UNDERSTANDING AIRSPEED

Hal Stoen, 6/19/2000

© Hal Stoen, 2000

Airspeed, indicated airspeed, true airspeed, actual airspeed,calibrated airspeed...the list goes on. Let's take a look at howthis "airspeed thing" works with airplanes.

The 90 degree angled piece of metal at the nose or wing ofthe aircraft is the Pitot tube. it is orientated to get the mostclear and least turbulent air flow available. It is usually heatedin most aircraft so that ice will not clog it.

Because the air gets thinner as you increase altitude thereis less and less air entering the pitot tube. In addition, barometricpressure has an affect because of the change in air density. And(wouldn't you know there would be an "and") temperaturealso has its input on the reading what with warm air being lessdense than cold. OK, that's a lot of variables- how can the thingeven start displaying information? Well, knowing all of this,a standard was set for airspeed readings.

That standard is:
1. at sea level
2. standard day (temperature, humidity)
3. a barometric setting of 29.92 inches of pressure

If any of these criteria are off then the indicatedairspeed will be different than the actual airspeed of the aircraft.Itís pretty obvious that most pilots will go their entirecareers without having the airspeed reflect the actual speed ofthe aircraft. Lousy deal, no? Well, not quite. The aircraft doesn'tcare! When an aircraft is flight tested it is loaded withinstruments that feed in all of these corrections so that whenthe plane stalls the INDICATED airspeed is recorded. Now, thatmeans given the air that was flowing over (and under) the wingson that day it stalled at, let's say 100 knots.

OK, let's dick with the standards and increase the altitudeby 10,000 feet. We know that the air is thinner up there. And,because of this there are fewer molecules slamming into the pitottube. And, because of that the airplane really has to go faster(if you were an observer on the ground you could tell that) toget an indicated airspeed of 100 knots. But, and here isthe key thing, the wing doesn't care. It will still stall at anindicated airspeed of 100 knots because the airspeed indicatorand the wing are both affected by the thinner air.

Let's increase the temperature. The air is thinner. The aircraftwill have to go faster to obtain an indicated airspeedof 100 knots (relative to you standing on the ground watchingall of this). But, it will still stall at 100 knots indicated.

The bottom line on this is its simplicity. The guy drivingdoesn't have to care about conversions for barometric pressure,humidity, temperature etc. All he cares about is that the airplanewill stall at 100 knots, indicated airspeed. It will stallat this indicated speed at the height of Mt. Everest or down atDeath Valley. So when the pilot glances down at his airspeed indicatorhe doesn't have to make any mental corrections- what you see iswhat you get.

So, what about those figures of "cruises at 494 knots@ 30,000 feet"? Well, that's where "true airspeed"comes in. Now, if indicated airspeed is only true & correctat those standards I mentioned before it holds that if you wantto find out what your true airspeed is you will have toaccount for any deviations. And that is exactly what is done.First the indicated airspeed is noted, a correction is made forthe outside air temperature, another correction is made for thebarometric setting, and unless you really want to get to the nitsthey ignore humidity (that comes into play with density altitude-another subject). So, figuring in all of these deviations thepilot is able to come up with his true airspeed.

An aircraft goes faster up high because the air is thinnerand causes less form drag on the aircraft. (the drag producedby lift is referred to as induced drag and pretty much remainsthe same in all flight regimes). So, higher means faster trueairspeed and less indicated airspeed.

So, outside of the sales department why the heck would anyonecare about true airspeed? Well, the pilot wants to know so thathe can plan his trip. Using charts he will pick what altitudehe is going to cruise at and find out what the true airspeed willbe. Factoring in winds aloft will give him his expected groundspeed and then the times can be calculated for each leg of thetrip. Also, ARTC (Center) wants to know so they can plug the informationinto their computers for tracking purposes. (they can recalculatebased on actual readouts, but it gives them a starting point)

So, the higher you go, the faster you go, the lower the indicatedairspeed is. Finally, you will reach a point called "thecoffin corner". It's usually associated with what is knownas "upsets". You're way up there in your Speedwing aircraftand have full power in...if you raise the nose the aircraft willstall, if you lower the nose you will descend- this is it, themax. you can get out of the old Speedwing. Oh, one other detailhere- the airspeed indicator will be indicating near-stall, eventhough you are now going the fastest the Speedwing is capableof going. Any turbulence will cause a stall and the aircraftwill fall a
considerable distance (20,000 feet or more is not unheard of)before the air becomes thick enough to recover from the upset.

This tutorial is available on a CD

This tutorial, along with additional content, is availableon a CD. Click here formore information.

Hal Stoen

© 2000

rev: 8/19/2000

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