According to Myers Jacobsen, "The first Pratt & Whitney R-4360-5P Wasp Major test engine was to have been handmade and delivered to Fort Worth in May 1943. Design improvements delayed it until October, but by December, the full-scale wind tunnel tests were begun in Wright Field's 20-foot wind tunnel... As it had been delivered from Pratt and Whitney, the R-4360-5P engine was equipped with a type 11-11A cooling fan, designed to be in operation continuously while the engine was running. The engine cooling tests were run through 1944 and into 1945, during which time seven different fans were tested. The 11-11A type was replaced with a 34-blade fan, and the air diffuser tunnel was belled out to increase its cross sectional area within the nacelle. This slowed down the air and greatly improved the cooling."
Nevertheless, when flight tests began, "two of the main problems to manifest themselves were lack of proper engine cooling and propeller vibration stress, although both of these situations had been extensively investigated in the wind tunnel test programs. Engine cooling became a problem that would result in the inability of the XB-36 to maintain high altitude operations for extended periods of time above 30,000 feet. A two speed cooling fan was later developed to keep engine temperatures within safe operating limits."
Air entering the upper intake on the wing leading edge went to the intercoolers and the engine cooling fan. The lower intake supplied the oil coolers. The turbos also inhaled through this intake. Each engine had two turbos and two intercoolers. Exit shutters for the latter were on top of the wing, in appearance similar to Venetian blinds either side of each nacelle.
Instead of cowl flaps, the B-36 had "air plugs" which were adjusted fore and aft by electric motors to throttle cooling air at the exit. A pattern of four diamonds on each air plug indicated its position to the gunners in the aft compartment, who could report this to the engineers. Observation of conditions on the wing trailing edge by the two lower rear gunners was so vital, there was an emergency interphone power circuit for them, the engineers, and the pilots. It allowed those six men to communicate in the event of total AC failure.
In addition to low and high fan speeds, late B-36s had a neutral setting to disengage fan drive. The flight manual says, "In the event that CHT cannot be maintained high enough for proper engine operation and if carburetor heat, almost closed air plugs, single turbo, etc. operation does not bring CHT up sufficiently; then, as a last resort neutral fan operation may be used." In neutral fan there were more restrictive limits on CHT and rpm since the hottest cylinder could much exceed one of the four you could select on the CHT gauge.
There were limits on high fan operation too. Below 15,000 feet it was possible to overstress the fan if rpm limits were not observed. Normally high fan went with high altitude. The manual says, "The rate at which the heat is removed from the head is dependent upon the weight of the cooling air flow and the temperature difference between the head material and the cooling air... These two factors work in opposing directions as the altitude of operation is increased. The decreasing temperature is favorable as it widens the temperature difference of the heat exchange. Unfortunately the decreasing air density, which is unfavorable, reduces the rate of cooling air flow at a rate that overcomes the beneficial temperature factor. If constant power is maintained at high altitude, the cylinder cooling problem becomes increasingly difficult."
On the other hand, during ground operation the Wasp Majors could become too cool: "During idling, an attempt should be made to maintain cylinder head temperatures between 170° to 220°C by use of the air plugs. This will minimize the possibility of spark plug fouling. Normally, high idle speed will not be used to keep cylinder head temperature high. However, if these temperatures cannot be maintained with the air plugs, it may be necessary to use higher idle speeds."
Low fan was the normal setting for all ground operation, except for a quick check of high fan during run-up.
References:
Meyers K. Jacobsen, "Convair B-36," 1997.
AN 01-5EUG-1, "Flight Handbook USAF Series B-36H Aircraft," 1953.
Convair B-36 H Aircraft Flight Manual 01-5EUG-1
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