PDA

View Full Version : TAS to IAS

thinman
08-26-2008, 01:15 PM
I have a print out from a previous thread showing how to calculate TAS/GS from the IAS, but is there a formula for calculating IAS from the TAS/GS?
When I do a flight plan it shows the GS (kts) estimate and I would like to figure out what the IAS is so I can set it prior to taking off.
Thanks,

____________
Bob

xxmikexx
08-26-2008, 01:51 PM
thinman,

Post the forumula your software is using and I should be able to come up with the reverse algebra that will answer your question.

thinman
08-26-2008, 05:11 PM
Hi Mike,

Not using software. The printout I have is an answer I received from a prior post concerning IAS vs GS. The formula is as follows.

TAS = ((IAS x 0.02) x (Altitude/1000)) + IAS

The example was:

At 4,000ft 140kts IAS = ((140x0.02) x 4) + 140 = 151kts TAS

By the way, this was supplied by Alastair back around 22 Jul 08.

____________
Bob

xxmikexx
08-26-2008, 07:08 PM
thinman,

I'm going to show the derivation of what you want so that other people will kind of understand what to do when similar situations arise in the future ...

xxxxxxxxxxxxxxxxxxxx

This is an algebra problem. In essence you have asked that we solve your equation for IAS as a function of TAS. Let's do it step by step as follows ...

TAS = ((IAS x 0.02) x (Altitude/1000)) + IAS <------- your original equation

TAS = IAS + (IAS x 0.02 x Altitude) / 1000 <------- rearranging and simplifying

1000 x TAS = (1000 x IAS) + (IAS x 0.02 x Altitude) <----- multiply both sides of the equation by 1,000

1000 x TAS = (1000 x IAS) + 0.02 x IAS x Altitude <------ remove parentheses because no longer needed

Oops, and now we see a problem ...

Unless I'm mistaken this equation can only be solved for IAS iteratively. That is, plug in a series of guesses for IAS until the resulting value of computed TAS is very close to the target value of TAS. Such an iterative solution can be set up in almost any spreadsheeting system. It can also be done by hand ...

Trouble is, whether by spreadsheet or by hand, this now becomes a problem in iterative numerical analysis and while I certainly could create the necessary algorithm given enough time (adaptation of Newton-Raphson), I instead used Google to search for

"ias from tas"

This resulted in the following link into a thread in an Avsim forum ... http://forums.avsim.net/dcboard.php?az=show_mesg&forum=122&topic_id=35469&mesg_id=36083&page=6 and one of the posts in this thread says "Found a formula to calculate IAS by myself on this site: http://williams.best.vwh.net/avform.htm#Intro (http://williams.best.vwh.net/avform.htm#Intro)." (I take "by myself" to mean "by hand".)

If you search within that second link for "IAS" you will come to a discussion about how to do it. Frankly, I haven't read the discussion. If you can't figure it out, let me know and over a several-day period I'll design an iterative hand solution that will do the job. If you're looking for a programmatic solution you could then use the hand solution as a guide to writing the code or spreadsheet formula.

But I'm intensely curious, thinman. Why are we doing this? Are you doing some kind of panel programming?

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I wish I had better news,
But I'll develop the by-hand solution myself if you really really really need me to,
But it would take me several days because this would be both difficult and low priority,

lnuss
08-26-2008, 09:20 PM
I have a print out from a previous thread showing how to calculate TAS/GS from the IAS, but is there a formula for calculating IAS from the TAS/GS?
When I do a flight plan it shows the GS (kts) estimate and I would like to figure out what the IAS is so I can set it prior to taking off.
Thanks,

____________
Bob

Keep in mind that the 2% per 1,000 feet to convert IAS to TAS is just a rough guesstimate when conditions are standard, that is, 59º F, 0% humidity, 29.92" Hg at sea level, and the lapse rate (temperature drop) is 3.5º F/1,000 feet of altitude change.

Any variation from those conditions throws off your calculations, which is why pilots use a flight computer (E6B, a circular slide rule or its electronic equivalent) to do that calculation. So with that rule of thumb you could be off by as much as 10-15%, maybe more under some conditions, though you may be very close, again depending on the existing conditions.

I have trouble visualizing a reason to do that conversion in reverse, unless you have the sim set to show TAS on your indicator, in which case you're losing the benefits of the airspeed indicator, which is to show stall speed, climb speed, never exceed speed, etc. corrected for the change in air density. We HAVE to use IAS in the real world, and only use TAS to figure times for navigation and fuel burn -- it's useless otherwise. Actually, we care more about ground speed than we do TAS, since GS is what actually affects our navigation and times. TAS is just an intermediate value used to get ground speed.

Oh, yes -- you keep saying TAS/GS -- they're only the same if you have NO wind whatsoever at your altitude. And that 2% rule gives you only a rough TAS. You must then take wind at your altitude into account to figure ground speed.

InsyleM
08-27-2008, 02:12 AM
Hi Mike,

Not using software. The printout I have is an answer I received from a prior post concerning IAS vs GS. The formula is as follows.

TAS = ((IAS x 0.02) x (Altitude/1000)) + IAS

The example was:

At 4,000ft 140kts IAS = ((140x0.02) x 4) + 140 = 151kts TAS

By the way, this was supplied by Alastair back around 22 Jul 08.

____________
Bob

I dont think that's right,
TAS is also compensated for tempature as well, and I don't see a
tempature varible in the equation......
Also keep in mind you need to know wind direction at the altitude your flying at...
IAS/TAS. If you have a side wind, you may have to use the pathergian therom and
vector calculations.
-Jonathan

bstolle
08-27-2008, 03:45 AM
Jonathan,

It might be a good idea to check the other responses before posting.
Especially the one right above yours from Inuss ;)
Furthermore in aviation there is no "side wind"....

tigisfat
08-27-2008, 04:50 AM
Jonathan,

It might be a good idea to check the other responses before posting.
Especially the one right above yours from Inuss ;)
Furthermore in aviation there is no "side wind"....

I beg to differ. Not only is there a "side wind", there are also "butt winds" and "jib winds" respectively.

xxmikexx
08-27-2008, 05:02 AM
Are we forgetting CAS -- calibrated air speed? Do we care?

bstolle
08-27-2008, 05:13 AM
Tig, sorry but my english isn't good enough to get these.
But I assume you can experience buttwinds only if you are flying a boeing peashooter ;)

Mike,

No we are not forgetting CAS, but 'we' don't care is there isn't any daily use for it....

xxmikexx
08-27-2008, 06:28 AM
Bernt/bstolle,

I don't know those terms either. They sound like they may have something to do with sailing -- and I'd rather be flying.

davehero
08-27-2008, 07:53 AM
The calculation isn't iterative. Your formula rearranged is as follows:

IAS = TAS/(1+0.00002*Alt)

However, as mentioned above, use with caution.

xxmikexx
08-27-2008, 12:53 PM
Folks,

Member davehero is correct. The formula is as he gives it. Now ...

All I can say is, I'm 64 and these things happen. Don't get old if you can reasonably avoid it. :D

(and no wonder "hero" appears in his forum name)

thinman
08-27-2008, 03:56 PM
[QUOTE=xxmikexx;1218501]thinman,

But I'm intensely curious, thinman. Why are we doing this? Are you doing some kind of panel programming?

QUOTE]

Hi Mike,

Not programming. It's that when I do a flight plan it gives me the distance, altitude, speed, etc., with speed being in estimated GS. Just had a wild brainwave and thought if I had a formula to convert it to IAS it would be a little easier. As you mentioned I could just convert IAS to GS until I got the GS, or pretty close anyway. I guess after doing this enough I would get a rough estimate as to what IAS to start with.

____________
Bob

xxmikexx
08-27-2008, 04:35 PM
thinman,

You can swag things as follows ...

If you're going to be cruising at around Mach .81 at around FL340 your indicated airspeed will be about 280 KIAS and your true airspeed therefore will be about 470 knots. To the TAS you can then add/subtract an estimate of whatever headwind/tailwind component is expected.

I personally never try to calculate anything in detail, I just base everything on reasonable simple corrections to this single situation depending on what's going on. For example, a descent at .60M is going to be .60/.81 of crusing airspeed, or about 3/4 of the cruise value. In this case TAS during the early phases of descent will be about 205 knots, but for realism you should be holding about .60M until IAS has risen to about 270 knots, at which point you should switch from Mach hold to IAS hold.

So ... From my viewpoint the only reason for wanting to know TAS is that it will be the same as zero-wind groundspeed. If you're really concerned but really lazy like me :D, during flight you can go into Aircraft, RealismSettings and switch from showing IAS to showing TAS. Then, knowing that number, you can switch back to IAS. (As in, Let FS do the calcuations for you.)

If member skylab, a former jet airline pilot, were here he would say what he has said so many times ... "Flying is not an exact science." By that he means that you don't have to be terribly precise.

I hope this helps,

thinman
08-27-2008, 05:23 PM
Mike,

It does help. Thanks.

Oh. And while we're on the subject of IAS and GS, I did a flight plan (FS2002)from Houston, TX to Tokyo, Japan. It had me flying at only 18500 at approximately 506 GS. Well at that altitude if you try to fly that fast by setting the appropriate IAS speed, that cop in the cockpit will be pushing on that overspeed button 'til his finger wears out.:)

_________
Bob

xxmikexx
08-27-2008, 05:42 PM
thinman,

It sounds as if you might have accidentally filed a VFR flight plan. Either that or the planner screwed up on alitude exactly as you have reported. In either case you can type an altitude request (demand, actually) into the flight plan.

Folks,

I've forgotten exactly where the text box for making that demand is located. I've forgotten because these days I'm using an external flight planner called FlightSim Commander X, discussed here ...

http://www.fspilotshop.com/product_info.php?products_id=1378&ref=135, \$41.95 US. Check out the screenshots at the bottom of the writeup followed by the features list.

Folks,

As it happens I'm running an older version of FlightSim Commander that will only couple to FS2004 for the moving map display, this version being out of print so to speak. The latest version couples only to FSX. However, it has all the features of its FS2004 counterpart ... And if you're running FS2004 and all you want to do is generate flight plans with waypoints, and/or to print schematic navigation charts, even Flight Sim Commander X will work for you as a free-standing standing utility. I love my older edititon of Flight Sim Commander to pieces and will not go back to the FS planner. This works for me because I don't care about either moving map or having the flight planner drive the aircraft via an FS-compatible flight plan.

You see, folks, we don' need no planner-driven autopilot. We generally hand fly from gate to gate, this being practical because because my flights tend to be only 1-2 hours in duration. In contrast, thinman is making long trips over open ocean. Here some kind of navigation automation is imperative. Otherwise thinman will not get enough sleep. :) Worse still, he might die of boredom.

The price for Flight Sim Commander X is as of 28aug-08 and is subject to change without notice. Go Broncos.

alexm
08-27-2008, 05:50 PM
And while we're on the subject of IAS and GS, I did a flight plan (FS2002)from Houston, TX to Tokyo, Japan. It had me flying at only 18500 at approximately 506 GS.Bob

The FS flight planner isn't particularly good at choosing appropriate flight levels for airliners. It has no way of knowing what your load is. You can get a fair idea by looking at RW flight plans at www.flightaware.com . Bear in mind that you won't know the load of these planes either, but it should give you a pretty fair idea of what flight levels are used.

Regards,

Alex M.

davehero
08-29-2008, 07:14 AM
Folks,

Member davehero is correct. The formula is as he gives it. Now ...

All I can say is, I'm 64 and these things happen. Don't get old if you can reasonably avoid it. :D

(and no wonder "hero" appears in his forum name)

LOL - age is an unfortunate side-effect of living for a long time.;) And "hero" is an official title - I'm struggling to get people to recognise it though.:D

alastairmonk
08-29-2008, 10:31 AM
I dont think that's right,
TAS is also compensated for tempature as well, and I don't see a
tempature varible in the equation......
Also keep in mind you need to know wind direction at the altitude your flying at...
IAS/TAS. If you have a side wind, you may have to use the pathergian therom and
vector calculations.
-Jonathan

Hi Jonathon,

You're quite correct that to more accurately determine TAS you need to take temperature into account, but the formula I provided will give you a good approximation.

From what I recall the subject came up when explaining why "overspeeds" at a particular speed value were happening, and the formula can help to show that relatively modest IAS values can "convert" to more significant "true" speeds at altitude.

Its much like the "rule of thumb" for converting temperatures. For Celsius to Fahrenheit "double it and add 30". For mild weather in the "middle range" of temperatures it gives a good approximation, but at elevated temperatures the differential becomes more marked:

Rule of Thumb: 20C = 70F
Accurate: 20C = 68F

Rule of Thumb: 35C = 100F
Accurate: 35C = 95F

Alastair

Tim_A
08-29-2008, 07:33 PM
Rule of Thumb: 35C = 100F
Accurate: 35C = 95F

37C = 98.8F (ie normal body temperature) is a pretty easy rule of thumb there :)

Jhew
08-29-2008, 08:56 PM
Hi Bob,

My suggestion is to do what real pilots do. As Larry (Inuss) mentioned earlier, there is a reason we use E6Bs. This one
http://www.mypilotstore.com/MyPilotStore/sep/2233
is the one I first used many years ago. It's only \$12.50. That's alot less than even the cheapest FS addons these days. This one's a plasticized cardboard but don't let that scare you off. Mine held up through many years of use until I inadvertently left it on the glareshield for a day on 100f day in an Archer. My guess is that you wont need to worry about that sitting in front of your computer.
With my E6B, I can compute TAS with one hand faster than you can input the numbers into a computer program. Once you have the TAS you can work out the IAS/GS. The instruction manual explains the process and gives you some sample problems to work out. The E6B is a true testament to the KISS principle.
Best of Luck.
Regards,
Jeff