Scaled Composites, manufacturer of the Proteus high-altitude long operation aircraft, plans later this month to expand the flight envelope to altitudes above 35,000ft (10,700m) and speeds up to 175kt (320km/h) indicated airspeed (or Mach 0.55) as part of its goal of achieving "loiter" operations at 64,000ft.

Proteus, which was designed as a platform for wideband communications and data relay antennas, has been limited to lower altitudes until the recent completion of pressure testing on the composite crew cabin and the development of the environmental control system (ECS). This is designed to provide the two crew with a 0.725bar (10.5lb/in2) pressurised environment and an 8,000ft cabin altitude at 60,000ft.

Scaled is confident of achieving its targets, following the failure of the pressure vessel at 2.53bar (0.828bar more than US Federal Aviation Administration requirements). Despite this, however, pressure suits will be worn by the test crew until the engine air bleed system is "refined and developed", says the company.

The unconventional tandem-wing aircraft "handles well", according to test pilot Mike Melvill ,who has taken it to 36,000ft and speeds ranging from 65kt (initial stall) to 245kt. During the 11 flights and 17h accumulated by late September, the Proteus had lifted off at weights up to 4,100kg (9,000lb) (gross weight is 5,675kg) and manoeuvred at loads up to 2.5g.

Rudder forces have been "light, but acceptable", says Melvill, who adds that it exhibits "-no adverse yaw, much to my surprise - in fact it flies as straight as an arrow". He adds, however that the large, but lightweight aircraft, which has a standard empty weight of 2,660kg, displays "-aeroelastic flexing, which can be a bit disconcerting in turbulence".

Minor changes already made to the Proteus include the introduction of a small bevel to the trailing edge of the aileron to counter high roll forces, and a shortening of the elevator span by 450mm to overcome a "hump" mode which led to elevator vibration encountered during the climb through 24,000ft. Ice shields put on the main gear doors were also reduced in size after they created a flow separation on the lower vertical tails. Melvill also notes that, during testing of longitudinal stability over a range of centre of gravity positions, "-elevator deflection is marginally neutral at the most aft positions, but speed stability is still strong".

One other major change being considered is the addition of a deployable parachute mounted on the tail. "We're looking at that," says Melvill. "At the moment, we get 2,000ft/min [10.16m/s] max with the gear down, but we really need 10,000ft/min in the event of an emergency."

Wing Drop Study after F/A-18E/F experience

NASA, the US Navy and Boeing have teamed on a long term study effort to develop tools that will help predict the costly phenomenon of wing drop on new aircraft designs.

Unexpected wing drop affected the F/A-18E/F and took considerable cost and effort to cure, adding to the overall development expense of the programme and prolonging the flight test phase which now stands at more than 3,000 flight hours amassed over 2,000 sorties. The problem revealed itself in abrupt, uncommanded rolls during manoeuvres at low angles of attack and high Mach numbers.

"It was one of scores of technical problems we dealt with, but if the wing drop fix affected all the other parts, then that was not acceptable-and we loved everything else the aircraft was doing," says Boeing test pilot Ricardo Traven. The final solution, ratified by the US Navy after exhaustive tests, involves the addition of a fairing over the wingfold fairing which is drilled with thousands of tiny holes of varying size.

Although modifications to the software controlling the scheduling of the fly-by-wire system's flight control laws produced an 80% improvement in handling, the aircraft still suffered "small wing drops" and was deemed unacceptable. Various aerodynamic solutions tested included wing fences, vortex generators, vortillons, combined vortex generators and vortillons and even stall strips. "This worked for wing drop, but was like flying with a sea anchor tied to the aircraft," says Traven.

Then the team flew a test aircraft (E-2) with "-the upper wing fold fairing removed to see if it was promoting flow separation". The wing drop problem was abruptly cured by high pressure entering the front and ejecting across the chordwise span. "We needed a fairing that would mimic this," says Traven. A NASA Langley-developed "passive porosity fairing", with holes in five discrete areas was tried, but, to the team's dismay, produced significant buffet with the aircraft carrying a centreline fuel tank and with empty pylons. "This is a bread and butter fleet load-and was sufficient to absolutely disqualify the porous door," says the team.

The final solution, just accepted by the US Navy, was a variable porosity fairing that incorporated reduced porosity near the wing leading edge and increased porosity further aft.

Source: Flight International