1. The concept
Conceptually envisages using ground provided power through devices mounted on the airfield to assist T/O or landing. The benefits perceived for such use are that the aircraft might then be able to fly using less installed power and to use less energy taking off and landing than would otherwise have been the case. The benefits would be expressed in reduced carbon emissions and in lower costs. The devices put forward include catapults, ramps, wind-tunnels, magnetic forces, cables and winches.
2. Delivering benefits – the take-off phase
An a/c requires forward velocity to take off conventionally. This is generally obtained by accelerating the a/c until the air velocity over the wing is sufficient for lift-off – perhaps with the use of high lift devices such as flaps.
One of the ways of providing ground power for part of this phase would be to hoist the aircraft to the top of a ramp and allow it to exchange the potential energy thus gained for kinetic energy as it rolls down the ramp. The benefits gained in this way do not appear to be sufficient for T/O as forward speeds of circa 150 knots are required.
Incorporating a second accelerating device could however, further enhance the elevated ramp. This would have some or all of the effect of a catapult in propelling the aircraft forward. Such devices could, for example, be maglev propulsion units in which the aircraft would engage with a trolley that was linked to the maglev track.
The a/c would be pulled back up the ramp engaged with the trolley and then both roll and be propelled down the track.
The maglev propulsive rail could, of course, be used with the ramp and the a/c would then take off from a substantially flat surface.
Ski-jump take-off ramps have been used in naval vessels to provide a vertical impulse to shorten the take off.
These units have, however, been most notably used with very high power-to-weight a/c like the Harrier that is able, on full power, to sustain flight even if sufficient forward velocity has not been achieved. The issue with a loaded airliner is that of achieving the necessary forward speed and a ski-jump is not thought likely to assist such an a/c substantially.
A concept with a very different approach is the "Tube Take-Off" device [TTOD]. This envisages a very large tube into which the a/c taking off is reversed. In the tube behind the airliner is a large and powerful fan powered from ground resources. The airliner stands in the tube, engines running, whilst the fan runs up to speed and sucks air past the airliner until the air velocity over the wings induces sufficient lift for flight. The airliner is then able to exit the tube and to take flight. The challenges to be overcome with this concept are concerned with the airflow into the mouth of the tube and the design of the tube itself. The a/c needs to accelerate out of the tube so that when it reaches the free, stationary air somewhere outside the tube it has achieved an adequate flying speed. The airflow into the tube will be such that stationary air will not occur immediately at the mouth of the tube. The length of time for the a/c to accelerate must be matched to the length of the tube from which it exits and the flow field distance outside the tube from which it may draw benefit. The flow-field may be optimised by the design of the tube mouth.
Accelerating winches may also be used to assist the acceleration of the a/c either alone or in combination with other devices. These winches would be larger versions of the devices used to launch gliders or the catapults used on naval aircraft carriers. The forces are very large. A typical medium a/c has a maximum TOW of about 73,000 kg. Augmenting forces of around 13,500Kg would be needed to reduce the t/o length by 300 m. The main purpose of ground power augmentation [GPA], however, is to reduce installed a/c power. In that scenario the same augmenting forces and the original t/o distance could reduce ground level thrust needed by up to a half which would be very significant for this phase of the mission taken in isolation.
Thrust calculations can, however, be extremely complex. One of the calculations necessary is to determine the size of engine that would sustain flight in the event of an engine failure. Clearly, it would be nonsense to suggest that by catapulting an aircraft with no engines into the air at flying velocity anything useful had been achieved.
So, the installed power on the aircraft has to meet several other constraint conditions. This situation is particularly dramatic on so-called big-twin aircraft where the calculation of engine failure on t/o is often the critical determinant of thrust. Airworthiness regulations require these a/c to be able to climb out on or after t/o with one engine failed. This leads most of these aircraft to have considerable excess thrust installed – in the limit approaching twice the thrust calculated to be necessary without engine failure. Four- or six-engine configurations have successively less need for such excesses of thrust since the t/o safety requirement is for failure of one engine. The four-engine configuration thus leads to each engine needing something less than 1.3 x the calculated figure. In practice the observed amount of "excess power" at take-off in a large four-engine aircraft is about 25%, equivalent to one of the engines.
One approach that has been proposed for overcoming this emergency requirement has been to install thrustaugmenting devices. These could, for example, be modern developments of the old JATO units that provide an emergency source of thrust or are routinely used for t/o assistance. In the GPA concept the installed thrust of an aircraft might well be determined by the single engine failure mode rather than the t/o thrust requirement.
With two JATO-like devices capable of providing (on our reference medium weight airliner) an emergency thrust of perhaps 2x 3000 Kg this consideration might allow a more closely balanced determination of installed engine thrust.
In conclusion GPA might have substantial attractions for aircraft designs where take-off thrust becomes the limiting condition. Where this is not the case, perhaps because of climb power needs or multi-engine configurations, the benefits on the aircraft are much less significant. The benefits of GPA might, however, still apply to the fuel consumed by the aircraft (and hence carried by it) during the take-off manoeuvre. Since t/o is a very significant part of the fuel budget this use alone has the prospect of useful savings. It is also worth noting that fuel used by the a/c at take-off also adds to the TOW and savings here have several benefits.
3. Delivering benefits – the landing phase
The concept is that fuel loads on aircraft can be reduced if the landing site can apply ground power to assist the aircraft in its approach.
Delivering benefits from such a concept is challenging. If the power to be supplied is some kind of force field, perhaps from very large electro-magnetic poles, the field would be very directional but in the free air space would become very weak. The power of magnetic attraction between two magnets reduces at high inter-pole distances according to a very high power of the separation. Only very weak field strengths are therefore possible at sensible aircraft separations even with very powerful magnets in both base and aircraft.
Another concept put forward was that of a winch system that pulls aircraft over long distances. The aircraft would be a wing-in-ground-effect (WIGE) machine that "flew" at low altitude as it was pulled by a very long winch rope. This too is a challenging concept. If we suppose that aircraft will not be especially useful in the future for journeys under 150 Km where trains will dominate then only at this distance should wire propelled units become effective. The ground effect thrust/tension needed for a medium air vehicle would probably be in the range of 15,000 lbs.
Two distinct problems would need to be overcome. The weight of a 5/8" (nominal strength 40,000 lbs) wire rope would be 0.72 lbs/foot. For a double length rope of 150 miles x 2 the weight to be moved would be 150 x 5280 x 2 x 0.72 = 1.14 million lbs. This 500 tonne rope would also need to overcome the forces of friction.
Accelerating it would lead to single point traction forces probably substantially in excess of the rope strength (more than 300 tonnes reducing to perhaps 50 tonnes in the "cruise" phase).To spread out this traction load implies both a heavier cable and multiple traction units. In the case of the craft considered here a traction unit every 15 miles and a rope capable of withstanding an operating load of 10-15 tonnes would probably be needed.
The system’s overall efficiency might also be rather poor, mainly on account of the friction of the traction rope. The friction could, of course, be reduced by investment in chains of roller supports. The ground investment demands would be very great.
More serious might be the effect on longitudinal stability of the WIGE. These craft are noted for having critical stability in pitch. The weight and tension of any traction and trailing wires would impose considerable forces upon the vehicle that might well not be predictable to the required levels for stable operation. This could be avoided by attaching the WIGE to a separate length of wire that joined the traction wire some distance ahead and below the craft. By joining this wire to the WIGE near to the longitudinal centre adverse effects might be minimised.
4. Policy issues
Some of the devices discussed in this section would have policy and regulatory issues. For example changing the customer experience to one in which new dangers and received and new forces felt would require regulations to control their design. The TTOD would certainly require intense testing and would need to be designed to a new code and for there to be appropriate training facilities. In all new devices considerations of circumstances outside the design parameters would need to be explored and safe responses to them tested.
For the wire traction WIGE craft not only would the safety of the WIGE need to be established but the terrestrial safety and convenience also examined and regulated. It appears likely that only certain routes would be suitable for such a device i.e. those with significant flat stretches over relatively barren land.
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