Post by acourt on Dec 4, 2011 23:57:36 GMT
Mark,
Gosh y'all are defensive sometimes I never said there was anything wrong with the HJG panel. In fact, it should be a complement that it's so accurate, and that actual procedures can be used. Here's what I was talking about...
One easy method of reducing takeoff power (again, to save wear and fuel) is to simply use performance numbers from a lower power variant of the same engine. The example I gave was for a DC-9-10 with JT8D-1B engines. Some carriers (I'm working from Eastern Air Lines data in this case) elected to use power settings for the JT8D-5 engine when able. Basically all that was required was to not push the throttles as far forward. There was no mechanical change made to the airplane, the engines, or (in most cases) the RAT/EPR Indicator. Takeoff performance charts were provided for both power settings, and Dash 1 power was always available if necessary.
With approval, a DC-9-31 with -9 engines could use the power settings and performance from a DC-9-31 with -7 engines. You could even have one engine of each type (a DC-9-51 with a -15 engine on the left side, and a -17 on the right side, for example). This configuration is referred to as an "intermix," and requires use of the lower rated engine's power and performance.
My first airline had two DC-9-81s. Both were fitted with JT8D-217 or -219 engines. But for contract and cost sake, they were operated at -209 power.
So with no change to the sim, aircraft, or panel, I may elect to use the -5 power settings with my -1 engined airplane. The performance works out just right. Again, it's a testament to how well the HJG DC-9 works.
Chris,
One of the many variables involved in determining takeoff v-speeds is thrust and acceleration. FAR Part 25.107 deals with determining these speeds, and is about as clear and concise as every other FAR. One of the main points is that "the takeoff decision speed [V1] is selected by the applicant." Right off the bat, different carriers may select different speeds. Again, I'm working with EAL data. Yours may be different.
When you change power settings, VMC changes, as does VMU (minimum unstick) and a host of other variables. But here's an example of how a speed may increase with reduced power...
VR may not be less than "the speed...that allows reaching V2 before reaching a height of 35 feet above the takeoff surface..." If I'm using reduced power, my acceleration is reduced. But I'm still required to reach V2 before 35 feet AFL. The fix is to arbitrarily increase VR so that I unstick at a higher speed and have less speed to gain before 35 feet AFL. This is possible if you aren't field length limited.
This evening I took the opportunity to try it for myself to make sure I'm not crazy. I ran our performance data first for maximum power, then for standard FLEX power, and then a TO2 derate. In every case, the numbers changed by at least one or two knots. This was at West Palm Beach, in a 717 with BR715-C1 engines.
When you reduce power, I can't say for certain whether the new speeds will be higher or lower. There's too many variables, and we don't get to see the math. All I know is that they will almost always change.
Gosh y'all are defensive sometimes I never said there was anything wrong with the HJG panel. In fact, it should be a complement that it's so accurate, and that actual procedures can be used. Here's what I was talking about...
One easy method of reducing takeoff power (again, to save wear and fuel) is to simply use performance numbers from a lower power variant of the same engine. The example I gave was for a DC-9-10 with JT8D-1B engines. Some carriers (I'm working from Eastern Air Lines data in this case) elected to use power settings for the JT8D-5 engine when able. Basically all that was required was to not push the throttles as far forward. There was no mechanical change made to the airplane, the engines, or (in most cases) the RAT/EPR Indicator. Takeoff performance charts were provided for both power settings, and Dash 1 power was always available if necessary.
With approval, a DC-9-31 with -9 engines could use the power settings and performance from a DC-9-31 with -7 engines. You could even have one engine of each type (a DC-9-51 with a -15 engine on the left side, and a -17 on the right side, for example). This configuration is referred to as an "intermix," and requires use of the lower rated engine's power and performance.
My first airline had two DC-9-81s. Both were fitted with JT8D-217 or -219 engines. But for contract and cost sake, they were operated at -209 power.
So with no change to the sim, aircraft, or panel, I may elect to use the -5 power settings with my -1 engined airplane. The performance works out just right. Again, it's a testament to how well the HJG DC-9 works.
Chris,
One of the many variables involved in determining takeoff v-speeds is thrust and acceleration. FAR Part 25.107 deals with determining these speeds, and is about as clear and concise as every other FAR. One of the main points is that "the takeoff decision speed [V1] is selected by the applicant." Right off the bat, different carriers may select different speeds. Again, I'm working with EAL data. Yours may be different.
When you change power settings, VMC changes, as does VMU (minimum unstick) and a host of other variables. But here's an example of how a speed may increase with reduced power...
VR may not be less than "the speed...that allows reaching V2 before reaching a height of 35 feet above the takeoff surface..." If I'm using reduced power, my acceleration is reduced. But I'm still required to reach V2 before 35 feet AFL. The fix is to arbitrarily increase VR so that I unstick at a higher speed and have less speed to gain before 35 feet AFL. This is possible if you aren't field length limited.
This evening I took the opportunity to try it for myself to make sure I'm not crazy. I ran our performance data first for maximum power, then for standard FLEX power, and then a TO2 derate. In every case, the numbers changed by at least one or two knots. This was at West Palm Beach, in a 717 with BR715-C1 engines.
When you reduce power, I can't say for certain whether the new speeds will be higher or lower. There's too many variables, and we don't get to see the math. All I know is that they will almost always change.