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1/25/2018 7:38:29 AM
Posted: 9/8/2010 8:21:03 AM EST
[Last Edit: 9/8/2010 8:22:00 AM EST by a555]
In my PPL training, I was always taught about best glide. I'm under the, possibly mistaken, impression that there's an airspeed that can give you a more favorable sink rate and forward speed when crashing into unknown terrain, such as IMC or night over water, where you may not be sure exactly when to flare, and there's no chance of being able to identify a favorable landing spot. For instance, if you're in a C172 and end up losing your engine in IMC crossing the gulf, or lose it crossing a swamp or over a forest, I would think you can slow to below best glide and get a more favorable rate of descent, or would it even be better to hold just above stall speed, keep the nose super-high, and come in tail first and with slow forward speed? The idea being that you minimize the amount of energy the plane has when you crash, and that you minimize the rate of descent for a 0/0 landing (you lose your engine crossing the great plains, hold the nose off, come down as softly as possible and hope for a field).

Those of you with knowledge; can you enlighten me on this?
Link Posted: 9/8/2010 8:45:01 AM EST
Adjust for minimum sustainable sink rate under dead engine conditions. The flight data for any given aircraft should tell you what that is.
Link Posted: 9/8/2010 9:15:21 AM EST
[Last Edit: 9/8/2010 9:27:58 AM EST by AeroE]
Link Posted: 9/8/2010 3:49:59 PM EST
attempt to plan a route and altitude that will allow you to glide to a alternate. I know its not always possible however.
Link Posted: 9/10/2010 4:10:47 PM EST
I'm not sure if your original question was answered but "best glide" speed is another way of saying "best lift over drag". It's the speed that allows the most DISTANCE. With both major types of drag, induced and parasitic, best glide speed is a compromise of the two. Any faster than best glide will have parasite drag dominate and any slower will have induced drag dominate. So, it's a very "efficient" speed which is why you will usually find Vy very close to best glide speed.

I'm not an engineer but I understand that minimum sink is usually slightly lower than best glide. Distance will be sacrificed for a lower sink rate. It may be unpublished in your flight manual but it's easy to use the VSI to make your own calculations. Keep in mind that it's one of the speeds that will change with weight like VS0, VS1, VX, VY, VA.

The weight change formula is found by dividing the current aircraft weight into the max weight, take the square root of that figure and multply it times the published value (usually the published value is at max weight)


Max aircraft weight (weight at which all published speeds are based)...2500 lbs
Actual aircraft weight...2000
2000/2500 = .80
Square root of .80 = .894
Thus a published best glide of 90 mph at max weight is actually 90 X .894 = 80.46

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