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TAS vs IAS/GS

Hongie

Member
Hi All, I've had a bit of a facebook discussion with a couple of flying friends regarding aircraft performance, culminating in a question about the TAS performance of one of said friends Tecnam.

Screenshots linked to discussion, it has been anonymized, so please indulge me all the pretty colors.

Let me know your thoughts please, because its driving me a bit insane :confused:

https://i.imgur.com/Vn2ZPxI.jpg
 
That's just going to keep going around in circles for a long, long time!
I had a similar discussion with a guy about QNH/QFE years ago...
 
I know this doesn't help your discussion but the first thing that came to my head was why not just get an RV? If you're looking for something that goes the speed of an RV it seems like that's the easiest thing to do but maybe I misunderstood the question.
 
:Dahhhh. So someone wants to keep up with RV's without getting an RV. What do you think the reason is for not getting an RV?
 
haha Not really sure about that.

I'm more questioning their reluctance to talk in TAS. To me, TAS at a give DA is the only true measure of cruise performance. these guys keep going on about GS and IAS. TAS is performance of the aircraft, GS is how well the weather worked out on the day.
 
I'm more questioning their reluctance to talk in TAS. To me, TAS at a give DA is the only true measure of cruise performance. these guys keep going on about GS and IAS. TAS is performance of the aircraft, GS is how well the weather worked out on the day.

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.
 
Resistance is futile.........?

Unless someone is interested in learning it probably is futile.

It is still fairly common for people to walk in to the booth at fly-ins and ask "did you fly one of these airplanes here"? After answering yes they ask "what were you indicating"?
When I answer "137 Kts, but the true airspeed was 165 Kts and with the tail wind I had, since I was eastbound, I had ground speeds of 190 Kts for quite a while." At that point their eyes begin to glaze over.....

There is a lot of the non professional pilot community that has very limited understanding of the difference between IAS and TAS. I guess that is understandable since a lot of the now aged pilots grew up flying airplanes that wouldn't very easily climb high enough that talking in TAS really mattered much. Particularly the ones who flew in the flat lands part of the country.
 
Ever since getting my RV and joining VAF, I have noticed the level of discussion to be leaps and bounds ( with regards to knowledge AND temperament) above those of former owner groups of which I have been involved. There is a professionalism to subjects and a thoroughness of explanation. I'm attributing that to the fact that, when building, one needs a higher level of understanding. I cold be wrong in my assessment but, whatever the reason, this place is GOLD! Thank you for teaching and explaining. I, for one, find the discussions invaluable.
 
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.
 
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.
 
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.
 
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.
 
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).
 
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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.
 
:mad:
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.
 
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Wow, isn't this stuff like the ABC's of the Private Pilot training? How could someone argue these points. There is only 1 right answer. If this person was a good friend and I could cut up with them without insulting them I would say, "You can't fix stupid", but If were just an acquaintance then I would respectfully walk away or end this discussion.
Some things are just not meant to be debated. Definitions of TAS, IAS and GS falls in that category.
 
Wow this blew up! I've been away training in my RV, thanks for your input all. I'm not wanting to insult either of these guys or anything, just wanted to be sure of my arguments. Reassuring that my brain still works sometimes.
 
Another gee wiz

Another thing that may bewilder the non-Van?s group is that on the Van?s designs, Vno is based on indicated airspeed regardless of altitude and Vne is based on true airspeed. Let them chew on that for a little bit.
 
Another thing that may bewilder the non-Van?s group is that on the Van?s designs, Vno is based on indicated airspeed regardless of altitude and Vne is based on true airspeed. Let them chew on that for a little bit.

Well, just like anything else on the interwebz - just because somebody says it, doesn't mean it's accurate.
 
I never could wrap my head around true airspeed until I started flying jets, so I feel the other guy's pain. For the guys who never climb over 4500' I could see it happening.
 
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