Air to Air Refuelling in Civil Aviation, An Opportunity & A Vision Dr ...

efficiencies drop off markedly. The high efficiency
of the short range aircraft can be maintained over
the longer ranges using AAR.
Reserve fuel policies vary between regulatory
bodies, domestic / International flights, airlines and
pilots. Pilots may opt for extra reserve fuel over and
above that required by relevant regulations.
Typical average values for X are 15000+ nm.
The current trend is for ever-larger more
powerful engines. Naturally, the emphasis is on
increasing fuel efficiency for the larger units. Some
of the favourable design developments can be
scaled for smaller engines, fuel efficiencies are
relatively less for the latter. Geared fans may help.
The development of Prop fans may result in
quieter and more fuel-efficient power-plants. The
effects on X values due to the resulting changes in
V and SFC need to be assessed.
3. AIR-TO-AIR REFUELLING (AAR),
CIVIL AIRCRAFT
3.1. Comparing 3000, 6000, 9000 & 12000 nm
Range Aircraft with and without AAR
Based on Refs.8, 11 & 12, the approach is to
design representative aircraft to carry the same
payload of 250 pax. over 3000, 6000, 9000 & 12000
nm and then estimate the fuel saved by using the
base 3000 nm range aircraft with AAR for the
longer ranges. The base aircraft requires less than
50,000 lbs of fuel per 3000 nm stage, dispensed
fairly easily from current tankers, provided the
operational tanker range is kept low. Each tanker
may accomplish 3-4 operations in a mission.
The relative sizes of the aircraft designed for 250
passengers over the different ranges are compared
in Fig.11. The fuselage size remains constant, the
wing area increases rapidly to contain fuel needed
and to maintain CL as design range increases.
The base aircraft weight variation over 3000 nm
is shown in Fig. 12. The block fuel used to carry
250 pax over this range is 46,147 lb (MTOW =
261,932 lb). An aircraft designed to carry the same
payload over 6000 nm, Fig. 13, uses 161,269 lb fuel
(doubling the range has more than trebled the fuel
required, MTOW = 505,438 lb). The increased fuel,
over and above that required for the doubled range,
is needed for the additional aircraft weight. This
arises from landing gear / wing structure required to
carry the additional fuel weight and provide the
extra tank volume. Fig. 13 also compares the weight
variations with range for the 6000 nm aircraft and
the 3000 nm aircraft refuelled at 3000 nm. Note the
fuel used and the savings offered by AAR, 42%
over 6000 nm (not including tanker fuel at this
stage). This amounts to saving in fuel cost of nearly
$190/pax (at $4.7/USG). With an allowance of
tanker fuel, the comparable figure is about $175.
Fig. 14 refers to the weight variations for 9000
nm range (250 pax). An aircraft without a refuelling
option would have MTOW of 656,262 lb, and burn
263,073 lb of fuel. With the 3000 nm aircraft and
two AAR operations, the block fuel would be
138,441 lb, a saving of 47% (without tanker fuel).
This amounts to saving in fuel cost of nearly
3
$350/pax (at $4.7/USG). Allowing for the tanker
fuel, the comparable figure is about $320.
3.2. Size. Cost & Noise Reductions Using Smaller
Aircraft
Even with current technology levels, using the
3000nm aircraft for all long ranges with AAR
implies not just size reduction but also associated
reductions in noise, cost, emissions, etc. We look
briefly at aircraft sizing, cost and noise trends.
Fig.15 shows the span correlation against
MTOW for several current aircraft (www.Boeing).
Ideally, wing Aspect Ratio and hence span should
be as large as possible for aerodynamic benefits.
For larger design payloads and ranges, the size and
span are subject to physical / structural / airport gate
limits (e,g. ICAO).
Fig.16 shows the acquisition cost trend against
OEW (lines C1 & C2 represent the lower and
higher quoted costs). Fig.16 shows an approximate
correlation against range. Such trends reflect the
economic situations (supply, demand, numbers built
etc.) and may be somewhat hypothetical. Essentially
large aircraft cost more.
Fig 18 shows ICAO Stage 3 noise certification
limits. Lighter aircraft have more stringent noise
requirements than heavier aircraft. In any case,
smaller aircraft do tend to be quieter because of less
installed thrust.
By using smaller, 3000 nm range aircraft with
AAR, the increases in costs, noise and pollution
associated with long-range flights can be avoided.
3.3. Cruise Climb or Constant Altitude Flight
It is useful to examine the cruise and altitude
relationships with the different range aircraft.
Aircraft wings are optimised for cruise at a
constant CL value of about 0.55. Assuming start of
cruise at 28,000 ft, if CL and M are held constant the
3000nm aircraft would climb to 32,000ft during the
cruise phase. The 6000 nm aircraft climbs to about
36,000ft and the 9000nm aircraft will reach
39,000ft, see Fig.19.
Air traffic control requires 2000 ft vertical
separation between aircraft and consequently
aircraft fly at constant stepped altitudes. Decreases
in CL, (in spite of L/D reduction) are accompanied
by a reduction in drag and thrust required. So the
compromises are lessened.
The long-range aircraft undergo the greatest
change in altitude during cruise. The fuselage has to
withstand an increased pressure differential with
obvious implications on stress, fatigue and initial
design weight. Short-range aircraft are naturally
better balanced for altitude effects and these could
be maintained over long ranges with AAR.
At constant CL, simple estimates indicate a 2%
degradation in L/D during cruise for a 3000 nm
range aircraft. For 6000 and 9000 nm range aircraft
the degradation is of the order of 7% and 12%
respectively. With 3000 nm aircraft and AAR on
the long ranges, the L/D degradations can be small.
With variable camber, constant CL need not be
maintained and altitude relationship become easier.
3.4. Fuel Burn & Operating Costs Issues
The variations of fuel burn and costs with design
range are summarised in Fig.20. The projected fuel