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14 Answers

Difference Between Vx and Obstacle Clearance Speed

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Aerodynamics, Student Pilot

The C172S publishes an obstacle clearance speed of 56 kts at max weight for short-field takeoffs. Why would the obstacle clearance speed be different from Vx?

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14 Answers



  1. Mark Kolber on Nov 04, 2014

    Flaps.

    Vx is published for a clean configuration. Obstacle takeoff is with 10° flaps.

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  2. Joshua Clark on Nov 04, 2014

    Obstacle clearance speed is not always with 10 degrees of flaps. The 1980 172RG limits the use of flaps above 2550# to 0 for all takeoffs, but the published speed is still 63 for a short field takeoff and 67 for best angle of climb.
    The difference comes in what the two speeds are trying to accomplish. Obstacle clearance speed is that speed at which you will reach 50 feet above the ground in the shortest distance from a stop. Best angle assumes you are already established in the climb and configuration over time.
    While it would probably be a really close tie with the above airplane example, waiting to reach 67 knots and then beginning the climb may give you a better angle over the long term, but you might reach that 50′ mark on the takeoff farther down the runway. 63 (obstacle clearance) is a shallower climb in the long term, but for the short term of the takeoff, it gets you to 50′ faster when you follow the short takeoff procedure described in the POH.

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  3. Mark Kolber on Nov 04, 2014

    Obstacle clearance speed is not always with 10 degrees of flaps. The 1980 172RG limits the use of flaps above 2550# to 0 for all takeoffs, but the published speed is still 63 for a short field takeoff and 67 for best angle of climb

    Gear.

    In the case of the 172RG, Vx is published for the clean configuration. Obstacle takeoff is with gear down until passing the obstacle.

    The original question was specifically about the 172S. If one looks at the airspeed table at the beginning of POH Section 4 one finds the difference. As does the same table in the 172RG.

    (And yes, of course there is the acceleration from a full stop issue in the equation. I thought it pretty obvious that one needs to accelerate on takeoff and one is not going to intentionally increase Vr and the associated ground roll in order to climb over a nearby obstacle)

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  4. Joshua Clark on Nov 04, 2014

    I believe Vx is an AOA based airspeed, and am doubtful that the gear would affect the AOA/speed to hold for the climb. Yes the flaps explain some of the discrepancy, but I dont think thats the entirety of the difference in the definition of the two speeds. As defined by the FAA Vx says nothing about configuration, and they are relatively silent on obstacle clearance speed. I could be wrong.

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  5. Kris Kortokrax on Nov 04, 2014

    Joshua,

    I’m ready to learn. Can you explain to me what an “AOA based airspeed” is? Can you explain to me what non “AOA based airspeeds” are based on?

    Drew,

    What needs to be considered is that you are looking at INDICATED airspeeds. If you go to the Airspeed Calibration table in Section 5, you will see that the CALIBRATED airspeed for 50 feet becomes 60.6 KCAS and Vx becomes 63.6 KCAS.
    A three knot difference. I doubt that you can hold your airspeed exactly to the knot.

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  6. Joshua Clark on Nov 04, 2014

    Kris,
    Vx and Vy, like best glide and stall speed are speeds at which a particular angle of attack will give certain performance. For small airplanes, these are usually calculated at maximum gross weight and given in knots. For example, the indicated airspeed for stall at maximum grossweight in the clean configuration is 48 knots for the 172S. This speed would change as weight is decreased, but it is not published. However, the angle of attack at which the aircraft stalls remains the same.
    For the climb speeds, Vx is the speed where the most excess thrust is available. The speed at which this happens may change, but the angle of attack would stay the same. I’m not 100% on Vx and Vy behaving like that, but I believe they do.
    An airspeed that is not angle of attack based would be a flap extension speed. The limitation here is the physical stress of the airflow on the flap, regardless of angle of attack, so that speed remains the same regardless of weight.
    Practially, we usually fly the different speeds that change with weight differently because the only speeds published are those at maximum weight. The best example of this would be best glide speed, which is 68 knots at maximum gross weight for the 172S. As weight decreases, so does the best glide airspeed. Maintaining 68 knots at a lower weight than maximum will actually decrease gliding distance. The difference at these weights and situations is usually minimal, and we are not necessairly able to calculate the weight and speed on a moment by moment basis. However, if you had an AOA indicator in the aircraft, and determined the AOA at maximum gross weight for best glide at 68, you could use that same AOA at lower weights and speeds to attain the same gliding performance.

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  7. Mark Kolber on Nov 04, 2014

    Excellent point, Kris. But when I interpolate the values I get a bit larger difference. The 63 KIAS clean Vx comes out to about 64.4 KCAS, while the 56 KIAS with 10° flap 56 KIAS comes out to 58 KCAS.

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  8. Steve Butler on Nov 04, 2014

    For the first 30-40 feet of your climb over the proverbial 50 ft. obstacle, you are still in ground effect.

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  9. Kris Kortokrax on Nov 04, 2014

    Mark,

    For Vx, I used the Flaps up lines. Vx is listed as 62 KIAS. The difference between KCAS of 62 and 70 is 8 knots. 62 KIAS is 20% of the way between 60 and 70 KIAS. 20% of 8 is 1.6. 62 + 1.6 = 63.6 KCAS.

    For barrier speed, I used the Flaps 10 degree lines. Barrier speed is listed as 56 KIAS. The difference between 57 and 63 KCAS is 6 knots. 56 is 60% of the way between 50 and 60 KIAS. 60% of 6 is 3.6 knots. 57 + 3.6 = 60.6 KCAS

    How did you work it?

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  10. Kris Kortokrax on Nov 04, 2014

    Joshua,

    I’m sorry. I was being facetious. Nowhere in any aviation related text that I have seen is there any mention of an “AOA based airspeed”.
    Where did you come up with this idea?

    If you are familiar with the equation for lift (L = dynamic pressure (1/2 rho V squared) X S (surface area of the airfoil) X Cl (Coefficient of Lift)), you will realize that for any given airspeed, there is one Coefficient of Lift that will keep the equation in balance. Angle of attack controls the Coefficient of Lift. So in other words, there is one AOA that will keep the equation in balance for a given airspeed.

    Every airspeed is tied to, but not determined by Angle of Attack, even limiting airspeeds.

    Addressing your second post:

    Performance for Vy is determined by excess power, not AOA.
    Performance for Vx is determined by excess thrust, not AOA.

    The angle of attack at which the airplane stalls will remain the same for a given configuration.
    The stalling Cl will be higher, but will occur at a lower angle of attack with flaps extended, as opposed to flaps retracted.
    (See Aerodynamics for Naval Aviators)

    If the speed for Vx changes, the angle of attack will not remain the same. AOA must change for the Lift equation to remain in balance.

    Steve,
    The reduction in induced drag varies from 47.6 % reduction at 1/10 wingspan to only 1.4% at 1 wingspan according to the Pilot’s Handbook of Aeronautical Knowledge. The benefit derived from ground effect diminishes greatly with height above the ground.

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  11. Mark Kolber on Nov 05, 2014

    Kris, turns out I copied an incorrect value to one of the cells in my interpolation spreadsheet.

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  12. Kris Kortokrax on Nov 05, 2014

    Mark,

    I programmed computers for many years. Garbage in, garbage out.
    I relied on pencil & paper, although I’ve made errors before when trying to multitask.

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  13. Mark Kolber on Nov 06, 2014

    Kris,

    …and I tested financial software for many years. Agree on GIGO 😀

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  14. Steve Pomroy on Nov 26, 2014

    Hi Drew.

    The reason for the two different speeds is that you are not in equilibrium on takeoff. Vx is the speed at which you have the most excess thrust (and therefore the steepest angle of climb), but it is predicated on being in equilibrium. During takeoff, you are accelerating, so it can be beneficial for the clearance of low obstacles to use a slower speed.

    You can read a bit more detail here: http://www.flightwriter.com/2012/08/clearing-obstacles.html.

    Cheers,
    Steve
    http://www.flightwriter.com

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