radials - question

[johnm]

I thought first morning start - a radial motor had to be pulled through to get rid of any oil in bottom cylinders - to stop hydraulic shock ? .................. or was that for compression ?

 

I can understand this is possible for a DHC2 beaver with wheels but what about a float plane - what about big radials ........ worse still a Mars or a Sunderland

 

..................... whats the answer aviators ?

 

 

[facthunter]

Winding them through for a number of turns on a starter with the right settings is used. It is to prevent the possibility of hydraulic locking for oil or fuel. Nev

 

 

[johnm]

FH - sorry I am no mechanic - what setting dodge hydraulic locking - do they have decompression thingy ?

 

 

[dutchroll]

Nev is quite correct. On the DHC4 Caribou we cranked the big P&W radials on the starter to check for hydraulic lock (there's no other way to do it). 15 blades for a cold start and 6 or 9 for a warm start (can't recall exactly - it was quite a while ago!).

 

The prop would of course stop virtually dead if you encountered hydraulic lock, so you were watching the rotation out the window very carefully and released the starter immediately. I had this happen once. You then remove the spark plug from the bottom cylinder/s and drain the oil out. In our case, almost an airsickness bag full! You can then attempt another start.

 

You never introduced ignition until after the required number of blades had passed. The biggest danger was if the engine actually fired, then encountered hydraulic lock. This does the damage. Running it on the starter alone wasn't a problem, so long as you release it when the prop stops! I'd suggest on a smaller radial though, there would be a higher probability of damage if you cranked it on the starter, hence obviously if you can pull it through by hand, this would be far more preferable.. The big radials were pretty solid engines.

 

 

[facthunter]

Sorry johnm. Only just caught up. There is no decompressor It is the starter torque setting. My view is that IF the starter was motoring the engine fairly fast it could still damage the motor. The risk is primarily with bending connecting rods and cylinder failure. Counting the prop blades is the way it is done as DR says. What determines that is the number of blades and the engine to prop reduction ratio.

 

Turning the actual crankshaft two turns as a minimum should check all cylinders. Very large engines can be turned by hand as long as they are not too high up..

 

While it might seem to be a hang over from past times I would still do it. If you are doing joy flights or such and the engine has not been shut down for long, perhaps skip it but why not just delay the introduction of fuel and ignition till the prop has turned the min number of times, and be safe. It protects the overall engine reliability. Nev

 

 

[dutchroll]

The engine of the plane in my profile photo requires pulling through 18 blades (3 full revolutions) on the first flight of the day to ensure no hydraulic lock. After that you're good to go for the rest of the day.

 

There are enough people out there who've discovered the cost of a teardown and conrod replacement! ;)

 

 

[facthunter]

There is a big flywheel effect in a radial You have large counterweights and the whole bunch of conrods and pistons hanging off the crankpin opposite as well. If there is a cupful of oil or fuel that gets into the cylinder It will do damage. The engine by it's design has a difficulty with oil draining in the centre of it It doesn't really have a proper sump for it to go to. When it's running the scavenge pump returns it to the tank and all is well. Most of them have a fairly high rate of oil consumption, and usually start with quite a bit of blue/black smoke. The modern Rusky/Chinese ones probably have better oil control. ( not as smoky) but still ,the same beast at heart. There's something special about a "Big round engine" Nev

 

 

[dutchroll]

As a matter of interest, the stock russian M14P can be quite bad for oil consumption and leakage, but mine has the Barrett Precision Engine mods which include brand new modern pistons and rings. This slashes the effect dramatically. It also has a shutoff valve at the oil tank, and a manually operated electric scavenge pump in the lowest line to pump back to the tank after shutdown. Despite that, the recommendation for pulling through the 18 blades at the start of the flying day is still maintained as a precaution.

 

 

[Old Koreelah]

If oil can accumulate in the lower cylinders of a radial, what about all the other upside-down engines? Many had the heads lower than the crankcase, not just on German and Russian aircraft. Does the same problem occur in Gypsy engines?

 

 

[David Isaac]

Lyle, the technique with Gipsys is to pull through 8 compressions for priming before you do the start pull, so you would soon know if there was a hydraulic lock.

 

 

[dutchroll]

If oil can accumulate in the lower cylinders of a radial, what about all the other upside-down engines? Many had the heads lower than the crankcase, not just on German and Russian aircraft. Does the same problem occur in Gypsy engines?

Radials by the nature of their design generally use a large amount of oil compared to most straight engines so their susceptibility has always been more than straight engines, even for those with cylinders below the crankcase. But it is certainly possible for any upside-down engine to get hydraulic lock.

Gipsy's have, I believe, been known to get hydraulic lock however they didn't really have a reputation for it, unlike some of the radials. I believe the cylinders on a Gipsy extend a fair way into the crankcase so it was never really routine to have much oil drain into them - at least not enough to have a major effect.

 

Notwithstanding that, I know there are Gipsy owners who will pull their props through if they've been sitting for a while, for the purpose of checking for hydraulic lock due to oil.

 

 

[facthunter]

Yes they are spiggoted further into the crankcase on the inverted in line and drain by gravity on the Gypsy 1-c into the oil tank.. The rocker boxes are only half full of oil and there is no actual oil feed to them. They are half filled at service, each 100 hours.

 

Some radial engine rocker boxes are flooded. They are much harder engines to oil, (and feed fuel to evenly).Nev

 

 

[Old Koreelah]

Thanks for the informative replies, fellas. I always thought it better to have the heads under the crank, because all that valve gear needs lots of lubrication. What is it about radials that makes them chew so much oil (I have read of 30 litres per hour)? Is it the sloppy clearances of air-cooled radials?

 

 

[facthunter]

I wouldn't call it "sloppy" but it must be adequate. . Aircooled engines NEED more clearance because they run hotter and if they have steel cylinders (normally do) it expands less than alloy ones would. When the engine is operating the running clearances for the pistons will be much less than when the engine is cold, but it still has to have a margin for when it has to work hard. Like METO with an engine fail case. .

 

Radials are always known as high oil use engines. There's not much volume in the crankcase so maybe a fair bit goes out the breather. The Planes I flew had an oil tank in the cabin to transfer oil to a selected engine in flight, that you use when you have to. The actual range of the aircraft was limited by the amount of oil carried as well as avgas in the tanks. You gotta have that antisqueek. Nev

 

 

[dutchroll]

Thanks for the informative replies, fellas. I always thought it better to have the heads under the crank, because all that valve gear needs lots of lubrication. What is it about radials that makes them chew so much oil (I have read of 30 litres per hour)? Is it the sloppy clearances of air-cooled radials?

Yes the clearances on radials had to be fairly generous by design. These were big engines with a lot of moving parts and a lot of oil required for lubrication and cooling being pumped and splashed in all sorts of directions. Also bear in the mind the era and technology around when these engines were designed.

In the case of the Russian M14P, it was designed not long after WW2 and needed to run on heavy, poor quality Soviet-era fuels and oils in cold weather ("Vladimir - ve run out of oil" "No problem boss, ve have ice block of best Siberian crude out ze back. I go get blowtorch to unfreeze it!"). The one I own has the Barrett Precision Engines mods, and they have introduced it to the 21st century by putting very high quality, fine tolerance racing pistons, rings and seals into it with some other improvements which have slashed the oil consumption to "flat engine" parameters.

 

On the old Caribou (P&W R2000 Twin Wasp) I remember the oil usage limit was 2 US gallons per hour. In 1994 I flew one up to Cape York in which one engine was burning 2 gallons/hr and the other engine leaked 2 gallons/hr (which made for a very messy main gear well, I can tell you). Flying away from our Townsville base on multi-day trips it was routine to carry multiple 20 litre drums of engine oil in the back. Upon arrival somewhere after a day of flying, while the flight engineer was attending to various things, one of us would usually assist him by refuelling or if you were feeling generous, re-oiling. Going through a whole 20 litre drum at the end of a long day wasn't uncommon. Often you had to crack another one open too! "Top up the gas, fill the oil!" was the catch-cry. The RAAF clothing store hated us, as we would regularly return formerly-green flying suits and gloves black and reeking of engine oil for exchange. Dirty work, flying a Caribou!