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Hello pbuford !
Just to precise the previous message, and provided that all the problems due to the overpowering of a Saphir airframe are solved, we can calculate the following performances related to the power of the engine :
- on the left, the rated power of the engine,
- in the middle, the best possible cruising speed (no extra drag added to the first prototype) à 75% of rated power and propeller efficiency = 80%,
- On the left, the endurance with 140 litres (37 US Gallons) of AVGAS and sparing the 45 minutes for safety.
HP kt Nm
160 150 544
180 156 490
200 162 444
260 176 343
300 185 293
360 197 235
400 204 204
Of course, I would not dare mounting a IO-720 in front of a Saphir firewall without competely redesining the airplane, but this is just to show the consequences on speed and endurance :
To fly 204 NM, the 400 HP Saphir would take exactly one hour, burning 80 litres (21 USGallons) of avgas
On the same flight, the standard 160 HP Saphir would land 21 minutes later, but would have saved 9.5 USGallons of avgas...
To fly 544 NM, the standard Saphir would take 3 hours and 38 minutes
On the same distance, the 400 HP Saphir would have spared 57 minutes of flight, but would that be enough to refuel 2 times?
Conclusion : Large overpowering is not the right way to fly fast.
A much better way is :
1/ to reduce drag to the minimum (not forgetting the drag induced by the engine cooling),
2/ to reduce the mass to the minimum (better climbing rate, reduced lift incuced drag...)
3/ to keep the fuel flow as low as possible (for money of course, but also to reduce the mass of fuel to be lifted...) by choosing a low specific fuel consumption engine.
4/ to fly higher (I mean to fly at the best altitude for ground speed taking into account distance and wind, but very often that means flying at a higher altitude than usual...).
For this purpose, if I had to repower a Saphir in North America, I would choose a 200HP turbocharged Deltahawk diesel engine with a CS propeller, and design the most streamline cowling that I could...
By the encreased power 200HP instead of 160 HP, but also by the turbocharging at medium altitude (let say 8,000 to 10,000 ft) and the reduced cooling induced drag I think that a corrected speed of 180 kt at cruising altitude could be obtained without reducing endurance.
Anyway... Enjoy flying !
_________________
Les fourmis sont des guêpes comme les autres !
Dernière édition par Philippe Dejean le Mer 14 Mai, 2008 6:48 pm, édité 1 fois.
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