Re: YASim Propeller rpm

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.  

-- 
Pete Stickney


_______________________________________________
Flightgear-flightmodel mailing list
[email protected]
http://mail.flightgear.org/mailman/listinfo/flightgear-flightmodel
2f585eeea02e2c79d7b1d8c4963bae2d