M-311 program is taking off

 - June 8, 2007, 12:22 PM

Visitors to Alenia Aermacchi, part of the Finmeccanica stand here at Le Bourget, will find the same M-311 lightweight jet basic/advanced trainer avionics demonstrator the company showed two years ago. However, the program has moved forward since then, with advancements in both the commercial and technical fields.

The M-311 is the only jet-powered training aircraft in its class and it has been down-selected in the United Arab Emirates and is competing against two modern
turboprops. Meanwhile, Aeromacchi has frozen the aircraft configuration and has started work on the prototype at its Venegono factory in Italy.

While the current avionics demonstrator is a basic S-211 format equipped with a CMC Electronics Cockpit 4000 digital avionics suite and other modifications, the M-311 prototype, which will exploit the earlier S-211 design, will be a new aircraft in many areas.

In the aerodynamic field, wing tips will allow improvements to the trainer’s Dutch-roll and roll rate performance and at the same time reduce induced drag. Its ventral fins are expected to improve lateral directional static and dynamic stability. The aircraft’s longitudinal static and dynamic stability will feature a redesigned horizontal tail plane that will also extend the center of gravity. Wing fences should deliver better stall characteristics.

One of the main changes will be to the air intakes, which have been modified to accommodate the increased airflow needed by the new version of its turbofan engine, the Pratt & Whitney Canada JT15D-5C. Compared to the -4C version powering the S-211 trainer, the -5C weight has increased from 575 pounds to 646 pounds but provides 3,190 pounds of thrust–both maximum takeoff and continuous–approximately 30 percent higher than the respective figures for the -4C model.

What is even more notable is that specific fuel consumption for the larger, higher performance jet has increased by less than 2 percent, from 0.562 pounds of fuel consumed to 0.573 for every horsepower generated during one hour of operation, while the average fuel consumption for similar missions has increased by less than 10 percent. The integral fuel tanks have been enlarged slightly for longer range.

Mission availability is the other prominent feature of the M-311; the engine helps here since its initial time between overhaul has doubled compared to the previous version (3,000 versus 1,500 hours). This should improve still further as the aircraft matures in service, with a forecast TBO of 4,500 hours.

Even when the M-311’s engine has to be removed, the operation should take approximately 90 minutes (including about 30 minutes run up time) compared to the two and a half hours with the previous model. The maintenance program consists of two levels, neither requiring a return to the depot. The standard O-level includes daily inspections, servicing and replenishment, as well as corrective maintenance on the aircraft. The turnaround time for this has been reduced thanks to the installation of a single-point pressure refueling/defueling system positioned under the left air intake. The more advanced I-level maintenance includes periodic inspections (every 150 and 600 flight hours), scheduled engine, replacement of line-replaceable units and selective corrective maintenance actions.

Design Changes
The engineers have minimized the number of life-limited parts and they have
relocated many components to ease accessibility. LRU adjustment and calibration has been significantly reduced. More than 100 panels and doors on the airframe provide easy access to all internal systems. All these measures, together with the installation of a state-of-the-art onboard oxygen generation system (OBOGS), have cut the maintenance burden for the M-311.

The airframe’s structure has been completely redesigned and includes a higher percentage of composite materials such as Kevlar, carbon-reinforced plastics and nomex than its predecessor. The M-311’s forecast calendar life is 30 years and its projected fatigue life is 15,000 flight hours.

The M-311 avionics suite features a night-vision-goggle-compatible glass cockpit with a head-up display and three multifunction displays in the front seat. The back seat is equipped with an additional MFD, which can be used as a HUD repeater.

Designers have optimized the final configuration of the cockpit from the ergonomic point of view compared to the current avionics demonstrator. For instance, they have increased rudder pedal adjustment and repositioned the HUD up-front control panel to provide an unobstructed view of the central MFD.

The M-311 will be equipped with a complete onboard embedded simulation suite like the one Aermacchi has developed with the MB-339CD and the M-346 trainers. The system will be capable of simulating sensors, weapons and tactical scenarios, and will be fitted for real-time datalink to create a net with ground stations and other aircraft, both real and simulated.

While these features can be found in most advanced turboprop training aircraft, Alenia Aermacchi officials stressed the advantages of the turbofan propulsion to provide a better selection and training capability. Forward visibility over the nose in a jet is not obstructed by the engine cowl and propeller hub, they said, and the forward positioned pilot in a jet enjoys lateral look-down visibility unobstructed by the wing.

The jet’s flying qualities are also substantially different from and the flight envelope is much wider than those of turboprops. For instance, the M-311 is able to reach a maximum speed of 400 knots at 20,000 feet–about 18 percent more than the best-performing turboprop trainer. They also have improved margins for specific excess power and sustained turn rates, especially in the high-speed regime which makes for an easy transition to a fast jet lead-in fighter trainer (LIFT).

Propeller-driven aircraft behave differently than jets largely because of the propeller gyroscopic effect, its torque, the helical flow that impacts the tailplanes and asymmetric thrust in certain conditions. To bring the aircraft’s behavior as close as possible to that of a jet, the designers have partially solved these problems using auto-rudder trim. However, if this system fails, it becomes a major danger for a student with limited flight experience because of the high installed power.

The engine response is also very different between a turboprop and a jet, which necessitates the provision of a power management system (adding complexity and potentially representing a safety issue in the event of failure). Another peculiar difference is the distribution of weight around the airframe, which in a turboprop sees most heavy components concentrated in the nose while in a jet these are distributed along the fuselage. This difference changes the moment of inertia and with it the behavior of the aircraft, especially in highly dynamic maneuvers.

According to Aermacchi, jet training offers a superior instructional effectiveness, a higher final skill level of trainees and a resulting lower cost per qualified pilot. According to some air force studies, turboprops may not be realistic enough to reveal which trainees are fit for the next phase of jet training. This results in more pilots having to start LIFT training before being diverted to transport aircraft, considerably increasing the overall cost of the single combat pilot. These studies sparked Aermacchi’s decision to completely redesign the S-211.