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Thread: No Do Overs! Defined Minimum Maneuvering Speed or D.M.M.S.

  1. #11

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    Default Re: No Do Overs! Defined Minimum Maneuvering Speed or D.M.M.S.

    So are the AOA's merely trying to you keep you at >1.3Vs?

  2. #12
    Senior Member PapuaPilot's Avatar
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    Default Re: No Do Overs! Defined Minimum Maneuvering Speed or D.M.M.S.

    An AOA shows you your margin from stall and compensates for weight. You can fly whatever AOA you want for approach and landing, but flying at 1.3 or higher gives you a safe margin. It will do the same if you want to use it for a DMMS reference.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  3. #13

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    Default Re: No Do Overs! Defined Minimum Maneuvering Speed or D.M.M.S.

    You say weight changes. How does it know the weight change?? I thought it was merely a stall plus factor gauge

  4. #14
    Senior Member PapuaPilot's Avatar
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    Default Re: No Do Overs! Defined Minimum Maneuvering Speed or D.M.M.S.

    This article talks about the relationship of airspeed & AOA:
    https://airfactsjournal.com/2015/03/...nt-understand/

    Unfortunately there is an incorrect statement in the article:
    "The five that are marked on the airspeed indicator are aerodynamic or structural limits. They are fixed and do not vary with weight."

    WRONG! Vs and Vso DO change with weight, air density and G loading. That is why airspeed is not a good indicator of stall margin. An AOA is always going to tell you how close you are to your critical AOA (stalling) regardless of all of these factors.
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  5. #15
    Senior Member rv9ralph's Avatar
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    Default Re: No Do Overs! Defined Minimum Maneuvering Speed or D.M.M.S.

    Correct Phil.

    How does it know the weight change??
    The wing stalls at the same AOA regardless. Density altitude can change, weight can change... but the wing will stall at the same AOA. So.. knowing the critical AOA, and having that displayed in the cockpit will give you the most important information on safety margins. Airspeed doesn't matter, as long as the AOA is above the critical point, with margin to spare.

    Ralph

  6. #16
    Senior Member 109JB's Avatar
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    Default Re: No Do Overs! Defined Minimum Maneuvering Speed or D.M.M.S.

    Quote Originally Posted by PapuaPilot View Post
    WRONG! Vs and Vso DO change with weight, air density and G loading.
    While you are right that the actual stall speed (TAS) itself does vary due to certain factors, Vs and Vso DO NOT change and are defined as the IAS (CAS) corresponding to the clean stall speed, or stall speed in landing configuration, both with maximum gross weight, forward CG, defined configuration and a 1g stall. So they don't change with weight because they are defined at maximum weight, they don't change with air density because the IAS stall speed is the same (TAS is different but IAS is same), and they don't change with G-load because they are defined as 1g stall speeds.

  7. #17
    Senior Member PapuaPilot's Avatar
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    Default Re: No Do Overs! Defined Minimum Maneuvering Speed or D.M.M.S.

    I think you are missing the whole point of the discussion. You are correct that Vs and Vso are defined in the conditions mentioned, but what happens to the stall speed when you are not at gross weight or at 1g? It changes.


    These are excerpts from Wikipedia: Stall_(fluid_dynamics)

    STALL SPEEDS
    Stalls depend only on angle of attack, not airspeed. However, the more slowly an airplane goes, the greater the angle of attack it needs to produce lift equal to the aircraft's weight. As the speed increases further, at some point this angle will be equal to the critical (stall) angle of attack. This speed is called the "stall speed". An aircraft flying at its stall speed cannot climb, and an aircraft flying below its stall speed cannot stop descending. Any attempt to do so by increasing angle of attack, without first increasing airspeed, will result in a stall.

    The actual stall speed will vary depending on the airplane's weight, altitude, configuration, and vertical and lateral acceleration.

    IN ACCELERATED AND TURNING FLIGHT
    The normal stall speed, specified by the VS values above, always refers to straight and level flight, where the load factor is equal to 1g. However, if the aircraft is turning or pulling up from a dive, additional lift is required to provide the vertical or lateral acceleration, and so the stall speed is higher. An accelerated stall is a stall that occurs under such conditions.

    Warning and safety devices
    An angle-of-attack indicator for light aircraft, the "AlphaSystemsAOA" and a nearly identical "Lift Reserve Indicator", are both pressure differential instruments that display margin above stall and/or angle of attack on an instantaneous, continuous readout. An AOA indicator provides a visual display of the amount of available lift throughout its slow speed envelope regardless of the many variables that act upon an aircraft. This indicator is immediately responsive to changes in speed, angle of attack, and wind conditions, and automatically compensates for aircraft weight, altitude, and temperature.


    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  8. #18
    Senior Member 109JB's Avatar
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    Default Re: No Do Overs! Defined Minimum Maneuvering Speed or D.M.M.S.

    I didn't miss the point at all. All I was doing is pointing out that Vs and Vso are defined with certain criteria and as such do not change.

    Vs and Vso are the worst case 1g stall speeds for clean and dirty configurations. I'm just saying leave them defined as such and if you want to talk about how the stall speed changes with weight, g-load, etc, just talk about the speed, not the narrowly defined V-speed.

  9. #19
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    Default Re: No Do Overs! Defined Minimum Maneuvering Speed or D.M.M.S.

    I watched this video and wondered why this isn’t part of basic pilot training. I’ve never had an engine out on takeoff, other than my instructor reaching over and pulling the throttle, but it would be a disarming feeling if you’d never experienced it before. This is a great video, to the point I’ve included an additional step in my check list when I’m pre takeoff. In bold letter: Think Engine Failure.
    David
    SS7 Builder

  10. #20
    Senior Member Rodney's Avatar
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    Default Re: No Do Overs! Defined Minimum Maneuvering Speed or D.M.M.S.

    I just finished installing an AOA yesterday - haven't flown it yet to calibrate it.
    (CYA-100)

    I understand the concept of DMMS - but no where in this discussion is the use of flaps mentioned. My CYA system says to place the aircraft in a landing configuration and reduce airspeed to just above a stall. You then push a programming button and that defines the angle of attack where the airplane stalls.

    Wouldn't it be natural, if you lost an engine to pull in at least one notch of flaps?

    I can understand folks with electric flaps wouldn't have time to mess with them in a take-off engine out situation, but our manual flaps can be pulled in so quickly that why wouldn't one pull them in?

    Any thoughts???

    Rodney

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