fairly interesting topic on its own, so lets not murder another one.

1 hour ago, UrgentSiesta said:

I stand to be corrected, but my understanding is, generally speaking, the lower top speeds at low altitudes are caused by high drag due to thicker air, rather than structural/safe operating limits.

Hence aircraft fly high because there is so much less drag that it offsets the loss of air for lift and loss of oxygen for combustion. So ground speed can be higher, or range longer, depending on priorities.

Though simulators can provide misleading instructions vs IRL, I've never seen a military addon that says to reduce power at low alts due to airframe fragility. It's always engine temp limits and fuel burn that drive power settings, with the more modern jets being practically unconcerned about temps, etc.

TBH, I make no claims to really having a clue about anything over a few thousand feet or a few hundred knots. This tomcat stuff is completely new to me, only time I ever put any time on a fast jet before now on a modern rig is Ace Combat 7 and those you get in the Battlefield series (and only then when someone stole my heli and I had to go sneak into the enemy spawn point to take theirs)

 Here's what I think I know

True mach - varies only with temperature

Indicated airspeed - measured by the pitot tubes, not accurate in many ways, but what matters when in comes to top speed, helis from retreating blade stall, planes from airframe damage or drag == thrust.

I guess there is also "indicated mach" - no idea how that instrument works...

groundspeed - how fast you are actually moving over the ground

True airspeed - https://skybrary.aero/articles/true-airspeed

So, on the "why aircraft fly high". AIUI - roughly speeking, the aircraft behaves according to IAS at low alt and TAS at high alt. The higher you go the higher the ground speed for the same TAS, so if the "mission" is to get somewhere as fast as possible, you want to be at as high an altitude as possible, in order that your groundspeed be as high as possible for the same fuel burn / TAS.

_____

moving into things I know even less about.

It looks like the F14 has a ~1400kts true airspeed limit, using 

https://aerotoolbox.com/airspeed-conversions/

1400kts CAS at 100 feet is mach 2.119

1400kts CAS at 30000 feet is mach 2.849

1400kts TAS at 100 feet is mach 2.117

1400kts TAS at 30000 feet is mach 2.376

Now, there are then other limits, like G limits, engine limits (air density coming in to the engine will obviously play some kind of role), afterburner limits, AIUI these start coming into "secret squirrel" type stuff, because the actual tests are not public, this was also a pretty big part of the storyline in the new TopGun:Maverick movie

Fabulous movie btw, if you haven't watched it, watch it. (I'll skip the spoilers as to why its on an F14 topic).

 

Edited December 14, 20223 yr by mSparks

1 hour ago, SAS443 said:

850KIAS

80% incorrect, the top speed of the F14, any variant is Waaaaay higher than mach 1.4 at 30,000 feet (although that could be 1400kts TAS.....)

aiui this is the "by the book" limits of full afterburner

There is a question of how hard of a limit is that engine limit. got any evidence its not just a "should replace the engine after landing" limit?

Edited December 14, 20223 yr by mSparks

Quote

top speed in mach altitude makes no difference - altitude just makes a difference in how efficiently it does the journey in terms of fuel consumption.

 

 

1 hour ago, mSparks said:

TBH, I make no claims to really having a clue about anything over a few thousand feet or a few hundred knots. This tomcat stuff is completely new to me, only time I ever put any time on a fast jet before now on a modern rig is Ace Combat 7 and those you get in the Battlefield series (and only then when someone stole my heli and I had to go sneak into the enemy spawn point to take theirs)

 Here's what I think I know

True mach - varies only with temperature

Indicated airspeed - measured by the pitot tubes, not accurate in many ways, but what matters when in comes to top speed, helis from retreating blade stall, planes from airframe damage or drag == thrust.

I guess there is also "indicated mach" - no idea how that instrument works...

So, on the "why aircraft fly high". AIUI - roughly speeking, the aircraft behaves according to IAS at low alt and TAS at high alt.

_____

moving into things I know even less about.

It looks like the F14 has a ~1400kts true airspeed limit, using 

https://aerotoolbox.com/airspeed-conversions/

1400kts CAS at 100 feet is mach 2.119

1400kts CAS at 30000 feet is mach 2.849

1400kts TAS at 100 feet is mach 2.117

1400kts TAS at 30000 feet is mach 2.376

Now, there are then other limits, like G limits, engine limits (air density coming in to the engine will obviously play some kind of role), afterburner limits, AIUI these start coming into "secret squirrel" type stuff, because the actual tests are not public, this was also a pretty big part of the storyline in the new TopGun:Maverick movie

 

 

54 minutes ago, mSparks said:

80% incorrect, the top speed of the F14, any variant is Waaaaay higher than mach 1.4 at 30,000 feet (although that could be 1400kts TAS.....)

aiui this is the "by the book" limits of full afterburner

There is a question of how hard of a limit is that engine limit. got any evidence its not just a "should replace the engine after landing" limit?

I'm just going to quote all of this so it can't be edited away.

Will be a good reference next time you start talking about the accuracy of flight models.

This is like watching a squirrel do algebra.

11 minutes ago, 2reds2whites said:

 

 

I'm just going to quote all of this so it can't be edited away.

Will be a good reference next time you start talking about the accuracy of flight models.

This is like watching a squirrel do algebra.

So you agree with everything then 🤣

Or just disagree with "Mcrit is the same at all altitudes."

 

Edited December 14, 20223 yr by mSparks

1 hour ago, mSparks said:

80% incorrect

Perhaps you should read up on what an indicated (or Calibrated) airspeed really is, and what VNE represents and why dense atmosphere is a limiting factor at low altitudes for reaching Mach 2.x or whatever you claim is possible.

I'm missing your 1.400 knots entry in the maximum speed diagrams.

Edited December 14, 20223 yr by SAS443

16 minutes ago, SAS443 said:

I'm missing your 1.400 knots entry in the maximum speed diagrams.

https://e6bx.com/tas/

100feet , 29.92inhg, 14'C OAT, 850KIAS = 850KTAS

30000feet , 29.92inhg, -52'C OAT, 850KIAS = 1400KTAS - the 850KIAS mach 2.2 in your diagram

https://aerotoolbox.com/airspeed-conversions/

1400KTAS@30000feet , 29.92inhg, -52'C OAT = mach 2.376  - the 850KIAS mach 2.2 in your diagram

1400KTAS@100feet , 29.92inhg, 14'C OAT = mach 2.117 - roughly the bottom right of your diagram

 

Edited December 14, 20223 yr by mSparks

Here's mach 2 at FL290, which also seems to be about as fast as it can go - IAS 738 on the HUD, 1267knots true on the map popup

Also, wow the cockpit looks fabulous in RC4

A lot of wrong info on the other threads about F14 max speed and supposed flight model flaws in X-Plane, so let's get things straight.

The max speed at very low altitude is mostly limited by either max structural CAS or engine limitations (max EAS, max inlet temperature).

Proof1:

Proof 2:

This is for the J79/F104.

As you can see, at low altitudes the engine would _theoretically_ be capable of producing much more thrust than that needed to go past M1.2.

The problem is that, in practice, a real turbojet engine would blow up if trying to do that. And even if wouldn't blow up, it would be the aircraft to have a structural failure for surpassing Vne.

You can also experiment yourself with this nice applet: https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/enginesim/

Load the F100 engine, and see how it will exceed temperature limitations just above M1.0 at sea level, while it can go to much higher Mach numbers at high altitude (the figures are not 100% accurate with the real engine data, but they give you a very good idea of the trends).

-----

So, the issue in X-Plane is NOT the aerodynamic modeling: it correctly takes into account air density for drag, contrary to what some people claimed in other threads.

The issue is that in X-Plane the engine can produce ever higher thrust at higher Mach numbers without blowing up, while a real engine would destroy itself.

Honestly, I don't know if the thrust of real engines is "forcibly" limited at low altitude or not.

For as we know, a real F14 could very well exceed M1.2 by far at low altitude, only to have its turbojets trashed or blown up. The airframe wouldn't structurally survive that speed though, as it _correctly_ happens in X-Plane if you have the option checked. (Speaking of which, are there structural failures for exceeding Vne in MSFS or DCS?)

Edited December 14, 20223 yr by Murmur

@Murmur, thank you for posting that data. I was going to posit the same idea in the now closed thread that the issue was temp and structural limitations.  
 

Also, and I don’t have the hard data to illustrate just how big a role this plays, but logically it is harder for an aircraft to achieve a higher Mach number in warmer temperatures close to the ground. Mach is the relationship between true airspeed and the speed of sound. The speed of sound increases as temperature increases so theoretically you’d have to fly at a higher TAS to achieve the same Mach number in warmer the air…which again would be higher at lower altitudes. 

1 hour ago, Murmur said:

The max speed at very low altitude is mostly limited by either max structural CAS or engine limitations (max EAS, max inlet temperature).

Which many of us pointed out to sparky.  That fact is nothing new. Air is too dense to accomodate MMO speeds or the like. it's aerodynamics 101. KCAS is limiting at low altitude, mach number at high. Irregardless of fighter jet or Boeing airliner.

1 hour ago, Murmur said:

The issue is that in X-Plane the engine can produce ever higher thrust at higher Mach numbers without blowing up, while a real engine would destroy itself.

So we have an issue with the flightmodel. (the 'thrust' part of lift-drag-thrust-weight and how they act on the aiframe)

1 hour ago, Murmur said:

For as we know, a real F14 could very well exceed M1.2 by far at low altitude,

Not M2.0. That was the statement given right?
Another issue is how are you able to safely control roll axis without damaging fins and wings due to excess G-load (aeroelastic divergence).

It was over a decade ago since I had aerodynamics classes in university. But I'm quite certain that M2.0 is impossible at sea level for the F14, for multiple reasons discussed. 

Edited December 14, 20223 yr by SAS443

2 hours ago, mSparks said:

1267knots true on the map popup

at FL290

and also 1267kts true at 100feet

28 minutes ago, SAS443 said:

So we have an issue with the flightmodel. (the 'thrust' part of lift-drag-thrust-weight and how they act on the aiframe)

While I said

8 hours ago, mSparks said:

I also dont have remove flight surfaces checked, and have been flying with a complete disregard for saftey.

check that box, and, oh look

The Vne is set to 800KIAS, so Mach 1.211 at 100 feet, Vmo at Mach 2.34

Sooo, what exactly are you complaining about again? Quote me exactly what you disagree with.

Edited December 14, 20223 yr by mSparks

23 minutes ago, mSparks said:

The Vne is set to 800KIAS, so Mach 1.211 at 100 feet, Vmo at Mach 2.34

Sooo, what exactly are you complaining about again? Quote me exactly what you disagree with.

glad you asked.

 

 

This is you mouthing off when I posted the VNE speed of the Tomcat. Which is not a mach number nor True Airspeed.  Which you should know. If you did, you would actually agree with me.

This was easy. Enjoy the rest of your day 🙂

Edited December 14, 20223 yr by SAS443

51 minutes ago, SAS443 said:

glad you asked.

 

 

This is you mouthing off when I posted the VNE speed of the Tomcat. Which is not a mach number nor True Airspeed.  Which you should know. If you did, you would actually agree with me.

This was easy. Enjoy the rest of your day 🙂

hence 80% incorrect. because 850KIAS is in virtually no way the determinant of top speed in mach

If you could do 850kts KIAS at FL530, then it would be doing mach 2.544 - which is above its MMo.

OTOH, its is also capable of doing 1400kts TAS - which is what determines the structural limits aiui... sooo

 

 

Edited December 14, 20223 yr by mSparks

2 hours ago, Murmur said:

Honestly, I don't know if the thrust of real engines is "forcibly" limited at low altitude or not.

Turbines generally behave similarly, described by a performance map that looks something like this (this is a canned map for a CF34-class turbine made by a 2013 grad student who's now apparently at Los Alamos):

 

SSL is static sea level. That's the baseline max thrust output of the turbine. Flight thrust is calculated by taking that max value and scaling it as a function of altitude and Mach. Building a map like this is something of a pain so Planemaker has them built in. These curves get shifted and scaled (safely, because they're pretty consistent across all turbines) as a function of various loss-generating factors that Austin describes here:

 

So, the user sets a throttle position, the sim calculates what %N1 they're requesting, simulates spool lag to get there, and scales the thrust per mach/alt along the way.

This is the one flight modeling aspect where XP and fs2020 are essentially on par with each other. The primary difference being that in the latter, if the dev is going to use a performance map like this, they have to create their own from scratch (hard to imagine Planemaker not being used there at times). There also exist datarefs in XP that can be used to tweak the maps 'on the fly', so to speak, to customize the thrust where needed.

Backing up in the video series a couple vids will show an option to set max afterburner thrust.

What I suspect is happening for the F14 is the afterburner thrust scaling needs to be tweaked. There's also a dataref that sets %afterburner application. Easy enough to create a function that sets that as a function of altitude/Mach. 

2 hours ago, SAS443 said:

Which many of us pointed out to sparky.  That fact is nothing new. Air is too dense to accomodate MMO speeds or the like. it's aerodynamics 101. KCAS is limiting at low altitude, mach number at high. Irregardless of fighter jet or Boeing airliner.

It was suggested in other threads that the reason the F14 can do Mach 2 at low altitude is because X-Plane is calculating the wrong drag at low altitude (i.e. it is not taking account air density), that is completely wrong so it needed to be corrected.

2 hours ago, SAS443 said:

So we have an issue with the flightmodel. (the 'thrust' part of lift-drag-thrust-weight and how they act on the aiframe)

I tend to see aerodynamics, propulsion, ground physics, aircraft systems, etc. as separate things in a flight simulator, so that was I meant when saying it's not an "aerodynamics" issue.

2 hours ago, SAS443 said:

Not M2.0. That was the statement given right?
Another issue is how are you able to safely control roll axis without damaging fins and wings due to excess G-load (aeroelastic divergence).

With all realism settings activated, the F14 in X-Plane can't reach M2.0. It will catastrophically fail around M1.5.

And you're right, aeroelastic divergence is the cause of structural failures above Vne. Very hard to model in a flight sim: X-Plane will simply destroy the aircraft when significantly above Vne, or when G limits are surpassed.

Not very realistic from a visual point of view. Still better than the other sims though.