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chiozza

Performance In Flight

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When flying the other night at 15k 50% fuel, I am indicating only 140knts with 11knt tailwind. This cannot be right. Across the ground showed 209 knts. Anyone else?


Chris Chiozza

 

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What was your power setting? At that altitude, you should be able to get to the torque limit, and probably about 190 - 195 KIAS.


Best Regards,

Kurt "Yoda" Kalbfleisch

Pinner, Middx, UK

Beta tester for PMDG J41, NGX, and GFO, Flight1 Super King Air B200, Flight1 Cessna Citation Mustang, Flight1 Cessna 182, Flight1 Cessna 177B, Aeroworx B200

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Your IAS and GS at that altitude could be correct. The 2 other parameters you need for an accurate calculation are altimeter setting and Outside Air Temp.


Joe Brown

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Joe's right, of course. The indicated airspeed and groundspeed could be correct, depending on your power setting.

 

A look at the performance tables for the C90B shows that you should expect to get about 1000 ft/lbs torque, and that should give you at least 167 knots indicated at 1900 RPM and ISA+37 degrees. That's worst case.

 

As Joe says, without knowing what the Outside Air Temp and altimeter setting are, we can't really calculate True Airspeed or groundspeed.


Best Regards,

Kurt "Yoda" Kalbfleisch

Pinner, Middx, UK

Beta tester for PMDG J41, NGX, and GFO, Flight1 Super King Air B200, Flight1 Cessna Citation Mustang, Flight1 Cessna 182, Flight1 Cessna 177B, Aeroworx B200

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Was your flight configuation "clean"? Flaps at 0, trim optimized? You should be able to achieve speed that approaches the barber pole on the airspeed indicator. You will note that the barber pole retards to lower and lower IAS as you gain altitude or lose temperature. You naturally lose temperature as you climb. You can by-the-way do both if you both gain altitude and fly from warmer into colder weather.

 

Find the thread here on the C90B Pilot Operation Handbook (POH) and buy it ($5). It has a series of tables with fuel flow, torque, and IAS & TAS. The tables are presented at intervals of 10C beginning with +30C and ending with -30C, each referencing their respective temperatures at 10,000 ft. Each table contains data at 2,000 levels from 0 to 29,000. There are columns for CAS (calibrated or indicated airspeed) and TAS (True Airspeed) for three different levels of loaded weight. You use the table that corresponds closest to the forecast temperature at 10,000 ft.

 

Yesterday on a full-fuel load flight from Florida to Maryland I was at 20,000 with an outside air temp indicated at between -15C and -19C. I used a fuel flow of around 265 lbs per hour and torque setting (prop pitch controlled) of around 1120. That provided an indicated airspeed of 175 knots. Using my trusty E6B (electronic model) and solving for TAS with those variables returns a True Airspeed of 237 knots.

 

Think of your power setting not as a percentage but as a combination of fuel flow and torque (from the tables). Max recommended torque in the handbook is 1315 up to around 15,000, depending on temperature. Then torque falls off steadily up to the max altitude of 30,000.

 

The standard table in the POH (the one with no temp reference at the top) shows Fuel Flow ranging from 342 PPH (per engine; 684 total) at takeoff from MSL to 180 PPH (per engine; 360 total) at 30,000 level cruise. Take note that table starts at 19C at MSL in the IOAT column, which is about 66F. When flying in the C90B you see a clear readout of torque on the panel. You can get the best reading on fuel flow by pointing to the fuel flow meters with your mouse. That will provide a pop-up reading, assuming you haven't pulled back too far on your 3D panel view. I use the graduated markings on my Saitek throttle quadrant as my approximate guides, then fine tune my levers based on the gauges.

 

At the 15,000 altitude you were at, the typical power setting with a surface temp of +20C (68F) should have been around 300 FF and 1280 on the torque. That is extrapolated from the 14,000 and 16,000 settings in the POH for that temperature. OAT at 15,000 for a surface temp of +19C should have been around -10C as the Normal Lapse Rate is loss of 2C per 1,000 ft of altitude, or +20 - (15 X 2) = -10. At 15,000 the barber pole should likely have been in the mid 190's on IAS.


Frank Patton
MSI Z490 WiFi MOB;  i7 10700k Comet Lake 3.8 Ghz CPU; Ripjaws 32 gb DDR4 3600; ASUS GTX 1070 TI Turbo 8GB; MasterCase H500M; Corsair H100i Pro cooler; Corsair RMX850X PSU; ASUS VG289 4K 27"; Honeycomb Alpha Yoke A+.  Former USAF meteorologist & ground weather school instructor; AOPA Member #07379126 
                        There is no limit to what a man can do or where he can go if he doesn’t mind who gets the credit! - Benjamin Jowett

 

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Frank Patton
MSI Z490 WiFi MOB;  i7 10700k Comet Lake 3.8 Ghz CPU; Ripjaws 32 gb DDR4 3600; ASUS GTX 1070 TI Turbo 8GB; MasterCase H500M; Corsair H100i Pro cooler; Corsair RMX850X PSU; ASUS VG289 4K 27"; Honeycomb Alpha Yoke A+.  Former USAF meteorologist & ground weather school instructor; AOPA Member #07379126 
                        There is no limit to what a man can do or where he can go if he doesn’t mind who gets the credit! - Benjamin Jowett

 

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