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I'm interested in the maximum L/D this plane can achieve. I can't find the necessary data to do the calculation myself.

If I knew the "long range cruise" engine power and assumed 85% propulsive efficiency I'd get pretty close to the real number, correct?

Is the optimal L/D achieved at the so-called "long range cruise" speed? According to Wikipedia this speed is 589 km/h (366 mph, 318 kn). Is this number correct?

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    $\begingroup$ Don't know the numbers but I remember from an article by Peter Garrison in FLYING magazine some years back that the Avanti maintains laminar flow on the fuselage to well aft of the cockpit, one of the secrets to its performance. $\endgroup$
    – John K
    Jun 21, 2021 at 18:18
  • $\begingroup$ L/D(max) is achieved at the best glide airspeed, which is certain to be well below cruise speed and even below V_y. I don't know what best glide is for the Avanti, but that's the number you're looking for. $\endgroup$
    – TypeIA
    Jun 22, 2021 at 21:03
  • $\begingroup$ A study of cruise missiles is in order. Setting max L/D parameter at cruise speed will result in tiny looking wings, with slats and flaps added for lower speed activities such as takeoff and landing. Aircraft with a wide weight range, such as cargo or larger airliner, must build in more wing for safety margin. Cruise thrust should answer your question. Keep in mind optimal cruising is drag (thrust, really fuel consumption) per distance (helped greatly by sufficient power to go high). $\endgroup$
    – Robert DiGiovanni
    Jun 23, 2021 at 7:10

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According to some flashcards posted on a random Internet site by an unknown user (i.e., not an official source), the Piaggio Avanti's glide ratio (which is equivalent to maximum L/D ratio) is 2.3 nm / 1000' or about 14:1. It can be expected to achieve that at around 135 knots indicated airspeed. (Give or take: best glide speed isn't actually stated, so I'm applying the useful rule of thumb that it's usually around halfway between best angle of climb and best rate of climb airspeeds.)

Maybe someone with access to a P-180 AFM/POH can chime in with the official numbers.

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