Flight test: GROB 109A

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

With refurbished RAF Air Cadet Vigilant T1s coming to the civilian market, Grob’s touring motor glider should become even more popular

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

Words: Dave Unwin | Photos: Keith Wilson

Skimming along just above the inversion I carefully set engine and propeller as per the useful placard on the panel, re-trim and sit back. It takes a while, but the ASI needle eventually settles on eighty knots, for a TAS (true air speed)of 86. “There you are,” said Mark Tolston proudly “exactly what I said it’d do, and its only burning twelve litres an hour too!”

It is self-evident that practically every aircraft ever built has an optimum engine size. Underpowered aeroplanes can be challenging (and at times frustrating) while overpowered machines are usually not much faster just a lot thirstier, albeit takeoff and rate of climb are improved exponentially.

This month’s test aircraft is a motor glider and provides object lessons in both aerodynamics and thermodynamics. Motor gliders can be sub-divided into being either of the self-sustaining, self-launching (SLMG) or touring (TMG) class, but to try and explain the difference is beyond the remit of this piece, so just accept the fact that the G109 is of the TMG type.

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

Not a lot of horses

Leaning heavily on its considerable experience in manufacturing sailplanes, Grob’s prototype G109 motor glider was beautifully made, aerodynamically efficient?and like too many SLMGs, woefully underpowered. The prototype flew in March 1980, fitted with a two-litre Limbach L2000 of only 80hp?not a lot of horses for something weighing 825kg?and it was found somewhat wanting. The logical thing to do was fit a more powerful engine, but instead of adding power, Grob did what sailplane manufactures always do, and increased the span, from fifteen metres to 16.6. Even with bigger wings (which although lowering the wing-loading also reduced the power-to-weight ratio further), a hot day with no wind or wet wings meant the earth’s curvature could figure prominently in takeoff performance calculations, and you always hoped that the ‘Dirt-Sniffer’ was functioning (see box opposite). And of course, even if you did manage to coerce the thing into the air, your problems were far from over. An encounter with even slightly sinking air was enough to turn the already-lamentable climb rate into a descent, and as the ASI and vario needles sagged and the oil and cylinder head temperatures redlined all you could really do was hold your nerve and look out for tall trees and double-decker buses.

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Despite the model’s reputation as a ‘ground-hugger’, Grob built around 150 G109As between 1982 and 1984, before switching production to the 109B. This featured myriad changes, most notably a much more powerful engine. The test aircraft, G-CINK, is a 109A, but what makes ‘November Kilo’ unusual (and a bit closer to the B in its performance) is its powerplant, as it is one of only thirty 109As that were fitted with a ninety horsepower 2.4-litre Limbach L2400EB.

Purchased in Hungary, the tale of bringing it home and then onto the G-register could fill many more pages than I’m allowed, but after many years and a lot of angst November Kilo is now a familiar sight in the skies over Lyveden, as Mark is the CFI of Welland GC.

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

Different to the 109B

There is a 109B at Saltby, my home airfield and as soon as I begin the preflight several subtle but significant differences between the two models become apparent. The 109A’s cowling is a squarer shape than the B’s, and features a controllable flap for the oil cooler inlet. Other notable differences are that the B has a significantly thinner wing section at the root, a relocated main undercarriage and a different canopy. Even before getting in, it seems to me that the B’s redesigned canopy is a retrograde step from the field of view perspective, as the A’s is essentially a bubble whereas the B has two separate doors.

Most 109As and Bs are fitted with a three-position, manually operated propeller. However, ’NK has an electrically-actuated, constant speed MTV unit. Having already had one thrill too many in another motor glider due to the slow pitch-change characteristics of such props, I ask Mark how long it takes to unfeather. “Longer than you’ll like!” he affirms.

The engine is quite tightly cowled, although judicious use of Dzus fasteners means access to the engine compartment is both quick and easy. Closely related to the L2000, the 2400 is an air-cooled flat-four derived?like so many other European aero engines?from the classic VW Beetle engine. It produces 90hp at 3,400rpm, turns a two-blade composite propeller and is fed from a single, eighty-litre fuel tank, directly behind the cockpit.

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

The wing section is another area worthy of note, and I soon appreciate why the 109B, which has only a three-position mechanical variable pitch prop, cruises faster. The A uses a modified Eppler aerofoil, which is notably thick at the root. You don’t need to be an aerodynamicist to appreciate this is what makes the B quicker.

The plain, narrow-chord ailerons are of GRP sandwich construction and are hinged internally. They are actuated by pushrods, as are the elevator and airbrakes, only the rudder being actuated by cables. The large airbrakes are of the Schemp-Hirth type and extend from the top of the wing only, at about one third of the span and 45% of the chord. Aft of the cockpit, the fuselage tapers quite sharply. The tail consists of a large swept-back fin which carries a T-tail and a surprisingly small rudder. There is a trim tab on the starboard side of the elevator.

The undercarriage comprises mainwheels fitted with hydraulic disc brakes and suspended from spring-steel legs attached to each side of the fuselage, and a tailwheel that steers through the rudder pedals. Deft application of differential braking will force it to break out, and it then castors through 360°.

Overall, the aircraft seems logically designed, but what really impresses me is the quality of the workmanship. It is beautifully made and in very good condition. It’s hard to believe its almost forty years old.

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

Good to go...

Time to get in. The plague would usually be an issue at this juncture, but Mark is a microbiologist and was able to run a Covid-19 test for both of us the morning we flew. Two negative results, and we’re good to go. The canopy opens forward and wide, almost to 90°, and is held open by well-damped gas struts. There are steps on the undercarriage legs so you climb in over the leading edge. I think Grob didn’t want people standing on those well-waxed wings! The cockpit is roomy and the seats adjust over a good range but must be set in place before you sit down, while the rudder pedals are ground-adjustable on spring loaded pegs.

Settling onto the comfortable seat I note that ’NK has all the standard flight instruments in front of P1, but they are laid out in a distinctly non-standard arrangement. These consist of (top row, left to right) a turn & slip, altimeter, variometer and ASI, with the attitude and direction indicators underneath. On the right side of the panel are the overly-large tachometer and a smaller manifold pressure gauge, with oil pressure and temperature, cylinder head temperature, fuel, volts and amps displayed underneath. There is a large silver lever which looks a bit like the throttle on an old-school motor-mower and opens the spring-loaded oil-cooler inlet, while the aircraft’s sailplane heritage is indicated by the ‘Canopy Jettison’ T-handle (glider pilots routinely wear parachutes, making this an essential provision?Ed).

In the centre is an Android GPS unit which runs both Sky Demon and XC soar. Cleverly, the GPS also has myriad pertinent papers saved on it (such as the POH, maintenance manual, ARC, insurance documents etc) and has WiFi and Bluetooth enabled, so you can access NOTAM and MET via tethering with a phone or a Wi-Fi router.

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

Be it a flying machine or a washing machine, one of my mantras has always been “don’t start anything until you know how to stop it” and the 109A has a brake system that could (and has!) literally tripped pilots up. Both pilots have an airbrake lever by the cockpit sidewall, and the wheel brakes are applied simultaneously at the end of the airbrake lever’s travel. Differential braking occurs at the end of each rudder pedal’s travel, while the large lever between the seats is the parking brake. Its quite possibly the largest aircraft parking brake selector I’ve ever seen, and it’s not hard to imagine how a pilot under training could easily grab it and put the aircraft on its nose. What’s that you say Mark? You’ve seen it happen? Oh! Who knows why Grob did it this way (talk about adding weight and complexity!) and it’s not surprising that it was discontinued on the ‘B’.

A central console drops down from the instrument panel and carries the control box for the electric CSU, the master switch and choke, cabin heat and carb heat. A minor ergonomic exasperation is that these three controls (round push/pull plungers) are all a bit samey. They are colour-coded but if they’d been made different shapes as well it would’ve been better. I like the location of the fuel selector (at the base of the central column and easy to see and reach) and the large map pockets on the cockpit sidewalls. Further stowage space is provided by the capacious baggage area behind the seats, which is accessible in flight and can carry up to 25kg.

Between the seats are a large yellow T-handle for the throttle and a smaller lever topped with a green knob for the elevator trimmer.

Taxying out reveals essentially good characteristics. The suspension seems reasonably well damped, the brakes adequate and the turning circle impressively small, once the tailwheel is forced to castor. The 109 does feel like it teeters from side to side a bit, but this is understandable with a wheel track barely a tenth of the wingspan. For a taildragger, the field of view is excellent, probably because the over-sized tailwheel ensures the ground attitude is quite shallow.

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

One real ‘gotcha’

Pre-takeoff checks are straightforward (you can’t do a mag check as the Limbach only has one) but you must ensure that the switch for the electric propeller CSU (constant speed unit) is set to ‘Auto’ and the rotary knob that controls rpm is set to ‘30’. Mark explains that if you use ‘Start’ it fixes the pitch in fine, but it’s possible to snag and flick the small toggle switch through Auto into ‘Segal’ (feathered). This can stall the engine, even at full power, and then you would have to get the prop all too slowly back into fine pitch before re-starting it. This is a real ‘gotcha’ so that’s why you leave the CSU in Auto!

Lining up on R25, I note the ambient conditions, particularly the northerly wind, which is creating a considerable crosswind from starboard. I consider taxying up to Runway 02, but instead elect to take off from the grass and keep the tailwheel on the ground to aid steering. Mark nods approvingly, as we both know that if a swing is allowed to develop it can be hard to catch.

The OAT is 15°C, which gives a density altitude of about 750ft, and 230kg of pilots and petrol puts us within fifteen kilograms of the 825kg maximum all-up weight.

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

The acceleration is good, and because I’m ready for it to swing, the 109 doesn’t. Lift-off occurs at fifty knots and the initial climb rate is around 600fpm. As we want to be above the inversion for the photographs, I climb at a higher power setting and slower speed than Mark would generally use. Consequently, and unsurprisingly the oil temperature soon starts to rise towards the redline, although interestingly the CHT remains well within limits. This makes Grob’s installation of an oil cooler flap even more sensible, and opening it does help lower the oil temperature - but what’s really needed is a bit more speed and a bit less power.

Al and Keith are airborne in the BGC EuroFox, and as soon as Keith’s door pops open I slide into close formation. One thing that can make a Pilot photo shoot tricky is when the camera ship and the subject aircraft are dissimilar and this is definitely the case on this occasion, as the EuroFox and 109 are distinctly dissimilar! I’ve set the propeller to Start (fully fine) to act as an additional airbrake) but it’s still not that easy to stay in formation, because I have the stick in my left hand and the throttle in my right but the airbrake lever is to my left! After a couple of minutes of constantly swapping hands between throttle, airbrake and stick I decide what is needed is a bit of CRM (crew resource management) and ask Mark to operate the airbrakes upon my command. This works well and Keith soon gets a fine selection of photographs.

With all the photos in the can, I start my examination of the general control and stability characteristics. Controllability is acceptable, harmony and stability are less so. The stick is quite short and the heavy ailerons require muscle at speed, while the elevator is relatively light. This is the wrong way around. The rudder is adequate and the breakout forces for all three primary controls reasonable. Roll-rate is acceptable and visibility in the turn (and indeed every phase of flight) excellent. An examination of the stick-free stability reveals it to be ‘soft’ laterally and longitudinally, and strong directionally. A ten-knot displacement from a trimmed speed of seventy results in a long-wavelength, low amplitude phugoid that eventually damps itself out after several lazy oscillations, while the spiral stability is just barely neutral. As you’d expect with any motor glider, the low-speed characteristics are very benign, and if the stall is approached at the classic one knot per second the 109 just mushes and wallows. Hold the stick on the backstop and it gently hunts in pitch. Pre-stall buffet is negligible. With the airbrakes fully extended it stalls at 47kt, and 41 with them retracted.

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

Optimum engine size

At the start of this article I opined that every aircraft has an optimum engine size, and this is especially true in the cruise. There’s a very useful chart on the 109’s panel which details the various power settings, so I check that the CSU selector switch is in Auto, set ‘24’ (hundreds of rpm) with the rotary knob and then 22 inches of manifold pressure with the throttle. This is Mark’s preferred power setting of forty per cent and the engine sounds very happy. Eventually the ASI indicates 80kt for a true air speed of 86 at 3,000ft and a very impressive fuel flow of 12 lph.

In car driving terms, that’s the equivalent of approximately 38 mpg. The 109 will go faster of course, and if you put the spurs to it and set 28/26 (75% power) the TAS goes up to 100 (at the same altitude) but the fuel consumption is now 18 lph. So, we can see that an extra fifteen per cent TAS requires fifty per cent more fuel flow and a commensurate reduction in range. Using 24/22 the range is a comfortable 500nm with thirty minutes’ reserve.

The only items left on the flight test card are to examine the gliding performance with the engine shut down and prop feathered, and then a few circuits. There is absolutely no vertical movement in the air at all and it’s as smooth as a millpond, but I’m interested to see what the handling is like with the engine off. Having ensured we’re within easy gliding range of Saltby (just in case the engine doesn’t re-start) I reduce power for two minutes to allow engine temperatures to stabilise and trim for sixty knots. Then its power to ‘flight idle’, mag off and slowly decelerate until the propeller stops turning, at around fifty knots, then set the CSU selector switch to Segel (German for sail, or glide).

I quickly begin to appreciate you’d need a good ‘thermic’ day to soar a 109 two-up and full of fuel because of the relatively high wing loading?although I bet its good fun on a strong ridge or in decent wave. The POH claims a best glide ratio of 30:1 at sixty knots, while the min sink is quoted at 220fpm at fifty. Both claims seem slightly optimistic, but then the airframe is forty years old...

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

Now we’re gliding, I’m in ‘glider mode’ myself?and really appreciate the fine field of view provided by the bubble canopy. It is irrefutable that the 109B offers many significant improvements over the A, but changing from a single bubble canopy to twin doors was not one of them.

By now we have lost over 500ft and it’s time to start thinking about either planning a circuit, starting the engine or both. Mark asks if I want to land engine-off as we are in a perfect position to do so but I decline, and for two reasons. Firstly, I want to shoot some touch ’n’ goes, and secondly should some unforeseen incident (such as a walker, horseman or dirt bike rider) choose that moment to transgress upon the runway you’d look such a fool if you’d pranged because you chose to land engine-off with a perfectly serviceable engine! I’m also keen to examine the unfeathering characteristics, which could be another definite gotcha.

Having joined on a base leg that is quite high, but not very wide, I turn on the master, move the CSU toggle switch back to Auto?and wait. And wait. The prop must be completely unfeathered before restarting the engine. However, as I’d suspected, this evolution takes what seems like a long time, and of course as the blades slowly begin to rotate they’re adding drag, and at precisely the most inappropriate moment. (If you were on a ridge, soaring low down and the lift suddenly switched off, you’d be well advised to have a field in mind.) Finally, the small green light in the CSU control panel illuminates to confirm that the blades have reached the selected rpm range, I press the starter button and the engine roars back into life.

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

The powerful airbrakes make precision approaches very easy. Simply close the throttle, trim for seventy and use the airbrakes to control the rate of descent. Sixty over the hedge is nice, and leaves enough energy for a flare and float, particularly if you just ease the brakes in slightly. (Incidentally this is good practice, otherwise you could land with the wheel brakes on). As mentioned earlier the over-sized tailwheel means that the 109’s ground attitude is quite shallow, and care must be taken to avoid over-rotating in the flare, otherwise you may land tailwheel-first. The trick is to just ease the airbrakes in slightly as you flare, hold the (surprisingly flat) attitude and touchdown on the mainwheels and tailwheel simultaneously. Pop the brakes back out when you’re down and you’ll stay down, but just remember the wheel brakes are on the end of the airbrake lever’s travel.

Overall I was favourably impressed with the 109A. The syndicate uses it mostly for touring, but for other gliding club basic training, navigation exercises and particularly field landing practice I much prefer it to the Slingsby T-61 Falke (Venture, in RAF service). The good rate of climb is especially advantageous when practising field landings?an essential skill for glider pilots?in these times of serial complainers. Finally, although most motor gliders can be landed in surprisingly small fields, the real trick is to get them back out again, as they’re often underpowered. With this aircraft, you should also be able to safely get back out again as well!

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

Dirt sniffer?

Many of the early motor gliders demonstrated a marked reluctance to leave the ground. This is a deeply unattractive trait in any flying machine – and some were so underpowered they were rumoured to depend heavily on a little-known device called a ‘dirt-sniffer’. Reputedly having its origins in the early jets (which were also often short of thrust) the dirt sniffer remained inert until it smelt the dirt beyond the end of the runway. Sensing the end of the runway and its own imminent demise induced it to produce a bit more thrust, thus allowing the aircraft to stagger into the air.

Grob 109A flight test

Grob 109A flight test - Credit: PHOTO: KEITH WILSON

Can I fly it on my PPL?

With a significant number of refurbished Grob 109s about to come onto the market doubtless many Pilot readers are wondering what licence and ratings are required to operate these fine flying machines. Well, as with practically everything in UK GA these days – it’s complicated. As the Grob 109 is an EASA TMG (touring motor glider) it can currently be flown on a UK NPPL (SLMG) or a UK PPL(A) with SLMG differences training. However, at the time of writing (an important caveat) it seems that from April 2021 you will need some form of EASA Licence. This could be either in the form of a TMG Rating on an EASA PPL(A) or EASA LAPL(A,) or a TMG Extension on an EASA SPL.

However, as the machines marketed by Aerobility were strictly speaking Vigilant T.1s, it is not impossible that they could be registered as Annex II aircraft by virtue of being ex-military. Called the Grob 109 Able these aircraft will be powered by Rotax 912iS engines and fitted with Garmin avionics.

GROB 109A

Dimensions

Length: 7.88m

Height: 1.68m

Wing span: 16.60

Wing area: 20.4sq m

Weights and loadings

Empty weight: 581kg

Max auw: 825kUseful load: 244kg

Wing loading: 40.4kg/sq m (8.26lb/sq ft)

Power loading: 12.31kg/kW (20.20lb/hp)

Fuel capacity: 80lit | Baggage capacity: 25kg

Performance

Vne: 130kt

Cruise: 100kt

Stall: 41kt

Climb rate: 600fpm

Takeoff over 50ft: 500m

Land over: 50ft 400m

Best Glide: 30:1@55kt

Min Sink: 220ft/min@50kt

ENGINE

Limbach L2400EB air-cooled flat-four, producing 90hp (67kW) at 3,400rpm and driving a MTV composite two-blade constant-speed propeller

Manufacturer: Grob Werke GmbH