Prop RPM and Torque
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Prop RPM and Torque
I would just like some opinions on this matter.
If your engine is developing max torque, (turbo-props) and you are not getting max prop rpm, would I be correct in saying that you will not be getting max thrust available?
Now, should your good engine and prop fail on take off, and having reduced rpm on the otherside, would this lead to reduced single engine climb performance?
I have my own ideas on this matter, but I would like to hear from others on this point before entering into a perhaps heated 'debate'.
Thanks....
If your engine is developing max torque, (turbo-props) and you are not getting max prop rpm, would I be correct in saying that you will not be getting max thrust available?
Now, should your good engine and prop fail on take off, and having reduced rpm on the otherside, would this lead to reduced single engine climb performance?
I have my own ideas on this matter, but I would like to hear from others on this point before entering into a perhaps heated 'debate'.
Thanks....
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Pitch?
Depends on prop pitch. What is the position of the pitch levers (fine/feathered?)
Will also depend on atmospheric conditions of the day/location.
Have a chat with your friendly engineer who will have a set of charts which will show expected minimum torque/rpm figures for various atmospheric conditions
Cheers
Will also depend on atmospheric conditions of the day/location.
Have a chat with your friendly engineer who will have a set of charts which will show expected minimum torque/rpm figures for various atmospheric conditions
Cheers
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Power curves etc
Max power available = max torque & max prop rpm
EFATO: Fly initially at V2 with max power available until flap retract speed.........after 5 minutes [this may vary] set Torque/ITT for max continous power.
Reduce TQ and/or prpm = reduced power.
For your heated debate, switch to the TECH forum.
EFATO: Fly initially at V2 with max power available until flap retract speed.........after 5 minutes [this may vary] set Torque/ITT for max continous power.
Reduce TQ and/or prpm = reduced power.
For your heated debate, switch to the TECH forum.
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Thanks for replies, but perhaps I did not explain the situation correctly.
If you have full fine pitch set on prop levers and you are not getting max rpm on a prop, is it correct to say that we are not getting max thrust from that prop?
The reduced rpm is not by choice, but a problem somewhere...so when the good side fails and you are left with an engine which is delivering max torque to the prop, but is not getting max rpm, is performance degraded?
If you have full fine pitch set on prop levers and you are not getting max rpm on a prop, is it correct to say that we are not getting max thrust from that prop?
The reduced rpm is not by choice, but a problem somewhere...so when the good side fails and you are left with an engine which is delivering max torque to the prop, but is not getting max rpm, is performance degraded?
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In a nutshell - yes.
Max power would require, amongst other things, max prpm.
If you cannot achieve max prpm with "props to max", then the appropriate tech log entry is required.
With say, Performance E aircraft, single engine performance is probably at best 200ft ROC. You need every bit of power to survive at EFATO.
This came home to me big time when I use to do C of A renewal flight tests on Seneca IIIs - the SE 5 minute climb was often interesting . And that was on a nice day!!
Max power would require, amongst other things, max prpm.
If you cannot achieve max prpm with "props to max", then the appropriate tech log entry is required.
With say, Performance E aircraft, single engine performance is probably at best 200ft ROC. You need every bit of power to survive at EFATO.
This came home to me big time when I use to do C of A renewal flight tests on Seneca IIIs - the SE 5 minute climb was often interesting . And that was on a nice day!!
Last edited by ecj; 24th Jun 2003 at 15:43.
Props are for boats!
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Max Torque plus Max Rpm equals MAX POWER.
Which is the main ingrediant for Performance, with both engines running, and especially in assymetric recovery. Max Power is Necessary for Best Performance. VMCA is defined with this very point, combined with corrrect control inputs and configeration.
What are you driving South Coast? Have a chat with your Mechanic, and tell him/her about it and put an entry in the Tech log. Go and do a ground run with him/her to demonstrate the problem. If you are not happy Ground it Bud.
Regards
Sheep
Which is the main ingrediant for Performance, with both engines running, and especially in assymetric recovery. Max Power is Necessary for Best Performance. VMCA is defined with this very point, combined with corrrect control inputs and configeration.
What are you driving South Coast? Have a chat with your Mechanic, and tell him/her about it and put an entry in the Tech log. Go and do a ground run with him/her to demonstrate the problem. If you are not happy Ground it Bud.
Regards
Sheep
I'm not convinced it's that simple.
Power = thrust x TAS.
Therefore Thrust = Power / TAS
Therefore Thrust = Power / (CAS / SQRT(relative density))
The power curve will vary with airspeed and density, so the relationship will be a fairly complex one. I wouldn't be surprised if best thrust is achieved below max RPM under certain circumstances, since in many aircraft types max RPM means some local supersonic flow at the tips and an associated loss of propeller efficiency.
Also VMCA is not defined by climb rate, it is the minimum speed at which a given level of control can be obtained - no more than 20° heading change following sudden failure of the critical engine, no more than 5° bank when trimmed for straight flight subsequently, the ability to roll at-least 20° each way, and yaw 15° both ways with no more than 150lbf foot-force.
The SEI / MEI best climb speed is a separate concept to VMCA but obviously will be no lower than VMCA (which itself incidentally isn't allowed to be less than 1.2Vs).
G
Power = thrust x TAS.
Therefore Thrust = Power / TAS
Therefore Thrust = Power / (CAS / SQRT(relative density))
The power curve will vary with airspeed and density, so the relationship will be a fairly complex one. I wouldn't be surprised if best thrust is achieved below max RPM under certain circumstances, since in many aircraft types max RPM means some local supersonic flow at the tips and an associated loss of propeller efficiency.
Also VMCA is not defined by climb rate, it is the minimum speed at which a given level of control can be obtained - no more than 20° heading change following sudden failure of the critical engine, no more than 5° bank when trimmed for straight flight subsequently, the ability to roll at-least 20° each way, and yaw 15° both ways with no more than 150lbf foot-force.
The SEI / MEI best climb speed is a separate concept to VMCA but obviously will be no lower than VMCA (which itself incidentally isn't allowed to be less than 1.2Vs).
G
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Our Metro 11 has an ITT limit of 885 degrees C with the RPM back at 96%. That means maximum power available is climb power with these reduced revs.
Compare this with Max take off power (5 mins) 100% RPM, max ITT 923 degrees & torque 2363 ft lbs.
or max continuous power at 100% RPM maximum of ITT 923 degrees & torque 2206 ft lbs.
So with lower than 100% available RPM you will cook the turbines at lower temps with corresponding lower available power.
Put it another way the slower the airflow through the turbine engine the less power is available.
I hope this helps
Compare this with Max take off power (5 mins) 100% RPM, max ITT 923 degrees & torque 2363 ft lbs.
or max continuous power at 100% RPM maximum of ITT 923 degrees & torque 2206 ft lbs.
So with lower than 100% available RPM you will cook the turbines at lower temps with corresponding lower available power.
Put it another way the slower the airflow through the turbine engine the less power is available.
I hope this helps
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Shaft power output of your engine is equal to Torque x 2Pi x RPM. If the advertised max take-off power conditions for your aircraft are power lever and RPM lever fully forward, giving max torque and max RPM, then this is what you should get, provided the atmospheric conditions are as specified in your manual. But any changes in density altitude will change the power output.
So if you have the power lever and RPM levers fuly forward and cannot get max torque and max RPM, then either you really are getting less than max power output, or one or more of your gauges are under reading.
If the problem is one of reduced power being generated by the engine, it would most probably result in reduced torque rather than reduced RPM. On the other hand if your engine is producing advertised power at a lower RPM, then the torque should be greater than advertised.
Genghis is correct about the difficulties of predicting what RPM will give best thrust. But if the power going into the prop is reduced, then the thrust output at any given RPM TAS or atmospheric conditions will also be reduced.
So the short answer to your question is yes performance will be reduced.
So if you have the power lever and RPM levers fuly forward and cannot get max torque and max RPM, then either you really are getting less than max power output, or one or more of your gauges are under reading.
If the problem is one of reduced power being generated by the engine, it would most probably result in reduced torque rather than reduced RPM. On the other hand if your engine is producing advertised power at a lower RPM, then the torque should be greater than advertised.
Genghis is correct about the difficulties of predicting what RPM will give best thrust. But if the power going into the prop is reduced, then the thrust output at any given RPM TAS or atmospheric conditions will also be reduced.
So the short answer to your question is yes performance will be reduced.
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Keith Williams is absolutely right....
Power formula for the PT6-A42 (KingAir B200)
SHP = torque X RPM X k
850 shp = 2230 ft/lbs X 2000 RPM X 0.00019058
20 rev less will mean 10 SHP less, 100 revs will cost 42 SHP
300 less (cruise for the B200) and you get 128 SHP less output.
Ask your local checker to try it on your next checkride:
If you do stall recovery with props, say in cruise pitch,instead of high rpm, youŽll need much longer to fly out of it.
(One of the mistakes one sees very often during checkrides is the forgotten prop lever on the working engine. )
Power formula for the PT6-A42 (KingAir B200)
SHP = torque X RPM X k
850 shp = 2230 ft/lbs X 2000 RPM X 0.00019058
20 rev less will mean 10 SHP less, 100 revs will cost 42 SHP
300 less (cruise for the B200) and you get 128 SHP less output.
Ask your local checker to try it on your next checkride:
If you do stall recovery with props, say in cruise pitch,instead of high rpm, youŽll need much longer to fly out of it.
(One of the mistakes one sees very often during checkrides is the forgotten prop lever on the working engine. )
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Seems to be crossed comments between T/O and climb.
On Take Off the the RPM is a function of the CSU
On climb the RPM is a function of the pitch lever.
On take off the prop should not be in fine pitch - it is constant speeding. (Thats how Max RPM is adjusted.)
The original question regarding a low Rpm on take off could be a prop adjustment problem or if the prop is fully fine ( i.e. unable to adjust up) then its most probably and engine power problem
On Take Off the the RPM is a function of the CSU
On climb the RPM is a function of the pitch lever.
On take off the prop should not be in fine pitch - it is constant speeding. (Thats how Max RPM is adjusted.)
The original question regarding a low Rpm on take off could be a prop adjustment problem or if the prop is fully fine ( i.e. unable to adjust up) then its most probably and engine power problem
