TAS changes with wind changes

[Jabawocky]

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:ok:

[tmax]

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

[Genghis the Engineer]

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

[tmax]

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????????

[Genghis the Engineer]

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????????


(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

[Jabawocky]

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.:eek:

Thanks again!

J:ok:

PS
But there is no relationship between TAS and head/tailwind component for a constant CAS value in perfectly level flight - just not possible.

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?

[Genghis the Engineer]

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 (http://bura.brunel.ac.uk/handle/2438/937) 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 (http://www.bmaa.org/techdocs.asp?DocumentTypeID=3&DocumentType=Forms&t_DAT_DocumentPage=2) (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?
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

[Jabawocky]

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. :ok:

I will keep an eye on it.....and if I remember to do it, I will post back here.

[Artisan]

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.

[Bullethead]

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.

[Jabawocky]

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!:ooh:

And yes I have EFIS / Air data and Bullethead may have a very valid point also!:ok:

All makes for interesting reading!:)

[Kelly Hopper]

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?????

[Cows getting bigger]

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.

[Brian Abraham]

Cows, think you have won the $64,000. And here was I thinking it was a rediculous proposition.

[Genghis the Engineer]

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

[DBTW]

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!:))

[Jabawocky]

Cows getting Bigger..........now that name is a crack up!!!!:ok: 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! :O

[Jabawocky]

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.

[Daniel_11000]

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

[Jabawocky]

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!

[Capt Pit Bull]

I (and several of my colleagues) noticed this when we first got our BRNAV fit. I recall chatting about it but never reaching a firm conclusion.

To summarise so far; there are several possibilities.

1. Its a figment of our imagination. An apparerent effect we notice, whilst mentally disregarding countary evidence. To reach a conclusion about this would require a rigorous experiment; as in data gathered in controlled circumstances.

2. Its an indication thing. Something to do with how the nav system performs its calculations. Something like Bulletheads suggestion, maybe related to generally getting less drift when you have a tailwind. Could it be as simple as someone wrote an algorithm for doing the calculation, and just simply got it wrong, and <insert your avionic manufacturer here> has been using it as a library routine ever since? I know it sounds unlikely, but .... wierder stuff has happened in IT before!

If its not one of those 2, then its a real effect, in which case:

3. Something to do with the air mass you're flying through such that a tailwind generally means your true altitude is decreasing, the pay off being an increase in IAS. As per Cows.

4. How about something to do with Earth Curvature? There is a very small performance increasing effect caused by this; in Level flight lift is actually slightly less than weight because the aircraft is not actually in a 'balanced forces' situation because its not actually following a stright line. However the effect is pretty minimal at aircraft speeds. However it does exist, so the faster your groundspeed the less lift you need, which would translate to less induced drag and a higher IAS. However, without doing the calculations, I would be gobsmacked if it could account for more than a fraction of a knot.

4 is a real effect, but just not big enough. 3 needs more investigation.

pb

[Genghis the Engineer]

I've now had a good chance to chat with a meteorologist - the chap in question used to be one of the flying observers on Snoopy and has also been a forecaster instructor at the Met Office college. In short, he knows sheds more than any of us about winds and weather.

His view is that there's no meteorological reason which explains what's being seen.


The more I think about this, the more I think that the phenomenon as seen is a function of the algorithms being used in certain common nav kit, and absolutely nothing to do with aircraft performance or meteorology. Cap'n Pitt Bull's No.2 in other words.

G

[TyroPicard]

Jabawocky - in your original post..
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.I think tmax was absolutely right to describe this as windshear. But you seem to be talking about something different...
???
1. Sideslip
2. Down/up draught - I saw ±7 kt in my first ever motorglider trip last month (flying level with power constant).

3. Are you sure the power setting is unchanged - how accurately can you measure that?

[Jabawocky]

Hi Tryo

Glad you asked........

Today while doing CAS/Static testing, fling the GPS boxes, I was focussed on other things. There was a 10 knot SE wind blowing, known about and verified by the GPS box tests, and you know what, on the second flight (did not notice the first one) the IAS and TAS and GS in the westerly direction was hard to keep constant. It was while trying to maintain straight and level, that the IAS was 3 knots quiker....... and took a long while to get stable, so I do think its an acceleration and inertai thing now, but again its hard to prove.

It defies the theory, but its real. If only I had time to follow it for 10 min in every direction........I would still be up there:eek:.

The more I talk to people the more they say.....yeah...i have seen that. The Jet drivers only ever notice it when in 120-200 knot machines.

J:confused:

[Daniel_11000]

JABA,
sure about thesame weight ?
Daniel

[Jabawocky]

Same weight????

what in about a minute from tur to turn.....say a kg difference?:confused:

[Daniel_11000]

JABA,
I was noticing that in all the example given on this topic, the speed ‘after this ‘ or ‘after that ‘ was always a grater speed.
It seems to me that never happened that a slower speed was reported (IAS/TAS/GS), so I was thinking about a lower weight effect on airspeed

[Jabawocky]

I have seen TAS/IAS reduce also after......certain events whatever they may have been.

But well spotted!:ok:

[411A]

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 .....

You are quite close, actually.
This was the theory behind pressure pattern navigation used many years ago on long range trans-oceanic flights.
These aircraft were normally equipped with a hi-range radio altimeter, which was used to find the true height, and thereby remain in a least headwind/greater tailwind situation, even though the actual ground track may be more nautical miles...the desired outcome was a minimum time track.

Now, on to position errors....and slightly faster airplanes.
With the original B707, rather large pitot/static errors were noted, so much so that the raw data info could not be reliably used.
Enter KIFIS...Kollsman Intregrated Flight Instrument System, an early sort of air data 'computer', that took in the raw pilot/static inputs, ran it through a series of 'corrections' and out came improved reliable instrument indications.
Usually.:rolleyes:
Then, enter the combined pitot/static probe.
This provided reduced position errors, and its use on some of the first generation wide-body jets (L1011, B747), combined with a true ADC system, provided very accurate airspeed/pressure altitude information.

For light GA aircraft, look for (sometimes) pronounced static errors due in large part to the location of static ports....in relation to the pitot probe.

[syseng68k]

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.

This may be a red herring, but if you are measuring tas, then you must also be measuring air temp and alt to get the right answer. If you aren't using a proper tat probe and just using a simple probe in the airstream, the temp measurement may not be accurate for all conditions of airflow. There may also an effect from the static port w/respect to small variation in measured alt value as the wind changes direction.

Do you get the same effect using a mechanical asi ?...

Chris

[Microburst2002]

Can It be an area with a wind shift and an updraft?
The first would explain the GS increase, the second would explain the IAS/TAS increase, (if level flight and power were maintained during the occurrence).
In addition, if pitch had to be lowered to maintain altitude, an updraft could be considered, as this would mean the IAS increment was actual, not just a wrong indication.
There would be propeller effects, too.
Expert glider pilots can "feel" where the gusts come from, and of course the updrafts and downdrafts.
A gust is transient, but an updraft can be sustained. Normally a head wind decrease will produce a decrease in IAS/TAS followed by an increase in GS until all is back to pre-gust conditions. But a simjultaneous updraft can mask the decrease in IAS or even overcome it and turn it into an IAS/TAS increase.
Of course I am just an amateur who has never been even close to a test pilot school, sorry if bothering you guys.

[Jabawocky]

That is a very good point indeed. you would not really think of "ridge soaring" in a high wing loading piston powered machine!

So it could be that you are surfng and a slight pitch down, and hence the IAS and TAS are affected.

This may not be the wildest thread on pprune, but it is one of the more interesting!

J :ok:

[Microburst2002]

Yes indeed!
There are many interesting threads. I have a lot of questios to make and misteries to solve, I hope those will bee interesting threads too...

[WeekendFlyer]

As an FTE and a specialist in air data and navigation systems, I think I may have something to add to this debate.

Firstly, I have to agree with Ghengis; if the airmass is stable (in terms of temperature and density), a change in wind speed/direction WILL NOT change the relationship between CAS and TAS. Windshear could temporarily alter the values indicated in the cockpit due to its effect on the pitot and static systems, but I do not believe windshear is a factor in this discussion.

If the aircraft encountered a sudden change in wind speed/direction, that suggests to me that there may have been a change in other things as well, e.g. QNH, air temperature/density, and perhaps also the vertical component of the relative airflow. If the relationship between pressure height and temperature changes, the air density will change, and thus the relationship between CAS and TAS will also change.

Another thought is that the aircraft in question (being a GA aircraft) probably does not have an air data system that calculates TAS from total temperature; therefore the GPS based navigation system will be trying to derive TAS from the difference between TRK/GS and HDG/CAS using the triangle of velocities and GPS height. This is an error prone method because: 1) the system will have to make assumptions about the atmosphere, particularly the relationship between height and the air density; and 2) it will be trying to do the maths assuming a reasonably constant wind. Also, if the system does not have an airspeed (or heading) data feed, it will also have to make assumptions regarding HDG/CAS. Therefore it is unlikely that the TAS and W/V estimates will be particularly accurate. Typically in these GPS based GA nav systems the information on TAS, W/V, etc is there to help the pilot with situational awareness but is not going to be so accurate that it can be treated as gospel.

To my mind the ONLY way an air data system can get an accurate derivation of TAS is to have accurate and properly calibrated air data sensors (pitot, static and total temperature). If this data is used in conjunction with an accurate source of TRK/GS (e.g. GPS/EGI or INS), and an accurate source of heading (such as an INS or AHRS), then and only then can an accurate instantaneous W/V be calculated.

If someone provides me with exact details of the systems integration on the aircraft in question (particularly the air data and heading sources that feed the navigation system), I may be able to provide a more specific explanation of where the problem is likely to lie.

Hope this helps.

WF

[Daniel_11000]

In many posts ,I read that local pressure or pressure difference is considered by the corresponding post’s Author to be conductive to a IAS /TAS increase or decrease. This QNH local differences must be ‘high’ to justify such fluctuation. However, please all keep in mind that a 1 mb / 1 hPa difference of pressure over , let’s say’ 50 km will result in winds of gale force (well , sort of), not suitable for air tests in stabilized conditions.
The same applies for windshear: it is so strong to have the anemometer significantly fluctuating by 10 KIAS , it must be of short duration(so no value for a stabilized flight) or the aircraft is for sure in/close to a thunderstorm, and also this condition is clearly not suitable for any performance speed test. If, instead , the windshear is not notable on the aircraft (no bumps, etc) in my opinion is ininfluential over the 5-10 minutes interval necessary for a stabilized measurement.
Daniel