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World War II
Discuss WWII and the era directly before and after the war from 1935-1949.
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B-25 top speed and redline
russamotto
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Utah, United States
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Posted: Sunday, April 03, 2016 - 12:58 AM UTC
I was reading this post http://www.militaryhistoryonline.com/wwii/articles/ormocbay.aspx discussing the 38th BG attack on Japanese shipping in Ormoc Bay. It is a very good read, but one comment caught my attention. It states the B-25s came in at roughly 400 mph, and that the pilots regularly flew well above redline as they knew the engines could take it. I can understand pilots pushing it as hard as they could as the bleed off in speed from firing 12 .50 cal guns would be significant, but the speed surprised me. I have started searching but can't find any other references to this. Just how far could the aircraft be pushed? I imagine the A-20 and B-25 would be pushed as hard as possible and the Wright radials must have been fairly rugged. I have also heard but can't find reference for that the opposite was said of the Allison V-1750. Push that too hard and it would cause the engine and plane to blow up. Does anyone have any better information or insight into this?
stooge
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South Australia, Australia
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Posted: Sunday, April 03, 2016 - 06:06 AM UTC
The controls would have been somewhat heavy in a B25 at 400mph, possibly more danger in not being able to pull out out or avoid the target than engine failure due to overspeeeding the prop / engine combo.
JPTRR
Staff MemberManaging Editor
RAILROAD MODELING
#051
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Tennessee, United States
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Posted: Sunday, April 03, 2016 - 06:54 AM UTC
Hi Russ,

I dunno about whether or not they often flew at red line, but short of an extended vertical dive, a B-25 would not reach 400 mph. Certainly not down in the thick air. Must be a typo or mis-speak. Page 102 of the below linked B-25 manual states the max. redline speed at 340mph.

https://ia801706.us.archive.org/12/items/PilotTrainingManualForTheMitchellBomber/PilotTrainingManualForTheMitchellBomber.pdf

IIRC, drag increases by the square of the speed increase. Double the speed, drag quadruples. There comes a point at which aerodynamic forces will prevent the speed from increasing without extra thrust, and it would take a lot of excess power to crush a B-25 through the air up to 400mph. But then bad things would happen, e.g., if somebody could 3D print a Sopwith Camel in titanium, the Camel could exceed Mach I if you strapped it to a Saturn V. But aerodynamic forces (such as aerodynamic flutter) would disintegrate it somewhere between 130 mph and Mach.

Consider the Arado Ar 234, smaller and lighter than the B-25, and much sleeker; yet it took two Jumo 004B-1 jets spewing 1,900 lbs thrust (it takes 2 hp to = 1 lbs thrust), or 7,600 hp to bump it up to 461mph, at 20,000.

Also the sleek light de Havilland Mosquito PR Mk 34 and PR Mk 34A could reach 425mph, but at 30,000, and with 3,380 hp of two Merlins kept perky at altitude by a special supercharger.

There's an incredible book out there I highly recommend to everyone: Wreaking Havoc: A Year in an A-20
By Joseph W. Rutter


Fun topic! ;-)

russamotto
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Utah, United States
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Posted: Sunday, April 03, 2016 - 06:48 PM UTC
Here is the section from the text.


Quoted Text

While still over the northern end of the Camotes Sea, Maurer watched on helplessly as his comrades in the 822nd got cut to ribbons. However, he continued to lead his squadron and in turn, the entire group further north to take advantage of the terrain. The formation flew up the west side of the Ormoc Peninsula, staying well out of range of the Japanese guns. This strategy allowed the strike force to head into the hills, gaining altitude, without being noticed by the enemy. Reaching Canaguayan Point, the three remaining squadrons made a starboard turn to the east and headed for the beach. When Maurer flying in B-25J #43-27971, made landfall, he had the group turn 90 degrees to the south, in the direction of the target. With the rugged terrain of the peninsula between the B-25s and the convoy, the formation stayed low, just above the trees, contour-flying the undulating landscape. Indeed, they were flying so low, that the propellers of some of the planes were chopping through the tops of the trees like a lawnmower. The bombers climbed up to 7000 feet, before gradually increasing power, while still remaining out of the line of sight of the Japanese ships. Flying down-slope, the aircraft were zooming along at about 400 miles per hour, with the engines way over the red line, when they crested the final ridge before reaching the sea again. That speed is way over the red line of the engines, however the pilots found out through experience that they could get away with it without anything coming apart. Although the tail did a lot of wiggling, it did not greatly alter the flight characteristics of the aircraft. At 1135 hours, the 22 strafers broke out over the water at full throttle, going like a bat out of hell. So complete was the surprise, that the first few flights attracted not a single shot from the enemy vessels before making their runs.



I had wondered if air pressure built up and gave a false reading, or if it was an exaggeration or misunderstanding.

The entire account makes for good reading and includes details of how the surviving crew members were rescued by locals and helped, at great personal risk.
Kepler
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Genova, Italy
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Posted: Sunday, April 03, 2016 - 07:48 PM UTC
Looks to me a little confusing. As far as I know, a rugged engine can be over revved for some time if it is not “loaded”, meaning it’s not producing power. Aerodynamic redline is reported on flight manuals as VNE, or never exceed speed, and usually the meaning is literal: never exceed, otherwise damage is inevitable, flutter likely and break up possible. If the B25 manual lists 340 mph as VNE, I doubt it could really do 400 without consequences.
JPTRR
Staff MemberManaging Editor
RAILROAD MODELING
#051
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Tennessee, United States
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Posted: Sunday, April 03, 2016 - 08:37 PM UTC

Quoted Text

I had wondered if air pressure built up and gave a false reading, or if it was an exaggeration or misunderstanding



My guess is misunderstanding or false readings. For decades legend held that a P-39 broke the sound barrier in a dive but that was due to bad instrumentation or interpretation thereof. There are different aircraft speeds - true, indicated and calibrated, those being based on air pressure & temperature, and mechanics; in a Ballentine Pocket book about the Spitfire is the narrative of a Spit pilot who went after a high-altitude recon Bf 109; he wrote that one had to be very careful in a dive at 40,000 because the wings would be approaching Mach while the IAS (Indicated Air Speed) might show a mere 150mph (he wrote something like, 'must be careful lest you suddenly be sitting in your seat without your socks or airplane on'.

Corky Meyer wrote that at sea level the F4U and F6F ran side-by-side at METO (Maximum Except for Take Off) power but the Corsair showed much faster - that was due to the placement of the static port. IAS (and altitude) errors will also be different depending on whether the pitot tube or the static port is blocked.

I also read a 1941 story in a U.S. publication about the Battle of Britain about RAF fighters 'hurdling skyward at 400 mph, then diving down spinning to put an 'impenetrable' cone of fire in front of formations of Luftwaffe bombers'.

Lots of interesting reading out there. Whether by pilots, propagandists, or armchair types, there's some interesting info.
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