Re: YASim Propeller rpm

On Tuesday 19 April 2005 17:17, Lee Elliott wrote:
> On Monday 18 April 2005 21:18, Jim Wilson wrote:
> > > From: Lee Elliott
> > >
> > > Hello all,
> > >
> > > I'm trying to work out the right prop RPM figures for the
> > > TU-114 - the airliner version of the TU-95 'Bear' - and I've
> > > arrived at a figure of around 494 rpm for the cruise
> > > setting.  I'm also using a value of 958 for the takeoff-rpm,
> > > which I calculated at 150 kts and 0 ft.
> > >
> > > This is based on 3.2m radius props and a cruise speed of 475
> > > kts at around 30000 ft.
> >
> > Off the top of my head...that'd be about 20m circumference,
> > which at 1000rpm would be 20km per minute.  Since an airliner
> > does about 15km per minute.  I think that'd be a bit over the
> > speed of sound...depending on altitude.
> >
> > > Could someone check my maths and confirm that these figures
> > > are about right i.e. the prop tips staying sub-sonic?
> > >
> > > While I can get the config to solve with these rpm values
> > > the stand alone YASim solver seems to go into a loop if I
> > > include the min-rpm & max-rpm tags.  Is this likely to be
> > > due to me trying to use impossible numbers in the config or
> > > is this a problem within the solver?
> >
> > Not sure.  Just make sure if you are using a ratio that you
> > scale everything that needs to be.
> 
> Thanks for that Jim.
> 
> I think I've got a much better understanding of constant-speed 
> props now and realise that I should be using the low value i.e. 
> around 494 rpm for both the take-off and cruise settings, the 
> turbine speed staying pretty much constant.  Heh - I didn't 
> realise how little I knew about turbo-prop engines.
> 
> What I'm trying to find out now is the high-altitude power of the 
> engine.  I've got a basic rating for the engines but I assume 
> that's at sea level.  I'm also assuming that the engine will 
> produce less power at high altitude but I don't know the factor 
> or what a reasonable ball-park figure would be.
> 
> Still, things are starting to make some sense.

Sorry for jumping in a bit late, but I think I can help.  I just dog 
though my files on the NK-12 turboprop that the beastie uses, and 
I've pulled the following data:
Engine RPM is 9250.
Prop RPM is a constant 850.  Yes, that leads to some inefficiencies at 
the cruise speed of Mach 0.85, and a horrendous case of 
"Contra-Clatter" prom the prop tips, but that's not the whole story.
Like pretty much every other turboprop, RPM is constant, and is held 
constant by the propeller governor, and power is controlled by how 
much fuel you burn - so the TET goes all over the map.  I don't know 
if those props have a high-drag "Beta Mode" to cut the thrust on 
approach and landing.
That's not the whole story though..Turboprops don't just turn the prop 
shaft, they also produce thrust from the hot gas, like a straight jet 
engine.  Don't forget the Power-Thrust-Speed relationship.  
Propellers are constant power systems - If you feed 1,000 HP to a 
propeller, it'll generate (Not counting efficiency in here) 3750 lbs 
of thrust at 100 mph, but 1000 lbs of thrust at 375, and, at the 
)Mach 0.85 cruise of a Tu-114, 668 lbs.  But, that same engine may be 
producing 300 lbs of thrust, which is mor or less constant throughout 
the speed range of the airplane.  Just how much of the gas 
generator's output goes to Shaft Horsepower, and how much to Jet 
Thrust is a design decision that has to balance low speed performance 
(Takeoff and climb), which favors the prop, and high speed 
performance, which favors the jet thrust.
In the NK-12's case, the base model had a takeoff power setting of 
12,000 SHP + 2645 lbs of thrust for an Equivalent Shaft Horsepower of 
about 13,000.  (Yes, later models were uprated, but the split's 
pretty much the same.)  That's part of the secret of the Tu-95's and 
Tu-114's high performance for a turboprop.

You asked about altitude performance.  In the NK-12's case, that's 
pretty interesting.  In order to get the needed performance at 
altitude, the engine uses a set of blow-off valves in the compressor 
section to limit the pressure going into the burner cans.  These 
blowoff valves close, keeping the discharge pressure constant as the 
airplane climbs.  As far as the engine is concerned, it's at Sea 
Level.  Or, to put it another way, you can consider the engine to be 
supercharged.
In the case of the base model engine, Max Power (Takeoff and 
Emergency), is 12,000 SHP + 2650 lbs thrust  from Sea Level to 
26,000' at 9250 RPM. (840 Propeller RPM)
Continuous Power is  8,000 SHP + 1760 lbs from Sea Level to 36,000', 
at 8250 RPM. (760 Propeller RPM)

Max Turbine Inlet Temperature is 637 Deg C, and Specific Fuel 
Consumption is 0.36 lb/ESHP/Hr.

If I've been unclear, please feel free to ask me to try again.
I hope this helps.
-- 
Pete Stickney




    


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