Re: YASim Propeller rpm

On Wednesday 20 April 2005 04:56, Peter Stickney wrote:
> 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.

That's a terrific help - thanks very much.

I probably will pester you again as I find other stuff I need to 
know :)

LeeE

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