Over Torquing
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well nice of the judge and jurors there to cast their sentence. Nothing got over torqued. Was just looking for an interesting conversation which some participated in. 
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handbag,
With 2 spool turboshafts that have the power turbine in the back of the engine (ie. the opposite end from the PTO), it's usually very easy to damage the power turbine shaft with torsional overload. The reason is that the power turbine shaft in these engines tends to be very slender due to space limitations, and thus usually has very small stress margins in torsion. To avoid this issue, small turboshaft engines sometimes put the power turbine at the front of the engine (like the Rolls/Allison 250).
Gearboxes/sprag clutches/TR drives usually are designed more robustly than the engine. So these components are less likely to be damaged by transient overtorques. The only exception that comes to mind is if a helo has undergone a significant engine power upgrade with the existing gearbox, where the only thing preventing gearbox overtorque is the pilot and/or FADEC. This type of engine upgrade would be done to improve performance at hot/high conditions, but could also cause overtorque at SL conditions if not carefully controlled.
Hope that helps to answer your question.
riff_raff
With 2 spool turboshafts that have the power turbine in the back of the engine (ie. the opposite end from the PTO), it's usually very easy to damage the power turbine shaft with torsional overload. The reason is that the power turbine shaft in these engines tends to be very slender due to space limitations, and thus usually has very small stress margins in torsion. To avoid this issue, small turboshaft engines sometimes put the power turbine at the front of the engine (like the Rolls/Allison 250).
Gearboxes/sprag clutches/TR drives usually are designed more robustly than the engine. So these components are less likely to be damaged by transient overtorques. The only exception that comes to mind is if a helo has undergone a significant engine power upgrade with the existing gearbox, where the only thing preventing gearbox overtorque is the pilot and/or FADEC. This type of engine upgrade would be done to improve performance at hot/high conditions, but could also cause overtorque at SL conditions if not carefully controlled.
Hope that helps to answer your question.
riff_raff
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HANDBAG
As already alluded to, depends on amount of torque pulled.
Most airframes have at least three limitations.
Take off power - up to 5 minutes at approximately 5 - 10% over 100% power.
Max continuous.
And transient as stipulated in the POH.
The engine itself has torque limitations but I think you would have overspeed or overtemp problems prior to reaching it.
RIFFRAFF
Were did you learn your helicopter maintenance theory?
It all sounds the opposite to what I was taught.
Torque limits generally pertain to the airframe NOT the engine, it has to be a massive over-torque before engine maintenance has to be carried out.
Reverse flow engines are used as it makes the engine shorter in length.
Cheers
BH
As already alluded to, depends on amount of torque pulled.
Most airframes have at least three limitations.
Take off power - up to 5 minutes at approximately 5 - 10% over 100% power.
Max continuous.
And transient as stipulated in the POH.
The engine itself has torque limitations but I think you would have overspeed or overtemp problems prior to reaching it.
RIFFRAFF
Were did you learn your helicopter maintenance theory?
It all sounds the opposite to what I was taught.
Torque limits generally pertain to the airframe NOT the engine, it has to be a massive over-torque before engine maintenance has to be carried out.
Reverse flow engines are used as it makes the engine shorter in length.
Cheers
BH
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Ah, but Blackhand I think you are wrong in this statement:
"Take off power - up to 5 minutes at approximately 5 - 10% over 100% power."
I don't know of helicopters can allow this period of time over 100%, but I stand to be corrected. I think you might be referring to transient limits that are generally around a few seconds duration, and usually not permitted for intentional use.
"Take off power - up to 5 minutes at approximately 5 - 10% over 100% power."
I don't know of helicopters can allow this period of time over 100%, but I stand to be corrected. I think you might be referring to transient limits that are generally around a few seconds duration, and usually not permitted for intentional use.
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In the models I've flown, torque is a transmission parameter, not an engine parameter. In most models, the engine(s) is capable of delivering much more torque than the transmission is rated for, at well below any engine limits, such as temp or N1, at least at or near sea level. Exceeding torque limits requires inspection of the transmission and drive train. Replacement of components may be mandatory, depending on the torque recorded. For most exceedances, it's just an inspection, and a few beans may be removed from the can.
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Ah - limitations!
They are complicated, and often not clearly explained. But you need to obey them, as they affect the airworthiness of the helicopter.
Transients are there only to keep you out of trouble. Using them on a regular basis is never a good idea - in fact in flight testing for certification, no credit can be given for transients.
Takeoff power is not always 100% - sometimes more, sometimes less. Whatever the number is, obey it.
And riff raff - I don't know any free turbine engine that has the power turbine ahead of the compressor turbine in the flow. The RR 250 series has the turbines arranged that way for reasons best known to the original Allison engineers - all long since retired.
And something for all to consider - if there is no equivalent engine indications of high power (such as N1 or TOT), is it really an over-torque? Remember that the torque meter is measuring drag on the blades, and a sharp increase in drag on the blades can cause a large change in torque - without the engine producing the equivalent power...
All the more reason to have a monitoring system like Intellistart if you don't have a FADEC / DEC.
They are complicated, and often not clearly explained. But you need to obey them, as they affect the airworthiness of the helicopter.
Transients are there only to keep you out of trouble. Using them on a regular basis is never a good idea - in fact in flight testing for certification, no credit can be given for transients.
Takeoff power is not always 100% - sometimes more, sometimes less. Whatever the number is, obey it.
And riff raff - I don't know any free turbine engine that has the power turbine ahead of the compressor turbine in the flow. The RR 250 series has the turbines arranged that way for reasons best known to the original Allison engineers - all long since retired.
And something for all to consider - if there is no equivalent engine indications of high power (such as N1 or TOT), is it really an over-torque? Remember that the torque meter is measuring drag on the blades, and a sharp increase in drag on the blades can cause a large change in torque - without the engine producing the equivalent power...
All the more reason to have a monitoring system like Intellistart if you don't have a FADEC / DEC.
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Shawn Coyle-
"The RR 250 series has the turbines arranged that way for reasons best known to the original Allison engineers"
The torsional strength, stiffness and low critical speed of a long, small diameter driveshaft passing from an aft mounted power turbine to a front end PTO presents a very difficult design challenge. Take a look at the lengths Turbomeca went through to avoid the issue. They located the PTO gear drive where it is surrounded by hot exhaust duct:
Pratt & Whitney took the same approach as "those long gone Allison engineers" with their PT6. They put the power turbine in front:
I was working on the design of a turboshaft drivetrain a while back. The engineers at the engine company were very nervous about any torsional vibrations or dynamic loads in our gearbox because their drive shaft only had an ultimate torsional MoS of 50% at MRP.
Regards,
riff_raff
"The RR 250 series has the turbines arranged that way for reasons best known to the original Allison engineers"
The torsional strength, stiffness and low critical speed of a long, small diameter driveshaft passing from an aft mounted power turbine to a front end PTO presents a very difficult design challenge. Take a look at the lengths Turbomeca went through to avoid the issue. They located the PTO gear drive where it is surrounded by hot exhaust duct:
Pratt & Whitney took the same approach as "those long gone Allison engineers" with their PT6. They put the power turbine in front:
I was working on the design of a turboshaft drivetrain a while back. The engineers at the engine company were very nervous about any torsional vibrations or dynamic loads in our gearbox because their drive shaft only had an ultimate torsional MoS of 50% at MRP.
Regards,
riff_raff
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Hi Shawne
was thinking of long ranger 5 min take off power.
Riff-Raff
depends how one looks at it. The power turbine on all the engines I know about are "after" the gas producer relative to airflow.
As far as reverse flow turboshaft engines are concerned, the power turbine may be in front of th GP turbine physically but not relative to airflow.
BH
was thinking of long ranger 5 min take off power.
Riff-Raff
depends how one looks at it. The power turbine on all the engines I know about are "after" the gas producer relative to airflow.
As far as reverse flow turboshaft engines are concerned, the power turbine may be in front of th GP turbine physically but not relative to airflow.
BH
