@ChiChiWerx Just to clarify, as there seemed to be a misunderstanding :}
Yes, lower altitude and thus higher air density is better for engine power output.
I suspect the mixup was in regard to where I said the "higher DA, the better", in which I only meant the calculated "DA" value that was being referenced for drag racing. Which even then, I could've mis-remembered. (and is quite likely!)
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Either way, I didn't mean to convey that a "higher altitude" was better. :P Sorry for that confusion.

No, Mach number depends only on temperature, it has nothing to do with air density or air pressure or anything else besides temperature. And lower density altitude is better for performance, not higher. Thicker air means engines that put out more power (more available air) and wings produce more lift. But don’t take my 30+ years of aviation experience flying high performance aircraft, just read the Wikipedia article on “Mach Number”. The graph about 1/3 of the way down shows the Mach (speed of sound) vs. temperature relationship.

@ChiChiWerx Thanks for clarifying, AND, for divising that formula for us! :D
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Having no idea what the values were, I started doing what I do, and just mucking with it further. Comparing back and forth between that NASA Calculator and the values my modifications were generating.
Again, I didn't know until just now, what it was all about, but I was coming here to follow up to say that I had managed to fine tune it further. Though, I only tested it at 33,000ft and 1,019ft:
TAS/(341-clamp((Altitude*0.0041437),0,43))
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However, at those two altitudes, it's:
@ 33,000ft - within a few-ten-thousandths off. (NASA Calc only returns to the thousandths 0.734, to my 0.7339)
@ 1,019ft - it's only roughly 2-thousandths off; 0.735 calced vs my 0.7339
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I assume that the one oddity I noticed while changing the "per meter of altitude" number, was that up at 33,000ft, anything over 0.0042 (regardless of the initial divisor used) resulted in the same output value.... However, as I check now while it's paused at 1,019ft, all of those readings are now.
Am I correct in my presumption that the reasons is due to the "correction" was running into your imposed clamped maximum of 43?

In the end I think I see where a lot of my initial issue with wrapping my head around it all, was coming from...
I was erroneously pairing temperature with air density on account that at Sea Level, temperature DOES impact air density. A couple decades back I remember some folk who frequent the drag strip, mentioning "DA" (Density Altitude), and that it was was (somehow) determined based on the altitude the track was at, and the air temp of that day. If I recall correctly, the higher the DA the, better, since it meant denser air, which for them meant they could run inject more fuel (they were all easily able to change the programming on their ECU).
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Fast forward to now, and while I'm sure that temperature is a factor, it no doubt is trivial in comparison to the altitude. Where the amount of atmosphere above that's weighing down is substantially less, causing less compression and thus a thinner atmosphere. (to put it crudely, as is all I'm capable of! lmao)

No, you’re incorrect. And since I came up with the formula @edensk is quoting, let me explain: Mach varies based on temperature, NOT air density. The reason why Mach varies with altitude is that it generally gets colder with higher altitude, that’s why Mach 1 is achieved at a slower TAS at higher altitudes. And that’s why your very rough TAS/343 formula gets further and further off with altitude. But SP doesn’t have temperature incorporated, only altitude and airspeed, so Mach doesn’t vary correctly. But there’s a workaround I came up with: I assumed a standard day temp at S.L. (around 15 degrees Celsius), with a standard temperature decrease of 2 degrees C per 1,000 ft/330 meters. So, I simply modeled the TAS to Mach variation based on standard day temperatures with increasing altitude. For example: I know that it should be 10 degrees C colder at 5,000’ than it is at sea level and the Mach 1 TAS speed is proportionally slower. The conversion factor is .003937 per meter of altitude. There are a couple of problems with my formula: When you get to the Tropopause at around 50,000’ (again, the altitude varies with latitude, from 34,000’ (at the poles) to 56,000’ (equator), so I just used a mid-latitude height) temperature remains constant as you continue to climb. It then gets colder again as you leave the Tropo. That’s why I clamped the correction at 43 m/s…it’s good for a Mach calculation from sea level to around 60,000’ for a mid-latitude location. Two of the four major SP islands seem temperate, so I think that’s a fair assumption. The second issue is that it assumes standard day conditions/temperature…hardly ever the case IRL, where it’s most often colder or hotter, but again, had to model using assumed temperature and since we use “standard day” as the starting point for our calculations in the flying world, also a good assumption, I think. Last, I’ve used that formula on both my Su and F-20 to model shock wave blanking of the horizontal stab, so you’ll find it on the rotators for the concealed surfaces on both those builds. It works fairly well, I think.

@Formula350
1. Because "altitude corrected TAS" is just IAS, which we already have.
2. In ISA conditions the speed of sound at sea level is 340.28 m/s which is about 340 m/s.
3. That would give you a meaningless number, as you would be dividing TAS by the reduction in the speed of sound vs altitude.
You're welcome, as always

@edensk Well, bummer. Do we know why Jundroo didn't opt to have TAS factor in altitude (density)?
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Also, out of curiosity, why does one want to use 340 instead of 343? Is it because of the inherent rounding error SP is prone to, where 340 accounts for that?
Or even a simpler reason: it's easier to remember and still "close enough"? lol
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Either way, thank you for that! And for the fact that you've always been a great help to anyone with coding question :D
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EDIT: Oh and then safe to assume that just removing /340 in order to have it output the Altitude-Adjusted-TAS speed?

It's not a bug as True Airspeed doesn't vary with density, which is why you have to factor altitude in.
The FT code for mach number in the ISA is this: TAS/(340-clamp((Altitude*0.003937),0,43))

@LieutenantSOT Thanks for the info :)
Hmm, IAS sort of is, but if that were true -- and honestly, it might be, but my ignorance is getting in the way, so I won't pretend it can't be an option lol -- then everything I thought I had understood about airspeeds was WAY way off! O_O
Which, again, is indeed possible. As only recently did I discover that Ground Speed is not a measure of your horizontal speed across the ground, where changing your AoA would impact your reported GS. Or to put it another way, I thought GS was more or less calculated by the amount of distance you traveled horizontally in a given time.
(Thus why I thought if you, say in a hypothetical scenario, where you had >1:1 T:WR and were pitched up at say 30deg, your GS would drop due to not covering the same distance as quickly from now going upwards too, even though your plane maintained the same velocity)
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But if you read the quote at the beginning of my comment down below, that definition for True Air Speed really does match what I was expecting OUR TAS ,in game, to be reporting to us. Obviously leading to this surprise that the calculated Mach results were off, and that error rate only grew as Altitude increased.
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In the end, though I hope not, it may indeed be a case where an elaborate script (FT equation) is going to be needed in order to factor in air density in the same way that NASA tool does..... If only I understood coding better, and wasn't absolute-trash at math, that might not be so hard! hahaha
On the plus side, I have an ulterior motive to figure that out, too, as it would (I think at least) provide the basis for allowing me to add another 'feature' to a plane I really wanted to.... So fingers crossed I can figure something out! (I'm not hopeful I'll be able to though, as my ""AG Toggled, Multi-Readout, Configurable Measurement System Air Speed Gauge"" [lol what a name] that uses FT, isn't working how it should. *sobs*)

This is a really cool find!!
I always felt like something was weird lol
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Anyways, I want to start off by saying that TAS is indeed based on the AIR MASS, but as you even mentioned, it is purely for how fast you are going in the Air. When I was doing some flight training in Florida, I asked my instructor why there were 3 types of speeds, as they seem useless in most cases.
He said that they each have their place. GS is how fast your aircraft would be ON THE GROUND, TAS is how fast you are going in the Air (taking wind into account) and IAS is what you're thinking of. Indicated Air Speed is based on Air pressure and speed based on ALTITUDE because of the pitot tube's static Air pressure

TAS in SP is only the speed relative to a hypothetical average particle of air. It only accounts for craft speed and wind speed

@Formula350 I’ll read it tomorrow maybe

@Chillybaconface deal with it.

@X99STRIKER
If my post is really long, take your phone with to the john!
When you're going #2, read it while you're on the loo!
:P

@marcox43 My problem is that if it IS True Air Speed, that only makes it far more likely to be a Bug, since for True Air Speed (emphasis mine):

The true airspeed is the speed that the aircraft travels relative to the air mass in which it is flying. The true airspeed is equal to the ground speed in cases where there is no wind, and is used mostly for flight planning and when quoting aircraft performance specifications. True airspeed can be calculated from either the equivalent airspeed, or the Mach number if the outside air temperature (OAT) is known.

(source: AeroToolbox)
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Worthy of note is that the middle part, is definitely true with SP. GS == TAS when Wind is disabled.
Or is that Jundroo's intention for TAS in Simple Planes... that its sole function is to provide an accurate speed when, and ONLY when, the player has Wind enabled??
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I was always under the assumption that it'd be tied to air density on account of the fact that clearly there is SOME density modeling included, since we experience it when passing through certain altitudes...
Around 11,000ft is the first density change I'm aware of.
Another I believe around 25,000ft
I think one around 39,000ft
I've not spend enough time in the 45,000-80,000 range
Pretty sure there's also one somewhere between 91,000-110,000ft
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That 11,000ft one is the more easily experienced density, particularly in jet powered craft, and especially if it's a faster plane.
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Actually! It just dawned on me that the density is modeled MORE than just in that way!
In my current build, which is a Piston Prop, I have the engine RPM-Limited to 3200RPM [maxRpm at 3232]. It's power output is 3500HP, and that's unfortunately way more than needed for SP's physics, for my build (modeled after a Race plane), as just about anything >75% throttle at low altitudes is wasted since its already at Max RPM.
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At 40,000ft, I can throttle down to about 18%, still achieving Max RPM and therefore suffer no performance loss. At various altitudes I have to use different throttle levels to balance that out, but the benefit of doing so is fuel economy -- legitimate in game economy, not just speaking to real life aircraft performance.
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My point summarized in a question: if density is modeled to the degree that it is... why wouldn't they be utilizing it with TAS? 🤔

I believe it means true air speed, at least regarding air density, I managed to get rather accurate data thankks to Edensk, but for air temperature, you'd have to input your own data ito the equation, AKA: adding the entire temp formula and all that stuff.

@X99STRIKER There's no point in being here if you're not reading it.

I’m not gonna read all that.