Re: YASim Propeller rpm

On Friday 22 April 2005 06:49, Peter Stickney wrote:
> On Wednesday 20 April 2005 11:33, Lee Elliott wrote:
> > On Wednesday 20 April 2005 05:19, Peter Stickney wrote:
> > > On Tuesday 19 April 2005 18:12, Lee Elliott wrote:
> > > > On Wednesday 20 April 2005 04:56, Peter Stickney wrote:
> > >
> > > <snip - a bunche stuff on the NK-12>
> > >
> > > > > 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 :)
> > >
> > > Please feel free to ask anything. (Well, within reason,
> > > anyway)  If I can help, I'll be glad to.
> >
> > --
> > Pete Stickney
> >
> > Hello Pete,
> >
> > I started working through some of the numbers you gave but I
> > can't see how the prop rpm you gave can be right:(
> >
> > The prop radius is around 3.2 m so the path of the prop tips
> > per revolution is just over 20.1 m.  At 850 rpm I figure
> > that the tips will be moving at just under 285 m/s when the
> > a/c is stationary.
> >
> > At a speed of 880 kph (475 kt/546 mph) the a/c is doing
> > around 244 m/s.
> >
> > Combining the two gives me around 375 m/s, which is
> > supersonic.
>
> That's correct.  Supersonic propeller tips aren't the "Kiss of
> Death", though.  There are efficiency losses that you need to
> account for, which usually make propellers uneconomical above,
> say, 450 mph, but they don't just stop working.  Nor are the
> particularly uncommon. The main source of a T-6/SNJ/Harvard's
> irritating buzz at takeoff power is due to the propeller tips
> being supersonic.  The Gas Generator of the engine turns at a
> certain speed, and it drives the props through a fixed
> gearbox, with a 1:11 step-down ratio,  That's where the prop
> RPM numbers come from.
> I don't dounbt that the numbers that I have (although a bit
> inconsistant at the top of the post, due to excavating
> references) are indeed correct, at least as far as Wright-Pat
> and Kuznetsev are concerned.  Let's look through them and see
> if they're unreasonable.
>
> My sources list a Max Level Speed (Vmax) for the Tu--144 of
> Mach 0.82 at 25,000'.  (You;ll have to forgive the mono-units
> stuff here.  I think in British Engineering Units, and I'm
> trying to dash this off quickly, so please forgive my skipping
> the conversions).  The propeller diameter I have is 18.42 ft. 
> (That may not be entirely correct, but it puts in in th eright
> area.)
> Pushing the numbers through the standard formulae, at Max
> Power I get a static propeller rotational speed of 810 ft/sec.
>  Mach 0.82 at 25,000' is 829.84 ft/sec.  Solving for teh total
> propeller tip speed. I get 1160 ft/sec, which is Mach 1.15 at
> that altitude.  A bit higher than optimum, but - the Tu-144's
> prop blades are well shaped, with a very low thichkness/chord
> ratio, and tapered tips.  I'll still go with the pessimistic
> side of teh numbers.
> Now - Using the N.A.C.A.'s tables for supersonic propeller
> efficiency, that high a tip Mach Number imposes a 0.82 factor
> penalty on the uncorrected propeller efficiency.  The Advance
> Ratio works out to 3.22, so it's still in the range for good
> propeller efficiency. Assuming the propeller's uncorrected
> efficiency is 0.85 (Not unreasonable, it's not off by more
> than a percent or so), that gives a corrected efficincy of 
> 0.697.  Not super great, but not all that bad.
> The engine's putting out 12,000 SHP + 2650# of jet thrust. 
> That gives us a propeller thrust of 5543 lbs, plus the jet
> thrust of 2650, for a total thrust of 8193# per engine, for a
> total of 32772 lbs for all engines.  (For comparison, the
> equivalant B-52 , output of around 4500#/engine at that
> altitude , for a total of 36,000.  Given thatt the Buff's a
> bit faster, (and about 25% bigger in size & weight) it seems
> very plausible.
>
> Running the Cruise condition (Mach 0.67) at 36,000', I get a
> ptopeller tip Mach Number of 1.02, not very extreme at all, as
> these things go. This yields a compresssibility correction of
> 0.905, the Advance Ratio's 2.72, and the corrected efficiency,
> assuming a 0.85 basic efficiency, is 0.77,  (Much happier.) 
> At cruise, the engine's producing 8.000 SHP + 1760#, the
> propeller thrust is 5350, and the total thrust is 7110#. 
> That's not bad at all.
> Don't forget that that exteme altitude rating skews
> everything. There's a lot of power to burn.
>
> > This was what Jim pointed out earlier and I'm currently
> > using a prop rpm value of 494.
>
> Again, don't assume that the propeller blade tips _need_ to be
> subsonic.  Flying, and airplane design, is all about
> tradeoffs.  In the case of teh Tu-95 adn Tu-114, Tupelev and
> Kuznetsov were willing to trade some efficiency at the high
> end for better overall fuel consumption and performance. 
> Given the shortcomings, especially in range, of the competing
> all-jet M-4 (ASCC Codename Bison), it looks like they made the
> right choice.

Thanks very much for going through that.  I was assuming that 
allowing the prop tips to go supersonic was a complete no-no.

I'll have a try with some of these numbers.

Thanks again.

LeeE


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