Apparently ground-based flight simulators are not as effective in training pilots as is generally believed and Buffalo, New York-based research organisation Calspan Corp has taken the idea one step further with its real-time in-flight simulator. The firm has modified a variable-stability LearJet 25 by replacing a front seat with variable electrohydraulic feel systems and incorporating a digitally-controlled analogue computer to give the pilot the feel of flying one aircraft while actually flying the LearJet. The plane comes over to the UK regularly to help with training at the International Test Pilot School at Cranfield Airfield in Bedford and is generally used to help already experienced pilots get a feel for different types of aircraft before they fly them for real or to help test out a prototype aircraft before it is actually built. One problem with ground simulators is pilot-induced oscillation, which occurs when the pilot gives and instruction, but because of the nature of the equipment, the response is not immediate. If the pilot immediately follows with a further instruction as the original request has been carried out, then an oscillation occurs as the instructions given don’t meet exactly with what the desired movement was. With a ground simulator, the pilot can compensate for this, but in the air the effect is more noticeable. The LearJet simulator can also be used to help develop software to avoid this problem. The software development environment is based on the Simulink graphical block diagram-oriented interface to Matlab from MathWorks Inc and can be used on a Sun Microsystems Inc workstation, Macintosh or personal computer. The user puts together a block diagram format of the plane or flight control system to be simulated, which once entered graphically, can be analysed on the desktop. The hardware consists of a 20-slot AT backplane and the host processor is a single-board 33MHz 80486 computer with 8Mb memory. Mass storage is provided by an internal 90Mb Bernoulli removable-cartridge disk drive from Iomega Corp. The flight simulator has one signal processor subsystem from dSpace GmbH and two others from Datel Inc that are used to implement the model-following flight control laws. The model following is an implicit three axis system which means the LearJet acts as if it were the aircraft that is being simulated. The LearJet is then forced to have the same accelerations as the model. At the Test Pilot School, the system is used to build a model with problems that the student has to identify and correct and then the students fly it and see whether they are correct or they can make modifications while in the air – although in the case of the Learjet being pushed over its limits there is an automatic safety cut out so the LearJet can be flown as itself.
