B-36 - Why a Pusher??

[OldSkeptic]

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.

Don't forget you get a loss of lift compared to a tractor. The fast air flowing over the wing from the propellers causes lift shortening takeoff and aids altitude performance.

Therefore just to take off you are going to need a bigger wing anyway for the same weight, wing area, power, etc.

Then you get into complex calculations, the larger wing has more drag (especially one as thick as that) raising the question of whether or not you are any better off than a well designed tractor setup.

 

[cimmex]

is there a big difference if the airflow is sucked or blown? I don't think so.
cimmex

 

[Elmas]

Once a Very Top Brass of the Bluesuiters was asked about wich, among all the aircrafts under his command, had been the best.
"B-36, no doubt", was the reply.
"But General, B-36 never dropped a single bomb in anger..."
"Exactly."

(From the Internet)

 

[OldSkeptic]

is there a big difference if the airflow is sucked or blown? I don't think so.
cimmex

If that was meant for me, I have clarified the post just in case of confusion.

Note that there are no 'sucking' forces, only 'pushing' ones when dealing with gases.

Though, possibly by accident, you may have raised an interesting issue about propeller efficiency in a pusher arrangement.
I'm thinking of turbulent air coming off the wing and hitting the propeller reducing its efficiency (and possibly creating vibration issues?).

Anyone else got any ideas on that?

 

[Aozora]

B-36 engine design features (from Graham White R-4630ratt Whitney's Major Miracle):

Intake ducting:

"Airplug" and engine diffuser duct:

Oil Supply and cooling:

Airflow and turbochargers:

NACA reports on designing the XB-36 engine installations:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050019376_2005009862.pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050019239_2005009918.pdf

Whichever way you look at it the B-36's engine installation, while a considerable engineering feat once perfected, was complex and naturally required thick, structurally massive and heavy wings. One wonders how the aircraft would have performed with tractor engines in aerodynamically efficient nacelles mounted on a thinner wing? eg; Republic XR-12:

 

[Aozora]

If that was meant for me, I have clarified the post just in case of confusion.

Note that there are no 'sucking' forces, only 'pushing' ones when dealing with gases.

Though, possibly by accident, you may have raised an interesting issue about propeller efficiency in a pusher arrangement.
I'm thinking of turbulent air coming off the wing and hitting the propeller reducing its efficiency (and possibly creating vibration issues?).

Anyone else got any ideas on that?

White believes that the position of the propellers above the centre of the wing trailing edges on the Northrop XB-35 resulted in airflow separating off the wings and creating asymmetric turbulence against the propellers, which exacerbated vibrations inherent in the long prop shafts required by the engines. In addition the sweep back of the wings meant that each side of the propeller disk received different degrees of turbulence from the airflow (White, pages 364-365):

The B-36's wings had straight trailing edges and the centres of the propellers were directly in line with the trailing edge airflow which reduced turbulence, although it wouldn't eliminate the problem entirely. In fact the early fabric covered magnesium flaps had to be replaced by heavier duralumin covered flaps because the turbulent airflow ripped off the fabric covering (White, page 429).

 

[11bwmech]

One small correction. When going aft, you had to hold yourself back. The aircraft was normally nose up to some degree. The pulling happened when going forward.

Ralph

 

[GregP]

Anyone who thinks there is no suction froma 19 foot propeller should try standing in front of one at full power ... and hope like hell you are tied back with a safety belt.

I'm pretty sure there was some extra airspeed going over the wings that would not have been there without the huge prop sucking in air ... though admittedly not near so much as the prop wash from a conventionallly mounted engine of the same power.

If you're still not convinced of the power of suction, stay away from the intakes of running larger jet engines or you'll find out different is a very bad way.


View: https://www.youtube.com/watch?v=5FsrNEeqd6Q

 

[FLYBOYJ]

One small correction. When going aft, you had to hold yourself back. The aircraft was normally nose up to some degree. The pulling happened when going forward.

Ralph

And how about trim?

 

[rinkol]

I recall that the B-36 had a laminar flow wing and that the pusher engine configuration was chosen in an effort to avoid disturbing the air flow over the forward portion of the wing as far as possible. This configuration also enabled the engines to be buried within the wings, an idea that was in vogue in the late 1930s and 1940s. The buried engine concept was a major consideration in the design of the Hyper engines, particularly the O-1230 and Tornado, but presented airframe designers with difficult design problems including structural and maintainability issues and finding space for the storage of fuel and the undercarriage.

 

[GregP]

A 200 pound man moving from the tail to a position in the nose would move some 150 feet. I bet that's enough delta moment to cause a significant trim change.

 

[FLYBOYJ]

A 200 pound man moving from the tail to a position in the nose would move some 150 feet. I bet that's enough delta moment to cause a significant trim change.

Of course, and that's why we have that big wheel to the right of the pilot's knee..

 

[Koopernic]

Of course, and that's why we have that big wheel to the right of the pilot's knee..

Amazing to think a single human could control such a large 180 ton aircraft using non powered flight controls (though with spring servo tabs). A similar sized aircraft of the era, the Bristol Brabazon went all hydraulic.

 

[OldSkeptic]

Anyone who thinks there is no suction froma 19 foot propeller should try standing in front of one at full power ... and hope like hell you are tied back with a safety belt.

I'm pretty sure there was some extra airspeed going over the wings that would not have been there without the huge prop sucking in air ... though admittedly not near so much as the prop wash from a conventionallly mounted engine of the same power.

If you're still not convinced of the power of suction, stay away from the intakes of running larger jet engines or you'll find out different is a very bad way.


View: https://www.youtube.com/watch?v=5FsrNEeqd6Q

Still a pushing force. Feels like being sucked into the prop/intake but you are actually being pushed by all the other air trying to fill the low pressure area right in front of the props/intake.
Nature abhors a vacuum and all that.

 

[GregP]

That's one way to look at it. But they have argued for YEARS about whether Mr. Bernouli was right or downforce causes lift, and they're still at it.

The rerason for the air moving in is low pressure caused by the suction of the prop, so it can be looked at a suction, too.

If you go study the SR-71 you'll find that 70% of the thrust at Mach 3 is caused by suction at the inlets, not air pushing into the inlets. At least Kelly Johnson thought os. I'd be inclined to agree with him.

 

[Shortround6]

What is going on in front of inlets/props at zero speed of the aircraft and what is going on at flying speeds may not be the same thing. And even flying speeds change what is going on or there wouldn't be much need for variable pitch propellers.

 

[swampyankee]

Amazing to think a single human could control such a large 180 ton aircraft using non powered flight controls (though with spring servo tabs). A similar sized aircraft of the era, the Bristol Brabazon went all hydraulic.

Boeing used servo-tabs on the flight controls for the B-17, the 707, and most versions of the B-52. I am mystified by that last, as I think the earlier B-47 used hydraulics.

 

[Koopernic]

Boeing used servo-tabs on the flight controls for the B-17, the 707, and most versions of the B-52. I am mystified by that last, as I think the earlier B-47 used hydraulics.

I think I can help there, the B-52 used small 'feeler' ailerons with servo tabs on the outboard section of the wing. Hydraulic boosted controls on main aileron and spoiler mirrored it. It's written in a book by Abzug on the history of flight control and stability. I think they may have changed the arrangement for the H.

The fear was something called PCO or Pilot Coupled Oscillation caused by flutter or shock-waves knocking the pilots control column around. The pilot would try and correct but because of the complexity of the phenomena only amplify the phenomena. What was wanted was 100% power controls that were 'irreversible'. The hydraulic system used in P-38J boosted ailerons only assisted the pilot rather than doing all of the work. This meant forces could get back to the pilot. F-86 controls latter went from boosted to irreversible.

What could go wrong is shown by what happened with the DeHaviland DH108 Swallow in which Geoffrey DeHaviland Jnr died. The aircraft initially had a blunt nose taken over from the vampire. Near the speed of sound it 'shock stalled' and smashed the pilot up against the canopy while also forcing the vibration from all this shaking about from pilots hands back into the controls, which worsened it. Eric Brown thinks DeHaviland, who was tall, broke his neck while he being shorter managed to survive.

Naturally this meant a loss of feel, so artificial feel was created to tension the controls on the basis of dynamic pressure (speed squared x pressure) from say the pitot tube. Stick shakers were added to vibrate controls on the basis of angle of attack instruments.

At some point Boeing learned how to make manual controls that were relatively free from these phenomena and so we saw a return to manual controls if only as a backup or on some of the flight surfaces. One could fly a 727 or 707 using pure manual controls via the servo tabs if there was a hydraulic failure however in the case of the B-747 one relied on the windmilling effect of the 4 engines to provide enough pressure. Many aircraft have a RAT ram air turbine, including the latter F-86.

 

[swampyankee]

I think I can help there, the B-52 used small 'feeler' ailerons with servo tabs on the outboard section of the wing. Hydraulic boosted controls on main aileron and spoiler mirrored it. It's written in a book by Abzug on the history of flight control and stability. I think they may have changed the arrangement for the H.

The fear was something called PCO or Pilot Coupled Oscillation caused by flutter or shock-waves knocking the pilots control column around. The pilot would try and correct but because of the complexity of the phenomena only amplify the phenomena. What was wanted was 100% power controls that were 'irreversible'. The hydraulic system used in P-38J boosted ailerons only assisted the pilot rather than doing all of the work. This meant forces could get back to the pilot. F-86 controls latter went from boosted to irreversible.

What could go wrong is shown by what happened with the DeHaviland DH108 Swallow in which Geoffrey DeHaviland Jnr died. The aircraft initially had a blunt nose taken over from the vampire. Near the speed of sound it 'shock stalled' and smashed the pilot up against the canopy while also forcing the vibration from all this shaking about from pilots hands back into the controls, which worsened it. Eric Brown thinks DeHaviland, who was tall, broke his neck while he being shorter managed to survive.

Naturally this meant a loss of feel, so artificial feel was created to tension the controls on the basis of dynamic pressure (speed squared x pressure) from say the pitot tube. Stick shakers were added to vibrate controls on the basis of angle of attack instruments.

At some point Boeing learned how to make manual controls that were relatively free from these phenomena and so we saw a return to manual controls if only as a backup or on some of the flight surfaces. One could fly a 727 or 707 using pure manual controls via the servo tabs if there was a hydraulic failure however in the case of the B-747 one relied on the windmilling effect of the 4 engines to provide enough pressure. Many aircraft have a RAT ram air turbine, including the latter F-86.

Thanks. Somewhere in the deep, dark past I saw some articles about flying these aircraft, and I remember it said the 707 and B-52 did not use hydraulic flight controls. Nice to get the right information..

 

[FLYBOYJ]

Many aircraft have a RAT ram air turbine, including the latter F-86.

I believe the F4J Fury had a RAT, I'm not sure that latter model F-86s did.