View Poll Results: Is Vy flight path angle greater than Vx?

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  • Vy flight path angle is greater than Vx

    1 11.11%
  • Vx flight path angle is greater tha Vy

    7 77.78%
  • Other opinion / no opinion

    1 11.11%
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Thread: Vy flight path angle greater than Vx ???

  1. #31

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    Quote Originally Posted by Ron Blum View Post
    The original poster (Waldo) posed a question relative to Vx, Vy and clearing a 50’ ground obstacle (regulation). Angle of climb (with respect to the air) is both irrelevant to aircraft performance and the original posters question/statement.

    Flying an airspeed of Vy will produce the highest rate of climb (altitude gain per time).
    Flying an airspeed of Vx will produce the highest angle of climb (altitude gain per distance travelled)
    If you accept that Vy has a lower angle of climb than Vx, then what's the discussion about? Angle of climb (with respect to the air) is what will determine angle of climb over an obstacle on the ground.

    The OP said:

    "In so doing I found that in all cases Vy yielded a steeper flight path angle than Vx."

    This is the incorrect part. Vy, as you know, does NOT yield a steeper flight path angle than Vx. This is what all of us took exception to, and it has nothing to do with distance over the ground, just distance (as defined by TAS per unit time).

    Because the OP used inappropriate information to attempt to DERIVE the Vx climb rate (since that information is not in any POH), he then got inaccurate data for climb performance over a 50' obstacle, which was his original goal. Conflating inappropriate data was the problem with his calculations. If he actually tested for Vx and Vy in a particular airplane and then measured the altitude above a 50' obstacle, he'd realize that climbing at Vx would get him higher over an obstacle than Vy would.

    And the OP stated, with respect to the POLL:

    "Poll is: agree Vy angle > than Vx
    disagree, Vy angle not > Vx, Vx is greater than Vy
    Other: ?"

    As you've clearly articulated, the angle of climb at Vx is ALWAYS higher than the angle of climb at Vy - there can be no other answer. And that's NOT a function of wind, ground, or anything else.

  2. #32

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    All: This thread has given me a great idea (yes, I am bias). I was just thinking of ideas for OSH forum topics. The making of AFM/POHs would be a great topic. Agree?

    i don’t want to mess up this thread, so email fly-in-home@att.net or call (316) 295-7812 with comments, questions or concerns. I’d even do it jointly with Marc as I think that he has done great work with his Cozt Mark IV, and we could both learn from each other ... I really think that we are saying the same thing. It’s just hard to put complex thoughts (and all the background/history/assumptions into short text.

  3. #33

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    This discussion has been very interesting. I have done flight testing on my Kitfox during its Phase 1 flight testing. Part of the Phase 1 testing is doing the timed climb testing to determine Vx and Vy. I did this several times at different weights and flap settings. After collecting the data you create a TAS vs. ROC graph like the one that Dana posted. Vy was very easy to determine, it is the point at the greatest ROC. Vy is determined by drawing a tangent line from the 0, 0 point to where it touches the ROC curve (just like the graph).

    What these numbers mean in flying, regardless of aircraft type, is:

    Vy gives us the greatest rate of climb, period.

    • We use it if we want to get to altitude ASAP
    • ROC changes with weight. Lower weight = higher ROC
    • Vy changes with altitude (it decreases as altitude increases)
    • Wind doesn’t affect Vy, but . . . we may chose a different speed due to:
      • Wanting better visibility over the nose
      • Desire to reduce engine temperatures (CHT, oil temp, etc.)
      • Terrain, turbulence, etc.

    • Most POHs only give us takeoff & climb charts only at gross weight, the worst case scenario.


    Vx is the best angle of climb and is always a steeper angle than Vy.

    • Vx has to do with climbing to get over an object (tree, mountain, antenna, etc.)
    • Vx is always given in a POH with zero wind, but Vx is actually wind dependent. We rarely fly that way.
      • The wind component can be drawn on the ROC graph. If you have a 20 knot headwind than you would draw the tangent line from the 20 KTAS point, which would intersect the ROC curve at a lower speed. Even if you don’t fly the lower speed you will have a better angle of climb due to the headwind component.
      • An extreme example would be a 50 knot head wind with a plane that has a Vso of 40 KIAS. If you climb at 50 KIAS it will be a vertical climb (10 knots above stall). If you increase your IAS above 50 you will be moving forward and have an angle less than vertical.
      • With a tailwind Vx increases. Again, if you had a 20 knot tail wind, you would draw a tangent line from the -20 KTAS point of the graph. It will intersect the ROC curve at a higher speed. With an increasing tailwind Vx increases, and approaches Vy.

    • Vx increases with altitude (Vx & Vy eventually will be the same speed when you can’t climb anymore)
    • Vx decreases with weight.
      • Theoretically speaking if you reduce the weight of a plane enough it will climb vertically (like an F-15).
      • Remember, as we decrease aircraft weight, Vso also decreases. Therefore, a different ROC curve has been established
        • Vso is lower
        • Peak ROC is higher
        • This changes where the tangent line intersects the curve (at a lower KIAS)

      • These are actual numbers from a Cessna TU206G POH
        • Best angle of climb / Vx – 55 KIAS
        • Weight vs. Lift off speed (from the Takeoff Distance tables)
          • 3600 lbs. – 55 KIAS (notice this speed = Vx @ GW)
          • 3300 lbs. – 53 KIAS
          • 3000 lbs. – 50 KIAS


    Lastly, the distance a POH gives for clearing a 50’ obstacle has several factors that make up the total distance.

    • The ground roll to liftoff (sometimes a liftoff speed is given, see to TU206G example above)
    • Continued acceleration to Vx (or another speed published)
    • Pitching from liftoff to an appropriate climb angle or pitch
    • Going from zero ROC at liftoff to the Vx ROC

    Due to these factors the published 50’ obstacle distance isn’t just calculated as (takeoff roll) + (horizontal distance in a Vx climb to 50’). It includes the transition from one to the other. I believe this number is determined by actual flight testing data.
    Last edited by PNelson; 02-23-2018 at 05:09 PM.

  4. #34

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    Quote Originally Posted by Ron Blum View Post
    The making of AFM/POHs would be a great topic.
    It would be. The main thing here, though, is understanding aerodynamic and aircraft performance basics, which in my experience as a pilot and engineer (and A&P) seem to be sorely lacking in a large minority of the pilot population.

    Quote Originally Posted by Ron Blum View Post
    I’d even do it jointly with Marc ...
    "Even", huh? :-). Let me know how I can help. I'd be happy to review documents, and as far as a presentation at OSH is concerned, I do the COZY forum each year on Friday, and generally get there on Wednesday, so it could be possible to squeeze something in.

    Quote Originally Posted by Ron Blum View Post
    as I think that he has done great work with his Cozy Mark IV...
    I followed in the footsteps of many others. Then, after working at Scaled for 6 - 7 years, I learned a lot more. The best Phase I test program document that I've seen (and I know that EAA is working on something, but I haven't seen a finished product yet) is Kevin Walsh's test plan for his COZY MKIV, available at:

    http://cozybuilders.org/docs/

    third bullet point down. It could obviously be modified for any aircraft.

    The results from the Phase I test program should be used to populate the fields in a generic POH for EAB aircraft.

    Quote Originally Posted by Ron Blum View Post
    and we could both learn from each other ... I really think that we are saying the same thing. It’s just hard to put complex thoughts (and all the background/history/assumptions into short text.
    I thank you for your civility. I should have been less strident in my postings, and for that I apologize.

  5. #35

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    I’m excited about this now ��. I’ll checkout the links you sent, look at the deltas between the homebuilt and certificated worlds and see what EAA is doing.

    I love when people tell me that they have 5 hours on their new airplanes and their box isn’t big enough to cruise around for the remainder of the test time.

    This is going to be fun. Thanks!

  6. #36

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    Quote Originally Posted by Ron Blum View Post
    I love when people tell me that they have 5 hours on their new airplanes and their box isn’t big enough to cruise around for the remainder of the test time.
    Don't get me started. I can't tell you how many times I've heard folks say "it's a Long-EZ - Burt tested it in 1980" or "what do you mean, did I test it at the rear CG limit?". Sigh. If it doesn't take you 30 - 40 hours to finish all the tests that should be statutorily required (IMO) then you haven't completed Phase I. Flying around in circles for 30 hours at mid-weight and mid-CG (and not having any idea what Vx and Vy, for example, and to keep us vaguely in the same state as the OP's mission statement) is not Phase I testing.

    And you kids get off of my lawn!

  7. #37

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    Quote Originally Posted by Marc Zeitlin View Post
    And you kids get off of my lawn!
    Amen!!! LOL LOL LOL

  8. #38

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    As the dumbest guy in the thread, can I see if I've sorted this out, as I'm knee deep in Phase I testing?

    Okay, so Vx - getting to altitude quickly to go over an obstacle - one is going to be more nose high, a tad slower, but with a higher climb rate. It's effectiveness over efficiency.
    Vy is getting to a higher altitude efficiently, so one is going to gain the most altitude over time. It's not as effective, though, in the short term, as Vx.

    Now, then, these are the "book" numbers.

    How we use those numbers in the real world are dependent on, well, the real world. Once an aircraft leaves the earth it is in the air and of the air, moving with it as much as through it.

    So if it takes me 200 feet at 60 miles an hour* to clear a 50 foot tall tree and I'm looking at a 10 mile an hour headwind, it really only takes me 165 feet in ground distance. The plane doesn't know any better, and thinks it's traveled the whole 200 feet.

    The opposite is true with a 10 mile per hour tailwind, as it would take me 235 feet on the ground to clear the same tree. Again, the airplane is ignorant of this, as it's only flown for the 200 feet. The air itself did the rest of the work.

    * As stated previously, this is a lie. The takeoff roll has to be taken into account. Determining this requires a patient and diligent helper (at least for me), or video taken from the ground, and one has to use an average. Bonus plan: air density matters.

    Between late October through April, my little airport in Alabama has an altitude of around 530 feet above sea level. Between mid-April to mid-October the density altitude can be as much as 3,400. What that means to me, a stick-and-rudder guy wearing goggles and a leather flying helmet, is that there's a lot of fudging on climb rates and distances in the summertime, as I don't have a graph with me. But knowing that graph as it relates to altitude is pretty dang important if I want to visit some of the short fields around here.

    So tell me if I got it right!
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  9. #39
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    Quote Originally Posted by Frank Giger View Post
    Okay, so Vx - getting to altitude quickly to go over an obstacle - one is going to be more nose high, a tad slower, but with a higher climb rate. It's effectiveness over efficiency.
    Vy is getting to a higher altitude efficiently, so one is going to gain the most altitude over time. It's not as effective, though, in the short term, as Vx.
    No, Vx has a higher climb angle, in degrees, but a lower climb rate, in feet per minute. Otherwise correct.

  10. #40

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    Frank: You’re not dumb, and you got it correct! (except as Dana pointed out in the second paragraph “climb rate” should be “climb angle”). Great job!

    Now you brought up that summer and winter performance is different. Yes! ��. Since climb rates (and angles) are based on excess power - power above and beyond what it takes to fly at that airspeed), rates (and angles) will decrease as engine horsepower decreases (due to altitude, temperature, humidity, etc. effects). OEMs have engine decks (programs that tell them power available under various conditions) and propeller maps to calculate power/thrust available. Not so simple ��. You’ll have to test under various conditions.

    Here’s a simple, totally made up example. If a 160 HP airplane takes 100 HP to fly 70 MPH, it has 60 excess HP to climb. We’ll say it climbs at 600 FPM. Now, if the engine is only putting out 150 HP, the airplane only has 50 excess HP to climb. Climb rate will decrease to 500 FPM (down 17%). At 100 HP the airplane will not climb (absolute ceiling).

    Be safe.

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