Reason for F6F-5 speed discrepancy?

[F4UDOA]

For comparison

F6F-5 1945 R2800-10W



F4U-1D 1946 R2800-8W



Check out 6. A about the low blower developing more HP than neutral on the F6F-5

[HoHun]

Hi Shorty,

>The Grumman and BuAer lines on the chart I posted from America's Hundred Thousand both show neutral gear useage in combat power climbs.

There are only Grumman speed lines, no Grumman climb lines on that chart.

Compare the two speed lines, the manufacturers is much faster but it has one blower change more while otherwise following the BuAer chart exactly in shape. Unless Grumman uses a 4th supercharger gear, the BuAer Combat Power chart uses only 2 speeds :-)

Regards,

Henning (HoHun)

[HoHun]

Hi Justin,

>I don't think the "jump" on the BuAer chart is entirely due to this reason(It's a helluva lot bigger jump).

The chart I posted is only a rough estimate, the jump might easily grow bigger once I enter more accurate data.

The jump is repeated in the Grumman data, by teh way.

>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

Ah, thanks. That means we don't know whether ram was used :-) We can probably figure it out from the power chart, though.

Regards,

Henning (HoHun)

[HoHun]

Hi F4UDOA,

>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.

Does the British AFDU show top speed as well? I'd like to include it in the overview graph I'm about to prepare :-)

Regards,

Henning (HoHun)

[HoHun]

Hi Greg,

All good points in your post! :-)

>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.

You nailed it :-) The engine is *not* capable of the required manifold pressure unrammed or rammed, it's operating above neutral gear full throttle height even at sea level.

That also explains the varying figures on boost pressure and power in neutral gear - the lower figures are what you can realistically expect, the higher figures are the engine's actual limits.

You can see it in the graphs I prepared - speed falls off from sea level upwards. That's the same effect we can see in the F4U-1D charts.

Regards,

Henning (HoHun)

[HoHun]

Hi F4UDOA,

>Check out 6. A about the low blower developing more HP than neutral on the F6F-5

Hm. The paragraph about derichment seems to suggest the opposite effect of the "weirdness" we have noticed both in the BuAer and the Grumman chart. Doubly weird!

Regards,

Henning (HoHun)

[ShortyDoowap]

Originally posted by HoHun
Hi Shorty,

>The Grumman and BuAer lines on the chart I posted from America's Hundred Thousand both show neutral gear useage in combat power climbs.

There are only Grumman speed lines, no Grumman climb lines on that chart.

Compare the two speed lines, the manufacturers is much faster but it has one blower change more while otherwise following the BuAer chart exactly in shape. Unless Grumman uses a 4th supercharger gear, the BuAer Combat Power chart uses only 2 speeds :-)

Regards,

Henning (HoHun)

Sorry, I meant speed lines.

Ok, I think I finally got what was obvious to you and Justin.  

You are saying this is the how it really is:


As opposed to the way I thought it was:


I don't know why the hell I didn't see that earlier, it was clearly explained.  And now that I look at it, it should have been obvious to me, too.  I guess I just got a notion in my head and didn't let go ouf it, even though it didn't make any sense.

[justin_g]

LOL, if only someone had drawn that picture at the start of the discussion - might have saved about a 1000 words!

[HoHun]

Hi Shorty,

>I don't know why the hell I didn't see that earlier, it was clearly explained.  And now that I look at it, it should have been obvious to me, too.

Hm, I'd say the critical point was the the virtual "fourth" stage which we now suspect as being due to propeller efficiency effects. Only the last chart you posted finally demonstrated its "virtual" nature, so you shouldn't be so critical with yourself :-) This was tough, and you found the decisive clue yourself!

Regards,

Henning (HoHun)

[ShortyDoowap]

It was too simple.  

My apologies to HoHun for thinking he was nuts, and especially to Justin, who I said was wrong when it was clearly I that was wrong.

Now that it is clear to me, all the stuff HoHun said about FTH and the other stuff Justin said makes perfect sense.  

The whole thing was like looking at one of those pictures with the weird designs on it.  Everyone tells you that if you just look at it right, you'll see a 3D dinosaur.  But you keep looking and looking and all you see if weird designs.  But they tell you it's right there, just look at it this way and you'll see it.  But you tell them they're full of crap.  And you are about to give up on it, you lay the picture down, get a Pepsi, walk past it again, the light hits it right, and that 3D dinosaur jumps out and pops you right on the pie-hole.

[justin_g]

R 2800-8W and -10W Combat power(WEP) output compared(from F4UDOA's charts with thanks!)





R 2800-8W and -10W Military power output compared

[gripen]

Well, the question still remains why the AH F6F has neutral blower FTH roughly 2k at WEP? Apparently the MAP of the WEP at neutral gear is much less than 58" or the engine had a quite bit different SC gearing.

gripen

[justin_g]

The Grumman data(see graph from AHT) has the same anomaly in neutral gear/WEP as the HTC version... it's very odd.

[HoHun]

Hi Justin,

>The Grumman data(see graph from AHT) has the same anomaly in neutral gear/WEP as the HTC version... it's very odd.

Here is a comparison of the F6F data we have found so far:



The Grumman data seems to have the highest low-level power.

The PTR 1107 report features data that's a bit strange - since it was only geathere during a rough side-by-side comparison, it might not be sufficiently accurate for a comparison of absolute numbers. The data points are marked individually since it seems to skip high gear full throttle height, and the continous line I provided there might be misleading.

The data from the Vought F4U-4 comparison report is odd, too, since the graph provides a sea level speed figure 10 mph lower than the tabulated data in the same report. Since the shape of the graph looks more sensible using the higher numbers, I have used the higher numbers near sea level.

The data point from the TAIC report sticks out, of course. It might be inaccurate for the same reason as the PTR 1107 data.

The BuAer data is the lowest performance data set. However, it's interesting that it seems to match the data provided by the AHT chart for the F6F-3.

Now I wonder if the BuAer might have used an F6F-3 using water injection as a basis for the F6F-5 data. (PTR 1107 suggests that there were F6F-3 aircraft with water injection, possibly retro-fitted.) That would in turn raise the question whether there were (aerodynamic) differences between the F6F-3 and the F6F-5 that would cause the newer version to be faster at the same power.

Regards,

Henning (HoHun)

[justin_g]

Thank you for preparing the chart!

Here's some of my thoughts:

1. PTR 1107 report lists the speed reached after 2 mins at full power - "It should be noted that the runs were made for about two minutes only, during which time full speed was probably not developed but serve for the purpose of comparison." I also suspect that the figures may not have been corrected to standard conditions.

2. The graph in the Vought data looks like 330mph at s/l to me?

3. Never seen the TAIC report. I gather that it is a comparitive report, so maybe it was not corrected to standard conditions as well?

4. The BuAer data states 2 things: "Performance is based on flight test of F6F-3 and F6F-5 airplanes." and "Performance based on engine power determined in flight test as follows:" I guess the author of AHT took it as good for a late production F6F-3, at the time there probably weren't as many data sources available as now.

5. From the Navy pdf:
"Provisions for water injection to increase combat power were added in late 1943, as production -10W engines fitted for it became available. Wing stub racks were added for bombs, or for additional fuel tanks, to supplement the single centreline belly tank that had become a standard operational feature.
   
In April 1944, when necessary strengthening of the rear fuselage and horizontal stabilizer were incorporated, along with other improvements - including engine cowling changes to reduce drag and aileron spring tabs to reduce stick force in combat maneuvers - the designation of the production aircraft was changed to F6F-5. With the structural changes, dive speed and pull out restrictions on the Hellcat were removed."

Here's a picture on which I've noted the details of changes from F6F-3 to F6F-5(thankyou  google image search!):