Density Altitude and stall speed

6 Answers

1. 
   

   
   

   Matej Dostal on Jan 05, 2011

   Simply put, your IAS stall speed remains the same, while the respective TAS increases with increasing density altitude.
   The fuel consumption rate depends significantly on the power setting you set with respect to the altitude at which you cruise. Optimum cruise altitude depends on your actual weight. Generally the higher you fly (within reason) the further you can get.

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2. 
   

   
   

   Kent Shook on Jan 05, 2011

   As Matej states, you will be going faster (true airspeed) at the stall at a higher density altitude, but your indicated airspeed will be the same (your airspeed indicator is affected by the thinner air the same way the wing is). That’s why it is very important to always rotate and climb out at the same indicated airspeed – A high density altitude takeoff will try to fool you into taking off too early (due to the groundspeed being much faster and the ground roll being much longer than you’re used to) and climbing too steeply (you’ll need a lower pitch attitude to maintain the same airspeed at a higher DA).
    
   DA does affect fuel consumption rate, assuming you set your mixture properly. The higher the DA, the leaner your mixture should be and the less fuel you’ll burn. On a long trip, it can save fuel to go up high if the winds are favorable or nonexistent. However, the higher fuel burn in the climb means that on a short trip you might as well stay low since otherwise you wouldn’t be at the high altitude (and lower fuel burn) long enough to make up for the extra fuel burned in the climb. Balancing these effects with the winds aloft and the minimum safe altitudes is part of the flight planning process. Have fun!

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3. 
   

   
   

   Brian on Jan 05, 2011

   …Qt: Jane — “I would think that since lift is reduced due to a decrease in air density, one would need to increase their speed in order to maintain adequate lift.”…
    
   In steady flight, lift is not reduced, as you implied, it balances with something. We know this something to be weight. Now, what you indicated as happening is one scenario. Here:
    
   Lift = speed * air density * angle of attack * wing area
    
   The left side, lift, is always equal to weight. The right side is in a never ending balancing act to keep this lift equaling weight. In other words, when air density went down, speed or angle of attack could be increase to negate the lose from reduced air density.
    
   In the case of air density decreasing and changing stall speed; If we assume a constant speed then angle of attack must increase to compensate for decreases in air density (wing area remains constant, assuming flaps and other lifting devices are not used). What this means is, for any given airspeed, reduced air density will require a higher angle of attack to achieve lift equal to weight. This is why higher density altitude, or less dense air, will result in a higher stall speed.
    
    

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4. 
   

   
   

   Wesley Beard on Jan 06, 2011

   I prefer to give out the lift equation to explain something like this.
   Lift = 1/2 * CL * p * SA * V^2  
   CL = Coefficient of Lift and for our example equals is proportional to the Angle of Attack
    
   p = Air Density
    
   SA = Surface Area of the wing
    
   V^2 = True Airspeed (TAS) squared.
    
    
   So if the air density decreases, either you need to have a higher angle of attack or your TAS must be increased.  Since we know TAS increases with altitude anyway, it helps balance with the decrease in density altitude.  Angle of Attack also increases as the airplane goes higher in the atmosphere.  This all assumes we have a constant indicated airspeed.
    
   TAS = calibrated airspeed corrected for density altitude = indicated aispeed corrected for installation and position error

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5. 
   

   
   

   Steve Pomroy on Jan 06, 2011

   Hi Jane.
    
   Following the theme of the lift equation set by the previous posters, it’s worth noting that the distinction between indicated speed and true speed is in that equation:
    
      L = CL * (1/2 * p * V^2) * S.
    
   The ‘V’ in this equation is your true speed.  The part in brackets, half the density times the speed squared, is the dynamic pressure.  The dynamic pressure is what your airspeed indicator actually measures — even though it indicates speed.
    
   The ASI is calibrated to indicate speed based on the assumption of standard sea level air density.  If you are at sea level on a standard day, the ASI will indicate your airspeed accurately — in other words, your true airspeed and your indicated airspeed will be the same.  However, if you are at higher altitudes, the changed (reduced) air density will create a discpepancy between your indicated and true speeds.
    
   This true v. indicated distinction is important because, as other posters have pointed out, it’s your true stalling speed that increases at altitude, not your indicated airspeed.  For a given weight / load factor / configuration / power setting / CG / etc., your indicated stall speed will be the same regardless of altitude.  But your true stalling speed will be higher at higher altitudes.
    
   As noted by Kent, it’s this increase in true stalling speed that leads to many performance changes (such as takeoff and landing distances) even though the indicated speeds we’re referencing are the same.
    
   So after all of that, the short answer to your question is:  Yes, true stalling speed is higher at higher altitudes, but No, indicated stalling speed does not change with altitude.
    
   Further, it’s worth highlighting and re-emphasizing Brian’s point about lift.  Lift is not reduced at higher altitudes.  Lift has to balance your weight and load factor no matter what altitude you are at.  So, at different altitudes, you produce the same amount of lift but under different conditions.
    
   NOTE:  For the sake of brevity, I’ve assumed here that there is no instrument or position error in your ASI.  So indicated and calibrated airspeed are the same thing.
    
   Cheers,Stevehttp://www.flightwriter.com

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6. 
   

   
   

   tariq hasan on Nov 01, 2013

   all the above answers are relevant till FL 200, After that the characteristics of stall are function of mach number especially after FL 260.Low speed buffet and high speed buffet are more relevant at those altitude and must be understood by the pilots who are flying at max computed altitude where maneuver margin is very less ,thats why that region is known as coffin corner.This region can easily be identified in EFIS aircraft.At that altitude the aircraft would enter into stall regime much before than the speed which would cause the aircraft to stall at lower altitude.

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