Category NEW DESIGN CONCEPTS FOR HIGH SPEED AIR TRANSPORT

Passengers like comfort, and especially on long trips some level of comfort is necessary But w hat is the special kind of comfort required for supersonic transports.’

Range:

Passengers select a faster transport to save time, and they pay for the time saved. But this is the time fiom airport to airport or even house to house. Therefore supersonic transport only makes sense when the flight time is an essential part of the trip time, i. c. for long ranges

And passengers do not like stop-overs, because they prolong flight time and arc even more annoying than flight.

Therefore an SCT must provide long range capability to connect at least the most impor­tant areas of the world Europe. US and eastern Asia, all separated by up to 5000 to 6000 nm

Speed:

Passengers pay for the time saved. Therefore overall trip time has to be reduced, as well by high cruise speed without stop-overs, but. as importantly by accelerated check-in / check-out.

Additionally, many passengers feel very uncomfortable during and after trips of more than 6 hours flight duration, especially children, or disabled and elderly people, who will feel circulatory trouble.

Space:

Passengers want space similar to comparable subsonic transports, i. e. medium range transports with same flight duration. The narrow Concorde fuselage is only accepted for Concorde’s exclusivity.

Range:

Concorde is only able to serve s<»mc of the medium range overwater routes. This is an inattrac – uvely limned part of the market. To gam a sufficient pan of the market, future SCT must serve the important overwater long range routes. This, for the traits-Pacific routes, is more than 5500 ran range. Range is even more important than speed! For its limited range Concorde did not find a market, although Concorde’s range was at the achievable limits Also, the American SST, the Boeing В2707. failed for its insufficient range |45] and its uncontrolled growth of weight, even when environmental concerns were cited to stop the program (46) (which luckily limned the li­ability of Boeing).

Operations:

Today, there exists a very expensive environment for air traffic operations airports with its buildings and installations, hangars and air traffic control (АТС). New aircraft may require some additional installations, hut the necessary investments must be paid by the money earned with those aircraft And at least for the introduction of the new aircraft, success depends on the ability to cope with the existing environment. Time spent at the airport for debarking and embarking of passengers, serv icing of the aircraft, refuelling etc. must be minimized

• Aircraft dimensions (length, span or occupied ground area) ниш be compatible with exist­ing installations at the relevant airports

• Aircraft accessibility (door*) must be compatible w ith existing airport installations, service ports should he compatible w ith usual procedures

• Aircraft accessibility must allow for parallel de-/cmhark. service, fueling, etc.

• Aircraft supply needs mast be compatible with existing (big) systems, e g. fuel type.

• To serve the many routes with overland legs, subsonic cruise performance must be about as good as the supersonic one. Concorde has only poor subsonic performance.

Cost and fares:

A new SCT will become a success only if the manufacturer of the aircraft and the airlines oper­ating this aircraft will earn money with this aircraft. This requires production of a sufficient number of aircraft at a profitable price for the manufacturer, and operating costs which arc lower than the money paid by the passengers:

• To reduce seat mile cost and to serve a large market, an SCT must transport many passen­gers (size effect), at least as much as subsonic long range aircraft (e. g. A340k (Smaller SCT w ould – at first – only serve full fare passengers, but after a short period airlines will begin to introduce reduced fares to fill empty seats – because empty scats arc the most expensive seats-, and this leads to the situation we have lixlay with mostly km fare tick­ets )

• To gain a sufficient part of the market, a future SCT must serve all classes.

• Speed pays (see cars, trains, air traffic) but a surcharge must stay in acceptable limits It seems that the travelling public accepts surcharges of about the money they would cam for the time they saved by increased speed, even for holiday trips.

• An SCT with those (low) fares must still allow a profit for the manufacturer and airlines. And the manufacturer must sell many aircraft in order to provide an acceptable price. This becomes impossible with pure exclusivity.

Safety:

Concorde has proven to be safe. But since Concorde certification some new rules were intro­duced by the authorities e g FAA, JAA. which a new SCT has to meet. The most obvious chal­lenge is cabin pressure loss.

It is required that aircraft and passengers can survive pressure loss in the cabin provoked by a sudden hole like a broken window. Therefore, sufficient pressure levels must be maintained in the aircraft when a hole opens After DC 10 accidents, when a burst of underfloor cargo doors destroyed the floor of the aircraft with the hydraulic systems, size of this hole was increased by pure geometrical definition (about door size); for a wide body aircraft it has а яze of 20 sqft (FAR 25). This poses extreme difficulties for flight altitudes above 40000 ft. It is impossible to fight against this requirement by building a stronger aircraft, because there is no requirement on strength or probability of creation of such a hole, but only a given hole size Concorde needs only to survive a hole in the hull of window size; and Concorde has small windows.

(environment:

• Concorde produces unacceptable noise at takc-ofT and landing, although having only 110 passengers. A new SCT. which will be of at least double the size of Concorde, has to meet current noise standards (FAR 36. stage 3) or even more stringent ones. For comparison: FAR 36. stage 3 is just met by the B747-200 But SCT engines will not have a bypass ratio of about 5 like the В747-200 engines [40]. but only one of about 2.

• Best aerodynamic performance L/D (lift/drag) is reached at elevated values of the lift coef­ficient (about Cl=0.5 for subsonic aircraft, below CL=0.15 for supersonic flight). To fly at good aerodynamic performance, to maintain acceptable pressure levels in the inlets and engines, and to fill the engines with air, dynamic pressure at supersonic cruise will be in the range of about 20 to 30 kPa. Therefore, flight altitude depends on cruise speed and weight, for a Mach 2 aircraft about 16 km And the higher the cruise altitude, the more sensitive is the atmosphere to pollution, especially the ozone layer. At the time being it seems that a Mach 2-SCT will not harm the ozonclayer; but this is based on calculations which arc still questionable. Supersonic aircraft will burn more fuel per passenger kilom­eter than subsonic transports. Although C02 is not altitude sensitise, it is a well known greenhouse gas. and the large amount of COi emitted has to be justified. In the future it has to he expected that the public will become even more sensitive to environmental impacts. Therefore a new SCT has to demonstrate, that its impact on the earth’s atmos­phere is tolerable.

• A body cruising at supersonic speed generates a sonic boom which follows the body. This boom is an annoying and startling noise in an area of about 20 to 40 km at both sides of the SCTs track. To avoid harassment or damage, civil supersonic flight will only be permitted over sea or perhaps uninhabited land. Because noise of the natural environment is so high on the sea. there is no harassment or damage known to people, animals or ships below Concorde routes In contrast to people on land, there exists no complaint about Concorde’s sonic boom over sea [44].

Operations:

• Any new aircraft must be able to follow сштепі and future AIC (air traffic control) proce­dures. In lire future, steeper descent angles may be requested in the airport area, which could be a challenge for SCT.

• New SCT must meet current ground load values, i. e. w heel number, size, loading and dis­tribution. High ground load values may damage some airports, especially on aprons with tunnels.

• Loads on passenger and crew during operation must stay within acceptable boundaries Especially long clastic fuselages provide strong vibrations during ground roll and in turbu­lence. and may impose high g-loads during rotation.

There are only a few Concordes, which serve a small, exclusive class of passengers over limited distances at high fares. For its rare presence Concorde is allowed to meet only elementary envi­ronmental criteria which will not become more stringent for Concorde itself. Especially for noise. ICAO Annex 16 and FAR 36 only require, that Concorde must not become even more noisy than it was at certification; and Concorde is very noisy For new supersonic airliners those rules will never be applied; instead, a new SCT must fulfill new requirements which arc not met by Concorde:

• It must comply with all valid certification rules.

• it must be economically viable.

• it must provide sufficient comfon.

A new SCT must be "just another aircraft” [431. In the following paragraphs only those points arc mentioned which will introduce significant new challenges compared to Concorde.

NVhen comparing engines we have tocompurc installed engine efficiencies (tip.1- although mostly specific fuel consumption (SFC) is used.

SFC divides the fuel flow (i. c an energy flow) by the thrust force; so physically it is not a meaningful value and can only be compared at the same speed (v * flight velocity)). In contrast Пр is the amount of energy flow provided by engine thrust divided by the energy flow provided by the fuel (H = calonfic value of fuel). Пр and SFC are connected via the equation

H a V/8FC (31)

When not using a consistent system of units like SI. the respective unit conversions have to be applied For Kerosene H is given by

H = 42.817 MJ/kg (32)

Now we can compare engines. At Mach 2 0-cruise. Olympus (401 and a proposed new supersonic engine arc given, as well as a modem high bypass transonic engine used for wide – body aircraft [41):

We see that Olympus is still very good. The improvements of subsonic engines, mainly achieved by strung!) increased bypass ratio, did not yet reach Olympus’ efficiency. Indeed, at supersonic cruise a modern optimized engine w ith very low bypass ratio would provide slightly better values than the one given above But this engine like Olympus will never meet the strin­gent noise criteria at take-off and landing which new SCT have to fulfill Probably a bypass ratio of about 2 and extensive noise suppression (damping plus cjectori will be applied to meet noise criteria; this will decrease engine efficiency at supersonic cruise to the values indicated above.

When comparing aerodynamic improvements since ihc Concorde’s lime, we find strong differ­ences in the aerodynamic efficiency between Concorde lime subsonic aircraft like B747 or B737 and modern aircraft like the B777. A340or A320. expressed by improved L/D (lift/drag). These subsonic aircraft fly at high subsonic Mach numbers, when the air flow at the aircraft locally reaches panially subsonic, partially supersonic Mach numbers, which ls named transonic flow. Physics of transonic flow include strong nonlmearities like shock waves, and the governing equations change from elliptic to hyperbolic type Whereas the old aircraft were designed using pure subsonic linear potential flow theory combined with simple sweep theory, improvements were provided exploiting nonlinear theory, the latter require modem high performance comput­ers not available for Concorde development

But flow around supersonic airliners like the Concorde is dominated by small distur­bances of the incoming flow, because strong disturbances would create high wave drag There­fore. design of supersonic airliners can mostly be based on linearized potential flow theory, as was the Concorde (slender body theory). Only for some pans nonlinear effects have to be respected for: strong interference effects like engine integration, fine tuning of the configuration and strongly nonlinear boundary layer flows. But. except for laminar flow, other strongly non­linear boundary layer flows like separation arc avoided because they arc connected with large drag increases Therefore, modem nonlinear aerodynamic theory can only provide limited improvements compared to Concorde (except for laminar flow).

Although, some aerodynamic improvements may be provided:

• New materials providing higher specific stiffness may allow a higher aspect ratio via inter­disciplinary effects.

• Local optimization for nonlinear flow phenomena will prov ide reduced interference drag, especially for engine integration.

• Supersonic laminar flow is the only new aerodynamic technology which can strongly improve performance. But it is still far away from realization for large transport aircraft.

The first example of technologies are weight improvements, listed m Table 2

OWE operating empty weight. PAY payload. F: fuel

PAX: passengers HIOW maximum Ukr-oft weight

Part of improvement was used to mcreavr range, part Ю increase payload

Performance comparison by foe! per passenger kilometer

About the same development generation Concorde B737-200. B747*IOO

Modem aircraft. 25 yean later В737-500. B747-400. A340-300E

•’Data base for the different 87 37-versions seems to be і act* sister*. because the more efficient engine of the B737-300 (CFM-56 instead of JT-SD foe B737-200) should improve aircraft efficiency, at least for long ranges

Boeing В737-200 and B747-100 were developed in parallel wiih Concorde (40). The new gen­eration of comparable size are B737-500. B747-400 and Airbus A340-300E |41); the latter being handicapped when compared with the B747-400. because it is a bit smaller Improvements of fuel per passenger-kilometers of more than 309r were achieved; but comparison to the old air­craft is difficult, because data base has changed (improved seating standards etc ).

When comparing Concorde’s structure weight, it is still comparable to the A340. (Comparison can only be made and was made by designing both aircraft with the same design tool. Assuming several structure technology standards, results showed the weights to reach the actual values of Concorde resp A340, when using the same structure standards for both designs (42)). But what arc the technologies leading to the subsonic weight improvement or Concorde’s advanced values?

• Because Concorde has only a small payload fraction, it is much more sensitive to weight increments; therefore more effort was spent for weight savings and expensive solutions became useful.

• Many (weight) improvements for transonic aircraft after B737-I00, B747-100 were pro­vided by interdisciplinary effects like

high bypass engines or

optimized (nonlinear) transonic aerodynamics (Figure 10) via. increased wing profile thickness and volume, reduced wing sweep.

which cannot be transferred to a Concorde type configuration.

• System weight was only marginally reduced since Concorde.

• Since Concorde’s time many improvements in structure technology were not transferred into weight improsements. hut offset by advanced safety requirements; e g. new require­ments for pressure losses, cabin evacuation or fire resistance of cabin equipment. But this is the world where a new SCT must fly.

increased Wng Tmefcnaa* and Vc*jm«

J. Merten*

Daimler-Benz Aerospace Airbus GmbH, Bremen, Germany

3.1 Concorde Technology Level

Concorde (Figure 9) is the only supersonic airliner which has been introduced into regular pas­senger service. It is still in service at British Airways and Air France w ithout any flight accidents, and probably will stay in service for at least for ten more years.

Concorde has experienced the most supersonic flight hours and flight miles of any air­craft Indeed, the twelve flying Concordes have accumulated more supersonic flight hours than die total of all military aircraft in the world Concorde s range is about 6 500 km, whereas the best fighters like the Su-27 or F-22 (so called “superc-misers") achieve about 200 km in sus­tained low supersonic cruise, and military supersonic bombers or reconnaissance aircraft like the B-l or SR-71 reach about 3.500 km without refuelling. But although the supersonic flight range of Concorde is by far better than for any other supersonic jet built, this range was the most important limning factor in the commercial success of the Concorde. A new tabic supersonic airliner, called Supersonic Commercial Transport (SCT). must be able to sene the important trans-Pacific market, requiring a range of 10.000 to 11.000 km. This is a tremendous improve­ment compared to the Concorde

What arc the differences between Concorde and a new SCT? Besides the larger size, which improves the range performance a bit, technology improvements arc cited to enable this big step forward. So. let’s look at the technology improvements we have achieved in aviation since the Concorde design As there is no other supersonic airliner, wc have to compare Con­corde’s contemporary subsonic airliners with the newest generation of subsonic airliners

Figure 9 Concorde

Aircraft flight performance is governed by aerodynamics, structures and engines All other specific disciplines, although often important for the viability of an aircraft, are only weakly related to flight performance of transport aircraft. Therefore we will look at the improvements in these three mam disciplines

So far we have focussed on engineering tradeoffs in an equilibrium market where the manufac­turer sells the required break-even production run and the airline sells the seats w ith the average load factor. Such a model may be very useful for preliminary economic evaluation of aircraft projects. In reality however, nobody wants to sell the same product at the same pnee as their competitors. In doing so they would invariably lower the price of the product offered

The tnck is to offer something at a lower cost and higher price. This can only be done by offering something new. As an example we will discuss the introduction of a supersonic air­craft under the assumption that it does not violate any existing performance or environmental regulations

The manufacturer is now faced with two more variables: The number and pnee of the aircraft he can sell. Since he – we hope, but this is for from sure • knows what his production costs are. he would then have an idea of whether he could make a profit.

The aircraft number of units versus price is determined by a complex macro-economic model, that aircraft manufacturers such as Airbus can use as a subroutine in an ovcral aircraft optimization program.

The price and number of aircraft are determined by the iteration shown in Figure 8. Its starts with an estimate of the price and number of aircraft sold. We can now use the manufacturer and airline cost models to estimate the (future) total operating cost of the aircraft

For a given aircraft service (speed i passengers arc willing to pay various amounts of money on various routes. Passengers will he inclmdcd to pay more for shorter travel times. Typ­ical business class passengers will also pay more than coach passengers, because their time is worth more. Figure К shows what fraction of passengers is willing to pay what as a function of the fare class The airline will try to sell the seats as expensive as possible So. as a function of the local route class distribution the airline will get more revenue. The airline has to sum all the revenue over all the routes an aircraft flics to decide whether to buy an individual aircraft

Ibis decision is made based on airline profit. The profit is the revenue minus the total operating cost. If the revenue of an individual aircraft purchase exceeds that of the aircraft it is replacing and there is no competing product, the manufacturer can expect to make a sale.

This ‘mixed fleet’ concept of supersonic and subsonic airliners is also shown in Figure 8 . Тік: curves shown here are typical for supersonic transports which command higher seat pric­es at higher operating cost than current subsonic transports

When very few aircraft arc sold, the airline is doing well. It sells all the seats of the su­personic №М$рОЛ at a premium to first class passengers. The premium is higher than the premi­
um they can charge in the more competitive subsonic markets, so the airline makes more profit with this fleet mix than with a uniformly subsonic fleet. As they buy more aircraft however, more and more business class and couch class travellers fill the scats of the supersonic transport. Since the profit per supersonic coach seat is less than the profit per subsonic coach scat, the airline’s overal revenues will go down. It is obviously also true that as the manufacturer raises the price of the aircraft the airlines profit goes down too.

For the manufacturer it is another story. The more he can ask tor his aircraft the better. It is even better to sell more aircraft than to increase the purchase price As he sells more aircraft his revenues go up steeply.

The price and number of units the manufacturer can sell are determined by tlie intersec­tion point of the airlines profit and the manufacturers profits. When the curse representing the intersection of manufacturer and airline profits goes up with the number of units, the market will keep buying the aircraft at relatively low prices. This is the case for most successful products. If the curve drops w ith the number of units, relatively few units w ill be sold at a premium. This is the case of supersonic aircraft which have a higher total operating cost than competitive subsonic transports.

The decision to launch a supersonic transport will be determined by estimates of the manufacturers future cash flow Depending on the cost of development, the manufacturer will have to go into deep debt to finance the enterprise. Even after he starts selling the plane his neg­ative cash flow will still increase since he will have to sell the initial planes at a discount while his actual cost – Wright curve – is much higher than the average production run.

It is therefore highly likely th3t a sealed up version of Concorde will suffer the same market fate as Concorde itself. Only when the total operating costs come down to values that arc very close to those of current subsonic transports will the projected sales be large enough to jus­tify launching such a project. International cooperation may reduce the cash flow risk for indi­vidual companies and ameliorate this situation somewhat.

The main problem with this macroeconomic model is thai its inputs are determined by inters icw ing the custumcr (airlines), and that his views may have changed by the time the aircraft is offered for sale Therefore probability distributions have to be attributed to different scenarios.

2.4 Conclusions

The present work provides a tool to tradeoff commercial aircraft design features assuming a bal­anced market for subsonic and supersonic jet aircraft up to Mach 2.2. The model integrates older methods such as the ATA ’67 and the American Airlines studies of the 1980’s with newer data into one consistent whole.

Furthermore, we have presented a simplified qualitative macroeconomic model that predicts aircraft price and the number of units sold. We showed that the simplest way to sell an aircraft is to offer a product with better or equal performance than the product it is replacing at the same price or to offer the same product perfonnace at a lower price. Trading off higher per­formance at a higher price is highly speculative in an unknown market.

Acknowledgements

The author would like to thank Mr Schwartz, Mr Zirnmermann and Mr. Kacskc of engineering cost and market forcasting at Daimler Benz Aerospace Airbus Hamburg lor their input.

. Journal of Aeronautical sciences, feb 1936

The DOC is directly related to the cost of development and the cost of production to the manu­facturer as well as the cost of operation Therefore the DOC of two aircraft analyzed with the same method and conditions will allow both the airframe manufacturer and the airline to select the better aircraft[5]. This is not the whole story, however. One aircraft may be able to attract more payload than the other. Since passenger carrying aircraft should be compared at the same level of comfort only the time saved by the passenger would make him pay more for a ticket. In many publications, for instance by Mizuno and others [34]. the additional revenue due to time savings w as found by means of a qucstionairc. They found the value of time for economy travellers to be $40 (1990) per hour. The question is whether economy passengers would stay an hour longer in the airplane if they were paid less than $40 dollars an hour to do so. In the author’s opinion more titan half the passengers would Based on Steiners (351 book 1 propose to use the average hourly wage as the time value of the economy passenger. To compare one aircraft to a reference aircraft

wc have to subtract the additional revenue due to time savings of the aircraft with respect to the reference aircraft.

In this expression $20 represents the estimate average hourly income of US air travel* lers in 1995. A corrected total operating cost for a balanced market can now be defined as:

TOC^r ~ DOC ♦ IOC – ^REV