Ram-Compression / Ice-Buildup

[fliger747]

The R2800 series engines used various supercharger arrangements (and the occasional turbocharger setup (eg. P-47)) . Directly attached to the engine and rotating at shaft RPM was a supercharger stage at which point the fuel was injected from the "injection carburetor". Think of this actually as sort of a single point fuel injection. Where carb ice could be an issue was at the venturi and throttle plate which disturbed both the air flow and metering setup. At small throttle settings, the airflow could become almost completely blocked!

However "neutral blower" which means no accessory stages were engaged was only used up to maybe 4-5,000' if high power was required. Both the F6F and F4U were equipped with two auxiliary stages, "low and high blower". Low blower might be used to say 12,000' and high blower above 18,000'. As noted before each blower stage will subtract shaft HP from the engine. A 2000 HP SL R2800 for instance only produces about 1600 HP max with high blower engaged as the blower uses some 400 HP! However where does this energy go, it is mostly turned into heat, a lot of heat. With any auxiliary blower engaged carb ice is quite unlikely! Fortunately as the higher blower stages are used at altitude where a normal lapse rate would make for low static air temperatures so thins helps the equation quite a bit. The aircraft were equipped with carb temp gauges, which was helpful in using both carb heat if necessary and more commonly managing intercooler flaps.

Carb ice can be formed at quite sultry temperatures as the pressure drop as air is drawn into an induction system cools it substantially. Even the lowly R985 could have carb ice on a nice 70F day!

 

[mikewint]

I'm confused here

Me too. Aircraft performance cannot be attributed to single factors.
The F6F-5 Hellcat was longer,wider, and shorter than the F4U. The Hellcat was also lighter (MTOW) than the F4U
While both had essentially the same engine the Hellcat had an P&W R-2800-10W 2,000 hp (1,491 kW) at 2,700 rpm at 1,000 ft (305 m); 1,800 (1,342 kW) at 2,700 rpm at 15,500 ft (4,724 m); up to 2,250 hp (1,677 kW) WEP with water injection. Similar to -8 series apart from downdraft PT-13G2-10 and PT-13G6-10 (-10W) carburetor.
While the F4U's P&W engine was a "C" series R-2800-18W developing 2,100 hp (1,566 kW) at 2,800 rpm at 1,000 ft (305 m); 1,800 hp (1,342 kW) at 2,800 rpm at 25,500 ft (7,772 m). This was the first series production variant of the "C" Series, which was a complete redesign of the R-2800. Some of the main changes were forged, rather than cast cylinders, allowing an increased compression ratio (from 6.65:1 to 6.75:1), a redesigned crankshaft, a single piece, rather than split crankcase center section, and a two section nose casing, incorporating hydraulically operated torque-monitoring equipment and an automatic, vacuum operated spark-advance unit. The supercharger used fluid coupling for the second stage. Updraft Bendix-Stromberg PT-13G2-10 carburetor.
Connors, Jack, The Engines of Pratt & Whitney: A Technical History, AIAA, 2009.

 

[tomo pauk]

Bent, Ralph D. and McKinley, James L., Aircraft Powerplants, 5th Edition, McGraw-Hill, 1985.

Edgars, Julian, A Guide to Turbos and Superchargers: A Comprehensive Guide to Forced Induction, Clockwork Media Pty Ltd., 2001.

White, Graham, Allied Aircraft Piston Engines of World War II: History and Development of Frontline Aircraft Piston Engines Produced by Great Britain and the United States, Society of Automotive Engineers, 1995.

Wisniewski, Jason R., Powering the Luftwaffe: German Aero Engines of World War II, FriesenPress, 2013.

Do ALL of them agree one with another? How well the last book covers Allied engines?
Please check out this data sheet, where only 'L' and 'H' 'speeds' are noted - just 2 speeds, not 3: picture. That is for last military Merlins, 130 and 131, as used on the Hornet.

 

[tomo pauk]

The R2800 series engines used various supercharger arrangements (and the occasional turbocharger setup (eg. P-47)) . Directly attached to the engine and rotating at shaft RPM was a supercharger stage at which point the fuel was injected from the "injection carburetor". Think of this actually as sort of a single point fuel injection. Where carb ice could be an issue was at the venturi and throttle plate which disturbed both the air flow and metering setup. At small throttle settings, the airflow could become almost completely blocked!

However "neutral blower" which means no accessory stages were engaged was only used up to maybe 4-5,000' if high power was required. Both the F6F and F4U were equipped with two auxiliary stages, "low and high blower". Low blower might be used to say 12,000' and high blower above 18,000'. As noted before each blower stage will subtract shaft HP from the engine. A 2000 HP SL R2800 for instance only produces about 1600 HP max with high blower engaged as the blower uses some 400 HP! However where does this energy go, it is mostly turned into heat, a lot of heat. With any auxiliary blower engaged carb ice is quite unlikely! Fortunately as the higher blower stages are used at altitude where a normal lapse rate would make for low static air temperatures so thins helps the equation quite a bit. The aircraft were equipped with carb temp gauges, which was helpful in using both carb heat if necessary and more commonly managing intercooler flaps.

There was just one auxiliary compressor on 2-stage R-2800s. 'Low blower' meant that 1st speed is engaged, 'high blower' meant that 2nd speed is engaged. Auxiliary stage couls also be disenagaged - 'neutral blower'. The main stage was always on, without the ability to change it's own gearing = singe speed.
R-2800-10 from F6F manual:

 

[mikewint]

Do ALL of them agree one with another?

Those are the books I used over all. The three speed Merlin was from the White book but I can find nothing to corroborate the "three".
So with no hesitation bow to your knowledge. As I posted earlier I suspect confusion with the Griffin engine

 

[mikewint]

Maybe I worded that badly,

Yes, I took your statement to mean equal volumes of air differing only in temperature. If you are going to vary the volume then 1000 L of cold air at 50% relative humidity contains more total water than 100 L of warm air at the same humidity.
The combustion of a gallon of gasoline requires a large (relatively speaking) amount of air so your point is valid in that sense.
At all altitudes air is 20.9% Oxygen thus it requires 124.9lbs (56.8kg) of air per gallon or in terms of volume 1625.4cu.ft. (46,026 L) of air.
A P-51 averages about 60 gallons per hour so in one hour you'd have to move 97,407 cu.ft. (2,761,506 L) of air through the engine. At full Mil. power that could reach 200 gallons per hour.

 

[fliger747]

Yes, indeed the "neutral" blower was alway engaged and there were variously in different dash numbers of R2800 single or two speed blowers. Selecting via a clutch and shift mechanism one could select no aux blower (neutral) or low or high blower. Some transport aircraft without need for high altitude performance might only have the low blower installed.

Ast to the above post it is important to note that high power aircraft engines were limited by the cooling available. For this reason takeoff and climb was usually conducted with an over rich mixture. The extra fuel evaporation both caused cooling, and the mass flow through the engine carried more heat out the pipes. Also the richer mixture burned at a lower temperature. The utilization of Water Methanol injection cooled the mixture, allowed a more stoichiometric mixture to be utilized as well as higher MP for better power. The mass of the water also improved the heat flow out the pipes.

Most WWII piston engines averaged about .45-.5 pounds of fuel per hour per HP generated. A 2000 hp engine might use about 155 gph at that rate. In actuality use of rich mixtures at high HP setting might make this worse.

 

[MIflyer]

Heating up air DECREASES its DENSITY, thereby partially defeating the reason for having a supercharger. An intercooler (P-38, B-17) or aftercooler (Merlin Mustang) helps improve that condition and also may prevent excessive inlet air temperature from causing pre-ignition and other ills.

Pull on the carb heat on a small aircraft engine and the RPM goes down; that is standard preflight check. It is like adding density altitude. My 1978 Celica came with the auto carb heat feature disconnected and I rigged up a manual carb heat control. It was amusing to see it idling at 1000 RPM and then switch on the carb heat and at the next stoplight see it at 800 RPM. By the way, on such automobiles the carb heat is ON all the time if the temperature is low enough and the auto system controls the carb inlet temp at 100F, even in the winter, which is hardly desirable in terms of good gas mileage.

 

[Zipper730]

R-2800-10 from F6F manual:

View attachment 522412

The bar type grill... that seems like something that would inhibit the build-up o ice, but it would probably reduce ram-compression: That the culprit?

 

[fliger747]

A trick with radials suffering from high cyl head temps (as in a climb) is to apply carb heat! This seems a little counter productive, but in actuality further enriches the mixture which lowers the combustion temperature and increases the extraction of heat via mass flow through the engine.

As to the F4U, the aircraft used the R2800-8 and 8W which was a "B" model. The "C" engines were not used till the F4U-4. This engine as indicated incorporated many improvements. One was a re engineering of the oil scavenging system. Supposedly at max RPM just slinging the oil around in the cranks wasted several hundred HP.

Cheers! T

 

[XBe02Drvr]

The conditions required for carburetor icing to occur are moist air with a temperature as low as 13 degrees Fahrenheit and as warm as 55 degrees F.

I had a student out on her third supervised solo in the pattern who had to chop throttle suddenly when a NORDO Aeronca Champ cut her off on downwind and she was about to hit him in the ass. It was a 50° spring day with a lot of snowmelt going on, and her engine promptly quit. She promptly turned a tight base, slipped off three hundred feet of altitude, dumped full flaps, and made the prettiest dead stick you ever saw, right on the numbers, and coasted to a stop right in the middle of the runway, cutting off the Champ on final. She immediately hopped out and tried to push the plane off the runway, but discovered she couldn't push and steer at the same time, so she got on the radio and asked for help. I told her to start the engine and taxi to the ramp.
"I can't, the engine died."
"Try it!"
"Lo and behold, hallelujah! How did you know it would start?"
"Did you pull carb heat BEFORE you pulled power?"
"Oh shit!.... I'm sorry, little airplane, I didn't mean to do this to you."
Cold blooded Continental O-200s,
don't you just love 'em?
Cheers,
Wes

 

[PStickney]

Tomo my source text stated:
Merlin engines that powered the Battle of Britain Spitfires had single-stage, single-speed superchargers. It was not until the Merlin Mk XX that a second speed was added, but not a second stage. When Stanley Hooker took over supercharger design at Rolls-Royce, he realized that air flows in the existing Merlin superchargers were imperfect. He improved them, and this resulted in a new single-stage two-speed supercharger for the Merlin Mk 45. This new design allowed output to be raised to 1,515 hp at 11,000 feet. The Royal Air Force put this new engine into the Spitfire Mk V airframe just in time to battle the new Bf 109F, which began to appear in large numbers in early 1941.

The arrival of the Fw 190 in late 1941 made even these engines obsolete. Fortunately, Rolls-Royce was ready with a two-stage, two-speed supercharger for its engines, beginning with the Mk 60 series. These engines powered the Spitfire Mk IX, which restored British parity with the best German fighters. These two-stage supercharged Merlins came considerably later than the two-stage R-1830. In compensation, this delay allowed the Mk 60 and engines to have not only two stages but also two speeds and eventually three speeds for greater pilot control.

I've done more checking and can find nothing to corroborate this statement EXCEPT:

The Griffon 60, 70, and 80 series featured two-stage supercharging and achieved their maximum power at low to medium altitudes. The Griffon 101, 121, and 130 series engines, collectively designated Griffon 3 SML, used a two-stage, three-speed supercharger, adding a set of "Low Supercharger (L.S)" gears to the already existing Medium and Full Supercharger (M.S and F.S) gears.

So I wonder if the first author confused Merlin and Griffon??

A bit of a quibbl, here. The first j2-Speed supercharged Merlin was the Merlin X, a contemporary of the Merlin I/II of the early Spitfires and Hurricanes, and the low altitude Merlin VIII of the Fulmar.
The Merlin X was developed to give grater takeoff power for heavily loaded bombers, like the Wellington Mk II, without sacrificing altitude performance.
The Brit production planners felt that the fighters weren't going to need that feature, due to their higher power to weight ratios.
(Note that this didn't just apply to engine selection - at the beginning of the War, the fighters made do with fixed- pitch propellers, getting 2-position props starting in late 1939, and constant speed the flight upgrades beginning in the late spring of 1940. This has large impact on performance throughout the flight envelope. (In fact, since a fixed-pitch prop is only at good efficiency at a single speed/altitude/power point, there's not much point to a multi-speed supercharger unless you are using a constant speed prop.))

 

[XBe02Drvr]

at the beginning of the War, the fighters made do with fixed- pitch propellers, getting 2-position props starting in late 1939, and constant speed the flight upgrades beginning in the late spring of 1940.

By 1939, the rest of the modern world had gone to double-acting hydraulic or electric constant speed props. Whatever were the Brits thinking, putting a fixed pitch prop on a fighter?? How could a country so forward in other technologies be so backward in propellers?
I know they build good props; our Fokkers had Dowtys on them and despite being expensive to maintain, they were rugged, reliable, and trouble-free.
Cheers,
Wes

 

[tomo pauk]

By 1939, the rest of the modern world had gone to double-acting hydraulic or electric constant speed props. Whatever were the Brits thinking, putting a fixed pitch prop on a fighter?? How could a country so forward in other technologies be so backward in propellers?

Easy does it.
British were ramping up production fast before the ww2, thus in 1939 and 1st six months of 1940 they out-produced Germany (and France combined?) in engine production. They were constantly out-producing Germany with regard to the engines.
Each engine needs a prop, bombers needed a better prop more than fighters did, so the bombers had the 1st call. Fixed props on fighters worked. Had the British production of engines and props been on level of Germany (let alone France) in 1939-40, would they won the BoB?

 

[fliger747]

Surprisingly wooden props aren't all bad. They are much lighter giving a much reduced gyroscopic effect, accelerate and decelerate much more quickly and can have some advantageous flex patterns. Not enough to compete with the constant speed props, but a lot better than you might think!

 

[PStickney]

Easy does it.
British were ramping up production fast before the ww2, thus in 1939 and 1st six months of 1940 they out-produced Germany (and France combined?) in engine production. They were constantly out-producing Germany with regard to the engines.
Each engine needs a prop, bombers needed a better prop more than fighters did, so the bombers had the 1st call. Fixed props on fighters worked. Had the British production of engines and props been on level of Germany (let alone France) in 1939-40, would they won the BoB?

Just so. Every phase of Aviation is a matter of compromises - not just design and employment, but planning and production, If your sources can only produce so many propellers, you'll have to decide where they go. If the fighters have adequate performance, then the bombers get them.
Note that the same thing happened with the introduction of the Merlin XX series. Originally the Spitfire Mk III was to get the Merlin XX, but the introduction of the Bf 109F meant that the Hurricane needed the better low level performance to survive - So the Spit III morphed into the single-speed Merlin 40 series powered Mk V, and the Hurricane II got the XXs.

 

[tomo pauk]

Note that the same thing happened with the introduction of the Merlin XX series. Originally the Spitfire Mk III was to get the Merlin XX, but the introduction of the Bf 109F meant that the Hurricane needed the better low level performance to survive - So the Spit III morphed into the single-speed Merlin 40 series powered Mk V, and the Hurricane II got the XXs.

Several claims that I don't agree with there.
Spit III morphed, too late, probably into Spit VII. Spit V (more or less Spit II + Merlin 45) didn't have any of improvements that made the Spit III sleek - internal BP glass, retractable tailwheel, fuly covered wheel wells.
Installation of Merlin XX on Hurricane was an attempt to adress lack of performance vs. the Bf 109E at all altitudes (test report). Merlin XX posessed much better mid- and hi-alt power than Merlin III, eg. above 15000 ft it gave 20% more power, and Hurricane IIa gained 20-30 mph vs. Mk.I above 17000 ft. What was actually important is that it more or less cancelled out the performance gap vs. Bf 109E that Hurricane I suffered.

 

[Zipper730]

Carburetors can form ice in them because they rely on a venturi in their design. Venturis are tube-like passages that are wide at one end, taper somewhere in the middle and then eventually widen back to their original dimension. The purpose of the venturi in a carburetor is to create a low-pressure region by increasing the speed of the air flowing through it. The low-pressure region, working in conjunction with the atmospheric pressure acting on the fuel in the float bowl, allows a carburetor to feed fuel to the engine.

So basically the airflow feeding into the engine is like a bell-mouth; but then further in narrows at the venturi to get the fuel in; then widens back out before reaching the engine and it's where the narrowing in occurs you can get the ice formation by both the pressure and temperature change and the fuel/air mix?

The R2800 series engines used various supercharger arrangements (and the occasional turbocharger setup (eg. P-47)) . Directly attached to the engine and rotating at shaft RPM was a supercharger stage at which point the fuel was injected from the "injection carburetor". Think of this actually as sort of a single point fuel injection.

It was meant to allow one to do push-overs and not lose the engine if I recall correctly.

I would have thought the pressurized set-up would have made the venturi unnecessary (the venturi seems to work like an atomizer, to "suck" the fuel into the airflow going past it) -- it might sound stupid.

Would the filters/heating elements play a role in affecting ram-compression figures if they were either present/not present or designed right/rong

 

[fliger747]

To explain a bit, in the Venturi the pressure drops due to the increase in velocity. In a normal carburetor this sucks the fuel out of the carburetor bowl, which uses a float to keep the fuel level constant. In the injection carburetor the venturi is used as a metering device to gauge the mass of the passing airflow. In neutral blower the carb venturi is ahead of the supercharger stage and so is somewhat vulnerable to carb icing. The R985 which has a standard carb before the supercharger is quite subject to carb icing effects and monitoring of carb temp and EGT values is part of the standard engine scan. The addition of auxiliary supercharger stages (or a turbo) generally pre heat the air (when engaged) such that use of the intercooler is a more common task.

 

[XBe02Drvr]

It was meant to allow one to do push-overs and not lose the engine if I recall correctly.

I would have thought the pressurized set-up would have made the venturi unnecessary (the venturi seems to work like an atomizer, to "suck" the fuel into the airflow going past it)

The T34 we had in the flying club had a Bendix pressure carb, which was sort of a "poor man's fuel injection". It had a sort of a venturi for measuring air mass flow, but nowhere near as pronounced as in a float carburetor, and the fuel was atomized by a pressure nozzle in the intake manifold well downstream from the carburetor. So the venturi effect cooling and the fuel atomization cooling were removed from each other, and the atomization occurred in a portion of the manifold that hugged the engine crankcase, thus keeping it warmer, and cooling the charge going into the cylinders in hot weather operation. Sort of a full time carburetor heat/intercooler.
The plane had carburetor heat, but it was never needed, and only served to make the engine run rough, put out black smoke, and burn vast amounts of fuel. It also had an "auto rich" function, which did a better job of controlling mixture than the average human pilot. You could manually lean it, but if you did, EGTs, CHTs, and oil temp would all run uncomfortably close to redline.
AND you could do pushovers all day long and up to 90 seconds of inverted flight without it skipping a beat. Truly a dilbert-proof machine!
Cheers,
Wes