Aquila Flight Test

The AQUILA is an exciting new design in the light aircraft category. Dave Unwin flies the prototype.

in: Today's pilot, issue 10, august 2001

Looking out along the flawlessly smooth wing and its instantly recognizable leading edge sweepback, I almost thought I was back in the cockpit of a Schemp-Hirth Discus Sailplane. The outstanding visibility, quiet, comfortable cockpit and well harmonized, responsive controls did nothing to dispel this illusion. In fact, I was actually at the controls of the prototype Aquila, one of an exciting new type of aircraft classification the Very Light Aircraft category. The Aquila first caught my attention during the AERO show at Friedrichshafen, an event that was notable for the number of new VLA designs on show. Due to over complexity within the Joint Aviation Authorities certification process, it is far simpler to certify a new aircraft to VLA standards than JAR Part 23 (Light Aircraft) standards. I next saw the AQUILA (Latin for Eagle) at North Weald during Aerofair and was again impressed by its smooth lines and high build quality. Unfortunately it was not possible to fly the aircraft during the show, so Siegfried and Gregor generously offered to bring it up to Sibson (the nearest airfield to the Today's Pilot office) in order for me to evaluate the Aquila in flight.

The aircraft is the first design from a relatively new company; AQUILA Aviation by Excellence AG. Three German aeronautical engineers, Peter Grundhoff, Alfred Schmiderer and Markus Wagner founded this company in 1995. All graduates from the Akafliegs, between them they had amassed a considerable amount of experience within the German aerospace industry, having worked with several well-known companies such as Stemme and Dornier. After debating precisely what kind of aircraft to build, they decided to focus on creating a trainer. However, they decided that their trainer would also be sufficiently fast and with a respectable and suitable large baggage capacity to double as a tourer. Finally, it would be also suitable for glider towing duties. My initial thoughts, as I looked at the Aquila, were that it really is a very handsome looking aeroplane. From the sharply pointed spinner, generously-sized cabin and elegantly tapered rear fuselage, I thought the Aquila looked very well proportioned. I´ve always maintained that a considerable amount can be learnt about aircraft during the preflight, and one of the first things I noticed about the Aquila is that a considerable amount of attention has been paid to reducing drag. The wings and fuselage are very well finished, indeed I would say that the build quality is comparable to that of a modern high performance sailplane. Gregor told me that the company has designed the aircraft using an integrated CAD/CAM system, while the molds had been produced using CNC milling techniques, resulting in incredible close tolerances and remarkable accuracy. The wing features a triple-tapered planform, which is strongly reminiscent of Schemp-Hirt´s famous Discus. It uses an HQ (Horstmann-Quast) series aerofoil that was especially designed for the Aquila. The spar and all other load bearing structures are constructed from carbon fibre reinforced fiberglass, while the shell structure of the wing is of foam sandwich construction covered with fiberglass skins. Large span electrically-operated Fowler flaps complete the wing. They offer three positions, ´up´, ´take off´ off 15 degrees and ´landing´ of 35 degrees. Overall, I was very impressed by the high built quality, particularly as D-EQUI is the prototype.

Access to the cockpit is via the trailing edge of the wing, and a useful step is provided just aft of the trailing edge of both wings. A good-sized door on the left side of the aircraft provides access to the baggage bay, which is also accessible in flight. The very large front-hinged canopy opens wide, allowing easy access to the cockpit which is surprisingly spacious for a two-seat aircraft. The seats are extremely comfortable and as they adjust over a good range it is possible to quickly make yourself very comfortable. I also liked the four-point inertia reel seat harness and the dedicated headset holders, while another nice touch are the small map holders built into both sides of the cockpit wall.

With seats adjusted, harnesses secured and the big canopy closed and locked, we started the engine. D-EQUI is powered by the ubiquitous 100hp Rotax 912S liquid - cooled flat-four. Because the Rotax produces its 100 horses at the relatively high engine speed of 5,800 rpm, power is conveyed to the prop via 2.43:1 reduction gearbox. This keeps the prop speed down to a much more neighbourly 2,400rpm. This combination of liquid-cooled engine, reasonable low prop speeds and an effective exhaust silencer, ensures a quiet aircraft, and this is precisely what is needed if GA is to survive in the 21st century. I've said it before and I'll say it again. Noisy aeroplanes will not have any future in the General Aviation environment of tomorrow. Already all German airfields levy landing charges based on the aircraft's noise output, while some very noisy aircraft, such as the Cessna Skymaster, are actually banned from many airfields. This is a trend that will not be reversed, indeed, l fully expect that some British airfields will soon start to copy their German counterparts.

As the engine was still quite warm from its flight up from North Weald it started readily. This was simply a matter of turning on the master switch and both magnetos before activating the electric fuel pump for four seconds to pressurise the fuel line. Then, having ensured that the throttle was fully closed with the choke 'off', the starter was activated and the Rotax engine burst into life. With the oil pressure rising the engine idling smoothly at 2,000 rpm it was simply a matter of releasing the parking brake. adding a touch of power and we set off towards the active runway with the geared Rotax emitting its characteristic muted whine. The undercarriage consists of steel struts for the main gear and a spring- suspended nosewheel strut. The nosewheel steers through the rudder pedals and all three wheels are very closely spatted. The nosewheel strut also features quite a lot of side area to the spat, and I imagine that this is to help align the nosewheel with the aircraft's longitudinal centreline in flight. An excellent design feature of the undercarriage is that three relatively large wheels of the same size are used. This means that only one size of tube and tyre needs to be retained in the spares inventory. This is an important point for both the private owner and the flying school operator. Taxiing was delightfully simple, with a fine view over the nose. If a very tight turn is required, differential braking using the toe-mounted hydraulic disc brakes can assist the steerable nosewheel.

Out at the run-up point, the pre-take off checks revealed the first feature that I found unsatisfactory. The pitch trim is adjusted by depressing a small threeposition rocker switch located on a consul between the seats, with the amount and direction of pitch trim applied indicated on a small vertical strip of lights mounted on the instrument panel. I found both the location of the trim switch and the trim indicator itself less than satisfactory, although in fairness I should point out that D-EOUI is the prototype and that minor niggles like this will almost certainly be addressed on production aircraft. In fact, all the system really needs is for the pitch trim switch to be re-located to the top of the control column and for 'nose up', 'nose down' and 'take off' to be clearly labelled next to the indicator lights. The actual trim operation is very efficient. Other pre-take off checks are made to ensure that the fullest of the two fuel tanks is selected for take off and that the electric fuel pump is on. The flaps also have to be set to the 'take off' position and the hydraulic governor for the constant speed unit must be checked to be sure that it is functioning correctly. The flaps are selected by a small guarded switch to the right of the rotary ignition switch, when the flaps have travelled to the pre-selected position this is indicated by one of three small green lights located coincident to the flap switch. A very neat system.

The relatively stubby sticks feature a slight backward crank and fall easily to hand, as do all the other controls and services. I was slightly surprised to note that the rudder cables are unshrouded, although this may well not be the case on production aircraft. A feature that I really didn't like, even on a prototype, was that the controls for the choke, carburettor heat and cabin heating were all identical in both appearance and operation. The carburettor heat control in particular, should be both a different shape and colour from the other two controls, particularly as they are all of the push/pull type.

I thought that overall, the cockpit was quite nicely laid out, although I would also have arranged the engine instruments differently. Just to the right of the avionics centre stack are seven gauges that indicate the health of the engine and electrical systems, plus the Hobbs meter. Personally I would have preferred to see these instruments arranged in horizontal pairs with the fuel pressure and contents at the top of the stack, then oil pressure and temp, amps and volts and finally coolant temp and the Hobbs meter. There are also three annunciator lights mounted directly above the radio. The left light indicates that the canopy is not closed, the middle light is the generator warning indicator and the light on the right side indicates low fuel pressure. Personally, I would have liked to see a light for high coolant temperature as well, but again it must be borne in mind that this is the prototype and that the actual cockpit design for production aircraft has not been finalised. With all the pre-take off checks completed, I lined up on the runway and smoothly opened the throttle. With around half fuel and no baggage we were well below the Aquila's maximum allup weight of 1,654Ibs. I would estimate that our actual take-off weight was probably around 1,500Ibs, which gave us a power-to-weight ratio of 15lbs per horsepower. This, combined with the inherent effectiveness of the constant speed prop to produce gratifyingly brisk acceleration, and I would guess that we used less than half of the grass runway's 2,300ft (700m). As the needle of the ASI slipped swiftly past the 50kt mark, I rotated and the Aquila literally leapt off the ground and quickly settled into a 65kt climb with the VSI indicating just over 1,000 ft/min.

At 500 ft above the ground I clicked the flap switch into the 'up' position and the flaps retracted quickly, with no really discernible change in pitch. Passing rapidly through 1,000 ft, I turned the electric fuel pump 'off' and swept the Aquila through a graceful curving turn and onto a southerly heading. At 2,000 ft I levelled out and set the Aquila up for highspeed cruise. Now, when I was a lad I was told that when flying an aircraft with a constant speed prop I should always 'rev up and throttle back'. Basically, always increase propeller rpm before opening the throttle and always reduce the throttle setting before reducing propeller speed. Furthermore, another hoary old maxim that was drummed into my brain was that it should always be 'prop on top'. This meant that for any given power setting, the manifold pressure (in inches of mercury) should not exceed the rpm in hundreds. Indeed, for many engines it was deemed entirely inappropriate for power settings in excess of the so-called 'squared' power settings (i.e. 2,400 rpm and 24 inches manifold pressure) to be used. In fact, as with so many areas of aviation, these are myths that have been carried over from days of yore. It is true that, particularly with older radial engines, it could be imprudent to exceed a 'squared' power setting. This was because very high manifold pressures could induce excessive bearing wear. These days, however, improvements in engine design, the use of better metals and also the introduction of modern lubricants, mean that with a contemporary opposed engine it is perfectly possible to use high manifold pressures alongside low propeller speeds.

Therefore, to set the Aquila up for highspeed cruise it was simply a case of leaving the throttle setting where it was and then adjusting the propeller lever until we had 2,100rpm with a manifold pressure of 27 inches. After adjusting the trim, the Aquila soon settled into a 120 kt cruise for a fuel flow of around four and a half gallons an hour. I briefly removed my headset to assess the ambient cockpit noise and am pleased to report that it was entirely acceptable. A couple of 360 degree turns and steep reversals revealed absolutely impeccable handling. With the sole exception of the rudder, the controls are all actuated by push rods, consequently the Aquila certainly has a very crisp feel about it. Both control response and harmony were entirely satisfactory, while the superb visibility really does make you feel as if you are flying a little fighter. Obviously, sitting under such a big canopy does mean that you really catch the sun, and I was glad that two generous vents supply fresh air to the cockpit.

Slowing down to assess the low speed end of the flight envelope revealed that, as I'd expected, the Aquila really is very slippery as it took a while to reduce speed. The electric stall warner was not functioning on the prototype, although in fact there is actually sufficient aerodynamic buffet to render it superfluous to requirements. Whether flaps are up or down, as the stall was approached, the turbulent airflow separating from the wing, buffets the cruciform tailplane with sufficient force to tell the pilot that something is clearly not right, and that unless something is done the aircraft is going to stop flying. The actual stall occurred at about 44kts and was accompanied by a very mild wing drop that was easily contained by the powerful rudder. As the Aquila is not approved for intentional spinning (it is not allowed under JAR-VLA certification) we stayed away from that particular corner of the flight envelope. However, I doubt that it would display any particularly nasty traits in that department. The rudder is of a good size and has a generous arm through which to apply itself.

The next item on the agenda was a couple of circuits, and as we were nearer to Conington than Sibson a quick call on the radio confirmed that they were more than happy for us to make use of their runway. As I've already observed, the very smooth fuselage and laminar flow wing have blessed the Aquila with near sailplane-quality aerodynamics, and this point was reinforced on the first circuit. A combination of turning in slightly too early plus the presence of a large thermal left me way too high on final approach, despite the Aquila's excellent Fowler-type flaps, A steep side-slip soon put us back on the glide slope, and a smooth flare saw me gently rolling the wheels onto the runway just past the numbers. Full power came back in very smoothly and we surged back up into the sky for another circuit. This time I deliberately extended the downwind leg slightly and extended full flap and selected high rpm that little bit earlier. Result - an easily controlled approach, leading to another very smooth touchdown. Probably the best part of being the Editor of Today's Pilot is all the wonderful aircraft that I get to fly. Obviously, one of the worst parts is that sooner or later I have to give them back! With a 120kt cruise speed the transit time back to Sibson was very short and I was soon lowering the Aquila gently onto Sibson's well-mown grass runway. The Aquila was scheduled to appear at Staverton later that day, so as Gregor and Siegfried prepared to leave, photographer Duncan and I wandered down to the departure end of the runway to assess for ourselves if it really was as quiet as they claimed. The measured noise output is said to be - 17 decibels below the already very stringent German limit, and as the Aquila soared past, both Duncan and I agreed that it really was very quiet.

Indeed, it seemed to be only producing about half as much noise as the Cessna 152 that preceded it. In conclusion, I have to say that I was very impressed by the Aquila. Students will love it for its docility and easy handling, while private owners will appreciate the generous baggage area, large comfortable cockpit and high cruise speed of 120kts IAS. While I doubt that 100hp will prove sufficient to tow large sailplanes from unpaved surfaces, I think that it will easily cope with lighter sailplanes, particularly if they are towed from hard-surfaced runways.

Currently the prototype - which has already logged 350 hours - is the only Aquila flying. However there are three more under construction and 19 aircraft have already been ordered. with the first deliveries planned for August. I think that the Aquila will become quite a common sight in the skies over Europe during the next few years.

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