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Post by Tom Goodrick on Nov 14, 2008 20:50:43 GMT -5
I intend to do some more work on the Avanti model. I had to change the MMO setting from 0.67 to 0.70 to comply with the newer version. But I think it needs better Mach drag and maybe a little Mach pitch for any who venture beyond the limit.
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Post by Tom Goodrick on Nov 15, 2008 18:02:17 GMT -5
The performance is not far off so you can fly the existing model without worry that it does not represent the real plane fairly well. You should change the max-mach=0.67 to 0.70 in the aircraft.cfg file. That gives a few more knots before the noise happens. But in careful comparison against the specs in the FLYING article the changes are slight. Fuel flow increases by 2%.
The tricky part is getting the performance change between 31,000 ft and 41,000 ft. At 31,000 ft you can still get pretty high power - about 86%. But by 41,000 ft this drops to about 67%. The speed should be 400 at 31,000 ft and 357 knots at 39,000 ft. Fuel flow drops from 770 pph at 31,000 ft and max power to under 600 pph at 41,000 ft (exact value is not given). The mach is increasing to max 0.70 at 41,000 ft with max throttle. There is probably a rise in drag along with this Mach increase.
Getting these numbers to come out right is the hard part. The model wants to give 370 at 31,000 ft and 365 at 39,000 ft.
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Post by Tom Goodrick on Nov 15, 2008 23:51:05 GMT -5
OK. My fiddling with the numbers has paid off. By checking performance from FL270 to FL410, it is clear why the Piagio performas as stated in the magazine. The pilot makes it perform that way by working with the constraints of MMO (max mach, operating) and VMO (max operating indicated airspeed). Before we give the exlanation, take a moment to look at the data.
MAX CRUISE
Flt Lev %Throt %Powr Mach KTAS PPH/eng KIAS<= 260
270___81______69.1___0.64__380___302______258
290___82______69.4___0.66__390___301______256
310___95______75.9___0.70__409___328______260
330___89______72.1___0.70__406___309______low
350___83______69.0___0.70__403___293______low
370___80______65.6___0.70__400___278______low
390___78______63.6___0.70__403___270______low
410__100______62.2___0.70__401___263______low
LONG RANGE / ECONOMY CRUISE
Flt Lev %Throt %Powr Mach KTAS PPH/eng
310____60_____57.0___0.61__357___245
330____60_____56.5___0.62__360___241
350____60_____55.4___0.63__363___235
370____64_____56.1___0.65__373___235
390____62_____53.7___0.63__361___237
410____80_____53.0___0.63__359___224
This is for an aircraft with full fuel at takeoff and a payload of 4x180+100lbs. There is margin for a little more weight.
Flight level 310 is a special place because that is where the two constraints meet: Mach 0.70 and VMO 260 KIAS. Below that altitude, you set cruise power according to the indicated airspeed of 260. Above that level, you set cruise power according to the Mach 0.70. If turbulence is expected you must, leave a marging below these values.
In the FLYING article, the true airspeed at FL310 did not quite reach 400 knots although Piaggio says it should. The reason was that the temperature was non-standard when the flight was made for the article. Non-standard conditions do different things to the two limits. I high temperature will reduce the Mach number at a particular true airspeed so you might think you could increase the true airspeed beyond what you would use in standard conditions. But if you do this, you would make the indicated airspeed violate the VMO limit.
I have not checked the estimated range for each of these conditions. The spec is 1420 nm which leaves a legal margin for IFR flight (All flight above 18,000 ft must be under IFR.) It seems there is a considerable variation, depending on conditions and on what ground speed you will accept. I would allow a wide margin for each leg with a nominal 1300 nm as a general number. You will have to pay close attention to the GPS ETE and the fuel computer's estimate of flight time remaining and adjust your flight plan if conditions vary from the assumed condition.
I'll update the posted download file on my web site tomorrow and will send out zips with the new FD files to Bill and Allen. Both aircraft.cfg and .air files were changed. Among other things, the MMO limit can be set in the aircraft.cfg file but the VMO limit must be changed in the .air file.
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