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
182J

my memory tells me the three turbines added a combined +/_ 825 hp or +/-
275 hp each in the most highly tuned version -- against the standard
R3350; and that they ran at under 10,000 rpm.

steve wolpin