TAS, VMC and Altitude

[uwochris]

Hey guys,

I got some more airspeed Qs, I hope you don't mind

#1. I realize that Vmc decreases with altitude. Is this because as you ascend, your engines don't produce as much power, and thus, the effects of the yaw will not be as great?

If this is the case, should not the rudders become less effective too, with increases in altitude? The reasoning here is that since the air is thinner, the rudders exert less force, and thus, should have a harder time to counteract the yaw.


#2. I realize that TAS increases for any given IAS, with altitude. I have been told that the reason is because of the lower amount of drag (i.e. resistance) acting on the plane.

If this is true, shouldn't the amount of thrust decrease in proportion with the decreased drag? ie) higher altitudes=thinner air= props exert less force=lower thrust?

TIA.

 

[bart]

#1

Your rudder is as effective at any altitude given your INDICATED airspeed stays the same. Vmc is measured with INDICATED airspeed, so the change in altitude effects no change in rudder effectiveness.

#2

Generally it is the powerplant that produces less power (however you want to measure it) as altitude increases, because there is less air and therefore less oxygen to burn fuel with. I will leave it to those with engineering degrees to discuss the efficiency of props and fans in changing air densities.

 

[sstearns2]

Hey guys,

I got some more airspeed Qs, I hope you don't mind

#1. I realize that Vmc decreases with altitude. Is this because as you ascend, your engines don't produce as much power, and thus, the effects of the yaw will not be as great?

Yes.

If this is the case, should not the rudders become less effective too, with increases in altitude? The reasoning here is that since the air is thinner, the rudders exert less force, and thus, should have a harder time to counteract the yaw.

The air is less dense at altitude, but your going thru it faster to generate the same IAS and the same rudder control power.

#2. I realize that TAS increases for any given IAS, with altitude. I have been told that the reason is because of the lower amount of drag (i.e. resistance) acting on the plane.

The reason TAS increases with altitude is that the air density is decreasing with altitude. If you go up to alaska in the winter at sea level your TAS will be less than IAS because the air is very dense because it is very cold.

If this is true, shouldn't the amount of thrust decrease in proportion with the decreased drag? ie) higher altitudes=thinner air= props exert less force=lower thrust?

Thrust increases directly with horsepower and decreases with the square airspeed.

Scott

 

[CLCAP]

#2

The pitot probe measures the amount of air particles hitting its membrane, thus it will decrease with altitude for any given TAS because of the lower air density.

Even though your props or jets eficciency decreases with altitude - the total decrease of drag on the airframe more than compensates for it.

An jet engine that was able to produce 340 KIAS (or more) at sea level might only produce 240 KIAS at FL410 (due to a decrease in thrust but that will correlate to approximately 460 KTAS. (To keep it simpler - I am disregarding the increase in drag due to transsonic speeds)

 

[100LL... Again!]

I'll answer #2 first:

You are actually asking two questions, and mixing them together.

In the first half of the question, you mention that TAS goes up with altitude for a given IAS. This is true. This is due to the fact that you have to move faster to create the same INDICATED airspeed.

You then mention that there is less power available at the higher altitude. This is true. But this does not affect the fact that TAS increases. It only means you can maintain a lower Indicated airspeed.

To summarize, TAS increases with altitude for a given IAS.

HOWEVER, actual TAS may eventually go DOWN with altitude because of the loss of power as you mentioned.


Example:

Suppose you are flying a large single engine airplane.

You fly at 3000 ft at reduced power. IAS = 100kt.
For sake of argument, lets say that TAS = 110kt.


Now you climb up to 10,000 ft.
Increase power to maintain 100kt.
For sake of argument, lets say that TAS = 120kt.

TAS goes up with altitude.


Now repeat the example in a wheezy old Cherokee 140.

At 3000 ft, IAS = 100, TAS = 110, as before.

At 10,000 ft (if you get there), you may be able to get IAS only up to 90kt. TAS might then equal 108kt.

TAS is now lower at the higher altitude.

See how power factors in? Normally power is disregarded when it is stated that TAS incrases with alt. I assumes that power and IAS can remain the same.

Hope that helps.

I'll get to #1 when I got time.

 

[Don]

Maybe this helps…

The short version….

TAS is ALMOST always higher than IAS. The difference is bigger the higher and faster you are.

Stalling speeds and effective force applied by a flight control is ALWAYS the same for a given IAS at any altitude.

VMC is lower in a normally aspirated twin when you go higher because of the reduced power. Not true in a turbo charged twin.

And now the long answer….

#1. I realize that Vmc decreases with altitude. Is this because as you ascend, your engines don't produce as much power, and thus, the effects of the yaw will not be as great?

Yes, unless you are in a turbo charged aircraft and then VMC will occur at the same indicated airspeed.

If this is the case, should not the rudders become less effective too, with increases in altitude? The reasoning here is that since the air is thinner, the rudders exert less force, and thus, should have a harder time to counteract the yaw.

Not really, see below

#2. I realize that TAS increases for any given IAS, with altitude. I have been told that the reason is because of the lower amount of drag (i.e. resistance) acting on the plane.

Not true, find a new CFI if you need to!

Airspeed is calculated as follows:

Your static port senses ambient static pressure, or the pressure of the air if you were not moving.

Your pitot tube senses ambient static pressure plus dynamic pressure, or the pressure of the moving air stream.

So say the static pressure is about 2000lbs per square foot and you are at sea level. Your static port senses 2000lbs psf. Now your pitot tube senses 2300lbs per square foot of pressure. The difference between the two is 300 lbs per square foot (your altimeter is a differential pressure gauge) and now we equate that to an airspeed.

The problem is that while at sea level densities, 300 lbs psf might equal 300 knots, at 40000feet 300lbs does not equal 300 knots because of the density of the air.

Airspeed indicators are set to read accurate on a standard day AT SEA LEVEL. 29.92 inches of mercury, 15 celcius or 2116 psf.

Deviate from those densities and an airspeed indicator does not read correctly, thus we correctfor nonstandard densities, or TAS.

On a standard day
Take a TAS at sea level of 300 Knots and that is a pressure difference of roughly 305 lbs/psf
Now to get 300 Knots at 40000 ft we only need a pressure difference of roughly 75 lbs/psf

So our airspeed indicator, calibrated for sea level reads 150 knots.

Clear as mud? Glad you asked?

Finally, as our airspeed indicator is calibrated for sea level, and the force applied by the rudder is a function of the coefficient of force (hasn’t changed from sea level), dynamic pressure which has changed, but our airspeed indicator still thinks its at sea level, and the surface area of the rudder which hasn’t changed, then for any given indicated airspeed at any altitude you will get the same applied force in pounds per square foot from the rudder.