Straight Dope Message Board > Main Air Speed versus...well....speed.
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#1
01-30-2004, 11:41 AM
 Philster Guest Join Date: Aug 2000
Air Speed versus...well....speed.

Could someone explain how air speed differs from other measurements of speed?

I recall some debate over how fast the Space Shuttle was travelling when it broke apart of the U.S. last year.

Measured in air speed, the Mach number was huge, but then others would jump in to dispute that and the number didn't seem so great.
#2
01-30-2004, 11:46 AM
 Redsland Charter Member Join Date: Feb 2001 Location: Yonder Posts: 275
Air speed is your speed relative to the air you are traveling through. So if you have a head wind, your air speed will be greater than if you have a tail wind.

Ground speed is the other common measure, and is calculated relative to fixed objects on the ground.
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#3
01-30-2004, 11:53 AM
 Jackknifed Juggernaut Guest Join Date: Oct 2000
I was once on a 747 that surpassed the speed of sound. But that was only because the tail wind was so high. Relative to the air, it was going substantially slower. I even asked a question about it on these boards.
#4
01-30-2004, 12:20 PM
 Absolute There are no absolutes. Charter Member Join Date: Apr 2000 Location: In flight Posts: 3,737
There's something special about Mach number, too (I'm probably getting the facts wrong, but here's my explanation from memory):

Mach number is how fast you are going, relative to the speed of sound at your altitude. So, if you're going Mach 2 at 80,000 feet, you're going twice as fast as sound would at 80,000 feet.

The problem is that sound travels slower at higher altitudes than at lower ones.

So, Mach 2 at 80,000 feet is slower than Mach 2 at 10,000 feet. That's why the shuttle can go Mach 22 on re-entry, but still have a comparatively lower actual speed.
#5
01-30-2004, 02:46 PM
 Xema Guest Join Date: Mar 2002
Quote:
 Originally Posted by Redsland if you have a head wind, your air speed will be greater than if you have a tail wind.
This is a trifle misleading. If you specify a constant gound speed, this would be true. But (except for special cases like kites) once a vehicle is airborne, it's lost its connection to the ground, so a truly constant ground speed would be unusual.

For essentially any airborne aircraft, it would be accurate to say that airspeed is unaffected by a headwind or a tailwind. Groundspeed, on the other hand, is directly affected by the wind.
#6
01-30-2004, 04:26 PM
 av8rmike Guest Join Date: Mar 2002
Let's see if I can't beat Broomstick in here...

Xema is essentially correct. Ground speed is the only thing that is affected by headwind/tailwind, not airspeed.

Indicated airspeed (IAS) is read off the airspeed indicator (ASI), and is the speed measured by the pitot tube (on an airplane, dunno about the Shuttle).

True airspeed (TAS) is IAS corrected for temperature and pressure, and is computed either with a E6B or other computer. I forget the exact number, but typically TAS increases by 2% per 1,000 feet of altitude. This is the number that is entered on a flight plan.

Lastly, Ground speed (GS) is TAS corrected for the vector component of wind. This number is used to compute travel times between waypoints, which are typically on the ground.

Mach number is speed relative to speed of sound at that altitude. Which speed? I think it's TAS, someone will correct if I'm wrong. Mister V explained how the Shuttle can be going Mach 22 at 80,000 feet; at that altitude the air is almost too sparse to carry sound, and will have very low sound velocities.
#7
01-30-2004, 05:50 PM
 Stana Claus Guest Join Date: May 2002
Air speed vs ground speed

The difference between airspeed and groundspeed is a critical factor when taking off and landing most fixed wing aircraft. Since these operations tend to be done flying into the wind when possible, you have to account for that head wind when executing your take off or landing. Say you are taking off into a steady 15 knot wind and your plane's stall speed is 80 knots. You wouldn't want to get airborn until your indicated air speed was at least 95 knots (stall speed + head wind) in case the wind suddenly died. If you were only doing 80 knots indicated air speed and the wind died, your sudden drop to 65 knots would lead to a stall and probably a crash.

The same is true for landings. I've heard stories of commercial jetliners landing in New Orleans ahead of an in-coming hurricane where the winds were in the 50-60 mph range. The pilot pretty much had to fly the plane into the ground at close to full throttle to maintain adequate ground speed for the landing.

SC
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#8
01-30-2004, 11:52 PM
 Richard Pearse Member Join Date: Aug 2001 Location: Kiwi in Adelaide Posts: 7,743
Quote:
 Originally Posted by Stana Claus The difference between airspeed and groundspeed is a critical factor when taking off and landing most fixed wing aircraft. Since these operations tend to be done flying into the wind when possible, you have to account for that head wind when executing your take off or landing. Say you are taking off into a steady 15 knot wind and your plane's stall speed is 80 knots. You wouldn't want to get airborn until your indicated air speed was at least 95 knots (stall speed + head wind) in case the wind suddenly died. If you were only doing 80 knots indicated air speed and the wind died, your sudden drop to 65 knots would lead to a stall and probably a crash. SC
Hmmmm yeah, not really. You generally take off at a speed comfortably above the stall speed regardless of wind. On most small aircraft you take of "when it's ready to fly", it's a feel thing. On larger aircraft there is a specific rotate speed which is more to do with single engine performance than anything else. A steady 15Kt headwind doesn't just stop suddenly unless there is some severe turbulence/windshear around. This would be indicated by various weather patterns (a thunderstorm close to the airfield would be a likely cause) and would be handled as a seperate issue to a steady 15kt headwind.

In actual fact the winds tend to increase as you go higher so you could expect your headwind to increae as you climb out after takeoff.
#9
01-31-2004, 06:14 AM
 Broomstick Charter Member Join Date: Mar 2001 Location: NW Indiana Posts: 17,037
Quote:
 Originally Posted by av8rmike Let's see if I can't beat Broomstick in here...
Only because I had to work late yesterday, ya know? (That's my story and I'm sticking to it...)

Quote:
 Originally Posted by Stana Claus Say you are taking off into a steady 15 knot wind and your plane's stall speed is 80 knots. You wouldn't want to get airborn until your indicated air speed was at least 95 knots (stall speed + head wind) in case the wind suddenly died. If you were only doing 80 knots indicated air speed and the wind died, your sudden drop to 65 knots would lead to a stall and probably a crash.
Um... you might want to re-think/re-phrase that post.

First of all, even when the wind is dead calm you don't take off at stall speed. I'm sure the engineer types have some sort of formula to figure this out, but taking the venerable C172 Skyhawk as an example, stall speed is 50 knots* and you climb out at 80 knots - which leaves you a comfortable margin above stall. If there are gusty winds you would add a "gust factor" to your aispeed, typically 1/2 the gust speed, to your airspeed. So if winds were steady at 20 and gusty to 30 knots you would add half the difference, or half of 10 knots in this case. You worry about this on landing, too. But for take-off you'd hold the airplane on the ground until your speed was where you wanted it, then you would let it go up.**

In any case, if you're taking off into a headwind (which is how you're supposed to do it), you don't correct for steady winds at all. It's airspeed that determines when you're ready to take off, not groundspeed. Which is why the little airplanes need to be physically tied down when left outside. An empty Cessna 150 (the Skyhawk's smaller counsin) WILL go airborne on it's own in a 40-45 mph wind, even with the engine stopped and a groundspeed of zero. If not tied down properly, it can turn into a 1000 lbs kite.

* For you nit-picky types, yes, I'm rounding off. I didn't want to take this down the primrose path regarding what affects stall speed like weight, precise model year, accelleration, etc.

** They way small planes are designed they will tend to stay on the ground when going too slow to fly safely, and simply lift off when ready to fly during normal operations. If you want to stay on the ground at a speed greater than normal lift-off speed, you actually do have to force it to stay on the ground. This is not a huge effort, but a noticable one.
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