Airspeed indicator

[LoneStarBuckeye]

I'm wondering about the red and white pointers on the airspeed indicator.  The docs state that the white pointer indicates "airspeed," and the red pointer indicates "true airspeed."  

I assume that one of those indicated speeds is with respect to the "earth" and the other is with respect to the air (and so is affected by, for instance, the wind).  Is my assumption correct?  If so, which is which.  If not, what's the story?

Another interesting point is that if my assumption is correct, the two speeds have very different implications.  It seems that one's energy state is defined by speed as computed with respect to the "earth," but for lift and stall calculations, it seems that the other speed, which relates to the speed of the wings through the air, is more relevant.

The real reason for this post is that, whatever the meanings of the two speeds, I have seen some dramatic differences between the two.  For instance, at one point last night, the white needle was indicating about 315 mph, but the red pointer was at 400 mph.  How can that be?  85 mph winds?  (Also, can anyone explain the wind indicator on the map?  If it's explained in the docs somewhere, just tell me to read the manual.)  

One more thing, why does the red pointer always indicate a higher speed?  It seems that the relative magnitudes of the red and white indicators should switch when one stops flying into the wind and begins flying with it.

If anyone has any insights, corrections, or otherwise, I'd be glad to read them.  Thanks.

[Sox62]

"Airspeed"(the lower number, is INDICATED airspeed,which is measured by airflow.
 "True Airspeed"(the higher number) is your speed in relation to the ground.

 An ex: If you are at 30,000 ft,Indicated(I know this is inaccurate,I don't have charts with me)your indicated airspeed may only show 200 kts,as the air is much thinner at that altitude,whereas your true airspeed may show 300 kts,as that is the actual speed of your plane in relation to the ground.

 It's best imho to rely on your indicated speed for judging what you want your aircraft to do.

 The only time both airspeeds should ever be the same,are if you are flying at sealevel.
 
 I hope this helps.  

[LoneStarBuckeye]

Thanks for the reply.  I'm not sure I understand, though.

Why (or how) is indicated airspeed affected by air density?  Does IAS reflect the ability of the wings to generate lift in the current air-flow environment, rather than merely the speed at which the air is passing over the wings?  

I'd really like to understand exactly what indicated airspeed is, because at my current level of understanding, I cannot explain what I observe (e.g., an 85 mph differential between indicated and actual airspeed).  Also, I'd like to know which number is important in different situations (e.g., when judging energy state or when lift potential).

Thanks.

[Sox62]

The reason there is an 85 difference,is that(let's use 30,000 ft again)at the higher altitude,since the air is so much thinner,there is much less air flowing over the wing(less lift),or through the pitot(which measures indicated airspeed).
 IAS does indicate the ability of your wings to generate lift,and is a better measure of your actual e-state.

[LoneStarBuckeye]

Thanks again for the reply.  Since I am not an aeronaturical engineer or a pilot, I did a bit of searching on the Web in an attempt to educate myself.  In short, I think that what you note above is correct.  Here is a summary of what I learned:

1.  IAS is calculated using what amounts to a differential pressure gauge.  Ideally, the gauge calcuates the pressure differential  between the pressure produced by the air striking a membrane attached to the plane (usually on the wing) and an approximation of the air pressure in the free stream  The pressure in the free stream generally cannot be measured directly, because to do so would require a probe well in front of the aircraft.  Nonetheless, the pressure sensor that indicates an approximation of free-stream pressure can be located on the aircraft such that the error under normal operating conditions (i.e., non-severe angle of attack, no flaps) is on the order of 1%.

2.  IAS is approximately equal to true airspeed when flying at sea level in no-wind conditions.  For a given true air speed, the corresponding IAS decreases by about 2% for every thousand feet (i.e., a plane with a true airspeed of 100 mph will produce an IAS of 100 mph at sea level but an IAS of only 90 mph at 5000 feet).

3.  IAS is a good estimation of a plane's capability (e.g., the wings' ability to generate lift) under most conditions, because the air-thinning effects that reduce IAS as altitude increases affect the plane's wings and control surfaces.  Thus, there is a "built-in" compensating factor.  I'm sure it's not exact, but the conventional wisdom seems to be that IAS is a good approximation of the plane's true-air-speed, sea-level performance.

[Sox62]

It's stated much better than the way I did,lol...but it's true.I've always used IAS as soon as I progressed beyond the dweeb stage eons ago in AW.
 When I was a brand new player,I had difficulty understanding why my plane wouldn't turn well at 300 kts indicated(true airspeed) at 25k+....switched to IAS and and realized that my plane was actually(performance wise) flying slower.
 Glad I could help.

[bozon]

some inacuracies:
neither IAS nor TAS indicate your speed relative to the ground. both indicate speed relative to the mass of air they plane is in.

TAS is indeed the "true" speed relative to the mass of surounding air and is used for navigation - still you have to add the wind speed to get your "true" ground speed.
If you want to know how long will it take to fly from A to B this is the number to use.

IAS is more like your aerodynamic speed. It is calculated, as described in the above posts, by measuring the difference betwin the dynamic preassure (created by moving the plane relative to the mass of air) and the standard static pressure at sea level (1013 mbar). This gives a false number as far as actual movement is concerned (always lower than TAS unless traveling below sea level), BUT this compansates for the drop in air condensity with alt. Since all aerodynamic behaviour is dependent on air condensity, you get the same aerodynamic behaviour for a given IAS in any altitude.
Thus, if you stall at 90mph IAS at sea level, you'll stall at 90mph IAS at 20k.
Use IAS to know your stall speed, corner speed, best glide speed and for approximation of max level speed - the parameters you consider when fighting.

Bozon

[LoneStarBuckeye]

Thanks, Bozon, for pointing out the inaccuracies in my description.  I think I now have a complete picture of how IAS and TAS work and relate to a plane's performance.

[Hammerhead]

I think its like this:-

True Airspeed:- Airspeed relative to the surrounding air.

Indicated airspeed:-Airspeed that is calculated by measuring the dynamic pressure (pressure due to motion of air) felt by the aircraft (the instrument is probably linked with the altimeter to get the air density).

Equivalent airspeed:- Airspeed that the aircraft would have to fly with at sea-level to feel the same dynamic pressure

Ground speed:- Speed relative to the ground.

I think the white needle shows the EAS, and because that uses the sea-level density it can be very different from the TAS.

I am no expert in historical aircraft, but perhaps their speedometers weren't linked to their altimeters and thus were calibrated on ground to standarad sea-level conditions???

Maybe some one can answer that!:)