De Havilland Canada Dash 7 - Design and Development

Design and Development

In the 1960s, de Havilland Canada was already well known worldwide for their series of high-performance STOL aircraft, notably the very popular DHC-6 Twin Otter. However, these aircraft were generally fairly small and served outlying routes, as opposed to the busier regional airliner routes which were already well served by larger, higher-performance turboprop aircraft such as the Fokker F27, Fairchild F-27, Convair 580 and 600, and Hawker Siddeley 748.

The de Havilland Canada company felt they could compete with these designs in a roundabout way. With their excellent STOL performance, their designs could fly into smaller airports located in city centres and smaller, outlying, more austere airports having runways that the other aircraft could not easily use (unpaved, unimproved). The original specification called for a 40-passenger aircraft with a fairly short range of 200 statute miles, operating from runways only 2,000 ft long (610 m).

With new noise restrictions coming into effect throughout the 1970s, an aircraft tailored for this role would also have to be very quiet. To meet this restriction, the new design used oversized propellers geared to spin at a slower speed than normal; much of the sound from a propeller is generated at the tips which are spinning near the speed of sound, and therefore, by reducing the number of RPM, this noise is reduced substantially. The Dash 7 often landed with only 900 rpm, and took off at only 1,210.

In other respects, the new DHC-7 was essentially a larger, four-engine version of the Twin Otter: the general layout remained similar, with a high aspect ratio high-mounted wing, most details of the cockpit and nose profile. Changes included the addition of cabin pressurization (requiring a switch to a fuselage with a circular cross-section), landing gear that folded forward into the inner engine nacelles and a large T-tail intended to keep the elevator clear of the propwash during take-off (the Twin Otter's empennage was a cruciform arrangement).

The Twin Otter incorporated "flaperons" that drooped the ailerons as part of the flaps, but these were not included in the Dash 7 due to weight and complexity. Instead, the ailerons were reduced in size to allow more flap area, and were augmented with two sets of roll spoilers, or "spoilerons": the inboard roll spoilers operate at all speeds while the outboard roll spoilers only operate at speeds less than 130KIAS to allow for more roll control at slower speeds. Upon touchdown, both the inboard and outboard roll spoilers extend in unison to aid in destroying lift created by the wing. Each wing also includes two ground spoilers which only extend on touchdown. Most of the trailing edge was spanned by a complex double Fowler flap arrangement for high lift at low speed. During a typical STOL landing, flaps would be selected to the 45° position; this flap setting allowed for steeper descents and slower approach speeds. Depending on weight, the V_REF speed with flaps at 45° would be between 70-85 knots. On touchdown, through "squat switches" in the main gear, the flaps would automatically retract to the 25° position and thus reduce lift once on the runway for better braking performance. The flaps would also retract to 25° when engine power was increased during a go-around procedure. The four-engine layout aided lift at low speeds due to the wide span of the propellers blowing air over the wing ("propwash"). When reverse thrust was selected on landing, the props reverse pitch, push air forward and slow the airplane effectively along with the anti-skid wheel brakes. More importantly, if an engine failed, the asymmetric thrust was much less than on a twin-engine layout, thereby increasing safety and allowing for a lower minimum control speed with an engine inoperative (V_MC).