Flying the Junkers Ju88

[wells]

The following comes from a book called, "The Luftwaffe in the Battle of Britain", by Armand van Ishoven (ISBN:1-55068-050-1)

 

The RAF got its first chance to examine a Junkers Ju88 in flight on 28 July 1940, when the direction-finding equipment of 9K + HL, a Ju88A-1 of 3/KG51 (WNr 7036), failed.  The pilot, Oberfeldwebel Josef Bier, lost his way and landed north of Bexhill, the Ju88 being undamaged.  It received the British registration AX919 and was test-flown by RAE pilots.  About the Ju88 they had the following to say:

The aeroplane has a number of interesting mechanical devices designed to assist the pilot in his handling of the aeroplane.

i.  The flaps may be lowered to two positions, roughly 25 deg for take-off and 50 deg for landing.  With the flaps up a small movable plate covers the slot gap on teh wing under surface, thereby reducing drag.  During the first 15 deg of the flap movement this plate is moved upwards, thereby opening the slot.

ii.  On lowering the flaps through their first 25 degrees the ailerons are both drooped about 15 degrees in order to increase Clmax.

iii.  On lowering the flaps fully, the tailplane incidence is automatically decreased by 5 degrees in order to counteract the change of trim and to help in getting the tail down on landing.

iv.  On lowering the dive brakes the elevator trimming tab is automatically moved up to counteract change of trim.  Pull-out from a steep dive during dive-bombing is automatic.

vi.  The rudder is fitted with a spring tab to assist in handling the aeroplane on one engine.  The tab comes into action when the pilot's foot load exceeds about 30 lb.  The same tab is used for trimming.

vii.  The aileron forces are reduced at large aileron displacements by swinging weights in the wing tips.

Take-off and Landing
Takeoff

The aeroplane must be taxied straight just before openeing up the throttles in order to lock the tail wheel in the correct fore and aft position.  Provided the throttles are opened up slowly and evenly, the tendency to swing is not very marked, while above 50 mph the rudder becomes very effective.  The aeroplane does not readily fly itself off, the control column having to be pulled back firmly to unstick.  Take-off run is rather long, but speed builds up rapidly once off the ground, giving full control in a short space of time.  The initial rate of climb is moderate.  25 degrees flap was used for takeoff.  

Approach

The undercarriage should be lowered at about 2000 ft.  At the appropriate time the flap lever is moved into the first position from neutral.  This half lowers the flaps and droops the ailerons; change of trim is small.  During the final stage of the approach the flap lever is brought to the second position.  This brings the flaps fully down and decreases the tailplane incidence, necessitating a push on the control column.  A certain amount of engine is normally used during the approach.  If the engines are throttled right back the best approach speed is about 115 mph; at 105 mph there is a sensation of sinking, and at 125 mph one of diving.  Stalling speed flaps down is 93 mph, and 116 mph with flaps up.  On lowering the flaps, control in general feels more 'sloppy', but is quite adequate on the glide.

Landing

The aeroplane is not easy to land.  It is very difficult to get the tail fully down, the brakes must be used with caution after a wheel landing, while there is a tendency to swing after touch-down which must be immediately corrected.

Baulked Landing

The engines can be opened up fully with flaps and undercarriage down, and the aeroplane climbed away without undue difficulty.  The undercarriage comes up in about 12 seconds.  On raising the flaps, which up in about 10 seconds, there is a marked sink, the nose drops rapidly and the speed increases quickly.  A hard pull on the stick ins needed to keep the nose up, and in the interests of safety the flaps should not be raised below about 1000 ft.

Taxying

The aeroplane handles well on the ground.  Care must be taken to see that the tail wheel is unlocked (as a safety feature, to prevent retraction of the tail wheel when skew, the hydraulics are automatically put out of action unless the tail wheel is loced central).  The brakes are particularly effective.

Longitudinal Stability

The center of gravity limits are from 2.48 to 3.55 ft aft of the root leading edge; since the aerodynamic chord is 9.18 ft, this gives a CG range of about 11.5% of the mean chord.  Trim curves at two CG positions suggest that even with CG aft the aeroplane has a large static margin in hand, stick fixed and stick free, of the order of 0.1C on the glide and slightly less at full throttle.

Flight on One Engine

If one engine is suddenly throttled back when cruising level at 220 mph and no corrective action is taken, the aeroplane banks fairly sharply and loses height, speed building up rapidly.  Recovery is very easy owing to the lightness and effectiveness of the ailerons.  The aeroplane can be trimmed to fly straight and level with feet and hands off, about quarter rudder being necessary.

Behaviour on throttling back one engine during a climb at 150 mph is far more violent, and unless prompt corrective action is taken to prevent the wing dropping, over 1000 ft may well be lost.  Provided the ailerons and rudder are applied promptly, however, correction is quite easy and there is no necessity to throttle back the live engine to retain control.  Enough rudder tab is available to trim when climbing on one engine.  

Recovery is rather more difficult if the flaps are set in the take-off position when climbing, as the ailerons are heavier and the manoeuvre somewhat more violent.  During all these engine cutting tests with virtues of the light spring tab rudder and the light ailerons were very apparent.

Landing with one engine dead is rather precarious, owing to the violent swing and bank accompanying opening p the good engine when correctgin the flight path at very low airspeeds.

'One control' Tests, Flat Turns and Sideslips

The aeroplane was trimmed to fly straight and level at 180 mph, 10000 ft.

Ailerons fixed central

On suddenly applying the rudder, swing and bank build up rapidly.  The rudder returns to central sharply on release, and after a few oscillations in yaw and roll the aeroplane settles in a banked turn.  Good banked turns can be done on rudder alone, the aeroplane banking unusually quickly in response to rudder movement.

Rudder fixed central

The ailerons can be applied violently without appreciable opposite yaw.  On releasing the stick it oscillates from side to side a few times before returning to central.  The aeroplane does a few oscillations in roll before settling down after release of the stick.  Excellent banked turns can be done on ailerons alone, with little sideslip in entry or recovery.

Steady Flat Turns

Full rudder can be applied, although considerable force is needed to get it fully on.  About 3/4 opposite aileron is required to hold the wings level.  Rate of flat turn is roughly 120 deg/min.  There is no pronounced nose-heaviness.

Steady sideslips when gliding

Gliding at low speeds with flaps and undercarriage up the maximum angle of bank in a straight sideslip is about 30 degrees, obtained with full rudder and 3/4 opposite aileron.  There is little nose-heaviness.  Behaviour is the same with flaps and undercarriage down, except that there is slightly more nose-heaviness.  In both cases the wing comes up rapidly on releasing all three controls, and the aeroplane quickly settles in a straight glide or a shallow turn.

Stalling Behaviour

Behaviour in a straight stall with the engines throttled back is fairly mild, flaps up or down.  With flaps up the control effectiveness decreases progressively as the stall is approached, and at the stall the nose drops abruptly with little tendency for a wing to go down viciously.  With flaps down more warning is given, since the loss of control effectiveness is more marked, and a buffeting is felt on the elevator at 96 mph, some 3 mph before the stall.

Flying Controls

Ailerons

Aileron control is excellent.  Response is adequate at low speeds and for dealing with a sudden engine cut, while at high speeds the stick forces are extremely light.  At 300 mph more than 3/4 aileron can easily be applied with one hand.  At 240 mph the time between the initiation of the stick movement and the stage at which the aeroplane reached 45 degrees bank in a steady roll was measured, when the pilot applied nearly full aileron vigorously.  The surprisingly low figure of 0.7 seconds to 45 deg bank was recorded.  This places the Ju88 quite out of the common run of bombers as regards aileron performance at high speeds.  The feature is, of course, of the utmost value in avoiding action.

The lightness of aileron control springs partly from aerodynamic balance and low stick gearing, and partly from an ingenious mechanical device for lightening the ailerons at large displacements.  A full description and discussion of this device will be found in Appendix I.  Briefly, swinging weights mechanically geared to the ailerons are mounted inside each wing tip in such a manner that when the aeroplane rolls, the centrifugal force on the weights generates a relieving hinge moment proportional to the cube of the aileron angle.  Such a device can be arranged to very appreciably relieve the pilot's effort during violent manoeuvres.

Elevator

The elevator control would be inadequate for landing were it not for the automatic coupling of tailplane and flaps which decreases teh tailplane incidence 5 degrees on lowering the flaps fully.  Even so there is barly sufficient elevator control for getting the tail down on landing with the CG forward.  The elevator is pleasantly light and effective at all speeds.  At 250 mph a normal acceleration of 4G can be put on by pulling fairly hard, without assistance from the tabs.  Vigorous avoiding action is thus possible.

Rudder

The rudder is considerably lighter than is usual on aeroplanes of this size, owing to the spring tab mechanism.  This is a very good feature from the viewpoint of engine cutting and evasive action.  The rudder, although still light, is not oversensitive in the dive.

Dive brakes

The procedure for dive-bombing with dive brakes out and automatic pull-out is interesing.  Before entering the dive a contacting altimeter, which sounds a horn at any selected altitude, is set for the height at which the bombs are to be released; and the bomb distributor is adjusted with the assistance of standardized tables.  The elevator trimmer is then set to a fixed mark which trims the aeroplane nose-heavy, and the dive is started.  As the speed builds up the dive brake lever is operated and the engines throttled back.

Operation of the dive brake lever lowers the dive brakes and at the same time moves the elevator trimming tab up through 2.5 degrees.  This moves the elevator down, and causes the aeroplane to drop its nose rapidly.  When a pilot first goes through the routine he finds this part of the operation quite alarming, since the aeroplane appears to bunt suddenly, with appreciable negative 'g' for a short period.  

However, the tab settings are so adjusted that the aeroplane automatically trims itself in a rougly 50 degree dive, during which the speed builds up to about 310 mph after a height loss of 6000 ft.  During this period the pilot can judge his dive angle approximately by means of inclined lines painted on one of the transparent cockpit panels.  He concentrates on getting his sights on target in a steady dive at the selected angle.

When the pre-set height is approached the contacting altimeter sets a horn going.  On reaching the selected height the horn noise suddenly stops.  The pilot immediately presses the bomb release button.  

The 2.5 degrees up tab movement obtained on lowering the dive brakes compresses a spring in the tab circuit.  The spring is tripped, via a solenoid device, on pressing the bomb release button, and the elevator tab is thereby abruptly jerked back 2.5 degrees down.  This moves the elevator up, and pulls the aeroplane out of the dive with a normal acceleration of about 3.5g.

During the pull-out, at a pre-selected time after the bomb release button which initiates the pull-out has been pressed, the first bomb is automatically released.  The whole purpose of releasing the bomb during the pull out is, of course, to compensate for the curved trajectory of the falling bomb.  The German technicians have attempted to make the process as definite as possible by reducing the possibility of piloting errors.  

The pilot is not restricted to a 50 degree dive; any angle in the range 30 to 70 degrees may be used.  Provision is also made for use of the automatic pull-out device with dive brakes retracted for steep dive bombing from very low altitudes.

[brady]

I have this book at home , it is a good one.It would be cool if we had the dive bombing systeam modeled in AH, and the dive bomb sight, which is conspicusly absent form or JU 88. I have a picture of it in a couple books at home but i am halfway across the state on a prodject ,anybody have a scan of it ?

       Nice post.