I thought I'd post the following excerpt from Graham Whites book on the R-2800, so that the younger generation raised on the revolting stench of kerosene, might understand the breed of aviators who took to the air with the aphrodisiac smell of high octane in their nostrils, and did battle with the elements, while tending with skill and care to these mechanical marvels of propulsion.
Operating, Service Difficulties
Sitting behind the power of one, two, or four R-2800s can be an intimidating and yet exhilarating experience. The pilot/flight engineer needs the dexterity and skill of an organ player as throttles, mixture controls, propeller controls, cowl flaps, magneto switches, boost pumps, and oil cooler flaps are manipulated in a mechanical ballet of flying arms and fingers. At the same time, the condition of the engine(s) is/are monitored via the oil pressure, cylinder head temperature, oil temperature, fuel pressure, tachometer, and BMEP meter. All R-2800s being operated now use direct crank electric starters. Engaging the starter emits that sweet sounding high pitched whine that could only come from straight-cut, precision reduction gearing. Bringing an R-2800 to life is like awakening a giant dinosaur: somewhat reluctant, but once in motion you have a tiger by the tall. Normal procedure is to motor the engine over on the starter for about four revolutions of the propeller, then energize the magnetos. During the initial cranking, a judicious amount of prime in the form of raw gasoline is injected into the intake system. This is where a skilled pilot/flight engineer can show his worth by judging what amount of prime is required based on ambient temperature and how long it has been since the engine was last shut down. As soon as the mag switches are energized, providing the correct amount of prime has been administered and the engine is in reasonably good mechanical condition, the cowl will shudder as the first couple of cylinders realize that a highly volatile and combustible mixture is trapped inside and a source of ignition has just been provided. Blue smoke from entrapped oil burning with the fuel/air mixture starts to emit from the exhaust stacks. As cranking continues, more cylinders come on line announced by further clouds of dense blue smoke. When all eighteen cylinders are hitting-some better than others, the blue smoke starts to diminish. Even so, it takes several minutes for the lower cylinders, the ones that are more vulnerable to oil build-up, to clear. This is evidenced by characteristic puffs of blue smoke from individual cylinders blowing out of the exhaust stacks, almost like smoke signals. Warm-up is then commenced at 800 to 1000 rpm. Anything less will not ensure adequate oil flow to the bearings and anything greater imposes too much thermal stress and loading on the engine.
Following is an operating procedure issued by Pratt & Whitney during the 1950s for CB engines. This procedure has been paraphrased and abbreviated by the author. Even so, it gives an idea of what it takes to operate an R-2800.
The following engine operating recommendations are intended to promote the most successful aw efficient use of the Pratt & Whitney R-2800.
1. Ground Operation
A. Ground Considerations- Since the R-2800's engine cooling provisions are not primarily designed for ground operation, additional consideration is required, not only to the engine but also to the accessories. To assist in maintaining adequate heat dissipation throughout the engine during warm-up and to provide maximum cooling thereafter, cowl flaps are left wide open during all ground operation.
The fuel booster pump is switched on prior to starting the engine. Especially on installations equipped with one-position boost pumps, the initial surge of boost fuel pressure, sometimes referred to as “hammer effect," due to "hydraulic-locking," is often sufficient to unseat the carburetor's mixture control plate and introduce a variable quantity of fuel into the intake system. This variable quantity of fuel may be sufficiently large to confuse priming requirements. By actuating the boost pump prior to starting, however, any excess fuel is dissipated during cranking, thus maintaining priming requirements at a relatively constant level.
B. Starting-Like any piston engine, a rich mixture is required for starting. The standard priming system is designed to provide sufficient fuel to start a cold engine during cold weather operation. In very cold conditions it may be necessary to actuate the primer continuously to get the R-2800 fired up. Conversely, in more temperate or tropical conditions, it is found that the standard system has a tendency to flood unless used intermittently with considerable caution. Therefore, like many aspects of the R-2800, sense of "feel" is required. The standard primer fuel flow is 250 to 300 pounds per hour. However, for aircraft operated in warmer climes this flow rate is reduced to 100 to 15 0 pounds per hour.
Prior to starting, the throttle should be positioned to yield 800 to 1000 rpm after the engine starts. Mixture is set to idle/cutoff and the propeller is set to fine pitch or max. rpm. At this point the engine is motored with the starter and magnetos off to purge the cylinders of residual fuel. This also insures initial lubrication of the reduction gearing is adequate. Should any hesitation or stoppage of propeller motion be noted, cranking should be discontinued immediately and the cause removed. This would indicate a classic case of a "hydraulic-locked" cylinder due to excessive fuel and/or oil. The primer switch should be actuated as required at, or just prior to, fifteen blades (six blades if engine was shut down within one hour). It is more desirable to underprime than overprime and risk the effects of a blower fire. A blower fire is due to a lean mixture backfire that causes fuel inside the supercharger to ignite. They are typically fairly harmless and will quickly bum themselves out as all the fuel is consumed. At fifteen blades (or six blades with a warm engine) the magneto switch is positioned to "Both" and cranking continued until the engine fires. The initial rpm surge should not be permitted to exceed 1100 rpm. After the engine fires, hold continuous prime, adjust the throttle to 800 rpm and note a rise in oil pressure. At this point, as the engine burns off the prime, the mixture control is slowly moved to the auto-rich position and priming discontinued when approximately 100 rpm decrease is noted.
The throttle is then adjusted to 1000 rpm and warm-up is considered complete when the oil temperature reaches 40°C.
C. Idling -Correct idle mixture is extremely important in obtaining a combination of proper engine operation with the least tendency toward spark plug fouling. In the idle range, fuel flow varies with throttle opening. Idle mixture must be set to accommodate desired ground idle as well as windmilling (touchdown), acceleration (wave off), and reversing operational characteristics. To best satisfy these requirements and minimize spark plug fouling, idle mixture should be adjusted at the speed most commonly used for ground operation (800 to 1000 rpm).
Idle mixture is not automatically compensated for changes in altitude. Therefore, when operating from a sea level base to a field above 4000 feet, it may become necessary to lean with the mixture control at the higher elevation, i.e., the mixture lever is positioned between auto-lean and idle cut-off. Of course, a pilot/flight engineer would need to exercise caution, however, to preclude taking off with a leaned mixture that would result in a severely damaged engine due to detonation.
When extended ground operation is necessary, it is helpful in preventing spark plug fouling to open the throttle to field barometric pressure for one minute during each ten-minute period of idle. Ground maintenance and flight personnel should be cautioned against excessive ground power and/or excessively long periods of power application during ground burnout procedures. If the procedure used is going to take effect, it should do so in ten or fifteen seconds. Also, excessively high power or long periods of power application on the ground when engine cooling is least effective will probably result in oil leaks from "cooked out" seals, possible ignition harness or low tension coil failures, or engine rubber mount core distress from excessive heat.
Again, as a concession to the marginal cooling available during ground operations, using reverse thrust for reverse taxiing is to be avoided due to its adverse effect on engine cooling. The cooling system and cylinder head temperature (CHT) locations were not designed to accommodate reverse flow.
D. Engine Run Up-Scuffed pistons have resulted by pulling power too quickly after starting cold engines. This is due to the aluminum pistons expanding at a faster rate than the steel barrel. Pilots have to exercise care not to exceed 1000 rpm until the oil temperature has reached 40'C. When the oil has warmed to this temperature, the propeller check may be started. The propeller check is performed at 1600 rpm.
While still at 1600 rpm and if desired, the spark advance check may be accomplished by switching to the "Cruise" (25 degrees of advance) position and then back to the "Takeoff and Climb” (20 degrees of advance) position, noting a BMEP fluctuation each time the switch position is changed. Usually, a satisfactory shift prior to the last descent will suffice, however, a visual check should be made to assure that the switch is in the "Takeoff and Climb" position. A takeoff made with spark advance in the "Cruise" position may result in a complete engine failure. This foregoing procedure is now all but redundant for the R-2800 because most, if not every, R-2800 operating in the world has had this feature disabled.
Engine power output is checked at a manifold pressure equal to field barometric pressure, as indicated by the manifold pressure gage prior to starting. Revolutions per minute at this mani-fold pressure should be approximately 2050-2200 rpm, depending on the propeller low pitch stop setting, the accessory load on the engine, and relative wind, regardless of field elevation. BMEP and fuel flow will vary with local atmospheric conditions. Should the rpm not fall within the prescribed rpm limits for the particular installation, a check for improper low pitch propeller settings and/or engine malfunction may be the culprit.
The blower shift check, if desired, may be performed by switching from low to high blower at field barometric and noting a rise (approximately of 2 in. Hg) in manifold pressure, a decrease in BMEP, and a momentary drop in oil pressure. The blower is then shifted back to the low position which is indicated by a drop in manifold pressure to the original value, an increase in BMEP, and a momentary drop in oil pressure. Should erratic indications of a shift be observed the cycle may be repeated after first idling the engine at 1000 rpm to allow the clutches to cool for two minutes. For en route run-up, a switch position check is usually regarded as sufficient.
The magneto check is also performed at field barometric pressure. The normal drop on one magneto is 50-75 rpm (5-7 BMEP) and should not exceed 100 rpm (10 BMEP). The maximum spread between the drops of right and left magnetos should not exceed 40 rpm (4 BMEP).
The ADI system is checked as follows, pump switch may be actuated at field barometric pres-sure and a partial system check made by noting the water pressure light, ADI pressure of approximately 26 or 30 psi (depending on installation), and a fuel derichment of approximately 100- 150 pph (pounds per hour) fuel flow. The ADI flow at lower powers is negligible and may be left on without appreciably depleting the ADI supply. In any event, it is desirable to switch on the AD1 at least several seconds prior to the application of takeoff power to purge the system of entrapped air. When ADI is needed, it is really needed!
II Flight Operation
A. Takeoff-Again, cooling is a prime consideration. A conscientious effort should be made to secure the lowest practicable cylinder head temperature immediately prior to the application of takeoff power for the following reasons: (1) the power available at 2800 rpm increases with decreasing CHT at an approximate rate of 30 bhp per 20 degrees Centigrade; (2) common types of spark plug deposits become more conductive with increasing temperature and their ground-ing tendencies can be reduced or eliminated through the control of peak CHTs; (3) since CHT will rise 40 to 60 degrees Centigrade during a normal takeoff, less cowl flap drag will be incurred in establishing the desired 190 to 200 degrees Centigrade CHT for climb; and (4) the tendency toward detonation and/or pre-ignition decreases with a cooler CHT. If a malfunction were to occur which could lead to possible detonation, operating at a cooler CHT would delay or prevent this detonation.
Minimum pre-takeoff CHTs may be attained: (1) through efficient use of the briefest possible engine run-up time necessary to ascertain normal functioning; (2) through use of minimum post run-up rpm commensurate with generator and ventilating system requirements; (3) by facing the aircraft directly into the wind when practicable; and (4) by delaying cowl flap closure to the takeoff setting until immediately prior to the application of takeoff power. Desired CHT before takeoff is on the order of 15 OOC. There is no minimum pre-takeoff CHT.
Although it is desirable that all types of aircraft are faced into the wind during run-up, this is particularly important for the Convair twins. In this case, a downwind component will not only reduce the cooling air flow over the engine, but it will also impede the efficient operation of the augmentor tubes, resulting in a further rise in CHT.
With a fixed throttle setting from the start of takeoff to the point at which the first power reduc-tion is affected, increasing airscoop ram pressure, resulting from the increase in airspeed, will cause a manifold pressure rise of from one to three depending on flight techniques. If lift-off is abrupt, a momentary decrease of one or two inches of manifold pressure will result due to the change in airscoop attitude. This situation is somewhat analogous to compressor stall with an axial flow gas turbine. These characteristics should be anticipated and the throttle monitored in an effort to maintain the desired manifold pressure.
During a wet takeoff, the ADI pressure, fuel flow, and BMEP are monitored to assure proper ADI operation. It is not uncommon to see the maximum "no flow" pressure limit (32 psi on Convair twins, 29 psi on DC-6s) when the ADI pump is first turned on. Although this condition is referred to as "no flow" it really means "no flow" to the engine. Actually a total flow of about 150 pph per engine is being pumped through the lines and returned to the ADI tank through the vapor vent line. As the throttles are opened beyond 36-38 in. Hg, a pressure drop (3 to 7 psi on Convairs, 1 to 4 psi on the DC-6), depending upon the particular engine installation, will occur, indicating that ADI is flowing to the engine. The desired AD1 pressure with flow is 22 to 25 psi. If at any time during a wet takeoff the ADI pressure drops below approximately 18 psi and the carburetor enriches to more than 1800 pph fuel flow, the rich mixture, resulting from both fuel and ADI flow, will probably cause a severe power loss which will be indicated on the BMEP gage. A slight throttle closure may reduce ADI flow requirements sufficiently to restore derichment pressure and result in a BMEP higher than that for dry operation.
In the event the foregoing procedure proved ineffective and conditions permitted a takeoff with dry power on the affected engine or engines, dry takeoff power is set and the ADI switch turned off.
Maximum flight safety dictates the use of full authorized power for every takeoff and retaining this power and takeoff flap setting until the gear is retracted and it is apparent that all obstacles will be cleared by a comfortable margin. The use of takeoff power is authorized for a period of two minutes and, subject to limiting cylinder head and oil temperatures, this authorization may be utilized when required by abnormal conditions. Normally however, takeoff power is used for much shorter periods that seldom exceed one minute even at maximum gross weight on a hot day. Of course, when this procedure was written, Corrosion Corner did not exist!
In the interest of preserving the supply of ADI fluid, ADI switches may be turned off at any time during the first power reduction when the manifold pressure is below the dry takeoff rating. An increase in fuel flow should be noted as the ADI is switched off. Approximately one to one and a half gallons of ADI are consumed per engine during a takeoff.
B. Climb-Climb power should be set with rpm and manifold pressure as selected from the appropriate climb chart for the existing carburetor air temperature and pressure attitude. Throttles should be adjusted to yield the same manifold pressure for all engines. By setting equal air flow or engine output in this manner, the resulting fuel flow may be more easily monitored. BMEP differences between engines with equal manifold pressure, rpm, carburetor air temperature, and fuel flow are due entirely to unequal accessory loads, engine condition, and/or instrument inaccuracy. Here it should be noted that on some installations cabin superchargers (pressurization systems) will absorb as much as 6-8 BMEP.
It is important that fuel flow be monitored throughout the climb to ascertain that it is within prescribed limits. The minimum fuel flow limit is not an engine limit at normal climb power. It is, however, a carburetor limit designed to obviate damage which might otherwise result at higher power, where the margin between a safe fuel flow and engine detonation is diminishing. At climb power, therefore, it is considered safe to continue operation when the fuel flow is at, or 50 pph below, the minimum fuel flow, providing CHT and CAT limits are observed. If the climb fuel flow falls more than 50 pph below published minimum, power should be reduced by increments of 100 bhp until the fuel flow is not more than 50 pph below the limit for that particular reduced power. CHT and CAT limits must still be monitored. Because fuel/air mixture strength versus air flow, schedules vary to accommodate individual aircraft airscoop and exhaust system characteristics, note that minimum climb fuel flows differ among aircraft and carburetor settings.
Power settings for each blower configuration are made with manifold pressure and rpm. Fuel flow is frequently checked to assure it is within limits. The carburetor knows what power the engine is pulling only by measuring the air flow that the engine pumps to produce this power. The carburetor then meters fuel according to the mass air flow it senses. In order to properly monitor fuel flow and evaluate engine performance, the engine should be kept on the pre-arranged schedule of air flow that can only be set by the proper rpm and manifold pressure for the existing CAT and pressure altitude. The fuel flow-limits given, therefore, are only valid at the prescribed rpm, MP, CAT, and pressure altitude schedule.
Blower Shifting-A blower shift may be accomplished as indicated on the climb chart even though low blower cruise is anticipated. This practice will eliminate one desludging shift during the subsequent cruise period. To shift from low to high blower, merely retard the throttle by three or four inches of manifold pressure, actuate the supercharger switch, and readjust the manifold pressure to that called for by the high blower climb chart.
Not surprisingly, during climb operation in hot weather the cooling air flow, required to main-tain desired CHT (190 to 200°C) is necessarily greater than in winter. Two choices are available to the pilot/flight engineer: (1) through higher airspeed, or (2) higher cowl flap opening. In general the former is preferable from the standpoint of total trip fuel and block speed. Larger cowl flap gap and maximum L/D (lift/drag) airspeed can only be Justified when a higher angle (as opposed to the high rate of climb) is required.
C. Cruise Operation-Upon reaching cruise altitude, climb power is maintained until the indicated airspeed slightly exceeds that anticipated for the particular altitude, gross weight, and cruise power to be used. This higher airspeed will afford a cushion so that the airspeed dissipation incurred during trim and power adjustments (blower shift, etc.) will not result in an airspeed at the start of cruise less than that anticipated for cruise.
From the cruise chart for the selected bhp find the appropriate manifold pressure for the existing pressure altitude and carburetor air temperature. This will be listed directly opposite the pres-sure altitude and under the carburetor air temperature. From this point move toward the right and downward staying within the parallel lines to find rpm, blower ratio, BMEP drop, fuel flow, and nominal BMEP. Cruise power will then be set in this sequence:
1. Set cruise rpm.
2. Shift blower, if required.
3. Set cowl flaps to the angle anticipated to yield 190 to 200'C cylinder head temperature when stabilized.
4. Adjust throttle to selected manifold pressure, allowing for any known gage error.
5. While carefully observing BMEP, lean mixtures individually until BMEP has been reduced from its maximum value (best power) by the amount of the prescribed BMEP drop. Since the transport carburetor has been specifically designed to facilitate manual leaning, a dis-tinct rise in BMEP should be seen during the initial leaning process. If this initial BMEP rise is not observed but instead an immediate decrease in BMEP is noted, the carburetor, even though in the auto-rich position, is at or slightly on the lean side of best power in the cruise range. In this case intermittent prime may be used to determine best power. Since the BMEP drop setting is based on a constant manifold pressure, it is essential that airspeed and altitude be held constant during this step. A change in airspeed at constant throttle affects ram and therefore manifold pressure and BMEP to the extent that an airspeed change of ten knots can result in as much as a five BMEP change. This gives an indication of the powerful power enhancing aspect of ram air induction.
6. Readjust cowl flaps to provide the desired CHT, 190 to 200'C. When stabilized, cross check engine instruments. With equal manifold pressure, rpm, carburetor air, and cylinder head temperatures, equal engine air flow is normally obtained. With identical BMEP drop settings, fuel/air ratio and therefore fuel flows will also be equal, regardless of the condition of the
ignition system. Any difference in fuel flow under these conditions must be due to instrument inaccuracy, either flowmeter or manifold pressure, or to a mechanical malfunction, such as a stuck valve or broken pushrod, which affects mixture flow. In summary, BMEP differences will be due entirely to unequal accessory loads, instrument inaccuracy, and/or mechanical discrepancies.
7. Once cruise power has been set and stabilized, the maximum difference in indicated BMEP, after allowing for that due to unequal accessory loading, should not exceed 10 BMEP. With discrepancies greater than 10 BMEP, the pilot/flight engineer would make a note of it to assist in troubleshooting.
Mixtures adjusted in this manner should remain substantially the same regardless of small throttle adjustments necessary to counteract small changes in airspeed, altitude, and/or CAT. Mixtures, however, should be periodically checked during cruise and adjusted as required particularly after appreciable changes in CAT, power, or altitude. Mixture strength or BMEP drop can be quickly checked by applying prime in varying amounts to determine best power or peak BMEP.
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