A few months ago I described in Microlight Flying how I first became acquainted with this exciting single-seater from the Czech Republic, having seen it at the Tannkosh event in September 2013, well before the changes in our SSDR rules had been finalized. As we now know, during the first half of 2014 those rules were reviewed with the CAA, with the BMAA taking a leading role, and this led to a new definition being agreed.  The final determinants were quite simple – single seat only, maximum take off weight 300 kg and stall speed not greater than 35 knots.  There was no continuing stipulation about wing loading, a factor which had held back development of fixed-wing SSDRs under the old regime, as the previous limitation had the effect of only allowing large and lightly loaded structures which were not well suited to typical British weather conditions.

 

As previously reported, in late Autumn 2014 I visited Igor Spacek, the designer and manufacturer, at his installation in Hodonin to take a closer look at the aircraft and to discuss the various options. The aircraft is available in several kit versions – a large number are under construction in Europe but I had already decided not to build one, so my focus was on buying one built in the factory. It was fairly clear that there would be quite a long lead time for this and this discussion led to an agreement for me to buy Igor’s factory demonstrator OK QUR 01.  The aircraft was delivered to me in November by road and registered as G-CIMA.

 

This background is important to the context of the flight test, as there are a number of configuration and engine options, and when one buys any second-hand aircraft it’s clear that those options are not going to be available.

 

G-CIMA is a taildragger version and is powered by the Kohler CH750 engine, equipped with a Sandera 1200 composite ground adjustable two-blade propeller. There are three main airframe options, the tail dragger, which is the lightest, a steerable nosewheel and castering nosewheel variants. Both the tail dragger and the castering nosewheel variants are fitted with heel brakes to each of the main wheels, which are equipped with small cable operated drum brakes. The cable operated brakes and rudder pedals can be adjusted to suit the leg length of the pilot. The steerable nosewheel version usually has drum brakes on each main wheel, operated by a single lever. On the engine side, the CH750, for which 31hp is claimed, is positioned in the centre of the power range. At the bottom end is the 24hp Briggs and Stratton and the most powerful option is the two-stroke Hirth F23 for which 50hp is claimed.

 

I’m aware that the two most popular engines have so far been the Kohler CH750 and the Hirth F23. Kohler is a large producer of industrial engines in the US, typically for use in industry and agriculture. They are robust and fairly low revving air-cooled wet sump engines. There appears to be good support in the UK for them. Hirth engines are well known in small aircraft installations and the F23 is a flat-twin two stroke.  The Kohler is quite heavy and the Hirth offers a much better power to weight ratio but uses nearly twice as much fuel – more of this later. The standard tank size of the Minisport is 28 litres, so consumption can be a factor, but G-CIMA appears to use fuel at less than 5 lph which is in line with expectations.

 

The flight test was conducted from my home strip at Whitehill Farm, which has a 280 m grass runway 03/21.  With a northerly of less than 10 mph, the runway of choice was 03. Slightly downhill. QNH 1036. Outside air temperature 3 degrees C.  G-CIMA has a manufacturer’s maximum take off weight of 240 kg. The empty weight is 137kg leaving just over 80 kg for the pilot plus a full tank of fuel. This empty weight includes a BRS system. Whilst our UK limit for SSDR is 300 kg (plus some more for the BRS), the stall speed will have been calculated at manufacturer’s MTOW so for this reason, and probably for insurance purposes, it would be sensible to operate within the manufacturers figure. If the pilot weighs materially more than 80 kg then my opinion is that it would be better to go for the higher-powered engine choice, which for the Hirth F23 also offers a weight reduction.

 

G-CIMA is named “Little Bird”, partly because her nose area is rather birdlike. And she is certainly little. Sitting on the apron the first impression she gives is that she is tiny, but formidable. The wingspan is less than 20 feet, overall length under 15 feet, but a very sleek profile and glossy finish to all her surfaces. The construction is traditional in wood, with most of the surfaces being covered in 1mm ply, which has been subject to a great deal of finishing to produce a surface resembling GRP, with the use of carbon fibre in the cowls and other composites elsewhere. There’s an old saying that if it looks right then it probably is right – not very scientific I know, but it’s the thought I had on first acquaintance and now reinforced on each trip to the hangar. The two bumps on the upper cowl conceal the V-twin cylinders of the Kohler CH750 and similar cowls are fitted to the other V-twin engine options. These bumps do restrict your direct forward view from the taildragger variant when the tail is down whereas the Hirth cowl is a flat shape on the top and has its bulges on the sides, being a flat twin.  These engine options can be viewed on the website at www.sdplanes.com

 

The well-fitting bubble canopy hinges on the starboard side and entry is gained by standing on the port wing, which is a very short step up from the ground.  There are two quadrant controls on the port side of the cockpit – one controlling the trim, the other the throttle and they both fall easily to hand. On the starboard side a further quadrant control operates the flaperons, with three positions – neutral, take off and landing flap positions. As can be seen, the flaperons are nearly full span and have a large area. The stall speed of 34 knots is only achievable because of the effect of these flaps at full setting. Ahead of the flap control there is a red handle for activating the Junkers Magnum BRS system. This rocket powered BRS is installed in the upper fuselage immediately behind the pilot and two of its four cables extend forward to the engine bay, cleverly concealed in cable runs just below the lower side edges of the canopy. In G-CIMA the panel is traditional but a great deal of clutter, and some weight, could be saved by installing a digital panel. There’s a good example of this on the US website at www.skycraftairplanes.com. In the centre of the panel is the fuel sight tube – a very simple fuel gauge device but one that complicates any removal of the panel for access to its rear. A minor criticism is that the level shown when the aircraft is parked is rather greater than when the aircraft is in a flying attitude. This wouldn’t affect the tri-gear versions though. My ASI is now in MPH but I retained the metric VSI, as the conversion to feet per minute is very easy. The rudder pedals  and the accompanying brake pedals can be adjusted for pilot leg length with a simple sliding mechanism. The rudder and tailwheel assembly is controlled by cables and all the other control runs take the form of rods.

 

The wings are easily detachable and the aircraft could be hangared with the  wings removed.  Removal and fitment of the wings can be carried out by one person, as each wing has a finished weight of only about 12kg. The carbon reinforced wing spars locate into slots in the fuselage and are secured with two pins, following a construction tradition favoured in modern gliders. The control rods for the flaperons are readily located and the installation is completed with a rear pin on each side.  I have taken the wings on and off several times and I estimate around 20 minutes which would be reduced by experience. Time would also be saved by installing quick release fittings to the seat pan in place of the four screws which retain it and which need to be removed for wing removal and attachment.

 

Luggage space is naturally rather limited – there would be more if a BRS wasn’t fitted – but on G-CIMA the main luggage area is under the back of the seat and therefore not accessible in flight. The space is triangular and equates to around 40 litres of volume. With care, some further luggage could be stored under the pilot’s legs but would need to be properly secured so as not to migrate down towards the pedals.

 

The safety harness is a four point arrangement and does need some minor contortions in the small cockpit. After a quick burst from the electric auxiliary fuel pump, the CH750 spins easily into life and the shortish time taken for it to reach operating temperatures – principally an oil temperature of 25 c - can be used for the usual preflight cockpit procedures. One important thing to ensure is that the canopy is properly latched. In one of my early flights I neglected to do this with the result that the canopy blew open at 90mph and 1000 feet with a very large bang. I was unable to move it back into position but regained control with the loss of only a couple of hundred feet and nursed the aircraft the two miles or so back to my airstrip. The breeze was a bit of a face twister, I have to say, but I was pleased to have returned safely to base even with the asymmetric drag on the starboard side. This was my fault entirely – and something to doubly check on each flight.

 

Using heel brakes was a new experience for me. The steerable and traditionally sprung tail wheel obviously has little effect at low speeds but the aircraft can easily be turned at low speed on its brakes. The narrow track of 1.08 m means however that this manouevre needs to be undertaken carefully, especially on a soft runway.

 

Having spent the last couple of thousand hours behind a Rotax 912 (both 80hp and 100hp variants) there has never been any issue with taking off – even with tail winds and uphill runways the robust push in one’s back has always been reassuring.  The Kohler powered SD-1 needs a different technique, especially on grass and in its taildragger format. Full power at the start of the take off run does not produce any discernible push in the back, momentum is gathered quite slowly and it’s essential to get the weight off the tail wheel at the start of the run, especially on soft grass.  I rotated at about 50 mph after around 150 meters on the grass.  It’s very easy to get on the wrong side of the power curve with the low powered CH750 and any temptation to raise the nose aggressively on take off and climb out must be resisted. My climb out showed 500 fpm at about 60mph – I had the feeling that the aircraft could perhaps do a bit better than this and in fact once clear of the strip area I noted around 600 fpm, which is close to the “book” figure. This is an area where the Hirth would offer much better performance, at around twice these rates according to other tests.  This performance would also have a bearing on the timing of go-rounds – with decisions having to be taken rather earlier.

 

The trim lever operates through a strong spring on the elevator axis of the stick, rather than through a controllable aerodynamic trim tab.  This positions the neutrality of the stick to the chosen position. There is, by the way, an anti-servo tab on the stabilator (the all-moving T-tail) deflecting in the same direction as the stabilator. These are common on stabilator installations as they provide a necessary increasing resistance to an increasing pilot input. The movements of the tab are connected to the movement of the surface so no additional pilot input is required and this does not affect the trim of the aircraft.

 

Having reached 2000 feet I put the aircraft through her paces. I had expected a much more twitchy performance, due to the short coupling and the size of some of the control surfaces. In pitch she was very easy to trim to straight and level at speeds between 60mph and 100 mph and I was able to fiddle around with things on the panel without having to worry about pitch corrections. The roll response is excellent – the time taken to switch from a 45 degree banked turn to the same turn in the other direction was just a few seconds, with very little stick movement required and easy to co-ordinate a balanced turn with the rudder. The stick itself is nicely positioned between the legs and there is not a lot of possible sideways movement – but then very little is needed. So I would describe the SD-1 as agile in manoeuvres but well controllable.  With flaps retracted I noted the stall at an indicated 48 mph. I should say at this point that some GPS/airspeed tests I performed did not show much of a difference between indicated and actual air speeds in the range 60 – 80 mph but above this level the indicated speed was optimistic. Performing a similar stall with flaps the indicated speed was 40 mph.  So these figures are more or less in line with the “book” figures and I experienced no difficulties with wing dropping or recovery. In a moderate turn the stall speed with no flaps increased to about 55 mph so this is a point to watch in slow flying – but overall I didn’t think the aircraft was very comfortable much below 60 mph, with the controls starting to get mushy, so I personally won’t be flying below that speed.

 

Putting the flaps into their full landing position results in a sharp drop in the nose, which greatly enhances the view forward for landing. One needs not to be tempted to raise the nose on the stick at this point as the aircraft appears to be in a shallow dive but there is no increase in airspeed.  In a sideslip, the very large ailerons easily overpower the rudder input. I didn’t try an aggressive sideslip but a fairly gentle affair was quite benign. Like many microlights there is a fair amount of adverse yaw, accentuated by the size of the flaperons and much more pronounced when the flaps are lowered.

 

With a very low wing format I have needed to change my approach pattern in a crosswind from wing-down to the crabbing style. The SD-1 has a fairly low tolerance to cross winds and a gentle introduction is recommended. The limit is stated as 5m/second, just over 11 mph. On the day of this flight test there was only a small cross wind component which required a gentle crabbing posture and I made the approach at around 65 mph with full landing flap. Landing on grass I found that much of the judgment I hope I’ve developed in a traditional tricycle undercarriage SkyRanger stood me in good stead in the SD-1. The landing attitude put her down in a classical three-pointer with nothing much to report but I’ve no doubt there will be other more challenging conditions in the future. G-CIMA does not have tie-down points in the wings and anyone building the aircraft would be advised to incorporate threaded sockets in the wing undersurface so that tie downs could be brought into use.

 

I think I will get accustomed to the relatively low power of the CH750, but if I had been ordering a new aircraft then the choice would have been more difficult, with the more powerful Hirth being an obvious alternative. At my normal cruising speed of around 85 to 90mph the Kohler is perfectly adequate, and using less than 5lph has much to recommend it. Some strips may not be straightforward or available to the heavier pilot though.

 

As I commented before, the SD-1 Minisport lies at the very edge of what is possible under our new SSDR rules. It’s a delightful aircraft and anyone who buys one will fall in love with it. There’s no better way of putting it.

 

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SPECIFICATIONS

 

 

DIMENSIONS

 

Wingspan      5.98 m

Length          4.35 m

Wing area      6.10 m2

 

 

WEIGHT & LOADING

 

Empty Weight                                                                       137kg

Maximum Take off Weight                                                  240kg

Useful load                                                                         103kg

Fuel capacity                                                                        28 litres

Baggage capacity       (equivalent)                                        40 litres

 

PERFORMANCE (from test and from Pilot Operating Handbook)

 

Vne     (never exceed speed)                                             129mph

Va       (maximum manouevering speed)                             95mph

Vfe      (maximum flap extended speed)                             70mph

75 per cent cruise                                                              85mph

Stall (clean)                                                                        48mph

Stall (landing flaps)                                                            40mph

Climb                                                                               600fpm

Take off distance (to clear 50 ft)                                       300m

Landing distance (to clear 50 ft)                                       250m

Range at cruise (with reserve)                                           400+ statute miles           

 

ENGINE & PROPELLOR

 

Kohler CH750 – Four stroke air cooled 31hp direct drive V-Twin

Sandera 1200 Composite two blade propeller ground adjustable

 

MANUFACTURER

 

Spacek SRO

Lesni 25

69501 Hodonin

Czech Republic

www.sdplanes.com