SI VIS PACEM, PARA BELLUM: L'AMSTc (Advanced Medium STOL Transport competition): concorso indetto negli anni ’70 dall'USAF per sostituire i C-130 Hercules.

L'AMSTc (Advanced Medium STOL Transport competition): concorso indetto negli anni ’70 dall'USAF per sostituire i C-130 Hercules; dopo anni di sperimentazioni, nacque finalmente il BOEING C 17 GLOBEMASTER III !

Uno dei due finalisti era il Boeing YC-14 era un bimotore turboventola da trasporto tattico ad ala alta con caratteristiche STOL realizzato dall'azienda statunitense Boeing negli anni settanta.

Proposto come il McDonnell Douglas YC-15 nel programma AMST della USAF rimase, come il concorrente, allo stadio di prototipo per l'avvenuta cancellazione del programma.

Storia del progetto

Nel 1971 la United States Air Force promosse una serie di programmi di sviluppo per la valutazione di nuovi velivoli atti a sostituire il parco velivoli esistenti, il Lightweight Fighter (LWF) per un aereo da caccia leggero ed il Advanced Medium STOL Transport (AMST) (aereo STOL da trasporto medio avanzato).

L'AMST mirava ad ottenere un velivolo in grado di sostituire i Lockheed C-130 Hercules in dotazione ai reparti da trasporto USAF. Le specifiche emesse riguardavano la necessità di poter decollare in una distanza massima di 2 000 ft (610 m) da una superficie semi-preparata e di poter operare con un carico di 27 000 lb (12 000 kg) in un raggio d'azione di 400 nm (740 km). Le aziende avrebbero dovuto fornire i progetti per la valutazione iniziale e, dopo la prima fase, due prototipi per le valutazioni comparative.

Al concorso risposero cinque aziende tra cui la Boeing e la McDonnell Douglas, poi contattate per la realizzazione dei velivoli, in quanto giudicati i più meritevoli allo sviluppo, ai quali vennero rispettivamente assegnate le designazioni ufficiali YC-14 e YC-15.

Le prove comparative si svolsero tra il 1975, anno in cui volò per la prima volta l'Y-15, fino al 1979, quando il programma venne cancellato in favore del Programma C-X, che nella decade successiva avrebbe generato il McDonnell Douglas C-17 Globemaster III. Anche se sia l'YC-14 che YC-15 si rivelarono un successo, nessuno dei due aerei venne avviato alla produzione in serie.

Impiego operativo

Il primo Boeing YC-14 (numero 72-1873) volo la prima volta il 9 agosto 1976. Vennero costruiti due aerei, il secondo con numero di coda numero 72-1874. L'avversario, lo YC-15 aveva iniziato a volare quasi un anno prima. i test di volo testa a testa vennero effettuati alla Edwards AFB agli inizi di novembre 1976. Al completamento dei test a fine estate 1977, i prototipi del YC-14 vennero rimandati alla Boeing. I prototipi non vennero demoliti, e uno venne immagazzinato presso lo AMARC, alla base aerea Davis-Monthan e l'altro venne messo in mostra al vicino Prima Air & Space Museum.

A questo punto il programma AMST era già in procinto di essere cancellato. Nel marzo del 1976 il generale dell'Air Force David C. Jones chiese allo Air Force Systems Command se si potesse usare un solo modello per entrambi i modelli di trasporto tattico e strategico, o alternativamente, se si sarebbe potuto sviluppare un derivato non-STOL per il trasporto strategico. Questo portò ad una serie di studi, che in sostanza mostrarono che una tale modifica non sarebbe stata facile, e avrebbe richiesto degli sviluppi sostanziali da inserire nel progetto per poter produrre un aereo più grande. Sia l'YC-14 che l'YC-15 raggiunsero o addirittura superarono le aspettative dell'AMST sotto molti punti di vista. Tuttavia, la crescita dell'importanza di un trasporto strategico rispetto ad uno tattico portarono alla fine del programma AMST nel dicembre 1979 a favore di uno nuovo che portò al C-17 Globemaster III.

L'USB (upper-surface blowing, i motori posti sopra l'ala e col flusso che viene diretto sui flap), utilizzato nella progettazione dell'ala del velivolo, rimase un concetto raro nell'uso, ed è stato usato solo su pochi altri aerei, come sull'Antonov An-72.

Utilizzatori: Stati Uniti - United States Air Force.

Il McDonnell Douglas YC-15 è un aereo da trasporto tattico quadrimotore, sviluppato nel corso degli anni settanta, concorrente nell'AMSTc (Advanced Medium STOL Transport competition), indetto dall'USAF per sostituire la propria linea da trasporto tattico composta da C-130 Hercules. Al termine delle valutazioni però, né l'YC-15 né il suo diretto avversario, il Boeing YC-14, vennero dichiarati vincitori. Tuttavia, i progetti dell'YC-15 furono in seguito impiegati per la realizzazione del più noto C-17 Globemaster III.

Storia

Sviluppo

Nel 1968 l'USAF intraprese lo sviluppo di una serie di prototipi che sarebbero stati presentati in occasione dei programmi AMST e Light Weight Fighter. Il bando di gara venne ufficializzato nel gennaio del 1972, ed i requisiti minimi richiesti ai velivoli furono quelli di poter operare da piste semipreparate di lunghezza massima pari a 2 000 m aventi un carico di 12 400 kg ed un raggio d'azione pari a 740 km. Altri velivoli furono proposti da Bell, Boeing, Fairchild, McDonnell Douglas e Lockheed/North American Rockwell. Il 10 novembre 1972, furono resi noti i nominativi dei velivoli vincitori i quali risultarono essere i prototipi proposti da Boeing e McDonnell Douglas. Alle aziende venne perciò richiesta la realizzazione di ulteriori due esemplari dei rispettivi velivoli. La McDonnell Douglas decise di designare, in questa occasione, il proprio velivolo con la sigla YC-15.

Tecnica

Il progetto della McDonnell Douglas prevedeva la realizzazione d'un velivolo con profilo alare supercritico, risultato di numerosi studi della NASA condotti da Richard Whitcomb. Quest'ala offriva una resistenza aerodinamica del 30% inferiore a quelle con profilo convenzionale, ed allo stesso tempo era in grado di generare un'eccellente portanza alle basse velocità. La maggior parte degli aerei fino ad allora prodotti erano soliti utilizzare ali a freccia per diminuire la resistenza nel fluido, ma ciò portava inevitabilmente ad un volo molto instabile alle basse velocità che li rendeva inadatti per operazioni STOL (Short Take Off and Landing).

La squadra di progettisti, scelse quindi di impiegare un rivoluzionario sistema d'ipersostentatori a soffiaggio esterno con lo scopo di aumentare la portanza. Questo sistema impiegava doppi flap per direzionare parte della spinta dei motori verso il suolo, mentre il resto dei gas di scarico veniva fatto passare attraverso questi seguendo il profilo alare generando un effetto Coandă. Il loro impiego tuttavia non fu realizzabile fino a quando non vennero introdotti i propulsori turboventola il cui flusso di gas di scarico risultava essere meno concentrato e rovente; il problema venne temporaneamente rimandato con l'adozione di ugelli più voluminosi. I propulsori impiegati furono i Pratt & Whitney JT8D, largamente utilizzati sul Boeing 727. L'YC-15 prese in prestito numerose parti dagli altri velivoli prodotti dalla McDonnell Douglas: la prua proveniva dal DC-8 mentre il cockpit dal DC-10.

Impiego operativo

Vennero realizzati due YC-15, uno avente apertura alare di 33,5 metri, l'altro di 40,2 metri. Entrambi erano motorizzati da quattro propulsori turboventola Pratt & Whitney JT8D-17, ognuno capace di 68,9 kN di spinta.

Il primo volo avvenne il 26 agosto 1975. Il secondo prototipo fu portato in volo nel dicembre successivo. Nel 1976, quando furono pronti anche gli esemplari prodotti da Boeing, entrambi i prototipi furono trasferiti presso la Edwards Air Force Base per una serie di test comparati, tra i quali figuravano il trasporto di carichi voluminosi, carri armati e pezzi d'artiglieria, dalla pista semi preparata del Graham Ranch.

L'YC-15 completò l'intero programma, pari a 600 ore di volo, nel 1977. Nel marzo 1976, il capo di stato maggiore dell'aeronautica statunitense il Gen. David C. Jones chiese all'Air Force Systems Command se fosse possibile utilizzare uno solo degli esemplari del progetto AMST per entrambi i ruoli cargo: tattico e strategico. Tutto ciò portò ad una serie di studi che constatarono la difficoltà nel realizzare tali modifiche, che potevano essere più radicali del previsto.

Entrambi i velivoli, sia l'YC-14 che l'YC-15, si rivelarono più efficaci del previsto rispondendo largamente alle richieste del programma AMST. Tuttavia, il bisogno di disporre d'un velivolo da trasporto strategico si fece sempre più strada tanto da portare alla chiusura del programma di sviluppo nel dicembre del 1979. Così nel novembre dello stesso anno, venne istituita la C-X Task Force, con l'obiettivo di riuscire a sviluppare un velivolo dalle caratteristiche differenti. Dal programma C-X venne selezionato un esemplare più capiente dell'YC-15 che fu poi sviluppato nel C-17 Globemaster III.

I prototipi dell'AMST furono destinati all'AMARC, presso la Davis-Monthan Air Force Base. Uno di essi venne successivamente esposto presso il vicino Pima Air & Space Museum. Il primo esemplare realizzato dell'YC-15, (matricola 72-1875) tornò in volo nel 1996 ed ancora nel 1997 per conto della McDonnell Douglas nell'ambito della realizzazione del Globemaster. Un problema tecnico legato al cattivo funzionamento di uno dei propulsori, reputato troppo oneroso, lo relegò per diversi anni presso un magazzino della Boeing presso l'Air Force Plant 42 di Palmdale in California. Attualmente è in mostra statica presso la Edwards AFB a cui è stato riconsegnato.

ENGLISH

The Boeing YC-14 was a twinjet short take-off and landing (STOL) tactical military transport aircraft. It was Boeing's entrant into the United States Air Force's Advanced Medium STOL Transport (AMST) competition, which aimed to replace the Lockheed C-130 Hercules as the USAF's standard STOL tactical transport. Although both the YC-14 and the competing McDonnell Douglas YC-15 were successful, neither aircraft entered production. The AMST project was ended in 1979 and replaced by the C-X program.

Design and development

In mid-1970, the USAF began a paper study, the Tactical Aircraft Investigation (TAI), with Boeing, McDonnell Douglas, and other companies to look at possible tactical transport aircraft designs. This study was a precursor to what became the Advanced Medium STOL Transport program. As a part of this program, Boeing began to look at various high-lift aircraft configurations. Boeing had earlier proposed an underwing externally blown flap solution for their competitor for the Lockheed C-5 Galaxy, and had put this to good use when they modified their losing entry into the Boeing 747. They had also done studies with the original Boeing 707 prototype, the Boeing 367-80, adding extensive leading and trailing edge devices using blown flaps. For the TAI studies, Boeing again looked at those mechanisms, as well as new mechanisms like boundary layer control. However, none of these studied designs were particularly appealing to Boeing.

The Boeing engineers were aware that NASA had carried out a series of "powered lift" studies some time earlier, including both externally blown flaps, as well an upper-surface blowing (USB), an unusual variation. In the USB system, the engine is arranged over the top surface of the wing, blowing over the flaps. When the flaps are lowered, the Coandă effect makes the jet exhaust "stick" to the flaps and bend down toward the ground. They searched for additional research on the concept, and found that half-span upper-surface blowing research had been conducted in the NASA Langley 12-foot (3.7 m) tunnel. An examination of the preliminary results suggested that the system was as effective as any of the other concepts previously studied. Boeing immediately started to build wind-tunnel models to verify the NASA data with layouts more closely matching their own designs. By the end of 1971, several models were being actively studied.

Another NASA project the engineers were interested in was the supercritical airfoil, designed by Richard Whitcomb. The supercritical design promised to greatly lower transonic drag, as much as a swept wing in some situations. This allowed an aircraft with such a wing to have low drag in cruise while also having a wing planform more suitable to lower-speed flight—swept wings have several undesirable characteristics at low speed. Additionally, the design has a larger leading edge radius that makes it particularly suitable for low-speed high-lift applications like a transport. Boeing incorporated the concept into their design, the first non-experimental aircraft to do so.

The request for proposal (RFP) was issued in January 1972, asking for operations into a 2,000-foot (610 m) semi-prepared field at 500 nautical miles (930 km) with a 27,000 lb (12,000 kg) payload in both directions with no refueling. For comparison, the C-130 of that era required about 4,000 ft (1,200 m) for this load. Five companies submitted designs at this stage of the competition, Boeing with their Model 953 in March 1972. On 10 November 1972 the downselect was carried out, and Boeing and McDonnell Douglas won development contracts for two prototypes each.

Wind tunnel tests continued through this period. In November, John K. Wimpress again visited Langley looking for an update on NASA's own USB program. Joe Johnson and Dudley Hammond both reported on testing and showed Wimpress data that verified the high-lift performance that Boeing had quoted in its proposal. By December 1975, Boeing and NASA Langley had arranged a contract for a full-scale USB testbed, which Boeing built at their Tulalip test facility consisting of a 1/4-scale wing with one JT-15D engine and a partial fuselage. Langley was particularly interested in the effectiveness of the D-shaped nozzle that directed the jet flow over the upper surface of the wing, as well as the resulting sound levels, at that time a major focus of NASA's civilian aerodynamics research.

Two major problems were found and corrected during testing. The first was a problem with air circulating around the wing when operating at low speeds close to the ground, which had a serious effect on the spreading of the jet flow through the nozzle. This led to flow separation near the flap, and a decrease in effectiveness of the USB system. In response, Boeing added a series of vortex generators on the upper surface of the wing, which retracted when the flap was raised above 30°. Additionally, the tail surfaces were initially placed well aft in order to maximize control effectiveness. This positioning turned out to interfere with the airflow over the wings during USB operations, and a new tail with a more vertical profile was introduced to move the elevator forward.

Operational history

The first Boeing YC-14 (serial number 72-1873) flew on 9 August 1976. Two aircraft were built, the second being s/n 72-1874. The competing YC-15 had started flights almost a year earlier. Head-to-head flight testing at Edwards Air Force Base started in early November 1976. During flight testing, the YC-14 was flown at speeds as low as 59 kn (68 mph; 109 km/h) and as high as Mach 0.78 at 38,000 feet (11,600 m). However, it was found that the YC-14's drag was 11% higher than originally predicted. Modifications developed in wind tunnel testing, comprising the addition of vortex generators to the upper aft portion of the nacelles, deletion of the nozzle door actuator fairing, alterations to the aft end of the landing gear pods and the addition of aft fuselage strakes, reduced this drag increment to 7%. The YC-14 also demonstrated the capability to carry the 109,200-pound (49,500 kg) M60 Patton main battle tank, something that was not demonstrated with the YC-15.

At the completion of testing in the late summer of 1977, the YC-14 prototypes were returned to Boeing. The prototypes were not scrapped; one is stored at AMARC, located at Davis-Monthan Air Force Base and the other is on display at the nearby Pima Air & Space Museum.

By this point, the seeds of the AMST program's demise had already been sown. In March 1976, the Air Force Chief of Staff, Gen. David C. Jones, asked the Air Force Systems Command to see if it would be possible to use a single model of the AMST for both strategic and tactical airlift roles, or alternatively, if it would be possible to develop conventional derivatives of the AMST for the strategic airlift role. This led to a series of studies that basically stated that such a modification was not easy, and would require major changes to either design to produce a much larger aircraft.

Both the YC-14 and YC-15 met or exceeded the AMST specifications under most conditions. However, the increasing importance of the strategic vs. tactical mission eventually led to the end of the AMST program in December 1979. Then, in November 1979, the C-X Task Force formed to develop the required strategic aircraft with tactical capability. The C-X program selected a proposal for an enlarged and upgraded YC-15 that was later developed into C-17 Globemaster III.

Upper surface blowing remains a fairly rare concept in use, and has been seen only on a few other aircraft, such as the Antonov An-72.

Aircraft on display

Aircraft serial number 72-1873 is on display at the Pima Air & Space Museum in Tucson, Arizona. The other is in storage at the 309th AMARG boneyard at Davis Monthan Air Force Base (32.17184°N 110.84743°W).

Specifications

General characteristics

Crew: 3
Capacity: 150 troops or 69,000 lb (31,400 kg) (STOL: 27,000 lb (12,300 kg))
Length: 131 ft 8 in (40.14 m)
Wingspan: 129 ft 0 in (39.32 m)
Height: 48 ft 4 in (14.74 m)
Wing area: 1,762 sq ft (163.7 m2)
Empty weight: 117,500 lb (53,410 kg)
Max takeoff weight: 251,000 lb (113,850 kg) (conventional landing and takeoff), 170,000 lb (77,270 kg) (STOL)
Powerplant: 2 × General Electric CF6-50D turbofans, 51,000 lbf (230 kN) thrust each.

Performance

Maximum speed: 504 mph (811 km/h, 438 kn)
Cruise speed: 449 mph (723 km/h, 390 kn)
Ferry range: 3,190 mi (5,136 km, 2,734 nmi)
Service ceiling: 45,000 ft (13,716 m)
Rate of climb: 6,350 ft/min (32.3 m/s).

The McDonnell Douglas YC-15 was a prototype four-engine short take-off and landing (STOL) tactical transport. It was McDonnell Douglas' entrant into the United States Air Force's Advanced Medium STOL Transport (AMST) competition to replace the Lockheed C-130 Hercules as the USAF's standard STOL tactical transport. In the end, neither the YC-15 nor the Boeing YC-14 was ordered into production, although the YC-15's basic design would be used to form the successful McDonnell Douglas (later Boeing) C-17 Globemaster III.

Design and development

In 1968, the USAF started work on a series of prototype proposals, which would lead to both the AMST project and the Light Weight Fighter. The official Request for proposal (RFP) was issued in January 1972, asking for operations into a 2,000-foot (610 m) semi-prepared field with a 27,000-pound (12,000 kg) payload and a 400-nautical-mile (460 mi; 740 km) mission radius. For comparison, the C-130 of that era required about 4,000 feet (1,200 m) for this load. Proposals were submitted by Bell, Boeing, Fairchild, McDonnell Douglas and the Lockheed/North American Rockwell team at this stage of the competition. On 10 November 1972, the two top bids (from Boeing and McDonnell Douglas) were selected. The companies were awarded development contracts for two prototypes each. McDonnell Douglas' prototype was designated YC-15.

McDonnell Douglas's design incorporated a supercritical wing, the result of NASA research carried out by the already famous Richard Whitcomb. This wing design dramatically lowers transonic wave drag by as much as 30% compared to more conventional profiles, while at the same time offering excellent low-speed lift. Most contemporary aircraft used swept wings to lower wave drag, but this led to poor low-speed handling, which made them unsuitable for STOL operations.

The design team also chose to use externally blown flaps to increase lift. This system uses double-slotted flaps to direct part of the jet exhaust downwards, while the rest of the exhaust passed through the flap and then followed the downward curve due to the Coandă effect. Although the effects had been studied for some time at NASA, along with similar concepts, until the introduction of the turbofan the hot and concentrated exhaust of existing engines made the system difficult to use. By the time of the AMST project, engines had changed dramatically and now provided larger volumes of less-concentrated and much cooler air. For the YC-15, four engines were used, versions of the Pratt & Whitney JT8D-17 widely used on the Boeing 727 and Douglas DC-9. The YC-15 borrowed components from other McDonnell Douglas aircraft, with its nose gear coming from the Douglas DC-8 and the nose section & cockpit being derived from the Douglas DC-10. Parts borrowed from other aircraft included the Universal Aerial Refueling Receptacle Slipway Installation (UARRSI), taken from a Fairchild A-10, anti-tipover stabilizer struts from the Lockheed C-141 Starlifter, pumps taken from the McDonnell Douglas F-15 Eagle, Lockheed C-5 Galaxy, DC-9 and C-141 and actuators taken from the C-5 Galaxy and DC-10. In addition, the environmental cooling system was composed of components taken from the DC-9, C-141 and Boeing KC-135.

Operational history

Two YC-15s were built, one with a wingspan of 110 feet (34 m) (#72-1876) and one of 132 feet (40 m) (#72-1875). Both were 124 feet (38 m) long and powered by four Pratt & Whitney JT8D-17 engines, each with 15,500 pounds-force (69,000 N) of thrust.

The first flight was on 26 August 1975. The second prototype followed in December. They were tested for some time at McDonnell Douglas as the Boeing entry was not ready until almost a year later. In November 1976, both designs were transferred to Edwards Air Force Base for head-to-head testing, including lifting heavy loads like tanks and artillery from dirt airfields at Graham Ranch, off the end of Runway 22.

In Phase II of the flight test program, a "refanned" Pratt & Whitney JT8D-209 was tested in No. 1 nacelle of 72–1876 and a CFM International CFM56 was tested in the No. 1 nacelle of 72–1875. In addition, a new wing with increased chord and span was flown on 72-1875.

The YC-15s completed a 600-hour flight test program in 1977. By this point, the seeds of the AMST program's demise had already been sown. In March 1976, the Air Force Chief of Staff, Gen. David C. Jones, asked the Air Force Systems Command to see if it was possible to use a single model of the AMST for both strategic and tactical airlift roles, or alternatively, if it was possible to develop non-STOL derivatives of the AMST for the strategic airlift role. This led to a series of studies that basically stated that such a modification was not easy, and would require major changes to either design to produce a much larger aircraft.

After the flight test program, the two aircraft were stored at the AMARC, located at Davis-Monthan Air Force Base. One aircraft (72-1875) was subsequently moved to the nearby Pima Air & Space Museum in 1981, but was returned to flying status by McDonnell Douglas in 1996; and was first reflown on 11 April 1997. On 16 April 1997, the aircraft was ferried to Long Beach, California to support the C-17 program. On 11 July 1998, the aircraft suffered a massive failure of the No. 1 engine during flight and made an emergency landing at Palmdale, California. On inspection, the aircraft was deemed too expensive to repair and was stored at Palmdale. In 2008, the aircraft was moved by road to Edwards AFB, where it is now on display at the Air Force Flight Test Center Museum's "Century Circle" display area, just outside the base's west gate. The other airframe (72–1876), which had remained on Celebrity Row at the AMARC for many years, was destroyed in place in April 2012.

Specifications

General characteristics