TAS changes with wind changes
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TAS changes with wind changes
Now this is a question I can not answer for myself, and others can not easily explain. So bare with me, as I am hoping there is a simple reason for what I have seen several times.
on two aircraft types, a J430 and now an RV-10 when a strong headwind or quartering headwind is experienced, the IAS and TAS would seem to be a few knots slower than turning and running with a tail wind component. I wish I could give more details of exact numbers, but in the past its been an observation and not a fact taking exercise.
This last week, while tracking a radial on a VOR, at low level (3000') we had a strong headwind around 20 knots, from the 10 oclock position, at a point in time we quickly entered a less headwind area, say 10 knots, the wind vectors all changed and our GS improved. The TAS improved also by several knots.
We have a pretty good system in this plane for instantaneous data, where as when I had seen this before it was just the ASI and mental maths, or the air computer in the Garmin 296, mind you the results were the same. A change in headwind component and or heading to get the tail wind instead, yielded same results.
Ask any questions........ challenge my observations, I just want to know where there may either be an error in my ways, or is there some slight efficiency of the airframe and prop in cetain conditions that we do not normally account for.
Thanks!
J
on two aircraft types, a J430 and now an RV-10 when a strong headwind or quartering headwind is experienced, the IAS and TAS would seem to be a few knots slower than turning and running with a tail wind component. I wish I could give more details of exact numbers, but in the past its been an observation and not a fact taking exercise.
This last week, while tracking a radial on a VOR, at low level (3000') we had a strong headwind around 20 knots, from the 10 oclock position, at a point in time we quickly entered a less headwind area, say 10 knots, the wind vectors all changed and our GS improved. The TAS improved also by several knots.
We have a pretty good system in this plane for instantaneous data, where as when I had seen this before it was just the ASI and mental maths, or the air computer in the Garmin 296, mind you the results were the same. A change in headwind component and or heading to get the tail wind instead, yielded same results.
Ask any questions........ challenge my observations, I just want to know where there may either be an error in my ways, or is there some slight efficiency of the airframe and prop in cetain conditions that we do not normally account for.
Thanks!
J
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That is cause your IAS/CAS changes as well with a shear of wind velocity and direction!!
Imagine that you are indicate 160 kts with a 20 kts head and all of the sudden wind now it is 10 kts head? what will be your IAS/CAS? (relative airflow) IAS decrease TAS decrease GS decrease!
IAS increase or decrease affects TAS correct? TAS is based of IAS /CAS corrected for temp/alt(Density).
Hope i helped
Imagine that you are indicate 160 kts with a 20 kts head and all of the sudden wind now it is 10 kts head? what will be your IAS/CAS? (relative airflow) IAS decrease TAS decrease GS decrease!
IAS increase or decrease affects TAS correct? TAS is based of IAS /CAS corrected for temp/alt(Density).
Hope i helped
The relationship in play is:
IAS --> CAS --> EAS -->TAS -->GS
These are low level subsonic aeroplanes, so you can ignore the CAS-EAS corrections, but the rest is still there:
IAS --> CAS (PEC chart corrections)
CAS --> TAS (correction for density altitude)
TAS --> GS (corrected for wind).
You are describing two aeroplanes which I suspect don't have PEC charts available of any quality, nor will they necessary comply with the CS.23 and CS.VLA requirements for 5kn/5% accuracy of IAS; so any air data system on board will be already working with suspect data, allied to which that any internal ability within these cheap GA systems for internal calibration is largely useless.
So, you're working with poor data in the first place. Now add into that, the following potential handling related issues;
- For constant IAS (and thus CAS), TAS will reduce slightly in turbulence
- Unless you are flying absolutely level there will be a change in speed for any given power setting as the aircraft climbs or descends.
Overall, what you're describing sounds to me like the sort of small errors that you'd expect to see due to inaccuracies in handling and in airspeed measurement, and possibly changes in local turbulence levels.
But there is no relationship between TAS and head/tailwind component for a constant CAS value in perfectly level flight - just not possible.
G
IAS --> CAS --> EAS -->TAS -->GS
These are low level subsonic aeroplanes, so you can ignore the CAS-EAS corrections, but the rest is still there:
IAS --> CAS (PEC chart corrections)
CAS --> TAS (correction for density altitude)
TAS --> GS (corrected for wind).
You are describing two aeroplanes which I suspect don't have PEC charts available of any quality, nor will they necessary comply with the CS.23 and CS.VLA requirements for 5kn/5% accuracy of IAS; so any air data system on board will be already working with suspect data, allied to which that any internal ability within these cheap GA systems for internal calibration is largely useless.
So, you're working with poor data in the first place. Now add into that, the following potential handling related issues;
- For constant IAS (and thus CAS), TAS will reduce slightly in turbulence
- Unless you are flying absolutely level there will be a change in speed for any given power setting as the aircraft climbs or descends.
Overall, what you're describing sounds to me like the sort of small errors that you'd expect to see due to inaccuracies in handling and in airspeed measurement, and possibly changes in local turbulence levels.
But there is no relationship between TAS and head/tailwind component for a constant CAS value in perfectly level flight - just not possible.
G
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But there is no relationship between TAS and head/tailwind component for a constant C
IAS/CAS will change brother if there id a wind sheer 20 to 10 or 40 to5 !!
IAS will inc or dec so as TAS understand????????
IAS will inc or dec so as TAS understand????????
Last edited by tmax; 11th Jul 2009 at 16:02.
(1) You're not my brother, he's a school physics teacher and can handle this sort of stuff quite well.
(2) Where did windshear suddenly come into the problem? We were talking about light aircraft at constant altitude, which is not somewhere I'd expect to see windshear effects.
(3) Please take the time to construct, spell, and punctuate your posts correctly or I'll start moderating them.
G
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Genghis
Thanks for your detailed reply, but to go back to a few points that need clarifying.
1. I think you would be shocked to learn just how accurate and reliable the equipment is that I am using, my avionics LAME commented during calibration tests and bench tests he never gets these results on the Boeing & DHC stuff he works on, so lets just say for the purposes of the exercise, they are near enough.
2. I agree with you on the theory of IAS/CAS etc and that your head / X-wind components should make ZERO difference. Thats why I am asking the question to see what sensible reasons for the observations are.
3. The observations regardless of what equipment and its accuracy shows a relative trend.....and that is really all that matters here.
4. Some have commented on a change in winds and windshear etc, and yes a slight and temporary increase or decrease due to the lag in instruments and inertia or whatever, but this would stabilise in seconds. What I have seen many times is a change that stays.
Yes turbulence and changing pitch and roll will have your speed going up and down all the time, but in reasonably smooth air in the same location doing a 5 minute leg Square Hold, it would seem that the results vary.
I will in future make closer observations, but its not me dreaming this up.....I hope.
Thanks again!
J
PS
Just a thought. Is there a difference in efficiency of the prop, airframe or something that changes with a different angle relative to track due to the X-W component? Does the pitot tube suffer from any effects of the same?
Thanks for your detailed reply, but to go back to a few points that need clarifying.
1. I think you would be shocked to learn just how accurate and reliable the equipment is that I am using, my avionics LAME commented during calibration tests and bench tests he never gets these results on the Boeing & DHC stuff he works on, so lets just say for the purposes of the exercise, they are near enough.
2. I agree with you on the theory of IAS/CAS etc and that your head / X-wind components should make ZERO difference. Thats why I am asking the question to see what sensible reasons for the observations are.
3. The observations regardless of what equipment and its accuracy shows a relative trend.....and that is really all that matters here.
4. Some have commented on a change in winds and windshear etc, and yes a slight and temporary increase or decrease due to the lag in instruments and inertia or whatever, but this would stabilise in seconds. What I have seen many times is a change that stays.
Yes turbulence and changing pitch and roll will have your speed going up and down all the time, but in reasonably smooth air in the same location doing a 5 minute leg Square Hold, it would seem that the results vary.
I will in future make closer observations, but its not me dreaming this up.....I hope.
Thanks again!
J
PS
But there is no relationship between TAS and head/tailwind component for a constant CAS value in perfectly level flight - just not possible.
I think it would take more than reliable and accurate avionics to shock me (although I'd admit to mild surprise), but however good it is, it's only as good as the basic pitot-static system. The errors caused by position / shape / (probably not) leakage / boundary layer effect on a pitot-static system are significant players in any aircraft of any size and, again in most aeroplanes, are the major factors in cockpit errors, not the errors within the instruments themselves. So, if you don't have significant errors I'd still be very surprised - but this is no reflection on the excellence (or not) of your instruments.
If you really want a deep look at this, a good starting point would be to take the aircraft up for an hour or so when you've got very smooth conditions suitable for calibration work and to conduct an accurate calibration of the whole pitot-static system. There's a reasonable guide to how this works here and my recommendation would be that the easiest approach is to use the racetrack method, for which you can download a data reduction form here (at form 43).
Shout if you want a hand with any of the planning or data analysis if you want to have go at that.
Theoretically since you're flying into/down wind, you're running parallel to the isobars, and wind strength may strengthen or weaken as they move closer together, or further apart. However, again you're dealing with relatively low performance aeroplanes so the odds of a significant effect within a noticeable period of time are low.
G
If you really want a deep look at this, a good starting point would be to take the aircraft up for an hour or so when you've got very smooth conditions suitable for calibration work and to conduct an accurate calibration of the whole pitot-static system. There's a reasonable guide to how this works here and my recommendation would be that the easiest approach is to use the racetrack method, for which you can download a data reduction form here (at form 43).
Shout if you want a hand with any of the planning or data analysis if you want to have go at that.
Just a thought. Is there a difference in efficiency of the prop, airframe or something that changes with a different angle relative to track due to the X-W component? Does the pitot tube suffer from any effects of the same?
G
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Thanks genghis
The racetrack method I am familiar with, and yes I agree with the pitot / static tests are important. The issue is in a good or bad system, the system does not change. its onlt the variation in result that has me curious.
The low performance bit..... well I would have thought the high performance military jets for example would hardly notice any effect while a 160 knot machine will notice 3-4 knots difference.
Your comments about running along or slightly across the isobars is food for thought indeed.
I will keep an eye on it.....and if I remember to do it, I will post back here.
The racetrack method I am familiar with, and yes I agree with the pitot / static tests are important. The issue is in a good or bad system, the system does not change. its onlt the variation in result that has me curious.
The low performance bit..... well I would have thought the high performance military jets for example would hardly notice any effect while a 160 knot machine will notice 3-4 knots difference.
Your comments about running along or slightly across the isobars is food for thought indeed.
I will keep an eye on it.....and if I remember to do it, I will post back here.
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Jabawocky,
You have not stated what type of equipment that you are using. i.e. is it a conventional ASI or an EFIS system with air data computer or just a GPS? Or conventional ASI and handheld GPS?
You seem to have a display of wind direction and strength, which leads me to believe that you are using an EFIS system and/or GPS.
You need to realise, that in these systems, the wind displayed is derived (by a mathematical algorithm) from your heading, groundspeed, TAS and track made good. So IAS/TAS affects the displayed wind, not the other way around.
You have not stated what type of equipment that you are using. i.e. is it a conventional ASI or an EFIS system with air data computer or just a GPS? Or conventional ASI and handheld GPS?
You seem to have a display of wind direction and strength, which leads me to believe that you are using an EFIS system and/or GPS.
You need to realise, that in these systems, the wind displayed is derived (by a mathematical algorithm) from your heading, groundspeed, TAS and track made good. So IAS/TAS affects the displayed wind, not the other way around.
Another thing to consider, with derived wind components, is that generally with these computer nav systems is that the displayed headwind component is that calculated along the aircraft longitudinal axis and not actually along track, so with a large drift angle the ground speed and the wind component will not add up exactly to the TAS, the larger the drift the larger the error.
The same goes for the crosswind component it is usually that calculated at 90deg to the aircraft longitudinal axis and not 90deg to the aircraft track.
It would be easy enough for the nav computer to be programmed to to make the correct calculations but in my experience it doesn't work that way.
Regards,
BH.
The same goes for the crosswind component it is usually that calculated at 90deg to the aircraft longitudinal axis and not 90deg to the aircraft track.
It would be easy enough for the nav computer to be programmed to to make the correct calculations but in my experience it doesn't work that way.
Regards,
BH.
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Interesting............
Not sure if I should post the contents of a private email, from a fellow ppruner.... but as a Military jet jockey he never considered much about what happens to lower performance machines............ but a later in life career has him in things such as ferrying Islanders.......
........... funny thing is he too has found a similar few knots variation......... and always better with the tail wind component. He just never bothered to find out why until now............ so I am not alone here!
And yes I have EFIS / Air data and Bullethead may have a very valid point also!
All makes for interesting reading!
Not sure if I should post the contents of a private email, from a fellow ppruner.... but as a Military jet jockey he never considered much about what happens to lower performance machines............ but a later in life career has him in things such as ferrying Islanders.......
........... funny thing is he too has found a similar few knots variation......... and always better with the tail wind component. He just never bothered to find out why until now............ so I am not alone here!
And yes I have EFIS / Air data and Bullethead may have a very valid point also!
All makes for interesting reading!
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Jabawocky
Hmmm. interesting this one.
I first saw this myself more than 20 years ago in a high perf turboprob. Flying the same routes at the same levels it was VERY noticable that tailwinds would give an extra few knots on the IAS. Not just sometimes but always.
I have spoken to many perf bods about this and they all say the same: "not possible."
So there you have it. It doesn't exist except I have witnessed it plain as day as you now have too.
Someone, somewhere must have a simple explaination?????
Hmmm. interesting this one.
I first saw this myself more than 20 years ago in a high perf turboprob. Flying the same routes at the same levels it was VERY noticable that tailwinds would give an extra few knots on the IAS. Not just sometimes but always.
I have spoken to many perf bods about this and they all say the same: "not possible."
So there you have it. It doesn't exist except I have witnessed it plain as day as you now have too.
Someone, somewhere must have a simple explaination?????
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Thinking outside the box (maybe a little too far outside the box ) I'm wondering whether air flow in weather systems has a part to play. Traditionally we think that air flows horizontal to the surface and along the isobars with PGF and Coriolis playing their part. Is there a small vertical element? My books don't say so but............
I'm also thinking that you're operating below the level where geostrophic flow is true. In other words, with a tailwind you are actually flying towards an area of lower pressure. You may be at a constant pressure altitude but will be descending relative to the ground. Keep the same 'power' setting, add a bit of Isaac Newton and .....
PS. I've had a couple of drams so don't take this too seriously. Just some meanderings.
) I'm wondering whether air flow in weather systems has a part to play. Traditionally we think that air flows horizontal to the surface and along the isobars with PGF and Coriolis playing their part. Is there a small vertical element? My books don't say so but............I'm also thinking that you're operating below the level where geostrophic flow is true. In other words, with a tailwind you are actually flying towards an area of lower pressure. You may be at a constant pressure altitude but will be descending relative to the ground. Keep the same 'power' setting, add a bit of Isaac Newton and .....
PS. I've had a couple of drams so don't take this too seriously. Just some meanderings.
Last edited by Cows getting bigger; 13th Jul 2009 at 22:08.
I must admit Cows has got me wondering as well. I'm working around a bunch of meteorologists at the moment - I'll see if I can get any sense out of any of them on the subject.
If Jabawocky is right, then we're presumably looking for an element of rising air downwind, descending as you fly into wind.
G
If Jabawocky is right, then we're presumably looking for an element of rising air downwind, descending as you fly into wind.
G
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If it's the TAS that's changing, surely we need to consider the source of the air flow? Would warm air on the tail and cold air on the nose make this kind of difference?....and is it relevant to this conversation? (I am only asking!)
)
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Cows getting Bigger..........now that name is a crack up!!!! You must be an ag pilot.....pull stick back - cows get smaller....push stick forward cows get bigger.....pull stick back again and hold- cows get smaller and smaller and ooooohhhhh cows getting bigger again! 
Seriously apart from a cool name I think we might end up chucking the Chocky frog to you sir! (or Maddam)
Can't wait to see what the met boys have to say Genghis.
And here you were all thinking time to flame the little Jaba from downunder!
You must be an ag pilot.....pull stick back - cows get smaller....push stick forward cows get bigger.....pull stick back again and hold- cows get smaller and smaller and ooooohhhhh cows getting bigger again! Seriously apart from a cool name I think we might end up chucking the Chocky frog to you sir! (or Maddam)
Can't wait to see what the met boys have to say Genghis.
And here you were all thinking time to flame the little Jaba from downunder!
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Just been rading some rigging material, and came across this and wondered how this effects the situation
On average, a propeller loses about 2% efficiency when the inflow of the air to the propeller
disk is not perpendicular to the disk, for angles less than ten degrees, and the loss in efficiency
increases rapidly for greater angles. The overall loss in propulsive efficiency is small
compared to the pitching or yawing moments that are produced, which require trim (drag) to
cancel out.
disk is not perpendicular to the disk, for angles less than ten degrees, and the loss in efficiency
increases rapidly for greater angles. The overall loss in propulsive efficiency is small
compared to the pitching or yawing moments that are produced, which require trim (drag) to
cancel out.
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Jaba,
unless you are flying in a sideslip (slip ball out-of-center) the propeller is receiving a perfect axial flow. So , if you are inferring that flying in a crosswind the propeller receive non-axial flow, the answer is : no, unless you are purposedly flying in a sideslip.
Daniel
unless you are flying in a sideslip (slip ball out-of-center) the propeller is receiving a perfect axial flow. So , if you are inferring that flying in a crosswind the propeller receive non-axial flow, the answer is : no, unless you are purposedly flying in a sideslip.
Daniel
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The ball is always in the middle............when it passes from one side to the other!
Yes good point, was clearly not thinking that one through at all. Back to the original topic then!
Yes good point, was clearly not thinking that one through at all. Back to the original topic then!