IAS / CAS. Or how to determine if your plane stalls at the correct speed or not

[Rapitor 10,702]

Hello, I would like to give a clear and detailed explanation about Indicated Air Speed (IAS) and Calibrated Air Speed (CAS). Moreover, this is critical in WT, especially when it comes to compare / Bug report the stall speed of a plane, supposedly over-/under-performing.

 

Let's start with the fundamental. How do we get the speed in a plane ? With a pitot tube. Basically you compare the static pressure [P_o] (the pressure around you when at rest) with the total pressure [P_s] (static pressure + the pressure induced by the wind blowing on a surface). This difference, using a relationship from Bernouilli, gives you the speed of the airplane.

This is what we call IAS. That's the speed "felt" by the plane.

 

Now, you should be like "ok, but this is IAS; and we have IAS IG, so what is the problem ?".

Well the problem is that on the drawings, we have the airflow being totally aligned with the entrance of the tube, totally perpendicular to the static intake, and totally homogeneous.

 

BUT what if one of those assumption is wrong ? You have an error. And errors are more common than perfectly working pitot tube on an airplane.

Look above: The green arrow is smaller than the black arrow. Thus, P_s is underestimated (the component of the air perpendicular to the tube won't do anything). And look at the blue arrow: it is actually affecting P_o. Worse, it is actually  increasing it by adding a dynamic component to the static intake. P_s is too small, P_o is too big, the velocity is too small (delta pressure is too small).

We are understimating the speed of the plane.

 

What is indicated (IAS) is not what the plane is really "feeling". That is very, extremely important to understand. It can be an overestimation, an underestimation, it doesn't matter, it is a local estimation of the air speed, supposedly representative of the whole airflow around the plane. What the pilot see though the instrument is not the real speed of the plane.IU There are many other reasons for wrong reading, but the reason are not important, the correction for them will take that into account.

 

That's where CAS enters in the picture: engineers are aware of this problem and will actually make measurments to tell you the real speed of the plane. The will tell you that for an IAS of xx kmh, your CAS is xx +2kmh, or xx - 3 kmh, it all depends on the plane and the speed. Those data, after being gathered, are put in a table called PEC (Position Error Correction). You have one PEC table per plane (at least and more when the pitot tube / airframe has been reworked).

 

Then, I have 3 questions and answers for you:

 

- Why don't we have CAS indicators IRL ?

-> Because it is too complex to implement, and it is not relevant. Fundamentally, the pilot doesn't care if he is really flying at xx or yy speed, what he want to know is when he will stall. It doesn't matter if that is the speed of the air around the plane or not, what he want is to know if he is close to stall or not, the rest is not critical.

 

- Why is that important in WT ?

-> Because error doesn't exist in here. We are not a pilot looking at the instrument to evaluate the local speed around the pitot tube, the game knows the speed of the plane and displays it on the speed indicator of your cockpit / HUD. It is not estimating a speed, it knows the airspeed of the plane.

 

- But why is it a problem then ?

-> If you look at a plane handbook, and it say "at 7650lb, the plane stall at 85knots IAS", that is NOT taking into account PEC. Like I said, the pilot doesn't care about it. But the game engine does. Because the game engine knows the speed of the plane, not the speed of the air around the plane (which is impossible to modelize). So basically, the game is such as IAS = CAS = air speed of the plane.

Back to the 85knots IAS stall speed from the manual: if your PEC is +4kts, then your CAS stall speed (IRL) is 89kts. And thus, your IG speed IAS to stall should be 89kts. If the PEC is -10kts, then your stall speed is 75kts CAS  (IRL) thus 75kts IAS IG.

 

I know this is very confusing, but it is also very important to understand this matter. If you take the raw IAS speed from the manual, and set the FM of the plane to match the same IAS, without taking into account the PEC, you can have huge errors, and over-/under-perfoming planes, turn / stall wise.

 

Side note: it is impossible for a dedicated flow computation software to properly modelize IAS for a 3D modelled plane, so don't expect WT to use something else than CAS for the simulation, because it is impossible.

 

So now, when you look at a datasheet and see a stall speed higher/lower than what is written in the manual, don't go straight for a bug report, document yourself, and find the PEC. Once you have it, you can draw conclusions about an improper stall speed for a given plane.

[Rapitor 10,702]

Typical exemple:

 

Tempest II VNE speed (IAS-manual): 420mph below 20'000ft. PEC table: add roughly 8mph (extrapolated) to get CAS. CAS (IRL): 428mph.

Tempest V VNE speed (IAS-manual): 450mph below 20'000ft. PEC table: substract roughly 26-30mph (extrapolated) to get CAS. CAS (IRL): 420-424mph.

 

Looking at manual (first glance) will make you sad "Tempest V can outdive Tempest II !!".

 

But as you can see, CAS is very similar and prooves you wrong. Moreover, Tempest II and V have the same airframe, aside from engine and minor tunings, and thus should have the same limit speed. Luckily, that's what I found there in my exemple.

 

Don't forget PEC, EVER

[jorelo 1,458]

Good information, i hope they pin it 

[blakeob 8,086]

Excellent work!

[fufubear 6,705]

This needs to be pinned.

[blakeob 8,086]

This needs to be pinned.

Your wish was made. Edited February 5, 2016 by blakeob

[Fuyukaze 22]

Is WT even that advanced? Don't they just copy paste whatever IAS number on the datasheets into their FM such that eg if it says X to stall then it stalls at X in game, compensation be damned?

[fufubear 6,705]

Is WT even that advanced? Don't they just copy paste whatever IAS number on the datasheets into their FM such that eg if it says X to stall then it stalls at X in game, compensation be damned?

War thunder has been using cas since the beginning.

[Rapitor 10,702]

Is WT even that advanced? Don't they just copy paste whatever IAS number on the datasheets into their FM such that eg if it says X to stall then it stalls at X in game, compensation be damned?

Yes and no.

 

VSN and Lassar, on a regular basis, answer to "why does xx have a stall speed of yy when it is written zz on the manual" with a "you forgot to take PEC into account".

WT FM designers always used the PEC in the FM. 2,5 years ago, when Nabutso was still a FM tester, PEC was already something the devs were aware of and used for FM tuning.

 

That obviously shows that CAS has always been used IG, and that's how it should be. Like I said, even USAF or Pentagon don't have the computational power to modelize a roughly correct flow around a plane, hence IAS is not something you can impement on a game. Plus, values like Cl, Cd etc are based on the TAS, which is, given the proper formula, related to CAS, not IAS.

 

This said, current P-51D (all version) have a datasheet made by SubRyan, which very likely simply c-p the manual into a datahseet. Why Am I saying that? look at the stall speed in the datasheet (101mph @7'000lb, 106mph @ 8000'lb). That is the very value from the manual, IRL IAS, and obvisouly lack the PEC.

 

That's actually when I personnally started losing it: FM stall above 106mph @8'000lb, Forum datasheet obvisouly says it does stall at 101mph, and the manual provide a PEC of +5mph.

We have currently 3 different stall values for the P-51D: IRL (CAS), IG test flight (IAS which doesn't fit IRL CAS), and Datasheet (IAS which doesn't fit IRL CAS but neither IG test flight).

 

P-51 is definitely not the only plane suffering from this, but then that's why I wrote this thread: to at least make a clarification (that Gaijin never did) so when someone tries to submit a Bug Report about incorrect stall speed, he won't stupidly give the IAS (IRL value), and thus make the dev lose their time, or worse, adapt the plane FM to a value which is wrong.

[Pony51 6,572]

Great work!

Wonder how many players heads exploded?  :)

 

If you have links to what VSN and Lassar said and SubRyan posts that would be great.

:salute: 

[Cpt_Branko 9,698]

Absolutely correct (and yes, what you see in game is really CAS, and yes it's taken into account). I'd like to add another thing when you make tests:

 

When we talk about stall speed we almost universally mean "stall speed at 1G". You can get some erroneous readings by varying the G load - either too high (over 1G) or too low (below 1G).

 

Here's an illustration of P-51D flight limits:

As you can see, without holding the load factor at 1g - our stall speed could be anything. We can easily see the G load in the browser thingy. Or you can guesstimate by zealously holding altitude during your stall test, but cheking it in the browser thingy is better.

Edited May 31, 2016 by Cpt_Branko