Reason for F6F-5 speed discrepancy?

[justin_g]

Shorty - "Full Throttle Height"(FTH) is just the British term for "Critical Altitude" - they are the same thing.

I agree with you to that extent, the chart is very odd. But I think HP has nothing to do with it. I think it has to do with incorrect critical altitudes being reflected on the combat power climb and speed lines. HP is incidental to the power rating.

That's basically what im trying to say.

The graphs in the BuAer data are calculated, using engine power outputs which were obtained in flight tests - they are listed on the last page of the document(link).

If you compare the critical altitudes listed(13,100ft & 18,000ft) to those in the Vought document(link) you can see that they match the critical altitudes for CLIMB POWERS. The critical altitudes for HIGH SPEED POWERS are about 2000ft higher(more ram air) and correspond to any F4U-1 chart at Combat power, with critical altitudes around 15,000ft & 20,000ft.

Frankly, I think the Combat Power climb should have a line similar to the combat power line in the F4U-1D chart I posted above. The -8W and the -10W engines had the same ratings and CAs. So the lines should be similar, even if showing less performance for the F6F.

If the BuAer had used engine power data with the correct critical altitudes in their calculations, then the graphs might have looked something like this:

But I don't agree that Neutral wasn't used. It may not have been used up to 10,000 feet, but I think it's clear it was used. To get best performance, it would have had to be used. Foregoing the use of neutral at sea level would have hurt performance.

Note that on the modified version I have included the use of Neutral blower only below 2000ft - as can also be seen on the BuAer charts for the F4U-1D, but NOT on those for the F4U-1.

[HoHun]

Hi Shorty,

>If Combat Power critical altitude is a FTH, it wouldn’t be a FTH for Military power, it would be above that.  THAT’S what I am trying to say.  

Look at Justins "corrected" graph - it meets your requirement.

Justin made a 2000 ft correction to get a perfectly logical graph. You had to make a 12000 ft correction in neutral gear full throttle height and got a graph that is far from perfectly logical. That's the reason I go with Justin's interpretation :-)

>On this chart, the CA of military power would be a FTH.  

Critical altitude always is a full throttle height. The reason I'm using the term "full throttle height" is that it is based on what really happens in the engine, while "critical altitude" is a term belonging to a rating system that does not reflect what's really going on in the engine (US system, constant rated power up to critical altitude).

>I’m simply arguing what that chart shows.  

There are two interpretations, both of which have merit. After a thorough analysis, Justin's variant is much more logical than yours (which is one that I initially suggested, too).

Regards,

Henning (HoHun)

[HoHun]

Hi Justin,

>Could you do one for WEP?

OK, here it is:



It's not particularly accurate as I had to extrapolate the power graphs though they weren't always linear. Escpecially the high gear is a bit of a guess. However, the result is still qualitatively accurate.

Note the break between MIL and WEP in high blower, with the higher rpm yielding a lower top speed!

This is caused by a propeller efficiency loss due to supersonic tip speeds. If you look at it, 2700 rpm only yields some extra 50 HP, and the higher tip speeds cost slightly more than that so that it's sensible to use 2550 rpm.

The break is not quite as large as in the BuAer chart, but it's there, and I believe propeller tip Mach effects are to blame for the BuAer chart's weirdness as well.

Regards,

Henning (HoHun)

[HoHun]

Hi again,

On the other hand, it could be that my analysis is completely wrong due to a KTAS/MPH confusion.

With the power chart broken down into three parts, there is so much manual labour required in my analysis that I have too many opportunities to screw up somewhere in the process :-/

Regards,

Henning (HoHun)

[ShortyDoowap]

The biggest confusion about the BuAer speed and climb graphs is that they are plotted inconsistently with each other.

Refering only to the Combat power lines:

On the speed chart 13,100 feet is plotted as the CA in neutral blower.
On the climb chart, 10,000 feet is plotted as the CA in neutral blower.

On the speed chart  15,700 feet is plotted as the shift point from neutral to low.
On the climb chart 13,100 feet is plotted as the shift point from neutral to low blower.  

On the speed chart 18,000 feet is plotted as the CA for low blower.
On the climb chart 15,700 feet is plotted as the CA for low blower.

On the speed chart 21,500 feet is plotted as the shift point from low to high.
On the climb chart 20,000 feet is plotted as the shift point from low to high.

On the speed chart 23,000 feet is plotted as the CA for high blower.
On the climb chart 21,000 feet is plotted as the CA for high blower.

Regardless of what altitudes are used for each phase, they should be consistent between graphs - speed and climb should show the same CAs and shift points.

[ShortyDoowap]

Here's anopther chart to ponder.  Forget the Navy data on the chart, I think it comes directly from BuAer.  The Grumman data is interesting:

[HoHun]

Hi again,

>On the other hand, it could be that my analysis is completely wrong due to a KTAS/MPH confusion.

I found I accidentally used a higher drag coefficient for some portions of the graph. It's fixed now.

The slight jump is still there, but a bit smaller in magnitude.

Regards,

Henning (HoHun)

[HoHun]

Hi Shorty,

>Regardless of what altitudes are used for each phase, they should be consistent between graphs - speed and climb should show the same CAs and shift points.

Not if there is any ram effect, which increase intake pressure and allows the supercharger to operate as it would at a lower altitude at a lower speed.

Regards,

Henning (HoHun)

[ShortyDoowap]

Originally posted by HoHun

Not if there is any ram effect, which increase intake pressure and allows the supercharger to operate as it would at a lower altitude at a lower speed.

Regards,

Henning (HoHun) [/B]

OK.

Ram has the effect of raising critical altitude.  Is there less ram effect when climbing then in straight lin speed?  Now that I think about it, it think there would be.  That explains why the critical altitude on the F4U-1D VMax graph are higher than those on the Climb graph.

But why, then, is the neutral-to-low blower shift point on the VMax graph at the same altitude as it is on the climb graph?  With ram, which the Corsair used in neutral, it would have been able to maintain power higher than without, or with less, ram, so it seems the shift point on the VMax graph should be higher.

[HoHun]

Hi Shorty,

>Ram has the effect of raising critical altitude.  Is there less ram effect when climbing then in straight lin speed?  Now that I think about it, it think there would be.  

Yes, ram pressure increases to the square of the speed, so the difference between climb and level flight is marked.

>But why, then, is the neutral-to-low blower shift point on the VMax graph the same as it is on the climb graph.  With ram, which the Corsair used in neutral, it would have been able to maintain power higher than without, or with less, ram, so it seems the shift point should be higher.

Hm, very good observation! That would indeed indicate the absence of ram pressure. Maybe we should search for a diagram of the F4U's intake system? (Zeno's site linked by Justin has a diagram of the F6F's system, and if I'd seen that earlier, I had not been as confused :-)

With regard to the gear change question: Did you notice that the Grumman graph you posted above features neutral gear on combat power? In the side by side comparison, it demonstrates that the BuAer data did not rely on neutral gear for combat power. The Grumman data looks a lot like the F4U-1D data, too, so I think it should finally solve the question.

The Grumman data also has the same "extra-step" weirdness between MIL and Combat Power! =8-O

With regard to neutral blower and ram pressure, here some data from my F6F-5 calculation:

Top speed at sea level, 60" Hg, neutral blower, no ram effect: 550 km/h
Top speed at sea level, 60" Hg, neutral blower, ram effect: 564 km/h

So I get 7 to 8 knots speed loss due to the lack of ram effect - far less than the 20 knots quoted by Corky Meyer. (Since I estimated ram effect based on the speed graph which might not perfectly match the power graph, there is a certain uncertainty about my number, but 20 knots seem a bit much.)

I believe my calculation currently over-estimates low-level speeds because it under-estimates high-altitude thrust. (Currently, it's based on shaft power rather than on indicated power because I took the quick and dirty route.)

To round off this post, I finally had a look at the different data sets we have for the F6F, and it seems that the BuAer set is the slowest of them all.

I have found:

- BuAer
- AHT graph (posted by Shorty)
- F4U-4 documentation (posted by F4UDOA)
- Fw 190 comparison report
- TAIC Zero report (posted by F4UDOA)

Do we have any more data? I'll try to prepare a graph showing all the figures combined.

Regards,

Henning (HoHun)

[ShortyDoowap]

The Grumman and BuAer lines on the chart I posted from America's Hundred Thousand both show neutral gear useage in combat power climbs.  Grumman data shows a MUCH earlier shift to low blower, about where I would expect it.  It also shows neutral blower CA is higher than sea level.

I'm still not convince the BUAer climb did not use neutral blower.   But I agree the Grumman line looks more reasonable.

[justin_g]

Originally posted by HoHun
Note the break between MIL and WEP in high blower, with the higher rpm yielding a lower top speed!

This is caused by a propeller efficiency loss due to supersonic tip speeds. If you look at it, 2700 rpm only yields some extra 50 HP, and the higher tip speeds cost slightly more than that so that it's sensible to use 2550 rpm.

The break is not quite as large as in the BuAer chart, but it's there, and I believe propeller tip Mach effects are to blame for the BuAer chart's weirdness as well.

I don't think the "jump" on the BuAer chart is entirely due to this reason(It's a helluva lot bigger jump). However, I do agree that 2700rpm would cause a performance loss at high altitude - you can see this effect on the BuAer F4U-1 charts, where Normal power(2550rpm) is actually faster than Military & Combat powers(2700rpm) at altitudes over 25,000ft.

Originally posted by HoHun
Maybe we should search for a diagram of the F4U's intake system? (Zeno's site linked by Justin has a diagram of the F6F's system, and if I'd seen that earlier, I had not been as confused :-)

F4U-1 uses wing root ducts for intake air, so it is always getting ram effect. It did however have a switchable unrammed intake which (it appears)could only be used with the aux. blower in neutral gear(item G). Diagram from Zenos

[F4UDOA]

A few things.

1. In later Combat climb instructions in the POH pilots are instructed to climb at 2500RPM at high alt because 2700RPM the prop looses efficiency at 20K and above.

2. The R2800-8W put out more HP especially at sea level in low blower. But even above that altitude the R2800-8W out performs the R-2800-10W. I will scan and post the 1945&1946 engine calibration charts for both.

3. The POH for the F6F-5 mentions that despite the engine cal curve that the low blower stage produces more HP in the Neutral stage. I have a copy of the British AFDU for the F6F-5 and they use the Low blower stage from sea level as well.

[gwshaw]

Originally posted by HoHun
With regard to neutral blower and ram pressure, here some data from my F6F-5 calculation:

Top speed at sea level, 60" Hg, neutral blower, no ram effect: 550 km/h
Top speed at sea level, 60" Hg, neutral blower, ram effect: 564 km/h

So I get 7 to 8 knots speed loss due to the lack of ram effect - far less than the 20 knots quoted by Corky Meyer. (Since I estimated ram effect based on the speed graph which might not perfectly match the power graph, there is a certain uncertainty about my number, but 20 knots seem a bit much.)

[/b]
This has been an interesting diversion, I'll see if I can do a workup on both the F6F-5 and the F4U-1. But note there is a lot of conflicting information on the R-2800-8/8w, listing anywhere from 57.5 in to 60 in Hg for WEP, and from 2100 to about 2300 hp in neutral blower.

But this doesn't make any sense HoHun. 60 in Hg is 60 in Hg, the engine doesn't know it is rammed or not, ie as long as the engine can manage 60 in Hg unrammed it will actually get HIGHER speed than 60 in Hg rammed would provide. You have to take compression heating of the charge air into account.

Lets call it 340 mph, nice easy number. Ram heating is:

340/100 ^ 2 = 11.56 deg F

SL standard temp is 518.69 R so charge air temp = 518.69 + 11.56 = 530.25

Density is going to be sqrt (518.69 / 530.25) = .9984 so the engine will lose .16% power rammed, not much but definitely going to be slower rammed than unrammed.

Offsetting that is the fact the the F6F-5 charge air was heated as it passed through the engine compartment to the carb, but I don't even have any empirical figures for how much that would be.

The point is, I don't know where the claims of a significant speed loss at SL came from. As long as the engine is capable of the required manifold pressure unrammed then ram is not going to provide an increase in power and speed, quite the opposite in fact.

The only possibility I can see is that while the engine is capable of 60 in Hg in neutral blower on a test stand, there are pressure losses ahead of the blower as installed in the F6F-5 that get multiplied by the neutral blower pressure ratio. The blower has approx a 2:1 pressure ratio, ie 60 in Hg / 29.92 = 2.005:1, so 2 in Hg pressure loss ahead of the blower drops MAP by 4 inches which could cost about 140 hp or so going from 60 in Hg to 56 in Hg. That could certainly cause a significant loss of speed in neutral blower.

Greg Shaw

[gripen]

Originally posted by gwshaw

The point is, I don't know where the claims of a significant speed loss at SL came from. As long as the engine is capable of the required manifold pressure unrammed then ram is not going to provide an increase in power and speed, quite the opposite in fact.

IMHO Meyer and others mean overall low altitude performance, say below 10k; the FTH of the Neutral blower is very low with high ratings and the F4U could use it 2-3k higher (due to RAM effect) than  F6F resulting better performance. Infact some F4U and F6F charts show that the neutral blower FTH is below sea level with high MAP (see here). In that case the F4U has a clear advantage until it activates the auxilary stage.

gripen