Each engine on the B-17 has a turbo-supercharger which boosts manifold pressure for takeoff and provides sea-level air pressure at high altitudes.
To operate the turbo-superchargers, engine exhaust gas passes through the collector ring and tailstack to the nozzle box, expands to atmosphere through the turbine nozzle, and drives the bucket wheel at high speed.
A ramming air inlet duct from the leading edge of the wing supplies air to the impeller, which increases pressure and temperature. However, in order to avoid detonation at the carburetor, the air supplied to the carburetor passes through the intercooler, where the temperature is reduced. The internal engine impeller, driven by the engine crankshaft, again increases air pressure at it enters the intake manifold. Higher intake manifold pressure results in greater power output.
Supercharger Regulator Operation
The amount of turbo boost is determined by the speed of the turbo bucket wheel. Speed of the bucket wheel is determined by the pressure-temperature difference between the atmosphere and the exhaust in the tailstack, and by the amount of gas passing through the turbine nozzles. If the waste gate is open, more exhaust gas passes to the atmosphere via the waste pipe, decreasing the tailstack pressure.
The boost lever at the pilot's control stand sets the turbo regulator which automatically operates the waste gate to hold constant pressure in the tailstack. High boost lever setting provides higher exhaust manifold pressure by closing the waste gate. The resulting higher bucket-impeller speed gives higher intake manifold pressure.
The regulator (operated by engine oil) auta-matically opens and closes the waste gate to maintain a constant exhaust stack pressure equal to the boost lever setting.
Electronic Turbo-supercharger Control
The electronic turbo-supercharger control system on late model B-17's consists of 4 separate regulator systems, one for each engine, all simultaneously adjusted by a manifold pressure (turbo boost) selector dial on the pilot's control panel. Induction pressures are controlled through a Pressuretrol unit connected directly to the carburetor air intake.
Electrical power for the entire system is derived from the 115-volt, 400-cycle inverter.
Each regulator includes a turbo governor which prevents turbo overspeeding both at high altitude and during rapid throttle changes.
The exhaust waste gate is operated by a small reversible electric motor which automatically receives power from the regulator system when a change in waste gate setting becomes necessary to maintain desired manifold pressure.
In case of complete failure of the airplane electrical system, or failure of the inverter, the waste gates on all engines will remain in the same position as when failure occurred, and the same manifold pressure will be available as was in use at the time of failure.
Operation: Before Starting
Set the turbo boost selector dial at zero to insure that the waste gate is open.
Taxi with the turbo boost selector dial at zero.
1. Set throttles at 1500 rpm on all engines.
2. Exercise propellers.
3. Check generators.
4. Electronic turbo control needs no exercising; therefore turbo boost selector remains on zero.
5. Return throttles to 1000 rpm on all 4 engines.
1. Run up engine No. 1 to 28" manifold pressure, and check the magneto.
2. Open No. 1 engine to full throttle, with the turbo boost selector on zero, to insure that waste gate is open.
3. Reduce No. 1 engine to 1000 rpm. Follow the above procedure on engines No.
2, No. 3, and No. 4.
4. Having checked the magnetos, open engine No. 1 to full throttle, and turn the turbo boost selector dial clockwise until the desired takeoff manifold pressure is reached. (If the electronic control has been properly adjusted, you will obtain this manifold pressure at reference point "7" on the turbo boost selector dial.) Return throttle to 1000 rpm.
Leaving the turbo boost selector dial as set, run up engines No. 2, No. 3, and No. 4, successively to check manifold pressure. The manifold pressure of each engine should equal that set on engine No. 1. If small adjustments are needed, they can be made with the small individual potentiometers by removing the black cap and turning the screw found underneath (1) clockwise to increase manifold pressure, or
(2) counter-clockwise to decrease manifold pressure.
1. Set turbo boost selector dial to reference point found correct in engine run-up.
2. After takeoff, re-set the turbo boost selector dial when reducing power to obtain desired manifold pressure, or to zero setting if boost is not needed. Adjust throttles to obtain desired manifold pressure for climbing.
3. Reduce rpm for climbing.
4. During the climb, continue to adjust manifold pressure with throttles until they are in the full open position. Then obtain desired manifold pressure by using the turbo boost selector dial.
During the before-landing check, set rpm and turbo boost selector dial on downwind leg, as outlined in checklist.
When flying at high altitude you may reach a point where further turning of the selector dial fails to produce an increase in manifold pressure. This means that the overspeed portion of the turbo governor is limiting the turbo speed to safe rpm. When you encounter this condition, turn the manifold pressure selector dial counter-clockwise until it controls manifold pressure again. This prevents undue wear of the overspeed governor mechanism.
Full emergency power (war power) can be obtained at maximum engine rpm and full throttles by releasing the dial stop and turning the turbo boost selector dial up to its limit. However, this setting places heavy strain on the engines. Use it only in emergencies and then only for periods not exceeding 2 minutes.
USE OF TURBO-SUPERCHARGER
To save wear and tear on the turbo-supercharger and to avoid excessive carburetor air temperature, maintain desired manifold pressure by advancing the throttles before using the turbos. At higher altitudes, definite turbo over-speeding may result from the use of part throttle and full turbo-supercharger operation.
You will note that during the climb manifold pressure tends to increase, making it necessary to keep retarding the turbo controls to hold constant intake manifold pressure in the climb. The regulator on the B-17 does not provide a constant intake manifold pressure during the climb but it does provide constant exhaust stack pressure. Any position of the supercharger control lever corresponds to a certain exhaust manifold pressure, and the supercharger regulator unit maintains that exhaust pressure by varying the waste gate opening. Therefore, as long as the control lever is not moved, the exhaust pressure is maintained at constant value, corresponding to the position of the control lever irrespective of altitude and reduced outside temperature and pressure.
Before starting the climb, the manifold pressure is set to a certain value with the control lever. Atmospheric pressure decreases rapidly during the climb. The difference between exhaust pressure and the atmospheric pressure thus increases with altitude and results in a greater pressure differential across the turbo. This increased turbo power is transmitted to the impeller, which utilizes it to further increase the differential between atmospheric pressure and carburetor pressure. The engine internal impeller then raises the carburetor air pressure to engine manifold pressure.
By this time both the carburetor and manifold pressure have exceeded the required values, primarily because of the tremendous output Hp of the turbo at higher rpm, which more than offsets the additional power requirements of the impeller. For example, the turbo Hp developed at 30,000 feet is almost five times that of 10,000 feet altitude with constant exhaust pressure. To correct for this excess turbo Hp, the boost control lever must be pulled back during the climb in small amounts, thus reducing the exhaust pressure.
The increase in pressure differential between exhaust pressure and outside atmospheric pressure across the turbo becomes so great at high altitude that to avoid overspeed you must decrease manifold pressure IV2" for each 1000 feet above the critical altitude.
Since you have manually decreased the manifold pressure, power is reduced proportionately, but the exhaust pressure and manifold pressure are not constant for all altitudes and you must continually readjust the supercharger controls while changing altitude. At sea level, the turbo turns at only about 10,000 rpm, while at 30,000 feet it turns at 21,300 rpm, which is the recommended maximum speed for continuous operation. Thus you must reduce the manifold pressure by the amount required to keep the turbo rpm constant with increased altitude above 30,000 feet.
As noted, 21,300 rpm has been determined to be the maximum operating turbo speed on the type B-2 turbo, with 5% overspeed allowance in emergencies. This would provide an emergency rating of 22,400 rpm.
The pressure in the exhaust stack, and therefore the pressure just upstream from the nozzle box, depends mainly upon the amount of the exhaust gas supplied by the engine. If the engine continues to develop leading power, the exhaust stack interior continues to maintain merely the same pressure, but the pressure on the outside of the turbo wheel is atmospheric pressure which continues to decrease with increased altitude until at 30,000 feet the pressure is only about 8.9" Hg. instead of 29.92", or less than one-third that of sea level.
The velocity of the exhaust gas past the turbo buckets, and consequently the speed of the turbo wheel, depends directly on the pressure differential between the inside exhaust stack and the atmospheric pressure, or the pressure differential across the nozzle. The decrease in manifold pressure therefore must reduce the exhaust gas pressure the same amount as the lapse rate in atmospheric pressure in order to keep the nozzle pressure differential approximately constant.
Note:—For constant turbo speed at 21,300 rpm, refer to Т. О. AN 01-20EF-1 for variations in manifold pressure with altitude.
You may find in certain conditions when using high turbo boost that there is a surge in manifold pressure. Turbo surge is caused by the action of the turbo-supercharger as a pressure pump. At a certain turbo rpm, the turbo will pump air into the induction system and continue to build up the induction system pressure to a certain point. Then the pressure will unload and effectively go back through the turbo against the centrifugal action of the blower. This reduces the pressure differential across the turbo and the turbo speeds up, tending to increase the induction system pressure again with its consequent reduction of turbo rpm and repetition of the cycle. This evidences itself in a surge in manifold pressure, resulting in inefficient operation and danger of temporary turbo overspeed.
The correction for this surge is to increase rpm until it discontinues. If turbo surge does not correct itself with an increase in engine rpm, the cause is probably a clogged or restricted governor balance line or a faulty governor.
Closed Turbo Waste Gate
If engine rpm is continually reduced with a wide-open throttle, manifold pressure falls off because of the closed turbo waste gate. At 25,000 feet, this begins at about 1650 rpm, while at higher altitudes it begins at higher rpm. This decrease in manifold pressure at full boost with a reduction in engine rpm takes place because as the engine rpm decreases the supply of exhaust gas from the engine is reduced. At a certain point the supply is not sufficient to drive the turbo fast enough to keep up the manifold pressure. As the rpm decreases from this point, the manifold pressure decreases also, since the turbo waste gate is closed and virtually all of the exhaust gas is going through the, turbo. Reducing the exhaust gas supply reduces turbo rpm, which in turn reduces manifold pressure.
You may find that in a condition such as formation flying at high altitude, using full throttle and high boost, power often will not increase again after throttles have been retarded considerably to avoid over-running the formation. This may be especially true if the rpm has been reduced by the throttle retardation. Restoration of full throttle and increase in boost will not bring up the manifold pressure.
This often results from the fact that the power may have been reduced to the region of closed waste gate, and insufficient exhaust gas is available to turn the turbo fast enough to bring up the manifold pressure. The correct procedure under this condition is to increase the rpm to 2500, if necessary, and keep the boost setting high until manifold pressure comes up. It usually comes up immediately.
HEATING AND VENTILATING SYSTEM
The B-17F airplane has a main and an auxiliary heating system, both of which operate on the same principle of heat exchange.
The main system supplies cabin heat through a glycol system in nacelle No. 2.
The heating system fluid (glycol solution of 55% diethylene glycol and 45% ethylene glycol by weight) is stored in a tank in the top of nacelle No. 2. The glycol flows from the tank to the engine-driven pump, which circulates the fluid at a rate of 55 to 60 U.S. gallons per hour. The flow is directed to a filter which removes impurities from the fluid. The glycol is then pumped through 3 heaters, which are installed in series and located in the exhaust stack, where it collects the heat of the exhaust gases.
A relief valve, between pump and filter, bypasses the glycol back to the supply line if high pressure is built up in the system during cold weather, or if the heaters are clogged.
The circulation of the glycol is continuous and therefore it must be constantly cooled. For this purpose there is a radiator between the spars in the left-hand wing gap. Ram air from the intercooler air inlet absorbs heat from the glycol at the radiator, and passes through the radiator and into the cabin. The cooled glycol passes into the supply tank. A controllable damper in the radiator may be operated to spill the air overboard if desired.
Auxiliary System (Some B-17F's)
The auxiliary heating system uses the same principle of heat exchange as that employed by the normal heating system and has a heater unit, filter, relief valve, pump and supply tank installation in nacelle No. 3 identical to the corresponding installation of the main heating system in nacelle No. 2. Eight radiator-fan assemblies are connected by glycol tubing to the heater units in nacelle No. 3. Five of these are the non-recirculating type (external radiator air supply) and the remaining are the recircu-lating type (internal radiator air supply). The non-recirculating type radiator-fan assemblies are in the astrodome, top turret, ball turret, and the tail gun enclosure. Each of these assemblies has a hand-operated damper which directs the flow of heated air to the gun and/or windows, or spills it overboard. The recirculating radiator-fan assemblies have overboard discharge ducts and damper tube controls for regulating the amount of heated air admitted to the pilot's, navigator's, and radio operator's compartment. Electric fan control is automatic.
Two thermo-switches, mounted on the glycol tubing under the flow of the pilots' compartment, turn 5 non-recirculating radiator fans on at 177°C (350°F) and the 3 recirculating radiator fans on at 77°C (150°F). The thermo-switches are capable of functioning when the master switch is "ON."
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