TAS vs IAS/GS

[Icarus]

Originally Posted by ChiefPilot

I agree with you.

Someone needs some quality time with a good CFI at their next flight review. It seems pretty clear that the individual lacks a good understanding of TAS vs. IAS vs. ground speed.

I've had this discussion twice this year, with similar outcomes. They walked away unchanged, one flying gps derived groundspeed on landing. They are out there, all I know is that I'm not signing one of their log books.

[ChiefPilot]

Originally Posted by Icarus

I've had this discussion twice this year, with similar outcomes. They walked away unchanged, one flying gps derived groundspeed on landing. They are out there, all I know is that I'm not signing one of their log books.

I hear you...I've had a couple of discussions like that myself. Children of the magenta can be pretty stubborn.

[longranger]

Originally Posted by Icarus

I've had this discussion twice this year, with similar outcomes. They walked away unchanged, one flying gps derived groundspeed on landing. They are out there, all I know is that I'm not signing one of their log books.

I had a friend once who would throttle back to prevent his GROUND SPEED from exceeding VNE. I worked on him for years before I left the area, but I don?t think I ever did get through.

[tomww]

I wonder if they realise one speed that should be monitored as TAS is VNE. that?s important, the rest is not that important. If someone chooses to use GS rather than IAS or TAS will it really matter?

Having said that I hope they know the speed to use on final.

[RV7A Flyer]

Originally Posted by longranger

I had a friend once who would throttle back to prevent his GROUND SPEED from exceeding VNE. I worked on him for years before I left the area, but I don?t think I ever did get through.

Was his name Daniel, by any chance?

[sailvi767]

TAS is the proper number to use for comparing aircraft performance.

[RV6_flyer]

Originally Posted by control

What I care most about is lowest possible fuel flow while maintaining 150 knots TAS in level flight

Originally Posted by sailvi767

TAS is the proper number to use for comparing aircraft performance.

Low fuel burn at or above 150 TAS sounds like me.

Typically I care about Ground Speed, True Air Speed, and only care about indicated airspeed when in the traffic pattern.

[TASEsq]

Hi all,

Seems like perhaps the place to ask..

EDIT: I take back my question: http://www.vansaircraft.com/pdf/hp_limts.pdf

Why do RV’s quote VNE as a TAS? I’ve never really understood that. It would mean, say for a 200 TAS VNE, that I would be indicating a lot less at altitude at my VNE. Do the RV’s have some issue at higher altitudes necessitating a reduction in IAS? (Flutter or something)?

All certified airplanes I have ever flown have always quoted VNE in IAS. Meaning the object of the day was to climb as high as you could so your TAS increased for better GS at the end of the day.

As far as I figured, VNE was an aerodynamic airframe strength issue - so it makes sense that it is based on IAS (I.e: the number of bits of air pushing against the airframe).

[paul330]

I am an ex-RAF QFI so, if I can think that far back, here goes:

Instrument Rating Exam Testers are Picked - IRET/PICD.....

IAS - what you see in the cockpit
Rectified AS - IAS corrected for Pressure and Instrument error
Equivalent AS - RAS corrected for Compressibility
TAS - EAS corrected for Density error

Technically, EAS is what you use for stall speed, approach speed, aerobatic speeds, lift, drag etc. It affects how the aircraft "feels" and "flies". It is a measure of the number of molecules going past the airframe and over the wing. So, in that respect, it is what we use for aircraft performance. Since we don't have air data computers to make the necessary corrections then we use IAS which is close enough for our needs.

As altitude increases, air density reduces. So to get the same IAS (number of molecules passing the aircraft), we have to fly faster - this is the so-called density error - that is, our true speed through the air is greater than indicated on the instrument. When looking at range and fuel consumption in still air, this is the number you use. It also affects TO and landing performance in that at high altitude/temperature, you have to accelerate to a higher TAS to get the same IAS for the necessary lift to take-off (plus the engine produces less power so there's a double whammy). Likewise on landing, for a given IAS, your TAS is higher and so the landing roll is longer.

Ground Speed is totally irrelevant to the aerodynamic performance of the aircraft. It is the TAS corrected for wind. It is also what is given by GPS. In that respect, it is what affects navigation performance but unless one is trying to get a Time on Target or similar, one does not "fly" GS. Other than changing altitude to get a more favourable wind, you cannot "fly" a particular GS to improve performance. You get what you get and use that to calculate headings and times.

So, you set your power which gives you an IAS. Using your whizz-wheel, you then calculate your TAS for your altitude/temperature. Using this number, you apply the wind and calculate drift and GS. Using these numbers, you then know your required heading to make good your track and the time to cover the leg distance. With and EFIS and GPS, this is all done for you.

As for the TAS Vne: Vne (in the VANS aircraft at least) is not due to a structural airframe issue, it is do to control surface flutter considerations. Since air density and therefore aerodynamic damping reduce with altitude, the IAS needs to be reduced - hence the TAS limit rather than IAS.

[Bevan]

Originally Posted by TASEsq

Hi all,

Seems like perhaps the place to ask..

Why do RV’s quote VNE as a TAS?...

...As far as I figured, VNE was an aerodynamic airframe strength issue - so it makes sense that it is based on IAS (I.e: the number of bits of air pushing against the airframe).

Yes, as I understand it... VNE is dictated by airframe strength but the RVs are fast and capable of altitudes higher than other normally aspirated aircraft. So, one of the limiting factors is flutter BECAUSE flutter has to with resonance which has to do with TIMING and natural frequencies of vibration of the structure, particularly the tail feathers. Exciting those frequencies (of airframe vibration) has to do with the ACTUAL speed of the air molecules of air flowing over the surface. That means the airspeed of interest is TAS not IAS. Since there can be quite a difference at the altitudes where our RVs can play, TAS becomes the one to watch. My understanding anyway.

Bevan.