You are not going high enough. 25,000 ft is still in the altitude where the engines still producing maximum power or close to it and the drag is much reduced. You posted that "it evens out" but it doesn't there is an optimum for all aircraft. Things like paddle blade props and fittings like bomb racks and tank supports do make a difference.
Fighter Escorts of B-29's over Japan & Pacific
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- #62
I never considered cooling flaps, or the issue of power/weight (though I would have figured that'd affect acceleration and climb)
The B-29 seemed to use several different R-3350 variants and by June 15, 1945, it was possible to push the horsepower up to 2500 for takeoffs. That said, by either late 1944 or early 1945, it seemed the ability to carry a 10000 lb. bomb-load was possible if the weather permitted, with typical loads being around 5000-10000 lb. during high altitude operations: The distance to Tokyo from Tinian was around 1280 nm or 1473 miles for a round-trip of 2560 nm or 2946 miles.
In the post-war period these figures would increase to 3095 nm for the B-29, and 3025 nm for the B-29A in the high altitude profile, which entailed climbing to 20000', and stepping up to 30000' prior to bomb release. I am not sure how much of the increase in range is caused by what.
I figured the issue would be climbing and accelerating longer. As for flying at higher altitudes itself, I figured the whole purpose was so drag levels would be lower, and that usually correlates to higher speed for altitude.
While I'm not an expert on propellers (and it clearly shows), I know the speed of sound is lower at altitude which pushes up the propeller tip-speed, but I ran some calculations for tip-speed and the B-29 with it's low gear-ratio is actually quite fine. I saw around 0.6425 during the cruise portion, with around 0.7027 during climb. I had to make some guesstimates as to what speed would be reached at what point during the climb, but I figured flaps would be brought up at around 500 feet, around 4000 feet before a full climb-speed would be reached and that would be held up to 20000' (for a high altitude flight), and then slowed back down to the appropriate cruise speed, then brought up to climb speed and settings for 30000', then brought back to cruise settings.
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- #63
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
The R-3350 was an eighteen cylinder engine where the R-1820 was a nine.
While it was generating double the heat of the R-1820, it was receiving less airflow over the cylinders due to the additional mass.
The problem with the B-29's engine cowling, is that they had to find a balance of minimal frontal area to reduce drag, while at the same time, get as much airflow over the engine as possible.
While it was generating double the heat of the R-1820, it was receiving less airflow over the cylinders due to the additional mass.
The problem with the B-29's engine cowling, is that they had to find a balance of minimal frontal area to reduce drag, while at the same time, get as much airflow over the engine as possible.
Shortround6
Major General
Well, you have to shove twice as much air through a B-29 nacelle as you do through a B-17 nacelle at the same speed.
Edit, I believe there is a B-29 manual in the manual section and it has some charts/graphs that show how much power (and fuel) is used to climb to certain altitudes as several different gross weights. A B-29 could go through hundreds of gallons of fuel just climbing to cruise altitudes.
It may answer some of your questions.
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- #66
I'll see what I can find, but the F6F, and P-47 both had engines with fairly high power-ratings (2000 vs 2200) and their cowl-flaps didn't seem to cause the drag issues the B-29's cowl-flaps caused.
I think it is that drag was more of an issue on a B-29. The first priority for a B-29 after take off was to gain forward speed not altitude, at any given continuous power setting a B-29 with full fuel and bomb load will be going slower than a single engined fighter, so there is less airflow into and through the engines. I presume this also meant the engine cowls were open more resulting in more drag.
This from "Blankets of Fire. US bombers over Japan in World War II" about the R-3350 woes.
"The engines continued to have technical problems and proved only marginal in operations. Although it is true that they improved as time went on (as they became better designed, constructed, maintained and operated), this improvement was somewhat mitigated by the ever increasing loads put aboard the B-29 for tactical purposes. At best, the R-3350 provided a small safety margin. Often this was inadequate, for if all four engines were not working at just about optimum performance, especially at takeoff, results could be serious, if not catastrophic.
Throughout its service life the R-3350 ran hot on the ground and during flight. This was primarily due to its design, although the cowling was a contributing factor. In April 1942 Boeing raised questions about the cowling and also objected to the prop blades the AAF had picked for the shorter four-blade prop as they cut the flow of air to the engines during ground operations. It should have come as no surprise that the initial B-29 engines operated at or above the desired temperature limits.
A number of measures were taken to cool the R-3350 engines. Aluminium-finned and forged barrels were fitted to the engines [*]. Moveable cowl flaps were designed to handle this situation (devices like flower petals were installed midway on the engine nacelles), but as the cowl flaps were opened to increase cooling on takeoff and climb, drag increased, requiring more power from the engines, which further aggravated the cooling problem and contributed to shorter engine life. One modification made the top two "petals" of the cowl flaps operable, which aided cooling on the ground, but caused buffeting when when fully opened during flight. To alleviate the buffeting, a spring device was fitted that gradually closed the flaps as airspeed increased. In addition the, all the cowl flaps were shortened by three inches and thus could be opened wider before buffeting occurred. After the war airmen discovered that the overheating problem was even worse than it appeared, which helps explain the high incidence of engine fires. They found that the cylinder where the engine's cylinder-head tempperature was measured was not the hottest for all operating conditions. For example, during ground operatiion, some cylinders were as much as ninety degrees hotter than the instrumented cylinder. The new cowl flaps, ducted baffles (to better circulate air), and oil crossover tubes (to better circulate oil) were put into engines at the Oklahoma City Air Depot beginning in September 1944 and sent in kits to the combat forces in late 1944. In 1944 a larger cowl opening in the nacelle went into production along with the cuffs on the prop blades and a better seal on the cowl. .As a result, engine temperatures could be kept comfotably below the desired limits, and the life of the engines began to increase."
* Introduced on the R-3350-23 "combat engine".
Another change that helped cooling was the switch to fuel injection, which stopped the backfiring, but only the 509th BG aircraft had these in Aug 1945 apparently.
"The engines continued to have technical problems and proved only marginal in operations. Although it is true that they improved as time went on (as they became better designed, constructed, maintained and operated), this improvement was somewhat mitigated by the ever increasing loads put aboard the B-29 for tactical purposes. At best, the R-3350 provided a small safety margin. Often this was inadequate, for if all four engines were not working at just about optimum performance, especially at takeoff, results could be serious, if not catastrophic.
Throughout its service life the R-3350 ran hot on the ground and during flight. This was primarily due to its design, although the cowling was a contributing factor. In April 1942 Boeing raised questions about the cowling and also objected to the prop blades the AAF had picked for the shorter four-blade prop as they cut the flow of air to the engines during ground operations. It should have come as no surprise that the initial B-29 engines operated at or above the desired temperature limits.
A number of measures were taken to cool the R-3350 engines. Aluminium-finned and forged barrels were fitted to the engines [*]. Moveable cowl flaps were designed to handle this situation (devices like flower petals were installed midway on the engine nacelles), but as the cowl flaps were opened to increase cooling on takeoff and climb, drag increased, requiring more power from the engines, which further aggravated the cooling problem and contributed to shorter engine life. One modification made the top two "petals" of the cowl flaps operable, which aided cooling on the ground, but caused buffeting when when fully opened during flight. To alleviate the buffeting, a spring device was fitted that gradually closed the flaps as airspeed increased. In addition the, all the cowl flaps were shortened by three inches and thus could be opened wider before buffeting occurred. After the war airmen discovered that the overheating problem was even worse than it appeared, which helps explain the high incidence of engine fires. They found that the cylinder where the engine's cylinder-head tempperature was measured was not the hottest for all operating conditions. For example, during ground operatiion, some cylinders were as much as ninety degrees hotter than the instrumented cylinder. The new cowl flaps, ducted baffles (to better circulate air), and oil crossover tubes (to better circulate oil) were put into engines at the Oklahoma City Air Depot beginning in September 1944 and sent in kits to the combat forces in late 1944. In 1944 a larger cowl opening in the nacelle went into production along with the cuffs on the prop blades and a better seal on the cowl. .As a result, engine temperatures could be kept comfotably below the desired limits, and the life of the engines began to increase."
* Introduced on the R-3350-23 "combat engine".
Another change that helped cooling was the switch to fuel injection, which stopped the backfiring, but only the 509th BG aircraft had these in Aug 1945 apparently.
Shortround6
Major General
OK, you have at least three different things going on and probably more.
A B-29 can use twice as much fuel per engine to climb to 25,000ft. and it took twice as long to get there. Amount of fuel burned per minute per engine wasn't that different. But you were cooking the engines for a lot longer.
What are the climb speeds? A 10% increase in speed gives 10% more airflow.
The smaller inlet on the B-29s cowling means less airflow unless you have the flaps opened up.
F6F with open flaps.
B-29 was trying to climb with each engine "lifting" 30,000lbs of weight, even at 100,000lbs each engine was lifting 25,000lbs of weight.
BTW. the C-69 had different cowls, no turbos, and max gross weight of 72,000lbs to start with.
The B-29 was under powered for what they were trying to do. It's wing was only 23% larger than the wing on B-17. It had a wing loading approaching 70lb per sq ft.
It HAD to fly on it's engines. And if the engine nacelles (cooling flaps) were making more drag than they figured on they were in trouble.
special ed
1st Sergeant
- 5,005
- May 13, 2018
Two bits of trivia:
The B-29s used on a post war flight around the world used different cowlings from any other of the B-29s.
The CAF B-29 "FIFI" flew without the nacelle side panels installed for years. Only recently have there been photos with the side panels installed.
The B-29s used on a post war flight around the world used different cowlings from any other of the B-29s.
The CAF B-29 "FIFI" flew without the nacelle side panels installed for years. Only recently have there been photos with the side panels installed.
A long time ago a poster put up the actual "regime" on ops when heavily loaded. The climb was a series of steps, climb a little burn off fuel, climb a little more burn off fuel in a series of 4 or 5 steps to desired altitude.
Shortround6
Major General
I don't doubt it.
We have number of tests where certain fighters had to take one or more "breaks" to the cool the engine when trying to do maximum climbs.
The B-29 just maybe the most notorious example of overheating the engines when climbing.
They may also have changed procedures after getting "combat" experience. Or the actual fight profile was dictated by actual air temperature conditions.
The Pilots manual under tropical conditions gives several warnings. One is that running the engines for too long on the ground will make the engines too hot for take-off.
Which normally is common sense but in the case of the B-29 they were expecting the engine temperature to go up 40C during the take-off run so if you started at 220C you could wind up in the danger zone by the time you lifted off. You were advised to have the cylinder temperature low enough to allow for that 40C temp rise.
The need to allow for hot weather is also included. Efficiency of up to 10% could be lost if taking off during the hot part of the day.
From what I remember and understand of it, the B-29 was the first plane where the flight engineer took more training than the pilot. The "steps" when on climb were not fixed but a result of calculations while in flight of the actual engine and air temperatures, air speeds fuel consumption, load out etc.
Shortround6
Major General
Well, we do have the Flight engineers manual over in the tech section 
When I become a flight engineer I will read it.Well, we do have the Flight engineers manual over in the tech section
Shortround6
Major General
Does that mean I can't read the Pilot's manuals anymore?
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- #77
Fascinating!
I wasnt being flippant, I have seen non pilots being schooled by pilots here on how to read a pilots manual. Since I have already posted that a flight engineer on a B-29 took longer to train than the pilot, I dont believe or have the fantasy that you can understand all the ins and outs of the subject by reading a manual. I can understand the principles of the problems but that doesnt make me a flight engineer, or even close.Does that mean I can't read the Pilot's manuals anymore?
Shortround6
Major General
I have read a number of flight manuals, doesn't mean anybody should let me loose in even a Piper cub.
Doesn't mean I can't get some nuggets of information or even get an understanding of how some things work.
I also understand that many of manuals are snapshots in time and just because a manual says to do something a certain way or a certain piece of equipment was mentioned that doesn't mean the manual wasn't revised the next week with a different procedure or different piece of equipment.
Some of use like to go through the manuals and some of us like to go through the unit histories or operations. Both (and other areas of study) help bring about a better understanding for all of us.
In regards to the B-29 it was a giant step in complexity compared to the bombers that came before it. Splitting the job of flying it into 3-4 men may have been the best idea with on man monitor/managing the mechanical systems, another man handling the navigation and 2 men actually flying the plane and interacting with the other two.
Doesn't mean I can't get some nuggets of information or even get an understanding of how some things work.
I also understand that many of manuals are snapshots in time and just because a manual says to do something a certain way or a certain piece of equipment was mentioned that doesn't mean the manual wasn't revised the next week with a different procedure or different piece of equipment.
Some of use like to go through the manuals and some of us like to go through the unit histories or operations. Both (and other areas of study) help bring about a better understanding for all of us.
In regards to the B-29 it was a giant step in complexity compared to the bombers that came before it. Splitting the job of flying it into 3-4 men may have been the best idea with on man monitor/managing the mechanical systems, another man handling the navigation and 2 men actually flying the plane and interacting with the other two.
FLYBOYJ
"THE GREAT GAZOO"
Think about the B-36!
Most of the time WW2 and post WW2 fight engineers begin their careers in maintenance centered around a specific airframe. They learn the airframe "backwards and forward," learned or have a working knowledge of all the systems and can troubleshoot many if not all maintenance issues encountered in flight. They are a link between the pilot(s) and maintainers. It's been said the Flight Engineers fly the plane, the pilots just steer!
