F8F Bearcat rate of climb (1 Viewer)

[Neil Stirling]

The SAC document is from 1949 and states that the fuel used is 100/130. I don't think that this is a mistake. For whatever reason the USN decided that it could operate the Bearcat with this fuel and 70"hg. Things were fine in low gear but again for whatever reason problematical when using high gear, hence the odd shape of the curve.

Neil.

 

[papashteve]

Thank you very much!!!!
O.K best guess.
Using Supercharger low gear.
Combat curve 4, Speed military power 54"hg , almost certainly without water injection.
Combat curve 2 WEP. Water injection 70"hg
Water injection on its own adds about 11kts to speed. The use of WEP drops the full throttle height from about 6,000ft to Sea Level, the height at which maximum speed is attained when using low gear. As the supercharger is now giving its best, speed falls off with altitude. When at about 12,000ft high gear is engaged and speed starts to increase, now at 15,000ft the curve speed decreases again? This could be due to high engine temperatures (Carb air intake?) or some other issue. Anyway shortly after that curve 4 bisects 2 and Military power is used.
The Standard Aircraft Characteristic doc does not match the power plant chart contained within the F8F-1 pilots notes, however, the F8F-1 SAC doc does state that its based on flight tests and calculations. I think we are missing some other document that may well clear things up.
For now I will go with the new information you have provided.

Thanks again.

Neil.

The missing document to explain this performance would most likely have been this one, it is from the 1947 F8F-1 manual (before the 1949 revision of the manual which used 115/145 octane). Note the use of 100/130 octane:

Sadly, combat power settings are not shown as it is written that it would be supplied at a later date. The later date was the 1949 revision of the manual, but they had moved on to 115/145 octane..

 

[Shortround6]

The SAC document is from 1949 and states that the fuel used is 100/130. I don't think that this is a mistake. For whatever reason the USN decided that it could operate the Bearcat with this fuel and 70"hg. Things were fine in low gear but again for whatever reason problematical when using high gear, hence the odd shape of the curve.

Neil.

I am not so sure that things were "fine" in low gear.

Things were "fine" in the sense that with both fuels military power didn't change with either gear. The problem is that the supercharger was already maxed out in low gear and simply would NOT supply any more air (or depended on RAM to do so) over a few thousand feet of altitude. Take-off power (dry) with 100/130 was limited to 53.5in (2100hp) and this could be held to just over 3000ft. Using 115/145 fuel allowed 58in for take-off (and another 200hp) but it could only hold this to 5-600ft and as the the altitude rose the pressure dropped to where, at a bit over 3000ft, the pressure was the same (53.5in) as using the lower grade fuel and by the time you reach 8000ft the pressure was down to 46in and power is down to about 1800hp. Water injection can help some but the supercharger is just not going to flow anymore air using the low gear ratio.

In high gear they may have limited the Military power to 49.5in instead of the 53.5/58in used in low gear due to the extra heat in the intake charge generated by the higher compression taking place. OR trying to balance the actual load in the cylinders.

 

[GregP]

Does anyone have any design documents on the supercharger used to suggest it was maxed out in flow at 2,800 rpm in low gear or is this guesswork?

To me it looks as if combat power produced significant extra power until somewhere around 17,000 feet at which time the Combat and Military curves join. Since both the Combat and Military ratings are at 2,800 rpm, the difference would have to be with water-methanol boost only. Otherwise, there will not be any change in manifold pressure since the engine is running at the same rpm for both power settings and hence the airflow is the same. That leaves only W-M injection as the difference maker in flow.

 

[Shortround6]

In the manual referred to earlier there are engine calibration charts on page 76.

At 2800 rpm in low gear the engine gave 58" at sea level to 500-600ft, it was down to 46" at 8000ft, 32" at 16,000ft and 30" at about 19,500ft. Chart gets little sparse but you might be down to 22-24in at 26,000ft (last line on chart in low blower) where the engine would be making a bit under 900hp. High gear kicks the Pressure up to 33-34in at 26,000ft and power is about 1200hp or a bit under.

The only "kink" in the lower gear, 2800 rpm pressure line is at just over 3000ft where the the engine has to be throttled to 53.5in if using 100/130 fuel to limit the engine to 53.5".

It was quite common to limit the boost in WEP by part closing the throttle below the FTH for WEP.

I doubt that water injection is going to boost the manifold pressure from 58" to 70" or 70+" just by cooling the charge while the supercharger impeller spins the same speed.

This 1949 manual gives WEP settings of 70" in HIGH gear from 6,000 to 10,000ft ( 3 lines on the chart ) from which we may assume that in high gear the engine could pull 70in at 10,000ft had had to be throttled back at lower altitudes to keep from exceeding 70in.

The engine calibration chart does NOT take RAM into account while the power plant charts given in the post and in the manual on page 41 do so there will be a discrepancy of several inchs of boost or several thousand feet.

 

[GregP]

Thanks SHortround,

My observation / question would then become along these lines.

The Combat and Military power are both at 2800 rpm and full throttle.

Both would have the throttle full forward at 2,800 rpm, and I am assuming auto-rich mixture. So, same throttle setting, same rpm, same mixture. What is the difference between Combat and Military, other than the water-methanol injection?

I am not arguing, I am confused. In the Standard Aircraft Characteristic pages above, the difference in pwoer is significant, yet both at at the same RPM. Combat power (or WER) doesn't mean "change the supercharger to high gear." It usually means "push the throttle fully forward and break the wire guarding the Combat or WER setting." All that does is engage water-methaniol injection, as far as I know. What belse would it do to enhance the horsepower? OPuching the throttle forward doesn't change gears ...

 

[wuzak]

In WER mode the throttle is opened wider at the same altitude, allowing higher boost to be reached by the supercharger. ADI is only used to prevent this from damaging the engine through detonation.

So, in the case of this R-2800 the throttle only becomes fully open at 6000ft in Military power, but is open fully from 0ft in WER power.

The throttle is used below critical altitude to prevent the engine from over-boosting.

 

[GregP]

Makes sense, Wayne. Thanks.

 

[Shortround6]

A lot of engines had an automatic boost limiter on the engine,that would limit the maximum boost of the engine to a preset value by closing the throttle butterfly. Think of it as gating down/restricting the intake. Like Early Merlins being set to 6lbs boost. The supercharger could supply 6lbs/42in (or close) at 16,250ft and no more. Below that height it could supply 12lbs/54in at 9000ft and 16lbs/62in at 5500ft and so on. That is were the WEP power came from. Extra supercharger capacity that was not being used at military power settings due to detonation limits of the fuel being used or concerns about engine life. If you change to a Merlin X with 6:89 low supercharger gears instead of the Merlin IIIs 8.588 gears and you "break the wire/punch the tit,etc" at low altitude your supercharger impeller is going to be turning about 80% as fast as the Merlin IIIS impeller which is going to seriously affect the max amount of boost you are going to get. Like 10lbs/50in at sea level?

With the Merlin III there was NO WER/combat power available above 16,250ft (except that supplied by RAM) because the supercharger was maxed out.

Much like the Allisons being able to pull 70" or so at sea level (depending on supercharger gear) but only being able to give 44-46in at 12,000ft or so. At 12-13,000ft there was no WEP but a sea level???? hold on tight

A lot of the American radials used two speed superchargers to increase take-off performance with low gear maxing out at 3-6,000ft which doesn't leave much room for extra supercharger capacity/ combat power in low gear.

I don't know how the WEP control set up on the Bearcat worked but just injecting water/alcohol isn't going to do a whole lot to increase pressure. A post war commercial R-2800 engine was good for 2400hp for take-off at 2800rpm using 56in boost and ADI. 2100hp at 3000ft at 2800rpm dry which matches the Bearcat engine very well. So the water appears to be good for about 100 extra hp while running 2" less boost on 100/130 than a -34W engine using 115/145.

And that is the 'secret' of water injection and WEP. It allowed higher boost to be used (if available) while preventing detonation.

The pilots manual doesn't have a real good section on emergency power for the -1 and the -34W engine but does say to use high gear at altitudes where power is not available in low blower and says to use high blower for combat power at altitudes as low as 6000ft in the power plant chart and while 70in listed until 10,000ft, F.T. (full throttle which suggests but does not prove that 70in was less than full throttle at the lower altitudes) is listed at the higher altitudes.
Again, I don't know how much juggling of throttles, boost controls, supercharger shift levers was needed. The -2 Bearcat went to a single lever control. Can't imagine why

 

[GregP]

In the case of the last piston fighters, going to a single lever afforded easier operation at the expense of fine control.

US pilots praised the single-lever power control of the Fw 190's BMW 801 in mock combat, but said it made formation flying very difficult because fine control was lost. It also made cruising around very economically impossible because there wasn't a lot a fiddling you could do with only one lever.

In combat I like the single throttle lever.

If you are NOT in combat ... which is 99+% of your flying time ... I'll take a throttle, mixture and prop lever anytime over a sinlge power lever for the extra options and control afforded. Of course, once wewent to jets, we had only one lever from then on forward per engine.

 

[BiffF15]

Greg,

I had heard the same thing about the Fw-190 single lever engine control. However, I always thought it was due to Tank looking at things through the eyes of a young pilot (how do I as the designer make it easier for the new guy) and as such downloaded that. Just reading all the numbers pilots had to remember for blower speed / altitude change over, constant adjusting of throttle, prop and mixture, etc. Also there was a point in which the average Luftwaffe fighter pilot wasn't doing much cruising since he was in fear of getting bounced at almost all times.

All the above is my opine only!

Cheers,
Biff

 

[BiffF15]

Gents,

Off thread so I apologize, but I'm trying to find out info on a fighter pilot named Thomas J Humphrey. Should be of Korean war vintage and possibly flew with Gabby G.

A google search didn't help much and am curious if there is a Korean War database for USAF pilots or something that could help. Any help would be appreciated!

Cheers,
Biff

 

[GregP]

I went to an online search, but they want $25 or $70 (5 or less pages, more than 5 pages) and a lot of information to retrieve a service record.

 

[BiffF15]

Thanks Greg. One of the guys I work with bought some stuff at an estate sale and wanted it to go to his family (like a signed autobiography of G. Gabreski to this individual).

Cheers,
Biff

 

[Neil Stirling]

Maybe this helps to explain why there is a mismatch between the SAC and pilots manual.

Neil.

 

[grampi]

I agree with Greg, unless the battle is more than 500 mls. from home......I'm just saying, That was the reason the Mustangs Allison/Merlin were so valuable.

....or if the battle were to take place above 20K feet...the Stang is a better performer at high altitudes...

 

[BiffF15]

Grampi,

I agree with your statement, however it's a little bit of apples to oranges comparison. The Mustang was a well designed fighter, very low drag. Add fuselage tanks and RR power and it went from good to great as it flew MUCH further than it's contemporaries and handled roughly the same as they did.

The Bearcat on the other hand was designed well after, was built to land on a carrier (tail hook, folding wings and their weight penalties) and designed to combat a different type of threat. Good low speed handling qualities a tremendous climb rate while fighting at lower altitude resulted in the Bearcat being what it is (and isn't) as compared to a Mustang.

They are probably my two favorite US fighters of WW2!

Cheers,
Biff

 

[grampi]

Grampi,

I agree with your statement, however it's a little bit of apples to oranges comparison. The Mustang was a well designed fighter, very low drag. Add fuselage tanks and RR power and it went from good to great as it flew MUCH further than it's contemporaries and handled roughly the same as they did.

The Bearcat on the other hand was designed well after, was built to land on a carrier (tail hook, folding wings and their weight penalties) and designed to combat a different type of threat. Good low speed handling qualities a tremendous climb rate while fighting at lower altitude resulted in the Bearcat being what it is (and isn't) as compared to a Mustang.

They are probably my two favorite US fighters of WW2!

Cheers,
Biff

Maybe that's why most people say the P-51H is most comparable to the Bearcat...both were designed around the same time and had similar performance numbers...the P-51D was designed and built several years earlier, but was the most widely produced and popular model, so many times it ends up being compared to the Bearcat, when in reality, that's more like comparing the P-40 to the P-51...

 

[Shortround6]

You also have two different Bearcats, or at least two rather different engines.

The second engine being able to produce power at 22,000ft that the earlier engine made at 17-18,000ft.

 

[CORSNING]

Agreed shortround. Kind of in the likeness of the Packard V-1650-3 vs V-1650-7 in the Mustang.

Jeff