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16 Answers

Stall Speed

Asked by: 18726 views ,
Aerodynamics, General Aviation, Student Pilot

I am having trouble visualizing stall speed (in regards to CG), and I am hoping someone can help me.

I know that a stall can occur at ANY AIRSPEED or ATTITUDE as long as the critical angle of attack is exceeded.  I've also read that a forward CG means a high stall speed and an aft CG means a low stall speed.

I know that one never wants to fly a plane outside of CG limits...however:

If I had a forward CG, my plane would be nose heavy.  In order to retain straight and level flight, I would have to pull back on the yoke.  Since the plane is nose heavy, it seems to me that I would need to INCREASE my airspeed in order to produce enough lift to equal the nose heavy weight.  Is this what is meant by a increased stall speed?

With an aft CG, I believe stall speed is lowered.  Since the nose will already be at a high angle of attack due to the increase aft weight, I am thinking that any great increase in speed will stall the plane more quickly.  Therefore, is stall speed considered lower with an aft CG because lift increased due to the nose is at a pitch-high attitude (allowing for slower flight)?  Or, is it considered "lower" because I would have to reduce my airspeed in order to bring the nose down?

I know that flaps lowers stall speed and increases increases lift.  If I did not use flaps on approach, but flew at the same speed as if I had deployed flaps...I would probably stall, correct?

Since stalls can occur at any speed, I'm not exactly sure what is meant by stall SPEED.

Does stall speed mean:

1) I have to stay ABOVE a certain speed in order for the plane to avoid stalling?

2) The plane stalls at a higher airspeed?

3) Or...the critical angle of attack is exceeded at a higher speed because the plane is going faster?

My question is hard to get across via computer, but please try to clarify this issue for me.

P.S. - Is it true that and aft CG reduces drag?  How can this be?  I would think the nose high pitch attitude would increase drag.

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16 Answers

  1. Brian on Nov 21, 2010

    …Qt: The Red Baron…Is it true that and aft CG reduces drag?  How can this be?  I would think the nose high pitch attitude would increase drag…
    I don’t have the time to detail this entire post for you right now, but it seems you entire lack of understanding is due to forgetting the purpose of the horizontal stabilizer. Consider it’s relationship to the wing when CG moves in relationship to center of pressure. I believe the most private pilot books cover this relationship in their aerodynamic section in decent detail.
    When you understand why the horizontal tail is there and exactly what it is doing then revisit your other questions. Since this relationship determines the AOA you will be flying at any given speed, it will also answer your stall speed questions.

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  2. The Red Baron on Nov 21, 2010

    Brian, thank you for your response.  However, I just need someone to help me claify and visualize “stall speed.”  I’m reading all of the material, but it’s just not making sense.
    Since I know stalls can occur at ANY SPEED or ANY ATTITUDE…what is meant by stall SPEED?
    When flaps are lowered, I know stall speed is decreased.  But what does this mean?  Does this mean the plane can fly at a SLOWER speed WITHOUT STALLING?
    Does a high stall speed mean a plane has to fly at a higher speed OR IT WILL STALL?
    If someone could just try to walk me through it, I would be very appreciative. 

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  3. The Red Baron on Nov 21, 2010

    Maybe I’m confused between Center of Pressure vs. Center of Gravity.  Please, help…

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  4. Brian on Nov 21, 2010

    Stall speed is the minimum speed in which the aircraft can be flown. It corresponds to the speed that, when weight is taken into account, will result in the aircraft exceeds critical angle of attack to achieve the lift equal to weight balance. One way to visualize it is with a basic lift equation:
    Lift = speed * angle of attack (Note: the actual equation is more involved, this is merely to demonstrate relationships using simple math.)
    Lift must always equal weight to fly. Angle of attack (AOA) can reach a maximum degree for a given airfoil, let’s say 10 is critical AOA for this example. Beyond 10 degrees the airfoil will be stalled.
    Airplane ‘A’ weights 100 pounds and airplane ‘B’ weights 150 pounds.
    Remember, lift must equal weight, when it no longer can we are at a stalled airspeed. We can use critical AOA for this as it is the maximum usable AOA; we called it 10. So:
    Aircraft A: 100 lift = speed * 10 AOA — Speed = 10. Any speed less than 10 results in a lift less than 100, so we are not getting lift equal to weight. We either will sink as a result, or pull back, increasing angle of attack, and stalling.
    The heavier aircraft, Aircraft B: 150 lift = speed * 10 AOA — Speed = 15. AOA cannot go higher than critical and our speed cannot go too low, otherwise lift cannot equal weight, or we stall.
    I purposely repeated myself a few different ways, I hope that helps. Sorry, I didn’t really see this as your primary question from your first post. Your confusion with stall speed and CG I still think can be solved with a refresher of the purpose of our horizontal tail.

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  5. Kent Shook on Nov 21, 2010

    I’m gonna take another crack at it, and try to answer in a different way than Brian (not that there’s anything wrong at all with his answer, in fact I like it).

    I’m also going to post two answers: One relating to the “stall speed” question and one relating to CG vs. stall. I think that in most private pilot training, the details of what’s going on here are often not fully covered by the instructor or understood by the student.

    First, “Stall speed.” This is a somewhat misleading term. The one and only thing that causes a stall is that you increase the angle of attack beyond that wing’s critical angle of attack. It is possible to stall at any speed, in any attitude, at any aircraft weight. That is a very important thing to understand.

    Two “stall speeds” are generally published in an aircraft’s POH: Vs0 is the stall speed in the landing configuration, and Vs1 is the stall speed in a “specified” configuration, usually clean. Both of these speeds are specified only for a certain condition: Aircraft is at maximum gross weight, furthest forward CG, with a load factor of 1 (generally, level flight). Also, depending on the manufacturer and the age of the POH, the stall speed may be published as either an indicated airspeed or a calibrated airspeed – Be sure you know the difference between the two! Often, the indicated and calibrated airspeeds differ from each other the MOST at stalling airspeeds due to the high angle of attack on the pitot tube.

    Now, some examples of when you can stall “at any airspeed, in any attitude, or at any weight”: If you’re in a steep turn, your load factor is increased, so you will stall at a faster speed than the published stall speed. If you were flying an aerobatic aircraft, you could be pointed straight down, pull very hard, and stall – It’s angle of attack that matters, not pitch attitude. The airplane doesn’t know which way it’s pointed! Finally, even if you’re light it’s possible to stall the plane – It will just stall at a slower speed.

    So, the idea of “stall speed” can be misleading. Stall is caused by angle of attack, and angle of attack only. What is meant by “stall speed” is “the speed at which an aircraft will stall under the given conditions.” The critical angle of attack never changes – It’s the factors that relate the speed to the critical angle of attack that change “stall speed.”

    That said, here are some of the things that can affect stall speed:

    1) Weight. A heavier aircraft needs to push more air downward to generate enough lift to maintain level flight. To generate more lift, you need to increase airspeed or angle of attack. At the same airspeed, a heavier airplane will need a higher angle of attack to maintain level flight. At the same angle of attack, a heavier airplane will need a higher airspeed to maintain level flight. Thus, “stall speed” is lower for a lighter airplane. When you practice maneuvers solo, you may notice that the airplane stalls at a slower airspeed than it does when your instructor is aboard.

    2) Load factor. To the airplane, this is the same as weight – If you’re in a constant-altitude turn, your load factor is increased because you’re generating lift in both vertical and horizontal directions, and to maintain altitude you need to keep the vertical component the same as it was in level flight. So, your total lift required is greater, thus the increased load factor has the same effect on stall speed as increased weight does in #1.

    3) CG. I’ll cover this in my next message.

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  6. Kent Shook on Nov 21, 2010

    Okay, on to center of gravity, its effect on angle of attack and stall speed, and the workings of the horizontal tail.
    The first thing to understand is that the horizontal tail surfaces, in normal flight in a small airplane, are generating a *downward* force. (It’s worth reading up on Center of Pressure at Wikipedia: http://en.wikipedia.org/wiki/Center_of_pressure )
    So, for the airplane to stay level in the air, the amount of lift generated by the wing is roughly equal to the weight of the airplane, <B>plus</B> the downward force generated by the tail. The farther forward the CG is, the more down force the tail needs to generate to compensate and keep the airplane from pitching down. This increased down force must be counteracted by increased lift from the wing. That, of course, means an increase in angle of attack or airspeed, just like how I explained above for weight. 
    So, when the CG is moved further forward, the down force created by the tail must be increased, and in turn the lift generated by the wing must be increased, necessitating a higher angle of attack for the same airspeed, or an increased airspeed for the same angle of attack – Thus, stall speed is increased the further forward the CG is.
    Hope this clears it up for you! :)

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  7. The Red Baron on Nov 21, 2010

    To add another element…The Center of Lift/Pressure (same thing?) changes as the angle of attack and/or pitch changes…but CG remains the same regardless of pitch/AOA.  Is this accurate?

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  8. Brian on Nov 22, 2010

    …Qt: The Red Baron…Lift/Pressure (same thing?)…
    Lift, after all, is nothing more than a summation of all air pressures acting on an aircraft. In other words, yes, they are the same.
    On that topic, aerodynamic force is often used in place of the center of pressure/lift. Aerodynamic force isn’t the same, but it is similar. Aerodynamic force is an accepted average of all the pressures for all angles of attack on an airfoil, typically used for rough calculation.
    For most sub sonic aircraft it is approximately 33 percent aft of the leading edge along the airfoils chord line. Knowing this is really useful if you intend on reading up and seeking out more aerodynamic information. Reason being is now you wont get hung up on silly terminology.
    Anyways, that said here is what I was getting at with understanding how the tail of an aircraft operates. Grab a golf club or baseball bat and with your left hand grab one end. Your left hand is to represent the aircrafts CG. In most aircraft the CG will be located ahead of the CP (longitudinally/pitch stable). So, with your right hand now representing a lifting force, push up just behind the CG. The club or bat will lift some, but it will also rotate. This is exactly what would happen to an airplane if the tail were removed, it would nose over itself.
    Unfortunately you don’t have a third hand to represent tail down force, but to balance this whole thing out you probably already guessed that you would need to push down on the opposite end of the club/bat. This is what the horizontal tail does, it is an upside down wing and lift is created in the negative direction to keep the aircraft in balance.
    Now, the further aft you move the CG, the closer it will be to the AC and the less rotation will be caused. Thus less tail down force will be needed and, as Kent pointed out, there will be less total upward lift needed. Recall that this is because wing lift equals weight + tail down force.
    To sum this all up: aft CG = less rotation because CG/AC are closer together = less tail down force needed = less total lift needed to overcome weight and tail down force. Finally, repeating Kent here, with less lift required we can now fly at a lower AOA for any given speed. This lower AOA means less drag for any given speed.

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  9. Flyer on Nov 26, 2010

    Is stall speed ever affected SOLELY by your power setting/RPM’s?  I would be so bold as to say “no”, not by power setting alone.
    I am a student pilot and have never flown a plane with a constant speed/variable pitch prop.  I wouldn’t know how the different combinations between propellor pitch and power setting would affect stall speed. 
    Guess I need to read up on it.  Can’t wait to fly a constant speed prop one day and learn how the different settings help increase performance.

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  10. Plane Crazy on Nov 27, 2010

    Dumb question: If you have an increase stall speed…this doesn’t mean that your nose necessarily drops any faster than if you stalled at a lower speed, does it?
    The thought just cross my mind for the first time…hmmm……..

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  11. Brian on Dec 03, 2010

    Flyer – Nope, the engines power has no effect on an aircrafts stall speed.
    Plane Crazy – If the reason you’re stall speed increased were due to weight increase, maybe. Otherwise, I don’t know. Really what you, as the pilot, are interested in, is the violence of the stall. This is most impacted by CG position and inherent aircraft design. See this example:
    Stall Propagation & Wing Design 

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  12. Steve Pomroy on Dec 23, 2010

    Power influences both your stall speed and your aircraft’s stall handling.  Increasing your power setting increased the airflow over the wing root induced by your propeller (assuming you’re in a single-engine tractor prop).  This increases the local airspeed while reducing the local angle of attack.  The effect is compounded because at high angles of attack, there is a component of your thrust acting perpendicular to your flightpath – i.e. – in the direction of lift, thus reducing the load on your wings.  The net effect is to reduce your stall speed.
    This is one of the reasons that stall testing for aircraft certification (and measurement of the published Vs) must be conducted at idle or zero-thrust power –it’s the worst-case condition and gives the highest value of Vs.  The difference isn’t significant at low power settings such as those used on approach.  But at high power settings such as takeoff or go-around, the difference can be appreciable in many aircraft types.
    The tradeoff here is that although your stall speed is reduced by power, your stall characteristics will be adversely affected.  The likelyhood of a violent pitch correction or a spin is increased greatly with power on.  The (main) reason for this is that the stall was delayed by preventing the wing root from stalling early.  So instead, a much larger portion of the wings stalls all at once.
    Designers go to great lengths (with design characteristics such as planform shape, washout, and stall strips) to prevent this type of stall — a stall starting at the wing root and spreading outward is less violent, quicker to recover, and allows you to retain roll control throughout the stall and recovery.  But much of the designers work is undone by carrying power into the stall.  A deeper stall with a larger portion of the wing stalled is especially problematic because it intensifies the aircraft’s response to roll and yaw disturbances and increases the risk of a spin.
    With regard to the original post about CG, understanding CG v. tail-down force is the key to understanding CG v. stall speed.  Refer to Brian’s explanation above.  He hit the nail on the head.
    With regard to the final question in the original post:  “P.S. – Is it true that and aft CG reduces drag?  How can this be?  I would think the nose high pitch attitude would increase drag.”  This also comes back to CG v. tail-down force:  Forward CG = more tail-down = more lift to balance = more induced drag.  Vice versa for an aft CG.
    You seem to be visualizing the aircraft flying nose high when there is an aft CG.  This is a common error new pilots make.  An aft CG means less lift will be required, so you will operate at a lower AOA.  All other things being equal, this means a lower pitch attitude.  The attitude and AOA are not controlled by your CG, they are controlled by your tail-down force (which you control with the elevator).
    Well, this has gotten a little bit long-winded.  But I hope it helps.

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  13. Maggot/CFII on Jan 02, 2011

    The “Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25A, Chapter 4, Aerodynamics of Flight, page 4-22 Stalls” would be good to read.
    Critical AOA rules – trying to visualize what the wing is doing; chord, relative wind, turning (feeling the load factor) vs not turning will be helpful.  Try “flying the chair” at home using both your open palms arms out and in front to visualize, lean back to simulate increases in pitch and think about power for a particular configuration.
    And think critical angle of attack – 16/18 degrees + or – to the relative wind.
    Bet if you do this enough you will feel the chair stall!
    And have your CFI cover the Airpseed Indicator and Attitude Indicator or PFD while you practice all the PTS stalls.
    As I recall, the PTS Stall Objectives, Power-Off and Power-On only mentions airspeed as the last step in recovery.
    If you learn your power settings for a particular training airplane for a particular attitude airspeed will take care of itself – get the head out of the cockpit for this work!
    When you start working on the Basic Instrument Maneuvers, as extra work, have your CFI work you through the stalls while under the hood/foggles.  You confidence level will increase.
    Good Luck!

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  14. alexandretp on Jan 19, 2012

    Guys, this is perfect to understand it:

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  15. Bbellur on Dec 13, 2012

    Your horizontal stabilizer works with the elevator to generally help keep the nose up. With out the horizontal stabilizers and elevators the the wing moment would cause the airplane to pitch down. So forward CG would cause the tail to work harder (because you would have the elevator up more than you need in an aft cg situation) to keep the nose pointed where you want to and therefore increases the Angle of attack and drag on the wings. With an airplane in an aft CG condition the elevator has less work to do to do to keep the nose up and therefore the wings have less drag and angle of attack to keep the same lift.

    Angle of attack is what the airspeed indicator measures for the most part. Higher the airspeed lower the angle of attack.

    Too much to say in a small text box but a great topic to discuss. A complicated topic indeed.

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  16. anusha on Feb 07, 2013

    im still unable to understandd “stall angle”…..its vry confusing…..can nyone xplain abt it even more detailed…plzz

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