Fighter Range-importance, philosophy, "records"?

[tomo pauk]

I have no problem with your intention. Just think your speed figures for UASF heavy bombers are wrong. Cheers.

Surely that is impossible to say apart from in a test, cruising speed depends on head winds, bomb load and the speed of the slowest.

Of course you're right. The SAC for the B-17G gives the 'average cruising speed' of 182 kt at 25000 ft, with 'military load' of 10000 lbs. 182 kt = 209 mph. Max speed in the same case being 277 kt = 319 mph.
Max bombs case eaquals military load of 12800 lbs, per same doc.

 

[stona]

Hi Tomo,

I was trying to not start the same old debate all over again and was mostly making the point that the speed of a fighter at any particular time in a mission was pretty dependent on the mission.

True, but also the opposition. The RAF's Spitfires were obliged to cruise at much higher and less economical speeds once the Fw 190 made its debut in order to have any chance of surviving.
Cheers
Steve

 

[drgondog]

The debate about cruise speed always must be bounded by the mission and the altitude and range and payload. Absent any of these criteria - the only answer can be 'It depends".

First - for ETO and MTO escort for 15th and 8th AF, the formation speeds were constant for planning purposes when the B-24 and B-17s were flying together in mixed distribution (i.e 2nd Bomb Division B-24s flying the same inbound route as the 1st and 3rd BD B-17s. In this case (high %) the slower B-17 at 25000 feet was cruising at 150IAS, which was about the fastest cruise speed for the Slowest B-17 with all four engines operation normally. If a B-24 Bomb Division is keeping pace between 20,000 and 22,000 feet (normal) it is cruising at a slightly higher IAS.

150mph IAS at 25000 feet STP, no winds aloft - is 223mph Ground speed. The B-24 Cruise at 25,000 feet is 180mph IAS (keeping pace with slowest operating 'functional' bomber) is 267mph Groundspeed when it did not have to worry about flying with slower B-17s

The True Airspeed of 223mph for 150mph IAS - is the same as True Groundspeed with zero winds aloft to slow the formations due to headwind or speed up due to tailwind - and was the 'normal B-17 formation 'footprint' speed on the ground below. It cruised under similar flight scenarios about 40mph Slower than the potential B-24 cruise speed 'footprint', when operating alone.

Ergo - 150 IAS is about 220 TAS/TGS (no winds) at 25,000 feet. The potentially faster B-24 has slowed down from180mph IAS capability down to about 155 mph IAS at 22,000 feet so as not to over run the B-17 BD's in front - or out run the ones behind. If the B-24 is at 25K, it is operating at 150mph IAS also

Those conditions establish the Operations planning for Escort versus Sweep for the escort. For escort, the flight plans include (99%) external drop tanks - sizing based on Combat Radius but normally in early to mid 1944, the P-51B/D carried 75 gallon tanks, the P-47 150 and the P-38 165 gallon tanks. These large tanks added a significant amount of drag at optimal cruise settings for endurance Until released to clean up the fighter for combat or when the fuel was gone. [NOTE: absent enemy fighters the formation leader may instruct to squadron/group to retain the external tanks even though drag is a factor, and return to base for re-use. absent that order, they would not drop tanks under same condition until everyone had drained external fuel so the formation flying with mixed clean versus attached was not required).

For this ONE condition of planned escort at 25,000 feet, the inbound best cruise setting for the P-51B/D with 1650-7 Merlin and 75 gallon tanks was between 190-205 IAS at ~2150-2250 Rpm and 32-34"MP. 190 IAS is 283 mph Groundspeed (no winds) at 57-58 gph. Boosting to 2250/34 pushes cruise speed to 205 IAS and 305 mph and 68 gallons per hour.

If the P-51 is escorting with one high squadron at 30,000 feet, their settings for optimal cruise w/75 gallon tanks are ~2350/34 for 190mph IAS and groundspeed of 310mph - So, their S turns to maintain position over a bomber stream at 25K running at 220mph is a more frequent S or a sharper S than the fighters at 25,000 feet.

THE KEY FACTOR IS THE 'MOVING FOOTPRINT' OF THE DISPARATE AIRCRAFT IN WHICH THE GROUNDSPEED IS THE COMMON DENOMINATOR. IF THE MUSTANGS ARE ESCORTING B-24S THAT DON'T HAVE TO SLOW DOWN TO KEEP PACE WITH B-17S, THE P-51S CAN ESS AT A MORE LEISURELY PACE - THEREBY 'SHORTENING' THE ACTUAL LINEAR DISTANCE TRAVELLED TO KEEP PACE WHILE MAINTAINING OPTIMAL CRUISE SPEED - AND THEREFORE HAVE SLIGHTLY MORE RANGE PER HOUR IN THE AIR.

In either case - the P-51B/D with externals is running significantly faster in optimal cruise for combat radius than the P-47 or the P-38 at those altitudes.

Coming home, with nobody to escort, after dropping tanks - the P-51D at economy cruise 2250/34 is ~248-250 IAS at 25,000 feet and 370mph groundspeed burning 68 gallons of fuel per hour.

You can see the performance and individual cruise settings as a function of altitude and fuel load out in the P-51D-5 Operating manual, approximately page 50-57. The P-51B/C performance was slightly better due to 600 pounds less GW for same load out.

 

[stona]

The high speed cruise employed by RAF Spitfires to counter the Fw 190 was an entirely tactical consideration.

Cheers

Steve