You can have multiple cylinders, each of which has good exposure to cooling airflow. Inline/opposed engines have some cylinders hiding behind other cylinders, so it's harder to get good consistent airflow to everybody...the rear cylinders tend to get pre-heated air from the front cyclinders.
Ultimately radials create a lot of form drag...it turns out that you don't normally need each cylinder to have full-frontal airflow from the slipstream. An inline design allows you to minimize the frontal drag, and duct only as much cooling air as you need to the cyclinders (in theory).
Also it's probably easier to start and manage combustion, and lubrication when all the cylinders are oriented in the same direction.
I can't think of any reason why a radial would have more parts though...it should have the same parts as other piston engines, just oriented differently.
The old radials actually spun around with the prop (think Sopwith Camel) and didn't have carburetors, they would cut out the ignition to reduce power. In the later years of radials they made some huge ones- 3350's etc. Nothing beats the sound of a round motor.
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!
Hey RAMA the engine on the Sopwith Camel was a rotary not a radial. The crank shaft was bolted to the firewall and the prop was bolted to the engine making them one unit. Also most early rotary engines had no throttle. Engine ran at max power rpm's were controlled by turning mags on and off.
There was a bigger radial than the R3350, Pratt & Whitney built the R4360. Four rows of 7 cylinders if you ever hear for the " corn cob " radial they are talking about the 4360. The B-36 bomber had 6 of them the KC97 tanker had 4 as well as the B-50 bomber. The very last model of the F4U Corsair WW2 fighter had one as well. If you really want to see some cool old engines the Udar Hazy Museum at the Dulles airport has a big display of engines as well as devlopment history. I think the airforce museum has like one entire hangar dedicated to powerplants
Yes, the 4360 had more, albeit individually smaller, cylinders. It also was not a turbo-compund radial, thus reducing it's complexity. A very cool engine, nonetheless!
Yes, The Udvar-Hazy museum is very good. However, I went there with a fellow pilot, who wasn't an A&P like myself and he wasn't as interested as I to look at everything from Curtiss OX-5 on up like I was. I'll have to do it again sometime.
Yeah the 4360 is basically 2 R2800's. I learned to operate radials on a howard 500 with 2 R2800cb16's
Howard 500, very cool.
I actually googled largest reciprocating engine, as I knew there was something by Lycoming, but I couldn't remember what the displacement was, and it was the Lycoming R-7755. It never was successful, as by that time the development of radials had reached it's zenith, and hair dryers, er, jet engines took over.