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Thread: Vx vs. Vy

  1. #1
    mel-bruce Guest

    Vx vs. Vy

    I probably should know this by now, considering the fact that in about 10 days I take my checkride, but better late than never to learn.

    Vx= best angle of climb speed or in plain english the greatest gain in altitude over a given horizontal distance. This speed is used to clear 50' obstacles and so forth.

    Vy=best rate of climb speed or in plain english the greatest gain in altitude over a given amount of time. Vy is used on normal takeoffs and such.

    I understand the definitions and the applications for their use, but what I don't understand is why are'nt these two speeds the same. In flying time = distance because you can't stop moving, so in my mind Vx and Vy would be the same.
    Then again I am a blonde, so this might explain the mental block on this subject. Any explanations to help me understand would be greatly appreciated.
    Melissa B.

  2. #2
    Think about it this way. With Vx you aren't as aerodynamically efficient... higher AOA, higher drag, so you aren't moving forward as quickly and in exchange gain more altitude for the distance you cover for the maximum climb angle. At Vy you at max aerodynamic efficiency for a climb so you gain the altitude quicker, but you also cover more distance which decreases the climb angle from Vx.

    From a practical reality, if at book performance figures the Vy climb angle would be insufficient to clear obstacles requiring the use of Vx... time to figure out a different way to depart. But it doesn't hurt to use Vx when in tight quarters. Just explain to your passengers there will be a higher than normal pitch.

  3. #3
    Quote Originally Posted by mel-bruce View Post
    I probably should know this by now, considering the fact that in about 10 days I take my checkride, but better late than never to learn.

    Vx= best angle of climb speed or in plain english the greatest gain in altitude over a given horizontal distance. This speed is used to clear 50' obstacles and so forth.

    Vy=best rate of climb speed or in plain english the greatest gain in altitude over a given amount of time. Vy is used on normal takeoffs and such.

    I understand the definitions and the applications for their use, but what I don't understand is why are'nt these two speeds the same. In flying time = distance because you can't stop moving, so in my mind Vx and Vy would be the same.
    Then again I am a blonde, so this might explain the mental block on this subject. Any explanations to help me understand would be greatly appreciated.
    Melissa B.
    I'm not sure exactly where your sticking point is, but to start, ask yourself these questions. Apply them to your airplane:

    1. If you don't climb at all after takeoff, will you clear the obstacle?
    2. What happens to your airspeed when you raise the nose and leave climb power in?

  4. #4
    mel-bruce Guest
    Quote Originally Posted by seandougherty View Post
    Think about it this way. With Vx you aren't as aerodynamically efficient... higher AOA, higher drag, so you aren't moving forward as quickly and in exchange gain more altitude for the distance you cover for the maximum climb angle. At Vy you at max aerodynamic efficiency for a climb so you gain the altitude quicker, but you also cover more distance which decreases the climb angle from Vx.

    From a practical reality, if at book performance figures the Vy climb angle would be insufficient to clear obstacles requiring the use of Vx... time to figure out a different way to depart. But it doesn't hurt to use Vx when in tight quarters. Just explain to your passengers there will be a higher than normal pitch.
    Ahhh, the light clicked on. I think I just needed a little more explanation. Now I completely understand. Thanx

  5. #5

  6. #6
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    Quote Originally Posted by mel-bruce View Post
    ... In flying time = distance because you can't stop moving, so in my mind Vx and Vy would be the same...
    Sure, you can always convert from time to distance, given your velocity. But in this case, your velocity in the two cases isn't the same, your Vy velocity is faster. So while you're traveling farther in the Vy case, you're also traveling faster.

    So imagine two ramps, a steep one and a shallow one. You're slowly trudging up the steep one, but driving a corvette up the shallow one. Which one will have you climbing faster?
    -harry

  7. #7

    ~

    Quote Originally Posted by seandougherty View Post
    Think about it this way. With Vx you aren't as aerodynamically efficient... higher AOA, higher drag
    Not quite, you're drag is actually lower at Vx than at Vy. Aerodynamic efficiency is better (if by efficiency we refer to drag). Take a look this graph:

    http://www.undaerospace.com/images/s...s/altitude.gif

    Notice that Vy is part way up the power required (green) curve.

    Take a look at the chart in Blackhawk's PDF. Sorry, I couldn't find a replica on google to link. Looking at the first graph on page one you see that the tangent line to the rate of climb curve gives us our Vx speed. The top of the rate of climb curve is Vy.


    Quote Originally Posted by mel-bruce
    why are'nt these two speeds the same.
    To increase our angle we sacrifice some of our climb performance to allow us to fly at a slower speed. Flying at a slower speed gives us more time to gain altitude. So Vx is at a slower speed to increase time available, over a given distance, to climb. As you put it, altitude/distance.



    An example, using the same first graph in the pdf mentioned above, (apologies for the numbers):

    Aircraft 'A' flies at Vx = ~68 knots giving ~640 fpm
    Aircraft 'B' flies at Vy = ~80 knots giving ~700 fpm



    After flying for 1 minute:

    Aircraft 'A' has covered 6886 feet in one minute, gaining 640 feet of altitude.
    Aircraft 'B' has covered 8101 feet in one minute, gaining 700 feet of altitude.



    Using some simple trig we find climb angles of each:

    Inverse tangent (640/6886) gives us Aircraft 'A' at a climb angle of 5.3 degrees.
    Inverse tangent (700/8101) gives us Aircraft 'B' at a climb angle of 4.9 degrees.



    You may ignore the math used to get to these numbers, the conclusion to draw here is:

    Aircraft 'A' was flying at Vx of 68 knots, rate of climb of 640 fpm, and a resultant climb angle of 5.3 degrees.
    Aircraft 'B' was flying at Vy of 80, rate of climb 700 fpm, and a resultant climb angle of 4.9 degrees.

    Math doesn't lie. Find this helpful or gibberish? TY blonde bombshell Melissa, right?

  8. #8
    Quote Originally Posted by mel-bruce View Post
    In flying time = distance because you can't stop moving
    time * speed = distance

    Similarly, time = distance/speed. Thus, speed and time are inversely proportional. That is, when speed goes down, time goes up. Assuming distance as constant, of course.

    I think harry was getting at the same thing?
    Last edited by shdw; 07-18-2010 at 15:26.

  9. #9
    mel-bruce Guest
    Quote Originally Posted by shdw View Post
    Not quite, you're drag is actually lower at Vx than at Vy. Aerodynamic efficiency is better (if by efficiency we refer to drag). Take a look this graph:

    http://www.undaerospace.com/images/s...s/altitude.gif

    Notice that Vy is part way up the power required (green) curve.

    Take a look at the chart in Blackhawk's PDF. Sorry, I couldn't find a replica on google to link. Looking at the first graph on page one you see that the tangent line to the rate of climb curve gives us our Vx speed. The top of the rate of climb curve is Vy.




    To increase our angle we sacrifice some of our climb performance to allow us to fly at a slower speed. Flying at a slower speed gives us more time to gain altitude. So Vx is at a slower speed to increase time available, over a given distance, to climb. As you put it, altitude/distance.



    An example, using the same first graph in the pdf mentioned above, (apologies for the numbers):

    Aircraft 'A' flies at Vx = ~68 knots giving ~640 fpm
    Aircraft 'B' flies at Vy = ~80 knots giving ~700 fpm



    After flying for 1 minute:

    Aircraft 'A' has covered 6886 feet in one minute, gaining 640 feet of altitude.
    Aircraft 'B' has covered 8101 feet in one minute, gaining 700 feet of altitude.



    Using some simple trig we find climb angles of each:

    Inverse tangent (640/6886) gives us Aircraft 'A' at a climb angle of 5.3 degrees.
    Inverse tangent (700/8101) gives us Aircraft 'B' at a climb angle of 4.9 degrees.



    You may ignore the math used to get to these numbers, the conclusion to draw here is:

    Aircraft 'A' was flying at Vx of 68 knots, rate of climb of 640 fpm, and a resultant climb angle of 5.3 degrees.
    Aircraft 'B' was flying at Vy of 80, rate of climb 700 fpm, and a resultant climb angle of 4.9 degrees.

    Math doesn't lie. Find this helpful or gibberish? TY blonde bombshell Melissa, right?
    This was helpful, as well as the other posts. There is something in the charts that I am unfamiliar with though, that is the power curve. I haven't learned anything about that in any of my groundschooling nor even from my instructor. I have a feeling that that is probably not a good thing.

    Math doesn't lie. Your starting to sound like my dad. (which is a very good thing, my dad is a great man) He's in love with numbers. Then again I'd suppose a guy with a quote from Lord Kelvin as his signature has no small crush on the subject

    As to being a blonde bombshell, beauty doesn't get me where I'm going safely, so unfortunately that doesn't really matter.

  10. #10
    Quote Originally Posted by mel-bruce View Post
    There is something in the charts that I am unfamiliar with though, that is the power curve.
    Are you comfortable with numbers, basic algebra? If so let me know and I can provide details through some formulas. For now I'll try to leave them out.

    To understand the power required curve you should know where it comes from. Look at this: http://upload.wikimedia.org/wikipedi...ag_Curve_2.jpg

    You're looking at the drag curve of a propeller aircraft. Now take a look at this: http://boundvortex.com/Images/Vy1.png

    Notice the similarities in the two graphs. Both have airspeed across the bottom. The green line (power require) on the later image is similar in shape to that of the total drag curve. However, on the left side one is marked power and the other marked drag.

    Think about the four forces for a moment, doesn't power have to equally oppose drag.

    So what is the power required curve? It is a curve derived from the aircrafts total drag curve and it gives us the power required to maintain altitude at any given airspeed. It also gives us some airspeeds, even more when compared with the power available curve. Here are some:

    1. Best rate of climb: Airspeed with the maximum distance between power available and power required curves.
    2. Best glide/economy speed: The tangent line to the power required curve.
    3. Minimum sink speed: Minimum power required.
    4. Max level cruise speed: The point of intersection of the power available and power required curves.


    If you ever get around to it, purchase emergency maneuver training by rich stowell or an illustrated guide to aerodynamics by H.C. Skip.

    If you do let me know and I'll reference you some pages to read through, they do a far better job explaining what this information has to offer than I do. It is my belief that knowing this stuff in this manner improves our safety because we no longer perceive something as truth, we know it is. Instinct reactions are going to be based on knowledge, not beliefs, IMO.



    Quote Originally Posted by mel-bruce
    As to being a blonde bombshell, beauty doesn't get me where I'm going safely, so unfortunately that doesn't really matter.
    But it gets you there in style, after your brain handles the safety part. lol

  11. #11
    mel-bruce Guest
    Quote Originally Posted by shdw View Post
    Are you comfortable with numbers, basic algebra? If so let me know and I can provide details through some formulas. For now I'll try to leave them out.

    To understand the power required curve you should know where it comes from. Look at this: http://upload.wikimedia.org/wikipedi...ag_Curve_2.jpg

    You're looking at the drag curve of a propeller aircraft. Now take a look at this: http://boundvortex.com/Images/Vy1.png

    Notice the similarities in the two graphs. Both have airspeed across the bottom. The green line (power require) on the later image is similar in shape to that of the total drag curve. However, on the left side one is marked power and the other marked drag.

    Think about the four forces for a moment, doesn't power have to equally oppose drag.

    So what is the power required curve? It is a curve derived from the aircrafts total drag curve and it gives us the power required to maintain altitude at any given airspeed. It also gives us some airspeeds, even more when compared with the power available curve. Here are some:

    1. Best rate of climb: Airspeed with the maximum distance between power available and power required curves.
    2. Best glide/economy speed: The tangent line to the power required curve.
    3. Minimum sink speed: Minimum power required.
    4. Max level cruise speed: The point of intersection of the power available and power required curves.

    I aced algebra in highschool, but algebra 2 is as far as I got, and to be honest with you math is not a subject I to much enjoy.
    I'll look for the books you mentioned, and if I get my hands on a copy I'll let you know. I am for learning as much about flying as possible, I believe the more you understand about your airplane and it's systems the better prepared you will be in the case of an emergency.

    BTW, what does the abreviation TY stand for?

  12. #12
    Quote Originally Posted by mel-bruce View Post
    what does the abreviation TY stand for?

    TY = Thank You.

    The Emergency maneuver training book has almost no math at all. The other, it is basic math. However, if you can understand it you will have a better grasp on aerodynamics than most.


 

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