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Old 05-26-2008, 04:24 AM
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Stonefly
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Joined APC: May 2007
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I worked on Lockheed P2V's as a crew chief for an airtanker operator that contracted with the Forest Service. We had two Curtiss-Wright R-3350's inboard, with two Westinghouse J-34 turbojets outboard.

A radial, especially one like the R-3350, has a lot more parts. Other than the increase in scale going from four or six cylinders to eighteen, you have the internal supercharger (blower), the reduction gear box, and in the case of the R-3350 the three power recovery turbines which make it a "Turbo-Compound Radial Engine." The PRT's are turbines that are spun by the exhaust, similar to how a turbocharger is spun. However, instead of using that power to compress the induction air charge (which is already done in this case by the internal supercharger), the turbines instead spin shafts that are connected through gearing to the crankshaft itself. This "recovers" approx. 100-150 hp per prt.

The ignition systems on large radials are more complex, utilizing a low tension system, rather than a high tension system like most small flat engines. Oil systems are more complex, especially around the bottom rows of cylinders.

Round engines are great. They look cool, sound awesome, and maintaing them is satisfying (but hard and dirty work). Starting them takes finesse, however. Basically, it goes something like this on a warm day with a 3350:

Mags off, Mixture cutoff, throttle open approx. one inch
Depress and hold starter switch, count eight blades
Mags on "Both"
This is where technique comes into play, there are variations that guys swear by. I swear by my technique!
Keeping one finger pressing on the starter toggle switch, with another finger depress the primer toggle switch for a count of four
Primer off for a count of four
Primer on for a count of four
At this point she should be coughing, puffing, belching and farting and should catch her breath. When she does, hold the primer toggle swith down while taking your finger off the starter switch.
Now, you can run her all day like this, with your one finger on the prime (different fuel circuit then mixture) and one hand on the throttle adjusting the amount of air coming in. But now, you want to bring the pressure carb in, so when she has smoothed out, bring the mixture forward (easy, or she'll flood and now you've lost her and have to start all over again), and you should get a drop in RPM as you've got fuel running from both circuits. This tells you theat the Carb is "on" and you can take your finger off of the prime.

Starting on a cold day, starting a floooded engine, etc., are whole other procedures, but you get the idea. I once saw a little sign hanging in the nose of a B-25 at an airshow. It read "Jets are for Kids". I agree! There is an "Ode to a Radial Engine" out there that i'll have to find and post here. It explins the difference between starting a jet engine, and starting a radial engine.

The first start of the day was always fun to watch peoples reactions, as she REALLY belched smoke, people would say "Is it on fire?" No, and yes the brakes are supposed to screech and squeal like that too when taxiing, that's how you know they're working!

Fun stuff!

Stonefly
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