Does Vx IAS increase with altitude or not?!
 

I have come across another discrepancy between my OAA textbook and my instructor who tells me that Vx IAS does increase with reduced density whereas my textbook insists that it does not!

Intuitively, I have to agree with my textbook, but my instructor tells me that, for the exams I should agree with him!

would love some further insight.

baleares :ok:

Does Vx IAS increase with altitude or not?!
 

If the exam can be proven to be wrong, file a complaint.

this is nice reading I found some time ago and still have in bookmarks (and still didn't read fully as TLDR :)


http://cospilot.com/documents/Why%20...20Altitude.pdf

thanks for that, but his explanation is of why TAS increases. I am being told that IAS increases which makes no sense to me.

Baleares

Wrong way round :E IAS decreases with altitude

pace

You are both missing something...what is staying the same. For example, if TAS kept constant IAS will decrease with altitude. If IAS kept constant, TAS increases with alt. Something to do with four fingers and eating chicken tikka massala if I remember correctly. (EAS, CAS, TAS, Mach)

At the ceiling of the aircraft, there is only one possible indicated speed. At that altitude Vx = Vy.
So Indicated Vx increases and Indicated Vy decreases with height.
It's pretty obvious in practice when you want to climb at FL100 .

Apparently it does increase with altitude, which I didn't know.

http://cospilot.com/documents/Why%20...20Altitude.pdf

Pace,

Sorry I don't understand. Are you saying that the IAS of Vx decreases with altitude ?

Theoretically, there should be no difference to the Drag curve with gain in altitude. As per the lift equation, for the same IAS, Lift and Drag will decrease in equal measure and must be accounted for by an increase in TAS.

There will, however, be a decrease in propeller thrust due to decreased density and so there will be a decrease in angle of climb but the best angle of climb will be achieved at the same speed.

The Rod Machiato article merely explains an increase in the TAS of Vx which is due to the need to fly faster due to reduced density. But it does not explain an increase in IAS

Since Vx is when you have the greatest excess thrust, the available thrust will decrease with altitude then the Vx speed will change as will Vy. They converge at the ceiling as was stated before.

An aircraft's climb rate is basically determined by how much power is left over, after fighting drag, that can fight gravity. Climb speeds (as most other performance speeds) are usually given in calibrated airspeed (CAS), which is indicated airspeed corrected for the errors of the instrument and the pitot/static system. After all, the wings and prop, as well as the engine, all behave in relation to a specific air density. So for our purposes we'll talk about indicated airspeed (IAS), which is the speed indication generated by ram air pressure into the pitot tube. Obviously, reduced air density at a given actual speed through the air (TAS, or true airspeed) gives a lower reading on the airspeed indicator.

Engine power (unless it's a turbocharged engine) also decreases with reduced air density. This reduced air density can be caused by higher altitudes, higher temperatures, or both. Because this is so highly variable, we typically relate aircraft performance to a "standard atmosphere" which is sea level with 29.92" on the barometer (or 1013 millibars, for metric users) and 59� F (or 15� C) and zero percent relative humidity. And we figure a 3.5� F (2� C) drop in temperature for each 1000 feet of altitude increase. This is how performance figures for aircraft are presented in the manuals, also.

Given the above, an aircraft responds in a certain way related to air density, not altitude, so at a given weight it will stall at the same indicated airspeed, regardless of the actual (true) airspeed, temperature, altitude, etc.

So Vy the best rate of climb airspeed, meaning that it is the best combination of engine power and drag (remember drag quadruples as the airspeed doubles) that leaves you with the most engine power remaining with which to climb. So long as you have the same amount of engine power available, this indicated airspeed for Vy will remain the same regardless of altitude/air density, which we call density altitude, meaning we correct altitude indications for pressure and temperature.

But in a normally aspirated (non turbocharged) aircraft, power is reduced as density altitude increases. So there will be a gradual reduction in the indicated airspeed for Vy as you climb (remember, you have less power to overcome drag, and drag changes with the square of the airspeed), so a slightly slower speed reduces drag but reduced power gives you a slower rate of climb.

Now about best angle of climb airspeed. This angle can also be viewed as how much altitude is gained per foot/mile/etc. of forward travel. Obviously, at a slower airspeed you cover less ground, so at the same vertical rate the angle would steepen. But we've already shown that there is only one speed at which you get the max rate of climb. However, recall that squared relationship between speed and drag -- it also works in reverse, so that drag decreases with the square of the speed reduction.

This means that at a slightly slower speed than Vy, the angle becomes a little steeper. So the manufacturers test and calculate until they find the optimum relationship between speed reduction and drag reduction that results in the airspeed that gives you the best (steepest) angle of climb (Vx), which is good for clearing obstacles. Just as Vy decreases with altitude, Vx increases with altitude.

As density altitude increases, remember, the engine power decreases, so these two values (Vy and Vx) slowly converge as you climb until they are the same at the absolute altitude of the aircraft, and Vy is the only speed which allows you to maintain altitude -- you can't actually climb from that point.

In a turbocharged aircraft, the two values will remain essentially the same up to what we call the "critical altitude," that is, the altitude above which the turbocharger no longer maintains sea level power, above which point the two values behave much as they do in a normally aspirated aircraft.

Drag will be reduced with as a square of airspeed, but so will Lift meaning the angle of attack will have to increase to compensate which will mean the same IAS - unless I am very mistaken!

Baleares

Sorry I didn't fully read the question when I said IAS will decrease with altitude;)
But it beggars the question of why you would want to climb at VX unless off a high altitude airport clearing obstructions

Pace

Pace, your previous comment missed a couple of critical points

1 - Vy is determined by the point of excess POWER - TAS is a factor in power
2 - Vx is (as you said determined by excess thrust), but this is also influenced by TAS
3 - for piston aircraft Vx is normally below Vmin drag - so increasing IAS at this speed will normally REDUCE total drag (but may either increase or decrease power required)


The key point is that it takes more power to cruise at 100 IAS at FL 100 than at sea level. Which is why power required goes up by the cube of TAS. It also means that for a given power output, the thrust will decrease at TAS increases (so thrust at F100 and 100 IAS is less than thrust as sea level and 100 IAS).



It is clear Vy and Vx come together at the service ceiling and when you work the math for a piston Vy (in terms of IAS) will decrease with altitude. it is less clear what Vx does! and I believe for most piston aircraft it marginally increases (in IAS terms) (but that is from adhoc reading not any systematic analysis)

Because Jets have a fundamentally different relationship between TAS, Thrust and Power (being approximately co stent thrust machines rather than constant power) the dynamics of Vx and Vy are quite different in the Jet world.

I remember the guy that bought the ex Concorde Air speed indicator was disappointed that it only went up to a few hundred Knots.

Does Vx IAS increase with altitude or not?
 

To find the answer, first look again at;

http://cospilot.com/documents/Why%20...20Altitude.pdf

Then, assuming ISA, and that we discount any position and instrument errors, convert the 10,000' speeds to CAS/IAS by dividing by 1.1 or 1.2, depending on which rule of thumb you adhere to. (The TAS and CAS/IAS at sea level are, of course, the same )

The result is that the conversion from TAS to CAS/IAS more than compensates for the increase in TAS with altitude, and that the CAS/IAS for Vx, far from increasing with altitude, reduces slightly, whilst the CAS/IAS for Vy reduces more quickly, both speeds eventually converging at the absolute ceiling.

So the answer is....... 'It depends, but almost certainly not' ;)


MJ:ok:

I am trying to work out the lift fairy theory on this one...

Standby :p;)

From a practical POV we climb at 160knts up to FL150 and then the speed gets lowered by 2knts per 1000ft above that in a twin TP.

Mad Jock

Fairies defy all normal laws of lift and gravity!:ok:

pace

It's the Fairies for me in future.:)

The Fairy theory can be adapted to any subject in the syllabus.

I love it!;)


MJ:ok:

The problem seems to be that Rod's reasoning in the linked pdf file is not consistent.

In one case he allows for the reduction in IAS with altitude but in the other he doesn't. From his own reasoning, both TASs increase but by less than the reduction of IAS due to altitude, so in fact both IASs reduce.

Using his own example, he says Vx at SL is 69KTAS (which is also 69 KIAS). He says Vx at 10k is 75KTAS which (using his 'math') would be 62 KIAS.

So both IASs reduce with altitude, but Vy decreases at a faster rate until it meets Vx.