Uh, I don't know how to say this politely, but the numbers posted above by Exaviator are not in agreement with the numbers I use - and I am the guy who writes the Twin Otter AFM!
The Twin Otter has been certified to many different standards - first to CAR 3, then later to SFAR 23, then to the BCARs (British standards in the 1970s, prior to JAR OPS-1), then to Australian DCA regulations (prior to publication of AFM amendment 19 in 2002). Exaviator indicated that he flew in PNG in the distant past, it is possible that he has quoted speeds, configurations, and power settings that were applicable to the Series 320 (Australian) variant aircraft prior to the publication of AFM amendment 19 in 2002.
FWIW, I've just finished writing an AFM supplement entitled "Performance to FAR 25 Standards" that will be published later this summer for operators who wish to conform to JAA 'Performance A' standards, and for Canadian and American operators who will be obliged to comply with the '2010 Rule' when it comes into effect next year. That will add yet another variable to what is out there...
Here are the current numbers for SFAR 23 certification standards. These are applicable to the Series 300 and variant standard landplane aircraft at the maximum weights permitted (12,500 lbs MTOW, 12,300 lbs MLW).
Takeoff - Flaps 10°, V1 73 KIAS, V2 80 KIAS, maintain V2 to 400 feet AGL minimum.
Initial Climb - following flap retraction, Vy is 100 KIAS, a higher speed may be used at the discretion of the pilot.
Initial Approach - no less than 94 KIAS if flaps are up, or no less than 85 KIAS with flaps extended to 10°.
Landing - Flaps 20°, Vref 80 KIAS or Flaps 37.5° Vref 74 KIAS
Cruise power is at the discretion of the pilot. At temperatures up to ISA +6° at sea level, 50 PSI torque can be used for cruise. This is not a common practice, because setting maximum cruise power results in a significant increase in fuel burn but an insignificant increase in speed. Many operators use about 480 SHP per engine for cruise, this can be achieved with a torque setting of 45 PSI and a propeller speed of 75% Np. At this power setting, fuel burn is approximately 300 pounds per hour (per engine), and a reasonable compromise between fuel burn and cruise speed is achieved.
Some other useful speeds, again, at maximum weight for the phase of flight, all in KIAS:
Best Rate of Climb 100 KIAS with flaps up
Best Angle of Climb 87 KIAS with flaps up
Best Single Engine Rate of Climb 80 KIAS with flaps 10°
Engine Failure after Take-off 80 KIAS, Flaps 10°
Enroute Descent Flaps up, speed limited by Vmo
Minimum Control Speed, One Engine Inoperative 64 KIAS. Vmc is always given in the take-off configuration, thus, flaps 10° is assumed.
Stall speed, landing configuration (Flaps 37.5°) - 56 KIAS
Stall speed, take-off configuration (Flaps 10°) - 66 KIAS
Stall speed, flaps up 73 KIAS
Manoeuvring Speed 132 KIAS from sea level to 18,000 feet. Limited by Maximum Operating Speed above 18,000 feet.
Glide Speed (both propellers feathered) for best range - 100 KIAS
Glide Speed (both propellers feathered) for best endurance - 77 KIAS
Maximum Operating Speed 166 KIAS for aircraft SN 271 and higher in landplane configuration, or earlier aircraft that are post Mod 6/1291 in landplane configuration. 156 KIAS for all other aircraft. Vmo begins to decrement above 6,700 feet pressure altitude, but this normally does not present an operational restriction.
Flaps Extended Speed Maximum 103 KIAS for flap extension up to and including 10°, maximum 93 KIAS for flap extension beyond 10°, both speeds quoted are for post Mod 6/1395 aircraft. Mod 6/1395 cut in at SN 290 and can be retrofitted to earlier production aircraft.
Hope this information is useful. If you happen to get out to Vancouver Island, drop in and visit - Twin Otters are back in production again.