https://www.simpleplanes.com/a/szk5m1/Lu-74-Bison-VTOL

General Description/Layout: The Lu-74 Bison is a medium-sized (30,885 lbs) three-engine VTOL cargo/gunship aircraft. The aircraft features a large fuselage (63.5 ft long) with the flightdeck placed high and forward with accommodations for three crew (pilot, copilot and loadmaster/gunner) in crash-survivable (rated to 15 Gs) seats. There are paratrooper-style seats for up to 24 troops and accessed via two large cargo doors on the left side. The wings, 83.9 ft long, support two turboprop engines located on rotating mounts at their tips. The standard cruciform tail includes a rather large horizontal stabilizer, to aid in low-speed pitch control. At the very end of the tail is a third, smaller, engine fixed in the vertical direction, which is activated to enable vertical takeoffs and landings. Additionally, a stabilization system with multiple thrusters placed in the nose, tail and wings aids low-speed flight. The tricycle landing gear includes a rather narrow pair of main gear placed in the fuselage.

Flight Profile: A vertical takeoff and climb to 5,000 ft, followed by airwork and both a conventional and horizontal landing was flown. A vertical takeoff was performed with the main engines tilted approximately 10 degrees forward from vertical to prevent "tip back" and the third engine activated. The "tip back" phenomenom may originate due to the fact that the main gear places the nose of the aircraft pitched slightly up (1 degree), instead of perfectly level. The takeoff was not difficult, the takeoff occurring after a very short roll, several dozen feet, to 15 mph. A bit of coordination between prop angle, trim and deactivating the tail engine was required to complete transition to horizontal flight, but was not difficult once the pilot had practiced it several times. Several "hops" were attempted to a hovering state, the aircraft was fairly stable at approximately 70-80% power, though substantial work was required to keep the aircraft in control and to prevent forward drift and acceleration. However, eventually, oscillations would result and several hard landings were experienced (test crew comment: "I thought the wings were going to break off!"), but the aircraft remained fairly intact due to its robust structure. The transition was completed by approximately 200 ft and acceleration to 300 mph was swift during the maneuver.

For high speed level flight, slight nose down trim was required. Rolls and maximum rate turns performed starting at approximately 420 mph revealed surprising agility for such a large beast, though airspeed bled off, but only down to approximately 320 mph, during the maneuver sequence. No adverse handling characteristics were observed during this phase, nose track remaining smooth even at full elevator throws. The test crew then slowed the aircraft for stalls, encountering a moderate break and pitch down at 5,000 ft, 10% power and 10-15 degrees nose up. Stall recovery was benign, requiring only pushing forward on the stick to decrease the angle of attack and effect the recovery and no adverse wing drop. Speeds up to 420 mph were achieved during the area work at 5,000 ft.

A quick weapons evaluation was flown in addition to the flight profile. The gatling-style tail gun provides a large volume of fire to protect the rear of the aircraft and the forward armament (unguided rockets, Inferno missiles, 2x.50 cal guns) provides substantial opportunity for suppression of enemy defenses. There are even 2x AAMs for self defense, though their utility might be slight if encountering an enemy fighter. The only complication proved to be a slight vertical nose track up or down when attempting to aim the unguided weapons (rockets/guns).

Upon return to the field, a conventional landing with the engines fully horizontal and the third engine deactivated, was attempted. The modest prop span allows these types of landings, providing a degree of safety should the engine rotation system fail, or be damaged, in flight. The test crew discovered 140 mph to be an ideal approach speed, as any slower and the tip-mounted engines would introduce oscillations which would rapidly grow if the crew attempted to counteract them. Flare control was adequate, but it was easy during touchdown to strike a prop on the ground, as the main gear is very narrow and not very stable to damp out unstabilized landings. Following the conventional landing, a transition to vertical flight and vertical landing were attempted. It was difficult to slow the aircraft; attempting to rotate the aircraft by tipping the engines aft produced dynamics which were difficult to control. Additionally, attempting to slow below 100 mph and definitely below 50 mph introduced increasingly severe roll oscillations, even with the stability system engaged, easily exacerbated by crew input, which led one crew to a loss of control accident, with the loss of the aircraft and original test crew in the ocean off of Murphy Airfield. Following the resumption of testing, the new crew found it most advisable to slow as much as possible in a conventional configuration, begin rotating the engines to horizontal at 120 mph and start the tail engine at 100 mph. By then staying above 85 mph, the average pilot has enough control authority to avoid pilot-induced oscillation (PIO) and put the aircraft where he or she would want to land. Vertical approaches and landings were best flown at 85-100 mph, 60-70% power as required with the flare aided by rotating the engines slightly (approximately 10-15 degrees) aft just prior to touchdown to further slow horizontal travel to approximately 50 mph. In this way, rollouts were well less than 200 ft.

Overall Impression/Recommendations: A well-executed aircraft suited for its stated mission. High performance and long range are the hallmarks of this aircraft. The test crew recommends first, that the main gear be repositioned in fuselage "sponsons" in order to allow a wider track and to lower the nose of the aircraft to level when sitting on the ground. This would eliminate any lingering difficulties with vertical takeoffs. Additionally, the manufacturer might be able to experiment with a slightly larger prop diameter, which would only benefit takeoff acceleration. The inherently difficult dynamics of vertical flight are somewhat mitigated by this design and a suitable safety margin may be maintained between a combination of operational restrictions (i.e., no approaches below 85 mph until flare) and training to make this a viable design. Well done!