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I recently started flying a SGS 2-33 glider. The POH lists the stall speed as 31 mph for solo flight. Min sink speed is 38 mph and optimal glide speed is 45 mph.

Being an old glider with imperfections, it stalls at 40 mph.

Knowing that the stall speed is 9 mph higher than described in the POH, how can I calculate a new optimal glide speed?

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    $\begingroup$ Perhaps cleaning and waxing the wings (and fuselage) might help. Effects of a poorly cleaned and scuffed surface are not unlike that of icing. Also, check to see if the CG is still in the proper range if repairs were made. $\endgroup$
    – Robert DiGiovanni
    Commented Oct 10, 2022 at 8:19
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    $\begingroup$ I wouldn't fly that glider at all, and tell the owner to red tag it. That is a very large and dangerous difference from the published stall speed at max gross weight for a low performance glider. It could be the airspeed indicator is not operating properly, and needs attention. $\endgroup$
    – Mike
    Commented Feb 14, 2023 at 2:10

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If only the maximum lift coefficient suffers from the imperfections, your minimum sink speed is very close to the stall speed and the best glide speed stays unaffected.

If a major mass increase (say, from unprofessional repairs) contributes to the higher stall speed, both the minimum sink and the best glide speed will be shifted to higher flight speeds. Use the dynamic pressure to scale the speeds with the glider's mass.

In general, such an increase in stall speed raises the question if the plane is still airworthy.

The "new" stall speed is at a dynamic pressure which is 66% higher than the old glide speed. This is much more than what could be caused by minor imperfections, so drag should also be affected. To really find out what the new speeds are, you should collect data to plot sink speed over flight speed for the whole speed range. Use early morning aerotows on days with very calm air (caution: High pressure weather means slowly sinking air mass; avoid those days!) and make sure you keep speed constant for as long as possible to measure the altitude loss over time. Correct for density and use the tangent method to find the best glide speed.

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