The NIMBUS-4 is a fundamentally newly develeoped single-seat Open Class competition sailplane, which incorporates the perfect realization of all aerodynamic potential currently available. At the same time, all technological progress practicable in sailplane construction is applied.
With the substantial performance data emerging from this research work (L/D>60), the NIMBUS-4 clearly ranks at the top of its class, whether as a pure glider or as a powerded sailplane.
The fuselage, wings and tailplane of the NIMBUS-4 are all designed uncompromisingly for the maximum in performance and yet are so perfectly harmonized (it behaves just like an enlarged VENTUS c) that even in the most extreme conditions, optimum and fatigue-free flight handling is achieved ... whether with full water tanks (capacity approx. 300 Liter, 80 US Gal.), in high mountains or, for instance, when flying long distances.
Thanks to the exceptional aerodynamic cleanness of the airframe and, as a result, the extremely low losses for speed-to-fly corrections, it is possible to cover extraordinarly long distances without any circling, over a wide speed range and with very little sink - even with only average thermals and at relatively low wing loading.
Other Open Class sailplanes will, therefore, find it difficult to keep up with the NIMBUS-4, even more so in booming weather at the associated very high flying speeds.
This superiority in performance became after the first comparision flights with the prototype in spring 1990, and was impressively confirmed shartly afterwards by being placed first in the World Chamionchips for Motor Gliders, and by winning the German Open Class Nationals where, in addition to the advantages in the glide, the immensely good climb performance of the NIMBUS-4 received high praise.
Apart from its span of 26,4 m (86.6 ft), the most significant feature of the NIMBUS-4 is its still unconventional multistage swept-back wing leading edge, the aerodynamic advantages of which (proven by the DISCUS-geometry) are even more enhanced by the slight dihedral of the wing tips. This wing concept guarantees a perfect utilisation of the span and offers excellent low speed handling qualities - no wonder that similar "natural" wing planforms are also found among sea birds, e.G. albatross and cormorant.
The six-piece wing (aspect ratio 38.8) is, of course, constructed from carbon fibers. It is fully sealed, and features a boundary layer control by means of turbulator tape on its lower surface.
The basic airfoil of the NIMBUS-4 wing is an improved version of the best section used on the VENTUS which, particularly at very high speeds, produces once again a distinct gain in performance.
The fuselage of the NIMBUS-4 is of a hybrid fiber construction, purposely designed for the ideal accomodation of a sustainer engine or a more powerful unit for self-launching. For this reason, the fuselage is slightly less tapered aft of the wing junction (than fashion demands) and transition softly into the tail boom. The resulting elegance of the fuselage contours is further enhanced by the newly developed and considerably lengthened cockpit, featuring a forward-hinged canopy.
Aramid (Kevlar) and carbon fabrics, incorporated in the cockpit structure, provide the maximum possible safety factor for the pilot who, on take-off and landing, is also protected from hard shocks by the sprung undercarriage, which has telescopic shock absorbers. Thanks to its large main wheel, stiff wings and an excellent aileron response, there is neither tendency for the NIMBUS-4 to drop a wing in these flight phases, nor is an abnormal effort in concentration required.
The cockpit is very roomy and offers a fantastic field of vision; even the tips of the elevator can be seen! Seating comfort and the arrangement of the control elements are in perfect compliance with the demand of competition pilots for a new development in the Open Class. Seat back and rudder pedals are adjustable in flight. The instrument panel swings up and down with the canopy. A large battery, essential for the powered sailplane variants, can be accomodated in the nose of the fuselage, thus compensating the tail-heavy moment of the engine.
A water ballast tank, integrated in the fin is, among a number of further benefits, included in the standard equipment of the NIMBUS-4.
Compared with the NIMBUS-3, the fuselage has been lengthened by approx. 20 cm (7.87 in.), and features a considerably öarger fin and rudder with a new airfoil. The coordination between rudder effectiveness and operating force leaves nothing to be desired.
The time required to reverse a 45 deg. banked turn of slightly less than 6 seconds is very crisp for an aircraft of this size, and is achieved with the aid of "spoilerons" as used on the predecessor of the NIMBUS-4. These additional control surfaces are automatically deflected (to a negative angle of up to 80 deg.), resulting in enormously safe flight handling at lower speeds.
The new horizontal tailplane (span 3,10 m | 10.17 ft) also contributes to this - its high aspect ratio, combined with a near elliptical lift distribution (two-stage trapezoid planform with swept-back leading edge), guarantees a maximum in effectiveness with the least possible additional drag.
Installing a drogue chute was not necessary. In landing configuration, the positive deflection of the flaps automatically increases to an angel of about 40 deg. when extending the enlarged and double-panelled airbrakes, thus compensating the loss in lift. With this combination, steep approaches and low touch-down speeds are not a problem.
The main wheel features a hydraulic disc brake which is very effective.
Rigging and de-rigging the NIMBUS-4 requires normal effort and may be accomplished by two persons using a wing stand.