Ram-Compression / Ice-Buildup

[mikewint]

both the pressure and temperature change and the fuel/air mix

Yup, under the right atmospheric conditions

 

[MIflyer]

Here are a couple of diagrams from AFM 51-42, "Aircraft Engineering for Pilots" showing how pressure carbs work. These were a major advantage for US aircraft in WWII and were later adopted by the British.

 

[DarrenW]

The Hellcat was also lighter (MTOW) than the F4U

I'm pretty sure that you are referring to the earlier F4U-1A and 1C models but from my recollection these versions often flew without fuel in their unprotected wing tanks, which lightened the aircraft considerably (by roughly 750lbs). The F4U-1D dispensed with these wing tanks altogether. Under these circumstances the Hellcat out-weighed the Corsair, no matter what the loading condition happened to be. But even with this additional weight the Hellcat had superior low-speed handling, which made it easier for the average pilot to bring aboard ship.

For example NAVAER documents published in 1945 for the F6F-3 and F4U-1D described the "offensive armament" for each aircraft as being similar, save for the seemingly greater fuselage payload of the F6F (which besides bombs included a "Tiny Tim" rocket or torpedo) while the F4U carried two more HVAR rockets on wing hard points.

The F6F had a somewhat greater gross weight than the F4U in every similar "loading condition":

http://www.wwiiaircraftperformance.org/f6f/F6F-3_1-Oct-45.pdf

http://www.wwiiaircraftperformance.org/f4u/f4u-1d-acp.pdf

Now the late-war F4U-4 was a totally different story, as its MTOW exceeded that of the F6F-5 by 860lbs, even with it having basically the same amount of internal fuel as the F4U-1D. This increase may be attributed to the improved power of the superlative R-2800-18W engine, which obviously gave it a greater load carrying capability over earlier Corsair models. By contrast, only the two prototypes for the F6F-6 project had this engine installed and I am quite certain that this "improved" Hellcat would have benefitted as well, if the project wasn't cancelled at the end of WW2.

And while the F6F-3 and -5 could theoretically carry 4,000lbs of bombs, it's quite apparent that fuel and ammunition had to be reduced from levels normally found in a "combat" condition in order for this to occur (half internal fuel/no ammunition, or a reduction of about 1440lbs). By performing simple math one can easily see that the maximum external load for the Hellcat with maximum ammunition and internal fuel was 2,560lbs. However, wartime tests were performed on a "overload" condition F6F-3 (max internal fuel and ammunition), carrying two 1,000lb bombs on wing hard points and a 165 gallon centerline drop tank. Take-off weight was 15,568lbs.

http://www.wwiiaircraftperformance.org/f6f/f6f-3-41588.pdf

The F4U-4 on the other hand could carry two "Tiny Tim" rockets and eight HVARs on wing hard points, and this is where the MTOW of 16,160lbs is derived from in the below document (3,680lbs external load). In this case no reduction of fuel or ammunition was required, unless the 3,680lbs was exceeded. In stark contrast to the Hellcat, no provision was made to carry bombs or tanks on fuselage hard points.

Note that both the F6F-5 and F4U-4 had the same maximum landing weight (15,000lbs).

http://www.wwiiaircraftperformance.org/f6f/f6f-5.pdf

http://www.wwiiaircraftperformance.org/f4u/f4u-4.pdf

 

[DarrenW]

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?

From what I understand, Grumman was only concerned about carb icing while using the main-stage blower. In this stage static warm air was drawn into the carburetor from inside the engine accessory compartment, thus negating the possibility of carb icing while flying at low speeds (as in a carrier take-off and landing). The auxiliary blower still used ram air, as shown in the diagram above. The grill was used to keep out debris that could get sucked into the blower system.

 

[Zipper730]

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.

So both of them use the venturi, but in different ways: The first use it so simply suck the fuel out as it passes; the second, to meter the mass of the airflow, which then is met by the pressure carburetor in order to match the fuel/air ratio specified?

In neutral blower the carb venturi is ahead of the supercharger stage and so is somewhat vulnerable to carb icing.

Because there's no build up in pressure?

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.

Wait a second, I get that the atomization occurring in an area that would hug the engine crank-case would heat up the atomized mix -- but how would the intake manifold neutralize the venturi and fuel atomization cooling from each other? I figure each would enhance each other...

View attachment 523707

The diaphragm is basially to keep the fuel and air mix under pressure right?

 

[fliger747]

The carb's used on the R2800 injected the fuel right at the base of the impeller of the supercharger. The idea here was the turbulence of the mixture going through the impeller would very evenly atomize and distribute the fuel. The Navy's studies of direct fuel injection eventually decided that the injection carb as described ensured better fuel distribution. In this case the weak cylinders would pull in less of the air fuel mixture and the stronger cylinders more, but maintain the desired mixture in all cylinders.

 

[XBe02Drvr]

I get that the atomization occurring in an area that would hug the engine crank-case would heat up the atomized mix -- but how would the intake manifold neutralize the venturi and fuel atomization cooling from each other?

The cooling that results from the metering venturi is localized at the venturi. The same effect that cools the air as it accelerates through the orifice rewarms it when it slows down and re-expands in the larger diameter downstream. By the time it reaches the atomization nozzle, it's in a warmer environment.
Cheers,
Wes

 

[Zipper730]

The cooling that results from the metering venturi is localized at the venturi. The same effect that cools the air as it accelerates through the orifice rewarms it when it slows down and re-expands in the larger diameter downstream. By the time it reaches the atomization nozzle, it's in a warmer environment.

So the venturi and the fuel atomizer are much further apart than usual in a pressure carburetor but the venturi still does it's thing as a metering device.

 

[XBe02Drvr]

So the venturi and the fuel atomizer are much further apart than usual in a pressure carburetor but the venturi still does it's thing as a metering device.

The atomizer nozzle isn't even in the carburetor body, it's in the intake manifold just prior to the "spider" where the manifold branches off to the six cylinders.
Cheers,
Wes

 

[fliger747]

Or 18 cylinders in the case of an R2800.

Really quite amazing what the engineers and manufacturers were able to develop back in the "Analogue Era".

 

[XBe02Drvr]

Or 18 cylinders in the case of an R2800.

A bit more complicated plumbing than for a 6 cylinder O-470! Probably a separate atomization nozzle for each row of cylinders, n'est-ce pas?
Cheers,
Wes

 

[fliger747]

The R2800 had a single impeller driven via gearing (to up the RPM) from the crankshaft at the rear of the engine case. The impeller was surrounded by a diffuser and all the induction pipes took off of the perimeter of this piece. My copy of Grahm White's "R2800" book is hiding somewhere with it's great illustrations.

 

[MIflyer]

I think I found yours lurking on my Engine Books shelf but can't locate mine. Was it still in the plastic wrapper? By the way it has such a dull cover that it does not stand out.

Here is a quote from the US Navy 1944 Training Manual on Aircraft Fuel Systems relative to the Stromberg Injection Carburetor:

"The fuel discharge nozzle is located in the adapter, if one is used, and as far beyond the throttle as possible. Since only air passes through the carburetor, this arrangement serves to prevent the formation of ice in the carburetor which might form as a result of the vaporization of fuel. However, the temperature of the air in the air scoop must be above freezing to prevent icing, if there is moisture present."

The manual has a very nice color cut away of the carb if anyone is interested. I guess I should consider scanning and supplying the whole document.

 

[mikewint]

Really quite amazing what the engineers and manufacturers were able to develop back in the "Analogue Era".

Ah yes Ye Golden Days of Yore when one could actually work on a car and tune it the way you wanted rather than some computer program.
My Marauder 390 cu in was bored and stroked out to about 450 cu in. A quad, even Holly's 1000 cu ft/min didn't really feed the beast. A popular option was a tri-power set up, i.e.: three 2-barrel carbs. The car ran on the center 2-barrel until you stomped on the pedal at which point the outer 2 carbs opened up. Problem was getting all thee to balance and work together. Then Holly developed the 6-pack, essentially a 6-barrel carb. The center 2-barrels were their 350 cu ft/min dual and the outer 4 barrels were their 500 cu/ft/min duals. Tromping on the pedal fed 1350 cu/ft/min. to the engine.
I tried this set up for a about 6-months but then a dual quad manifold for Edelbrock's 1000 became available and I switched set-ups to the dual quads. The beast was now getting 2000 cu ft/min

 

[XBe02Drvr]

The beast was now getting 2000 cu ft/min

One question, Mike: How do you put all of that howl on the road and keep it straight?
Cheers,
Wes

 

[mikewint]

put all of that howl on the road and keep it straight?

4.10 positraction rear end with 11.00 X 17 rear tires backed by 50-50 shocks and three extra leaves to the springs. Front shocks changed to 90-10 with coil springs from an ambulance. Tuned ceramic headers to the lakes pipes

 

[XBe02Drvr]

4.10 positraction rear end with 11.00 X 17 rear tires backed by 50-50 shocks and three extra leaves to the springs. Front shocks changed to 90-10 with coil springs from an ambulance. Tuned ceramic headers to the lakes pipes

Shit hot!

 

[BiffF15]

Shit hot!

Picture of the rest of it?

Cheers,
Biff