B-36 - Why a Pusher??

[wuzak]

An earlier turboprop, the experimental XT-31
1980lb, 2300hp.

 

[ccheese]

I just saw this thread, and had to go all the way through it. I am probably the only person on the forum who has actually flown in the B-36. I was stationed at Wheelus AFB, in Tripoli, Libya 1954-1955. I made two flights on the B-36 as an observer (non-crew). Both times we went from Wheelus to somewhere over Europe, then turned around and came back by a different route. I thought it was an enjoyable plane to fly in. On the down side, on both flights we had to shut down the #3 engine. First time for overheating and the second time for a pitch control failure of some kind. The plane flew just as well without it !

As for the pros and cons of the plane, as in the discussion of the thread, I honestly don't know that much about the technical aspects of the plane.

Charles

 

[chipieal]

Hello,
For years - I have pondered the reason for pusher type aircraft? Especially in the B36 configuration. Any thoughts?

 

[Matt308]

Reduction in drag hiding those 6 radials in the wing.

 

[OldSkeptic]

Hello,
For years - I have pondered the reason for pusher type aircraft? Especially in the B36 configuration. Any thoughts?

Stupidity, idiocy, insanity???? There are no gains here at all. Harder to make, harder to service, poorer lift on takeoff, poorer climb, worse stall, cooling issues (especially on an air cooled engine) and so on.

As for the cruising drag issue of a twin tractor it is just down to careful engine nacelle/wing design (near the engine of course), do that then it becomes a non issue.

Bit like the Do-335, a tribute to monomania about a single issue and showed a complete lack of creativity.

Not even sure this even a solution for more economical high speed cruising at high altitudes, as you will need a bigger wing for the lift (=more drag) and/or a higher angle of attack (=more drag).

Write this off as a dud..... Brewster Buffalo or Defiant stuff. Sort of like the ideas we all get after a few too many drinks and get all excited about ... then write off in the cold light of the day (after the hangover had gone).

 

[cimmex]

Stupidity, idiocy, insanity???? There are no gains here at all. Harder to make, harder to service, poorer lift on takeoff, poorer climb, worse stall, cooling issues (especially on an air cooled engine) and so on.

As for the cruising drag issue of a twin tractor it is just down to careful engine nacelle/wing design (near the engine of course), do that then it becomes a non issue.

Bit like the Do-335, a tribute to monomania about a single issue and showed a complete lack of creativity.

Not even sure this even a solution for more economical high speed cruising at high altitudes, as you will need a bigger wing for the lift (=more drag) and/or a higher angle of attack (=more drag).

Write this off as a dud..... Brewster Buffalo or Defiant stuff. Sort of like the ideas we all get after a few too many drinks and get all excited about ... then write off in the cold light of the day (after the hangover had gone).

what is your statement based on? The B-36 program had cost a lot of money and I think the men who made the decision knew what they are doing. The best aircraft engineers of that time were working on the pusher design and it is kind of strange when 70 years later someone criticize this without any proof.
cimmex

 

[tyrodtom]

A wing without the propeller in front to mess up the airflow is going to be more efficient.
But a propeller in the rear, in the turbulent air coming off the rear of the wing, isn't going to be at it's best possible efficiency.

On the B-36 the designers evidently thought that they'd gain more on the first, than they'd lose on the second.
Were they right ?

Also the propeller being in the wing's slipstream was the reason for the B-36's throbbing drone, I've read.

 

[swampyankee]

The B-35 had a different problem. It's difficult to get enough yaw stability and pitch and yaw damping with a flying wing, and the pusher propellers, well behind the center of gravity added both.

 

[OldSkeptic]

what is your statement based on? The B-36 program had cost a lot of money and I think the men who made the decision knew what they are doing. The best aircraft engineers of that time were working on the pusher design and it is kind of strange when 70 years later someone criticize this without any proof.
cimmex

Sadly, being a big project and costing a lot of money, with lots of people working on it is no guarantee that it is not a completely dumb project.
The world is awash (and history replete) with really dumb 'big projects'.

Note that the military does not have a monopoloy on this by any means (though in recent times you might struggle to find a non 'really dumb military project that costs a lot of money and is completely useless'.

Take for example here in Australia. In the State I live in we built the world's biggest desalination plant at the cost of 10s of billions of dollars, that almost certainly will never be used and even it ever is the water it produces will probably be undrinkable anyway. Huge amounts of 'smart' people 'who knew that they were doing' worked on that one.

I could list some of the 'really dumb' military projects in recent decades .... but I don't think a 100,000 word post would be appreciated.

By modern standards I suppose the whole B-36 saga is a drop in the ocean, after all in the end it sort of worked (ish), which is more than you can say for many things.

But never forget my 1st law of life: never underestimate human stupidity.

 

[GregP]

My first two laws are:

1) Never piss into the wind.
2) Never stand next to anyone pissing into the wind.

All else follows these two.

 

[Matt308]

There is an "intolerable people" thread in the miscellaneous section. I would highly suggest that you curmudgeons check it out. You will fit right in with the rest of us.

 

[OldSkeptic]

what is your statement based on? The B-36 program had cost a lot of money and I think the men who made the decision knew what they are doing. The best aircraft engineers of that time were working on the pusher design and it is kind of strange when 70 years later someone criticize this without any proof.
cimmex

Being more serious, this reminds me of the Do-335. To solve a single problem (similar sort of logic to the B-36) of drag caused by a normal tractor, wing mounted twin engine setup, they went to incredible time and effort to create a whole new set of problem for themselves.

In the end they came up with something that was not any faster (the whole purpose of the design) than a Dh Hornet, with far poorer overall performance such as agility, climb, etc plus additional maintenance issues and their own problems with overheating (even though they used liquid cooled engines).

DH took the sensible way of dealing with this issue, design the engine/nacelle/wing interface properly to minimise drag (did the same with the Mosquito) and maintain all the advantages of the twin tractor design (plus be a lot easier to make, debug and maintain).

Dornier took the dumb way and produced a pig that in the end never worked, DH took the smart way and produced a superb aircraft, that was successful in service (and if the jet age hadn't come in then would have had a very long career).

Following this logic the designers of the B-36 should have followed the same process, especially when they were using air cooled engines and it was a given from the start that overheating would be an issue.
Some of those late model US air cooled engines were very marginal in cooling anyway, plus they were designed for an airflow coming in from the font.

The advantages would have been immense, for a start they could have got away with a far smaller wing.
But they didn't and in trying to solve that one issue they created a whole new set of problems for them. Much better to focus on that one issue and minimise all the other problems.


One of the ways (to take a single example) of minimising drag from the tractor design was to make sure the propeller was well away from the leading edge of the wing, reducing turbulance. An example is the British Halifax which long had problems with its Merlin version because the props were too close, causing turbulence and vibration problems, as opposed to the Lancaster which maintained a good distance and never suffered those issues.

The engine nacelle design was critical to maintaining good airflow with minimal drag, this is very important on both the upper and lower parts of the wing. Look at the pictures I have posted on the Mosquito and see the carefully designed engine pod/nacelle. The end result of that (and other careful attention overall) was that a clean light Mossie (eg the early PR ones) was faster with the same power/wetted area ratio than a Spitfire.

So if I was designing a B-36 I'd be looking at all that first, especially the frontal drag caused by using an air cooled engine and good non turbulent air over the top and bottom of the wings. Do all that and you could easily get the same low drag, high altitude performance, but with easier construction, easier maintenance and, almost certainly, a smaller wing.

 

[Shortround6]

Wing size may have had very little to do with engine placement as the engines were not actually IN the wing but in the nacelles at the rear of the wing.

There are no extension shafts. Propellers are mounted on a "normal" propshaft/gearbox and the engines are pretty much in line with the flaps. Engine power section (cylinders) is much smaller than the whole nacelle, The turbos, intercoolers and ducting taking up quite a bit of space.

See: http://www.angelfire.com/dc/jinxx1/B36/B36J522217_10.jpg

 

[drgondog]

A couple of simple facts:
1.) until the B-52 it was the Only bomber the US had which could strike Moscow with a Thermonuclear weapon in 1950, multiple nuclear weapons in 1947.
2.) When powered properly it was fast and could go a long way at high altitude.
3.) No nuclear wars occurred on its watch.

The wing was pretty low drag/high CL through all cruise speed angles of attack - good L/D despite fat wing

The pusher does provide less drag over the forward 25% of the airfoil than conventionally mounted engine/prop/nacelle combinations mounted on leading edge.

The B-35 and B-49 also demonstrated excellent L/D

 

[Timppa]

until the B-52 it was the Only bomber the US had which could strike Moscow with a Thermonuclear weapon in 1950, multiple nuclear weapons in 1947.

The first thermonuclear explosion was the "Ivy Mike" shot in 1952. On February 28, 1954, the U.S. detonated its first deliverable thermonuclear weapon (which used isotopes of lithium as its fusion fuel), known as the "Shrimp" device of the Castle Bravo test.

The Mark 17/24 were the first mass-produced hydrogen bombs deployed by the US, yielding 25 megatons, produced between October 1954 and November 1955.

The world's first air-dropped fusion bomb test was RDS-37, 22nd November 1955 at Semipalatinsk Test Site, Kazakhstan, yield 1.6 Megatons
It was the Soviet Union's first test of a two-stage radiation implosion (aka Sakharov's "Third Idea", and Teller-Ulam) design.
The bomb's yield was reduced from its design yield for the test by about half by replacing part of the Li-6 D fusion fuel with "a passive material" (probably ordinary lithium hydride). The yield was within 10% of the predicted value.

 

[drgondog]

You are correct Timppa - relying on my memory at the age of 5-7 and the cover of Life magazine is not reliable. Having said this, the B-52 fleet was not ready in 1954.

 

[swampyankee]

I wish people would echoing the "radials have tons of drag" meme. It is false. Properly designed radial engine installations have no more drag than properly designed liquid-cooled engine installations.

One way is to look at this logically: a 1,000 shp piston engine has to get rid of so much waste heat (probably three or four times its shaft power). Some goes out through the exhaust, some through oil coolers, and some has to be rejected to the air, either directly, through cylinder fins, or indirectly, through radiator fins. Proper cooling design is highly non-trivial for piston-engined aircraft, and it may be a bit easier with radiators, but, reading the literature will show that cooling drag correlates well with power (not a surprise), but there is no distinct advantage to conventional liquid cooled engines vs air-cooled ones. Some unconventional systems, like the surface radiators used on the Schneider Trophy aircraft are much different, although even those will have cooling drag (heating a surface will tend to destabilize a laminar boundary layer, thereby increasing skin friction).

 

[Shortround6]

That is the theory, practice took a while to catch up with theory although by the time the B-36 took to the air practice and theory were pretty much in agreement. This was NOT so in late 30s and only so in early 40s in rare situations. ( FW 190 being a leader)

 

[Timppa]

In June 1948, Convair delivered the first operational B-36A to SAC's 7th Bomb Group at Carswell Air Force Base, across the runway from its Fort Worth plant. Big as the B-29 Superfort was, it could nearly fit beneath one wing of a B-36. Despite the difference in size, the two airplanes had similar vertical tails, and they had slim fuselages, like cigarettes, round in cross-section, with two pressurized crew cabins separated by two bomb bays and connected by a tunnel.

But the wings were different. The Superfort's were thin, straight, and glider-like, while the B-36's wings were more than seven feet thick at the root, enough for a crewman to crawl in and reach the engines or the landing gear in flight. The wings were tapered, with the leading edges swept back, and the effect of that, combined with the wings' location so far back on the fuselage, made the airplane appear out of balance. Strangest of all, the B-36's six Pratt Whitney Wasp Major engines were faired into the trailing edges, with the propellers located aft in the pusher configuration. Although it was supposed to reduce the propeller swirl's turbulence over the wing, the pusher design was rarely used on U.S. aircraft. Apparently it worked, though, because the B-36 had very low drag. The main drawback was that air for cooling the engines was ducted from intakes in the leading edge of the wing, and there was never enough of it, especially at high altitude.

The propellers were 19 feet in diameter, and to keep the tips from going supersonic they were geared to turn less than half as fast as the engines. The engines and propellers produced an unforgettable throbbing sound when the B-36 flew overhead. A friend of mine remembers the sound from his boyhood as a "captivating drone. The noise went down to your heels, it was so resonant. It just stopped you in your tracks. You looked up into the sky to try to find this thing, and it was just a tiny cross, it was so high." Others remember that it rattled windows on the ground from 40,000 feet.

The propellers were reversible for braking on landing, but sometimes they reversed in flight or while the airplane was straining to take off--at least once with fatal consequences. The stainless steel firewalls enclosing the engines cracked. The cylinders overheated. Lead in the gasoline fouled the spark plugs at cruising speed. Each airplane had 336 spark plugs, and after a flight lasting a day and a half, a mechanic would have to haul a bucket of replacement plugs to the airplane to service all six engines. The engines leaked oil, and sometimes a flight engineer had to shut one down because it had exhausted its allotment of 150 gallons.

Then there was the "wet wing." The outboard fuel tanks were formed by the wing panels and sealed at the junctions, and after the wing flexed for a few hundred hours the sealant was apt to fail. Jim Little recalls that one airplane leaked so badly "the ground underneath was just purple [from the dye in the high-octane gasoline]--it was raining fuel under that airplane."
Pilot opinion of the B-36 tended to run to the extremes, but most crew members loved it--"this big, wonderful old bird," Jim Edmundson calls it. As a colonel in the early 1950s, Edmundson commanded a B-36 group at Fairchild Air Force Base near Spokane, Washington. But even he admitted that the airplane could be a chore for its pilot--"like sitting on your front porch and flying your house around."

Of course most of the pilots were young and eager, and the older men had flown worse contraptions during the war. "It was a noisy airplane; it was big," former radioman/gunner Raleigh Watson recalled at a B-36 reunion at the Castle Air Museum in Atwater, California last September, "but it was comfortable, and I think we felt it was a safe airplane, a very well-built airplane." Moxie Shirley, a pilot with more than a thousand hours in the B-36, loved the airplane, declaring that it "kept the Russians off our backs." But he went on to add, "Every crew that ever flew that airplane had stories that would make your hair stand on end."

Altogether, 1951 was a good year for mega-bombers. Margaret Bourke-White rhapsodized over the B-36 in a photo-essay for Life magazine, with photographs taken at 41,000 feet, where the sky "was a color such as I've never seen, the darkest blue imaginable, yet luminous like the hottest cobalt, too brilliant for the eyes to bear." She photographed fluffy white contrails streaming from the reciprocating engines, a 55-foot scaffold used to repair the rudder, and (from both ends) the marvelous flying boom that refueled bombers in flight

B-36 crews speak of 45-hour missions, presumably with fuel cells instead of nukes in the rear bomb bays; at cruise speed, a "featherweight" could travel almost 9,000 miles in that period. The official ceiling was 41,300 feet, but again, crews say that they routinely flew higher than 50,000 feet, and one man--John McCoy, quoted in Thundering Peacemaker--boasted of soaring to 58,000 feet. On missions over China, McCoy said, his RB-36 was chased by MiG fighters that couldn't climb anywhere near it. U.S. fighter pilots of that period also recall B-36s cruising comfortably well above their own maximum altitude. Not until the advent of the "century series" fighters--the F-100 and up--would the B-36 be challenged. Whether the RB-36 ever overflew Russia is anyone's guess, but it was the U.S. altitude and distance champ until the Lockheed U-2 came on line toward the end of the decade.

The whole article:
B-36: Bomber at the Crossroads | History of Flight | Air Space Magazine

 

[GregP]

Shortround, when I crawled all over the B-36 that used to be at Chanute AFB in Champaign-Urbana, Illinois, the engines were in the wing. The nacelles didn't extend far enough aft of the trailing edge to do anything but prevent the props from cutting into the wing.