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

Voters
9. You may not vote on this poll
  • 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%
Page 3 of 6 FirstFirst 12345 ... LastLast
Results 21 to 30 of 52

Thread: Vy flight path angle greater than Vx ???

  1. #21

    Join Date
    Oct 2014
    Location
    Tehachapi, CA
    Posts
    189
    Quote Originally Posted by Waldo Pepper View Post
    To simplify this whole discussion does anyone know why those numbers are wrong in all the POHs (looked at anyway)?
    They're not wrong. You're just misinterpreting what they mean.

    To quote Inigo Montoya from "Princess Bride:

    "... I do not think it means what you think it means."

  2. #22
    Dana's Avatar
    Join Date
    Jul 2011
    Posts
    704
    Quote Originally Posted by Waldo Pepper View Post
    Hi Dana,

    The graph you presented is obviously for illustration purposes.
    Yes, of course. But ignoring the numbers, it looks just like the real curves for real aircraft, I posted it simply to illustrate why Vx is always slower than Vy.

    While all this is I'm sure fascinating, saying that we don't know how the numbers in a POH were arrived at so we can't empirically derive numbers is the same as saying we can't rely on the number/data in a POH,
    ESPECIALLY when you consider the 50' number and the ground roll number and the total distance over an obstacle number - if those 3 numbers aren't reliable what numbers in the POH could be reliable?
    It's not that those numbers aren't "reliable", but they don't contain enough information to derive the information you're looking for. It's probably like the "maximum demonstrated crosswind" in the POH, which is not necessarily the max the plane can handle, but that max that was demonstrated, and intended to be representative of what a typical pilot in typical conditions can safely expect to handle.

  3. #23

    Join Date
    Jul 2011
    Location
    Clarklake, MI
    Posts
    2,274
    Quote Originally Posted by Waldo Pepper View Post
    To simplify this whole discussion does anyone know why those numbers are wrong in all the POHs (looked at anyway)?
    Define "wrong"

    The numbers accomplish exactly what the manufacturer intended.

  4. #24

    Join Date
    Aug 2011
    Posts
    204

    Confusing Terms

    Dana posted a great graph of ROC (rate of climb) versus airspeed. It shows the relationships between Vx (best angle of climb) and Vy (best rate of climb).

    The best rate of climb (Vy) is simply the highest value of rate of climb. This value is independent of wind.

    The best angle of climb (Vx - for clearing obstacles) is the point on the ROC data curves where a line drawn from the origin (zero airspeed, zero rate of climb) tangents the ROC curve. The ROC at Vx is always lower than the ROC at Vy (or Vx would be Vy by definition). Wind greatly affects the ANGLE of climb, but the best angle of climb is always at Vx.

    MANY people confuse angles and rates of climb. Angle of climb (altitude change per distance) is based on an earth reference, which is why wind changes the angle of climb (increases with headwind; decreases with tailwind). Rate of climb (altitude change per time) is independent of wind.
    .

  5. #25

    Join Date
    Oct 2014
    Location
    Tehachapi, CA
    Posts
    189
    Quote Originally Posted by Ron Blum View Post
    Wind greatly affects the ANGLE of climb...
    Only with respect to ground position - NOT with respect to aircraft performance. We're ONLY talking about the aircraft's performance here, NOT where it ends up over the ground.

    Quote Originally Posted by Ron Blum View Post
    ... but the best angle of climb is always at Vx.
    Yes, by definition, as Dana and others have pointed out multiple times. Wherever the best angle of climb is is by definition called Vx.

    Quote Originally Posted by Ron Blum View Post
    MANY people confuse angles and rates of climb. Angle of climb (altitude change per distance) is based on an earth reference, which is why wind changes the angle of climb (increases with headwind; decreases with tailwind). Rate of climb (altitude change per time) is independent of wind.
    See above. They're BOTH completely independent of wind when discussing AIRCRAFT PERFORMANCE. The airplane does not know if or which way the wind is blowing (unless we're going to start having downwind turn discussions again).

    If you're talking about clearing an obstacle on the ground, then you care about which way the wind is blowing. If you only care to know what your airplane's best climb angle is, then the ground and the wind are meaningless.

  6. #26

    Join Date
    Jul 2011
    Location
    Shiloh, IL
    Posts
    50
    Quote Originally Posted by Waldo Pepper View Post
    Hi Keen9,
    I looked through some of those CAFE reports (Thanks for the link) and didn't run across the one(s) you are referring to for Vx and Vy, would you mention which ones do?
    Thanks
    The RV-9A Report (clearly the finest of the aircraft tested) has the climb data on page 11. I'm sure there is similar data in the others.

  7. #27

    Join Date
    Aug 2011
    Posts
    204
    Quote Originally Posted by Marc Zeitlin View Post
    If you're talking about clearing an obstacle on the ground, then you care about which way the wind is blowing. If you only care to know what your airplane's best climb angle is, then the ground and the wind are meaningless.
    Marc: By your definition of aircraft performance, Vx is not an aircraft performance number because it is the airspeed to fly to produce the largest climb ANGLE (with respect to the ground).

    I am curious to know what your aircraft performance climb angle is referenced to?

  8. #28

    Join Date
    Oct 2011
    Location
    New Hampshire
    Posts
    1,224
    All aircraft performance numbers are referenced to the air mass that you are flying in. When we are close to the ground, or we are trying travel across the ground, then we factor in the wind vector. But when calculating or demonstrating a pure performance number, we assume that we are moving in an air mass whose horizontal motion (N-S & E-W) is not a factor.

    Best of luck,

    Wes

  9. #29

    Join Date
    Oct 2014
    Location
    Tehachapi, CA
    Posts
    189
    Quote Originally Posted by Ron Blum View Post
    Marc: By your definition of aircraft performance, Vx is not an aircraft performance number because it is the airspeed to fly to produce the largest climb ANGLE (with respect to the ground).
    You do understand that the aircraft's performance is a function of the air that it's in, not the ground that it's over, yes? If my TAS is 100 Kts (169 ft/sec) and my climb rate is 800 fpm (13 ft/sec), then my climb angle is:

    sin^(-1)(13/169) = 4.5 degrees

    For my COZY MKIV at Vx, SL, standard day, if my TAS is 80 Kt. (135 ft/sec) and my climb rate is 1300 fpm (21.7 ft/sec), then my climb angle is 9.2 degrees.

    Same plane, same day, Vy, TAS of 95 Kt. (160 ft/sec) and a climb rate of 1400 fpm (23.3 ft/sec), my climb angle is 8.4 degrees. Climb RATE is higher at Vy, but climb ANGLE is lower.

    See? Vx is steeper than Vy, and I don't care about the ground, or wind, or trees, or 50 ft. obstacles. It's the airplane's performance we're measuring - nothing else.

    Quote Originally Posted by Ron Blum View Post
    I am curious to know what your aircraft performance climb angle is referenced to?
    The air in which the aircraft is flying. Same as ALL aircraft performance, except landing and takeoff rolls. Once the wheels are no longer touching the ground, the air is all that matters. TAS is not referenced to the ground; climb rate is not referenced to the ground; glide ratio is not referenced to the ground; etc.

    Does wind affect the travel over the ground? Sure. But the OP's question was about Vx and Vy and climb angles, not distance traveled over the ground for a given altitude increase.

  10. #30

    Join Date
    Aug 2011
    Posts
    204
    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)

    Yes, I understand aircraft performance. Heading Flight Test, Aerodynamic and Engineering departments and being an FAA Flight Analyst DER at major GA manufacturers across the country over the past 30 years, I think I know aircraft performance, but I am ALWAYS open to learn from others.

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •