Boeing YC-14

https://en.wikipedia.org/wiki/Boeing_YC-14

The Boeing YC-14 was a twin-engine 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.

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 Coanda 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.

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.


Role
Tactical airlifter

Manufacturer
Boeing

First flight
9 August 1976

Status
Retired

Primary user
United States Air Force

Number built
2

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 (109 km/h; 68 mph) and as high as M=.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 decrement to 7%. The YC-14 also demonstrated the capability to
carry the 109,200 lb (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.

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.

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 (227 kN) each

Performance
Maximum speed: 504 mph (438 knots, 811 km/h)
Cruise speed: 449 mph (390 knots, 723 km/h)
Ferry range: 3,190 mi (2,734 nmi, 5,136 km)
Service ceiling: 45,000 feet (13,716 m)
Rate of climb: 6,350 ft/min (1,935 m/min)





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