DCS Radials - P-47, I-16 and Fw 190 pilots take note!

[DD_Fenrir]

Found this gem on the ED forums:

https://forums.eagle.ru/topic/260841-engine-bearings-and-how-you-can-keep-them-happy/?tab=comments#comment-4558624

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This seems to be a hot topic lately, so I've put in some research to learn as much as possible about this, and what we can do to mitigate the problem with wrecking our bearings.

Background on the problem:

This is a problem that is unique to radial engines, and is not much of a factor in inline or V engines, in that reciprocating loads can damage bearings. Under normal conditions, combustion happens, pushes the piston and connecting rod down in the cylinder, which turns the crankshaft. Crankshaft turns the reduction gears, gears turn the propeller, propeller creates thrust. This is true for both radial and inline/V engines.

Lets briefly explore V engines, like the famous Merlin 61 V-12, used in warbirds such as the Spitfire and Mustang. The crankpins on the crankshaft were only attached to two connecting rods, and being a four stroke engine, only take the force of a power stroke only once per revolution. This is the key concept here, but bears repeating, the crankpins only take the force of a power stroke only once per revolution in a V engine. Here's a picture illustrating this. Note that two pistons share a crankpin.

 

 

Radial engines are a whole different animal compared to their V shaped cousins. The crankshaft layout is a lot simpler, with one row of cylinders sharing one crankpin. The cylinders are then spaced evenly around that point, and nearly always feature an odd number of cylinders in that row. Having an odd number of cylinders simplifies the ignition timing, as every other cylinder fires in turn. (a nine cylinder would have the firing order of 1-3-5-7-9-2-4-6-8) Since all of the cylinders have to share one crankpin, the crankshaft is built much larger and stronger to handle the forces involved. The connecting rods also have a unique arrangement, with one serving as the master rod which connects to the crankshaft, and the other cylinders rods are articulating rods that bolt to the master rod. Looks something like this:

 

 

On to our R-2800 engines now. The cylinders in one row, nine of them spaced 40° apart, share one crankpin, and every other cylinder fires as the engine runs. The crankshaft thus takes the force of a power stroke every 80° of rotation, or 4.5 times every revolution. Contrast that with the V engine where a crankpin only takes one power stroke every revolution. That's a lot of force on the crankshaft, but fortunately Pratt and Whitney thought this through and added a hole for pressurized oil to flow in between the crankshaft and master rod assembly at the correct place where a power stroke would be pushing down on the crank (the thrust side). You certainly don't want any metal on metal contact, especially in the one spot on the crankshaft where power strokes are pounding down on it. A lot of radial engine crankshafts featured this oil hole, not just ones built by P&W.

 

Here's what the R-2800 crankshaft looks like. There are several bearings pointed out here, the mains and the crank journals. The mains are supported by the crankcase, and are not the ones that we damage by running the engine improperly. Those would be the crank journals that take the damage. There are only two, one for the front row, and one for the rear. In the photo, if you look at the rear crank journal (right hand side) you can see the oil hole for that bearing facing downward.

 

This (and a lot of others) radial engine were very well designed, and provided many hours of reliable service (combat damage and ham fisted pilots not withstanding). So how do the bearings get damaged? By reversing the reciprocating load on them, or in other words, windmilling the prop.

 

The engine is designed to provide power to the prop and is built to do exactly that. When power is reduced and sufficient wind speed exerts more force on the prop than what the engine is providing, the loads in the engine are reversed by 180º. The prop is now driving the crankshaft, which is now moving the pistons around. The crank journal is taking the load on the side opposite of the oil hole, where there is very little oil. This leads to metal on metal contact, an overheated bearing, and metal in the oil, loss of power, and given enough time, total engine failure. This is likely amplified somewhat in that since the engine is still turning it has oil pressure, and that is likely exerting some hydraulic force on the thrust side of the crank, pulling the opposite side in a bit closer. Oil pressure eventually falls and the oil temperature rises due the engine wear and friction (not sure if DCS models this behavior). While this is happening the piston rings are also fluttering in their grooves, leading to damaged ring lands and broken rings (not sure if DCS models this either). How quickly damage accumulates is a function of severity and time. A high RPM steep dive would damage the engine more rapidly than a moderate RPM shallow dive.

 

Now that we know the how and the why the bearings get damaged, how do we prevent it, and why does this happen more during landings? And with that, how do we know when the engine is being windmilled?

 

The point where the engine starts to be windmilled is different between aircraft (weight, speed, props, RPM, and MP settings). Aircraft that had a BMEP gauge or torquemeter had a decent idea of when this happened as those instruments were a good direct measurement of power output. Other aircraft just had to make do with the RPM and MP settings. One of the old "rules of thumb" was to keep at least one inch of MP for every 100 RPM. If you're doing steep dives link the prop lever with the throttle and pull them both back during the dive. Don't close the throttle entirely during the dive, keep some power on.

 

For landing techniques, there are two schools of thought. The military method and the airline method. I'll elaborate on both, but keep in mind the time frame of this (1930's to 1960's), the heyday of radial engine aircraft.

 

The military technique mostly utilized the overhead break landing pattern, with a high RPM setting during the approach. The Mustang usually had 2,700 RPM set, the Jug set 2,550 RPM. The Air Corps/Air Force preferred to have its pilots ready for a go around, hence the high RPM setting. During the war, if the engine could be used for the next mission, great. If not, a new one was installed. This mentality carried on into the Cold War when the military had a fairly generous budget. Keep in mind that jets were up and coming as well. Accident rates with them were higher, mostly due to pilots transitioning from piston engines to jets. Piston engine can deliver power pretty quick when you push that throttle up, early jets not so much. Jet engine spool up times took a lot longer than pistons, especially if one was on short final. It took a while for the Air Force to adopt the stabilized approach (high drag, high thrust) with jets. The high RPM approach with pistons, stayed with them. Better to risk engine damage and a go around in order to use the plane (and pilot) again. Maintenance costs were not of much concern.

 

If you are going to use the military high RPM approach, follow the 1" MP per 100 RPM rule of thumb. With RPM set at 2,550 RPM, don't let the MP fall below 26", until you are on final and near the flare. By then your airspeed should be between 90-110 MPH, and your RPM should naturally fallen some. The prop should be sitting on the low pitch stops and there won't be sufficient wind speed to drive the prop.

 

The airline technique was much different. They had to stay profitable and keep happy customers, and work within much tighter budgets than the military. Pilots wrecking engines would eat up maintenance budgets with replacement engines, not to mention taking a bird out of service and cancelling flights. Aircraft only make money when they are flying. Their technique with descents and landings was to start down sooner (shallower descent) while keeping the engines at cruise RPM settings (around 1,900 RPM). and also not allowing the MP to fall below the 1" per 100 RPM. This would be maintained during landing, with the the RPM brought up at the flare when the throttles are closed when there was no risk of the prop driving the engine. This led to quieter operations and higher engine longevity.

 

This method gives some more flexibility with power settings as you are not constrained to a fixed RPM during the descent and approach.

 

The key points in summary:

 

• Radial engines are pretty tough, but can be easily damaged if its pilot is not careful with the engine settings.
• Pull the RPM back before entering a steep dive and keep a little bit of power on.
• During descent and approach, keep at least 1" of MP for every 100 RPM to ensure the engine is providing power and not being driven by the prop.

 

 

[BluBear]

Thanks old bean, very informative indeed!
Coincidentally, I had just read the following article over at VAE which touches on some of this:

https://vintageaviationecho.com/richie-radials/

 

[Painless]

Both very interesting reads thanks chaps. It always fascinates me the way a Radial engine crankshaft and con rods work, it’s poetry in motion.

1” of MP per 100rpm as a rule of thumb, would that be transferable to inline engines, just as a good habit ?

[Jabo]

It certainly looks like there's a lot of information there, but I don't understand the vast majority of it. Clearly more reading is required.

[BluBear]

3 hours ago, Jabo said:

It certainly looks like there's a lot of information there, but I don't understand the vast majority of it. Clearly more reading is required.

I found that reading the left-hand sides of the paragraph with my left eye and right with my right proved most fruitful; although there was often a fight for the last word when they met in the middle.

[fruitbat]

Nice article.

 

Now I'll understand better how I broke my engine in future! 

[FoolTrottel]

Ah, so, this could be why I regularly and mysteriously make the I-16's engine lose power?

But, he mentions this twice: "not sure if DCS models this"

And this "1" MP per 100 RPM rule of thumb." is not helpful in the I-16, as it not do MP in inches!

Anyways, I'll lower revs in a dive, and keep some throttle open ... Thanks!