Confusion with cruising performance of DV20 (Manifold pressure/RPM combinations)
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Confusion with cruising performance of DV20 (Manifold pressure/RPM combinations)
I'm getting thru DV20 AFM and the page with cruising performance chart is really confusing me.
1) What did they mean by saying increase/decrease "pressure altitude"? What if I fly below Transition altitude and use regional QNH instead of "pressure altitude" (aka QNE aka standard pressure of 1013mb)? How do I calculate how many feet do I need to add or subtract?
2) I also can't clearly understand what they wanted to say in the NOTE section. What is "economical cruise"? Why do I need to pick something between 2300-2200 RPM and a MP of 1-2 in.Hg below max possible at chosen altitude when I already have _the lowest Liters/Hour number_ from the table?
What is "fast cruise" ? Why do they say to select 2400 RPM and a MP that will be 0.7 in.Hg less than maximum possible MP to reduce fuel consumption? The chart is already showing that max possible MP on all altitudes (except 0 ft) will stick to 2400 RPM. I don't get it
Please, explain it to me!
1) What did they mean by saying increase/decrease "pressure altitude"? What if I fly below Transition altitude and use regional QNH instead of "pressure altitude" (aka QNE aka standard pressure of 1013mb)? How do I calculate how many feet do I need to add or subtract?
2) I also can't clearly understand what they wanted to say in the NOTE section. What is "economical cruise"? Why do I need to pick something between 2300-2200 RPM and a MP of 1-2 in.Hg below max possible at chosen altitude when I already have _the lowest Liters/Hour number_ from the table?
What is "fast cruise" ? Why do they say to select 2400 RPM and a MP that will be 0.7 in.Hg less than maximum possible MP to reduce fuel consumption? The chart is already showing that max possible MP on all altitudes (except 0 ft) will stick to 2400 RPM. I don't get it
Please, explain it to me!
Last edited by super jet 100; 8th Aug 2017 at 18:49.
Basic stuff!
Pressure altitude = effectively means measurements done at a certain pressure and temperature defined by the altitude (ie pressure) in a nominal standard atmosphere.
Economical/fast cruise - I hope you understand that when in the cruise an aircraft can either go faster and burn more fuel (fast cruise) or slower, burn less fuel and have a longer range (economical). The Paragraphs ringed in red are explaining the recommended power settings for these two options.
The paragraphs talking about ISA +-10 are telling you how to modify the values for different atmospheric conditions.
Pressure altitude = effectively means measurements done at a certain pressure and temperature defined by the altitude (ie pressure) in a nominal standard atmosphere.
Economical/fast cruise - I hope you understand that when in the cruise an aircraft can either go faster and burn more fuel (fast cruise) or slower, burn less fuel and have a longer range (economical). The Paragraphs ringed in red are explaining the recommended power settings for these two options.
The paragraphs talking about ISA +-10 are telling you how to modify the values for different atmospheric conditions.
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I'm getting thru DV20 AFM and the page with cruising performance chart is really confusing me.
1) What did they mean by saying increase/decrease "pressure altitude"? What if I fly below Transition altitude and use regional QNH instead of "pressure altitude" (aka QNE aka standard pressure of 1013mb)? How do I calculate how many feet do I need to add or subtract?
1) What did they mean by saying increase/decrease "pressure altitude"? What if I fly below Transition altitude and use regional QNH instead of "pressure altitude" (aka QNE aka standard pressure of 1013mb)? How do I calculate how many feet do I need to add or subtract?
As an example, it's winter, ISA - 10 degrees (so 5 degrees on the ground), you are looking at 65% power at 1000 feet MSL. However as we have already stated, the weather is ISA - 10 so we need to increase our pressure altitude by 500 feet so you would be cruising at an effective Pressure Altitude (for performance calculations) of 1500 feet. The difference in fuel flow between sea level and 2000 feet MSL is 0.3lph so you would extrapolate the difference and cruise at 2100 RPM, 13.2 LPH and 24,4 inches MAP (24.7 minus 0.3 due to ISA - 10), you would then be flying with 65% power.
2) I also can't clearly understand what they wanted to say in the NOTE section. What is "economical cruise"? Why do I need to pick something between 2300-2200 RPM and a MP of 1-2 in.Hg below max possible at chosen altitude when I already have _the lowest Liters/Hour number_ from the table?
What is "fast cruise" ? Why do they say to select 2400 RPM and a MP that will be 0.7 in.Hg less than maximum possible MP to reduce fuel consumption? The chart is already showing that max possible MP on all altitudes (except 0 ft) will stick to 2400 RPM. I don't get it
Please, explain it to me!
What is "fast cruise" ? Why do they say to select 2400 RPM and a MP that will be 0.7 in.Hg less than maximum possible MP to reduce fuel consumption? The chart is already showing that max possible MP on all altitudes (except 0 ft) will stick to 2400 RPM. I don't get it
Please, explain it to me!
Economical cruise - fewer bangs per minute, slower speed. Here you would choose an RPM to suit you - vibrations, noise level etc. Certain planes have certain engine rpms which 'feel' smoother - you just choose which fits best from that range.
Fast Cruise - best airspeed - so 2400 RPM.
The table attached shows normal cruise settings.
Concerning MP - when you adjust the throttle, you are restricting the flow of air into the cylinder. So they are recommending you allow less air into the cylinders with economical cruise whereas with fast cruise, they are suggesting you open the throttle wider to ensure max airflow into the cylinder. More air needs more fuel.... so your consumption would be higher....
