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Some Flight Testing Concept Questions

macrafic

Well Known Member
Trying to get a handle on determining best glide speed and glide ratio, but have some questions that beg simple answers please!

I believe glide ratio is affected by wind (greater distance with a tailwind vs headwind). So, when POHs list glide ratio (usually in charts), how is this determined (tough/impossible to find still air around here in Minnesota!)?

Similarly, is the best glide speed (or best sink rate speed) affected by the wind?

Basically, I am searching for a method to establish those speeds/ratios in my current test phase.
 
Trying to get a handle on determining best glide speed and glide ratio, but have some questions that beg simple answers please!

I believe glide ratio is affected by wind (greater distance with a tailwind vs headwind). So, when POHs list glide ratio (usually in charts), how is this determined (tough/impossible to find still air around here in Minnesota!)?

Similarly, is the best glide speed (or best sink rate speed) affected by the wind?

Basically, I am searching for a method to establish those speeds/ratios in my current test phase.
Your belief is correct. ‘Best glide speed’, in terms of covering the greatest distance over the ground before you run out of altitude, depends on wind and aircraft weight. Most POH’s quote only no-wind, gross weight situations.
For the no-wind case, you want to reduce gross weight speed by the square root of the (actual weight/gross weight), e.g., if 10% under gross, reduce published best glide speed by about 5%. For a tail wind, slowly decrease that speed with increasing wind strength, but never less than minimum sink speed. For a headwind, a rough approximation is to add 1/3 to 1/2 of the wind speed to the nominal speed. Notice that many training situations at an airport have an under-gross airplane (calling for a smaller speed than best glide at gross) but on final a headwind (calling for an inceased speed) so often published gross weight best glide speed works pretty well.
To take data for your airplane, measure true airspeed (calculate from indicated if necessary)/rate of descent. Where that ratio is a maximum is your best no-wind glide speed, at whatever weight you were flying that day.
This is independent of horizontal wind, but subject to errors if there are updrafts or downdrafts.
 
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Here?s another way to calculate best glide speed from the same data. It may be more accurate because it lets you do some smoothing.
Take the data as above, and plot the data points: vertical speed on the vertical axis, true airspeed (IAS is okay if all the data is taken at the same density altitude) on the horizontal axis. Draw a smooth curve thru this data, it is roughly a U shape. Lay a ruler along the horizontal axis. Pivot it around the origin, and rotate the right end up until it just touches the curve. Read the airspeed below the point where it touched the curve, that is best glide speed, no wind. The angle of the ruler is proportional to the glide path. For a tailwind, let?s say 20 knots, repeat this but pivot the ruler around a point 20 knots to the left of the origin. For a 30 knot headwind, move the pivot point 30 knots to the right of the origin.
 
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