Calculating TAS & frequencies

[Mak]

Hi all,

Does anybody know how TAS is calculated from IAS,temp,ALT (and possibly pressure since PPLs tend to fly more on altitudes than flight levels)?

Also is there a freely available (up-to-date) list of RT frequencies?

I'm designing my own flight planning spreadsheet and these are the only bits missing.

Thanks.

Mak

[Aussie Andy]

We should know the answert on TAS I suppose, but admit I'd either have to use the whiz-wheel or look it up in the book (e.g. Trevor Thom).

Airfield frequencies (and some enroute ATC frequencies) are published in the AIP (www.ais.org.uk then login then go to Publications -> AIP) - but listed separately for each airfield, see the AIP section AD2 here (http://www.ais.org.uk/aes/pubs/aip/html/aipad2.htm) (you'll need to login).

Hope this helps,

[Dave Gittins]

As Aussie Andy says, I would have to look at the whiz wheel, read it up in Thom or see if it's in the E6B bit of my nice new Pilot 111 GPS. I suspect that most folk in light aircraft at 2000 ft or so just don't bother as the answer is so close to the IAS.

As far as the frequencies are concerned, most of the useful ones for initial call are on the sides of the half mil and quarter mil chart which you MUST have ???

I also have a BAM-K Bottlang for the UK with an update service which means I am always up to date with frequencies and changes. Generally it means that for my £30 or 40 quid a year if I have to divert from Fairoaks to .... Papa Westray, Barra or someplace in Western Eire I am well ahead of the game.

I have a little A5 "PLOG" sheet which I produce in Excel with my local frequencies and VORs (and space for weather, airfield data, clearances, Hobbs etc.) and modify it for any different journeys I undertake. I also keep copies with the headings and distances for particular VFR routes for re-use already in the appropriate boxes. Then I just need to do the wind calcs and pencil them in.

[Aussie Andy]

Hey Dave g, I would love to swap my MS-WORD format PLOG for a copy of your EXCEL formatted one if interested! I so much prefer the flexibility of being able to make my own PLOG to suit me and the job in hand... would like to see how it is in Excel.

Andy

[Dave Gittins]

No problem. I haven't got a copy with me so It'll have to wait until I get home. I can EMAIL you 2 or three. (Fairoaks, Luton and a couple of trips) My EMAIL Address at home is [email protected].

Also got weight and balance calc sheets on Excel for Cherokee 140, and PA-28-161. These are aeroplane specifics from the weight and balance schedules of particular aircraft and need to have mods made for others., A couple of them do also do a nice plot onto the weight and balance CG graph.

I suppose I should say (before somebody else does it for me) that these are offered without guarantee, for use at your own risk etc. etc.

Like anything else when it comes to my own safety I don't trust non-commercial products ... and sometimes not even those ... without a comprehensive check and a few trail calcs by other means. I expect others to be equally circumspect.

I don't have any urge to contribute to "I Learned About Flying from That "........

:)

[FlyingForFun]

This website (http://www.csgnetwork.com/tasinfocalc.html) suggests adding 2% per thousand feet to your indicated airspeed to get a good approximation of true airspeed.

I don't recall ever seeing the actual formula, taking density altitude into account, anywhere, including my ATPL notes. And of course if you want to do it properly you'd need to include compressibility effects for speeds over 300 knots... although I suspect that's probably not going to be too relevant! I'd guess that you might get a better response to this one in Tech Log, or maybe Questions. But the 2% per thousand feet would probably be good enough for real-world flying - I doubt any of us could fly more accurately than that anyway.

FFF
--------------

[Mak]

The idea of the TAS calculation and frequencies is precisely to create an automatic inflight log (what does plog mean anyway?) so that's why the wheel and lists of frequencies on paper aren't much good. I've added the most common frequencies on paper but it would be nice to have a larger database of frequencies that I can update regularly. Another thing I'd like to add in the future is a database of positions so that tracks can be calculated too.

Mak

[Rote 8]

Mak

The following is all copied directly from a gripping document that I have floating about which provides a range of formula for problems relating to navigation. If you can understand it all then you are a better man than me but most is reasonably straightforward. Should hopefully be what you are looking for. If you are interested in the remainder of the document then drop me an e mail and I will send you a copy.

Cheers

Rote 8

Mach Number (M) = TAS/CS
CS = sound speed= 38.967854*sqrt(T+273.15) where T is the OAT in celsius.
TAS is true airspeed in knots.

Because of compressibility, the measured IAT (indicated air temperature) is higher than the actual true OAT. Approximately:

IAT=OAT+K*TAS^2/7592

The recovery factor K, depends on installation, and is usually in the range 0.95 to 1.0, but can be as low as 0.7. Temperatures are Celsius, TAS in knots.
Also:
OAT = (IAT + 273.15) / (1 + 0.2*K*M^2) - 273.15

The airspeed indicator measures the differential pressure, DP, between the pitot tube and the static port, the resulting indicated airspeed (IAS), when corrected for calibration and installation error is called "calibrated airspeed" (CAS).
For low-speed (M<0.3) airplanes the true airspeed can be obtained from CAS and the density altitude, DA.

TAS = CAS*(rho_0/rho)^0.5=CAS/(1-6.8755856*10^-6 * DA)^2.127940 (DA<36,089.24ft)

Roughly, TAS increases by 1.5% per 1000ft.
When compressibility is taken into account, the calculation of the TAS is more elaborate:

DP=P_0*((1+0.2*(IAS/CS_0)^2)^3.5 -1)
M=(5*( (DP/P+1)^(2/7) -1) )^0.5
TAS= M*CS

P_0 is is (standard) sea-level pressure, CS_0 is the speed of sound at sea-level, CS is the speed of sound at altitude, and P is the pressure at altitude.
These are given by earlier formulae:

P_0= 29.92126 "Hg = 1013.25 mB = 2116.2166 lbs/ft^2
P= P_0*(1-6.8755856*10^-6*PA)^5.2558797, pressure altitude, PA<36,089.24ft
CS= 38.967854*sqrt(T+273.15) where T is the (static/true) OAT in Celsius.
CS_0=38.967854*sqrt(15+273.15)=661.4786 knots

[Example: CAS=250 knots, PA=10000ft, IAT=2C, recovery factor=0.8
DP=29.92126*((1+0.2*(250/661.4786)^2)^3.5 -1)= 3.1001 "
P=29.92126*(1-6.8755856*10^-6 *10000)^5.2558797= 20.577 "
M= (5*( (3.1001/20.577 +1)^(2/7) -1) )^0.5= 0.4523 Mach
OAT=(2+273.15)/(1 + 0.2*0.8*0.4523^2) - 273.15= -6.72C
CS= 38.967854*sqrt(-6.7+273.15)=636.08 knots
TAS=636.08*0.4523=287.7 knots]

In the reverse direction, given Mach number M and pressure altitude PA, we can find the IAS with:

x=(1-6.8755856e-6*PA)^5.2558797
ias=661.4786*(5*((1 + x*((1 + M^2/5)^3.5 - 1))^(2/7.) - 1))^0.5

Some notes on the origins of some of the "magic" number constants in the preceding section:
6.8755856*10^-6 = T'/T_0, where T' is the standard temperature lapse rate and T_0 is the standard sea-level temperature.

5.2558797 = Mg/RT', where M is the (average) molecular weight of air, g is the acceleration of gravity and R is the gas constant.

0.2233609 = ratio of the pressure at the tropopause to sea-level pressure.

4.806346*10^-5 = Mg/RT_tr, where T_tr is the temperature at the tropopause.

4.2558797 = Mg/RT' -1

0.2970756 = ratio of the density at the tropopause to the density at SL (rho_0)

145442 = T_0/T'

38.967854 = sqrt(gamma R/M) (in knots/Kelvin^0.5), where gamma is the ratio of the specific heats of air

[bluskis]

Surely your actual cruise airspeed will be dependent on the power settings you chose to use, therefore for flight planning you know your true airspeed you will use for the flight.

The indicated airspeed will differ from true, but this should not affect flight planning.

On my ASI I just turn the knob to line up OAT with altitude, and indicated becomes true. Usually 190mph. Reassuring that nothing has been left down by mistake.

[Mak]

Thanks for the data guys. I think I have all I need now.

bluskis: I'm afraid the so far only place where I've seen a TAS adjustable ASI has been in FS2000 :)