Airtest — Sukhoi Su-31

Sukhoi Su-31

Much More Than Just an Uprated Su-26!

After flying the Sukhoi Su-26 single seat and Su-29 two seat aerobatic aircraft and watching world class unlimited aerobatic pilots fly them to near perfection, I thought that an improvement in performance of the Sukhoi line-up was not needed.

An improvement in control harmony, maybe, but not performance. Fortunately for aerobatic pilots, (and unfortunately for my wife’s plans for our house and life savings), the Sukhoi design bureau did not rest on the laurels of the prize winning Su-26 series, and has recently added the Su-31 single seat aerobatic aircraft to the stable. The Su-31 is not an ‘uprated/modified’ Su-26, but an extensive redesign. The wing section is changed, the aileron profile altered, the rudder enlarged and the fuselage modified. Bolted to the front of this airframe is the same Vedeneyev nine cylinder radial, but with power options of up to a whopping 400hp (300kW)! This aircraft offers a marked improvement in performance and control harmony over the Su-26, giving an aircraft which has awesome performance, but is extremely nice to fly. Nigel Arnot of Camden based Ultimate Aerobatics imported the first Australian owned Su-26 over two years ago. This aircraft has since been sold, along with three other Su-26/29 aircraft, and Nigel has proudly added the Su-31 to his distributorship. The Su-31 tested here is the 360hp (270kW) version and is number seven off the production line and number two in the Western world. Visually, the Su-31 is very appealing, even more so than the Su-26. And the adage that “if it looks right, it flies right”, is true here. The clean straight lines which taper from the radial engine back to the trapezoidal shaped fin and rudder, enable easy judging of the aircraft reference axis from a distance, aiding those ever picky aerobatic judges to see those attitude errors and wobbles you were hoping to hide. The tapered wing has a very slightly swept back leading edge, and is attached just below the mid section of the fuselage assisting that “I don’t care which way I am up” handling. The new aerofoil section has its point of maximum thickness more rearward than the old Su-26 wing. This has moved the aerodynamic centre back and improved the longitudinal stability. The new ailerons also have their points of maximum thickness more aft, increasing the aerodynamic balancing and radically reducing the lateral stick forces. These lower lateral stick forces have been achieved with one less aileron spade set than on the Su-26. The increased rudder area improves the directional controllability, making knife edge flight and other manoeuvres which require marked amounts of sideslip angle, such as rolling circles, a breeze. The supercharged Vedeneyev nine cylinder radial produces 360hp (270kW) in its basic form and 400hp (300kW) in the uprated version. This extra 40hp (30kW) is achieved by increasing the supercharger ratio and raising the manifold pressure, but has led to a reduction in engine TBO. The 360hp (270kW) engine requires a 750 hour TBO, while the 400hp (300kW) engine ‘life’ has decreased to a 500 hour inspection requirement, with replacement of components ‘On Condition’, thence, 50 hourly inspections. The ‘life’ may be increased as the boosted engine proves itself over time. However, all these inspection requirements are present only when doing unlimited class aerobatics. If not, the engine TBO is 2000 hours. The airframe has no defined life but has an overhaul period of 2500 hours, whereupon certain components are replaced. The wings have a carbonfibre mainspar, titanium (that’s right, titanium) ribs and are proven to an ultimate load of ±23 g while the rest of the airframe is typically of Russian strength with a solid titanium undercarriage to boot. What is not typical of previous Russian aircraft is the overall finish. The Su-31 finish is excellent. All fairings and cowlings fit properly, and the interior cockpit structure is lined with moulded fibreglass. Interior linings are almost unheard of in an aerobatic aircraft, and it was a pleasure to see them introduced here. Internal fuel capacity of the Su-31 is also increased to over 280 litres total, of which 70 litres is available in the aerobatic tank. This gives the Su-31 a good 40 minute plus aerobatic endurance, with over five hours of cross country time. The airstart system has a capacity for a good six attempts when full, and is continually replenished in flight. External jacks are present to allow recharging of the air start system and for connection to an external electric power supply. There are no special requirements for this aircraft when operating from remote airfields. It can even be hand started! When stepping into the cockpit, one immediately receives the ‘Russian aircraft smell’. A unique blend of the odours from the materials used in construction, not unpleasant, just uniquely ‘Russian’. The seating position is excellent and all controls and instruments are easily to hand. The seat is not as ‘laid back’ as in the Su-26 and Su-29 allowing better rearwards visibility, but still has enough slope to improve ‘g’ tolerance. The seat cushion and parachute are very comfortable to sit on (I didn’t want to get out!) and the double harness ensures you are well restrained in the cockpit with no discomfort. The cockpit is well laid out. The throttle and propeller levers are combined, allowing simultaneous movement and all instruments are easily read. There is no mixture control as this is automatic. Checking full and free movement of the controls reveals that the control circuit has almost no friction and the controls are all well balanced. Visibility is excellent, especially rearwards, which assists greatly in vertical rolls when checking the box axes, and the two piece canopy has very little framework to restrict your forwards view. The start-up sequence is straight forward. Pump up the fuel with the hand pump, prime the engine, magnetos on and press the start switch. One propeller blade passes in front and then the radial bursts into life amidst a cloud of ‘radial blue’ smoke. Initial warm-up is at 40% revolutions and taxying commenced when the CHT has reached the amber arc. The tailwheel is fully castoring, so some brake is necessary when taxying, but overall ground handling is good. Run-up to 70% is commenced when the CHT and oil temperature are in the green, whereupon the cooling gills and oil cooler flap are opened. After the usual checking of the propeller governor and magnetos, a brief cockpit scan is all that is required before takeoff. Takeoff is a simple matter. Lock the tailwheel, add some power and the aircraft accelerates rapidly. The tail can be raised early in the takeoff roll and a touch of left rudder is all that is required to keep the aircraft straight. Fifty four knots (100km/h) is reached in less than 100 metres, whereupon rotate to the takeoff attitude is commenced. Initial climbout attitude with full power applied was a sustained 40 to 45 degrees nose up! And the aircraft was still accelerating. The immediate control impressions I received were all very positive. While the controls are still sensitive and powerful, they are not as ‘touchy’ as in the Su-26. Gone too is the disparity between the longitudinal and lateral stick forces that was ever present in the Su-26 and Su-29. And gone is the requirement to use two hands to get full stick deflection when rolling. Gone is the tendency to overpitch and stall when looping. This aircraft has a beautiful harmonisation of all three flight controls, and feels much more ‘Westernised’. The Su-31 is indeed a delight to fly. Full aileron deflection roll rate has increased to over 400 degrees a second but the rolling inertia of the aircraft seems to have decreased. This improves roll acceleration and the starting and stopping of rolls. The increase in longitudinal stability has reduced the tendency to over-control in pitch, but with no loss of angle of attack authority. Increased rudder area has improved directional control and the improved harmonisation of the three flight controls enables the transition to the Su-31 from other aircraft to be much less traumatic than in the Su-26. However, it is an aircraft that must be flown with your fingers as any tensing or unwanted input, no matter how small, gives an instant, amplified response from the airframe. Neil Williams’ statement that “You cannot lie to yourself in aerobatics” is reinforced greatly in the Sukhoi. Aerobatically, the aircraft is unbelievably capable. All competition unlimited figures and any combination of the basic manoeuvres are easily completed. And the power of the Su-31 is awesome. Apart from the takeoff, I never used any more than about 90% power during the whole sortie! The enormous power of the Sukhoi becomes most noticeable at low speeds, where the aircraft seems to hang onto that big geared propeller and claw its way through the sky. The fast pitch and roll rates combined with the tractor like capabilities of the aircraft enable multiple vertical rolls to be performed with plenty of energy left to fly off level at the top of the vertical line. The stall is totally docile and aileron control is still available beyond the critical angle of attack. The spin is also very docile and controllable. To demonstrate the power and controllability available in the Su-31, I initially stalled the aircraft in level flight, pulled the nose up to about 45 degrees and added power.

The pitch attitude increased to about 70 degrees nose up and the aircraft, though still stalled, actually climbed and started to accelerate. It even tends to accelerate during the cartwheel of a stall turn when the nose is still above the horizon!

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