F4U vs F6F & Top-Speed: Let's Settle This

[Zipper730]

Things I should have asked earlier but didn't

Fuel/Air Ratio

Rich mixtures don't GIVE you more power, they help you SUSTAIN more power by cooling the cylinder temps and holding off detonation.

At the risk of sounding stupid: Does this apply with carburetor/pressure-carbs only or with direct fuel-injection as well?

F6F Speeds

F6F-5 BuNo.58310 reached 395 mph @ 18,750 feet while utilizing Combat power, and this was with a starboard wing pylon installed (known to reduce speed by roughly 5 mph at this height). So we have an honest 400 mph F6F-5 here in a 'clean' condition....

I should have addressed this earlier, but looking at that speed figure, 400 mph seems completely plausible if no wing-pylon was installed (not sure how common this was at the time of the test).

That's why I've always considered Hellcat's actually achievable maximum speed to be over 400 mph(about 410 mph). for example, according to AHT, ' Jan. 44, A modified F6F-3, including some of the features to be included in the later F6F-5 version, is flown at a speed of 410 mph at 21000 feet altitude. '

Not bad.

 

[XBe02Drvr]

At the risk of sounding stupid: Does this apply with carburetor/pressure-carbs only or with direct fuel-injection as well?

"The stupidest question is the unasked question." (I forget whose quote that is.)
It's basic internal combustion facts. Regardless of the induction system, the best theoretical power output comes at a mixture that generates more heat than most engines cooling systems can dissipate and is on the ragged edge of detonation. So high power settings require a richer mixture than the ideal, or "stochiometric" best power mixture, the extra fuel serving to cool the engine and create a safety margin between operation and detonation. That's Fuel Mixture 101 from the perspective of us peons who only fix them and fly them.
To get the real skinny, you should consult Cullum (Snowygrouch, as he's known here on the forum) for the engineer/designer perspective. Wait..he'll probably see this, jump on here, and set the record straight.

 

[Zipper730]

D Deleted member 68059

 

[Deleted member 68059]

This is a bit messy so I`m not 100% sure whats being asked - however I think it would be instructive if I shared whats called a "mixture response curve"

This was developed for aero engines in the late 1930`s, and was at least published first by German engineers (this is not necessarily proof of others ignorance on it).

The mixture response curve was the fuel chemists creation after they started realising how worthless the Octane number was once you get down to very detailed development, because the Octane number (pre about 1942) was done at ONE air/fuel ratio, ONE engine speed, ONE air temperature ONE ignition advance. It didnt take long before engineers realised that if you vary those parameters, the detonation limit (analogous to the Octane number) jumped around all over the place, and in certain circumstances some engines made LESS power on fuels with higher octane fuels (once you get down to looking at differences of jus a few points).

So they decided to measure knock resistance across the full range of air/fuel ratios and make a graph (a more primative version of this is what the Allies essentially did when the 100/130 PN grade fuel specifications were jointly agreed between the UK and USA, which measured octane at TWO air/fuel mixtures not ONE, hence 100 Octane lean/130 PN at rich, its essentially the same fuel as 100 Octane from 1940, just measured with greater diligence).

Germans being Germans decied that a couple of points were no good, they wanted a complete curve, so they measured it at perhaps six or more points.

An example is below, in short you can make a LOT more power at rich mixtures for two reasons, firstly the cylinder may be kept cool by the extra fuel (a lot more than
can actually be burned, potentially) which raises the knock limit so you can increase boost, and secondly you can obviously burn more fuel because there are more
fuel carbon and hydrogen molecules floating about inside the cylinder to combine with oxygen.

Of course, if you have an air cooled engine it may be advantagous to run at a rich mixture a lot of the time to help keep temperatures down, which is a reliabilty factor,
but this doesnt mean that you CANT make a lot more power at rich mixtures if you are able to do so.

 

[Zipper730]

D Deleted member 68059 ,

I remember something that X XBe02Drvr saying something awhile back about how rich mixtures tend to see flames shoot out the stacks: I'm curious if that contributes to a significant increase in overall exhaust thrust?

 

[Deleted member 68059]

D Deleted member 68059 ,

I remember something that X XBe02Drvr saying something awhile back about how rich mixtures tend to see flames shoot out the stacks: I'm curious if that contributes to a significant increase in overall exhaust thrust?

It probably would as the gas will have higher temperatures, which means greater energy.

 

[DarrenW]

F6F Speeds

I should have addressed this earlier, but looking at that speed figure, 400 mph seems completely plausible if no wing-pylon was installed (not sure how common this was at the time of the test).

By the fall of 1944 wing pylons and fuselage bomb/drop tank attachment points were a very familiar sight on fleet Hellcats (both F6F-3 and -5 models). Rocket launching hard points were common as well. Much of this had to do with the aircraft being increasingly tasked with the fighter-bomber role.

Maximum level COMBAT speed of the F6F-5 has been quoted in USN ACPs to be 378 mph at 18,000 ft with wing pylons/fuselage bomb attachment points and 389 mph in clean condition. From what I can figure, dual wing pylons lowered maximum speed by 5 mph, while fuselage bomb attachment points reduced this further by another 6 mph.

So yes, 400 mph was certainly possible with a factory-fresh F6F-5 in clean condition but due to operational needs this wasn't a normal configuration for Hellcats by this period of the war.

It's interesting to note that an F4U-1D with two uncapped wing pylons and fuselage drop tank rack had a maximum COMBAT speed roughly equal to the F6F-5 in a 'clean' configuration.

 

[XBe02Drvr]

It probably would as the gas will have higher temperatures, which means greater energy.

Hey, an "afterburner" equipped piston pounder!

 

[Barrett]

I'm late returning to this thread and, considering how many screens are posted, I might have missed something.
But I'll post a couple of sources anyway, from RAE Farnborough

Home page:
WWII Aircraft Performance

F6F-5 performance from Boscombe Down

https://www.wwiiaircraftperformance.org/f6f/jv224-b.pdf

375 mph @18k ft, 60 in MP

==

F4U-1 performance from Patuxent River
420 mph @18k ft, 60 in MP
https://www.wwiiaircraftperformance.org/f4u/f4u-1-50030.pdf

 

[Irregualr23]

One has to be noted that most files account F6F's maximum speed with WEP at 18,000ft as the FTH altitude, while using 20,000ft as the FTH of the Corsair using WEP. (Talking about the ACP file) Which shouldn't be correct, both the Hellcat and Corsair has high blower FTH using WEP varying from 18k to 21k ft, and that causes a lot of performance discrepancy if we ignore the FTH value.

In one specific case, the VF-13 had clocked their newly received F6F-5 to be 425-438mph @ 21000ft using water-injection, which is unusually fast. If we assume -20knot installation (sourced from F6F manual in 1944) and -6mph compressibility effect (from JV224b test), we get a number around 396-409mph, which is plausible and also explained 409mph@21,500ft been tested in the PTR-1111 comparison with Zero52 , and the late F6F-3's claim 410mph@21,000ft. It should be noted that the VF-13 had adjusted the regulator of the Aux blower before the test, whether this is the cause of the rise of FTH is unknown. Although this record is still questionable, because the TAS been recorded at 2,000ft looks quite accurate.

Since the F4U-1A Buno.17930 with tail-hook removed and 13'1'' propeller installed had obtained 430mph @ 21,000ft, an typical production F4U-1D should obtain 420-425mph at this altitude, if its high-blower obtain full-throttle at this altitude either by extral adjustment or the change of testing environment.

In most cases, we should conclude F6F and F4U had around similar rammed air effect for their Aux blower, and thus similar FTH when using water-injection, in some cases the Hellcat would attain a higher FTH than the Corsair (Chance-Vought test and PTR-1111).