ANT-25 (RD) long-range record-breaking aircraft
The Tupolev ANT-25 was a three-seat all-metal low-wing monoplane powered by a single M-34 (M-34R) engine. The requisite range was to be achieved by a blend of design features intended to give the airframe and its engine a very high degree of aerodynamic and weight-to-strength sophistication for its time. The main aerodynamic characteristics were the wings’ high aspect ratio (13:1) and taper (5.3).
The high aspect ratio cut the induced drag at cruising speed, and the taper increased the rigidity of the wing and suppressed flutter. The reduced weight and enhanced rigidity of the wing were achieved by using a three-spar structure with interspar fuel compartments – from which the fuel would be drawn in strict sequence in flight. This relieved the wings of air loads and allowed the structural weight of the wings to be significantly reduced.
The forward section of the fuselage was of truss structure and was integral with the wing centre section. The three-seat crew cabin was separated from the engine bay by a firewall. The rear fuselage was of semimonocoque stressed-skin construction. Behind the crew cabin, the upper fuselage contour ran back smoothly to the base of the fin which carried a variable-incidence tailplane that could be adjusted in flight. The fuselage had a smooth skin, but the tailplane, fin and rudder had corrugated skins. The wing roots and inboard sections were originally intended to have a corrugated skin as well, but this was later covered with perkal’ linen fabric. The twin-wheel main landing gear units retracted aft into the wings and were provided with fairings; the tailwheel was carefully faired. All these measures aimed at cutting drag had a direct effect on the aircraft’s ability to attain its specified range.
A total of 6,100 kg of petrol and 350 kg of oil was carried to enable the aircraft to fly non-stop for 75 hours with a maximum take-off weight of 16,500 kg at a cruising speed of 165 km/h. The most up-to date flying and navigational equipment was installed as vital for prolonged non-stop flights: an artificial horizon, a gyro-magnetic compass, a direction finder, a sextant and a solar course indicator. The onboard radio with a fixed long-wave tuning could transmit over distances up to 5,000 km and a ground fix could be made on the aircraft at a distance of 2,000 km. In preparation for the flights by the ANT-25, the longest paved runway yet built in Russia, 1,800 m long and rising to 12 m in height at the start, was constructed at Shcholkovo airfield east of Moscow (now Chkalovskaya airbase).
Two aircraft were built: the first prototype (ANT-25-1, or RD-1) and very shortly afterwards the second prototype (ANT-25-2, or RD-2). The RD-1 powered by an M-34 engine without reduction gear driving a twoblade propeller was completed on 13th June 1933 and first flown by M. M. Gromov on 22nd June.
The RD-2 with a geared M-34R (normal output 800 hp, 900-950 hp with boosting) with a different cowling and a three-bladed propeller was ready on 1st September the same year and first flown by Gromov on 10th September. The RD-2 also differed from the RD-1 in having a revised rudder. Subsequently, the RD-1 was modified to make it identical to the RD-2.