Post by Tom Goodrick on Sept 22, 2010 9:31:48 GMT -5
I used FS2002 on my laptop to run these tests. The performance of this model is reasonable but it not identical to the model I have in FS9 and certainly may perform differently compared to the real aircraft. A few years ago we had an LJ45 pilot advising us on our model. But the sim has changed since then and I cannot be sure that the model used here is sufficiently accurate to represent real performance. But the real performance is known to have features like this though the detail values shown may differ.
The "Coffin Corner" for a jet is at the completion of climb to a high altitude where the thrust is so diminished that the aircraft cannot maintain enough airspeed to avoid a stall during the transition to level flight for cruise.
I loaded up the LJ45 with 8 x 200 lb pax and 400 lb of bags and did two tests. First I wanted to determine the stall condition for this exact loading. Then I flew it from takeoff directly to 45,000 ft where it did indeed expeirence stall. Note that many climb methods call for a level off somewhere near FL300 to gain speed before continuing the climb. Also, it is clear that I could have leveled off safely at FL400 or 410 and gained full cruise speed before continuing the climb. The LJ45 has an operating altitude of 51,000 ft under current certification. But that does not mean that you can climb directly to that altitude under all loading conditions.
Below are the data for the stall test showing angle of attack versus indicated airspeed during a deceleration to stall at FL150.
Note that the stall occurred at 105 KIAS when AoA began increasing quickly and passed 16 degrees. Recovery took a few thousand feet. The autopilot was used for this test until stall occurred. I did try a 120 degree turn on autopilot at 120 KIAS and it was successfully completed.
This shows the data during the climb from takeoff.
The climb method used is an approximation of an optimal climb method in which, after passing FL100 with its speed restriction, we accelerate to a high speed, 300 KIAS in this case and then resume a high climb rate. Climb rate is used to control the airspeed while operating on autopilot in altitude mode with thrust at a high level. The optimization is to minimize fuel cost while minimizing the time taken. This method works well for a wide range of aircraft. A similar method is often used with the 747. But different companies have their prefferred optimizations. One or two methods are normally programmed into the Flight Management Computers (FMC's).
The "Coffin Corner" for a jet is at the completion of climb to a high altitude where the thrust is so diminished that the aircraft cannot maintain enough airspeed to avoid a stall during the transition to level flight for cruise.
I loaded up the LJ45 with 8 x 200 lb pax and 400 lb of bags and did two tests. First I wanted to determine the stall condition for this exact loading. Then I flew it from takeoff directly to 45,000 ft where it did indeed expeirence stall. Note that many climb methods call for a level off somewhere near FL300 to gain speed before continuing the climb. Also, it is clear that I could have leveled off safely at FL400 or 410 and gained full cruise speed before continuing the climb. The LJ45 has an operating altitude of 51,000 ft under current certification. But that does not mean that you can climb directly to that altitude under all loading conditions.
Below are the data for the stall test showing angle of attack versus indicated airspeed during a deceleration to stall at FL150.
Note that the stall occurred at 105 KIAS when AoA began increasing quickly and passed 16 degrees. Recovery took a few thousand feet. The autopilot was used for this test until stall occurred. I did try a 120 degree turn on autopilot at 120 KIAS and it was successfully completed.
This shows the data during the climb from takeoff.
The climb method used is an approximation of an optimal climb method in which, after passing FL100 with its speed restriction, we accelerate to a high speed, 300 KIAS in this case and then resume a high climb rate. Climb rate is used to control the airspeed while operating on autopilot in altitude mode with thrust at a high level. The optimization is to minimize fuel cost while minimizing the time taken. This method works well for a wide range of aircraft. A similar method is often used with the 747. But different companies have their prefferred optimizations. One or two methods are normally programmed into the Flight Management Computers (FMC's).