Swindled on my ground speed

[piperboy84]

Got a question,

My wee puddle jumper cruises at 120mph indicated at 2400rpm, today there was a very light westerly wind for my east bound journey at 1000msl. The airspeed showed 120, the VFR GPS was reading 115mph which I figured was about right given the light headwind.Turned around and started heading back west but climbed to 6000 thinking I will get my usual 120 GS plus the stronger tailwind wind at that altitude,plus a bit more speed because of the altitude when I looked the air speed it was what i expected at 120 but theGPS ground speed was 103, I thought it would be up around 130 plus.

Now why would that be? Is it be because the wind gradient at that altitude may remove the tailwind? Or is the GPS is less accurate at altitude or is it the horsepower from 2400 rpm at 6000 is less than the hp at the same rpm at lower levels or is it just the way things are and I,m thinking to much about stuff and should just look out the window and enjoy the beautiful view of the snow capped highlands and leave the answers to those ground speed anomalies to others.

[ShyTorque]

Simple answer is that the wind hasn't read the met. theory books. ;)

[Unusual Attitude]

Some funny things going on today with the wind, was up at 7k over Glamis area and it was slightly easterly.

At Perth on the ground it was down 27 at about 5kts but I was operating off 21. On the way back in A/G suggested I might be better off on 03 as upper winds reported as northerly but on finals for 21 a quick cross check showed 120 indicated vs 110 ground speed.

I'd suspect being very cold and light winds means no vertical mixing of air so different directions at different levels, certainly felt like that today.

Cracking day though.

UA

[GipsyMagpie]

Err, if you are eastbound with a light westerly you have a tailwind! Your gauge sounds like it is over reading or you are mixing your units. You got a lot less going west because you were in a head wind plus original error. Wind at 1000 by day will not match that at surface too.

[piperboy84]

Yeah your right I got it bass ackwards , better just enjoy the view and bollox to the speeds

[AirborneAgain]

Or is the GPS is less accurate at altitude

No.

or is it the horsepower from 2400 rpm at 6000 is less than the hp at the same rpm at lower levels

Yes. If you have a fixed-pitch propeller you will have to increase rpm with increasing altitude to maintain the same power. Take a look at the power setting charts/tables of your POH.

[UV]

Piperboy
I think what you are saying is that on the EASTbound leg you had an EASTERLY wind and thus a lower groundspeed. Then you climbed, turned WEST, and found you had a WESTERLY wind? Correct?

Is so then that is standard.

Eastlery winds DECREASE with Altitude and become Westerly as you get higher in the Northern Hempisphere (Earth Rotation).
Only Westerlies become stronger with Altitude.

[AirborneAgain]

Eastlery winds DECREASE with Altitude and become Westerly as you get higher in the Northern Hempisphere (Earth Rotation).
Only Westerlies become stronger with Altitude.
This is not correct. Of course you can find specific weather situations where the wind behaves as you say, but in general there is no such correlation.

Addendum: Upper winds are generally westerly (in both hemispheres), but that doesn't mean that an easterly surface wind will generally turn westerly with increasing altitude -- certainly not at 6000 ft.

[Pace]

I am surprised you had 120 kts IAS at 1000 and 120 kts at 6000?

Pace

[m.Berger]

I suspect that your pitot was on the downwind side and masked by the airframe , giving a lower reading.

[BackPacker]

If there is a strong difference between lower and upper layer winds, you can typically detect this through either windsheer (which will be like turbulence with a sudden drop or increase of IAS) or an abnormal temperature gradient (inversion for instance), or both.

Did you notice any of these two effects when climbing or descending?

[Maoraigh1]

I suspect that your pitot was on the downwind side and masked by the airframe , giving a lower reading.
Sideslip wasn't mentioned.:hmm:

[4forward8back]

ASI in mph and GPS in kts?

[piperboy84]

As Gyspy pointed out I got my east and west backwards when explaining the issue in my OP, cutting to the chase, what I meant was a slight headwind flying west from forfar to Perth showing 120mph indicated and 115 on the GPS, hang a 180 climb to 6000 and get the same airspeed but a slower ground speed,wind socks at both fields which are about 25 miles apart showed a slight westerly, I guess I am just surprised that in such a small geographical area on a very calm/stable air day with both fields being in the same long valley there was such differential in the wind between the altitudes.

I think Unusual Attitude may have explained it as that is the exact area (Glamis) where I noticed it.

Everything in MPH ( no particular reason it's just the way I roll and the Mph markings on the ASI are easier to read on my half knackered eyes)