B-36 - Why a Pusher??

[Piper106]

Does anyone have any data or links to technical papers on why the B-36 was built with pusher engines??

I assume that the Convair engineers (and the Northrup engineers working on the B-35) calculated that the drag reduction by not having the propeller airflow over the wing more than made-up for the losses resulting from the propeller having to work in the wake from the wing, but was the difference significant??

I notice that some other 'clean sheet of paper' very long range aircraft (Me 264, and the Nakajima G10N) were conventional tractor designs, so I assume the difference in efficiency between tractor and pusher was pretty close, but that is just a guess.

Comments??

Piper106

 

[iron man]

Does anyone have any data or links to technical papers on why the B-36 was built with pusher engines??

I assume that the Convair engineers (and the Northrup engineers working on the B-35) calculated that the drag reduction by not having the propeller airflow over the wing more than made-up for the losses resulting from the propeller having to work in the wake from the wing, but was the difference significant??

I notice that some other 'clean sheet of paper' very long range aircraft (Me 264, and the Nakajima G10N) were conventional tractor designs, so I assume the difference in efficiency between tractor and pusher was pretty close, but that is just a guess.

Comments??

Piper106

First off...The "Peacemaker" (while impressive, within it's own context) was a massive waste of money.
The entire program was funded on an "we're already too far into this to walk away" basis.
Aerodynamicallly? Pushers had their merits.
Realistically? The power plants themselves were a major PITA for those who had to deal with them...this is well documented.
Forwards or backwards, the enginewas a nightmare to deal with.

A "Bridge to Far", IMO.
'
Not that the B-47 was that much of a improvement either

Interesting times indeed.

There are tons of "B-36" resources on the net...do a little reading. You might find specifics that relate to the question you're asking. I've read boatloads of material on the B-36, yet I've never seen this particular issue addressed.

Perhaps look for NACA reports, with regards to this line of inquiry?

Love the "Magnesium Overcast"...what a STATEMENT.

It now seems silly how we were "duck and covering", when the USSR were actually so far behind the curve, as to make the whole thing kind of laughable (in hindsight) ...

 

[FLYBOYJ]

First off...The "Peacemaker" (while impressive, within it's own context) was a massive waste of money.
The entire program was funded on an "we're already too far into this to walk away" basis.
Aerodynamicallly? Pushers had their merits.
Realistically? The power plants themselves were a major PITA for those who had to deal with them...this is well documented.
Forwards or backwards, the enginewas a nightmare to deal with.

A "Bridge to Far", IMO.
'
Not that the B-47 was that much of a improvement either
Interesting times indeed.

There are tons of "B-36" resources on the net...do a little reading. You might find specifics that relate to the question you're asking. I've read boatloads of material on the B-36, yet I've never seen this particular issue addressed.

Perhaps look for NACA reports, with regards to this line of inquiry?

Love the "Magnesium Overcast"...what a STATEMENT.

It now seems silly how we were "duck and covering", when the USSR were actually so far behind the curve, as to make the whole thing kind of laughable (in hindsight) ...

Agree to a point but disagree about the B-47 - entered service in 51', phased out as a bomber till 65 and finally retired in 77, it flew like a fighter and for a while scared the crap out of the Ruskies. It was a good plane and served well Compared to the B-36 was a maintenance godsend

 

[Siegfried]

A B36 must have had 28 x 2 x 6 = 168 spark plugs. Essentially ubmaintainable. It's no surprise that commercial operators avoided this engine.

 

[tyrodtom]

Siegfried, that's a fail on arithmetic. 28x2x6=336.
The Wasp Major R-4360 used in the B-36 was used by plenty of other aircraft so it was maintainable. The B-50, C-97, C-119, C-124, and about 20 other well known aircraft used the same engine, but not as a pusher.

The B-36 was the only aircraft in the American arsenal that could carry the early largest H-bombs until the B-52 came along.

 

[woljags]

i always understood that the B47 was for a while faster than the Russian fighters of the time it entered service or is that incorrect

 

[JoeB]

Does anyone have any data or links to technical papers on why the B-36 was built with pusher engines??

I assume that the Convair engineers (and the Northrup engineers working on the B-35) calculated that the drag reduction by not having the propeller airflow over the wing more than made-up for the losses resulting from the propeller having to work in the wake from the wing, but was the difference significant??

I haven't ever seen quantitative papers or books about this design decision, but qualitatively speaking it's more or less as you said. The B-36 specification heavily emphasized long range and thus cruise performance. As a rule a pusher configuration will be more efficient in cruise, subject to considerations like rotation on take off, which might force the pusher prop to be smaller diameter to clear the ground on rotation. However in B-36 case they found that pusher was definitely more efficient in cruise. Some early concepts leading to the B-36, and some wind tunnel models even, had push-pull nacelles or conventional tractor nacelles. but the designers believed pusher would win, and at least at the state of the art in wind tunnel testing at the time the tests proved them right.

The disadvantage of pusher is not so much that wing downwash will actually make it less efficient net, but the vibration considerations of uneven flow, not only wing downwash in cruise, but also the engine exhaust stream flowing through the prop. These were issues with the B-36 which had flight restrictions related to prop vibration. Also, besides the cruise configuration, the wing interaction issues become more serious with flaps down. Note in photo's of B-36's even fully extended flaps are continued right in front of the props. And also in low speed flight, the tractor prop helps generate lift which the pusher doens't to the same degree. And then there's more potential for FOD from stuff thrown into the props by the main gear; again see B-36 photo's: the main gear legs are within the radius of the inboard props.

Re: B-47, can't see how it could be compared to the B-36 in practically any way. The whole concept of its use was different, relying on friendly bases close to the USSR (many B-47's were homebased in CONUS, but practically speaking would need staging bases in Allied countries, even with aerial refueling). And it was an example of much more advanced state of art. The planes shared the J47 engine, but those were just an add on to the B-36, rather than the engine techology around which the B-47 was designed. And the B-47 was close enough in speed to enough of the Soviet interceptor force to make large scale B-47 nuclear raids essentially unstoppable (and there were lots of B-47's), even if groups of subsonic MiG's could intercept a single B-47 as they proved in certain RB-47 intrusions into Soviet territory or nearby. Although, even the B-36's capabilities were adequate to make the US nuclear deterrent credible v the USSR into the late 1950's. It's one thing to shoot down a bomber or cause a few % losses per sortie and make a conventional bombing campaign too costly. It's quite another to shoot down enough planes to make a nuclear attack's results tolerable: that was extremely difficult to do, requiring almost perfection for the defense and giving every advantage to the offense.

Joe

 

[GregP]

The B-47 was a great plane. I have friends who flew them and they heap praise upon it as fast, easy to fly, able to hit the target, and just a plain old kick to be assicoated with. As a first-generation jet bomber design after WWII, it was among the best of the lot. If Boeing hadn't developed the B-52, who knows? We might still be flying it.

There are a lot of people out there who are not familiar enough with the B-52 to even begin to understand why we are still flying it. suffice to say that if you vew it from above, all that wing area is not there for nothing. It can hit you from a LONG way up in the air, with iron bombs, smart bombs, cruise missiles, antiradtion missiles, and even sir-to-air missiles. It may be old, but it still has a great set of teeth!

 

[Siegfried]

Siegfried, that's a fail on arithmetic. 28x2x6=336.

Yes but I can pose the correct question, even with 1/2 bottle of Cabernet.

The Wasp Major R-4360 used in the B-36 was used by plenty of other aircraft so it was maintainable. The B-50, C-97, C-119, C-124, and about 20 other well known aircraft used the same engine, but not as a pusher.

The B-36 was the only aircraft in the American arsenal that could carry the early largest H-bombs until the B-52 came along.

Yes, but no civilian customers.

 

[Jenisch]

Yes, but no civilian customers.

It was used in the Boeing 377 and it's Mini Guppy and Pregnant Guppy variants, and in the SNCASE Armagnac.

 

[tyrodtom]

Yes but I can pose the correct question, even with 1/2 bottle of Cabernet.



Yes, but no civilian customers.

LOL, Since when do we judge the success of a military engine or aircraft by how successful it was on the civilian market?

 

[Jenisch]

I don't know if we can say the B-36 was a "failure". The plane was developed to attack Germany from the US and Canada if Britain fell. It would be avaliable earlier if needed. It also could attack any target inside the Soviet Union in case Stalin turned out against the West (this for justify it's development during WWII). So, I think it gave flexibility to the USAAF/USAF.

 

[tyrodtom]

The B-36 program got put on the back burner pretty early in it's developement program, by mid 1944 everybody, probably even Hitler, knew Germany was on the downhill road to defeat.

 

[Siegfried]

It was used in the Boeing 377 and it's Mini Guppy and Pregnant Guppy variants, and in the SNCASE Armagnac.

56 + 9. I wouldn't call that commercially successfull in any way.

 

[Jenisch]

56 + 9. I wouldn't call that commercially successfull in any way.

Indeed. The 377 had to make scales because oil would be running low, not fuel. The turboprop put those nice but complex and expensive to maintein radials in the grave.

 

[Siegfried]

LOL, Since when do we judge the success of a military engine or aircraft by how successful it was on the civilian market?

It would have been a maintenance and resource hog and that also suggests a militarily inefficient aircraft. By the time it could have been ready it would have been rendered ineffective by SAM missiles, ( an easy target for wasserfall )should it have been needed it was also well within interception capability of any 1945 or 1946 Luftwaffe. A more creatively designed jet aircraft could have exceeded its range by 1949 anyway by which time the b36 was barely ready anyway. It should have been caned and the US Navy had its super carriers funded. They'd still be around.

 

[Jenisch]

Lycoming XR-7755 was the largest piston-driven aircraft engine ever produced; with 36 cylinders totaling about 7,750 in³ (127 L) of displacement and a power output of 5,000 horsepower (3,700 kilowatts). It was originally intended to be used in the "European bomber" that eventually emerged as the Convair B-36. Only two examples were built before the project was terminated in 1946.

The resulting design used 9 banks of 4 cylinders arranged around a central crankshaft to form a four-row radial engine. Unlike most multi-row radials, which "spiral" the cylinders to allow cooling air to reach them, the R-7755 was water-cooled and so each of the cylinder heads in a cylinder bank were in-line within a cooling jacket. Contrast this with the Junkers Jumo 222, which looked similar from the outside but ran on a V-style cycle instead of a radial. The XR-7755 was 10 ft (3 m) long, 5 ft (1.5 m) in diameter, and weighed 6,050 lb (2,740 kg). At full power it was to produce 5,000 hp (3,700 kW) at 2,600 rpm, maintaining that with a turbocharger to a critical altitude that was apparently never published.

Each cylinder bank had a single overhead cam powering the poppet valves. The camshaft included two sets of cams, one for full takeoff power, and another for economical cruise. The pilot could select between the two settings, which would shift the camshaft along its axis to bring the other set of cams over the valve stems. Interestingly, the design mounted some of the accessories on the "front side" of the camshafts, namely two magnetos and four distributors. The seventh camshaft was not used in this fashion, its location on the front of the engine was used to feed oil to the propeller reduction gearing.

The original XR-7755-1 design drove a single propeller, but even on the largest aircraft the propeller needed to absorb the power would have been ridiculously large. This led to a minor redesign that produced the XR-7755-3, using a new propeller gearing system driving a set of coaxial shafts to power a set of contra-rotating propellers. The propeller reduction gearing also had two speed settings to allow for a greater range of operating power than adjustable props alone could deliver. Another minor modification resulted in the XR-7755-5, the only change being the replacement of carburetors with a new fuel injection system.

Lycoming XR-7755 - Pictures Photos on FlightGlobal Airspace

Now imagine this monster people.

 

[tyrodtom]

It would have been a maintenance and resource hog and that also suggests a militarily inefficient aircraft. By the time it could have been ready it would have been rendered ineffective by SAM missiles, ( an easy target for wasserfall )should it have been needed it was also well within interception capability of any 1945 or 1946 Luftwaffe. A more creatively designed jet aircraft could have exceeded its range by 1949 anyway by which time the b36 was barely ready anyway. It should have been caned and the US Navy had its super carriers funded. They'd still be around.

Of course, as usual, you're presenting something that never made it beyound the prototype stage of developement, with your usual magic enhancement of it's capabilities, to someting that got built and actually put on operations, but in the B-36's case after WW2. Because it wasn't needed to defeat Germany as it WAS.

 

[drgondog]

To the B-36 detractors - It was the ONLY weapon the US had through the early 50's which could carry the big boomers (internally) and destroy anything, anywhere on this earth. The late 40's brought out the MiG 15 which was the only Soviet fighter capable of getting an altitude advantage and it was a point interceptor which would have to be reasonably close to the course track to intercept the late model 36's.

Waste of money? How does one determine that, had the 36 not been around in strength in 1948-1950? The Russians did NOT pile on in Korea for a reason.

 

[Jenisch]

BTW, how the USAAF would obtain weather information to attack Germany from the US like the B-36 was planned? I can think of submarines and ships providing today's SHIP reporting for the Atlantic, but don't know more details about the meteorology in those days.