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 1 of 6 123 ... LastLast
Results 1 to 10 of 52

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

  1. #1

    Vy flight path angle greater than Vx ???

    A while back I decided to analyze runway design for obstacle clearances (angle to 50' vs distance vs climb rate vs speed, etc.) using my trusty spread sheet.
    In so doing I decided to incorporate data from various POHs.
    In so doing I found that in all cases Vy yielded a steeper flight path angle than Vx.


    Apparently Vx is NOT the "airspeed for best climb angle" it is the airspeed to use at LIFT OFF to give the shortest ditance to 50' of altitude.
    In other words; the sortest distance to 50' from the BEGINNING OF THE GROUND ROLL is attained by lifting off at the Vx speed.
    The basic idea to clear 50' on takeoff is to get off the ground as soon as possible and establish an IAS of Vx.


    This is very different from being "the best angle of climb speed" since while inflight if you need to clear the obstacle above you, you would go to Vy, not Vx.


    Comments, agree/disagree welcome.

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

  2. #2

    Join Date
    Jul 2011
    Location
    Clarklake, MI
    Posts
    2,258
    Quote Originally Posted by Waldo Pepper View Post
    In other words; the sortest distance to 50' from the BEGINNING OF THE GROUND ROLL is attained by lifting off at the Vx speed.
    Actually, the Cessna max performance takeoff procedure in later FAA approved AFM's calls for "lifting off" at a speed less than Vx.

    Additionally, at 50 ft the recommended speed is > than Vx for envelope protection in turbulent conditions. I believe the distance numbers in the AFM reflect the published procedure.

    Also agree there is probably a lot of confusion on when, where and what speed to use. Therefore, all points considered, I voted "other"
    Last edited by martymayes; 02-20-2018 at 06:04 PM.

  3. #3
    Quote Originally Posted by martymayes View Post
    Actually, the Cessna max performance takeoff procedure in later FAA approved AFM's calls for "lifting off" at a speed less than Vx.

    Additionally, at 50 ft the recommended speed is > than Vx for envelope protection in turbulent conditions. I believe the distance numbers in the AFM reflect the published procedure.

    Also agree there is probably a lot of confusion on when, where and what speed to use. Therefore, all points considered, I voted "other"
    Agree Marty, but the question wasn't about flying techniques, it was just simply "If you fly at the POH Vx speed is your flight path angle greater or less than if you fly at the POH Vy speed?"

  4. #4
    Dana's Avatar
    Join Date
    Jul 2011
    Posts
    690
    I don't know what numbers are in any particular POH, but by definition Vx is the speed for the steepest climb angle, so unless Vy happens to be at the same speed, Vx will be steeper. The POH numbers may include fudge factors for expected pilot skill, averaging for varying conditions, etc. But I also see errors in your spreadsheet; for example, the formula in column J should be ATAN not TAN... not that it greatly affects the results at these small angles (as the tangent of a small angle is very close to the angle itself in radians).

    I suspect that the POH numbers to clear the obstacle, from which you take the C-150 Vx climb rate (I didn't look further) include the time it takes to initiate the climb (accelerate the aircraft upwards) from zero ROC on the ground, i.e. the actual ROC at Vx is probably greater than you calculated from the published obstacle clearance distance. The faster the aircraft, the more ground you'll cover during that transition.

  5. #5
    Quote Originally Posted by Dana View Post
    I don't know what numbers are in any particular POH, but by definition Vx is the speed for the steepest climb angle, so unless Vy happens to be at the same speed, Vx will be steeper. The POH numbers may include fudge factors for expected pilot skill, averaging for varying conditions, etc. But I also see errors in your spreadsheet; for example, the formula in column J should be ATAN not TAN... not that it greatly affects the results at these small angles (as the tangent of a small angle is very close to the angle itself in radians).

    I suspect that the POH numbers to clear the obstacle, from which you take the C-150 Vx climb rate (I didn't look further) include the time it takes to initiate the climb (accelerate the aircraft upwards) from zero ROC on the ground, i.e. the actual ROC at Vx is probably greater than you calculated from the published obstacle clearance distance. The faster the aircraft, the more ground you'll cover during that transition.
    Good catch on TAN v ATAN, I guess I was too busy coloring the spread sheet.
    But as you pointed out for small angles it doesn't make much difference in the result (some of the angles over ~4 degrees decreased by 0.1 degree).

    I too always thought Vx was the speed yielding the steepest climb angle (as it's label states), but the data says otherwise.

  6. #6

    Join Date
    Jul 2011
    Location
    Clarklake, MI
    Posts
    2,258
    The value for Vx is just a point on a curve plotted on a graph. The theoretical value may not = the practical value.

  7. #7

    Cessan POHs Link

    For anyone wishing to check the numbers in the SS or add some new planes, here's a link to Cessna POHs:

    https://www.manualslib.com/brand/cessna/aircrafts.html

  8. #8

    Join Date
    Oct 2014
    Location
    Tehachapi, CA
    Posts
    187
    Quote Originally Posted by Waldo Pepper View Post
    A while back I decided to analyze runway design for obstacle clearances (angle to 50' vs distance vs climb rate vs speed, etc.) using my trusty spread sheet.
    In so doing I decided to incorporate data from various POHs.
    In so doing I found that in all cases Vy yielded a steeper flight path angle than Vx...
    I would be very interested in understanding where you got the #'s for climb rate when at Vx. I looked at two different C-172 manuals:

    https://static1.squarespace.com/stat...C172N-+POH.pdf

    and:
    http://www.lanierflightcenter.com/wp...s/172S-POH.pdf

    Both of them have tables for Vy rate of climbs, and both have tables for Vx and Vy at different altitudes, but NEITHER have a table of actual climb rates when flying at Vx. If you don't have this #, you cannot calculate the angle of climb.

    Maybe some of the other aircraft have these #'s, but you claimed ALL cases to have Vy steeper than Vx (which others have pointed out is the opposite of the definition of Vx and Vy).

    When testing my COZY MKIV in Phase I, I did complete climb tests at airspeeds from 60 KIAS to 140 KIAS and at fwd, center and aft CG, as well as lightweight, mid-weight and MGW. In ALL cases, the maximum climb angle occurred at a speed lower (by about the right amount) than the maximum climb rate speed - IOW, Vx is always below Vy, and Vx's climb angle is always higher than the climb angle at Vy.

    So I think that you're confusing something in the POH with the climb rate when at Vx.

    Can you point me to a specific POH where they give the climb rates at both Vx and Vy, and where, if you calculate the climb angle at both speeds, you come up with a steeper angle at Vy?

  9. #9
    Quote Originally Posted by Marc Zeitlin View Post
    I would be very interested in understanding where you got the #'s for climb rate when at Vx. I looked at two different C-172 manuals:

    https://static1.squarespace.com/stat...C172N-+POH.pdf

    and:
    http://www.lanierflightcenter.com/wp...s/172S-POH.pdf

    Both of them have tables for Vy rate of climbs, and both have tables for Vx and Vy at different altitudes, but NEITHER have a table of actual climb rates when flying at Vx. If you don't have this #, you cannot calculate the angle of climb.

    Maybe some of the other aircraft have these #'s, but you claimed ALL cases to have Vy steeper than Vx (which others have pointed out is the opposite of the definition of Vx and Vy).

    When testing my COZY MKIV in Phase I, I did complete climb tests at airspeeds from 60 KIAS to 140 KIAS and at fwd, center and aft CG, as well as lightweight, mid-weight and MGW. In ALL cases, the maximum climb angle occurred at a speed lower (by about the right amount) than the maximum climb rate speed - IOW, Vx is always below Vy, and Vx's climb angle is always higher than the climb angle at Vy.

    So I think that you're confusing something in the POH with the climb rate when at Vx.

    Can you point me to a specific POH where they give the climb rates at both Vx and Vy, and where, if you calculate the climb angle at both speeds, you come up with a steeper angle at Vy?


    Hi Marc, Very insightful questions,

    As I think I noted in the SS none of the manuals I looked at gave the climb rate for Vx, nor did they give the ground roll distance for a Vy TO.
    Looking further into why all the manuals had pretty much the same format and types of data (or lack thereof) I found that GAMA has a spec for POHs.
    It seems to be the source for not specifying the inclusion of those 2 essential bits of information:

    Link to GAMA Specification No. 1 for POHs (+ others):
    https://gama.aero/facts-and-statisti...pecifications/

    Not having those numbers available from the POHs I derived them as follows:

    Vx gives the Vx IAS, ground roll distance and total distance over a 50' obstacle, 4 pieces of info.
    1) Subtracting ground roll distance from total distance gives the distance from lift-off to the 50' obstacle, taking the ATAN of 50/that delta distance gives the flight path climb angle.
    2) Converting the IAS in mph or Kts to fpm for common units then correcting (reducing) the air speed to Ground speed due to the climb angle.
    3) Now knowing the GS from lift-off to the obstacle and the distance from lift-off to the obstacle I ratio distance to speed and get the time (fraction of a minute) from lift-off to the obstacle.
    4) Now knowing time from liftoff to the obstacle and that the obstacle is at 50', I divide the fraction of a minute into 50' and get the Vx average rate of climb from Lift-off to 50'.

    One set of numbers in the spread sheet (left side) assumes that the Vy Lift-off occurs at the same lift-off point as Vx (clearly not true) - but the point here is to determine which speed gives the greater angle of climb so the answer here is valid.

    Having some concern about how much longer the unstated Vy ground roll might be than the Vx ground roll a friend of mine assumed a constant acceleration to Vy, thos numbers are the yellow and blue high lights to the right.

    The Vy numbers were then derived similarly to the Vx numbers.

    I realize it's a PITA to try and figure out what the thinking is behind someone else's spread sheet formulas, hope these alleviates the pain somewhat.

  10. #10
    Dana's Avatar
    Join Date
    Jul 2011
    Posts
    690
    Quote Originally Posted by Waldo Pepper View Post
    I too always thought Vx was the speed yielding the steepest climb angle (as it's label states), but the data says otherwise.
    Then the data is wrong, or includes other assumptions not factored into your calculations. Vx is, as I said above, by definition the speed for steepest climb, but the distance given to clear an obstacle (from which you calculated the ROC at Vx) may include other factors.

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
  •