GVFlyer
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- Feb 22, 2002
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The short answer is, "Embraer wouldn't, no manufacturer would."bigD said:
Part 25, Airworthiness Standards, Transport Category Aircraft requires manufacturers to define the edges of the operating envelope:
c) The controllability, stability, trim, and stalling characteristics of the airplane must be shown for each altitude up to the maximum expected in operation.
Many of the tests require the manufaturer to rise to meeting the required standards. One of the tests that has been challenging for some airframers and is always entertaining to perform is for determining VD:
b) Design dive speed, VD ...
(1) From an initial condition of stabilized flight at VC / MC, the airplane is upset, flown for 20 seconds along a flight path 7.5° below the initial path, and then pulled up at a load factor of 1.5g (0.5g acceleration increment).
Even Gulfstream had difficulty in one of the trim tests when certifying the GV. The regulation calls for," One pound of centering force for each six knots of displacement from a trimed condition." In one obscure corner of the operating envelope for operations in the landing configuration, where a rational pilot would never find himself, the GV couldn't overcome the hysteresis to meet this standard. Subsequently, Gulfstream had to add elevator downweight springs to the flight controls. This was really a shame because prior to this modification to meet Part 25 standards, the GV had the control harmony of a fighter.
LegacyDriver was correct in stating that the Legacy encountered early Mach effects, buffet and buzz prior to encountering elevator flutter. Such effects are characteristic of flight controls that are not hydraulically boosted . I participated in the certification and military conformity of the C-21 Learjet. It also had unboosted flight controls. Mach Crit for the 39'6" wing was at M 0.75. At that speed you could see the shock wave form up on the wing's upper surface. Mmo was M 0.81. At altitude, at M 0.825-0.83 a shock wave would form up on the ailerons which could not offer hydraulic resistance for stability and they would buzz and cause control wheel snatching. If you went faster or rolled into a turn, Mach buffet would occur followed by a pronounced rumble. On the GV, even when it was inadvertently flown to Mach 1.07, there was no buzz, no buffet, no rumble and no roll off.
Going fast in airplanes is a critical science. The following is a synopsis of what happened to a well known member of SETP during devlopmental test for the Sino-Swearingen SJ-30-2:
Before the test flight, a mission briefing was conducted via conference call between the San Antonio-based personnel and the telemetry van personnel in Rock Springs, Texas. During the briefing, the project’s flutter consultant (a DER) noted that he had, during previous discussions, advised that for the purpose of flutter testing, if Beeler (left) ran out of aileron or elevator trim, the tests could still be completed, even if the pilot had to hold aileron/elevator force to steady the airplane. He further stated, however, that continuation of the testing would never override the pilot’s decision as to whether the control forces were unacceptable or hazardous.
SJ30-2 N138BF, S/N 002, departed for the second flutter test on an IFR flight plan–in VMC–from San Antonio International Airport at 9:11 a.m. on April 26. The airplane climbed to 39,000 feet and set up for a shallow dive to accelerate to the Mach 0.884 (indicated) test point. When the airplane reached indicated Mach 0.875, Beeler called “Mark” on the radio to duplicate the previous day’s test point. Beeler then initiated a single pulse input to the elevator.
After checking the telemetry strips, the consultant then gave Beeler the go-ahead for a single pulse to the aileron, followed by another “Go” for a single pulse to the rudder. Telemetry van personnel noted that all the modes excited were “well damped.” Telemetry van personnel also reported that after the pulses were completed, Beeler said the uncommanded roll to the left–which had been experienced on the previous flight–did not occur. There was also no mention of a rumble. In addition, the chase-plane pilots confirmed that there were no mechanical anomalies evident on the airplane.
Beeler subsequently began to set up the SJ30-2 for the dive to reach Mach 0.884. Discussions between Beeler and telemetry van personnel indicated that this test point might be the last one of the mission due to fuel concerns, especially for the chase airplane.
Following telemetry lock, the airplane began a shallow dive. According to most of the participating personnel, before reaching the test point of indicated Mach 0.884, Beeler said that he had put in full trim and couldn’t let go. At indicated Mach 0.884, he called “Mark.” Each control surface was again pulsed, and the responses were again “well damped.”
After the final pulse, Beeler was cleared to the next test point, indicated Mach 0.894, but Beeler responded that the airplane was rolling to the right, and he couldn’t stop it. Telemetry was lost about 20 seconds later. The chase pilot called “get out” twice to Beeler, but he said that he couldn’t get out, that there were too many g’s. This was his last transmission. The airplane was at about 30,000 feet at this time and appeared to be in a shallow right bank, according to the chase-plane pilot.
Walls in the T-38 saw the SJ30-2’s nose to be “a little low,” and in an approximately 30-degree right bank after reaching Mach 0.884. A few seconds later, the test airplane entered a “barrel roll-type maneuver” to the right, then continued to roll, and increased its dive angle until it hit the ground at about 10 a.m. No parts were seen separating from the airframe as it descended. The chase airplane was not close enough for its crew to see the SJ30-2’s control positions.
When Walls saw the initial roll, his first thought was, “What did he do that for?” Then he saw that the airplane “came around and made another barrel roll. It was not around a point like an aileron roll; and it was not real fast; it looked lazy.”
The telemetry van was receiving reference system airspeed. One of the telemetry personnel said that as the accident sequence approached, the airplane’s airspeed readout was consistently between Mach 0.881 and 0.882. He subsequently noted that he did not believe the airplane exceeded Mach 0.882 before the telemetry signal was lost. (The airplane had achieved its planned Mmo of Mach 0.83 late last year without any reported anomalies.)
It is now known that the Williams-Rolls FJ44-2A-twinjet had problems on its first flight in the same series on the previous day. That flight was curtailed after the airplane encountered an uncommanded roll to the left and a possible encounter with Mach buffet.
This occured in an airplane designed to go M 0.83; not the Legacy's design limit speed of M 0.80.
GV
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