Thoughts about the RR250 Series?
Well put Adam.
I feel that the containment ring on C20 series is another rip off cost, I am not saying it is not a good idea but the cost is crazy, because they can.
Engineering-wise, there is nothing today that suggests they would be that hard to make. The tolerances are far lower than in many other industries where none such price hiking appears. Let's not forget they were certified and approved when they were still hand made on lathe's and manual mills. Today we have robotic CNC vertical mill stations that have more productivity and much higher tolerances. Neither are the materials used very exciting or expensive. I don't believe for a second these aren't downright cash cows for RR, GE and P&W. Nobody can with a straight face tell me a 250 or a PT6 costs $600.000 to make. The R&D and the certification costs were recouped decades ago. What we're left with is corporate bullying. "
OEI-Dave
I have some pics of a failure (pre ring) SCARY.
Has anyone had an uncontained failure of these turbines yet? I know an AD was issued for a containment ring, AD 2005-10-13. This only applies to the series 2 engines (ie, C20,B17,C20R ect). Which in theory, should have "contained" a failure of the hot section. Somehow I doubt this after inspecting a containment ring and doing some maths on the rotating inertia of the turbine section.
Anyone had anything with the series 3 and 4?
"But isn't this all a bit of inflating one's importance with turbines? Let's be honest here - they can charge these kind of money because they're on the type certificate as the only engine that particular aircraft can use. It's what is called a 'captive market'.
I feel that the containment ring on C20 series is another rip off cost, I am not saying it is not a good idea but the cost is crazy, because they can.
Engineering-wise, there is nothing today that suggests they would be that hard to make. The tolerances are far lower than in many other industries where none such price hiking appears. Let's not forget they were certified and approved when they were still hand made on lathe's and manual mills. Today we have robotic CNC vertical mill stations that have more productivity and much higher tolerances. Neither are the materials used very exciting or expensive. I don't believe for a second these aren't downright cash cows for RR, GE and P&W. Nobody can with a straight face tell me a 250 or a PT6 costs $600.000 to make. The R&D and the certification costs were recouped decades ago. What we're left with is corporate bullying. "
OEI-Dave
I have some pics of a failure (pre ring) SCARY.
Has anyone had an uncontained failure of these turbines yet? I know an AD was issued for a containment ring, AD 2005-10-13. This only applies to the series 2 engines (ie, C20,B17,C20R ect). Which in theory, should have "contained" a failure of the hot section. Somehow I doubt this after inspecting a containment ring and doing some maths on the rotating inertia of the turbine section.
Anyone had anything with the series 3 and 4?
"But isn't this all a bit of inflating one's importance with turbines? Let's be honest here - they can charge these kind of money because they're on the type certificate as the only engine that particular aircraft can use. It's what is called a 'captive market'.
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you'd see massive sales
Consider that in the 50+ year history of the 250 series, only about 30,000 engines have been manufactured, or about 500 per year, and this includes all applications: new acft, replacements, etc. For perspective, something like 60 MILLION motor vehicles (cars, truck, motorcycles) are produced annually, each with a piston engine.
If you examine what per centage of the selling price the engine cost represents in a new light acft, either piston or turbine, if this cost went to zero, it would make only a modest differencce in acft price and therefore sales volume. Examples: new Cessna Skyhawk about $300K, Lyc piston engine about $40K; new B206 about $1.6 million (when you could get one); new 250-C20 about $350K. (Aircraft piston engines benefit to some degree from what is sometimes called "shared technology" among all piston engines.)
Would be great if turbine engine prices were much lower, but unless demand is in the million plus units per year range, isnt likely to happen, IMHO.
Last edited by EN48; 18th Jun 2011 at 16:17.
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Ironically, the target price for the original 250 shp T63 of 1958 was $4,000 (about $30,000 in today's money). Bill Castle and the Allison design team soon realized just how badly they had low-balled their proposal once the contract was awarded.
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All pretty good points. If production was up to a reasonable rate the cost would come down some. While we have a lot better machining technology today, the 250 was made using dedicated machines. That is, each operation had a dedicated machine and that reduced costs a lot. While you can do more nowadays with more capable machines, it doesn't reduce costs as much as you might think compared to production line methods.
RR right now isn't making their cost bogeys for the RR300, and is, consequently, losing money on each engine. This is because they have high costs and low productivity in a union environment and their costs are higher than if they bought all the parts and just assembled the engines. RR gets a big number for the other Model 250 engines and makes a good profit on them.
I'm sure the PT-6 could be sold for less if P&W had some competition, but they don't so the price is what the traffic will bear.
Nobody has mentioned the cost impact of product liability, which is a big deal for engines. You have to factor that in to the price too. It isn't just the cost of making and sending an engine out the door and covering the warranty costs.
Turbines can be less expensive if the engine is designed expressly for low cost production using modern production techniques. But you aren't going to ever see a 300 hp turbine with a cost similar to a 300 hp recip. The reason is that the nickel base alloys in a turbine expensive, and all of these nickel based castings for the hot section pieces are vacuum investment castings and those are expensive.
Don't try to compare model turbojets to larger multi-spool man rated turboshaft engines. Those engines are very simple machines with relatively low mechanical loadings. Consequently the performance is poor and they don't operate at higher temperatures or require high performance and costly materials. They don't have power turbine sections and gearboxes and don't have fuel controls that would ever be capable of being used on a real engine. They aren't man rated and I'm sure the makers aren't carrying the product liability insurance that would be required for a real engine. They are basically toys when compared to real engines, and you are paying the price of a toy for what you are getting.
RR right now isn't making their cost bogeys for the RR300, and is, consequently, losing money on each engine. This is because they have high costs and low productivity in a union environment and their costs are higher than if they bought all the parts and just assembled the engines. RR gets a big number for the other Model 250 engines and makes a good profit on them.
I'm sure the PT-6 could be sold for less if P&W had some competition, but they don't so the price is what the traffic will bear.
Nobody has mentioned the cost impact of product liability, which is a big deal for engines. You have to factor that in to the price too. It isn't just the cost of making and sending an engine out the door and covering the warranty costs.
Turbines can be less expensive if the engine is designed expressly for low cost production using modern production techniques. But you aren't going to ever see a 300 hp turbine with a cost similar to a 300 hp recip. The reason is that the nickel base alloys in a turbine expensive, and all of these nickel based castings for the hot section pieces are vacuum investment castings and those are expensive.
Don't try to compare model turbojets to larger multi-spool man rated turboshaft engines. Those engines are very simple machines with relatively low mechanical loadings. Consequently the performance is poor and they don't operate at higher temperatures or require high performance and costly materials. They don't have power turbine sections and gearboxes and don't have fuel controls that would ever be capable of being used on a real engine. They aren't man rated and I'm sure the makers aren't carrying the product liability insurance that would be required for a real engine. They are basically toys when compared to real engines, and you are paying the price of a toy for what you are getting.
![](http://[img]http//i98.photobucket.com/albums/l259/500d_2006/IMG_9071.jpg%5B/IMG%5D"[/IMG]Lost photo[/URL]<br />
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Ugly!
Seen similar on a B206. The pieces went down through the engine pan through the the boot and out the bottom. Missed the T/R control!
I can't remember what the reason was after all these years but there was a conclusion as to why.
A classic one that often gets missed and not implying anything to do with the above but after a module change a start and ground run up to flight and subsequent shutdown is imperative. Do NOT start, ground and then fly. It seems the engine likes to go through a heat cycle just to get everything lined up properly.
Couple of things on the five -
Leaking no.8 U seal will leak air into the oil system when it is running (it will run slightly hot) and oil onto the wheel after shutdown causing shock cooling to the wheel.
Leaking check valve in the oil cooler will let oil drain back through the scavenge and flood the sumps and then onto the wheel. Did it ever leak oil out of the burner drain? The burner drain has to be clean to leak the oil so you can tell! Often they are not!
Seen similar on a B206. The pieces went down through the engine pan through the the boot and out the bottom. Missed the T/R control!
I can't remember what the reason was after all these years but there was a conclusion as to why.
A classic one that often gets missed and not implying anything to do with the above but after a module change a start and ground run up to flight and subsequent shutdown is imperative. Do NOT start, ground and then fly. It seems the engine likes to go through a heat cycle just to get everything lined up properly.
Couple of things on the five -
Leaking no.8 U seal will leak air into the oil system when it is running (it will run slightly hot) and oil onto the wheel after shutdown causing shock cooling to the wheel.
Leaking check valve in the oil cooler will let oil drain back through the scavenge and flood the sumps and then onto the wheel. Did it ever leak oil out of the burner drain? The burner drain has to be clean to leak the oil so you can tell! Often they are not!
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The biggest issue with the 250 is the limited low cycle fatigue life of the turbine wheels.
The wheels are integral castings. That means that the wheel is made up of a disk that is cast in one piece with the blades. The blades are not removeable. What happens is that on startup and as the engine accelerates, the rim of the disk gets hot quicker than the bore (the center of the disk). This causes the rim to yield in compression. When the engine is shut down, the disk is hot and the rim cools quickly and the disk rim now goes into tension and cracks.
To help reduce the amount of thermal stress in the disks, it is very important to cool down the engine after you land and before you shut it off. This allows the disk bores to cool off and minimizes the temperature difference between the disk rim and the bore. The shutdown procedure includes two minutes at flight idle before shutting down the engine, and to get the most out of your engine it should be followed to minimize the damage to the disks. Going from a high power hover to a quick shutdown is a really bad thing to do to these engines.
If you don't properly shut down the engine the cracks, which grow from the rim of the disk downward toward the bore, can progress to the point where the disk will fail and you will burst the disk.
The disks crack and the crack growth is well understood, so there are time limits on the disks and periodic inspectons. In all it is a safe system when the procedures are followed correctly.
The wheels are integral castings. That means that the wheel is made up of a disk that is cast in one piece with the blades. The blades are not removeable. What happens is that on startup and as the engine accelerates, the rim of the disk gets hot quicker than the bore (the center of the disk). This causes the rim to yield in compression. When the engine is shut down, the disk is hot and the rim cools quickly and the disk rim now goes into tension and cracks.
To help reduce the amount of thermal stress in the disks, it is very important to cool down the engine after you land and before you shut it off. This allows the disk bores to cool off and minimizes the temperature difference between the disk rim and the bore. The shutdown procedure includes two minutes at flight idle before shutting down the engine, and to get the most out of your engine it should be followed to minimize the damage to the disks. Going from a high power hover to a quick shutdown is a really bad thing to do to these engines.
If you don't properly shut down the engine the cracks, which grow from the rim of the disk downward toward the bore, can progress to the point where the disk will fail and you will burst the disk.
The disks crack and the crack growth is well understood, so there are time limits on the disks and periodic inspectons. In all it is a safe system when the procedures are followed correctly.
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The manual says to cool for two minutes at idle. But if you notice the TOT at flat pitch with the throttle full open, it's the same and perhaps lower than at idle.
Why don't they let you count time at flat pitch in the cooling time?
And if two minutes is good, three minutes isn't better.
Why don't they let you count time at flat pitch in the cooling time?
And if two minutes is good, three minutes isn't better.
Shawn, I *believe* (meaning: maybe I read it once...somewhere...) that there is a high-wind shutdown procedure for the 250 in which the cooldown is done at 100%. I mean, I agree with you, it's TOT that makes the engine hot, right? And if the TOT is at its lowest point, why DON'T they let you count the time at 100%/flat pitch?
Me, I'm old school. I start my cooldown when the N2 gets to idle, like I was taught many, many, many years ago, no matter how badly I have to pee by that time (and usually I do!). Old habits and whatnot.
Engine-eer, it's interesting that you point out the temperature issues with the wheels themselves. I would imagine that most 250 drivers believe the cooldown to be *only* an issue of oil temperature and coking.
I have a young friend who flies a 206. He knows EVERYTHING about it, or so he says. He believes he has learned more in his 500 hours of 206 time than I have in my 30 years of flying them. Whatever. Perhaps he has. This youngster apparently views the 2-minute time as a maximum, or perhaps just a "suggestion" from Allison/RR, because I have never seen him do a complete 2-minutes; I have however seen him give it as little as a minute and a half. Perhaps he starts his countdown clock from some point prior to throttle roll-off, as Shawn posited. But hey, it's his ship (or his boss's), not mine and he views any input from this old-timer as gratuitous criticism of his fine piloting skillz. Some fights just aren't worth fighting.
Then again... I have heard from sources I consider fairly reliable that modern engine oils (like Mobil 254), can allow a shorter cooldown time than 2-minutes. One "expert" did actually suggest that 1.5 minutes is fine for a 250, and maybe my young friend also heard that advice and took it to heart.
So I don't know. Sometimes the tales we hear from those old wives are wrong. I'll just keep giving RR-250s a proper 2-minute cooldown until I'm officially told by RR that it's not necessary anymore.
No matter how badly I have to pee.
Me, I'm old school. I start my cooldown when the N2 gets to idle, like I was taught many, many, many years ago, no matter how badly I have to pee by that time (and usually I do!). Old habits and whatnot.
Engine-eer, it's interesting that you point out the temperature issues with the wheels themselves. I would imagine that most 250 drivers believe the cooldown to be *only* an issue of oil temperature and coking.
I have a young friend who flies a 206. He knows EVERYTHING about it, or so he says. He believes he has learned more in his 500 hours of 206 time than I have in my 30 years of flying them. Whatever. Perhaps he has. This youngster apparently views the 2-minute time as a maximum, or perhaps just a "suggestion" from Allison/RR, because I have never seen him do a complete 2-minutes; I have however seen him give it as little as a minute and a half. Perhaps he starts his countdown clock from some point prior to throttle roll-off, as Shawn posited. But hey, it's his ship (or his boss's), not mine and he views any input from this old-timer as gratuitous criticism of his fine piloting skillz. Some fights just aren't worth fighting.
Then again... I have heard from sources I consider fairly reliable that modern engine oils (like Mobil 254), can allow a shorter cooldown time than 2-minutes. One "expert" did actually suggest that 1.5 minutes is fine for a 250, and maybe my young friend also heard that advice and took it to heart.
So I don't know. Sometimes the tales we hear from those old wives are wrong. I'll just keep giving RR-250s a proper 2-minute cooldown until I'm officially told by RR that it's not necessary anymore.
No matter how badly I have to pee.
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And if two minutes is good, three minutes isn't better.
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It's turbine INLET temperature that matters, not turbine outlet temperature. The cracks start at the leading edge of the rotor. If the main rotor is taking out work (even though it is in flat pitch it has some drag), then the turbine is doing work and the inlet temperature is higher than the outlet temp. When you are at idle and doing no work the turbine inlet temp is pretty much equal to the outlet temp.
You don't measure burner outlet temperature since that would toast thermocouples, but generally the less fuel you are burning the lower the combustor outlet temp is.
It's possible that the engine could be more efficient (compressor and turbines into a more efficient operating range) that the temperatures could be cooler at a higher idle. I've seen some small turbojets that had a flat EGT curve from 20% speed up to 50% speed, so it's possible, but you want the combustor exit temperature to be as low as you can get it and that usually happens at a lower idle point, except in a high wind situation where you have the wind acting to back pressure the exhaust.
You don't measure burner outlet temperature since that would toast thermocouples, but generally the less fuel you are burning the lower the combustor outlet temp is.
It's possible that the engine could be more efficient (compressor and turbines into a more efficient operating range) that the temperatures could be cooler at a higher idle. I've seen some small turbojets that had a flat EGT curve from 20% speed up to 50% speed, so it's possible, but you want the combustor exit temperature to be as low as you can get it and that usually happens at a lower idle point, except in a high wind situation where you have the wind acting to back pressure the exhaust.
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An interesting story regarding RR catastrophic failures here.
http://abcnews.go.com/Business/wireStory?id=12349267
http://abcnews.go.com/Business/wireStory?id=12349267
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As bad as the failures seem I still think these engines are quite robust. We've had engines come in from foreign militaries for low power only to find that there was a bullet hole in the occ and pieces of copper/lead in the cubustion case and on nozzle leading edges. The even crazier thing is that they put 100hrs on the engine before they decided to pull it!
One of the well known manufacturers did thousands of engine offs in flight with no cool down . When the engines went in for maintenance i am told there was no difference or damage at all . Not advocating ignoring the 2 min ......
Probably the most significant issue with cooldown time on the 250 engines is the 6 & 7 oil drain which goes through the lower support strut. If you shut off with the strut still hot the oil "fries" in the strut and eventually blocks it. This will lead to flooding of the bearing cavity and the oil exiting through the labyrinth seals onto the wheel. Not good to quench the wheel and also stop the oil flow to the bearings. From memory the 6 & 7 sump should be removed periodically and the flow checked and the strut eyeballed to see that it is clean. The No. 8 "hook" jet can also suffer the same.
Some oils are better than others and results have varied over the years with supposedly the same oil but different manufacturing batches.
Another area that also used to suffer was the PT coupling shaft. Carbon buildup would lathe off the "pea shooter" shaft and disconnect the compressor!
Definitely guaranteed to make your eyes water in one way or another............
Some oils are better than others and results have varied over the years with supposedly the same oil but different manufacturing batches.
Another area that also used to suffer was the PT coupling shaft. Carbon buildup would lathe off the "pea shooter" shaft and disconnect the compressor!
Definitely guaranteed to make your eyes water in one way or another............
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I recall seeing an Allison chart showing the core temp of the turbine wheels and after 1 min 45 secs the temp had stabilized so I can only assume the published Bell 206 '2 minutes' added a little margin for those challenged by time-keeping..
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Some oils are better than others and results have varied over the years with supposedly the same oil but different manufacturing batches.
In addition you should alwasy use the newer 3 micron oil filter. Life of a number of components in the gearbox and sumps was drastically improved by going to the 3 micron filter. In addition, by filtering out more particles from the oil, there was a lower tendency to produce deposits in critical parts of the system.
