Max ground speed 1494 kn?

[draconus]

@Nahen I don't know why would you deny my own, Vindicator's or Ironhand's tests that say otherwise - you can exceed the 1494kts GS in DCS F-15C - not in level flight but in descent. This is true without the wind in the mission. But I already said I tested flying with the wind and the wind speed was not added to GS at all at max speed, which I suppose is a bug.

[Ironhand]

13 hours ago, Nahen said:

I can assure you that I have been flying this module for several years - I ONLY FLY THIS MODULE, I risk saying that I know it like my own pocket. I am not interested in how faithful it is to the real F-15C. I know all its capabilities as a DCS module. Currently, it is impossible to exceed the speed of 2.606 Mach / 1494 knots in level flight. Just like in "diving", you will not exceed this speed either. No matter what angle you descend, whether the is 3 degrees or 45. You will maintain a speed of 2.606 Mach / 1494 knots until you reach the altitude at which simulated air resistance due to its increase in air density begins to slow down the plane.
You do not believe? Give it a try.

 

And one more thing - if you want to check something thoroughly, turn off the wind. Because only under such conditions does it make sense. if you set a strong wind and you fly "against the wind", perhaps the DCS physicist will add a few knots. But from experience - sometimes yes, sometimes not.

 

I don’t doubt your experience in the sim but you seem to doubt mine. I never include wind in a test mission unless it’s a test parameter. So it wasn’t included in the test mission that led to my statement earlier.

Perhaps you can bring your superior time in the F-15C to bear and help me solve the following conundrum:

 

Edited July 6, 2022 by Ironhand

[Ironhand]

I said that I wasn't going to look into this further but I should know that I can't leave a puzzle unfinished. One tidbit to add: 36,300 ft is the magic altitude. Above that you are locked at 1494. Below that (and I don't know how far) it can increase. I don't know if they are getting the speeds from a table or what but...the calculation stops at a lower number (1494) at 36,300 ft. That might explain why different people are experiencing different things. I've seen something similar occur on some TAS/Mach calculators. In fact, some will even warn you about it.

Edited July 6, 2022 by Ironhand

[Frederf]

Mach is basically the square root of temperature. If the airplane is being Mach limited then changes of mission sea level temperature will change the troposphere temperature but the stratosphere temperature may be hard coded. In hot temperatures X Mach will be more knots for a given X.

[Nahen]

7 hours ago, Ironhand said:

I don’t doubt your experience in the sim but you seem to doubt mine. I never include wind in a test mission unless it’s a test parameter. So it wasn’t included in the test mission that led to my statement earlier.

Perhaps you can bring your superior time in the F-15C to bear and help me solve the following conundrum:

 

 

I'm paying some honor to you - but you still haven't exceeded Mach 2.606 in the video;)
Granted, I was wrong, it is possible to exceed the speed of 1494 knots - the film shows that by a whole 9 knots. Anyway, in the DCS F-15C it behaves "strangely" under speed limit conditions and certainly, diving from an altitude of 50,000 feet, you should be able to accelerate much more without any problems.

[-Vindicator-]

Outstanding, Ironhand.  And thanks for the magic number.  That’s the kind of specific info I was looking for.  So I guess a remaining issue is whether an F15c can exceed 1494 knots ground speed in real life.  If it can’t, then obviously the radar screen of DCS’s F15c is wrongly displaying 1503 kn gs.  If an F15c can exceed 1494 kn gs in real life, the 1503 makes sense.  I don’t know whether info on that real life issue is readily available, so I may have to just assume that the display simulates real life.  
 

Dying to get back home on my computer and fool around with this lol…. Thanks to all who posted on this topic.

[Ironhand]

2 hours ago, Nahen said:

I'm paying some honor to you - but you still haven't exceeded Mach 2.606 in the video;)
Granted, I was wrong, it is possible to exceed the speed of 1494 knots - the film shows that by a whole 9 knots. Anyway, in the DCS F-15C it behaves "strangely" under speed limit conditions and certainly, diving from an altitude of 50,000 feet, you should be able to accelerate much more without any problems.

 Exceeding M2.606 was never my goal. I’ll leave that to you F-15 jocks. I only got involved in this conversation in the first place because Vindicator’s issue intrigued me.

There are certainly issues, the biggest of which is that TAS calculations cease at 36,300 ft and default to 1494 as the highest value above that altitude. As I noted earlier, I’ve seen some Mach/TAS/CAS calculators do the same thing. The altitude it happens at, though, is somewhat higher. I don’t know if the calculations become unreliable above a certain altitude or what the reason is.

 

1 hour ago, -Vindicator- said:

… So I guess a remaining issue is whether an F15c can exceed 1494 knots ground speed in real life.  If it can’t, then obviously the radar screen of DCS’s F15c is wrongly displaying 1503 kn gs.  If an F15c can exceed 1494 kn gs in real life, the 1503 makes sense…

Of course it can exceed 1494. Just add a 20 knot tailwind and the GS will be 1514 in the real world. That’s the problem with being so fixated on the GS. It varies, not in the DCS F-15 but in the real world. If you’re top speed in the aircraft were 1600 kt, your GS would vary, quite literally, with the speed and direction of the wind.

BTW, that 1600 is probably somewhere around the aircraft’s practical top speed. Various websites report various numbers but 1600 or slightly more is a safe bet. Get your hands on the aircraft’s real world flight manual, if you want more precise numbers. But I guarantee, max GS will not be among the listed stats.

Edited July 6, 2022 by Ironhand

[Cmptohocah]

16 hours ago, Ironhand said:

... But I guarantee, max GS will not be among the listed stats.

 

There is no maximum or minimum Ground Speed in a practical sense. Depending on the wind GS can theoretically be 0 or even negative. It's always TAS+/-wind.

It has no performance sense and the only thing it tells you is how fast you will reach some fixed point on the ground.

This is true for level flight. If you dive down at an angle of 90deg, the GS will be zero.

Edited July 7, 2022 by Cmptohocah

[Ironhand]

2 hours ago, Cmptohocah said:

There is no maximum or minimum Ground Speed in a practical sense. Depending on the wind GS can theoretically be 0 or even negative. It's always TAS+/-wind.

It has no performance sense and the only thing it tells you is how fast you will reach some fixed point on the ground.

This is true for level flight. If you dive down at an angle of 90deg, the GS will be zero.

 

Which is why I made the statement.

[-Vindicator-]

Got home, still foolin with it but got to 1509 kn gs lol

[Ironhand]

To infinity and beyond!  

[Ironhand]

For your viewing enjoyment, start at 35000 ft and enter a Mach number. I used 2.6. Then keep increasing the altitude by 500 and watch the TAS behavior. Look familiar?

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

Edited July 8, 2022 by Ironhand

[-Vindicator-]

Lol thanks!