Turboprop Torque as power indication????
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Been reading and re-reading all the helpful and interesting contributions. I now find myself struggling on another aspect:
When you complete the climb and level off in the cruise, I assume you don't reduce the "power" levers, rather you pitch to level, accelerate, and ONLY reduce your propeller condition?? Compressor sits around 96% most of time as pointed out.
(And) If you are more or less always at 96% (ish) the free/power turbine is always being subjected to a lot of energy isn't it? Surely 96% + power turbine + 1390 RPM means you are always going to be a "max speed and power."
Hope my puzzlement makes sense! Am I looking too deep?
When you complete the climb and level off in the cruise, I assume you don't reduce the "power" levers, rather you pitch to level, accelerate, and ONLY reduce your propeller condition?? Compressor sits around 96% most of time as pointed out.
(And) If you are more or less always at 96% (ish) the free/power turbine is always being subjected to a lot of energy isn't it? Surely 96% + power turbine + 1390 RPM means you are always going to be a "max speed and power."
Hope my puzzlement makes sense! Am I looking too deep?
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Basically the right idea, pull back prop rpm for cabin noise, and at cruise altitude, the power turbine can absorb all the available gas energy from the gas generator. Just watch Ng & ITT limits.
Not untrue, but mind the engine limitations in this process. Looking at a free turbine engine here.
Consider what the power and condition levers do to the engine - the power lever basically governs the gas generator speeds (type dependent, it normally has direct control on propeller pitch in beta mode as well), while the condition lever sets the RPM the propeller is attempting to maintain (again, type dependent, it usually also governs the fuel shutoff valve for starting and shutting down the engine).
And these two are very much interdependent. Remember the formula "Power = Torque * RPM" - this time let me change it to Torque = Power / RPM to hopefully make it clearer. Now let the gas generator run at constant 96% Nh and pull back the condition lever - as the power provided from the gas generator is constant but the propeller is spinning at a lower RPM, You will see the torque value skyrocket, likely exceeding the engines torque limitation. Also, You will find the exhaust gas temperature (whatever it may be named in Your engine) to have risen - when the power turbine is spinning slower, it will exert a greater drag on the gas flow, causing the temperatures to rise. One might simplify it and say that the ability of the prop to digest the engines power is dependent on its RPM - the higher the RPM, the more power it can accept from the gas generator.
So normally, reducing the propeller RPM will entail also reducing the gas generator speed in order not to overtorque the engine. On the DH8-300, setting climb power was normally done using both hands, one on the power and one on the condition levers, as just pulling back the condition lever with the engines delivering normal takeoff power at 1200rpm and 95% torque would immediately have overtorqued them.
Consider what the power and condition levers do to the engine - the power lever basically governs the gas generator speeds (type dependent, it normally has direct control on propeller pitch in beta mode as well), while the condition lever sets the RPM the propeller is attempting to maintain (again, type dependent, it usually also governs the fuel shutoff valve for starting and shutting down the engine).
And these two are very much interdependent. Remember the formula "Power = Torque * RPM" - this time let me change it to Torque = Power / RPM to hopefully make it clearer. Now let the gas generator run at constant 96% Nh and pull back the condition lever - as the power provided from the gas generator is constant but the propeller is spinning at a lower RPM, You will see the torque value skyrocket, likely exceeding the engines torque limitation. Also, You will find the exhaust gas temperature (whatever it may be named in Your engine) to have risen - when the power turbine is spinning slower, it will exert a greater drag on the gas flow, causing the temperatures to rise. One might simplify it and say that the ability of the prop to digest the engines power is dependent on its RPM - the higher the RPM, the more power it can accept from the gas generator.
So normally, reducing the propeller RPM will entail also reducing the gas generator speed in order not to overtorque the engine. On the DH8-300, setting climb power was normally done using both hands, one on the power and one on the condition levers, as just pulling back the condition lever with the engines delivering normal takeoff power at 1200rpm and 95% torque would immediately have overtorqued them.
Last edited by Tu.114; 11th Aug 2012 at 12:12.
Hey there wangus, hopefully what we do with the CT-7 may be of help. What aircraft are you studying? Our procedures have the condition levers/prop RPM at max only for takeoff, landing, goaround and abnormal situations where maximum power is required. Obviously this is all type and SOP-dependant, but our procedures are takeoff, prop RPM at max (1396) and power levers advanced to set slightly below the required torque with the CTOT system (constant torque on takeoff) adding extra fuel to maintain the constant commanded power level for the takeoff run. At 1000 ft agl we set climb power which involves switching off the CTOT system, reduce power if necessary to a safe level and then move the condition levers to MIN (1230 PRPM) - this is to avoid the type of situation that Tu.114 describes where simply pulling the condition levers to MIN from takeoff power would over-torque or over-temp the engines. After that then manually advance power levers to the required torque. From there to configuring the aircraft for landing we wont touch the condition levers in normal operations.
When I level out and determine the required power from the power chart, typically I dont need to change the power by more than a few percent from the climb setting, we're just going from power taking us up and across to power taking us across a bit faster. Another aside - as the aircraft speeds up from maybe 150 KIAS to ~220 KIAS (or whatever you get on the day) there is a ram effect as, due to airspeed, more air enters the intake, and you might need to reduce the power levers by a couple of percent to maintain the specified torque level. At 1230 PRPM you wont be at max power there. An example might be cruising at 76% torque, 1230 PRPM, 96% Ng and 800 degrees ITT, but the condition levers have been at MIN since 1000 ft AGL. Your procedures may vary of course!
When I level out and determine the required power from the power chart, typically I dont need to change the power by more than a few percent from the climb setting, we're just going from power taking us up and across to power taking us across a bit faster. Another aside - as the aircraft speeds up from maybe 150 KIAS to ~220 KIAS (or whatever you get on the day) there is a ram effect as, due to airspeed, more air enters the intake, and you might need to reduce the power levers by a couple of percent to maintain the specified torque level. At 1230 PRPM you wont be at max power there. An example might be cruising at 76% torque, 1230 PRPM, 96% Ng and 800 degrees ITT, but the condition levers have been at MIN since 1000 ft AGL. Your procedures may vary of course!
