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In the DA20, the POH lists for glide speed (flaps up) only one speed giving the best glide angle / furthest distance (73 KIAS).

But what about, if I don’t need that much distance to glide (e.g. because I’m high above an airport) and rather want to optimize for the maximum time airborne, i.e. the lowest sink rate. Which speed would I need to fly for that? I couldn’t find any L/D diagram in the POH either where I could find it from.

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    $\begingroup$ Are Vx and Vy given in POH? $\endgroup$ – Cpt Reynolds Apr 29 at 15:50
  • $\begingroup$ @CptReynolds yes, Vx 68, Vy 75 (both KIAS) $\endgroup$ – Florian Apr 29 at 15:51
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    $\begingroup$ I asked the manufacturer and they said, pilots should just fly best glide speed, because of the drag of the windmilling prop that's the best recommendable procedure. They also told me that they don't publish the actual best sinkrate speed nor the L/D diagram since regulations don't require them to. $\endgroup$ – Florian Jun 4 at 2:29
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Admittedly, my prop performance theory is a tad rusty, so maybe someone else can provide proof here.

I seem to recall that - assuming a quadratic drag polar which might not hold true for your aircraft - minimum sink speed has a theoretical value of .76 x best glide speed, so in your case that would be around 56 KIAS.

If you’re comfortable and consider it safe to do so, why don’t you (at a safe training altitude) trim for 73 KIAS and establish a stable idle descent, note vertical speed descending through a predetermined altitude, reposition to initial altitude and repeat in maybe 5kts decrements down to stall speed or some safe margin above - the former of course only if flying right down to stall is still trained and allowed in small aircraft. This will give you at least a rough idea of where minimum sink speed lies and what rates to expect.

The DA 20 Approved Flight Manual doesn’t help much, by the way.

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    $\begingroup$ Never pictured Vx and Vy as a min sink rate and Vbg diagram turned sideways by adding power. That min sink rate curve for gliders seems to be a combination of the linear AOA / Coefficient of Lift graph and the lift / square of speed graph. That would put the maxima a little further away from stall than I had imagined. Looking at old stuff a new way, thanks! $\endgroup$ – Robert DiGiovanni Apr 29 at 23:01
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    $\begingroup$ Thanks for this idea how to find it, I might do it next time I have the possibility to do so. And regarding that the AFM doesn't help: The manufacturer's answer to that is that an L/D diagram or the like isn't mandated by regulations and thus won't be published $\endgroup$ – Florian Jun 4 at 2:31
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The DA 20 Katana is a powered aircraft, but the rate of descent vs airspeed graph from gliders still applies. Generally, the lowest rate of descent (vertically) is slightly lower than Vbg. This makes sense as higher AOA will generate more lift at the expense of increased drag, which slows forward motion. Vertical component grows larger, allowing lower speed, horizontal vector grows smaller, therefor less distance covered.

The relationship holds all the way down to near the stall point, any slower, Coefficient of Lift begins to decline and sink rate increases. A little more dangerous to fly here, as a gust of wind or a turn could lead to a stall.

If you are high over the airport and approaching to land (getting near the ground), "going slow" or being on "the back side of the power curve" is not as safe. Better to hold speed and drop flaps.

But if you are higher and wish to try minimum rate of descent (flaps up or down), "real" numbers for your particular plane can be had. Keep that carb heat on too, if needed.

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  • $\begingroup$ Minimum rate of descent would probably be closer to Vx. $\endgroup$ – Robert DiGiovanni Apr 29 at 20:15

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