DCS F-15C Combat guide for beginners by Sryan [very IMG heavy!]

[Decibel_dB]

@klarsnow

And slight correction, the Speed of sound changes with the ambient air pressure, it is not a static speed, so up at 40,000Ft your IAS showing 500kts is definitely over the mach. Whereas down at sea level you may not even be able to break the mach at 650 Knots IAS.

I am sure you mean temperature because you know the pressure is irrelevant for the speed of sound, right?

[AeroMechanical]

@Decibel_dB

pressure is irrelevant for the speed of sound, right?

Ooof.

Well, you’re not wrong, for an ideal gas the speed of sound is entirely dependent on temperature, given that you’re hopefully not in a situation where the adiabatic index (gamma) or mass (M) are changing:

Which, in a stable, dry atmospheric region like the upper stratosphere or mesosphere, that’s probably an okay assumption BASED ON YOUR FLIGHT CONDITION **. Buuut…

https://www.grc.nasa.gov/www/k-12/airplane/sound.html

The best quote from that page is the one at the very top: “Speed of sound (a) depends on the type of medium and the temperature of the medium.” When you’re down in the weeds of the Troposphere that medium is a sloshing mess thanks to weather and terrain, and while the speed of sound is predominantly going to be determined by temperature still, the assumption that the other terms in the equation above (gamma, M) are going to remain constant is probably shaky, especially as you zip through the medium at the speed of heat.

TL;DR: my eyes widen when a hard adjective like “IRRELEVANT” is thrown around in something as complex as aerodynamics/atmospheric modeling/fluid dynamics. Assuming what you meant was, “In most atmospheric models, temperature is the dominant variable in determining the speed of sound in stable atmospheric conditions,” yes, you’re right.

*Side note, NASA’s got amazing references on the foundations of aerospace engineering, check 'em out: Index of Compressible Aerodynamics Slides

** http://web.stanford.edu/group/uq/pdfs/conferences/AIAA-2010-0707.pdf

[Decibel_dB]

I should have wrote " Irrelevant unless your on a spaceship" perhaps

[komemiute]

Aaaaaaand I’ve lost you totally, guys…

[fearlessfrog]

Well I’m inspired to add Math equation LaTex support to the forum software now…

https://meta.discourse.org/t/mathjax-plugin-supports-math-notation-using-latex/12826

[AeroMechanical]

Yessssssssssss!

[Sryan]

@fearlessfrog

BROKEN ON CURRENT VERSION OF DISCOURSE

[Sryan]

@near_blind

Optimally, you want to be looking outside of the cockpit as much as possible without neglecting the your other instruments. In the example of the BVR fight, the radar will be the primary instrument for determining the bandits position and state, but peaking outside will hopefully allow you to pick up on what his shots are doing (are the tracking? are they dumb?, whether or not he’s launched any IR missiles, and depending on what radar mode you are in (STT), what his wingman is doing, and if any additional threats have picked up (did he have an additional wingman on the deck or some place?). Most importantly it allows you to maintain a sight if worse comes the worse and your BVR engagement develops into a WVR engagement.

@klarsnow

typical inside/outside time they teach IRL is bout 20/80% varying depending on what part of the engagement you are at.

This is also because you will be maintaining a tactical formation, looking out for any thing else you might see as NB talked about, and in general keeping eyes outside can help a lot in keeping your SA up, as opposed to getting sucked into the green goo ( as we call it).

You guys are totally right. It’s also why I wrote the apendix to my reply to dB that looking outside is still important. What I see a lot of people do though is put their plane so that the bandit sits in the HUD (no contact visual, just the green box), preferably dead center of it. This is not always the ideal thing to do. Hence I am trying to get people to head down a little and work the bandit from the radar screen a bit as well. experience (or training for guys like you ) should dictate how much time you should spend looking outside. I’ll make sure to clarify it a bit better. The green goo stuff goes in the guide too

@klarsnow

And slight correction, the Speed of sound changes with the ambient air pressure, it is not a static speed, so up at 40,000Ft your IAS showing 500kts is definitely over the mach. Whereas down at sea level you may not even be able to break the mach at 650 Knots IAS.

Completely right. Even at a specific altitude the Speed of Sound is not static due to temperature. However with IAS being calibrated to 0 ASL… Hence my statement that a speed of 500 knots is not supersonic when looking purely at IAS and nothing else (the altimeter and mach indicator proving otherwise). Reading back now I think I’m trying to confuse myself to justify my claims, so I’ll correct this.

@klarsnow

GS is useless unless you want to navigate somewhere on the ground. TAS is what you really want for BVR intercepts, because its the only way you can compare what you and the bandit are doing

True, when writing, I forgot the VSD displays
bandit TAS as well, and not GS. I will be correcting this![quote=“klarsnow, post:19, topic:2063”]
Other interesting thing about mach, it is actually a very good indicator of True Airspeed as well. A common rule of thumb used when navigating is your mach is how many miles a minute you are going.

So .9 mach = 9 miles a minute, 2.0 mach = 20 miles a minute. This rule of thumb holds true at pretty much all altitudes, and for navigation purposes works out great.
[/quote]

Excellent stuff! I’d like to put this in with your permission. Quoted if you prefer.

@klarsnow

IAS is an indicator of how many molecules are going over/under the wings, so at 250 Kts IAS, whether its at 40,000Ft or 2,000 ft the plane should (mach effects and other effects aside) behave the same.

I thought I had this one covered! Albeit without the molecule theory, wich is true ofcourse. I thought I’d leave that out in order to keep things accesible. I’ll have to think about it, still appreciate bringing up the point. I’ll reinforce my arguments so that all aircraft behaviour is similair, bar for Mach effects, at a specific IAS regardless of altitude.

@Sryan

At higher altitude, the pressure is lower. So the device gives a lower reading than the actual True Airspeed is. Useless right? It turns out to be an advantage. The pressure is related to the density. If the pitot tube is detecting a lower pressure, the wings will feel like as if less air is moving across them, and they will want to provide less lift. This means that critical flight regimes, like stalling, occurs at the same indicated speed at all altitudes!

Thank you @near_blind and @klarsnow for the honest, constructive feedback! keep it coming.

[fearlessfrog]

As Discourse (the forum software) is all open source, what I do is fix things up and then submit back fixes. I’ll fix up MathJax. A couple of the bits here we use I wrote (the Steam and some shop URL expanders) and then they get rolled back into the main Discourse release to help everyone else. It’s a good deal for all.

Sometimes other provide improvements for things we find here too - @Sryan you’ll recognize this:

Feature Request: Tall and Thin Image Handling

(Apologies for OT, we’re now back…)

[Sryan]

I didn’t know that. While we’re still OT can we maybe forward one more request? Maybe it’s possible to allow posts in specific sections of the forum to use the full screen estate? Instead of some of the nifty functions we see left and right of current posts. This would specifically be for for the AAR screenshot section.

Just a thought Now we’re really back on topic!

[Sryan]

I didn’t know that. While we’re still OT can we maybe forward one more request? Maybe it’s possible to allow posts in specific sections of the forum to use the full screen estate? Instead of some of the nifty functions we see left and right of current posts. This would specifically be for for the AAR screenshot section.

I also forgot you where a game developer so you can fix things like that.

Just a thought Now we’re really back on topic!

[fearlessfrog]

@Sryan

Maybe it’s possible to allow posts in specific sections of the forum to use the full screen estate

I’m a bit of a Forrest Gump in ruby (the language the forum is written in) as I’m a C++ guy all my life, plus have the aesthetic eye of Genghis Khan, so not much of a web designer, but I am working on a nicer image gallery component - that should give us nicer full screen displays in a responsive way.

[AeroMechanical]

@komemiute

Aaaaaaand I’ve lost you totally, guys…

Don’t worry, all your questions and more will be answered in the upcoming Mudspike series,

Compressible Flow is a Jerk: How I Learned to Stop Worrying and Love The Sound Barrier

[near_blind]

@AeroMechanical

Compressible Flow is a Jerk

There used to be an… eccentricity in the F-15C’s PFM where any roll in the transonic region equaled total, instantaneous loss of stability. One second you’re happy as a clam, setting up for the crank. The next you’re tumbling rear over tea kettle at 20,000 feet and 250 Kias.

[komemiute]

Got to love you guys…

[BeachAV8R]

Really, really technical discussion guys. But I’ve found that my Mach number (a blistering .755 at my absolute fastest speed…sigh…) is not dependent on temperature, altitude, or pressure at all. There are three variables…but they are:

M# = V(b) - SWS

For all you non-engineer types…that (of course) translates to:

Mach Number = Volume (bladder) - Shifted Wind Speed (all aviators know this term…it is that miraculous windshift that turns that 100 knot headwind that you’ve been fighting in one direction…into a similar headwind going the other direction…how that is possible, I don’t know…but it is a real phenomenon…)

So Mach number is largely a function of bladder volume.

Second variable is (of course)

M# = HQ / tM

Or Mach Number is = to Hunger Quotient divided by (time) since last (Meal)… Often can be read not only off the Mach meter, but by the crew irritability index.

Third variable is just a correction factor…

M# correction factor = DTR - 20 minutes to = RON

or…

Mach Number reduction = Duty Time Remaining - 20 minutes = Remain Overnight

Slow planes = more overnights.

They don’t call em’ Slow-tations for nothing…yawn

[klarsnow]

I find the Mach number also decreases proportionally to how many piddle packs you have versus how full your bladder is. Reaching infinitely slow speeds once you have a full bladder and no piddle pack

This is also proportional to how long the taxi back will be and what will break in the jet that the ground few will have to fix before you can shutdown

If you are wearing a poopy suit as well, the bladder curve increases at an exponential rate whilst following the other standard rules

[PaulRix]

@BeachAV8R

Third variable is just a correction factor…

M# correction factor = DTR - 20 minutes to = RON

or…

Mach Number reduction = Duty Time Remaining - 20 minutes = Remain Overnight

Slow planes = more overnights.

Except when you have large fuel tanks. That usually results in an overnight regardless of the speed of the airplane .

[Hangar200]

@Sryan

The F-15 is a great plane to get started with.

Yeah…starting out with a Piper Cub didn’t work well for me…my wing caught fire when I launched my first AIM-9 and I couldn’t seem to get airborne when carrying a MK-84.

Seriously, GREAT tutorial! BZ

[Decibel_dB]

Ok now I understand