Airspeed vs Ground Speed


#1

If I understand correctly, If a plane is traveling at, say 300 knots IAS, the ground speed is higher. How is this possible? I know 1 nautical mile at, say 30,000 feet is longer than 1 nautical mile at sea level, but both are 1 nautical mile. So how can a plane cover 300 nautical miles in the air, but cover more than 300 nautical miles of land in an hour? Did I get something wrong?
-Josh


#2

It has something to do with the curvature of the Earth; but, in dredging up my HS and college physics electrons, I would think that the opposite would be true: Going 300nm at altitude would be going less than 300nm on the ground. That’s just how it should seem to work, if I remember the Coriolis effect correctly. (The object further to the outside of a circle or sphere must travel farther to make 1 revolution than a more-medial object in the same circle/sphere.–Heavily abbreviated, of course.)

I’m probably wrong on this. If you know you’re correct, please accept my apologies and disregard the above brain babble.


#3

Planeaholic - I agree with you. You’d think the higher you are, the farther you have to go.

I don’t know for sure that the ground speed is higher than airspeed, but that’s what I got from some sites I was looking at. I mighta got it wrong.


#4

No, you apparently don’t understand correctly as you don’t automatically have a higher ground speed.

From Wikipedia - “Ground speed is the speed of an aircraft relative to the ground. It is the sum of the aircraft’s true airspeed and the current wind and weather conditions; a headwind subtracts from the ground speed, while a tailwind adds to it. Winds at other angles to the heading will have components of either headwind or tailwind as well as a crosstrack component.”

The medium through which you’re travelling (the airmass) isn’t static.

If you’re flying at an IAS of 300KTS but flying into a 60KT headwind, what’s your ground speed?

If you’re flying at an IAS of 300KTS with a 60KT tailwind, what’s your groundspeed?

Your ASI indicates how fast your are travelling through the airmass based on the ram-air differential between the pitot tube and a static pressure port on the ASI. But, unless the air is absolutely still with no winds aloft at all, there’s no direct relationship between IAS and groundspeed.

This is a very simple response to a matter that has enough variables, including density altitude, that one normally uses a flight calculator to determine it.


#5

Also, you used IAS above. This is Indicated Air Speed. As the plane flies higher, the air is less dense. So the Indicated Air Speed, which is a product of the Pitot Tube(s) measuring the pressure of the ram air versus the pressure from the static port(s) isn’t measuring your actual speed (like you said covering 300nm each hour). Ignoring wind completely, an airplane flying:
at 300kts indicated airspeed
at a pressure altitude of 20,000ft
at -20C
will have a TRUE airspeed of 404kts. This means that the airplane will actually be traveling at 404kts through the air but the airspeed indicator’s inherent errors will show a smaller value. Once again, ignoring wind completely, the ground speed will be 404kts. THEN throw wind into the equation to get a higher or lower groundspeed.
For instance: the Pilatus cruises at around 265kts True. The maximum operating limit (Vmo) is 236kts Indicated. So when we are up at altitude cruising along at maximum cruise power, the airspeed indicator will only show somewhere around 165kts or so. (try explaining all of this to passengers who paid a lot of money to go 260kts when they come up front to see how the flight is progressing)
(another example, just for fun… A 172 flying at 6,000ft and 15C indicating 110kts is actually going 123kts true.)


#6

This would be why we have that cute little knob thing we can turn on the Airspeed Indicator that moves those microscopic numbers at the top of the AI left and right.


#7

Hence my link to the E6B in my reply. As density altitude rears its ugly head this matter becomes a can of worms for most non-pilots, and even for some active pilots.

We had a math-challenged individual in my CAP squadron years ago who never could wrap his head around this matter in ground school, even with a calculator.

Pop


#8

1 Nautical Mile = 1 Nautical Mile regardless of altitude.

The only reason (to make short what others said long) you can cover 1 NM faster or slower at a different altitude is because your speed over the ground is faster or slower depending upon whether winds are adding to (i.e. tailwind) or subtracting from (i.e. headwind) from the speed shown on your instruments.


#9

Is THAT what that does!??
:wink:


#10
  • True airspeed (TAS). This is the speed the aircraft is “truly” traveling through the air. TAS corrects for all instrument error and environmental factors. If there was no wind at all, TAS would be the speed you cover the ground. TAS is the correct expression of an airplane’s capability.
    • Indicated airspeed (IAS). As the name implies, this is the speed read from your airspeed indicator (ASI) without any corrections applied. Since the ASI measures impact air pressure, if the airplane maintains a constant TAS its IAS will decrease as altitude (and air pressure) decreases. Similarly, if IAS remains constant, the higher you go the higher your TAS… you have to fly through space more rapidly in thin air to get the same impact air pressure to show a constant IAS.
    • Calibrated airspeed (CAS). CAS is IAS corrected for installation error and indicator error. Flight in some configurations and/or some angles of attack may result in IAS error. CAS is an engineer’s way of compensating for this… from the pilot’s seat, those errors are built into manufacturer’s recommendations for reference IAS. CAS more correctly describes the amount of air flowing over the airplane’s wings.
    • o Example: Generally liftoff in a light multiengine airplane is done at VMCA, the minimum controllable airspeed with one engine inoperative, plus five knots. This gives the pilot a margin above VMCA should the critical engine fail right at liftoff. In the Beech 58TC Baron I flew for several years, VMCA is 81 knots. Because of instrument error in the 58TC 86 knots IAS (VMCA +5) is 81 knots indicated. Lifting off right at the "red radial" VMCA speed IAS, as recommended in the Pilot's Operating Handbook, provided the five-knot safety margin in CAS.
      
    • Ground speed (GS). GS is TAS corrected for wind. For a given TAS, GS increases with a tailwind, and decreases with a headwind. GS is usually what you read from a GPS. Since GS is entirely dependent on non-aircraft factors (the wind), it is not an honest value for comparing one aircraft’s performance to another’s. Brag about seeing 200 knots on your Skylane’s GPS with a tailwind if you like, but it’s still slower through the air than the Columbia 400 flying into the wind in the other direction.

aero-news.net/index.cfm?ContentB … b91d1da10&


#11

OOOoooh! OOOOooooh! Check THIS out! I have an awesome real life example:

After a flying lesson and returning back to home base, my instructor was looking out the window and commented, “Wow! We’re really moving! I don’t think I’ve ever gone this fast in a [Cessna] 152 before!” He commanded, “Let me have the airplane - I wanna try something.” We were pretty near the airport, so he turned away for a few minutes and then turned the plane directly into the wind. He began fine-tuning the throttle and trim, and after a couple of minutes or so, he got it so that we were actually HOVERING in a fixed-wing airplane! He told me to pick out a point of reference on the ground, so I noticed a white house right along the edge of tire of the plane, and sure enough it remained pretty much motionless! I think we “hung around” in that location for a good five minutes or so. I think he had the flaps down to 30, so we had to be near Vso (minimum controllable airspeed) - about 35 to 40 kias - but we were not going anywhere over the ground!

Now, turn the plane around 180 and we’ll be back in the tailwind again, get back up to cruising speed of 90 kias - add that 35 to 40 knots to our indicated speed and we’re moving across the ground at about 125 to 130 knots or about 145 MPH!

My instructor scolded me for not checking the weather, but I did check and as it turned out, it was a freakish thing that caught a few experienced pilots off-guard as well. There were two other planes still checked out from our FBO when we got on the ground.

That was quite a lesson for me! It was a good thing the wind direction was very much in line with the runway heading at home base.


#12

I was in a G-1000 C-172 on my way to Ohio when me and my friend decided to see how fast we could get it. We got to 181.8 knots ground speed(G-1000s record top ground speeds so we could check it) which works out to be 209 mph. Another thing we like to do is see if we can “put the plane in reverse” which isn’t hard w/ Kansas winds.


#13

None of our 152’s had GPS, LORAN, or even DME, so I don’t know my best speed in the 152, but in the 172 my personal best is 9kts backwards. I would guess my personal best in the 152 is somewhere around 12kts backwards or so. I would call my buddies in the FBO to go outside and watch us as we flew backwards over the runway at 1000ft. (all the while getting paid to screw around!)


#14

JHEM: Thanks for the simple definition. It makes total sense.

Offspring: Thanks for destroying the simplicity of your father’s definition, and giving me a headache. I would be hard-pressed to question your accuracy on such a topic! Thanks for the correct answer. :wink:


#15

My pleasure. Just FYI, I think the old man prefers to use “spawn” versus offspring. Either way, thanks for that. :wink:


#16

Getting amazing groundspeeds with a tailwind? Think it’s the fastest anyone’s gotten that A/C type? Check out http://www.groundspeedrecords.com
One of our pilots got the PC12 to 439kts groundspeed.


#17

Keep studying, Vso is the stall speed with full flaps. [http://www.airodyssey.net/reference/glossary.html#v]](http://www.airodyssey.net/reference/glossary.html#v]).

At Vso the aircraft will not fly, it will stall. Typically we use about 1.3Vso for approach speeds, there should be no reason to fly slower then that until your in the flare.


#18

I suppose it all depends on where we get our information.

Guess I should have simply said, “MCAS”, or “minimum controllable airspeed”. I pulled the Vso reference from Wikipedia

Vso - the stalling speed or the minimum steady flight speed in the landing configuration.

But now we have a wonderful opportunity for discussion…

A YAHOO SEARCH of the quoted definition turns up 5 pages of hits. Granted they could all be misquoted from one original bad definition.

Are V-speeds defined in the AIM? Anyone have a copy of it handy? (I’m at work right now and don’t have one here). Just wondering what the OFFICIAL definition of Vso is, and where it can be found…


#19

VSO: Stalling Speed in the Landing Configuration

From FAA Glossary of Acronyms & Abbreviations


#20

Thanks for pointing out the error of my ways, leardvr - and thanks for confirmation, J.R.