At AirVenture last summer, Remos introduced its new all-composite GX Special (factory-built) Light Sport airplane. The GX replaces the Remos G-3, which we noted in this column in January 08.
Major features of the GX are a completely new all-carbon-fiber wing that folds quickly, a pair of sticks and throttles, 110-knot cruise speed and a provision for a Magnum ballistic airframe parachute.
Also introduced at Oshkosh was the company’s newly appointed chief engineer, Christian Majunke. Before he arrived, Remos had designed the all-composite wing for the GX, which replaces the fabric-covered wing on the G-3.
Majunkes first job, he said, was to optimize the aircraft, which included fine-tuning the design of the GX control system. He joined the flight-test team after the aerodynamic design was frozen. He flew performance tests in Germany and in the United States. Wing structural testing had been completed when Majunke arrived, but he participated in testing the GXs vertical and horizontal tail.
Two of the most dangerous experimental flight-test activities are multi-turn spins and flutter testing. In fact, test pilots who specialize in this type of work are often employed for only these tests, and they tend to be bachelors who are willing to wear a parachute and use it if necessary.
Flutter usually happens to control surfaces without warning and may result in loss of the surface-and sometimes the structure to which it is attached-in a fraction of a second. Failure of the original Tacoma Narrows Bridge (in Washington state) and the twisting of a stop sign in a high wind are two well-known examples of low-frequency flutter. The cause is known as aeroelastics, where at some airspeed, something sets a movable or flexible part in motion, and it vibrates at its resonant frequency. If there is insufficient damping, the initial buzz may increase in amplitude until the part or the hinge fails. In the case of elevator flutter, it might take the horizontal stabilizer with it.
To preclude flutter, airplane designers make supporting surfaces such as wings and fixed tail parts stiff, and they add weights to the control surfaces so that the controls nearly balance on their hinges. Aircraft-design software also helps engineers avoid high-airspeed flutter, but two kinds of testing (ground and flight) are needed to verify the design at airspeeds higher than Vne.
Ground vibration testing (GVT) is required in Germany, and it is conducted on the entire airframe as it sits stationary at the test site. The test airframes wheels may be placed on inflatable pads to minimize the damping effect of sitting on the landing gear.
Ground vibration testing looks for resonances that could lead to flutter and airframe failure. The technique is to attach speaker-like pulsers and sensors to various parts at prescribed locations, drive the pulsers with variable-frequency oscillators and then look for resonant frequencies. The hope is that the normal flight envelope will yield no resonances that could amplify and cause structural failure due to flutter. Good structural stiffness results in rapid damping of the vibrations rather than amplification of them at a resonant frequency.
A specialist in interpreting GVT results analyzes the data and usually determines a maximum safe airspeed that will avoid the flutter regime. If flutter appears likely in any part of the normal flight envelope, new design, probably including extra stiffening, will be prescribed. In the case of the Remos GX, GVT analysis determined a possible problem with the flaps. The solution was to modify the flap hinge slightly to stiffen the flaps when they are retracted.
Ground vibration testing takes considerable time to set up, but the tests themselves are quick. Majunke said that setup for the GX required about three days, but the tests were completed in 2 hours. In Germany, four groups are authorized to perform GVT, Majunke said: EADS and DLR (the German equivalent of NASA minus the space programs)-both involved in big-airplane projects-are primary players. An independent GVT expert, Professor Norbert Niedbal, performed the Remos tests, which determined that the GX is theoretically safe to an airspeed of 400 kph (214 knots), well beyond Vne.
Experimental Test Flying
Flight-testing to Vd (maximum design velocity) comes after the GVT analysis; Vd is 10% beyond Vne, which is 249 kph (133 knots) for the Remos GX. Flight testing was to 288 kph (154 knots). Testing for flutter is begun at high altitude for safety purposes. A common procedure is to dive in smooth air to a predetermined airspeed and pull up before the pilot purposefully raps the stick to induce a vibration. That way, if something flutters, the airplane is already decelerating because the nose is high. These tests take time, as the test sequence may call for increasing the airspeed by only 2 knots or so on each successive dive and pull-up. The airplane performed very well at this top speed, Majunke said. Of course there was no flutter, but also no vibrations.
GX development took about two years, and testing was embedded in that process. At first, the composite wing was much too heavy and more rigid than it needed to be. A wing that can support 12 to 15 G is much too heavy, Majunke said. It gives up performance and payload. So the wing was redesigned for an ultimate of 8 G at 1320 pounds [maximum gross weight for an LSA], which means that much load would have to be applied to break it. The design load [the limit the pilot is required to observe] is 4 G.
Majunke found flying performance data to be the most fun part of the project. You’re flying on the edge of the envelope and expanding it, he said. As of late July 08, he had accumulated nearly 50 hours of flight-test time.
Majunke also flies production test flights on customers new airplanes, and he had flown 10 new Remos G-3s as of last July. He described the process.
The test time required in the production test phase on average depends on the avionics, Majunke said. If the avionics package is quite simple and everything checks out, 20 minutes to half an hour will be enough. I had one aircraft that was absolutely perfect, and I was disappointed that the single test flight was over and I was back on the ground in 20 minutes. Of course, the ground crew was happy. But with multiple glass screens and actual IFR capability, the production test pilot needs to do an ILS approach, so the minimum will be about an hour of flying time.
Before the flight phase of a production test, the pilot checks weight measurement and c.g. calculation and does a normal thorough preflight check. I then look at many details, because the production test pilot is the first person to fly the new airplane, Majunke said. As you would expect, control systems, engine installation and servicing are checked especially thoroughly.
You look twice at the engine and three times at the rescue system, Majunke said. Then its time to check that every system operates well. Lights. Seat belts. Everything else. You fire up the engine and make it warm enough to take full power. You count numbers: oil pressure, rpm, coolant and other systems. A very careful takeoff follows, and the airplane is kept over the home field initially. Once I gain confidence, I may apply up to 2.5 to 3 G to make sure everything is in place.
Then you’re checking performance. How much is the airspeed at a given rpm? I check that at four different rpm plus full power, where the rpm is noted, he said. For airplanes with the full avionics package, Majunke flies to the nearest airport with an ILS and makes an approach while checking the avionics and instruments. Coming home, I may fly full throttle over the hangar and pull up. The ground crew gets the idea: Another Remos is nearly ready for delivery to a customer.
For more information, call 877-REMOS-88 or visit www.remos.com.
Dave Martin served as editor of this magazine for 17 years and began aviation journalism evaluating ultralights in the early 80s. A former CFI (airplanes, gliders, instruments), hes flown more than 160 aircraft types plus 60 ultralights (including a single-seat, no-basket hot air balloon). Now living at a residential airpark in Oregon, he flies his Spacewalker II homebuilt as a Sport Pilot.