Reason for F6F-5 speed discrepancy?

[ShortyDoowap]

Originally posted by justin_g
Consider this. You have marked the BuAer graph as using Neutral blower with Combat power up to a critical altitude of 10,000ft when climbing.

According to the R2800-10 charts you have provided - at 10,000ft and 2700RPM - Neutral blower is only capable of 34" MAP and just under 1500hp. MAP has fallen well below the required 60" for Combat power because this altitude is far above the critical altitude of Neutral blower!

I didn’t provide R-2800-10 charts.  The charts are for the R-2800-8.  And they show power at military power.  

At the same altitude in Low blower, output is 1600hp with only Normal power rating(2550RPM & 49.5" MAP*). At Military power(2700RPM & 53" MAP*) the output would be about 1750hp. Combat power(2700RPM & 60" MAP*) would yield around 2000hp. The BuAer document tells us that Combat power produces something between 2030-2110hp at this altitude.

At altitudes below 2000ft or so, Neutral blower would certainly make more power than Low blower - but for reasons unknown, this was not used for the graphs in the F6F-5 document.

* power settings come from SEFC at http://www.zenoswarbirdvideos.com/More_F6F_Stuff.html [/B]

[/quote]

If there is a discrepancy between the charts I provided and the BuAer documents, then consider the fact that we are talking about different engines.  The -5 Hellcat used the R-28800-8W with water injection.  It should produce more power at Combat Power than the -8 engine.

[ShortyDoowap]

HoHun,

The Combat Power climb and Combat Power speed chart show highest possible performance.  (I know about other sources, let's just concentrate on the BuAer -5 Hellcat chart.)  Combat power represents maximum boost.

You cannot get more boost than Combat Power (unless your plane is tweaked, or you you exceed maximum allowable MAP in neutral - which I am not sure is possible, unless your plane is tweaked).  And you cannot get better performance by shifting the gears differently.   By shifting differently, you can only hurt performance.

If you shift too soon, then you shift out of a higher horsepower than the next stage can develope.   In other words, best performance is achieved by climbing to critical altitude then letting the power bleed off until it is equal to the maximum power the next stage can develope.   The trick is to maintain highest possible power at all times.  If you shift too soon, you may be shifting from 2,000 directly to 1,800 hp when you could have let the power simply bleed off from 2,000 to 1,800 thereby flying at 1,995hp, 1,900hp, 1,850hp down to 1,800hp.  

Likewise, if you shift too late, then you've allowed the horsepower to fall below that which can be developed by the next gear.  In other word, if you stay in neutral too long, you may let your horsepower slip to 1,700 hp.  Then when you shift to low, you have to wait for the horsepower to build back yup to 1,800 hp.

There is no benefit, only detriment, to shifting at other times.

[HoHun]

Hi Shorty,

>I didn’t provide R-2800-10 charts.  The charts are for the R-2800-8.  And they show power at military power.  

Hm, the charts you posted are labeled "Altitude Characteristics of P & W R2800 - 10 Engine". The BuAer data quotes the R-2800-10W for the F6F-5, too, so we seem to have a good match-up here. (Of course, R-2800-8 charts would be just as welcome :-)

Regards,

Henning (HoHun)

[ShortyDoowap]

Argh, your right.  I was boogering around thru my stuff looking for -8 charts when I read that and had that on my mind when I posted.  Never mind what I said.

Yes, those are R2800-10 charts, not R2800-8 chart as I wrote.   Those charts come from the F6F manual.

Nevertheless, the BuAer chart is for a F6F-5 with R-2800-10W with water.  The engine charts I posted are for the R-2800-10 without water.

[HoHun]

Hi Shorty,

>Combat power represents maximum boost.

Combat Power represents maximum permissible boost. That's the vertical line.

The horizontal lines represent maximum achievable boost, regardless of whether its permissable or not.

Anything above the respective gear's horizontal line can only be achieved in a higher gear.



The BuAer diagram is confusing because somehow, Combat Power brings a slight downward jump in full throttle height.

However, there can be no doubt that the indicated climb graph is achieved in low blower, not in neutral blower.

Neutral Blower has a full throttle height that is below sea level for the high boost level of Combat Power. You can verify this in the graph you posted: Just extend the 2700 rpm line of the Neutral Blower chart to 60 "Hg. With a full throttle height of 2800 ft for 50" Hg, and 1800 ft for 52" Hg, increasing boost by 2 "Hg decreases full throttle height by 1000 ft. At 60" Hg, we end up with a full throttle height 4000 ft below that for 52" Hg, or about 2200 ft below sea level.

So at sea level - above its full throttle height - Neutral Blower power has dropped so far that it does not yield much of an improvement over Low Blower, and thus it's left out of the BuAer chart.

Regards,

Henning (HoHun)

[ShortyDoowap]

Originally posted by HoHun

However, there can be no doubt that the indicated climb graph is achieved in low blower, not in neutral blower.

Henning (HoHun) [/B]

No, HoHun, this is wrong.  Leave the manifold pressures out of it.  They confuse things.  Manifold pressures are incidental to all this.

There is no question neutral, low and high blowers are used in both the military power and combat power climbs.

http://members.cox.net/us.fighters/blower3.jpg

[ShortyDoowap]

Look at it this way.  In order to get three distinct stages to each speed and climb line, three distinct supercharger gears had to be used.

See:

http://members.cox.net/us.fighters/blower4.jpg

The red line represents the stage of the climb where neutral gear was used.

The green line represents the second stage of the climb where low gear was used.

The purple line represents the last stage of the climb where high gear was used.

If only low and high gears were used, there would only be two stages to the lines.

[HoHun]

Hi Shorty,

>No, HoHun, this is wrong.  Leave the manifold pressures out of it.  They confuse things.  

Engines are powered by combustible fuel, mixed with air that provides the oxygen for combustion. How much air you get into the cylinder is determined by the boost pressure.

You can't just magically increase the neutral blower's full throttle height by calling it "Combat Power". If you keep the speed of the engine constant and increase boost pressure, full throttle height will drop.

Your graph shows a neutral blower full throttle height that does not drop, but actually jump up by thousands of feet when you increase boost pressure. This is not going to happen.

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.

The engine working point on the full throttle height of that branch is 2700 rpm, 60" Hg. All you have to do is to find that working point on the neutral blower chart.

It's 2470 HP @ -2200 ft - safely subterranean :-)

Regards,

Henning (HoHun)

[ShortyDoowap]

You have a misunderstanding and I don't think I can explain it any better.

Forget the "full throttle height" terminology.  TIf the supercharger is used correctly, there is no full throttle height at military power until above the high-blower critical altitude - when it takes full throttle to maintain military power.  The only way you have a full throttle height situation below the high-blower's critical altitude is if you misuse the supercharger and fail to shift at the right time.   You MUST shift the supercharger at the right time to avoid this.  Failure to do so will result in a loss of performance.

And the entire combat power climb is a "Full throttle" climb as max boost is being generated at all stages of the climb.  

The BuAer chart shows neutral is used in both military climb and combat climb (and speed).  There is simply NO question about it.  I've shown several times that it is is used.  The engine charts I posted also confirm that low blower simply cannot generate the same power as nuetral blower below low blower's critical altitude.  Using low blower ANYWHERE below neutral blower's critical altitude will result in a loss of power and performance.  

If you want to see what a climb line looks like using just low and high blower, look at the chart for the F4U-1 (NOT F4U-1D) Justin posted a link to.

[HoHun]

Hi Shorty,

>The only way you have a full throttle height situation below the high-blower's critical altitude [...]

Each supercharger gear stage has its own full throttle height. Remember, full throttle height is the same as critical altitude.

Neutral gear critical altitude is below the ground. You would see three steps in the climb graph just as you expect if the graphs were extended below the ground to show performance in a sufficiently deep valley (think "Dead Sea").

>I've shown several times that it is is used.  

You have painted red lines. But I have painted blue lines ;-)

>Using low blower ANYWHERE below neutral blower's critical altitude will result in a loss of power and performance.  

We agree on that :-) However, the reason neutral blower is not used on the BuAer chart in combat power is that its critical altitude is below sea level.

Regards,

Henning (HoHun)

[ShortyDoowap]

Originally posted by HoHun
Each supercharger gear stage has its own full throttle height. Remember, full throttle height is the same as critical altitude.

Using that term, which I don’t like, isn’t really American terminology, and which I think mucks things up, I’ll explain it using the definition of FTH provided earlier.

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

For instance, at military power.  The throttle is partially open from sea level up to the critical altitude.  The throttle is no more open at critical altitude than it was at sea-level.  Critical altitude is the highest altitude at which rated power can be maintained using that blower setting.   ABOVE THAT POINT, power begins to fall off.  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.

Neutral gear critical altitude is below the ground. You would see three steps in the climb graph just as you expect if the graphs were extended below the ground to show performance in a sufficiently deep valley (think "Dead Sea").

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

I finally found a R-2800-10W (water injection) power chart.  This is from America’s Hundred Thousand:



For the R-2800-10W at Combat Power, Critical Altitude for neutral blower IS at or below sea-level.  BUT THAT DOESN’T MEAN IT WASN’T USED.

REMEMBER – above critical altitude, power begins to DROP.  Once critical altitude is reached, power in neutral doesn't just end, it gradually drops.  

Look at the chart.  Look at the Combat Power line.  You can see that EVEN THOUGH the critical altitude for neutral blower was at or below sea level, it was still used.   The red line represents the neutral blower stage.  At combat power in neutral blower, power began to drop immediately above seal level.  But it didn’t drop to below low-blower power until about 5,000 feet.  It would not have made any sense to use low-blower at sea level because it couldn’t generate the same power neutral blower did.  

ONLY WHEN NEUTRAL BLOWER COULD NOT MAINTAIN THE MAXIMUM POWER THAT LOW BLOWER COULD DID THE SUPERCHARGER GET SHIFTED FROM NEUTRAL TO LOW.  

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.


--

editted because I keep saying R-2800-8

[ShortyDoowap]

That secon sentence should say "...and get more power."

[justin_g]

But you have marked the BuAer chart as using Neutral blower up to 10,000ft in Combat power - this is simply not possible - neutral blower is only capable of 1500hp at 10,000ft! Low blower has over 2100hp at 10,000ft with Combat power!

The BuAer F6F-5 charts do not use Neutral blower with Combat power - if they did, then they would look more like the F4U-1D chart - with speed initially falling from s/l up to ~2000ft as Neutral blower falls of in power from about 2250hp down to 2000hp,  then speed is increasing again after Low blower is engaged, giving 2000-2100hp up to 15,000ft.

[ShortyDoowap]

Justin, you are wrong.  You need to study the charts some more.  

Again, the BuAer chart showing the combat power climb of the F6F-5 shows three DISTINCT stages.  EACH stage represents a different blower stage.  There are three blower stages - neutral, low and high.  You CANNOT have three distinct stages if three distinct blower stages are not used.   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?"

Now, look at the combat power climb for the F4U-1.  That is using TWO stages.  Just two.  Low and high blower stages.  There is ONE distinct shift - from low to high.  

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

The BuAer F6F-5 climb and speed charts at combat power, DO, without question, show the use of neutral blower.

[ShortyDoowap]

Originally posted by justin_g
But you have marked the BuAer chart as using Neutral blower up to 10,000ft in Combat power - this is simply not possible - neutral blower is only capable of 1500hp at 10,000ft! Low blower has over 2100hp at 10,000ft with Combat power!

One:  I didn't make the BuAer charts.  The charts speak for themselves.  I colored the lines on the chart to help HoHun understand them.  But it is the BuAer chart, not me, that indicates neutral was used in the combat power climb.

Two:  the terms COMBAT POWER and MILITARY power ARE NOT  defined by the amount of horsepower made.  Horsepower is INCIDENTAL to these terms.  

Combat Power and Military Power are defined by specific manifold pressures and prop RPMs.  Horsepower is, again, incidental.    

A plane at COMBAT POWER in neutral blower at sea level may be at 60" hga and 2800 rpm.  It MAY be making 2,300 hp.  At its critical altitude, it will still be 60" hga and 2800 rpm.  But it may only be making 1,800 hp.  Neverthless, it is still at combat power - because it's still at the settings that define Combat Power - 60" hga and 2,800 rpm.

Above critical altitude, hga will begin to fall.  Once it falls to point where the actual hp is equal to that which can be generated by low blower, the supercharger stage is changed.  Low blower raises the hga back up to 60" hga - though HP at this point will still be lower than it was at sea level.  

The point of the shift is set in the flight manual.  It was determined through testing and calculation.  There is no horsepower guage in a plane.  The pilot knows when to shift because his manual tells him at what altitude to shift at a particular blower stage and power setting.

Again, Combat Power and Military Power (as well as Normal and Cruise) have nothing really to do with the amount of HP being made.  Those settings are defined by the MAP and RPM.  Your plane would be at Combat Power at 60" hga and 2,800 rpm whether it was churning out 2,300 hp or 500 hp.  As long as the MAP and rpm are at the defined points, you are at combat power.