F4U vs F6F & Top-Speed: Let's Settle This

[Zipper730]

There's been all sorts of discussions about the performance of the two aircraft which often revolve around position and compressibility error corrections. The claims that seem to pop up are, as follows

• The F6F & F4U in top speed and level flights by test pilots in WWII seemed to show much less of a speed discrepancy than listed on the charts: Supposedly this had to do with the fact that Grumman went to rigorous lengths to ensure that the speeds were calculated as accurately as possible, whereas Chance-Vought did a shoddier job. A counterpoint is that all these claims came from Grumman which raises questions as to bias and competition.
• Claims from Grumman stated that the F6F-3 & F6F-5, despite reading a difference in 10-15 miles an hour, were actually much closer in top speed due to a repositioning of the pitot-static system so it would be closer to the F4U-1. It seems an odd thing to lying about admitting to (as it seems to be confessing to fraud).
• Pilots who raced the F4U's and F6F's after the war said they were fairly close in top-speed as well: This one seems to have a legitimate claim as they might not have had the same biases as Grumman and Chance-Vought (though pilots can be partial to aircraft). That said, I'm not sure what manifold pressures were used in the post-war period, and race-planes are often souped up beyond all recognition.

I'm basically interested in facts and figures, and I'm curious if there were any speed discrepancies in the F4U-4 as well.

drgondog S Shortround6 W wuzak X XBe02Drvr

 

[XBe02Drvr]

Claims from Grumman stated that the F6F-3 & F6F-5, despite reading a difference in 10-15 miles an hour, were actually much closer in top speed due to a repositioning of the pitot-static system so it would be closer to the F4U-1. It seems an odd thing to lying about admitting to (as it seems to be confessing to fraud).

The devil is in the details, my friend. There is nothing cut and dried about pitot-static system design, as it can get damn close to FM (F_ _king Magic). At best they're dart game compromises between accuracy, durability, and complexity. The biggest issue is coming up with a static air reference value that is unaffected by airspeed, angle of attack, sideslip angle, propwash, disturbed air flow, etc, etc. The surfaces of an airframe are a patchwork of constantly varying "static" air pressures as all of these variables change, and without an accurate static pressure your pitot system has no meaningful reference to compare its sampled dynamic pressure to. The charted calibration factors are approximations at best. Even changing the position or location of the pitot tube (for durability's sake, maybe - ever hear the term "hangar rash"?), will cause a different airspeed indication, even if all other conditions remain equal. Matching Chance Vought's pitot mounting installation might have been Grumman's attempt at an apples to apples comparison, not necessarily with fraudulent intent.
Cheers,
Wes

 

[tomo pauk]

People can invest a hour or two looking at speed graphs produced by independent entities (is this case - US Navy, testers in the UK) and see that F4U was faster on same horsepower and altitude.

 

[Zipper730]

The devil is in the details, my friend.

I kind of figured that to an extent: That's kind of why I posted it under technical.

There is nothing cut and dried about pitot-static system design . . . . The biggest issue is coming up with a static air reference value that is unaffected by airspeed, angle of attack, sideslip angle, propwash, disturbed air flow, etc, etc.

It's kind of amazing that they can make the device read accurately with AoA factored in.

As for slipstream, I figured that's why most propeller aircraft have the tubes mounted on the wings.

ever hear the term "hangar rash"?

Actually, I'm not sure I have. I have heard of road-rash though, but they're almost certainly different things. Regardless, I don't think either can be fixed with latex...

People can invest a hour or two looking at speed graphs produced by independent entities (is this case - US Navy, testers in the UK) and see that F4U was faster on same horsepower and altitude.

Did they use the same pitot-static systems on both?

 

[tomo pauk]

...
Did they use the same pitot-static systems on both?

The true air speed table/chart was a product of reading the IAS recording, temperature and altitude, transforming that into non-corrected TAS table, that table was then corrected for compressibility and 'position error'. The 'position error' was different on each aircraft type, and it was dependant on the location of pitot tube.

 

[XBe02Drvr]

As for slipstream, I figured that's why most propeller aircraft have the tubes mounted on the wings.

That's true, but the real issue is the static port(s). They have a way greater effect on accuracy than pitot tubes do, and are so much harder to get right. Getting a reliable and accurate static pressure in the midst of a virtual hurricane of moving air is a real challenge.

 

[DarrenW]

Have to agree with Tomo on this one. I have exhaustively examined test data from many sources and it's glaringly obvious that the F4U had a distinct speed advantage over the F6F at all altitudes and power settings. The US Navy was quite thorough with it's testing procedures and I personally wouldn't question their accuracy. And although there may have been some errors induced by pilot inability to accurately read US instruments (concerning the British tests) this probably occurred with both airplane types equally. Grumman relocated the pitot static port from the wing tip and placed it on the starboard side mid-fuselage of the F6F-5 which obviously changed the position error and charts reflect this in pilot manuals.

Given the larger wing area and fatter fuselage of the Hellcat it would be logical to assume that it would be slower than the Corsair. Speed differences were more pronounced in neutral blower due to the slightly greater horsepower afforded the Corsair via the ram air affect. Auxiliary blower critical altitudes of the two types were close but not identical and when similarly configured/loaded contemporary models are compared (F6F-3 vs F4U-1A & F6F-5 vs F4U-1D) you could see an average speed differential from around 10 to 30 mph, depending on power settings and altitudes involved:

Performance with wing pylons and centerline racks:

http://www.wwiiaircraftperformance.org/f6f/f6f-5.pdf

http://www.wwiiaircraftperformance.org/f4u/f4u-1d-acp.pdf

Export version of F4U-1D and F6F-5 (probably in 'clean' condition):

http://www.wwiiaircraftperformance.org/f4u/corsair-IV-ads.jpg

http://www.wwiiaircraftperformance.org/f6f/hellcat-II-ads-a.jpg

 

[Zipper730]

That's true, but the real issue is the static port(s).

Where are they located on the F6F-3 and F4U-1?

Have to agree with Tomo on this one. I have exhaustively examined test data from many sources and it's glaringly obvious that the F4U had a distinct speed advantage over the F6F at all altitudes and power settings.

What I was basically curious about had to do with the fact that...

1. They calculated based on the plane's IAS
2. Then corrected for altitude and temperature
3. Then they corrected for position and compressibility errors

How did they generally determine the latter item? I assume they either had another plane with properly calibrated instruments, or towed some kind of probe behind them.

And although there may have been some errors induced by pilot inability to accurately read US instruments (concerning the British tests) this probably occurred with both airplane types equally.

Why would the British have any trouble -- the US and UK both used miles per hour or knots at that point in time.

Grumman relocated the pitot static port from the wing tip and placed it on the starboard side mid-fuselage of the F6F-5 which obviously changed the position error and charts reflect this in pilot manuals.

Do you have information on the F6F-3 & F6F-5 regarding position/compressibility/both errors? Also, do you have anything on the F4U-1?

Given the larger wing area and fatter fuselage of the Hellcat it would be logical to assume that it would be slower than the Corsair.

The F6F's propeller might have been more efficient: I remember reading that they started swapping the F4U's normal 13'4" propeller with the 13'1" or 13'2" propeller used on the F6F. They showed a difference in top speed and climb-rates.

 

[DarrenW]

Why would the British have any trouble -- the US and UK both used miles per hour or knots at that point in time.

This had been discused in British tests concerning the P-47C (see item 14):

P-47 Tactical Trials

 

[DarrenW]

Where are they located on the F6F-3 and F4U-1?

The static port is located near the wing tip on the F6F-3. I believe that the F4U series had them located on the fuselage but someone here may know for sure.

 

[DarrenW]

Do you have information on the F6F-3 & F6F-5 regarding position/compressibility/both errors? Also, do you have anything on the F4U-1?

Check with pilot manuals for the type in question.

 

[DarrenW]

The F6F's propeller might have been more efficient:

This was true.

 

[DarrenW]

How did they generally determine the latter item? I assume they either had another plane with properly calibrated instruments, or towed some kind of probe behind them.

This is way above my knowledge level.

 

[Zipper730]

This had been discused in British tests concerning the P-47C (see item 14):

P-47 Tactical Trials

This doesn't seem to have to do with units of measurement, so much as the technique of reducing the data. This actually came up on another post, regarding discrepancies in climb performance. I have no idea how they were reducing the data, and I got some discrepancies in the rate of climb.

The static port is located near the wing tip on the F6F-3.

Right around the same spot where the static port is...

I believe that the F4U series had them located on the fuselage but someone here may know for sure.

That's an odd spot to put them, you'd be in the slipstream. At least the pitot tube was located on the wing-tip.

Check with pilot manuals for the type in question.

I'll see if there's any on this site...

This is way above my knowledge level.

Yeah, I don't have a clue on that either...

 

[DarrenW]

That's an odd spot to put them, you'd be in the slipstream. At least the pitot tube was located on the wing-tip.

I think they were concerned with reading air flow in a relatively undisturbed area. Depending on speed and other factors there could be a lot of wake turbulence generated at the wing tips ( aka vortices) that distort the readings.

 

[Joe Broady]

What I was basically curious about had to do with the fact that...

1. They calculated based on the plane's IAS
2. Then corrected for altitude and temperature
3. Then they corrected for position and compressibility errors

Your steps 1 - 3 are out of sequence, which is understandable since misinformation has been put out elsewhere in this discussion. First off, the chart in the airplane manual gives the correction to convert IAS (what the gauge in the cockpit reads) to CAS (what it would read, if it responded to pressure exactly as designed, and received perfect samples of pitot and static pressure). The F6F pilot handbook gives this:

IAS F6F-3 F6F-5
100 +12 -5
120 +12 -6.5
140 +13 -7
160 +13 -10
180 +14 -11
200 +14 -13
220 +15 -15

For example, if you're flying at -3 Hellcat at 140 KIAS, that equals 140 + 13 = 153 KCAS. That's the correction for position error. (Back in the day it was commonly called "installation error".) To convert to true airspeed, adjust the calibrated airspeed for temperature and altitude. For example, at 10,000 feet and air temperature 10 C, 153 KCAS = 183 KTAS according to my E-6B computer.

The only part specific to the airplane is the IAS to CAS conversion. I've listed the Hellcat figures in the clean configuration. The numbers are different for flaps down. Even the weight can have significant effect due to the greater angle of attack necessary to maintain flight at a given airspeed. The B-36 and C-124 charts show correction curves for different weights.

I know little about how the corrections are determined. As someone else has said, most of the position error comes from inaccuracies in static pressure. Grumman took advantage of that when they relocated the Hellcat static port to address complaints about the Hellcat speed. At least, that's what long time test pilot Corky Meyer said in the book he co-wrote with Steve Ginter. I wonder, were Navy pilots so ignorant of installation error, they were fooled by this change? Of course when you correct IAS to CAS the plane is going no faster than before. And the correction chart is right there in the pilot manual.

Getting back to where those numbers come from, often in flight test operations the plane is modified with a boom on the nose to get air data from a point of undisturbed airflow. The F-100 is one plane which retained the boom on production aircraft. I think it was so long it could be folded on the ground.

Another method is to lower a small bomb-shaped device on a cable. Fins keep the "bomb" pointed straight, and a pitot-static probe samples the air. Famously, the XB-36 lost one of these over Fort Worth. It plunged through the skylight of an elementary school restroom. Several boys were slightly injured by flying concrete fragments from the shattered floor and a toilet was knocked out of commission.

A trailing cone is sometimes employed to get an accurate static pressure when testing large aircraft. A long plastic tube is unreeled from the rear in flight. At the end is a short perforated metal tube to take the pressure sample, which is conducted forward through the plastic tube and sensed by a transducer in the aircraft. At the rear of the assembly is a cone which generates enough drag to keep the long tube taut and stable.

Still another method is to employ a pacer aircraft flying in formation. Often it's equipped with the previously mentioned boom, and its instruments have been calibrated with special care.

References:

Corwin Meyer and Steve Ginter, "Grumman F6F Hellcat," 2012.
(In the book Meyer admits he laid a tremendous egg in the relocation of the static port from its co-location on a boom with the pitot port to the fuselage. Grumman engineers had no previous experience with such an installation, so they put a port on the left side only. But Navy test pilots discovered indicated airspeed would drop to zero in a left sideslip in landing configuration. As senior engineering test pilot, Meyer should have caught that. The fix was to use a static port on both sides of the fuselage.)

Meyers K. Jacobsen, "Convair B-36: A Comprehensive History of America's Big Stick," 1997. (Photo of smashed toilet on p. 34.)

 

[XBe02Drvr]

Grumman engineers had no previous experience with such an installation, so they put a port on the left side only. But Navy test pilots discovered indicated airspeed would drop to zero in a left sideslip in landing configuration.

Back in the day, Cessna 150s had a single static port, located on the starboard side just forward of the doorpost. One of the things you had to make sure students understood before getting in to slips and crosswind landings was how much that affected IAS. In level cruise flight you could yaw the plane with rudder enough to indicate any speed between Vstall and Vne. Better be sure your student is past the "white knuckle" stage before you try this.
Another neat feature of this static port was that you could easily cover it with a near-invisible piece of shear scotch tape to test their vigilance in preflight inspections. If they didn't detect it and actually got airborne, it was a good opportunity to hammer home the value of noting and remembering power settings, attitudes, and aircraft performance, sound, and feel so they could detect and cope with instrument malfunctions. They got used to the idea of something going "wrong" on most flights.
Cheers,
Wes

 

[DarrenW]

(In the book Meyer admits he laid a tremendous egg in the relocation of the static port from its co-location on a boom with the pitot port to the fuselage. Grumman engineers had no previous experience with such an installation, so they put a port on the left side only. But Navy test pilots discovered indicated airspeed would drop to zero in a left sideslip in landing configuration. As senior engineering test pilot, Meyer should have caught that. The fix was to use a static port on both sides of the fuselage.)

Corky's explanation of the Hellcat's pitot-static system has always troubled me. From everything I've read the F6F-3 had the static orifice located on the pitot head. For the F6F-5 the one static line was moved to the starboard (right) side of the fuselage, along station #97. Whatever he was experimenting with apparently didn't make it to any production aircraft.

Here is an excerpt from the report concerning flight testing performed by the US Navy on the 310th production F6F-5. It describes the change in placement of the orifice:

http://www.wwiiaircraftperformance.org/f6f/f6f-5-58310.pdf

IAS F6F-3 F6F-5
100 +12 -5
120 +12 -6.5
140 +13 -7
160 +13 -10
180 +14 -11
200 +14 -13
220 +15 -15

The correction data above is from the flight manual dated 1 June 1944 but by publication of the manual dated 1 May 1946 there were major changes to these figures (as before flaps are retracted):
IAS F6F-3 F6F-5
100 +4.5 N/A
120 +4.5 -2.5
140 +6.0 -2.5
160 +7.0 -4.5
180 +7.0 -4.5
200 +8.0 -4.5
220 +9.0 -4.5
240 +9.0 -4.5
260 +10.5 -3.5
280 +11.5 -3.0
300 +11.5 -2.5

As you probably notice, further tweaking of the pitot-static system resulted in much more accurate instrument readings, with the F6F-5 showing the greatest improvement. This was most likely due to the better placement of the static port (starboard side of fuselage and not on wing tip).

Now if one compares performance data of the F6F and F4U found in documents created after these modifications the Corsair still maintains roughly the same speed advantage over the Hellcat. This to me proves that during the original flight tests the correct instrument error was taken into account and the test data is indeed accurate and comparable.

 

[tomo pauk]

Enjoy some bacon, Darren.

 

[Zipper730]

Back in the day, Cessna 150s had a single static port, located on the starboard side just forward of the doorpost.

This would have been in the 1970's to 1980's right?

One of the things you had to make sure students understood before getting in to slips and crosswind landings was how much that affected IAS. In level cruise flight you could yaw the plane with rudder enough to indicate any speed between Vstall and Vne.

Then how did you know how fast you were going at? I was reading a NATOPS manual on the F-8J and they mentioned that there was a discrepancy in airspeed reading (I think it was 3.5 knots) at high AoA, though in that case, the way one would carry out an approach almost certainly revolved around simply focusing on alpha and glide-path to determine this.