Headwinds become Tailwinds

[Papa9571]

I have done some experiments using your ideas and these are the results.

Settings were as follows....wind speed all altitudes 0
                                          KIAS 350
                                          Altitude 10,000
                                          Aircraft B747-400

Shift z gave the following..Wind 336 at 7 kts
                                         KIAS 350
                                         Altitude 10,000

GPS Speed Indications..course 336    385kts
                                     course 154    397kts

Conclusions...

1. Weather settings are not quite correct when you set user weather...suprise suprise

2. Programming issue dealing with aircraft speeds. I can not see how your ground speed can be faster than your airspeed when flying into the wind, especially by 35 kts. It looks like the speed computaions for FS9 need tweaking. Does that mean we have found another problem? Or do we live with it like all the others?

Inquiring minds and bald heads want to know

 ;D

[-sam-]

I

[Papa9571]

Ok..lets make that assumption and use the 350kias with a 7 kt headwind as a base. The GPS reads 385 kts. Altitude and KIAS remain the same

When you turn around and you headwind becomes a tailwind shouldnt your ground speed increase by 7 kts to 393 kts, not 397?

[beefhole]

This isn't going to help papa with his problem, but that statement is false.

Ahh, but I already said that it's not true for smaller aircraft, where you'd be flying at 100 kts. Papa is refering to larger jets-I'm sure someone can back me up, their GS is almost instantly higher than their IAS once they're off the ground, except in the case of an extreme headwind right after takeoff.

[JerryH]

Papa,

Just to confuse the situation, there's something missing in this discussion. True AirSpeed, KTAS.

Your airspeed indicator is displaying Indicated AirSpeed, KIAS, described earlier as a pressure measurement. As the altitude increases and the air gets thinner, the true airspeed must increase just to maintain a constant KIAS. If the KIAS is also increasing, then KTAS must increase even faster.

At any altitude above sea level, KTAS is always higher than KIAS. And with no wind, the ground speed will be identical to KTAS.

Unless I've missed something (and I'll certainly hear about it), I believe your speeds are correct.

Regards,
JerryH

[Papa9571]

So in effect.... are KTAS and GPS indicated ground speed the same? If so thats what I am experiencing and that would make sense. (As the light begins to dawn on the befuddled bald brain of Papa) But my other question still hasn't been answered. Does 1+1 still equal 2 in terms of airspeed with tailwinds and 2-1=1 with headwinds?

Or did math change when I was sleeping through all those math classes ?  ;D

[JerryH]

KTAS and Ground Speed are identical if the wind speed is zero.

If the air mass is also moving, the ground speed will be different than KTAS. Stating the obvious, tailwinds increase the ground speed and headwinds decrease the ground speed.

If the wind speed is steady, wind does not affect KTAS or KIAS. Gusts and directional changes are not part of this discussion.

I hope all of this means that 1+1=2 and 2-1=1.

JerryH

[Papa9571]

In that case I would agree with you. However in my test run my kias was 350 with a head wind of 7. That translated to 385 kts on my GPS. Now I understand  those indications and their relationship to altitude.

But with altitutde the same (10,000) , barometric pesssure steady, winds steady at 7 with no direction changes and turning 180 so the headwind becomes a tailwind should the GPS indicate 392 (385+7)? Why would it indicate 397? Or am I not comparing apples to apples here and trying to add KIAS and wind speed (MPH) to get GPS indicated in knots?

[beaky]

I have obviously forgotten about the effects of pressure on IAS... my  low-altitude VFR  myopia, sorry.I think  you might possibly see an IAS higher than your GPS groundspeed with a tailwind, depending on conditions (pressure altitude).  However, it's important to note that TAS + the tailwind component (or minus the headwind component) = GS, regardless. Wind plays a major role.
And... you said you flew a course of 336 at 350 kts IAS into the wind,  which was blowing at 7 kts from 336. Right? Then you "made a 180" to 154 for the return leg. Well, that doesn't make sense, because 336-180 is 156, not 154.  You didn't quite make a 180-degree turn.  Is that 2-degree difference enough to show you something confusing? Sure, because TAS (or IAS, doesn't matter) plus the tailwind COMPONENT (not shouting; can't work the italics here), gives you your groundspeed. The angles have to be figured in. If you were running straight downwind, not 2 degrees off, the component would just be the wind speed added to your TAS (forget the GPS readout for a minute).
 The wind component involves wind direction (which is true, not magnetic, enroute), wind velocity, TAS, True Course,(the line on the ground), and your Aircraft or Compass Heading (TC plus or minus wind correction angle, plus or minus magnetic variation). There could possibly be some confusion here between Heading and True Course, also... airplanes almost always fly a little sideways relative to their True Course, due to wind angles.
Starts to make more sense, in light of all that, although it's making my head hurt...
I need to brush up on my flight computer skills, so I'm dying to check this for you.  But I need your TAS, and to get that for 10,000 feet indicated, I need your Outside Air Temperature. Then I can check to see if those numbers are legit. Or maybe I'll take the 747 up and see what I get...

[Papa9571]

That actually makes sense....

But that still doesnt answer the question even with a two degree difference.

How do 385+7= 397 assuming all measurement are in kts?

I would tend to believe that a two degree variation would not account for a difference or a speed increase of 5 kts

And if I am right 7 mph = 6.08 kts so that cant be the answer either.

It may not matter in the long run but I would like to know where the extra 5 kts came from.

Any wonder why I'm bald?  

[JerryH]

Papa,

In your test, you showed a GPS ground speed of 385 and 397 knots for the two directions. That says the true airspeed must be right in the middle at 391 KTAS. Because of roundoff of all the numbers, the wind might be closer to 6 knots. Add or subtract 6 knots from 391 KTAS and you get the GPS values.

That's close enough for me. I still don't understand how you can zero out the wind and still end up with wind, but then nothing in our world is perfect.

Cheers,
JerryH

[Papa9571]

Ok....so KIAS at 350 with GPS indicating 385, factor in 10,000 ft altitude, outside air temp at 59, wind speed and direction, and that makes KTAS = 391. And I am sure there is a chart or formula somewhere that takes all of this into consideration.

Which means I gotta get  a hair transplant before I go to ground school.  ;D

I though programming Industrial Control systems was tough.  As sim users we dont deal with all of the flight planning, pre flight and post flight checks,and other stuff they deal with on an everyday basis. My admiration for commercial pilots and the job they do just jumped dramatically.

[beaky]

OK papa; made my own little test:
Departed Honolulu Int'l (PHTO) in the 747-400, default fuel/payload, and set up the AP to put me at 10,000 indicated at 340 kts indicated, on a direct route to Hilo (PHNL).
Weather: no wind, barometer standard (29.92"), temperature at 10,000 set to freezing (0 deg. C).Don't care if any of that is realistic; it's just a test. Zero is easy to remember.
In order to steer a straight line to PHNL, the AP took up a mag heading (as confirmed by GPS and wet compass) of 110 deg. The multinav (?) display showed the same for true course (remember, VORs don't take wind and magnetic fields into account). I don't have a proper chart of that area, so I don't know the magnetic deviation there... let's assume there is none. It happens, some places.
Indicated airspeed: 340 kts.
Groundspeed as per GPS: 383 kts.
OK, it's the pressure thing, but to be sure, I break out the whiz wheel (flight computer: get one and learn to use it if you're gonna fly for real). 0 deg. C at 10,000 indicated (which, at 29.92", is close enough to exact pressure altitude for our purposes), indicated airspeed is 340...  gives me a TAS of 395 kts. Twelve kts more than the GPS.
Hmmm. Not sure what's up with that, but without wind, TAS should match GS. Maybe the fudge factor I'm accepting for pressure altitude (and then there's calibrated altitude!   :-/  ) is greater than I think. The good news: when I set up a GPS course back to PHTO, everything jibes, as it should with no wind... groundspeed and mag heading.

Now I add some wind: 20kts at 150 degrees, from the ground to about 13,000 ft. I reverse course again towards Hilo, and when she settles down  I get:
True course (nav display):110
Mag heading (GPS, compass): 112
IAS: still 340
GS (GPS): 367
Seems logical,groundspeed reflects a quartering headwind, but lemme check with the flight computer.
TAS is still 395, wind 150 at 20 kts, squeak, scribble, etc., etc...got it!
According to the whiz wheel, with this quartering right headwind I should have:
Mag heading:112
GS:375
Again, the groundspeed is off (this time by only 8 kts), but the angles work out as they should.
Now I'll turn around and head downwind, back to Honolulu; same altitude, IAS, etc. The autopilot gives me:
True course: 290
Mag Heading: 288
Groundspeed (GPS): 398
According to flight computer:
Mag heading:288
Groundspeed: 410. Again, just like the first run with no wind, it's off by  12 knots. .And again, like the upwind leg with the quartering headwind, the angle is correct.
Bear in mind that this is a mechanical whizwheel and I didn't really sharpen the pencil first, so those groundspeeds could be off by a knot or two.Or even three...
And of course this doesn't definitively answer the question of whether or not there's innacuracy in indicated altitude at standard pressure or GPS groundspeed in FS9, but it may put to rest your concerns about your own install.  However, if you're really seeing  two different groundspeeds along the same course line with no wind (are you absolutely sure it was set at zero knots?), I don't know what to tell you...
If anybody has an electronic flight computer, I'd like to see results of a similar test done with that (more precise)...


A flight computer does all the number-crunching for you: even a mechanical whiz-wheel (basically a multifunction circular sliderule) does the job nicely if more slowly than an electronic one, and requires no batteries. They're used not only for planning, but for finding unknown data in flight, as well. Very handy.

[Nav]

Papa, I don't know whether it can be this simple - but the air is thinner at altitude, so 'Indicated Air Speed' will always be less than true speed, once you're off the ground.

[Papa9571]

Rotty......
Its nice to know there is a 12 kt discrepancy between KTAS and GPS indicated while flying with with a tailwind. That jives with what I am seeing. I did discover that even with winds set to 0 for all altitudes I still had a 7 mph wind from 336, as shown by using the Shift+Z, at 10,000 ft. I also went and flew a 737 I got a while ago that has an indicator for winds built in to the MFC and that confirmed I had a 7mph wind from 336.

I also believe winds are in MPH not KTS and that is where the problem lies. A 7mph wind works out to 6kts (actually 6.08 )

I think we have found a math error in the speed calculations for FS9 or a weather error but now that we know it is there we can deal with it. It might be kinda fun to sit this weekend and play with wind speeds and see if FS9 adds 7 mph to that component.
yeah...yeah..some of us have a diffenert concept of fun...

Nav....

I agree with you statement about the winds. But until I tried Shift+Z I thought the winds were at 0 (based on my weather settings). Since they were not that was one source of my confusion.

Also I didnt know what standard practice was for speed at altitude. I just assumed (we all know what that means) that speed in kts was the standard and thats what I have been using. That explains my excessive fuel usage and also explains why beefhole made the statement he did about 313kts being really fast for a 747 at altitude. I understand now why he did.

I thank you all for the lessons learned here and will apply them to my future flights.


BTW..am I too bald to take flight lessons now?