cooldowns
Not sure about turning the blades backward, but nowadays most of the engines are of free turbine type, meaning that you will turn the free turbine backward, not the gas generator which is subject to the internal high temp and drives all accessories unlike the free turbine.
My 2 cents.
My 2 cents.
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In a Rolls Royce engine there is no benefit at all. In fact you do more harm than good. I have a graph from Rolls which shows that after 2 minutes things start to heat back up again. If more computer savvy I'd post it for you. So in your 407 2 minutes is it. They don't spend millions of dollars on research in these companies to have someone go "well 3 must be better than 2!" Can't answer for your Astar though.
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Sorry, I am not an engineer so this is just my understanding. It seems to me that if you are not shutting down, only waiting, then you have oil pressure and flow. As I undertand it the problem arises when you shut down, lose oil pressure and oil flow, and then excess heat will cook the oil remaining.
If that is right then sitting at IDLE with the intention of returning to FLIGHT should not cause a problem. Or did I just misunderstand the problem?
John
If that is right then sitting at IDLE with the intention of returning to FLIGHT should not cause a problem. Or did I just misunderstand the problem?
John
Horses for courses.
RR Allison -
An Allison will probably sit there all day at IDLE with no detrimental effect.
The run down time can commence from flat pitch. i.e from stable lowest TOT. I am always surprised to see people doing ground runs for TR balance etc doing a 2 minute rundown. Unless you had been flying - why?
The issue is the temperature in the scavenge which is in the lower support strut of the 6 & 7 bearing. If this is too hot and you have no flow of oil the remaining residue will turn to carbon and over time block the strut and subsequently the oil will try and exit via the labyrinth seals and into the gas flow or onto the hub of the turbine wheels quenching the wheel unevenly. This phenomena is cumulative. NOT GOOD. Much of this treatment and do not be surprised to see the wheel and/or or blades exit the engine. Think Hand Grenade.
Carbon in an Allison can block the small lubricating jets as well. These jets are designed around a high oil pressure which the Allison incidentally only needs to make the torquemeter work, and are very small and can block very easily.
TM -
The Arriel originally had no concept of IDLE. The issue in the TM is that cool down has found to be beneficial but the oil system doesn't really work well at idle so you end up with low flow. After about 30 seconds the temperature in an Arriel can do all sorts of unpredictable things. It is a trade off of cooldown and low flow. IDLE for long periods of time is not real smart in an Arriel due to low flow and poor scavenging. If you have idled for a period of time run the engine back up to flight then shut it off. Different brands of oil can even make a difference. (Note - do NOT mix different brands of oil in a turbine). Sometimes even batches of the same brand can cause issues.
PWC 200 - PWC206B - EC135P1 30 seconds. PWC206B2 - EC135P2 and up - none required. These engines are a later design and have a lot of the pitfalls of earlier engines removed at the drawing board stage.
Above all - operate in accordance with the approved and current version of the RFM and service data. Any questions call your local manufacturers tech rep. Thats what they are for and it is free. Good support here
RR Allison -
An Allison will probably sit there all day at IDLE with no detrimental effect.
The run down time can commence from flat pitch. i.e from stable lowest TOT. I am always surprised to see people doing ground runs for TR balance etc doing a 2 minute rundown. Unless you had been flying - why?
The issue is the temperature in the scavenge which is in the lower support strut of the 6 & 7 bearing. If this is too hot and you have no flow of oil the remaining residue will turn to carbon and over time block the strut and subsequently the oil will try and exit via the labyrinth seals and into the gas flow or onto the hub of the turbine wheels quenching the wheel unevenly. This phenomena is cumulative. NOT GOOD. Much of this treatment and do not be surprised to see the wheel and/or or blades exit the engine. Think Hand Grenade.
Carbon in an Allison can block the small lubricating jets as well. These jets are designed around a high oil pressure which the Allison incidentally only needs to make the torquemeter work, and are very small and can block very easily.
TM -
The Arriel originally had no concept of IDLE. The issue in the TM is that cool down has found to be beneficial but the oil system doesn't really work well at idle so you end up with low flow. After about 30 seconds the temperature in an Arriel can do all sorts of unpredictable things. It is a trade off of cooldown and low flow. IDLE for long periods of time is not real smart in an Arriel due to low flow and poor scavenging. If you have idled for a period of time run the engine back up to flight then shut it off. Different brands of oil can even make a difference. (Note - do NOT mix different brands of oil in a turbine). Sometimes even batches of the same brand can cause issues.
PWC 200 - PWC206B - EC135P1 30 seconds. PWC206B2 - EC135P2 and up - none required. These engines are a later design and have a lot of the pitfalls of earlier engines removed at the drawing board stage.
Above all - operate in accordance with the approved and current version of the RFM and service data. Any questions call your local manufacturers tech rep. Thats what they are for and it is free. Good support here
