Reason for F6F-5 speed discrepancy?

[ShortyDoowap]

Look, from the Hellcat's manual.  Not only does it show that the Neutral is used at combat power, it shows when the shift should take place:





note: Adobe has superposed some number errors, but the proof remains still

[justin_g]

Yes, it says that at Combat power(WEP) to shift to Low blower at 3,000ft - and then to High blower at 16,000ft.

You have indicated on the BuAer charts that Neutral blower is used up to 13,000ft before shifting to Low blower, which is maintained until 22,000ft!

The reason there are 3 steps in the Combat power graph is because of AN ERROR in the making of said graphs.

It appears that the engine powers used to calculate the speed graph at Combat power were for climbing and not level flight, thus the critical altitudes displayed are 2000ft too low. The first critical altitude should be at 15,000ft and not 13,100ft, and the second at 20,000ft and not 18,000ft as displayed on the chart. See next post.

[justin_g]

Originally posted by HoHun
The BuAer chart is weird, and it's easy to get confused. However, armed with the chart you posted, I can tell you for certain that neutral blower is not used on the Combat Power branch of the BuAer chart.

I think I know why the BuAer chart has such a weird shape.

The BuAer F6F-5 chart has incorrect critical altitudes at Combat power(2000ft too low, they should be the same as F4U-1, ie: 15,000ft & 20,000ft).

The only clue I see as to why is in the power charts:



The engine powers listed for Combat power are climb powers - the critical altitudes listed(13,100ft & 18,000ft) are a close match for those seen in F4U-1 climbrate charts, while the Military power and Normal power settings have the correct critical altitudes for level flight.

My theory is that the performance figures shown on the graphs were calculated - using the engine powers listed(which were obtained in seperate flight tests and then somehow got mixed up, re: climb/level speed powers) - hence the incorrect, lower critical altitudes shown at Combat power, which make the graph curves so confusing.

Here is my interpretation of how the graphs should appear:




Note that the critical altitudes on the speed chart have been raised to 15,000ft for Low blower and 20,000ft for High blower, and the effect of Neutral blower has been added below 2000ft.

[HoHun]

Hi Shorty,

>FTH IS NOT the critical altitude at anything under combat power.  It would be ABOVE critical altitude at power below combat power.  

It is exactly the lowest altitude at which the throttle is fully open, in any gear, at any power setting. Thus, full throttle height varies with engine speed as well as with boost pressure.

>The throttle is no more open at critical altitude than it was at sea-level.

Quite wrong. Don't think of the throttle lever, think of the actual throttle valve which is operated by the automatic boost regulator even if all the pilot does is to push the level to the firewall and leaving it there.

>If you are climbing at Military Power, and you climb above critical altitude, your power will begin to fall off.   If you don’t have another supercharger stage to shift to maintain power, the only thing you could do to maintain power would be to increase throttle.  Sooner or later, the throttle would be wide open.  At the point where the throttle has to be wide open to achieve military power – THAT would be full throttle height.   THAT is ABOVE critical altitude.

Hm, why do you think the power falls off with increasing altitude in the first place? The reason is that the supercharger is unable to provide enough air to the engine to maintain the boost. Power begins to fall off exactly at that altitude where the throttle is fully opened for the first time.

Critical altitude and full throttle height always coincede.

>Not for the R-2800-10.  Critical altitude in neutral blower is above seal level.  It’s proven by the BuAer Charts.

By what aspect of the BuAer charts? The step from MIL to Combat Power certainly contradicts that opinion.

>Neutral blower at Combat Power could achieve around 2,300 hp at sea level.  Low blower could not even make 2,000 hp at sea level.  Therefore, it only makes sense to use neutral blower from sea-level on up until it couldn’t maintain low-blower sea-level power.

Even the AHT chart (which is rather inconsistent in several points) shows neutral blower full throttle height as subterranean. Your interpretation shows neutral blower full throttle height as 10000 ft. There is no way to resolve this contradiction.

Regards,

Henning (HoHun)

[HoHun]

Hi Shorty

>There are two distinct shift points, from neutral to low, and from low to high.   If neutral was not use, what do you think that last shift was to?   From "high" to "higher?"

The third jump is what makes the analysis so difficult. However, from the analysis of full throttle heights (blue lines), it's certain that the last jump in the graph was not a supercharger gear change.

>All this talk about FTH and MAP numbers confuses the issue.  

Actually, it's the key to the solution - provided that if you know enough about engines to understand that there is a real physical background behind the curves.

Regards,

Henning (HoHun)

[HoHun]

Hi Justin,

>Yes, it says that at Combat power(WEP) to shift to Low blower at 3,000ft - and then to High blower at 16,000ft.

>You have indicated on the BuAer charts that Neutral blower is used up to 13,000ft before shifting to Low blower, which is maintained until 22,000ft!

You have nailed the critical point here :-)

Regards,

Henning (HoHun)

[HoHun]

Hi Justin,

>The only clue I see as to why is in the power charts:

Hm, these power charts don't match the ones provided by Shorty all too well, regardless of the power setting.

(Of course, I'd like to find a physical explanation for the weirdness of the BuAer graphs, but currently I have no idea at all.)

>Here is my interpretation of how the graphs should appear:

That's a good way to remove the weirdness :-) However, it is still unexplained.

Regards,

Henning (HoHun)

[justin_g]

Originally posted by HoHun
Hi Justin,

>The only clue I see as to why is in the power charts:

Hm, these power charts don't match the ones provided by Shorty all too well, regardless of the power setting.

(Of course, I'd like to find a physical explanation for the weirdness of the BuAer graphs, but currently I have no idea at all.)

>Here is my interpretation of how the graphs should appear:

That's a good way to remove the weirdness :-) However, it is still unexplained.

Regards,

Henning (HoHun)

The engine power charts posted by Shorty are without ram effect. The powers listed in the BuAer document were collected by actual flight test, and vary for several reasons. The most important difference is that the figures listed of course include the effect of ram air, which raises the critical altitudes.

The engine power chart from the Vought documents below shows Combat power critical altitudes of 13,700ft & 18,300ft in climb, and 15,300ft & 20,100ft in level flight for the F6F-5. The the AHT R2800-10W engine graph posted previously shows critical altitudes of 15,000ft & 20,000ft for Combat power(which would be for level flight too). Every chart for the F4U-1 shows critical altitudes in level flight of around 15,000ft & 20,000ft at Combat power.

Why does the BuAer chart for the F6F-5 level speed at Combat power have critical altitudes of 13,100ft & 18,000ft then? Because they calculated the Combat power level speeds using engine powers achieved in a climb - and not the level flight powers that they should have used(which they did use correctly for the other power settings).

Vought chart:

[HoHun]

Hi again,

Here is an analysis based on the power charts posted by Shorty:



I have calibrated F6F-5 drag to match the speed at low gear full throttle height.

Power settings are from the SEFC provided by Justin.

I'm a bit suspicious against the way power drops off below full throttle height, resulting in exaggerated sea level speeds. Maybe it would be necessary to apply a temperature-modified pressure altitude to the engine calibration chart - I have seen something similar in the B-29 manual. That's only a thought, of course.

Please consider the entire analysis preliminary :-)

Regards,

Henning (HoHun)

[justin_g]

Could you do one for WEP?

[gripen]

justin_g,
Use the chart from page 4 of the Vought doc. It shows speeds and FTHs at same power in high speed flight. The chart is a bit unclear but you can roughly read:

F6F-5
Neutral gear FTH 2k
Low gear FTH 16k
High gear FTH 20k

F4U-1
Neutral gear FTH 4k
Low gear FTH 16k
High gear FTH 20k

The chart correctly shows higher neutral gear FTH for the F4U-1 due to RAM effect while low and high gear FTHs are about the same for F6F-1 and F4U-1.

The speed values itself are quite unrelevant but the point here is that the AH F6F-5 has higher neutral gear FTH than the F4U-1D. In the real life the F4U had higher FTH for the neutral gear due to RAM at given MAP.

gripen

[justin_g]

Engine power chart on page 2 shows the same thing(see above) - but what I'm mainly trying to prove is that the BuAer charts for the F6F-5 are porked for Combat power due to incorrect hp figures used in the calculations.

[ShortyDoowap]

Originally posted by HoHun
Hi Shorty,

>The throttle is no more open at critical altitude than it was at sea-level.

Quite wrong. Don't think of the throttle lever, think of the actual throttle valve which is operated by the automatic boost regulator even if all the pilot does is to push the level to the firewall and leaving it there.

Please understand that I am speaking in terms of that BuAer F6F-5 chart – that’s all.

The reason I assert what I do is because Military Power and Combat Power in  Neutral, Low and High have different critical altitudes.   Military power will always have a higher critical altitude than Combat Power.  But the BuAer chart does not reflect that – and that’s where I think it’s screwed up.

The Combat Power critical altitude is  FTH.  If the chart is showing Military Power critical altitude as the SAME height at the same blower mode, then it’s wrong in that regard.  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.  

So, I agree there is something fishy about the chart.  Nevertheless, the chart itself shows neutral was used in both Military and Combat Power.


Here is the BuAer chart on the F4U-1D – powered by the R2800-8W engine – essentially the same as the Hellcat’s -10W engine.  In fact, I think they have exactly the same ratings.  



Now this chart DOES reflect the FTH’s of both Combat Power and Military Power in the 3 blower modes in the way you describe.  In this case, it shows the critical altitude of military power in neutral, low and high as higher than the CSA’s of combat power in the same blower modes.    On this chart, the CA of military power would be a FTH.  There would be no way of obtaining combat power in neutral blower at Military Power/Neutral Blower CA.

What I’m getting at is that on the F6F-5 chart, the FTHs (and I hate that term) the critical altitudes of  Miltiary Power and Combat Power are shown as the same (in low and high).  If it’s FTH for Combat Power, IT’S NOT a FTH for Military Power, it would be above the CA reflected on that chart.  

You guys are arguing numbers – I’m not.  I’m simply arguing what that chart shows.   I think it’s screwed up to.  But nevertheless, the chart shows neutral was used.   In reality, it may not have been used to the extent that chart reflects, but all other evidence I’ve seen shows it should have been used.  Again, it would make no sense at all to use low blower from sea-level when neutral blower generated more power for several thousand feet.  

So, while I am not denying the chart is odd, I think it would have been silly to use low blower at sea level.

[gripen]

I have not really looked the BuAer data but the so called "combat" rating for the F6F-5 in the BuAer data seem to be based on lower  MAP than "war emergency" rating for the F4U-1 and F6F-5 in the Vought doc.

gripen

[ShortyDoowap]

Originally posted by justin_g
Engine power chart on page 2 shows the same thing(see above) - but what I'm mainly trying to prove is that the BuAer charts for the F6F-5 are porked for Combat power due to incorrect hp figures used in the calculations.

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.  

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.

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.  

The "FTH" thing HoHum was arguing doesn't make any sense with respect to that F6F chart - and it's on that I base my arguments - solely.   HoHun's arguments make perfect sense with repsect to the F4U-1D chart I posted.   The F6F chart shows incorrect CAs for either Military power, or Combat power, or both.  FTH for Combat power is not the the FTH for military power- Military Power FTH is above Combat Power FTH (if I'm using that term correctly).  On the F4U-1D chart, the Critical Altitudes appear to be are properly marked.

I've clearly not made myself very understandable.   The whole FTH term is new to me and maybe I'm not understanding how its used.