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Thread: Kitfox abrupt departure sensitivity to low speed stalls

  1. #61
    Senior Member rv9ralph's Avatar
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    Default Re: Kitfox abrupt departure sensitivity to low speed stalls

    James,
    I am not an aerodynamicist nor am I a test pilot. However, I think you are getting in the weeds trying to decipher this.

    The testing of an aircraft starts using a stabilized and constant airspeed at least 15 knots/mph above the tested and demonstrated stall for that aircraft. Then the timed climb is conducted starting at a predetermined altitude maintaining a constant airspeed for one minute noting the altitude at the end of one minute. Repeat starting at the same altitude, with a decrease in airspeed by 5 knots and climb one minute then note the altitude. Repeat again, and again When this data is plotted, there will be an increase in the rate or angle of climb up to a point and then it will drop off. This is the data point that will give the appropriate Vx and Vy for that tested aircraft. With certified aircraft, it is considered to apply to the whole fleet of the same type of aircraft (hence the purpose of "type certified") with experimental aircraft, each one is considered unique and needs to be tested to demonstrate their performance.

    This climb is not hanging on the prop, just above stall. The test is conducted using airspeeds (remember, you start 10 knots above stall), in increasing increments until the performance drops off. The position over ground is not considered in making these tests.

    That is it for now.
    Ralph
    Last edited by rv9ralph; 06-11-2017 at 09:27 PM.

  2. #62
    Senior Member PapuaPilot's Avatar
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    Default Re: Kitfox abrupt departure sensitivity to low speed stalls

    James,
    I have to agree with what Ralph is saying. An aircraft's stall speed has nothing to do with the Vx flight testing. The reason the 10 mph is mentioned in the AC is to keep you from stalling during the testing.

    The AC tells you to start at the testing at 15 mph over the estimated best rate of climb. You do a bunch of successive climbs while reducing the speed in 5 mph increments until you get to 10 mph above stall speed. You write down all of the times to climb and plot them on a graph. The highest point on the graph is Vy and the tangental point from 0 is Vy.

    Ideally you want to get more than one point slower than Vy to make a more accurate curve. Even if you only get one data point you can always plot 0 rate of climb at your aircraft's stall speed and estimate the curve it would make. With my Model 5 the estimated Vy was 65 mph and my Vs was 46. I flew it at 5 mph increments from 80 down to 55 (following the guidelines in the AC) and I got a pretty accurate graph.

    You are correct that you do not reference Vx from the takeoff point.

    Vx is used for obstacle takeoffs or out climbing obstacles/terrain in flight. If you have an obstacle on take off you accelerate, liftoff, continue to accelerate to Vx, pitch to hold Vx until you are over the obstacle. Then you accelerate to Vy or a cruise climb.

    Vx doesn't have anything to do with stalling or stall margin, it is merely for climbing over something.

    Using Vx puts us in a vulnerable position at low altitudes if the engine were to quit or if you encounter a gust. There may not be time to push the nose over and regain your speed before hitting the ground. You also need to consider reaction time, the possibility of doing nothing or the wrong thing.

    When you do an obstacle takeoff in a certified aircraft you should use the POH's takeoff performance charts and calculate the distance it will take to clear the obstacle using the actual conditions. The stated distance assumes you will be doing a maximum performance Vx takeoff.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  3. #63

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    Default Re: Kitfox abrupt departure sensitivity to low speed stalls

    Quote Originally Posted by PapuaPilot View Post
    James,
    I have to agree with what Ralph is saying. An aircraft's stall speed has nothing to do with the Vx flight testing. The reason the 10 mph is mentioned in the AC is to keep you from stalling during the testing.

    The AC tells you to start at the testing at 15 mph over the estimated best rate of climb. You do a bunch of successive climbs while reducing the speed in 5 mph increments until you get to 10 mph above stall speed. You write down all of the times to climb and plot them on a graph. The highest point on the graph is Vy and the tangental point from 0 is Vy.

    Ideally you want to get more than one point slower than Vy to make a more accurate curve. Even if you only get one data point you can always plot 0 rate of climb at your aircraft's stall speed and estimate the curve it would make. With my Model 5 the estimated Vy was 65 mph and my Vs was 46. I flew it at 5 mph increments from 80 down to 55 (following the guidelines in the AC) and I got a pretty accurate graph.

    You are correct that you do not reference Vx from the takeoff point.

    Vx is used for obstacle takeoffs or out climbing obstacles/terrain in flight. If you have an obstacle on take off you accelerate, liftoff, continue to accelerate to Vx, pitch to hold Vx until you are over the obstacle. Then you accelerate to Vy or a cruise climb.

    Vx doesn't have anything to do with stalling or stall margin, it is merely for climbing over something.

    Using Vx puts us in a vulnerable position at low altitudes if the engine were to quit or if you encounter a gust. There may not be time to push the nose over and regain your speed before hitting the ground. You also need to consider reaction time, the possibility of doing nothing or the wrong thing.

    When you do an obstacle takeoff in a certified aircraft you should use the POH's takeoff performance charts and calculate the distance it will take to clear the obstacle using the actual conditions. The stated distance assumes you will be doing a maximum performance Vx takeoff.
    Hello Sir, I just re-built a kitfox 5 with an io-240 and I am ready to start ground testing. Would you be ok with sharing with me all the specs and performance specs for yours so that I could use them as reference? I would appreciate that being that u have a similar set up and we are rare.

  4. #64
    Senior Member PapuaPilot's Avatar
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    Default Re: Kitfox abrupt departure sensitivity to low speed stalls

    If you are rebuilding a plane those numbers should be in the logbooks. It is a requirement to put the stall speeds in there at the end of the phase 1 flight testing.

    Here are my V speeds (mph):
    Vso 46 @ 1550 lbs
    Vs 53 @ 1550 lbs
    Vx 60
    Vy 65
    Vfe 80
    Va 105 @ 1550 lbs
    Vno 120
    Vne 140
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

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