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7 Answers

Best Glide Speed

Asked by: 26161 views Flight Instructor, General Aviation, Private Pilot

I am instructing in a Aero Commander Darter - and the POH does not have Best Glide Speed listed.  I am wondering how to handle this with an examiner when he ask.  I asumme it will be a little above Best Angle speed but I need some rationale to provide my Student when he faces the Examiner.

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7 Answers



  1. Wesley Beard on Dec 09, 2010

    The best glide speed is defined as the speed where the lift to drag ratio is the greatest.  You will find it only a couple knots faster than Vx at sea level and at max gross weight.  You can figure out what the best glide speed is in the airplane impirically by doing the calculations for both lift and drag at different airspeeds or practically by flying different airspeeds and noting the descent rates.

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  2. Kent Shook on Dec 09, 2010

    IME, best glide is closer to Vy than Vx. In fact, in the plane I’ve been flying most recently, Vg is actually higher than Vy.
     
    But, it’ll be in that range somewhere – Use that as a starting point and try some glide tests to find the actual Vg.

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  3. Micah on Dec 10, 2010

    I agree with the idea of testing it yourself.  Spend some good time with your student on a calm wind day and do long descents at specific speeds.  Make small changes (2 KIAS or so) and measure the time.  Don’t just watch the VSI, but track total descent (xxx feet) and time and write it down.  Practice large scale descents (1,500-2,000 feet) and repeat.  Depending on your student’s skills, you may need to fly these yourself and let your student write down the measurements.  That sounds like a very good lesson and if your student doesn’t understand the value of that time then you’ve got an obvious teaching situation. 
    At the very least, if I were an examiner and I had an applicant tell me “There’s no published Vg so I just guessed what speed to fly in case of emergency…” or “There’s no published Vg so my instructor spent more than an hour with me in flight and later on the ground showing me how subtle changes in pitch and trim made noticeable differences during the descent.  We can’t be certain, but we think Vg for this airplane is really close to…”, I think I know which explanation I would prefer.  

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  4. Brian on Dec 11, 2010

    I’m going to quote and explain two passages out of Aerodynamics for Naval Aviators. There has been some discussion here that, to me, seems to relate best glide speed to subsequent climb speeds. These speeds are not related.
    Before getting to these quotes, (Qt=quote by the way, since I’ve had a few people ask me this now) I agree with the experiential method. However, I caution recognizing the speed you are finding given the parameters discussed thus far.
     
    The speed that yields minimum rate of descent IS NOT best glide. This speed would be the speed that correlates to minimum power required and is synonymous with a speed we call minimum sink.
     
    Minimum sink is the speed that will keep you in the air the longest, but it doesn’t give the greatest glide distance. That speed is best glide, or the point of minimum drag. Best glide speed occurs approximately 5 or so knots greater than minimum sink speed in the typical single engine trainer aircrafts of today.
     
    In other words, go up and do the tests as described. Once you have the speed that gives minimum rate of descent, add 5 knots to it. You can use the VSI without bothering to calculate and plot so long as you give the aircraft sufficient time to stabilize at each given speed. There will be some error, but unless you are really good with math it will probably be less error than using a calculator anyways.
     
    Besides, shifts of 5 or so knots in either direction of best glide speed have minimal effect on glide performance. Now onto clarifying why Vx, Vy, and Vbg cannot be related. Here:
     
    Qt: Aerodynamics for Naval Aviators – Page 155-156…”The power curves for the representative propeller aircraft…maximum rate of climb for this aircraft will occur at some speed near the speed for (L/D)max. There is no direct relationship which establishes this situation since the variation of propeller efficiency is the principal factor accounting for the variation of power available with velocity…
     
    More or less, this is stating that, since propeller efficiency effects climb speeds, climb speeds cannot be accurately related to glide speeds.
     
    …Qt: Aerodynamics for Naval Aviators – Page 156…”In an ideal sense, if the propeller efficiency were constant, maximum rate of climb would occur at the speed for minimum power required.”
     
    Recall that I just told you the speed for minimum power required is minimum sink, which occurs about 5 knots below best glide speed. Best glide speed is higher than minimum power required speed. This quote states that Vy would occur at minimum power required speed and explains why Kent has found an airplane for which Vbg is faster than Vy. 🙂
     
    Kent, would you mind posting the aircraft you’re flying now that has this relationship? I’d be interested in doing some research into that aircraft to better understand these relationships and, as of yet, I haven’t encountered an aircraft with best glide greater than Vy. Thank you. 

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  5. Kent Shook on Dec 11, 2010

    Brian,
     
    I was not saying to use Vy (or Vx) specifically, just that that would be a good starting point for testing since Vg is usually not very far away from Vy (within 10-15 knots on most airplanes).
     
    <blockquote>The speed that yields minimum rate of descent IS NOT best glide. This speed would be the speed that correlates to minimum power required and is synonymous with a speed we call minimum sink.
     Minimum sink is the speed that will keep you in the air the longest, but it doesn’t give the greatest glide distance. That speed is best glide,</blockquote>
     
    Exactly. For testing, you’ll want to set up a spreadsheet that calculates the glide ratio. Enter the airspeeds in one column, vertical speeds in another (though it is best to actually time how long it takes to descend a couple thousand feet at each speed so you get a more accurate vertical speed), and convert the knots (airspeed) into feet per minute (1 knot = 101.27 feet per minute) so that you’re using the same units. Finally, divide the forward speed in feet per minute by the vertical speed in feet per minute to obtain your glide ratio.
     
    I have a spreadsheet that does this and a few other things to determine whether I can make it across a particular body of water and remain within gliding distance to land. Unfortunately, I don’t think we can post files here.
     
    Brian, the plane I mentioned is the Diamond Star (DA40). Vx isn’t published in the POH, but it shows as 64 KIAS on the G1000. At max gross, Vy is 66 KIAS and Vg is 73 KIAS. They also list 73 KIAS as a cruise climb speed. I fly from a large airport, and the pitch attitude is very high at 66 (note the very small difference – 2 knots – between Vx and Vy), so I tend to go straight to 73 KIAS after takeoff for improved visibility and passenger comfort. The plane accelerates very quickly after liftoff as it is, so to keep it from accelerating past 66 requires a pretty swift nose-up.
     
    Using the G1000’s data logging capability and plugging some of that data into spreadsheets, I’ve been able to determine that it has a phenomenal glide ratio – Slightly better than 13:1 – And successfully tested it recently, pulling the engine to idle 20.5 miles from my destination airport at 7500 feet (field elevation 754) and gliding it the whole way in. (If you do the math – I used 68 KIAS as my glide speed, since I was pretty light, and yes, I had a hefty tailwind too, ~40 knots average at all altitudes, which I factored into my calculation to pull the engine at 20.5 miles out.) Fun stuff, for this airplane geek! 🙂

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  6. Gary on Mar 14, 2011

    I was not able to find a glide speed listed with all of the Vspeeds either, however Pilot Operating Handbooks have performance graphs somewhere towards the back.  One of those graphs should address glide performance and give a set calabrated airspeed.  In the one I’m looking at right now it says 85mph for a PA-28-151.  Hope that helps.

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  7. Dan Morke on Apr 26, 2014

    A Navajo 310 stalls speed with flaps … down is 73 , how do you find its glide speed. Its take off weight is 7600 , empty is 3950. Its best angle dpeed is 106. Some one told me its 1.2 timed stall speed……..Thankyou…downis

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