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Member
Posts: 16
Location: El Paso, Texas | Here's a new one, at least to me. A respected allison/rolls repair shop advises against "motoring only" to bring the temp down to start range.
Checked with Ole, and he agreed. They recommend letting the engine cool down on its own, which in the summer can be quite awhile. Since I've been using the procedure out of the POH for over five years now, it was quite a shock to hear this new advice. Anyone else have any info on this? |
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Member
Posts: 45
| I was told, that motoring the temp down and start is ok, however one should not motor down only without subsequent start.
Kurt |
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Member
Posts: 27
Location: stuart, florida and the U.S.Virgin Islands | Lowenfield /Kurt;
Strange and mysterious. Ole, Carl and Ron all advocated motoring as the means to reduce temps after a short stop and shutdown. 1 1/2 years ago. Ron did downplay the need to cool to<150 c. I saw him just start up with temps much higher than that.He said, 'no problem'.
Did the respected RR guru indicate what the downside of motoring is??? Other than running down the battery, what can be harmed??? We do it all the time when washing the compressor no???
N-50DH |
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Member
Posts: 45
| My Guru is a Heli Operator. He said, if the engine is hot and has time to cool down, one should not motor it down. But if a start is necessary, cool down by motoring is ok.
If you wash the engine, it is cool, therefore disregard this discussion. |
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Member
Posts: 16
Location: El Paso, Texas | Well, we certainly got some action here.....remember that you are getting this from a geriatric neophyte whose a&p skills are limited to checking the oil. To quote the "respected" source......"the T-wheel is cast, and cooling down by motoring may cause shock cooling and crack the wheel." I don't want to believe it....... |
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| It may have to do with the caution while trying a restart at altitude. That the' motor may seize
and require a period of time cooling before a start'. I never did completely understand this as
the turbine does not seize while it is running. When shut off, metals cool=contract, not expand.
N-50DH |
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Member
Posts: 27
Location: stuart, florida and the U.S.Virgin Islands | This one has been bothering me since I have owned my S.E. I have had two occasions where the generator has gone off line, in flight ,leaving me 'powerless" from a charging standpoint, but with all still working, from the massive battery reserves.
First time,I shed all non essential electrical load, landed immediately, phoned Ole, and he explained that carbon dust sticks the gen. brushes, needs blown out with compressed air, and all is right with the world. I now do this every 50-75 hrs of operation.
My question for you more experienced owner drivers, is what is the downside to simply continuing your flight, as in a piston plane ?? As the fuel pump(s) are electric, will I simply run out of fuel and the fires go out or what??? I do believe that if you loose the engine driven fuel pump, things will get quiet immediately, that the electric pumps can not
make up for that failure. But total electrical failure is still somewhat of a mystery. N-50DH |
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Regular
Posts: 67
| Gerry suggested doing a test - switch off the Master while on the ground and see what happens!
I have flown without the electric pumps running - by mistake, with no problem.
P |
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Member
Posts: 27
Location: stuart, florida and the U.S.Virgin Islands | Paul;
Turning off the master while on the ground at ground idle , is not the same as when fuel flowing at 2030RPM , but I get the general idea . The motor will continue to run . Thanks . Perhaps the danger would be if the ignitor is needed and you have no elect power to fire it . Am I safe though in assuming, that should the non redundant engine driven pump go out, one had better start looking for a landing place??? N-50DH |
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Member
Posts: 27
Location: stuart, florida and the U.S.Virgin Islands | Lowenfield;
I definately do not believe that we have any 'cast pieces' in our 250 B17F motors
turning at 53,000 rpm. If that were true, I would be building a scattershield under the cowling.
N-50DH |
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Member
Posts: 5
| What does RR factory say. What is the procedure with Prat & Wh. on quick turns? Al Hill |
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Member
Posts: 45
| My friend has a Piper Mirage modified with a Pratt and Wh. He says that the POH indicates no time limitations for a restart. It seems that this turbine is more tolerant to cooling. |
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New user
Posts: 1
| I apologize for being the new guy resurecting old posts, but I felt this needed to be addressed.
The reason hot-motoring is verbotten, is that it results in cool air being blown onto a cherry-hot 1st stage turbine wheel.
This shock cools the wheel and accelerates crack propagation.
ALL turbine wheels crack the first time the engine is run at the factory. Your 3rd and 4th stage turbine wheels have cracks 'installed' at the factory. The 1st and 2nd stage wheels do not, but are assumed to start cracking at birth, and crack a little bit more at each thermal cycle (start). The rate of crack propagation is very well known and understood...it's this rate that gives you your GG Turbine overhaul interval. When you pump cool air into a hot turbine, you introduce an unknown variable into the known rate of crack propagation.
In the lab, if they want to crack a turbine wheel for testing, the technicians simply heat a wheel up to operating temperature...then blow some cold air on it.
Rolls-Royce has an excellent RR250 engine course that costs less than an annual inspection. This is where I picked up these little nuggets of information. |
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motoring
