From The Cockpit – Grob G1098

The G109B is a motor glider built for two, the side by side seating being accessed by a pair of clamshell doors, upwards hinged and forming a large percentage of the – canopy transparency area. Apart from the door supporting frames, the canopy has unrestricted 360 degree vision. On a G109B used solely for daytime flying, the overhead panel between the clamshell doors can also be a clear perspex segment (or tinted). For night-time flying, it is replaced by a filled in piece which accommodates a pair of overhead lamps for pilot and passenger lighting. Accommodation is cosy but comfortable, the moulded seats having a variable recline angle. Behind the seats the cockpit has a large and useful shelf area. Each seat has its own flying control stick and rudder pedals are duplicated. The aircraft’s fuel tank of 100 litre usable capacity is located under the shelf behind the seats. Between the seats is a pedestal with engine, park brake and elevator trim controls. The propeller controls are above the throttle and consist of separate fine/coarse pitch and feathering controls, both pull activated.

In size, the panel is just that for which the small 3 inch diameter Becker radios were designed. Thus configured, there is adequate space for a full suite of NVMC flight and nav instruments and switches. Fuel contents and engine condition instrumentation are centrally located.

Start and operation of the 67kW (70bhp) Grob 2500 engine is straightforward. It has throttle and carb heat control only, there is no manual mixture adjustment. Taxying and ground manoeuvring hold no surprises. In spite of the tailwheel configuration, visibility over the nose on the ground is adequate.

Due to the relatively low engine power and the reasonably small propeller diameter (reducing its static thrust), ground roll acceleration on takeoff at a typical two-up weight of 800kg is rather meagre. It is, however, adequate because takeoff speeds are low due to the small wing loading of the aircraft. With 40 knots IAS on the clock the tail can come up, and shortly thereafter the machine can be lifted off at 48 knots. Putting the nose to horizon provides for an initial climb speed of 52 knots. Once safely clear of the ground, an early selection of coarse pitch can be made, taking care not to pull the feathering control (although located near each other, incorrect selection is guarded against by the fact that the two T-handled controls are very different in size – the fine/coarse pitch control being much larger); thereafter a cruise-climb speed of 60 knots at 2,500rpm yields 500ft/min ROC. Cruising, an engine speed of 2,450rpm yields 82 knots IAS, 2,550rpm gives 90 knots. Cruise nose attitude is very low, and visibility excellent, including to the rear. Due to its low wing loading, aircraft motion is quite susceptible to gustiness, but not disconcertingly so. The G109B’s flying controls are conventional – ailerons, elevator and rudder. Aileron control forces are pretty high, I suspect heavier than desirable for serious and long duration gliding. However their roll authority is excellent, considering the degree of roll damping that the high aspect ratio wing provides – a ninety degree (45 to 45) roll reversal taking about three seconds. And, surprisingly, there is very little adverse yaw produced by the attendantly large deflections of the ailerons.

Aileron control is effective but elevator control is sensitive. A delicate touch is needed in order to maintain the desired pitch attitude. In steady rate steep turns, the elevator control force is likewise low but visibility is great and the view of the cowling enables the airplane to be held in a precise attitude- all of which are desirable qualities for manoeuvring the aircraft when thermalling. The pitch trim change with power is minuscule, one of the lowest I’ve seen, thanks largely to the T-tail removing the tailplane from the slipstream and near-wake downwash. The directional trim change with power is also small. Both of these qualities assist in the easy control of the aircraft. The elevator (bias) trimmer is also very effective and requires a delicate touch to operate it.

The magnitude of rudder control forces is just right. Spiral stability at cruise speed is neutral. Dutch roll motion is of the snaking type (yaw oscillating without rolling) and is moderately damped, taking 21/2 cycles to subside.

The sideslip characteristics of the G109B are peculiar and not conducive to using the manoeuvres in the normal operation of the aeroplane. Even at cruise power/speed, upon entering both left and right steady heading sideslips, at about half rudder deflection there occurs a sharp pitch break and nose down trim change, accompanied by moderate buffet, and requiring a large input of up elevator to balance. Likely cause is upper surface flow separation.

Certainly sideslipping is not necessary because of the wing upper surface speedbrakes. These are mechanically operated, normally locked by over-centre linkage action. Upon unlocking, air pressure forces them fully out, requiring the pilot to rigidly hold the control to restrain them from fully deploying rapidly and causing damage. Deployment also tends to pop open the ventilator panels in the canopy sides- an unfortunate annoyance. Effective though they are, increasing the ROD to 1,000ft/min at 70 KIAS with sufficient power to keep the engine warm. Stalling characteristics of the G109B are innocuous. With two on board and half fuel- throttle closed, the aircraft reaches a minimum speed of 43 KIAS before wing rocking develops, itself controllable with aileron; the only effect of power is to make an actual stall discernible- with 1,500rpm and once again the stick fully back, a definite nose down stall break occurs at 43 KIAS, followed by wing rocking as before.

The speedbrakes make the aircraft flexible in the circuit pattern. Final approach is best flown at about 60 KIAS with a constant throttle setting and using variable speedbrake to make good the threshold at a target speed of 50 KIAS, thereafter using only a touch of elevator to flare- effectively a sailplane approach technique.

All this, and- when the thermals are rising, or the wave lift is rising – close the throttle, pull the feathering control and take advantage of the Grob’s 30:1 glide ratio for as long as your heart desires. The G109B can take you places regardless of soaring conditions for, without shutting down the engine its low consumption of about 12 litres/hour will get you 809nm at 97 knots (an easy Adelaide/Sydney) before you will need to rely solely on the aircraft’s aerodynamic qualities and the weather.

The first G109 didn’t fare quite so well in either department-powered or unpowered. With certification under JAR 22 in seventeen countries, series production began in mid 1981. Production totalled 160 before Grob switched to the 109B. The G109, with a 56kW (75bhp) Limbach fitted, was somewhat underpowered. Its gliding capability also required enhancement. Apart from the switch to the Grob 2500 engine, wingspan was extended to 17. 4m (57ft). Production commenced mid 1983 and established at a mean rate of about 10 aircraft per month.

The entire aircraft is of composite construction – primarily fibreglass, except for the rudder and some of the wing, which are carbon fibre. The 19sq m (205sq ft) wing has an Eppler E580 sectionof 16% tic, set at 3 deg dihedral. Thisis a low value and probably accounts for the aileron centring spring that is installed (presumably to enhance static lateral stability), which in turn adds to the heaviness of the ailerons.

The mainwheels are size 380x150mm and are fitted with effective hydraulic disc brakes. The aircraft has no need for any other system, except electric, which is comprehensive. The pitot-static system derives air pressure sensing from the combined head atop the fin, forward of the tailplane leading edge.

The composite structure is strong (g limits +5. 3/-2. 65), but typically heavy- at 620kg the empty weight is 73% of MTOW, substantially higher than an aluminium airframe. Best glide ratio speed is a moderate 62 knots, so that the lack of a water ballast system is not sorely missed. The minimum sink rate is 216ft/min at 57 knots. Field distances are trivial at 600 odd feet takeoff ground roll, so that should an outlanding be at any time necessary, the likelihood of a suitable field size should be good. The wings are foldable rearwards.

Grob discontinued production of the G109B. However, in response to an order for 60+ of the type placed earlier this year by the UK MoD for the RAF Elementary Flying Schools fleet replacement (ousting the ubiquitous Chipmunk, which will be the next From The Cockpit subject aircraft), Burkhart Grob Flugzeugbau of Mindelheim will be laying up another batch. It could well be the time to add an individual order, or alternatively to pick up a used 109B before their market prices firm from the relative scarcity of the dual purpose glider.