Part 5 - Dynamic pressure

by Ihab Awad, Founder of Airball.aero

If you missed it: Part 4 - Displaying the relative wind.

Every pilot is used to flying by indicated air speed (IAS). We know it's not how fast we are actually going, but it's the important quantity for flight. We also know that we have a true air speed (TAS) which is the actual speed with which we are moving through the air. But why all this mystery? How can there be more than one "speed" of something? After much tribulation, our CFIs convince us that IAS is the speed that we would be at, at sea level on an ideal day, at the same conditions. But what conditions? The mystery only deepens.

So let's get this straight. There are two important quantities. The first is just speed. This is how fast we are whizzing through the air. It is measured in miles per hour, or knots. This is what our CFI told us is called TAS (true -- get it? what does that make the other one?).

The second is dynamic pressure. It is the force applied by the airstream as it rushes at your airplane. It is measured in pounds per square foot.

The faster you go, the more dynamic pressure you feel. And, at the same speed, the denser the air, the more dynamic pressure.

So "indicated air speed" is just a measure of dynamic pressure. 100 knots IAS means, at the dynamic pressure you are feeling right now, if you were at sea level on a perfect day, you would be going 100 knots. But that's actually not super useful. Why?

At 100 kias, you are feeling 32 pounds per square foot of pressure. Go down to half that speed, 50 kias, and you are feeling 8 pounds per square foot of pressure.

What??? Half the speed, one quarter the pressure???

Well yes. Dynamic pressure changes with the square of the speed. You didn't know? Well, you are not alone. Neither do most pilots. And even if you "know" it in theory, our practice and speech still tempts you to think like 50 kias is "half of" 100 kias. Now can you see how IAS should be called UAS - untrue air speed?

An important quantity used to describe airplanes is wing loading. This is how much the airplane weighs per square foot of wing. It is also measured in pounds per square foot. Here is how typical airplanes rank:

Piper Cub: 6.8 psf
Piper Cherokee 140: 12.2 psf
Cessna 172R: 14.1 psf
Cirrus SR22: 23.5 psf
F-16: 141.1 psf

Compare that to your 8 psf of dynamic pressure at 50 kias, and 32 psf at 100 kias. Do the numbers "feel" like they make sense? They should. A wing has a particular ability to turn dynamic pressure into lift, and while it can do a really good job, it is not magical. You expect the SR22 to stall somewhere between 50 and 100 kias, and the Cub to stall well below 50 kias.

We can fix this first and foremost by not using IAS any more. Instead, we should use dynamic pressure. Remember the X-15 control panel? Here is a tiny zoomed in piece of it:

There it is -- the mathematical symbol for dynamic pressure, q, in pounds per square foot! These 1950s rocket scientists knew what they were doing!

In a future society, maybe we will fly with true air speed and dynamic pressure. Perhaps we will say things like, "I am flying this approach at 10 pounds; on short final I go down to 8.5 pounds then flare". Viva la revolución?

But meanwhile, back here on Earth, the Airball visualization can help! It turns out that our eyes tend to see area more than diameter when we look at something. So if the diameter of the ball is scaled to our IAS, its area will be correctly scaled to dynamic pressure. And so the Airball looks like this:

The 100 kias version "feels" way larger than twice the 50 kias version. Which it should. And this intuition should help us even as we use IAS for daily flight because this is how we do things today.

Next time: Part 6 - One true instrument.