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Old 26th Jan 2011, 07:31   #1 (permalink)
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Why are Radial Engines so Hard to Start.

In this video of some P-47 Thunderbolts starting up, they pilots seem to have a devil of a time getting them started, and even after thay start the have a propensity to quit.

As well, they often admited puff of smoke after about 30 seconds. I know radials are smokey after on startup, oil pooling in the lower cylinders etc. But why the puffs?

It also seems that the Merlins on the Mustangs start up and go much like a modern engine.

Thoughts anyone?

YouTube - P-47 Start-up
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Old 26th Jan 2011, 09:19   #2 (permalink)
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Radials hard to start

It is a long time since I had anything to do with a Radial, however it is not so much a problem starting the engine but rather that they are rotated through several revolutions before introducing ignition to ensure that any remaining oil in the lower cylinders is purged before actually firing up. We used to count off the blades before selecting the ignition switches to ON. The puffs of smoke are simply small pockets of oil getting past the rings while the engine is cold and clearances greater than when normal operating temperature is reached.
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Old 26th Jan 2011, 09:28   #3 (permalink)
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My experience is with old pratts and curtiss wrights.
Cranking the thing thru so many blades to clear the oil and lower the risk of blowing the heads off then tickling the primer balancing the throttles and the ignition.Been many many years but it was an art form to get them started without stalling it, over priming, under priming ignition to early blam fart bang crash ...The risk of fire was pretty high too!

I guess it is mainly the oil issue with dirty plugs.They sound a bit clunky until the boost kicks in too. cant beat the sound of big piston engines when going full noise.
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Old 26th Jan 2011, 10:17   #4 (permalink)
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The oil problem was hydraulic locking, and therefore not an ignition issue ne'st pas?
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Old 26th Jan 2011, 10:43   #5 (permalink)
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Radials tend to be technology 60 years old and more.

What you are maybe overlooking is that at that time and before, almost ALL reciprocating engines were hard to start. On road vehicles the first diesels were such a nightmare that truck and bus operators (the early advocates of them) would tend to leave them running all night if they could. Early railway diesel locomotives were likewise.

I you've ever tried to start a petrol lawnmower after winter storage, or to get an old car going with a crank handle, you'll regard radials as a cinch to start !
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Old 26th Jan 2011, 11:16   #6 (permalink)
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Even a newly built IO-360 can be difficult to start (might be because the design is decades old... )...
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Old 26th Jan 2011, 14:51   #7 (permalink)
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Part of the starting issue is that to get a reciprocating aircraft engine to start, you have to get the mixture into a runnable range without the benefit of modern digital technology.
To get a reliable start, normal procedure is to start either too lean or too rich and transition to the opposite condition and if the ignition system is working, you will get a start somewhere in the middle. With big radials, a start near the lean condition will often cause a damaging backfire, so it is usually better to start from the too rich condition and transition towards lean. At least that is what we did in the T-28 and C-1A if I remember correctly. On the R-1820 we were supposed to turn the engine over by hand first before attempting to use the starter to avoid the oil hydraulic lock problem and resultant blowing off the cylinder head.
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Old 26th Jan 2011, 15:01   #8 (permalink)
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Why are Radial Engines so Hard to Start.
Because starting a radial (or most any other reciprocating engine) required the operator to have some "feel" for the machinery, something severely lacking amongst most of those those weaned on Nintendos.
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Old 26th Jan 2011, 15:09   #9 (permalink)
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Even a newly built IO-360 can be difficult to start (might be because the design is decades old...
Nope, it's because people don't know how to start them.
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Old 26th Jan 2011, 15:11   #10 (permalink)
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Even a newly built IO-360 can be difficult to start
I agree. However, my old Hughes 269B with an IO-360 had a throttle that a bit more resistance just after you opened it. When you twisted the throttle you could feel a slight bit of resistance at which point you pushed the starter button. It never failed to start if you followed this procedure.
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Old 26th Jan 2011, 19:39   #11 (permalink)
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If someone had put an impulse magneto on a radial it would be a pussycat to start. Actually the R-985 is not bad to hand-prop - I have a treasured video of my Dad (age 70+) propping one.
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Old 26th Jan 2011, 21:24   #12 (permalink)
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Starting large radials

In the dim past I flew a 4 engine a/c which had Wright R3350 engines, which are similar to those on the P47. The R3350 had oil seals in the supercharger which did not seal properly until there was significant oil pressure. Consequently, there was always a large puff of smoke on start.

It's also true that large radials, due to the less controlled cylinder/head temperatures (compared to liquid cooled engines) had larger tolerances between moving parts, and this led to more oil getting past the valves and cylinder rings. For the R3350, acceptable oil consumption in flight was 3 gallons per hour, although we would not normally see more than 1. Our aircraft had 80 gallon oil tanks per engine.

As for being hard to start, I don't have any recollection of that, at least in summer temperatures. Arctic conditions are a different story, and starts were always sporty there, even when we had taken precautions. There is a reality show running called "Ice Pilots NWT" which features a Yellowknife based airline operating WWII transports. The frequent starting issues shown on that show seem realistic.

Merlins, BTW, were no easier to start in the Arctic. They do at least have the advantage of a wonderful sound when running.
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Old 26th Jan 2011, 21:30   #13 (permalink)
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Winter starting required huge amounts of prime, which led to exhaust flames up to 20 feet long. This led to some complacency about flame on start up. One pilot of my acquaintance had an event during which the intercom equipped ground crew monitoring start was saying, for some time "torching.. torching.." upon which the captain looked at the engine and replied "Torching my a**, that engine's on fire" - at which point the ground crew dropped his fire extinguisher and left the scene! Crew had to use the on board engine fire extinguisher.
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Old 27th Jan 2011, 06:58   #14 (permalink)
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Just this week a student had trouble starting the 985 in a Beaver. He tried twice but it would not catch. It was pretty cold so I told him to prime again and give it another go. He primed, turned on the starter but still no joy. The prop came to a complete stop, he looked at me as if to say "what do I try next?", when the engine started, all by itself, no starter engaged.
Great old engine.
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Old 27th Jan 2011, 10:25   #15 (permalink)
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Hey..Old Fella...I think the number was 9, wasn't it!

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Old 27th Jan 2011, 10:59   #16 (permalink)

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I've operated mostly R985's, R1340's, R2600's, R3350's, and R4360's. Smoking on start is due to oil, primarily in the lower cylinders, but also due to excessively rich mixtures and a lot of unburned fuel initially passing through the engine.

Unlike a turbine engine in which one or several burner cans can typically be lit at the same time and easily, the radial engine may have 9 to 28 cylinders to individually light off.

During the start process as prime is applied, it's dumped into the supercharger as raw fuel, slung around the supercharger, and dumped into cylinders. Even in a small piston engine, mixture isn't consistent from cylinder to cylinder, and the mixture needs to be within an easily ignitable range for the cylinder to fire. Too rich or too lean, and it wont' fire. During start when airflow is low through the carburetor, establishing a consistent mixture is compounded a lot more for each of the cylinders on the engine.

The spark plug in each cylinder needs to be able to fire, and plugs that are fouled with lead or oil tend to fire weakly or not at all. The end result is that some cylinders will fire while others won't.

You may have noted that some radials tend to catch fire on start more than others. This is partially due to starting techniques, and partially due to the construction of the engine itself. Particularly in the case of cold weather starts, large amounts of prime are sometimes used, and improper starting techniques can result in a lot of gasoline and fires. On some motors, we would have five gallon buckets under the engine during the start to catch fuel pouring out of the supercharger drain, as the technique involved flooding the engine then starting it off the mixture.

Unlike "modern" horizontally opposed recip engines, the radial offers a fairly poor induction distribution system with respect to establishing a consistent mixture at each cylinder. This is more pronounced at start. The first few blades (counted as blades passing a given point, by whomever is turning the starter on a particular airplane) are done with ignition off to clear the engine, ensure no hydraulic or liquid lock (particularly in the bottom cylinders), and to begin drawing fuel into cylinders (eventually); the initial part of the start looks slow to an observer because the engine is being turned through a number of revolutions before the start actually begins. In some cases, this is done by hand to protect the engine, but in most cases it's done with the starter to protect the engine (because the starter clutch will slip before bending a connecting rod in the event of a hydraulic lock). The particulars depend on the engine and the operator practice.

Once fuel and ignition is introduced, each cylinder must be individually awakened. This isn't necessarily a fast process. Whereas there's a lot of mass for the starter to turn over, it doesn't happen quickly, the mags aren't turned rapidly, and mag points don't open rapidly at first, and spark isn't necessarily as hot. Plug fouling and inconsistent mixture mean that each cylinder will "wake up" at a different moment, and enough cylinders need to be firing (and firing consistently) enough to keep the engine turning. The result is that the engine tends to lope and cough on start, belching smoke, often banging and after firing (and in some cases, undesirably, backfiring (the difference between burning through the exhaust or through the induction; a backfire can separate induction tubes and cause an engine fire, whereas an after fire does not).

Starting some radial engines is part science, part art. The start can vary, depending on how much oil is in the cylinders. Some engines can be dry consistently, and then one start be locked or wet ("wet," with a radial, can be expressed in terms of both excess fuel, and oil) on the next. Some engines can sit for months and never liquid lock, while others can sit for five minutes and have hydraulic lock problems, or oil pooling in the lower cylinders. For some radial engines, especially in cold weather, starts can be more art than science.
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Old 27th Jan 2011, 13:31   #17 (permalink)
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There's a series of Youtube videos about starting a R-985 after storage - see:
YouTube - Starting a Pratt & Whitney 985 Part 1 (Intro)

...then look for others in the series.
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Old 27th Jan 2011, 13:49   #18 (permalink)
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Merlins, BTW, were no easier to start in the Arctic. They do at least have the advantage of a wonderful sound when running.
I don't know, I think the sound of a big radial waking up one cylinder at a time is way better than a merlin.

And of course sometimes they'll just start right up fine.

Last edited by Peter Fanelli; 27th Jan 2011 at 14:00.
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Old 27th Jan 2011, 14:02   #19 (permalink)
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How do you preheat a Merlin, or for that matter, any water cooled aircraft engine? Is there such a thing as a block heater for these engines?
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Old 27th Jan 2011, 16:59   #20 (permalink)
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Some radial installations had an oil dilution switch to reduce cold weather starting torque and a boil-off procedure to restore oil viscosity after start.

After an excellent landing you can use the airplane again!
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