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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.I'm confused about something here regarding air-miles per pound or air-miles per gallon.
These stats come from a thread about the most overrated aircraft of WWII and includes figures for the P-51D-5, P-38L, and the P-47D-25 at different altitudes and weights
P-51D-5, 9,600 to 8,000 lbs, wing bomb racks only, maximum range cruise conditionAt 15000 feet: 260 TAS, 44 GPH = 5.91 air miles per gallonAt 20,000 feet: 280 TAS, 48 GPH = 5.83 air miles per gallonAt 25,000 feet: 305 TAS, 52 GPH = 5.87 air miles per gallonP-38L, 17,400 to 13,500 lbs, tank supports only, maximum range cruise conditionAt 15000 feet: 229 TAS, 61 GPH = 3.75 air miles per gallonAt 20,000 feet: 248 TAS, 66 GPH = 3.76 air miles per gallonAt 25,000 feet: 267 TAS, 71 GPH = 3.76 air miles per gallonP-47D-25, 14,200 to 12,000 lbs, no external load, maximum range cruise condition (preliminary data)At 15000 feet: 266 TAS, 88 GPH = 3.02 air miles per gallonAt 20,000 feet: 288 TAS, 95 GPH = 3.03 air miles per gallonAt 25,000 feet: — no figures given —
Why do the fighters show a performance benefit at high altitudes? Fuel burn is higher, but they also fly faster at higher altitudes and, on the fighters it evens out. It doesn't seem to be related to superchargers or turbocharges as the first only has a supercharger, and the latter two have turbochargers. My guess is it's got something to do with acceleration and time to climb.
I never considered cooling flaps, or the issue of power/weight (though I would have figured that'd affect acceleration and climb)No, the climb is not relevant because the figures are for the cruise condition, i.e., after the stated altitude has been attained. The B-36 figures for best range speed at various altitudes, which I posted earlier, show a similar effect of increased true airspeed with no range penalty as altitude increases — up to a point.
My theory is that the "point" occurs when the bomber must begin taking extra steps to keep its engines cool. The fighter doesn't reach that point because it doesn't have to push its engines so hard due to its more favorable power to weight ratio. My copy of the P-38 manual agrees with the values above, and also shows auto lean as the cruise mixture setting at all altitudes.
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.It's even worse because the higher you fly, the more horsepower is needed to obtain the airspeed for best range.
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.Why did the cowl-flaps cause so much trouble on the B-29? Boeing had built the B-17 with ones that seemed to work a lot better for the same altitude (climb-rate was about the same at normal rate)
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.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.
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.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'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.
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.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.
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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.
I don't doubt it.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.
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.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.
When I become a flight engineer I will read it.Well, we do have the Flight engineers manual over in the tech section
Fascinating!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.
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?
Think about the B-36!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.