TAS/CAS/IAS

[USMCmech]

Time to reveal my emberasing gap in aircrat knowledge.

Can anybody give me a good simple explanation for the differance between indicated and true airspeed.

I understand about the relationship between CAS & TAS, but IAS and TAS baffels me.

[jetman]

USMCmech

The way i understand it:the airspeed indicator measures the dynamic pressure in the pitot
as the plane moves forward through the air [.really it measures the difference between dynamic and static systems pressures]
The value of the dynamic pressure is not only dependent on the airplane speed [ it increases with an increase in speed]but also on the air density[ atmospheric prss and temp.] If we flew at sea level .29.92 '' and 15C IAS and TAS would be the same,as we climb, the air is less dense and exert less dynamic press even if we mantain the same FORWARD SPEED THROUGH THE AIR [TAS] so you would see a decrease in the IAS .Rough correction : for every 1000' of pressure alt. add 2% to the IAS to calculate TAS Dont know if this may help,regards
Jetman

[USMCmech]

[ QUOTE ]
for every 1000' of pressure alt. add 2% to the IAS to calculate TAS

[/ QUOTE ]

Much clearer.

Thanx

[Grabo172]

Good explaination Jetman, I just have one minor correction...

The pitot tube feels Total Pressure (Total Pressure = Dynamic Pressure + Static Pressure)

IAS is based on Dynamic pressure. To get Dynamic Pressure from the pitot tube you have to do some quick algebra and get the equation Dynamic Pressure = Total Pressure - Static Pressure.

The Airspeed Indicator mechanically does this by opposing the measured Pitot Tube Pressure (Total Pressure) against the Static Pressure.


Then the manufacturer calibrates the face plate of the Airspeed Indicator to relate the measured Dynamic Presssure to an IAS.

[seagull]

Just a tidbit to add, that Dynamic Pressure (Q factor), being V^2 * rho/2, is really just the kinetic energy of the air itself, substitude rho for m in the standard KE= m/2 * V^2. Less mass means that higher V is required to maintain the same dynamic pressure.

[JayAre]

[ QUOTE ]
Just a tidbit to add, that Dynamic Pressure (Q factor), being V^2 * rho/2, is really just the kinetic energy of the air itself, substitude rho for m in the standard KE= m/2 * V^2. Less mass means that higher V is required to maintain the same dynamic pressure.

[/ QUOTE ]

What the "F"

[E_Dawg]

Just don't bust redline. And don't stall.

You'll be fine

[CaliforniaSurfer]

Amen Ed!

[C650CPT]

[ QUOTE ]
Just a tidbit to add, that Dynamic Pressure (Q factor), being V^2 * rho/2, is really just the kinetic energy of the air itself, substitude rho for m in the standard KE= m/2 * V^2. Less mass means that higher V is required to maintain the same dynamic pressure.

[/ QUOTE ]

The scarry thing is that I actually understand what you are saying. Isn't that part of the lift equation?

Jim

[Grabo172]

Has some of the same stuff as the lift equation, but not the same equation.

It'd be alot easier to see if we could type actual equations in their actual form... we use Mathtype at work... goes with WORD.

[seagull]

Lift equation uses Q-factor (dynamic pressure) for part of the total. The lift equation is really just measuring the amount of pressure over the area of the wing and the factor for the shape (or lift producing ability) of the airfoil. This is the root of the "newton causes lift" argument, but it has to be remembered that this is not the _cause_ of lift, it is just the way we _measure_ it!