Re: Rules of thumb...

On Monday 20 June 2005 15:22, Lee Elliott wrote:
> On Monday 20 Jun 2005 19:52, Andy Ross wrote:
> > Lee Elliott wrote:
> > > does anyone know of any rule-of-thumb method for determining
> > > the most efficient cruise speed of a propeller?
> >
> > Propeller efficiency curves are pretty flat.  For lightplanes,
> > you should probably pick a specified cruise speed/rpm from a
> > POH.  For props with a wider range of use, pick something from
> > the middle.
> >
> > Generally, especially with constant speed propellers, it
> > shouldn't matter much.
> >
> > Andy
> 
> I'm still trying to get the TU-114 I've been working on to behave 
> credibly and playing with the cruise-speed is the most recent 
> thing I've tried.  I found that by lowering it from the typical 
> cruise speed of 431kt @ 36000ft to 300kt @ 1000ft has improved 
> the take-offs no end and it's also allowed me to reduce the 
> camber on the wing which was causing sudden pitch-downs at low 
> altitude/low weights/high-speeds.
> 
> However, I've not been able to get a config working if I specify 
> a condition lever anywhere and I imagine that this points to a 
> serious problem in the config.

Lee, part of the problem is that you're starting out with an anomalous 
case.  The engines on the Tu-144, (and the Tu-95, or course) are 
pretty much unique as turbine engines go.  With the variable pressure 
ratio that the blowoff valves in the compressor section provides, it 
develops Sea Level power all the way from the deck to 40,000'.  
You've also got a very healthy dollop of jet thrust, as well, which 
becomes as significant factor as speed increases.

I'm not so sure that the powerplant model will handle it correctly. 
It's a weird case. 
Perhaps something with a sea-level rated powerplant, like a P-3 (Or an 
L-188 Electra airliner), or a C-130B or E model would be an 
interesting test.

In fact, that's one of the reasons I've joined the list.  Most sims 
just don't do powerplants as well as I'd like. I find too many 
"unnatural" things going on.  This isn't a slap at the other folks 
who've been doing it - it's hard.  Especially when propellers, 
superchargers, (Especially the mechanical ones) and mixed-mode stuff 
like turboprops are involved.) Having criticized, it's only fair that 
I throw myself off that cliff, too. :), So I'm trying to get up to 
speed and see in I can help with some improvement.

> I'm also not clear on how the prop-pitch and condition levers are 
> used together.  I understand, I think, the prop-pitch control 
> but I'm still confused by the condition control and how constant 
> speed props fit into the equation - there seem to be three 
> variable factors here, with control over pitch and condition, 
> and the CS prop taking care of itself.

This is one of those places where turboprops are different.  Becasue 
of the slow spool-up and deceleration times of a turbine-propeller 
combination, the prop is very strongly governed to a single RPM 
(Which is what the Condition Levers are basically for), and the power 
is controlled by varying the Turbine Inlet Temperature. (Which is 
what happens when the Power Lever is used to adjust the engine's fuel 
control.  So - the RPM stays constant, and it you want to go faster, 
the TIT goes up.
As the speed of the airplane increases,  the propeller has to change 
pitch to absorb the given amount of power for that particular Advance 
Ratio.  Pitch gets quite high (65-70 degrees wouldn't be out of line) 
at high speeds and altitudes.  If you're trying to figure out the 
specifically correct prop pitch for every power/airspeed/altitude/RPM 
combination, you'll drive yourself bats.  It's best to just have the 
outer envelopes of the Efficiency curve, or the Cp and Ct curves, 
depending on how you are computing it, defined, and if you stay 
within its conditions, you're operating at the optimum point.  That's 
what the propeller governor does in real life.  It doesn't spend time 
figuring, it just twiddles the pitch to keep the selected RPM, until 
it hits either the high or low pitch stops.

> I was given max and cruise prop rpms and I can control the actual 
> rpm to match these numbers by using the pitch control but this 
> doesn't seem right to me.

That does seem a bit odd.  You should be telling the Prop Governor to 
hold a specific RPM, and it should just do it (Modulo the Pitch 
Stops)  Set it and forget it, pretty much

-- 
Pete Stickney


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