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 how this "airspeed thing" works with airplanes.

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

Because the air gets thinner as you increase altitude there is less and less air entering the pitot tube. In addition, barometric pressure has an affect because of the change in air density. And (wouldn't you know there would be an "and") temperature also has its input on the reading what with warm air being less dense than cold. OK, that's a lot of variables- how can the thing even 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 indicated airspeed will be different than the actual airspeed of the aircraft. Itís pretty obvious that most pilots will go their entire careers without having the airspeed reflect the actual speed of the aircraft. Lousy deal, no? Well, not quite. The aircraft doesn't care! When an aircraft is flight tested it is loaded with instruments that feed in all of these corrections so that when the plane stalls the INDICATED airspeed is recorded. Now, that means given the air that was flowing over (and under) the wings on that day it stalled at, let's say 100 knots.

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

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

The bottom line on this is its simplicity. The guy driving doesn't have to care about conversions for barometric pressure, humidity, temperature etc. All he cares about is that the airplane will stall at 100 knots, indicated airspeed. It will stall at this indicated speed at the height of Mt. Everest or down at Death Valley. So when the pilot glances down at his airspeed indicator he doesn't have to make any mental corrections- what you see is what 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 & correct at those standards I mentioned before it holds that if you want to find out what your true airspeed is you will have to account for any deviations. And that is exactly what is done. First the indicated airspeed is noted, a correction is made for the outside air temperature, another correction is made for the barometric setting, and unless you really want to get to the nits they ignore humidity (that comes into play with density altitude- another subject). So, figuring in all of these deviations the pilot is able to come up with his true airspeed.

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

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

So, the higher you go, the faster you go, the lower the indicated airspeed is. Finally, you will reach a point called "the coffin corner". It's usually associated with what is known as "upsets". You're way up there in your Speedwing aircraft and have full power in...if you raise the nose the aircraft will stall, if you lower the nose you will descend- this is it, the max. you can get out of the old Speedwing. Oh, one other detail here- the airspeed indicator will be indicating near-stall, even though you are now going the fastest the Speedwing is capable of going. Any turbulence will cause a stall and the aircraft will 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 available on a CD. Click here for more information.

Hal Stoen

© 2000

rev: 8/19/2000

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