I’m really curious as to what the real best L/D speed is for the Lancair IV’s and Legacies.  I was told in my training at the factory that the best glide speed is approximately 120 knots (for the IV), knowing full well that this speed is dependent on the weight of the aircraft.  I also remember someone telling me that best glide speed in any airplane is the same as the best rate of climb speed, Vx.  In the IV’s that speed is approx. 135 kts.  That’s a big difference.  I also know from my sailplane racing days that you never want to be slower than the best glide speed when trying to maximize distance.  The penalty for being too slow is much greater than the penalty for being too fast.  So has anyone actually done some testing to determine the appropriateness of the 120 knot figure?  I’m beginning to suspect that its really the speed at which you get the minimum sink rate.  Isn’t 120 knots the reported figure for best angle of climb?  Here’s another data point – the CAFÉ test of the Legacy tested its best L/D as 135 kts — hmmm.

The AOA indicator (that some have wisely installed – I haven’t) has a small ‘x’ on the second bar from the yellow bars that’s reported to be the best L/D.  How is that number calculated?  Does the setup procedure determine the best glide point for that aircraft’s glide polar?  And can someone with this equipment perform a flight at the AOA angle of the “x” and report back to us the indicated airspeed and approximated aircraft weight at the time of this reading?

The best solution would be to do some engine off glides at various speeds to track the sink rates at those speeds.  Has anyone done this?

Given the same situation as faced Shannon, I certainly would like to maximize my distance by flying the best speed for the conditions (wind, weight) and give myself the best chance at making it to a suitable landing spot.  I think that we all need to armed with this knowledge to give us the confidence that we can make it to that next airport, or, not make it and choose a field or road instead.

Hal Woodruff
IV-P