Silence Aircraft

Silence - The Micro light Spitfire

Jochen Ewald flies this elegant German micro light.

A Report in "Today's Pilot" Jan 2003

Have you ever dreamed of flying a Spitfire? If the answer is 'yes', you should take a look at the new Silence, manufactured by Silence Aircraft in Verl near Bielefeld, Germany. With this aircraft, you get all the fun and good looks of the Spitfire on a micro light budget!

When work began in 1996, the Silence team set a high standard for their first aircraft, originally intended to become their one-of-a-kind private aircraft. The planned single-seater had to look attractive, feature excellent aerodynamics, be very light in weight and easy to manufacture, and offer plenty of safety features. To keep the required amount of certification time and expenses low, the company decided to certify it in the micro light category, though it also had to meet certification standards for 'real aircraft' in order to retain the option of certifying it as a homebuilt 'experimental'.

The maiden flight of the prototype, equipped with a 65hp Wankel Rotary engine took place in autumn 2000.

At the Aero Trade Fair 2001 at Friedrichshafen, the Silence was displayed in a corner of one of the halls not for sale purposes but simply to show how modern materials could be constructed in a homebuilder's workshop. The great public interest shown in this design finally persuaded the team to continue optimizing it towards serial and kit production, and to found a manufacturing firm. On September 4, 2002, the first Silence of the serial production version took off for its maiden flight, this time powered by the Mid-West AE 50 Harrier 50hp rotary engine, which weighed 77lb less than the engine used in the prototype. The Harrier engine is JAR-22 certified and also powers the Schleicher retractable engined motor gliders.

The fact that the shape of the Silence is so reminiscent of the famous Spitfire is for good aerodynamic reasons the simple, elliptical wing offers nearly optimum lift distribution, the smoothly rounded fuselage promises low drag and, at the same time, the whole thing looks attractive! The main building material Nomex (aramide fibre) honeycomb plates, sandwich-covered with GRP has not often been used in light aviation. Though not particularly cheap, this material does have some clear advantages. It is possible to build strong, stiff components, uniting high surface quality and strength with low weight. The use of glass fibre as a covering sandwich layer results not only in good protection against damage, but makes the aircraft easy to construct, simple to repair and straightforward to inspect for damage. Additionally, this material absorbs a great deal of energy in case of a crash. For high load-carrying internal structures, such as the wing spars or engine carrier, carbon fibre rovings are used, well protected against exterior damage. The highly energy-absorbent Aramid fibres are used for those parts, such as the inner cockpit shell, which protect the pilot in the case of a crash.

To keep the building costs as low as possible, the wing and elevator halves have a fully symmetric airfoil, so just one mould each is needed to manufacture both the upper and lower shells of these parts. The lightness and strength made possible by this method of construction can be seen from the weight of the wings including the 8.8 Imp gal integrated fuel tank, flap and aileron, they weigh only 36lb each!

The elegant fuselage shows several clever refinements. The front section hosts the compact Mid-West rotary engine, which is installed from the top between the carbon fibre engine carriers. The upper cowling, with its shark-like cooling gills, is held by two spring steel wires, which are pushed in from the front side and locked by snapping them behind a safety plate. Beneath the engine, the air intake and exhaust section is covered with a removable cover. In between engine and cockpit is a ballistic recovery system, mandatory for micro lights in Germany. In an emergency, it fires upwards through a lid held on only by clear adhesive tape. The cockpit itself forms a modern safety cell, the basic idea of which has been borrowed from Formula 1 racing car designers. Into the outer honeycomb structure of the fuselage, an Aramid fibre cell is fitted. The carbon fibre seat-pan with its stiff backrest to protect the pilot's spine can be removed to enable the control systems to be reached. Behind it, in the rear fuselage tube, a baggage compartment can be fitted, its loading capacity limited by the centre of gravity, depending on the pilot's weight (he sits behind the C of G). The pilot is strapped to the safety cell by a modern four-point-harness, and the cell is directly fixed to the BRS. This is a very safe cockpit, professionally manufactured.

The electrically retractable main undercarriage is hinged at the fuselage and rotates backwards around a diagonal axis. While the 'legs' remain outside at the wing root bottom, the wheels with their hydraulic disc brakes disappear almost completely into wells which are half in the wing and half in the fuselage. Doors screwed to the legs cover the wheels when retracted. The small covered tail wheel is double-sprung a spring acting telescopically in the fuselage takes the vertical loads, and a rubber block just above the wheel allows it to swing backwards when small obstacles on the ground are hit.

The control system is simple and well-thought-out. Elevator and ailerons are operated by pushrods, the rudder by cables and the flaps via a torsional drive by an electric motor in the fuselage. The spring elevator trim is also electrically actuated. Like a glider, the Silence can be rigged and derigged for hangaring or transport in only ten minutes. Rigging is simple, and, if the fuel tanks are empty, may be carried out by only one person. Once the wings are slid into their slots in the fuselage, one eccentric bolt for each wing underneath the seat pan serves to pull the wings in by swinging the bolt's lever behind the securing spring metal sheet hook. The torsional drive of the flaps connects automatically during assembly, while the aileron pushrods have to be connected to the lower end of the stick by bolts with securing Fokker safety pins. The fuel hoses, the wires of the fuel indicators of both wings and the pitot pressure hose from the right wing are connected by reaching through the undercarriage openings below the fuselage. Finally, the tail plane halves are slid onto the aluminium tube spar ends at the fuselage and secured by a spring steel wire pushed in from behind, the elevator connecting automatically via a hexagon head torsional link. If the rudder has also been taken off, it is inserted from above and secured by two Fokker pins. A look underneath the cowling to check whether there is enough cooling liquid and oil in the reservoirs (the Mid West Harrier uses about 2.5% of the fuel consumed as 'lost greasing oil'), before refuelling the tanks, and the Silence is ready to fly.

The cockpit is easily entered from the front side by stepping onto the tire and then the narrow fuselage section of the wing root. After pushing forward the canopy locking lever at the outside fuselage surface, the canopy can be swung open towards the right side. The pilot then slides easily into the well-upholstered, ergonomically-designed, but non-adjustable seat: there is enough space here even for tall pilots. A head-rest had not yet been installed on the one I saw, but this can easily be done. The rudder pedals can be adjusted separately by turning two knobs in the instrument panel: the designers are considering connecting these two knobs together to avoid asymmetric settings. The cockpit's large instrument panel is well equipped. The BRS handle at the front frame is within easy reach and can be secured with a lock. At the left cockpit wall, there is one small fuel cock. In spite of the fact that there are two tanks, one fuel cock is enough because the tanks are inter-connected.

The engine takes the fuel from the left tank, which has no ventilation and is refilled from the ventilated right tank. This system is very simple, but comes with the disadvantage of asymmetric wing loading during flight. However, as the tanks are close to the fuselage, this is not expected to greatly affect the flying characteristics. Nevertheless, the manufacturer plans to install separate fuel cocks in later aircraft to preclude the possibility of asymmetric wing loading. The throttle is placed in the spot where you would find the pilot's left hand in a relaxed position. The two buttons for trim operation and also the transmitter button are placed on top of a fighter-like control stick. The trim setting is indicated by an electronic instrument. A knob in the instrument panel serves to set the flaps to 0, 10, 20 or 30, and a covered switch besides it operates the undercarriage. In case of a failure of the electrical system, there is a red emergency extension handle between the pilot's legs. If this handle is pulled, the undercarriage is disconnected from its spindle drive and then pulled out, assisted by gravity, until it is in the 'down and locked' position. The status of the undercarriage (up, in transit and down) is indicated by LEDs, other LEDs warn if the engine is operated outside its normal parameters, although there are also conventional instruments fitted.

When I test-flew the Silence V2 at Bielefeld-Windelsbleiche airfield on September 5, 2002, it was not yet in final serial condition in fact, it had only made its maiden flight the day before. The wheel covers for the undercarriage legs were not yet fixed in place and the propeller was still a shortened, fixed two-blade Helix unit. The planned three-blade electrically adjustable Silence propeller, already used on the first prototype where it rotates right-hand, was not yet available in the left-hand rotating version needed for the Mid-West. Finally, the engine power was not yet fully available, and resonance problems still required some adjustment to the air supply system.

On closing the canopy, it can be locked by pushing backwards the small knob on the left cockpit wall. The main switch and starter function are united in an ignition key lock similar to that on a car. After switching the key to 'ON', the two ignition switches and the two fuel pump switches beside the key are switched on. With the Mid-West engine nothing runs without fuel pressure, so the second pump is a mandatory safety feature for take-off and landing. With the throttle set to a little above idle, the engine starts immediately on turning the key to the 'start' position. It runs a bit rough below 4,000rpm and only becomes smooth above 4,000rpm typical behaviour of one-disc rotary engines. To protect the ears from the relatively high frequency engine noise, a good headset is recommended. The time needed to warm up is short, and after checking the ignition and fuel pumps, the Silence is ready for take-off. During taxiing I found the separate toe brakes effective and easy-to-operate, and the tail wheel, spring-connected to the rudder, easy to steer. By using differential braking, quite tight turns are possible. My eyes were just at the height of the narrow front cowling tip, so I did not need to taxi in a zigzag to get a decent view.

This Silence weighs exactly 441lb empty. With me in the cockpit and 11 Imp gal of fuel (standard 92 oct car fuel), it weighs 683lb: even with full tanks I would still be below the maximum permitted 750lb. The centre of gravity is in the forward range. For take-off, I turn the flaps knob to 10 and set the trim to neutral. After pushing the throttle forwards until I reach 1cm behind the front stop (because of the resonance problem mentioned earlier, the engine starts coughing with more throttle), I accelerate quickly with the stick held slightly forward.

As soon as the tail wheel lifts off, I need a small amount of left rudder to compensate for the propeller torque. At 43kts it lifts off, and I let it accelerate to 60kts before starting to climb. At a safe height I retract the undercarriage, set the flaps to 0 and switch the second fuel pump off. In this clean configuration, I find the best climb rate at 65kts, and although the engine performance is not fully available it climbs at about 790ft/min. With the engine problem solved, I expect the aircraft to climb in excess of 1,000ft/min. The heavier prototype with the 65hp engine which I flew in the spring climbed at nearly 1,180ft/min. Trimmed to the best climb speed of 65kts, only a small amount of left rudder is needed to keep it flying straight. On reaching 2,500ft, the engine's three minutes of permitted full power operation have passed and the cooling liquid temperature comes close to its maximum. I reduce the power to the maximum continuous setting of 7,100rpm and continue climbing at 70kts, with the result that the temperatures remain in the green arc.

During cruise flight, this rpm limit is already reached at 86kt with the current Helix propeller. The prototype, equipped with the Silence-developed three-blade variable pitch propeller, reaches more than 108kts with no problems. In this aircraft, and probably also the Mid West-powered ones equipped with the new propeller, the 17.6 Imp gal of fuel makes possible 620-mile flights at a cruise speed of around 100kts.

I am particularly impressed with the handling of this little aircraft. The control forces remain very low, though positive, for just about the whole speed range. The Vne of the 'Experimental' version is calculated as 162kts, though it is only 140kts for the micro light version due to the limited operating speed of the BRS. The sensitivity of the Silence gives the impression of flying a fully-aerobatic aircraft, and trimmed to cruise speed it flies beautifully stable in the 'hands off' position. However, if the right fuel tank is empty and the left one full, a small amount of aileron deflection is needed to keep the wings level.

These excellent general handling characteristics are further supported by the excellent view through the big canopy and the good fresh air supply through the nozzles in the front cockpit frame. Cabin heating is also installed, though in the current warm weather I feel disinclined to test its efficiency. The only thing I miss is a small 'direct-vision' panel in the canopy I once experienced outside cabin icing when descending with a cold glider through an inversion layer into warm, humid air, and opening this window then saved me from being 'blind' and lost... How does such an elliptic, fully-symmetric wing behave when it is stalled?

I was quite astonished at the gentleness of the stall. With the engine running at idle and the flaps at 0, the Silence's controls start feeling soft at 43kts: at 40kts the stall starts buffeting and speed increases to 43kts again in the staggering stall, where it can be held by using the rudder. With the elevator further back, staggering increases and I start feeling as if I am sitting on a wild horse, though I can still prevent it from departing into spin with the rudder. I think this effect is caused by the turbulence separating behind the wing root (which was probably stronger than usual due to the uncovered retracted wheels underneath the wing) hitting the tail. With its forward C of G position, the Silence starts dropping the nose, but immediately the airflow comes back to the tail plane, the game starts again.

Setting the flaps results in the same behaviour, with the minimum indicated speed decreasing by about 2kts per 10 of flaps set. With full throttle, the indicated speeds are again 1kt lower. The ASI of this aircraft had not yet been calibrated and seems to over-indicate slightly with the prototype, a calibrated airspeed of 35kts at 750lb AUW has been demonstrated during the certification. When I flew this prototype with an aft C of G and the wheel covers fitted, the staggering was less pronounced. It did try to drop a wing, although the incipient spin could be stopped immediately by relaxing the stick and applying opposite rudder. The Silence shows excellent control harmony. With the engine running at idle and trimmed to 70kts, it raises the nose and starts climbing at 65kts once the throttle is opened fully. Setting the flaps (below 65kts) to different positions does not really influence the trimmed speed. Its roll rate is impressive: when full rudder and aileron are applied, even at the slow speed of 59kts it rolls from 45 to 45 bank within 1.5 seconds without side slipping. This roll rate is practically aerobatic, and care has to be taken not to stall the lower wing when applying full aileron and not enough rudder against the rolling direction. But even in such a situation the Silence is straightforward a kick into the rudder against the rotation direction stops the initial snap roll immediately.

The manoeuvring speed, up to which full control deflections are permitted, is 97kts, and the maximum allowed speed to fly through stronger gusts is 119 kts, clearly above cruising speed. Compared to other fast micro lights, in which you should cut your flying speed considerably when flying on a day with strong thermals, this gives a good margin of safety. Of course, these flying characteristics may tempt some to try a few aerobatics. However, the rules of the micro light category set clear limits aerobatic are generally prohibited in these aircraft. Anyone with ambitions in the direction of aerobatics could always buy the strengthened kit Silence Aircraft offers. If the builder completes it at home and certifies it as an aerobatic aircraft in the 'Experimental' category, there are no limits to gambolling around the sky (and probably outclassing the Tipsy Nippers and RF-4s you can hardly ever find in the second-hand market...).

The electric flap drive acts rapidly. Within three seconds the flaps reach their maximum of 30: the undercarriage needs 7.5 seconds for retraction and 9 seconds for extension. The maximum speed in landing configuration with the flaps set to 30 is limited to 65kts. Under calm weather conditions, I find an approach speed of 56kts comfortable and perfectly safe. Before landing, I switch the second fuel pump on again. The flaps are effective, and with the engine running at idle it is easy to control the glide path. Side slipping is very easy and effective, permitting very steep approaches and further improving the pilot's (already good) view of the runway it also feels as if it's the right way to land an aircraft resembling a Spitfire!

Provided you stopped side slipping after holding the Silence off until the nose was at the correct attitude, and that you centralized the rudder before the wheels touched the ground in a proper three-point attitude, the Silence will land gently. If you do not follow the advice given above, it will 'dance'

a little on the runway, due to the un-damped, but sufficiently sprung undercarriage. Even a strong or gusty crosswind causes no problems, though some tail dragger experience is useful to produce a good landing on a concrete runway (and to keep the aircraft there afterwards!) For new Silence pilots, I would recommend making your first few landings on a more forgiving grass strip. The rudder has to be used carefully during the ground run the separate toe-operated hydraulic disc brakes are easy to use and quite powerful. As the main undercarriage is well in front of the C of G, the risk of nosing over is relatively low.

As a single seater of the upper micro light class, the Silence is not for everyone, and it does require the pilot to have a certain level of tail dragger experience. Flying it is enormous fun as it offers a high level of performance combined with excellent safety features. The Mid West 'Harrier' single disc rotary engine suits its slim appearance, though it consumes a relatively large amount of fuel and is not the smoothest of engines, particularly at low rpm. With this in mind, Silence Aircraft is considering equipping the aircraft with the 75hp Walther Minor IIIB air-cooled in-line four cylinder engine. Used as a micro light, the Silence's operational range is fairly limited by the definition of this aircraft category however strengthened versions certified in the 'Experimental' category, built at home from a pre-fabricated kit, may be used for aerobatics and show the full range of this design's abilities. The great interest that these first 'little Spitfires' attracted whenever they appeared demonstrates clearly that a large market is out there waiting for the Silence!

On reaching 2,500ft, the engine's three minutes of permitted full power operation up to 7,750rpm have passed, and the cooling liquid temperature comes close to its maximum. Reducing the power setting to the permanently permitted 7,100rpm and continuing climbing at 70kts results in the temperatures staying in the green arc. Right Straight from the box the kit is well-produced and has the benefit of considerable prefabrication.

specifications
 
powerplant
propeller
length
height
wing span
wing area
seats
empty weight
useful load
gross weight
fuel capacity
range
80 HP JABIRU 2200
2.blade GT propeller
x
x
x
x
1
500 lb.
x

880 lb
21 US gal
800
nm

performance

takeoff distance, ground roll
rate of climb
max speed
cruise speed
landing distance, ground roll
service ceiling
x
1500 ft/min
x
127 kts
x
x

limiting and recommended speeds

design manoeuvring speed (Va)
never exceed speed (Vne)
stall, power off (Vsl)
landing approach speed
135 kts
196 kts
41 kts
x

All specifications are based on manufacturer's calculations