Now I see it , thanks

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Originally posted by V-Tail:

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Originally posted by Pipe Smoker:

And altitude.

Temperature, too.

IIRC, from a high school class many, many years ago, 1,125fps @ 32F @ sea level.

Airspeed can be much different than speed over the ground. Aircraft are travelling within an air mass, which is also moving constantly. An airplane moving at an airspeed of 100MPH into a 120MPH headwind is actually travelling backwards over the ground below. Same aircraft moving at airspeed 100MPH with a 120MPH tailwind is making 220MPH over the ground. Naturally, none of these conditions remains constant for any predictable duration.

Crosswinds, updrafts, downdrafts all contribute to the task of air travel. Correction to compensate for ambient conditions is a constant.

had that happen in a sail boat on a river once... speed through the water was probably about 5 knots the only problem was the current in the river was around 8 and going in the opposite direction. Boat was healed over and sailing great only problem was the 'ground speed' was -3 knots....

A guy I knew in college flew a Piper Cub backwards. The headwind was 40 mph and the Cub had a stall speed of 37 or 38 mph.

He was taxiing from his tiedown to the fuel pump, lifted off and backed up (deliberately, according to him).

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Originally posted by wrightd:

I didn't comprehend all of the physics, but that is fascinating.

Aerodynamics is really fun stuff.

There are 4 kinds of airspeed. Indicated, true, mach number, and ground.

Ground speed is just how fast you're going across the ground, which is greatly affected by the winds, which is what the original article was about. We will ignore ground speed for the rest of this.

True airspeed is how fast the airplane is actually moving through the air. If a helium birthday balloon is floating in the air and you fly by right next to it, your true airspeed is how fast you pass it.

Indicated airspeed is how fast it feels to the airplane. For standardization, at sea level with standard conditions (15 degrees celsius, air pressure 29.92), indicated airspeed equals true airspeed. With no winds (a completely still air mass), your ground speed, true airspeed, and indicated airspeed are all exactly the same.

Air density changes with higher altitude, getting thinner as you go up. Temperature gets colder, too. Thus the wings "feel" like you are going slower than you actually are, because the air is thinner up higher. Fewer molecules of air are passing over the wings, which produces less lift and less drag. It "feels" slower aerodynamically.

Thus as you go higher your indicated airspeed will be less for the same true airspeed. e.g. at sea level your indicated airspeed is 200, and your true airspeed is 200. It feels like, and is, 200 to the airplane. Up higher, if your indicated airspeed is 200 then your true airspeed will be faster, say 250 for this example. The airplane "feels" 200 but you're really going 250.

That's why we fly high, to go faster without using more fuel or needing bigger engines.

Air molecules zip around at basically the speed of sound. Hotter air means faster moving molecules. So the speed of sound is faster in hot air than cold air. Since the atmosphere gets colder up higher, Mach 1 is slower up at airliner altitudes than at sea level.

As air flows over the wing it speeds up a bit, which is part of what makes lift. As airflow gets close to the speed of sound it compresses and it has abrupt pressure changes called shock waves. These change the lift and drag characteristics dramatically. Airliners are not designed to fly transonic or supersonic, so there is a maximum Mach number which assures no shock waves will form on that particular aircraft.

This max Mach number applies at any altitude.

So our hypothetical airplane takes off at sea level and is going 250 indicated. Its true airspeed is also 250.

As it climbs at a constant indicated 250, the air is getting thinner, so the true airspeed is increasing. It is factually moving faster compared to the air, but it doesn't "feel" faster to the airplane.

But going even higher and faster into colder air means getting closer to the speed of sound. Somewhere up around 30,000 to 32,000 feet the airplane reaches the maximum mach number, perhaps 0.77.

Now if we level off here we are indicating 250, our Mach number is 0.77, and our true airspeed will depend on the air density but will be around 430. It feels like 250 aerodynamically to the airplane, but the air is so thin we are really going 430, and Mach shock waves don't quite form.

If we climb higher we must keep our Mach number at 0.77, but our true airspeed will decrease because the air gets colder and the speed of sound gets slower. Thus our indicated airspeed gets slower because we are actually going slower, and we are in ever thinner air as we climb. The airplane "feels" it is going slower, with less lift and less drag.

We might cruise at 37,000 feet with Mach .77, indicating 220. Our true airspeed would depend on altitude and temperature, and is somewhat less than the 430.

So, down low we are so far away from Mach 1 that indicated airspeed is all the airplane cares about. Don't go too slow or there won't be enough lift to fly, and don't go too fast or you break things off. Up high we must not exceed the Mach limitation or we could lose control due to shock waves. This was the challenge in breaking the sound barrier initially.

Thanks Fly-Sig! That is a great education for those who may know a little bit about airspeeds, but have no experience with Mach numbers.

quote:

Originally posted by Fly-Sig:

There are 4 kinds of airspeed. Indicated, true, mach number, and ground.

Add another one: Calibrated airspeed (CAS) -- indicated airspeed, corrected for (calibrated for) errors caused by installation in the aircraft. Section V of a typical POH (Pilot's Operating Handbook) will have a table or listing for conversion from IAS to CAS.

quote:

Originally posted by V-Tail:
Add another one: Calibrated airspeed (CAS) -- indicated airspeed, corrected for (calibrated for) errors caused by installation in the aircraft. Section V of a typical POH (Pilot's Operating Handbook) will have a table or listing for conversion from IAS to CAS.

Good one!

One of the airplanes I used to fly had stall speeds listed in CAS, which seemed particularly silly.

The jet airliners are all so computerized that the IAS is already corrected, though labeled on the instrument as IAS. But I am not certain that it is compensated for compressibility... hmm. In the airline world we flew IAS or Mach number. If there was an ATC instruction we just did as instructed, which in a few instances were weird such as issuing an IAS up at cruise instead of giving us a Mach number.

IOW, we were trained monkeys and just had to be sure to stay away from the red markings on the instruments. You'd probably be quite disappointed in the modern airline training philosophy, as I am. We used to enjoy all the details. We could (and had to be able to) draw detailed systems diagrams, and recite all the technical specifications.

So here's a side story of Operation Bongo II, in 1964 the Air Force had 8 sonic booms a day over Oklahoma City as a test. This is during the time when the airlines were making orders for the Concorde. When the announcement was made (FAA didn't even ask the local OKC city government), OKC threw a party thinking it would be fun. Five months later the residents were pissed off, the two tallest buildings in downtown had a 147 broken windows, there were over 4,000 claims of damage to buildings and after 1,200 sonic booms the project was cancelled.

quote:

Originally posted by Fly-Sig:

The jet airliners are all so computerized that the IAS is already corrected, though labeled on the instrument as IAS. But I am not certain that it is compensated for compressibility

That brings up Yet Another: EAS (Equivalent Air Speed), which is CAS (Calibrated Air Speed, corrected for compressibility.

So, we have the following order

• IAS: Indicated Air Speed. This is what is shown on the airspeed indicator.
  
  
• CAS: Calibrated Air Speed. IAS corrected for instrument and installation errors, typically found in paperwork accompanying the specific aircraft.
  
  
• EAS: Equivalent Air Speed. CAS corrected for compressibility.
  
  
• TAS: True Air Speed. EAS corrected for density altitude. For rough estimates flying piston-powered airplanes below 18,000', my rule of thumb was to use IAS and add 2% per thousand feet.

Not an air speed, but the one that we're most interested in for things like "when are we going to get there?" and "do we have enough fuel?" is GS (Ground Speed), which is the sum of the TAS / heading vector and the wind vector, after fooling around with the fact that heading as read from a compass is magnetic, and winds are reported in true.

This airplane generated hundreds of sonic booms per minute without even flying.
https://en.wikipedia.org/wiki/...Thunderscreech#Noise