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8 Answers

Power-On and Power-Off as defined by Vs0 and Vs1

Asked by: 46204 views ,
Student Pilot

I am hoping someone can provide me with some additional clarity concerning power-on and power-off stalls.  I know what the difference is (and how to perform them)...however, my confusion lies in the textbook definitions of Vso and Vs1.

According to both Jeppesen Private Pilot textbook and the FAA Handbook of Aeronautical Knowledge:

Vso is defined as the calibrated POWER-OFF stalling speed or the minimum steady flight speed at which an aircraft is controllable in the landing configuration.

Vs1 is defined as the calibrated POWER-OFF stalling speed or the minimum steady flight speed at which the aircraft is controllable in a specified configuration.

We practice POWER-ON stalls using the V-speeds of Vso and Vs1.  However according to the definitions listed above, Vso and Vs1 are the calibrated POWER-OFF stalling speeds.

Perhaps I am quibbling over semantics, but I have always been a bit confused as to why Vso and Vs1 are defined in this manner.  If these definitions are taken at face value, it would seem that "power-on" stalls are achieved when at "power-off" stalling speeds...which is true...yet, confusing by these textbook definitions.  Thoughts?

Secondly, what is the difference between Vs and Vs1?  I have also never been able to get much clarity on the exact difference between these two V-Speeds.

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8 Answers



  1. skyboyCFI on Jan 22, 2011

    Vso is flaps down aka landing configuration.
    Vs1 is flaps up (clean, gear up, etc.)

    The aircraft stalls a different speeds based on its configuration.

    Remember, flaps increase lift, but also increase drag. And the speed at which the wings stalls is not the same.

    Large aircraft may have a V2 or V3 or more. It’s referencing the stall speed for an aircraft for the different configurations possible.

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  2. skyboyCFI on Jan 22, 2011

    Oh, one more thing. You’re never quibbling. Ask anything, so you understand it completely. Correct understanding in your own mind is what will make you a good pilot. Rote understanding is bad.

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  3. Kent Shook on Jan 22, 2011

    Many things change stalling speeds, one of which is power. Vs0 and Vs1 are two speeds that give you a reference for how fast the airplane will stall in a given condition – No more. You say “we practice power on stalls using the V-speeds of Vso and Vs1” but you don’t really practice stalls using a V-speed – You are using a technique and waiting for the airplane to stall, not recovering at a certain speed, right?
     
    Anyway, using Vs0 and Vs1 as a reference, you should be able to develop some sort of idea of what your stall speed will be. Many conditions can change your stall speed – Aircraft weight and balance, load factor, configuration, power, etc. An aircraft with power on will stall at a lower speed than with power off if you’re holding altitude because the high angle of attack (which corresponds to a high pitch-up attitude if you’re holding altitude) means that you’re getting a lift component from engine thrust as well as from the wing. That reduces the amount of lift the wing needs to generate, leading to a lower angle of attack necessary to hold altitude with power on.
     
    Finally, the difference between Vs0 and Vs1 is simply the aircraft configuration. Hope this helps!

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  4. MaggotCFII on Jan 22, 2011

    Some of your understanding the difference between these airspeeds are on the Airspeed Indicator for you to see – color coded markings.
    Take a look in the Pilot’s Handbook of Aeronautical Knowledge, CH 7, at pg 7-9.
    Airspeed Indicator Markings – the White Arc and the Green Arc.
    Vso:”Lower Limit of the White Arc (Vso) – the stalling speed or the minimum steady flight speed in the landing configuration.  In small aircraft, this is the power-off stall speed at the maximum landing weight in the landing configuration (gear and flaps down.”
     
    Vs1: “Lower limit of the green arc (Vs1) – the stalling speed or the minimum steady flight speed obtained in a specified configuration.  For most aircraft, this is the power-off stall speed at the maximum takeoff weight in the clean configuration (gear up, if retractable, and flaps up).”
    The speeds are right there on the airspeed indicator.  Good to know if the DPE asks what the “bottem of the white arc” means!
    And please remember the wing stalls at the critical angle of attack.
    “The stalling speed of a particular aircraft is not a fixed value for all flight situations, but a given aircraft always stalls at the same AOA regardless os airspeed, weight, load factor or density altitude.” (Pilot’s Handbook, page 4-22)
    So perhaps the color coding on the airspeed indicator will help you see the “exact difference between these two V-Speeds”.
    Blue Skys and always ask the questions!

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  5. skyboyCFI on Jan 22, 2011

    I understood the question to be a simple one. What’s the overall difference. White arc, green arc,, too in depth. Simply, they are variable stall speeds based merely on different aircraft conditions, and set up.

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  6. Brian on Jan 24, 2011

    …Qt: Tomato Flames — “If these definitions are taken at face value, it would seem that “power-on” stalls are achieved when at “power-off” stalling speeds…which is true”…
     
    I’d like to better understand your thinking with this question. Please elaborate when you have time, thank you. One point here before moving on, a restatement of what Kent has already mentioned: stall speeds with power are not the same as stall speed without.
     
    …Qt: Tomato Flames — “Secondly, what is the difference between Vs and Vs1?  I have also never been able to get much clarity on the exact difference between these two V-Speeds.”…
     
    I wonder if your lack of clarity comes from misunderstanding the variables nomenclature. I noticed in this question you use Vs in place of Vs0, but did you know you could or did you do this by accident, I wonder.
     
    These two can be used interchangeably as the 0 is only there to let us know that this is the first reference of this variable, Vs, or stall speed. Scientists use numerical subscripts to identify different instances of the same variable, usually arranged chronologically.
     
    For instance, if we were to drop an object from some height and wanted to know the speed at various heights we would use this system. Our starting height could be h0, after 10 seconds we might reach h1, and 20 seconds into the fall we could call h3. All of these are representations of the variable height, but each has a different meaning because some part of the condition has changed. I.E. height 3’s condition is the object has already fallen for 20 seconds versus h (or h0) being the starting point.
     
    So how does this apply to an airplane stall speed? We have our variable, Vs, which represents stalling speed. However, because we use various configurations when flying, it might be nice to know this Vs speed given a couple conditions. To do this, we typically choose to use Vs (or Vs0) as the stall speed given a clean (no flaps/ spoilers and gear up if able) configuration. Our second instance of Vs, called Vs1, is typically the stall speed given a landing configuration. To find out how these speeds are defined for your aircraft, visit section one of the pilot operation manual for that aircraft.
     
    Just remember, the only difference between Vs, Vs1, Vs2, Vs3, etc is the stipulation on aircraft configuration. Each of these is a stalling speed, but the way the aircraft is configured is different for each instance and can be found in the aircrafts POH. Typically, only two conditions for stall, Vs (or Vs0) & Vs1, are published for our small aircraft.
     
     

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  7. Brian on Sep 01, 2011

    “Many things change stalling speeds, one of which is power.”
     
    Time to contradict myself. In my above statement I noted Kent’s accuracy in stating this. However, in a way it is also an inaccurate statement. Power typically causes our stalling speed to change only because thrust in many of our smaller, high-wing, aircraft contributes to the lift. Stalling speed for a wing, baring this stipulation, is only a result of one thing:
     
    Vs = L / Sqrt (1/2 p S Clmax) where we know lift is equal to weight so we can rewrite this as:
     
    Vs = W / Sqrt (1/2 p S Clmax)
     
    Notice thrust is not a factor in an elementary analysis of a stalled wing. The variables above are air density (p), wing area (S), and lift coefficient maximum (Clmax). Intuitively, all we’ve done is rewritten the lift formula, substituted weight for lift since we know they are equal, and set Clmax for Cl since we know that a stall occurs at this predetermined angle of attack.
     
    What I’m getting at is that, where the aircrafts thrust does not appreciably effect the velocity of the air over the wing (low wings/twins), stalling speed is less impacted from changes in thrust than our typical high wing Cessna. In the case of jets, where the thrust does not impact any aerodynamic surface, stall speed doesn’t change at all from thrust changes. In other words, stall speed is only changed by power changes because the wing area around the engine is experiencing a higher air velocity than what actually exists. This prolongs the stall to a slower indicated/true airspeed. 
     
    Hope I didn’t confuse anyone too much! Fly safe.

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  8. n266ac on Feb 03, 2013

    According to the Commercial test prep 2013 edition, Brian’s definition of Vso and Vs1 are exactly reverse.

    Vso is stall in the landing configuration ie with flaps and gear down (bottom of white arc)
    Vs1 is stall in specified configuration ie flaps and gear up (bottom of the green arc)

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