This is an interesting discussion.......
Subject: Re: Pratt & Whitney R-3350 for sale reduction ratios on PRTs
Date : 11/25/08 02:37 PM
" I would dearly like to know the step down ratio from the exhaust
turbine to the prop shaft ".........
The PRT's are unique to the 3350. When I took my FAA writtens there was
a question on it asking how much hp each made. I am afraid I cannot
remember the exact answer but would guess approx. 125 ea. They are
geared right to the crank if my memory is accurate.
I used to frequent the Save-A-Connie museum when I lived in Kansas
City. They had a cut-away R-3350 (actually TC-18) on a stand that they
could rotate with an electric motor. For some reason, the number I
recall is 150HP each for a total contribution of 450HP to the engine.
There's a lot of gearing in those big radial engines. The crank is
geared relative to the prop. The accessory case is geared relative to
the crank. The supercharger is geared relative to the accessory case.
And so it goes.
Figuring out the gear ratio between the PRT's and the prop would be a
bit of work. Suffice it to say it's a lot of gearing. The PRT's
function efficiently at turbo-charger speeds, which can run up close to
100,000 RPM. The PRT shafts run through a fluid coupling anyway. It
looks just like a mini torque converter out of an automatic
transmission in a Buick.
I would ballpark it at more than 50,000 PRT RPM for 1500 prop RPM; for
an overall ratio of more than 30:1. But the moving parts along the way
go faster and slower at each step and I could easily be off by an order
of magnitude.
Like a turbo, the PRT's are probably turning at a rate similar to the
supercharger. That's where they're efficient.
David Bunin
David-
The Curtiss Wright R-3350 972TC18DA-2 or DA-4 on the DC-7 (believe the
Connie used the R3350 972TC18EA-2) put out 3250 BHP with the PRT's
nearly 1 BHP were pound of engine. Each engine has 3 of them. The
basic engine was 2600 BHP and then they had a water injection model
that Developed either 2900 or 3000 BHP. I'll have to get out my DC-7
manual for the exact amount of HP the PRT's provided. Some engine
differences were due to the prop on the airplane. The DC-7 had a four
bladed prop and the Connie had a longer 3 bladed prop. Prop difference
was due to airplane gear height and ground clearance.
The drive coupling for the PRT was manufacturered by Fluid Drive,
Corp., in my home town of Holly , Michigan, a divison of Nelson
Manufacturing(Now defunct). One problem was the shaft from the coupling
to the PRT turbine was rather long and would create a whip in its
bearing and end up in a PRT failure.
The R-3350 with the PRT was an engineering miracle and a very good
engine, but not with a ham handed pilot at the controls--you had to
treat it like a new girl friend--nice . It was an injected engine and
if you advanced the throttle forward too fast you could twist the
engine accessory tail shaft that drove the injection pumps (two of
them). The engine was like an Indianapolis race car compared to NASCAR
boilerplate type car. The P&W R-2800 on the Convair 240/440, Martins
404 and Douglas DC-6 was boiler plate compared to the R-3350.
I flew them for 11 years and never had an engine failure. Alot of
shorted secondaries and plug lead and coil changes (the system was low
tension ignition with a high tension coil and short lead on the
cylinder) but no gut jobs. Watching the exhaust flame from the top PRT
on a night takeoff was a sight to behold. When rich at takeoff power
the flames were blue orange and the length varied--when you reduced to
METO power the flame sucked right in near the PRT exhaust and was a
blue white beauty. You could set the power almost by the exhaust it.
Doug 1
182J
Dave
Thanks for the info. I think you gave me the information I needed. I was
interested in the gearing ratio from the PRT to the fluid coupling only
actually. If the coupling is actually a torque converter, which is possible
since Packhard used one in 1949, then the actual rpms going into the
converter might well be a bit higher than the engine crankshaft rpms. It is
sensible that the gear ratio relationship between the PRT and the
crankshaft would follow different power curves. Using a torque converter
(unless it locked up) would result in large efficiency losses however.
http://en.wikipedia.org/wiki/Torque_converter
Torque converter elements
"A fluid coupling is a two element drive that is incapable of multiplying
torque, while a torque converter has at least one extra element the stator
which alters the drive's characteristics during periods of high slippage,
producing an increase in output torque. In a torque converter there are at
least three rotating elements: the pump, which is mechanically driven by the
prime mover; the turbine, which drives the load; and the stator, which is
interposed between the pump and turbine so that it can alter oil flow
returning from the turbine to the pump."
I am trying to work out the best ratio to use after the traction drive for
my turbo compound design and I suspect on the R-3350 set-up, there had to be
a rpm mismatch and perhaps the designers assured that the rpms going into
the torque converter were always higher than the engine crankshaft speed.
This is only my opinion though and I would like to confirm it as I am in the
process for finally getting a handle on the turbine choice and final gear
ratio.
On the other hand if if just a oil dampener, or fluid coupling that
essentially locks up, then that is an entirely different design concept and
would infer that the engine and the PRT power curves a or rpms are matched
close enough.
Doug 1,
So you have actually flown behind this engine. Wonderful. Please tell me
how you went about dialing in the engine for best performance at cruise
speed and high altitude. I understand you had a BMEP guage to aid in the
process. What was the order in leaning and adjusting the super charger deck
pressure?
Doug 2 in Japan.
----------------------------------------------------------------------
Dave or Doug 1 is not on here. This message was uploaded from I don't know
where by Marc Wiese.
The R3350TC had a simple fluid coupling. Not a torque converter. Torque
converters are horribly inefficient when actually converting torque. On the
order of 65%. I know..we used to use them in our Chaparral race cars.
Modern torque converter transmissions have lock up clutches
and these types of auto transmissions are disappearing in the world. They
are being replaced by servo shifted manual transmission. Some with dual
clutches because they are more efficient over the entire speed range.
I appreciate the need for a variable gear ratio but I don't think torque
converters are the way to go. The R3350 TC had a two speed geared
supercharger.
I have changed my mind about using radial inflow turbines. They primarily
work on pressure differences. Further turbine study on my part indicates
they are not as efficient as an axial flow blow down turbines. There is
still some confusion in my mind exactly what a blow down turbine now looks
like as the name has been changed over the years. The R3350 blow down
turbine looks a lot like what is now known as a as an axial impulse turbine
in Barber Nichols speak.
This I think is why the Scania TC is not as effective as the Detroit Diesel
DD15
turbo compound engine. The Scania used a radial inflow turbine and the DD15
uses
an axial flow turbine. Radial inflow turbines require considerable back
pressure
to work while properly designed axial flow blow down turbines work on
kinetic
energy alone.
This is not to say a variable vane nozzle on an axial flow turbine will not
work. It will but at the expense of some back pressure.
Paul Lamar
--I will get you David's and Doug1's direct emails, Then you can
speak directly. Apparently there still remains a lot of experience out there
with this massive engine. While at A/P school I too saw a working cutaway of
this engine (we had one geared to an electric motor that turned the entire
motor-what an intricate mess), but we worked mostly on turbines......and
smaller piston engines. I think they still have it. I think Paul has
pictures of this same cutaway (how many did they do that to?)
Marc Wiese
The 3350 was leaned to 10% BMEP under a certain threshold of power (I don't
remember what it was) because of the power enrichment valve in the carburetor.
Above that power setting, but below METO (maximum continuous power setting below
takeoff power) and climb power, you used "Auto Lean".
The two injection pumps, initially, were connected with a synchronizer bar that
would change with temperature and vibrations but were later replaced with flow
dividers that equalized the fuel to each cylinder.
The engines were BMEP or manifold pressure limited on takeoff, whichever limit
was reached first, and you shifted to high blower when you ran out of throttle
at low blower in climb. Karibian
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