What are Vx and Vy? Why are two different speeds given for climb?

Q:
Now, its our turn. These questions are for the pilots in the group.

1. What does Vx, and Vy mean (explanation)?

2. Why is Vx (best angle of climb) given in a speed?

3. Why would there be two different measurements for climb?

4. How do you use them?

Thanks,
Brian


Ed. Note: This was from a Feb. 2003 thread in the Outer Marker forum, around which time there were a number of informative questions and posts about flying, aircraft responses, techniques and other things about real aviation. Marvelous discussions ensued. The answer below condensed many, many posts by many knowledgeable people into one explanation.

A:
Vx is best angle of climb

Vy is best rate of climb

The difference is how you measure the altitude gained.

Imagine two planes took off on parallel runways one at Vx the other at Vy. If you looked at their altimeters when they crossed the departure end of the runway the plane at Vx would be at a higher altitude. If you checked the altimeter in 10 minutes after the takeoff the plane at Vy would now be at a higher altitude.

Vx is used to clear obstacles around the airport. The problem with Vx is that your deck angle is pretty steep and not only is your vision restricted, but also cooling to the engine.

Typically when I take off VFR I accelerate the plane to Vy then climb out at that rate until I get about 2000ft agl, then i switch to cruise climb for better visibility.

Note ! These numbers are related to the gross weight of the airplane. The lighter the plane the lower the speed. The speeds in the handbook typically are for a plane at gross weight

Bob K. [flyinggriffin - Ed.]



Takeoffs make at Vx and Vr will usually be made at full power in light aircraft.

Power reduction is made in accordance withe the operating manual. Fixed pitch prop aircraft use full power to climb. With a constant speed prop you will pull back the manifold pressure and prop RPM to a value that the manufacturer recommends when you're at a safe altitude.

Okay..for takeoff at a short field, with trees at then end. You deploy the flaps to the recommended amount. Taxi right to the very end of the runway you don't want to leave an inch of usable strip behind you. Hold the brakes, full power, check the gages for problems and release the brakes. Hold some back pressure on the yoke and start to rotate about 5 knots before Vx. At liftoff, climb at Vx until clear of the obstacles then lower the nose, climb at Vy and retract flaps SLOWLY! The rest is a normal climb

Bob K.



Grif has done a pretty good job, but I might amplify a couple of things.

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 is, as Grif said, 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.

Hopefully I've now answered (along with what Grif said) your questions 1, 2 and 3.

For number four, as Grif said, best rate is used when you need to get upstairs as quickly as possible. Best angle is used only for obstacle clearance, trees at the end of the runway, for example, then you ease the nose down to best rate.

As Grif also mentioned, sometimes engine cooling and forward visibility become concerns, so that you may instead use a cruise climb, which will be faster (more airflow, so better cooling) and with a lower deck angle (nose further down- better forward visibility), yet you are still climbing at a reasonable rate.

Again, as Grif mentioned, the low-power aircraft such as Warrior, Cessna 150/172, etc. don't have very much of the problems with engine cooling or too high a deck angle, but when you get into more powerful aircraft such as the C-182, Mooney, Bonanza, etc. these become of more concern.

Hope this helps.


Larry N.






See also the Wikipedia entry V speeds.

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