But they are completely different flight models.

https://www.digitalcombatsimulator.com/en/support/faq/general/

Having been ED's full fidelity flag ship during recent years and being 5 years newer the level of modeling is expected to be higher for the Hornet though.

[GGTharos]

17 hours ago, Rex said:

P.S.  After doing some testing, it appears that all FC3 aircraft exhibit this "omnipotent-rudder" effect.  I'm a little confused as to what reality should be here.

 

@KlarSnow and @draconus have given you very good answers.

 

I'll only say this to make it really short:  There's no aerodynamic damage model implemented for the F-15.  The flight model is good, but there's no DM for g/aero forces.

[Rex]

Thanks for the replies.  I understand about the damage not being modeled.  I think what's caused me to wonder about the underlying flight model is that I was able to crab 15 degrees left or right at Mach 2.0+, and maintain that flight attitude for as long as I held it.  I'm sure it would have rolled had I not held the stick offset.  It "feels" like it would be impossible, and from the responses here, it looks like it's the lack of damage that allows it.

 

Essentially, I could do things that I don't think are physically possible. 

 

Oh, and to add to the confusion, every FC3 aircraft more or less behaves the same way.   Stomp the rudder at maximum speed, and it'll fly crabbed at 1.5+ Mach.  That's what I meant by not modelling the rudder.  The rudders perform at high speed very much the same as the perform at low speed.  At least from my limited experience.  As you pointed out, though, with most of my time being spent in the F18, much if it is likely just pilot error/misunderstanding.

 

I appreciate the replies.  I learned several things about the F-15 that I didn't know.   I'm looking forward to the F-15 Strike Eagle full fidelity plane which is coming, I think next year.  Or that was the rumor at one point.

 

Thanks again!

On 6/21/2021 at 2:01 AM, draconus said:

Hornet both use PFM - meaning similar level of modeling. But they are completely different flight models.

https://www.digitalcombatsimulator.com/en/support/faq/general/

Having been ED's full fidelity flag ship during recent years and being 5 years newer the level of modeling is expected to be higher for the Hornet though.

 

Ah-ha ... got it.   This makes sense. Thank you.

[razo+r]

What are your game settings? Are you using game/simplified avionics/flight model? 

[pepin1234]

To clarify. Mig-29, Su-27/33 and Su-25A brake wings if you pull the stick in certain Gs/Speed/Load wigs. F-15 is the only UFO FC3 that can do whatever you want. We have point that out in other threads after PFM added but seem F-15 is the forgiven bad boy of DCS and no change until date about. You win a dogfight with fuel tanks... 

[Marduk879]

Hi,

Draconus and Klarsnow claim that full rudder deflection above mach 1,5 isnt possible due to system limiting rudder deflection but Rex is able to get full deflection at mach 2.

I dont understand how is this possible.

Probably the mentioned system isnt implemented or isnt working then?

[draconus]

11 minutes ago, Marduk879 said:

Draconus and Klarsnow claim that full rudder deflection above mach 1,5 isnt possible due to system limiting rudder deflection but Rex is able to get full deflection at mach 2.

I dont understand how is this possible.

You can always put the pedal to the metal but the control systems will limit the actual flight surface deflections in some conditions. So it was not full deflection - just full rudder input.

[Spurts]

1 hour ago, gortex said:

After observing that the Eagle can reach 12g and 13g loads again and again without consequence

That is an issue of time, one we do not have to worry about in a sim.  Being a 9G certified plane means the Eagle was static tested to a 13.5G load under the determined critical condition and it did not fail.  In reality, Eagles have hit 14G due to turbulence and survived.  A 747SP, a notional 2.5G airliner, once pulled 5G and landed and went on the have a full life after repairs.  

 

As as long as the F-15 has sufficient dynamic pressure (altitude and speed related) it can absolutely reach greater than 9G, and if this was real life the plane would be downed for maintenance inspections and possibly repair afterwards, but these are aspects not simulated in DCS.  I would expect that a low hours Eagle, with a superhuman pilot, could indeed pull several seconds of 12-13G flight without damage.

[GGTharos]

Yeah, it's fighter ace alright, you're a total fighter ace when  you blow all your energy on a 13g spike, black out and go into the drink.  Yep, if you pull 12.5g for a few seconds you'll damage the airframe IRL at some GW, at much higher GW you will disintegrate it.

 

In DCS that won't happen, but you will disintegrate your smash or your SA or both.   IMHO you picked a pretty poor hill to die on.

[Spurts]

2 hours ago, gortex said:

 Every other non FBW jet in the game punishes you for pushing past its limits. 

I see F-5s doing 12G in a break.  I took a Harrier to 9.1G once.  The F-16 can pull 9G with full A-G stores/tanks with no issue (F/A-18 is the only FBW jet that accounts for weight in setting the G-limit).  This is not a "only F-15 is like this!" issue.  AFAIK only the F-14 and the Su-xx break for over G.  I have had other planes break for overspeed (A-10 and warbirds)

 

And you are failing to grasp that at design weight 13.5G is the Ultimate Load Limit under which nothing SHOULD break.

Edited August 18, 2021 by Spurts

[Spurts]

I don't fly the Eagle, I fly the Harrier and the Tomcat, both of which have nasty departure modes.  I am simply pointing out that A) MANY modules don't punish you for exceeding the Design Load Limit and B) 13G is under the Eagles Ultimate Load Limit.

 

 

*Edit*  I was reminded that the F-15 was not built as a 9G design load airframe but as a 7.33G one, with an 11G ultimate load at critical conditions (airspeed altitude combo).  It was later fitted with the OWS to allow it to fly at 9G at conditions farther away from that critical condition.  Under these conditions the Ultimate Load Limit might be 13.5 or they might be 11 and it all depends on how the plane was designed.  It would literally take a structures engineer who worked on the early Eagles to tell us this.  So, I capitulate that anything between 11 and 13G is suspect.  I was also informed that some people are hitting 17G in the break.  That is more than I have ever seen, but that does not make it untrue.

Edited August 20, 2021 by Spurts

[Hulkbust44]

 
The difference is jarring enough to make it feel like I'm playing a different game when I fly the Eagle.  After observing that the Eagle can reach 12g and 13g loads again and again without consequence I quit flying it.  Such a shame because the real thing is a great airplane.

There's was a Pilot in ODS the went into a split S after a MIG, didn't realize till after the fact that he pulled 12G. The aircraft is perfectly capable of it. Going past a g limit is hardly ever going to cause a catastrophic failure. A Rhino over G'd to 9.3 and had a few dented belly panels.

Mobius708

[Hulkbust44]

Sure, over g happens, but it is bad piloting, and some aircraft have broken up mid-air.  In our game, habitual over-g is an advantage for the pilots of some aircraft (F15 and F18) and not others (fly the Tomcat recently?), and that is wrong.  It is not fair that some aircraft suffer consequences for it and others do not.

Hmm, it almost like they're built better.

If there's anything in DCS with a habitual over G issue it's the Viper. There's no G limiter for stores.

Tomcat was susceptible to catastrophic failure from over G. Modern FBW won't allow that. Hornet will be the aircraft least impacted by stores overstress once implimented, it's got a variable limiter.



Mobius708

[GGTharos]

44 minutes ago, gortex said:

Sure, over g happens,

 

More often than some want to admit.

 

44 minutes ago, gortex said:

but it is bad piloting,

 

This is true.

 

44 minutes ago, gortex said:

and some aircraft have broken up mid-air.

 

And the vast majority have not.  Certainly not from the over-g's being complained about.

 

44 minutes ago, gortex said:

It is not fair that some aircraft suffer consequences for it and others do not.

 

Too bad.  Even when they will suffer them (I'm sure it'll come) it's still going to be worse for some that it is for others.

[GGTharos]

12 minutes ago, gortex said:

The rl pilots, like mover and lex, have given up arguing with sim pilots on this topic.  You offer a good demonstration why.

 

You offer a good demonstration of not knowing the subject but just wanting things to be equal 

[Hulkbust44]

Right, because Viper pilots do competitive BFM with bombs on their wings.

No, but you may have a fuel tank, or something that can't be jettisoned like a FLIR or HTS... Those have their own specific G limits.

Mobius708

[Snappy]

7 hours ago, gortex said:

Sure, over g happens, but it is bad piloting, and some aircraft have broken up mid-air.  In our game, habitual over-g is an advantage for the pilots of some aircraft (F15 and F18) and not others (fly the Tomcat recently?), and that is wrong.  It is not fair that some aircraft suffer consequences for it and others do not.

Are you also active in the subforums for the Hornet , F-5, ,F-16 , and so on complaining about their DM?Don‘t think so.

Because the habitual bashing of the F-15 gets boring by now.

 

Sure it would be nice if it had a better DM , but same goes for the other planes. Can’t count the amount of times I‘ve seen people

over G the F-18 to 11 plus G and more habitually, because they want to win. Just hop on any BFM server and watch any given F-18 fight..

Yes it’s bad flying I agree with you, but that doesn’t preclude me from enjoying the F-15 from time to time. If I want „realism“ with it, I can still try to fly it just within its envelope.

 

 

 

[Rex]

On 9/28/2021 at 12:45 PM, gortex said:

Sure, over g happens, but it is bad piloting, and some aircraft have broken up mid-air.  In our game, habitual over-g is an advantage for the pilots of some aircraft (F15 and F18) and not others (fly the Tomcat recently?), and that is wrong.  It is not fair that some aircraft suffer consequences for it and others do not.

 

I fly the Hornet primarily, and it's hard to truly over-G the plane for more than transiently.  The FCS limits the plane to 7.5 G's (slick), and even if you pull the pinkie switch, it only gives you 33% more ... a bit less than 10Gs total.  Even pulling the switch, I rarely see 10+, and the rare times I do, it's extremely brief, maybe a quarter of a second.

 

I could be wrong, but I don't think you can fly the Hornet in a way that will result in immediate airframe damage (sans colliding with something, of course).  I imagine this is why it's not modelled?  Not to mention, a few seconds at max-G gets the Hornet < 200kts *very* quickly. 

 

BTW, I agree that pulling the pinkie switch in dogfights is lame, but you'll do nothing but go down in flames if you don't because everyone else is pulling it. Two wrongs don't make a right, but one wrong makes for a pretty unenjoyable, and possibly even pointless dogfighting session.  That's why they banned using the pinkie switch from that Folds of Honor contest last year.

 

 

Edited October 1, 2021 by Rex
brief edit

[henshao]

Quote

 

3) Set Altitude hold

 

 

this function is still broken for me, my jet will wander wildly in altitude. it has been reported many times but somehow they don't believe us. My personal guess is that the autopilot is looking at inputs without deadzones so unless you have exceptional equipment IRL the jet will wander in the game even with autopilot on

[razo+r]

4 minutes ago, Rex said:

 

I fly the Hornet primarily, and it's hard to truly over-G the plane for more than transiently.  The FCS limits the plane to 7.5 G's (slick), and even if you pull the pinkie switch, it only gives you 33% more ... a bit less than 10Gs total.  Even pulling the switch, I rarely see 10+, and the rare times I do, it's extremely brief, maybe a quarter of a second.

 

I could be wrong, but I don't think you can fly the Hornet in a way that will result in immediate airframe damage (sans colliding with something, of course).  I imagine this is why it's not modelled?  Not to mention, a few seconds at max-G gets the Hornet < 200kts *very* quickly. 

 

BTW, I agree that pulling the pinkie switch in dogfights is lame, but you'll do nothing but go down in flames if you don't because everyone else is pulling it. Two wrongs don't make a right, but one wrong makes for a pretty unenjoyable, and possibly even pointless dogfighting session.  That's why they banned using the pinkie switch from that Folds of Honor contest last year.

 

 

 

You can easily get more than 10Gs if you pull certain CB's. Of course, nobody in real life would do that, but hey, it's DCS. 

[Rex]

49 minutes ago, henshao said:

 

this function is still broken for me, my jet will wander wildly in altitude. it has been reported many times but somehow they don't believe us. My personal guess is that the autopilot is looking at inputs without deadzones so unless you have exceptional equipment IRL the jet will wander in the game even with autopilot on

 

Even with the Warthog HOTAS setup, I had to set deadzones or autopilot wouldn't work.  It does work now, though, quite well ... but yeah, it does need dead zones.

[henshao]

1 hour ago, Rex said:

 

Even with the Warthog HOTAS setup, I had to set deadzones or autopilot wouldn't work.  It does work now, though, quite well ... but yeah, it does need dead zones.

i have big giant dead zones setup and mine is still very unreliable but also bear in mind I'm comparing it to the way it was in LOMAC-fc2 when, if you press altitude hold, your airplane will freaking hunt that altitude down to the foot. it used to allow up to a ~2.6g turn while holding altitude, fairly significant bank. this was a major workload reducer

[wilbur81]

On 9/28/2021 at 3:01 PM, Hulkbust44 said:

If there's anything in DCS with a habitual over G issue it's the Viper. There's no G limiter for stores.

Mobius708
 

Indeed. It will be interesting to see if this is ever implemented for the Viper's stores. My understanding from talking to a couple Viper guys is that the pilot has to do the over-g calculations in their head for the stores/weight that they're currently carrying whereas the Hornet's FCS makes these g-limiter calculations for the pilot. Without a true FLCS or FCS, I'm sure the Eagle driver has to make all those stores/over-g calc's in his head as well.