Simviation Forums

Hi.

Does anyone know how i can convert an airspeed, in knots obviously, combined with altitude into a ground speed?

I wasnt sure where to put this so please move if neccessary.

Cheers
Danny

Well first you need to know the wind speed.  That is the difference between airspeed and groundspeed.  Plus there are many different airspeeds, including indicated and true.

Ok, well my question was about the IAS, say an IAS of 250 knots at an altitude of 47000ft.  Do you need any other information?

I was asking as i was interested in whether being as high up as nearly FL500 the ground speed would really be that much faster than a high IAS of more like 450 at a much lower altitude.

Well the easiest way to do that is use the GPS, and ensure the winds are zero.  The speed on the GPS will indicate ground speed, which in a no wind situation is the same as true airspeed.

You're going about this the wrong way, if I understand the question...

True airspeed is higher than indicated airspeed the higher you go, but that is useless in calculating your groundspeed (the speed at which your shadow moves over the ground, so to speak).

In order to calculate your groundspeed, all you need to do is note the time between checkpoints on your course. That time factor is the only reason you'd want to know your groundspeed, anyway: will you get to your destination ahead of that front? Will you have enough fuel?

If the two points are 100 miles apart and it takes you an hour to go from A to B, your groundspeed is 100 mph, regardless of wind or anything else.

All you need to solve any navigation problem is compass, chart and clock- that's what the CPU of a GPS uses.  ;D

Maybe I misunderstood, but it seems he was trying to figure out how much altitude affects indicated airspeed vs. true airspeed.  

What i was trying to work out was, like rottydaddy said, how fast your shadow moves over the ground.  As at a lower altitude of around 10000ft for example, if you were to fly london to new york (obviously you wouldnt at that height, but humour me here) at 300kts, then you wouldnt get there as fast than if you were flying at 300kts at a height of 40000kts... am i right?

I was trying to work out how i can calculate the 'speed my shadow moves accross the ground' dependant on IAS and altitude.

Danny
:-?

What i was trying to work out was, like rottydaddy said, how fast your shadow moves over the ground.  As at a lower altitude of around 10000ft for example, if you were to fly london to new york (obviously you wouldnt at that height, but humour me here) at 300kts, then you wouldnt get there as fast than if you were flying at 300kts at a height of 40000kts... am i right?

I was trying to work out how i can calculate the 'speed my shadow moves accross the ground' dependant on IAS and altitude.

Danny
:-?

It does depend on wind (it's usually stronger the higher you go), so a wicked headwind at FL400 would spoil things, but yes, assuming no wind, 300 kts indicated (not corrected for altitude) at 40,000 will yield a higher groundspeed than 300 indicated at 10,000.

So, assuming you're trying to estimate your groundspeed before the flight, you'll need to figure your true airspeed- based on indicated airspeed (from aircraft's performance tables; even indicated max. airspeed will vary with altitude a little, then there's weight, etc), then you need to "plug in" the winds aloft (angle and velocity).

What's needed for this kind of flight planning is a flight computer... if your plane has a GPS, it will initially give you a no-wind estimate as soon as you enter the route data, but this will not be very accurate.

You need to start flying to get an update, but of course at that point all you need to do is note the time between two given points once you are at your desired cruise altitude, which eliminates the need for all the fancy mathematics.

A flight computer, even the humble "whiz wheel" , which is simply a circular slide rule, can tell you before you fly what your true airspeed will be... then it can tell you, based on reported winds aloft, what your ground speed will be. When I'm flying "seriously" in FS9, I actually use my "whiz wheel" to figure all this out beforehand, to estimate fuel range, etc. Believe it or not, there are still airline pilots who do the same, and not just the old ones. But many also use the electronic version of the whiz wheel, and I believe many aircraft have flight directors capable of the same calculations.

But as I said before, once you get going, you must verify this estimate by actually timing your progress.

Mass, the GPS shows your true groundspeed (bottom left of the main screen, under 'GS'). It will also show your true track ('TRK', at the top).

Pressing 'Shift-Z' a couple of times will bring up the wind direction and speed in red at the top of the screen (plus a lot of other useful information). You'll find that you can quickly get the hang of seeing how windshifts, power changes, etc. affect your progress over the ground (press 'Shift-Z' twice more to clear).

Yes, rottydaddy, that was primarily my aim, to work out the speeds and therefore timing and fuel needed prior to flight as im not particularly useful when it comes to the different computers that you find in the complicated panels around.

Well as it goes, using my citation x, which is my most often used flight, i never go further than london-new york route and i know that the aircraft can do that on full fuel.  

Is there an online or downloadable system i can get which will allow me to do those calculations though? If i could get the hang of using one then i would definately help to bring an extra sense of reality to my sim.


Cheers!
Danny

Yes, rottydaddy, that was primarily my aim, to work out the speeds and therefore timing and fuel needed prior to flight as im not particularly useful when it comes to the different computers that you find in the complicated panels around.

Well as it goes, using my citation x, which is my most often used flight, i never go further than london-new york route and i know that the aircraft can do that on full fuel.  

Is there an online or downloadable system i can get which will allow me to do those calculations though? If i could get the hang of using one then i would definately help to bring an extra sense of reality to my sim.


Cheers!
Danny

I'm not sure what's available... try a search in the Utilities downloads here (or wherever).

But you may also want to get yourself a "whiz wheel"... they're inexpensive and work just as well as an electronic E-6B. Once you get the hang of using it, it's very quick and easy to figure everything out before you take off, as well as updating your calculations in flight.

http://www.sportys.com/acb/showdetl.cfm ... &CATID=171

Is there an online or downloadable system i can get which will allow me to do those calculations though? If i could get the hang of using one then i would definately help to bring an extra sense of reality to my sim.

Mass, there's a Flight Planner already in FS9. Just press 'Alt/Flight Planner'! You'll find that you can enter in your origin and destination, and go to 'Map View' and drag the course line across to VORs or NDBs to create waypoints if you like.

The Flight Plan comes up in a table which you can print out, or alternatively call up any time in flight. It will give you information on speed, fuel required

hey mass ..i did a serch for you and found a online site that should give you what you are looking for..outside of going out and purchsing a e6b flight computer this site will give you the same imfo the e6b will give you ..www.lowapproach.com/e6b.html   or   www.csgnetwork.com/e6bcalc.html  the second site might be better for you

Cheers for all the help guys

Nav, with the in sim flight planner, i find the fuel consumption and all that to be very inaccurate as yuou said, which is half of the reason why i was asking the question, so i can get a much more accurate idea of the flight time, fuel consumption etc.

I will deffinately check that site in detail over the next week or so 727, looks good!  And given my current money crisis i always do love something free!

Cheers guys!
Danny

Nav, with the in sim flight planner, i find the fuel consumption and all that to be very inaccurate as yuou said,

Trouble is, Danny, however accurate any 'planner' you can find is, you will still be dependent on the accuracy with which the fuel consumption of a given aeroplane is modelled in FS.

I'm a bit nuts about 'range' as my favourite flights are long-distance (most often 'round-world'). My own approach is first of all to fly any new aeroplane on a 'stock flight' I have saved - Melbourne-Brisbane via Canberra and Sydney, about 800 miles). During the flight I use 'Shift/Enter' to check the percentage of fuel remaining, and also the wind strength and direction, and that gives me a good idea of the maximum range I can expect from that type.

I was pleased with that P51 Mustang because the 1944 droptank version could indeed manage around 2,000 miles (Berlin and back!) at economy cruise - so it's modelled exactly right.

The main problems with the FS Flight Planner are that it tends to over-estimate fuel consumption and that it further tends, with some aeroplanes, to show unrealistically-high estimated 'true' airspeeds. It also, of course, can't allow for the extra fuel used climbing to height (often about 10% in a jetliner) but that doesn't matter since it tends to even out over the whole flight.

The other problem is that no two pilots will ever get exactly the same performance out of a given aeroplane - especially in props, where you can monkey around with mixture, propeller pitch, and throttle settings.