Vx and Vy

[Chris767PL]

Gentlemen,

Hopefully I am not asking a question that was answered in another thread. For the record, I did a search and did not find anything.

So my question to the experts: Why does Vx increase with altitude, and why does Vy decrease with altitude? I know both speeds have to do with topics such as total drag, thrust available, excess power, etc, but am having trouble incorporating it into a well developed explanation, and am not sure if I fully caught on myself.

Thanks in advance.

[BOAC]

Google is your friend? (http://www.rodmachado.com/Articles/Article%20of%20Month/Articles-of-Month/Vx%20and%20Vy/Vx%20&%20Vy.html)

[Chris767PL]

Done that already. Thanks for the suggestion.

[hawk37]

Chris, you may have noticed that the second last paragraph in the article makes no sense at all. It's reproduced here, complete with the spelling mistake.

"Even thought these Figures don't represent a specific airplane you can see that at 10,000 feet Vx equals Vy (both are 75 knots). Therefore, we can say that, for this fictitious airplane, 10,000 feet is the airplane's absolute ceiling."

Just look at figure 2. For 10,000 feet, this shows Vx well less than Vy (the tangent line through the origin, versus the tangent line for zero slope). Further, as shown in figure 4, since there is an excess of power, the aircraft cannot be at it's absolute ceiling.

Hawk

[TheGorrilla]

Quite. Seems to be an excess of about 30HP and a difference of about 10Kts. He doesn't state what his definition of absolute ceiling is either. I thought it was based on rate of climb rather than Vx and Vy becoming coincident, which, according to the tangent method used there could only happen at a zero rate of climb (i.e. the tangent is horizontal).

I wouldn't blame google for this mistake though! :O

[hawk37]

No real experience here, but by the sound of "absolute" ceiling, I can only think that means where Vx = Vy, and the rate of climb ROC = zero at both. In other words, if you fly a bit slower, or faster, you'll end up descending. Service ceiling, IIRC means something along the lines of capable of 100 fpm, although there may be different definitions for prop and jet.

Hawk

[Chris767PL]

Woops, I just realized that that link was to a specific website. I thought it was directing me to google.

I read that article over and over again, and added it to my collection of AC, and aerodynamic articles, before I posted my question on here. The school I am with that are preparing me for my CPL checkride are requiring me to explain WHY everything happens. And that article doesn't answer it. I agree with how the second to the last paragraph is misleading.

So my question is, why does Vx continue to increase with altitude? Is it because of increasing TAS as you continue to climb, decreasing the prop efficiency? And increasing the speed changes your angle of attack and allows you to produce the same amount of lift at for example 5,000 than at 1,000, by permitting more air over the wing? Or maybe it’s because you’re trying to fulfill the second part of the definition of Vx: for a given horizontal distance? If you maintain the same speed of Vx at 5,000 when it’s designated for sea level, that given horizontal distance would change (talking in calm air conditions of course). It would get shorter. I have a hunch it's really the second answer, but am not sure.

Vy is simply based off of excess power. The higher you climb the amount of thrust decreases due to decreasing prop efficiency because of relative wind. The part I don't understand is WHY. Rods article, however, says it is the greatest deflection on the VSI, which confuses me and makes me think that your trying to climb as fast as possible, implying that that speed should be used for short field takeoffs on the initial climb. So I just blacked out that sentence on the printed article in my binder. Doesn’t Vx give you the greatest needle deflection? Does Vy decrease because while your climbing, TAS is getting greater, which increases your Total Drag (Power required), and Power available decreases? That doesn't really explain why your IAS would decrease though.

Finally, TAS increases with altitude due to air density decreasing and more airflow required to fly over the wings in order to produce the same amount of lift at higher altitude, correct?

Your help is much appreciated gentlemen.

[SNS3Guppy]

Chris,

You're talking about two different climbs here...steepest climb, and fastest climb. Without getting wrapped around the axle about needle deflections, put it in a place that means the most; your windscreen. When you take off, if a tower or big rock is in front of you and you can't go around it, then you want whatever climb will get you over it.

Steepest means you move toward the tower more slowly, while climbing as steeply as possible. This is Vx. It may take you longer to get over the tower, but it will also take you longer to get to the tower, and your only priority is clearing the tower. Vx is a good obstacle speed when climbing as steeply as possible is the most important thing in the world to you.

Fastest means you move toward the tower faster, while climbing faster. You'll get to the tower faster, and when you do, you won't be as high. In a given time period you'll climb faster which is why Vy is a good speed when you want to get to a higher altitude as soon as possible...it's just not necessarily the best speed to choose when you're trying to get over an obstacle that's right in front of you. You won't climb as high in as short a distance travelled forward, as you would at Vx.

Clear as mud.

Vx occurs at your minimum drag speed. Remember the charts showing parasite drag and induced drag? One speed exists for a given airplane configuration and air density (altitude) where induced drag will equal parasite drag, and that's your minimum drag speed. Vmd. It's also the point where you will be able to climb at the steepest angle, because climbing is based on excess thrust. You won't necessarily be able to climb the quickest...your rate of climb may not be the highest, because minimum drag doesn't necessarily represent the point on the drag curve where the biggest surplus of power exists over drag...all it does for you is represent the steepest angle you can climb.

One way to think about it is pointing your aircraft straight up. If you have an aircraft that weighs a thousand pounds, and you can produce a thousand pounds of thrust, in theory you'll float there all day long. Add two thousand pounds of thrust, and now you're going up, up, up. You have climb performance. Lower the nose a little bit, and now you're not only going up, but forward. Keep lowering it a little at a time, and you'll eventually mark a point where the aircraft climbs the greatest gain in altitude for the shortest distance forward...the steepest climb. This isn't straight up, for your airplane because you're going to hit a point as you climb when lift is helping you, drag is at a minimum, and you've found Vx.

This won't be the speed that will get you to altitude the fastest, however. When going straight up, with some unholy amount of thrust, yes, you would be steepest...but few aircrft have that capability, so we balance it with forward speed. Rather than using pure thrust to hold the airplane up, we use some lift, some thrust. The speed that produces minimum drag makes for the steepest practical climb in your aircraft, but at a higher speed you'll come across a point where you will achieve the highest rate of climb. Vy. At this point, you are producing slightly more parasite drag than you were at Vx, but you've also got more excess thrust and more lift...you're climbing faster.

Your question centers around why Vx and Vy come together. Think of Vx as an aerodynamic question; it's a function of drag. The minimum drag speed will vary with angle of attack, and as you climb higher and higher you need a higher and higher angle of attack, or a higher and higher airspeed, to produce the same lift.

Think of Vy as a function of power. As power available decreases with altitude, Vy also changes. Power decreases in a piston airplane as air density decreases and thus engine output decreases, and as propeller efficiency decreases as the air density drops (air gets thinner as you go up).

At the same time, you're flying at a higher and higher angle of attack. Eventually the place where minimum drag and excess thrust meet comes together and you've reached your absolute ceiling. At this point, you can't go faster, you can't go slower you can maintain altitude. You have no more excess power, you can only maintain altitude.

[Chris767PL]

SNS3Guppy,

Thank you for your help. You cleared a lot of things up for me.

Best Regards,

-Chris

[Islander2]

SNS3Guppy,
Vx occurs at your minimum drag speed.That's a little bit of an oversimplification ... Vx actually occurs at the speed which gives maximum excess thrust (i.e. maximum difference between thrust and drag).

True this is immaterial for a jet, where thrust is reasonably constant with speed, because Vmd and Vmet will coincide. It's a different story for a propeller aircraft, however. Here, thrust decreases with increasing speed, so Vmet will be much closer to Vs than in a jet and can be significantly lower than the minimum drag speed.