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Effect of Weight on Glide Ratio

Posted by on June 19, 2010 11 Comments Category : Flight Instructor Blog Tags : ,

Every once in a while I get a question on askacfi.com that I’m pretty sure is a homework question but I still don’t mind answering it.   Like the one I just got from Javier who writes asking:

Two airplanes are flying at the same altitude, the same day, same conditios, except airplane A is much heavier than airplane B.  Both of them start a power off descent at the same time at the same position. Which airplane will arrive first to the ground?  The heavier? The lighter? Both at the same time? Will both airplanes arrive at the same point? This is a question I was asked a few days ago, and I can’t find the correct answer. I would really appreciate if you could help me out.

Holy Smokes that’s a good question! But I think I found your answer from:

http://www.skybrary.aero/index.php/Glide_Performance

Variations in aircraft weight do not affect the glide angle provided that the correct airspeed is flown. Since it is the lift over drag (L/D) ratio that determines the gliding range, weight will not affect it. The glide ratio is based only on the relationship of the aerodynamic forces acting on the aircraft. The only effect weight has is to vary the time the aircraft will glide for. The heavier the aircraft is, the higher the airspeed must be to obtain the same glide ratio. If two aircraft have the same L/D ratio but different weights and start a glide from the same altitude, the heavier aircraft gliding at a higher airspeed will arrive at the same touchdown point in a shorter time. Both aircraft will cover the same distance but the lighter one will take a longer time to do so.

So based on the information above.  Here are your answers:

  • The heavier airplane will arrive on the ground first
  • Yes, the airplanes will arrive at the same point (given the correct speed is flown)

Pilots, do you have your aircraft’s glide speed memorized? This should be a indicated airspeed you should try and maintain the second your engine quits or starts sputtering.   This speed is important as it represents the bottom of the drag curve.  Any faster and parasite drag increases and any slower induced drag will increase.  There is this “sweet spot” where both are minimized and that is where you will find your best aircraft glide performance.

Ever wonder how far you could glide given a certain altitude?  Try this Aviation Glide Calculator.  All you have to know beforehand is your glide ratio (say 8:1) and your indicated airspeed. (Use your L/D max speed)

Paul.

11 Comments



  1. Jim Wolper on Jun 20, 2010

    Instead of best glide SPEED, memorize best glide ATTITUDE.

    This applies to singles and multis.

    This frees you to make the decisions and troubleshoot.

    A little extra airspeed has minimal effect, but a little too slow is bad.

    JSW (CFII, MEI, CFI-G)



  2. Paul on Jun 21, 2010

    Thanks Jim. You’re right, flying the correct attitude is a much better approach to flying the aircraft during emergencies than trying to focus on maintaining a specific indicated airspeed.



  3. Joe on Jun 29, 2010

    Come on Paul. You have to explain that a little better than that. This just doesn’t seem to jive. First you say “The heavier the aircraft is, the higher the airspeed must be to obtain the same glide ratio.” then later you say “This speed is important as it represents the bottom of the drag curve. Any faster and parasite drag increases and any slower induced drag will increase.”

    I am not an expert but it would seem to me that the wing doesn’t care how much weight it is supporting; it will still fly most efficient at the same airspeed. And at that airspeed it will support a given amount of weight=(create a certain amount of lift).

    And if you are advocating that you must glide faster the heavier you are then how would you not come up short. You would have to have a different attitude (sight picture) to achieve this. Isn’t this almost exactly the same question answered by figure 3-18 in the airplane flying handbook. Do you disagree? Why? I am all about learning so help me out here.



  4. Brian on Aug 14, 2010

    “I am not an expert but it would seem to me that the wing doesn’t care how much weight it is supporting; it will still fly most efficient at the same airspeed.”

    –spacing–

    Not airspeed Joe, Angle of Attack. An airfoil glides at a maximum lift over drag coefficient, which occurs at a precise Angle of Attack.

    Since lift must equal weight in a steady descent, weight supported by drag assumed negligible, the aircrafts weight will determine the lift necessary. Lift (L) is a product of half of the velocity (V) (speed) squared times wing area (S) times air density (ρ — pronounced rho) times coefficient of lift (Cl):

    L = 1/2 V2 * S * ρ * Cl

    Since best glide occurs at an exact Cl, we know Cl is a constant. In other words, the Angle of Attack for best glide will not change. We can also assume that wing area (S) is constant and air density (ρ) changes can be neglected. Thus, the only remaining variable left to change our lift is velocity (V), or airspeed.

    So what does all this gibberish mean, right? Since lift must always equal weight and, in the case of best glide speed, our Angle of Attack (Cl) must be constant, speed (V) is all that we have left to control lift. Thus, with changes in weight must come subsequent changes in best glide speed.

    ~Brian



  5. Christopher Ian on Feb 29, 2012

    Yes, L/D is L/D always.
    Aircraft weight, air density (e.g. altitude) have no effect on the glide ratio.
    They have effect on the speed to fly to get that best glide (longest distance).
    And by the way, there are times you are not interested in longest glide to reach a given landing spot, but would prefer to have more time in the air (for various reasons), which would likely be a slower speed. Ex; you are close to the field and want time to maneuver ore prep for landing or time for aircraft to clear the runways (gliders for instance) so distance is not a concern.

    The first quoted paragraph said;
    ” Variations in aircraft weight do not affect the glide angle . . . ”
    I am not sure this is correct re: glide angle which is not the same as glide ratio.
    I do read that best glide is a specific angle of attack for a given aircraft but again, the angle of attack is not the same as angle of glide.

    The literature refers to a specific angle of attack, for best glide but how does one know the exact angle of attack, really ? Especially when the angle is so slight.
    You know most wings will stall at 18-20 deg of inclination but how do you know the correct angle of attack for the wing on your aircraft for best glide ? ? ?

    A C172 w 9.5/1 glide ratio (L/D) has a 6.01 deg. glide path.
    Can you or anyone judge 6.01 deg glide path (not angle of attack)
    rather than 7.01 or 5.01 ? Really ?
    This is important.
    A 5 deg angle (glide path) gets you 1909 feet farther while
    a 7 deg angle (glide path) gets you 1366 feet less ~
    This can be the difference between going home !

    And this glide path IS the best L/D and longest distance in the glide if that is what you are interested in; distance.
    BUT
    glide path is not the same as attitude / angle of attack (AoA).
    So tell me (Jim and Paul) what is the AoA for my Citabria or C172 ?
    And how do I judge that angle ? ?

    So Joe, Jim and Paul are correct but they have not told us how they determine
    that specific attitude.
    And in fact I would challenge that concept of choosing an ‘attitude’ or angle of attack.
    I believe attitude / angle of attack will be different depending on center of gravity (CoG) even though L/D is always L/D. But I am not sure of that.

    Brian; ” Not airspeed Joe, Angle of Attack. ”
    The airspeed and angle of attack are related directly, yes ?
    Just as when you are under power.
    I just read elsewhere that altitude / air density does not effect glide ratio but it seems your explanation says it does effect glide ratio.

    That is the lesson we learn about slow flight and stalls.
    Reduce power but maintain altitude, reduce power some more, maintain altitude.
    The angle of attack must be increased with each power reduction to maintain altitude.

    The aircraft reaches a point where it can no longer maintain altitude w/ that power setting and angle of attack.
    At a critical point – angle of attack, the aircraft can maintain altitude only by increasing power and is said to be behind the power curve.

    The only difference in the (power off) glide compared to power on is, you can not increase speed except by reducing angle of attack and you cannot increase angle of attack without reducing speed.

    Perhaps I am to understand that angle of attack stays the same in best glide,
    while the glide angle can change but glide ratio will stay the same.
    Is all that possible ?
    Thanks



  6. rizwan on Aug 23, 2012

    hi i just wanted to ask that while practicing a flameout landing.. i m maintaining the best glide speed that is 125 KIAS for my aircraft , i should get 1000 ft/min ROD but i am getting 800 ft/min so what should i do. should i maintain the same speed or should i increase my airspeed by increasing dive angle to get 1000 ft .min ROD



  7. Mark on Jan 10, 2013

    This is great!! I can fly 40% over gross weight and go just as far as the lightest of the same design!



  8. Mark on Jan 10, 2013

    Now that I think of it, I could carry an extra 5,000 lbs. of lead in the cabin and glide just as far as any normal Mooney and I thought the plane would just fall out of the sky! Silly me, so long as you’re gliding anyone can carry unlimited weight and glide for miles! Yippee!



  9. donald cilbup smollo on Jan 26, 2013

    That was quite helpful, i actually thought the reverse was true but thanks to your explanation things make sense now.:) keep up the good job



  10. houston on Feb 28, 2014

    Mark, not exactly. Remember weight does affect stall speed. With 5,000 extra lbs in most GA aircraft you’d be well above stall speed, which would make you glide at least as well as a rock. All bets are off when the wing stalls.



  11. Airwolf319 on Mar 08, 2014

    Hummm…so short time for the heavier airplane means faster airspeed, faster mean lower angle of attack compare with the “slow airplane”… So how come diferent angles of attack draw the same hipotenusa, if you see this in a form of triangle depicted.?
    Is posible descent the same distance from the same altitude and land at the same point with diferent angles?… i dont see it.

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