Translated from: Raunio Jukka, 1998

JK = Junkers 88 A-4


Starting, Taxiing and Take-off

JK was started with batteries or using hand-crank. Manual recommended "Kaltstart", if outside temperature was below -5 degrees. At temperatures below -10 heat blower should be used to warm up the injection-pump and starter to body temperature. When starting electrically propellor was was first rotated without ignition, main fuel was then switched on and fuel lines were pressurised with hand or electric pump. Pitch was set to fine. Ignition was switched on and throttle was moved very slightly forward from "Idle" position. 6-12 presses on the riching pump where applied when starter was started(warm engine should not be enrichened). Starter handle was pressed for 10-20 seconds to give it enough rpm, and then pulled to engage it with the engine.

Cold engine was let to run at 600rpm until oil pressure was registered. After that rpm was risen to 1000rpm and warming up was continued until oil temperature rose to 30 and cooler fluid to 50 degrees. Rpm was again risen to 1600rpm and cooler fluid temperature was controlled with radiator flap and oil temperature was observed. Quite long pre-fligh checks were done while warming the engines. Brakes were tested by rising the rpm to 1800 and plane stayed still. Landing flaps were opened and pulled in many times alone and together with elevator. If divebrakes were installed they were tested. Autopilot, generators and propellors were checked.

In the test running full boost 1.4 ATA was taken, and rpm should stay within normal. Magnetos were tested at 1.25 ATA, allowed drop was 100rpm. After test running engines couldn't be dropped to idle because of the sparkplugs which would get dirty and endanger the take-off. Radiator flaps were opened for taxiing and opening lasted about 8 seconds. Turning should be done saving the brakes, with power and rudder; turning on one wheel was prohibited. Control column was kept centered to prevent tail wheel reaction. Oil temperature had to be followed especially in the summer and take-off should be made with out unnessary delays. Before take of fuelpumps were engaged and flaps lowered to 25 degrees.

Throttle was opened carefully, as Jumo was a fast accelerator. If rpm rose over 2600, propellor pitch had to be pulled backwards. JK tried to turn to left quite strongly, and for that reason pilots often pushed left throttle ahead of the right one. Nose was slowly pushed to horisontal level, wheer the plane was let to accelerate to 170-185kmh; push required quite a lot of power like did the pull to lift the airplane up. During start trim would be rotated backwards. According to the manual take-off distance with 12.5 ton mass was 570m and over 20m high obstacle 1280m; but on our soft fields (Finland) it often was much longer than that. In practise the take-off couldn't be aborted on our 1000-1200m long fields but in the very beginning of take-off, critical stage being in the half way of the field.

After taking off climbing was continued in a shallow angle for the speed to build up to 200kmh and over - over the theoretical one-engine climb speed. Power was dropped to the 30 minute area 2400rpm/1.25ATA. Wheels were braked and pulled in right after the take-off (max speed 265kmh). In the 9 lamp instrument the highest 3 lights turned on when wheels were up and wheel control switch was returned to "0" position when hydraulic pressure was 85-95ATA. Retracting landing gear took 18-20 seconds. Flaps were pulled in at safe altitude because plane would drop when doing it. Climbing speed was set to 250kmh.

JK wasn't quite the racer that it could be thought of from it's appearance and stories told about it. Large weight killed the Schnellbomber's performance values: power/weight ratio was some 30% worse than that of a Blenheim. Take-off was the most critical part of JK operation and it continued to the heigh of 200m. Also climbing speed was modest.


Climbing and performance in level flight

Supercharger switched automatically at 3km altitude to a higher gear, although pilot could also change manually. Automatic control was sometimes problematic when flying at 3-4km altitude and caused some small engine trouble.

Initial climbing speed was about 3m/s and with best climbing speed variometre would show 4m/s. Picture 1(kuva 1) shows climbing times from the manual at 13 ton take-off weight. Theoretical ceiling was 8500m. JK taking off from Onttola to battles on Carelian Isthmus would most often reach the diving altitude 4000m somewhere over Punkaharju-Parikkala.

In Rechlin tests highest momentary level speed with bombload was 465kmh at 5km altitude and 395kmh at low level. In Finland top speed was considered to be around 450kmh. It depended strongly on the position of radiator flaps. For example test pilot Uuno Karhum�ki got a top speed for JK-268 in may 1944 to be 405kmh(TAS) at 1500m and 440kmh(TAS) at 5000m altitude. Outside air temperature was +4 degrees and radiator could be kept close. On a evening of a very hot day JK-253 could achieve only 385kmh at 1500m and 410kmh at 5000m. Air temperature when taking off at 20:45 was still +22.5 degrees and opening the flaps proved necessary. Cruise speed(2250rpm/1.0ATA) on this flight was recorded to be 350kmh and 370kmh in before mentioned altitudes. (flaps open)

JK that had dropped it's bombs could evade fighters on a level speed of 400-440kmh depending on the altitude. On lean mixture highest possible cruise power was 2250rpm/1.15ATA with which heavily loaded plane would achive 350kmh on low level and at high level 400kmh TAS. Fuel expenditure was 600-650l/h depending on altitude, and operating time with 1680l fuel load about 2.5 hours. With full(inc. fuselage tanks) 2900l load JK could cruise 4.5 hours and travel 1600km.Oil expenditure was at maximum 24l/h. At range power setting 2000rpm/1.0ATA speed was 290-350kmh depending on weight and altitude, fuel expenditure 500l/h and operating time more than 3 hours.

Finnish saved the sensitive engines and used 340-350kmh as cruise speeds, so it was about before mentioned power settings, and with those flying distance was 1150km. Return bombing trip from Onttola to Isthmus took normally about 2 hours so the three-hour fuel load gave enough reserve. Often it was the maximum that could be afforded: empty plane weighed 8750kg, bombs typically 1500kg and crew with equipment 400kg. Take-off weight came thus to12 tons which was about the maximum for short and soft fields. Although 13 ton weights were flown as well.



Flying characteristics

Despite large size and weight JK wasn't clumsy - quite the opposite. Controls, especially ailerons and rudder felt remarkably light and effective through the whole speed range, and aileron friction was small so there was little need for rudder in banking turns. Aileron effectivity was also excellent and plane did a fast half-roll at over 500kmh speeds. Measured aileron force with full deflection was at cruising speeds 10kp and so very little for a bomber. It should be mentioned that the German captain Seibert did make loops, Immelmanns and rolls in 1943 over Onttola even though aerobatics were prohibited in the manual.

Elevator was heavier than the ailerons, and the stick force per G was in the "bomber class", but automatical landingflap-trim function and small need for trimming of the long tailed plane made also elevator controls quite pleasant. Relative agility of JK  is best described by the revelation that the best defense against enemy fighters was violently pulled diving turn. Climbing turn shouldn't be tried ever because of the poor climbing of JK. The result of the sensitive controls was moderate stability in instrument flying. Especially at high altitudes elevator control lightened and intrument flying did take some concentration.

Automatic direction pilot helped on long flights. Direction automat was engaged by matching the directional gyro with the magnetic Patin-compass and then selecting a course in the displaying device. On long flights pilot then could rest and let the elevator and aileron controls to the observer. Observers stick like control column, which was normally held in the side of the cockpit, was attached to its socket. Pedals were not duplicated.

When directional automat operated pilot could adjust the direction and perform shallow banking turns with a direction control switch in the right horn of the control column. It had three positions for 1, 2, and 2.7 degree/second turns. In JK equipped with Lofte also observer could control the direction automat with his own direction control. Direction automat could be disengaged in emergencies by kicking the pedals.

Dive bombing automat worked with setting the height of the target from sea level and wind correction from planned attack direction in it before the flight. Drop altitude could be set during the flight also.On approach pilot would set mixture to rich, closed the radiator flaps, set pitch on coarseand set the trim to the red markings in the trim indicator. On diving throttles were pulled to "idle" and automat was engaged, which deflected the trim tabs to turn the plane in to a dive. Observer would constantly input the the speed to the system(not required in all models), and pilot would hold the target in the crosshair. When approaching the dropping altitude buzzer would alarm, and pilot would pull slightly to move the target to the "Kravatte" on the sight and release the bombs by pressing the button on the left horn. Bombs would drop and trim tabs turned to pull out from the dive at 2-3 G acceleration. If pilot helped by pulling the stick G would easily rise up to 4-5, and vision would start to blacken.

Instead of the original deep 60-80 dive Finnish started using 45 degree dive angle. Also dive brakes had been removed and resulting higher speed (over 700kmh IAS) helped to avoid enemy AA fire. After 600kmh JK actually seemed to suck more speed and the pilots couldn't more often hold it within permitted limits. To minimise the casualties dropping altitude was often ordered to be 2500m; after which it took 500m or even more to pull out. High bombing altitude reduced accuracy compared to smaller dive bombers. But experienced crew could achieve 10 metre accuracy in good conditions; largest problem being the wind and living of the plane in the dive, if there was movement when the bombs were dropped the movement was transmitted to the bombs as well.



Critical one-engine performance and landing

In the event of engine failure throttle was pulled to "idle", fuel main lever for the engine was closed, ignition was turned off, and pitch selector was pressed to position "Segelstellung" for about ten seconds until the propellor was feathered. Before feathering plane pulled strongly to the side of damaged engine. Direct flight could be done just and just, but turning to the side of the working engine wasn't possible. Altitude dropped before all the preparations were done. Radiator flaps were closed and both tanks were used to cool the working engine. Simultaneously reducing weight was started by dropping the bombs, cockpit armour and all not fixed devices out of the floor hatch.

JK could fly with maximum weight of 10.5 tons with 30 minute power setting. Best speed was approx. 240kmh and theoretical one-engine ceiling was 1500m. In one engine landing flaps were opened in the very last momentand approach was flown at 210-220kmh. Pulling up with one engine wasn't possible anymore with landing gear in down position. With 10 ton weight JK would stall in 180kmh with clean configurationand, in landing configuration in the speed of 150kmh. Stall could be sensed before from the vibration in the elevator after which the nose fell down. Speed of descent was large.

During approach direction automat was switched off, supercharger was kept on automatic and mixture set to normal. Radiators were opened in case of aborting the landing and flaps were lowered  25 degrees when speed dropped below 255kmh. Trimm changes when lowering gear and flaps were small, but landing flap caused large difference in flight attitude compared to cruise. Speed would be kept under 255kmh because otherwise flaps would automatically retract. Speed on the downwind leg was 220kmh and the gear was dropped, which took about 25-28 seconds. In the final about 1-2 kilometres from the field full flaps were dropped and speed was reduced to 200-210kmh.

Landing was made in three points with reducing throttle, and the JK would sit down at 165kmh speed. During flare angle change was very large and could cause some problems for beginners; directional stability was decreased asa the rudder got into the fuselage slipstream. Landing took some 500-700 metres and the overall distance over a 20m obstacle was 700-900m. Brakes had to be used conservatively, and they had to be let cool down before the next use during taxiing. "Airplane card" made by Airforce Headquaters describes JK as follows:
"Flying and steering characteristics are very good. Requires long airfield. Climbing characteristics bad, speed satisfactory. Diving characteristics are good. Agile, good turning and landing characteristics. Good positioning of weapons. Engine power satisfactory. Engine durability satisfactory. Use of the plane: Long range and dive bombing, can be used also as long distance reconnaisance and night defence."

(Jan Niukkanen 2003)