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Grappling a Grob

With the Royal Air Force’s new Military Flying Training System about to come into effect in the next couple of years, key.aero tries out the military’s basic trainer, the Grob 115E Tutor, also popular with private pilots.

13-Aug-2009


All images Key - Steve Fletcher.

The Tutor is certainly a handsome-looking aeroplane. Selected by the UK Ministry of Defence to replace the Bulldog with both the Royal Air Force and the Royal Navy’s Fleet Air Arm (which calls the type the Heron), the prototype Grob 115 first flew in November 1985.

However, early flights revealed a number of design deficiencies and when the second prototype flew the following year it had been substantially redesigned and featured a taller fin and rudder as well as a relocated tailplane.

Build quality appears to be very high. The aircraft is constructed almost entirely from composite materials, primarily carbon-fibre reinforced plastic (CFRP). The fuselage is quite interesting in that it is of a semi-monocoque design constructed from two vertically split halves. Aerodynamically, it is very clean, with the only anomaly being a relatively large ventral strake under the fin. The wings and tailplane are of the ‘sandwich’ type, with a honeycomb core covered with CFRP skins, while the flying controls are made from rigid foam covered with fibreglass.

An ‘Eppler’ aerofoil section is used for the mainplane while, rather curiously, a NACA section is used for the tailplane. Only the elevator trim can be adjusted in flight, although both the ailerons and rudder feature ground adjustable trim tabs. The electrically actuated flaps are of the plain type.


The three-blade Hoffman constant-speed prop is spun by a 180hp Lycoming AEIO-360-B1F air-cooled flat-four.
Power is provided by the ubiquitous 180hp Lycoming O-360 (actually an AEIO-360), which is fitted with a Christen inverted oil system and spins a three-blade Hoffmann constant-speed prop. Fuel is contained in a pair of integral wing tanks, which have a total capacity of 33 Imperial gallons, with the engine being fed from a 1.2 gallon collector tank. The aircraft is approved for up to three minutes of continuous inverted flight. A tricycle-type undercarriage is fitted, with the nosewheel connected to a gas-damped strut, while the mainwheels are suspended from a cantilevered spring-steel arrangement. As I was to discover, the Tutor’s undercarriage is on the firm side and (in common with other aircraft fitted with a spring-steel undercarriage) is not particularly well-damped. All three wheels feature relatively close-fitting fibreglass wheel spats.

One of the flying instructors at Headquarters, Elementary Flying Training Services unit at RAF Cranwell, had generously brought an aircraft down to Sibson for a flight test. Unlike purely civilian aircraft, which generally only require a headset to fly, as the purpose of EFTS is to train pilots for the military their aircraft need more equipment to fly. I got into the flying suit, adjusted the helmet to fit and strapped the parachute on. Up until this point, the Tutor had been making a very favourable impression.

The walkround had revealed a sturdy, well-made airframe, while my initial look inside led me to believe that the cockpit and instrument panel were equally well-designed. However, my pilot suddenly produced three different coloured cushions of varying thickness! Although the rudder pedals are adjustable, the seats aren’t. Call me a bluff old traditionalist, but in my humble opinion, I really don’t think it is that difficult to design an adjustable seat! If I bought a car I would expect the seats to adjust, so it beggars belief that a modern, certificated aircraft does not offer the same facility. I’m fully aware that non-adjustable seats can be made that much stronger, but I still think that this design was far from satisfactory. The different sized cushions (for both seat back and seat pan) are colour coded and along with the adjustable rudder pedals offer a very wide range of adjustment.


The instrument panel is very well laid out; with the primary flight instruments arranged in the time-honoured ‘Basic T’ directly in front of the pilot, along with the tachometer and combined manifold pressure/fuel flow gauge. The left side of the panel contains an altimeter and artificial horizon for the instructor, a VOR/ILS and GPS plus the various engine monitoring gauges. The curved control sticks are topped with comfortable pistol grips and fall nicely to hand, as do the engine control quadrant levers for throttle, propeller and mixture. These are mounted in a neat central console, which extends aft from the base of the instrument panel back between the seats. Apart from the power control levers the console also carries the flap switch and its co-located indicator, the elevator trim wheel and its adjacent indicator, the knob for alternate engine air, the three -position fuel valve and also the fuel on/off cock.
That criticism aside, I thought the rest of the cockpit was excellent. In keeping with the military philosophy, the cockpit is configured to be flown solo from the right seat, so having slid the large sliding canopy fully aft – and installed the correct cushion – I dropped down onto the right seat and began to strap myself in. The five-point harness is very good, possibly the best I’ve ever seen in an aircraft in this class, and having strapped myself in I adjusted the rudder pedals via the knurled wheel mounted in the floor. I then began to assess the overall layout of the controls and instruments; the curved control sticks are topped with comfortable pistol grips and fell nicely to hand, as did the engine control quadrant levers for throttle, propeller and mixture. These are mounted in a neat central console, which extends aft from the base of the instrument panel back between the seats. Apart from the power control levers the console also carries the flap switch and its co-located indicator, the elevator trim wheel and its adjacent indicator, the knob for alternate engine air, the three -position fuel valve and also the fuel on/off cock.

An interesting anomaly was that the flap switch has four pre-selected settings: (0), TO (15°), Land (45°) and Full (60°). I found this rather curious, as almost all aircraft in this category land with full flap. Two clues indicated that, despite its ‘G’ registration, this aircraft normally operates in a military environment – namely there was a small UHF radio tucked away at the bottom of the panel below the avionics stack and there were no keys. I thought that the instrument panel was very well laid out; with the primary flight instruments arranged in the time-honoured ‘Basic T’ directly in front of the pilot, along with the tachometer and combined manifold pressure/fuel flow gauge. The left side of the panel contains an altimeter and artificial horizon for the instructor, a VOR/ILS and GPS plus the various engine monitoring gauges.

Two features that I definitely approved of were the ‘needle and ball’ type Turn and Slip (better, in my opinion, than the Turn Coordinator that is more usual these days) and also the parking brake, which was easy to both reach and operate. This may sound trivial, but I’ve flown at least two aircraft with awkward parking brakes. The cockpit is sufficiently wide to avoid constantly rubbing shoulders with the other occupant and the generous baggage bay behind the seats is accessible in flight.

So far so good, but as the large canopy slid shut I suddenly became aware that there was a considerable blind area caused by the combined thickness of the arch of the canopy bow and the windscreen frame. Grob has a great many years of experience working with composite materials and I just couldn’t help but feel that neither the canopy bow nor windscreen frame needed to be quite so thick. Conversely, I thought the canopy locking latch was excellent. It is easy to operate and also obvious when the canopy is locked.

Engine start was the usual fuel-injected Lycoming procedure, and we were soon taxying out. The Tutor promptly revealed itself to have very pleasant ground handling characteristics with excellent visibility and powerful progressive toe-operated hydraulic disc brakes. The nosewheel steers through the rudder pedals and can be turned nine degrees either side of neutral, although if differential braking is used for a tighter turn, the nosewheel castors up to 47° each way.


A NACA aerofoil section is used for the tailplane. Only the elevator trim can be adjusted in flight, although both the ailerons and rudder feature ground adjustable trim tabs.
Thus far, the Tutor had seemed to be entirely conventional in both construction and operation, but as we taxied out to the run-up point, I was surprised when my pilot announced he would lean the mixture prior to take-off. Directly in front of him is a small table that indicates what the fuel flow should be at maximum power for various altitudes, and as Sibson is only 100ft ASL, we used the figures for sea level, which equated to a fuel flow of 55 litres per hour.

With the parking brake on and prop set for max rpm the throttle was smoothly opened up to 25 inches of manifold pressure and then the mixture slowly leaned until the gauge was indicating the requisite fuel flow. With the rest of the checks completed and the flaps set to the take-off position, I taxied out onto Sibson’s runway 07 and opened the Lycoming up to full power.

Acceleration was good but not great, although, to be fair, as the fuel tanks were full we were only about 30lbs below the maximum take-off weight of 2,182lb. A reasonable amount of right rudder was required to track the centreline and as the needle of the ASI swept through 60kts a touch of backpressure on the stick lifted the Tutor off the ground and into the air.

The airspeed increased rapidly towards the Vy of 80kts, at which point I retracted the flaps and eased the stick back to hold 80 with the VSI indicating just over 1,000ft/min. Having turned towards the south in order to keep clear of the busy Wittering Military Air Traffic Zone, I continued the climb up to 6,000ft, as I was keen to try my hand at aerobatting the Tutor. But first I needed to explore some of the Tutor’s basic handling characteristics. Stability around all three axes was good, with the longitudinal stability in particular being very positive. Control around all three axes was equally satisfactory, with light, powerful ailerons, a slightly heavier though no less authoritative elevator and an effective rudder. Control harmony was also spot-on, with the ailerons being the lightest control and the rudder the heaviest.

I was slightly surprised at how little rudder was required in flight, but then realised that there is an interlink between the ailerons and the rudder. Pushrods are used for the elevator and ailerons – consequently (and unlike designs which use cables for the primary controls), there is no ‘slop’ in the control circuit at all. This is one of the great advantages of using pushrods over bellcranks and cables, and is almost certainly a by-product of Grob’s long association with sailplane design and manufacture.

A look at the cruise performance came next, so I set the recommended 19 inches of manifold pressure with the throttle and then eased the prop lever back to 2,500rpm. The indicated airspeed quickly settled on 100kts, for a fuel flow of just over nine gallons an hour. Frankly, I was a little bit disappointed by this, as (on paper at least) I felt that a modern composite airframe fitted with a laminar flow aerofoil and powered by a 180hp engine turning a three-blade constant-speed prop should really do better. However, the Tutor’s canopy is fairly tall (in order to accommodate the ‘bone domes’ worn by the instructors and students) and this must increase the drag. While we’re on the subject of the canopy, I have to report that the large blind area caused by the design of the windscreen frame and canopy arch continued to make itself felt. I thought that this was a great shame, as overall the visibility is excellent and only marred by something which probably could have been avoided.


Control around all three axes is very good, with light, powerful ailerons, a slightly heavier though no less authoritative elevator and an effective rudder. Control harmony is also spot-on, with the ailerons being the lightest control and the rudder the heaviest.
A look at the slow end of the speed envelope revealed no surprises, with the Tutor exhibiting exemplary stall characteristics. Even with the flaps fully retracted there is plenty of pre-stall buffet, while with full flap the airframe really shakes! The difference in the stall speed with flaps fully extended was only about four knots slower than a clean stall, and the ailerons remained effective even deep in the stall. The Tutor’s spin behaviour was equally benign, with recovery from a two-turn spin beginning almost immediately spin recovery action was taken.

Moving swiftly on to the aerobatic part of the flight, my pilot suggested that he demonstrate each manoeuvre first, and that I should then try to replicate that manoeuvre. We started off with a loop, which I managed to copy quite successfully, before moving onto a barrel roll. After a reasonable reverse Cuban and a passable half Cuban, we just had time for a pair of flick rolls before returning to Sibson for a couple of circuits. All the circuits were essentially similar and concluded with respectable landings. The Tutor is easy to trim and is very speed-stable. I flew at 80kts on the downwind leg and 70 on final, bleeding back to a ‘last look’ speed of 65, with the flaps set to the ‘land’ setting of 45°.

Landing with full (60°) of flap, instead of the recommended setting of 45°, produced a slightly shorter flare, although it didn’t seem to make as much difference to the landing as I’d predicted. The rather stiff-legged undercarriage is certainly no flatterer, and I failed to produce a real ‘greaser’, although all the landings were perfectly acceptable.

In conclusion, I think that the Tutor is an excellent trainer. It possesses reasonable performance and excellent handling, combined with a relatively low-maintenance airframe and large, comfortable cockpit. There are a few areas where improvements could be made, most noticeably in terms of the visibility, as the blind spot caused by the over-large canopy arch and windscreen frame is definitely sub-standard. Overall though, I was quite favourably impressed with the Tutor, and wouldn’t mind owning one!

Extracted from an article first published in Today's Pilot magazine January 2003


Filed Under Military Aviation Features, General Aviation Features.

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Lego said on the 4-Sep-2011 at 15:07

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