From Leadership Profile: Vertiflite November/December 2015
Klausdieter Pahlke, Rotorcraft Research Area Manager, German Aerospace Center (DLR)
In 19 years at the German Aerospace Center DLR (Deutsches Zentrum für Luft- und Raumfahrt) at Braunschweig (Brunswick) in central Germany, Dr. Klausdieter Pahlke advanced from being a mathematician-researcher in Computational Fluid Dynamics (CFD) to managing a range of rotorcraft research projects. “The DLR portfolio covers aerodynamics, acoustics, rotor dynamics, flight control, flight guidance, pilot assistance, crashworthiness, bird strike, etc,” explains Dr. Pahlke. “Our DLR experts have special skills in those areas — CFD developers, measurement technology experts, experts in flight mechanics, rotor dynamics, flight control, human factors, flight guidance, crashworthiness and certification.” Rotary-wing investigations employ about 50 full-time scientists and share another 40 researchers with fixed-wing programs. Dr. Pahlke notes, “The DLR rotorcraft teams are an international mix of scientists from many nations. There is still a majority of Germans, which is decreasing year-by-year. New engineers come to us from German and European universities.”
One recent investigation was “ECO-HC” (HC for “Helicopter”), partnering DLR with the University of Stuttgart in an Airbus Helicopters-led project (and partially funded by the German Ministry of Economy) on higher-performance, lower-noise, fuel-efficient rotor developments that are now flying on the Airbus Bluecopter demonstrator. “The DLR rotorcraft program is working in close cooperation with Airbus Helicopters Deutschland (AHD),” says Dr. Pahlke, “but DLR is by no means the extended workbench of AHD. We do many basic research activities, which are clearly knowledge/understanding-driven and not directly connected to any product.”
For example, ALL-In-Flight (Assisted Low-Level Flight using In-Flight Simulation Capability) seeks improved handling qualities and aims at a fully-automatic flight from takeoff to landing using DLR’s own fly-by-light/fly-by-wire EC135. DLR contributions to CleanSky’s Green RotorCraft “Environment Friendly Flight Path” (GRC5) subproject aim at low-noise flight procedures and a helmet-mounted display that shows the pilot a noise-optimized flight path. “In terms of helicopter acceptance, a significant improvement in safety is most important,” says Dr. Pahlke. “This is very difficult because safety depends on the actions of many persons and not simply on the rotorcraft design, although the rotor design plays a crucial role. The second element to improve acceptance is noise reduction. We need to make further progress here.”
Dr. Pahlke observes, “Today we have highly-sophisticated CFD tools, which allow the very accurate prediction of rotor behaviors under medium load, including blade-vortex interaction. But these methods consume too much computer and engineer time — we need more efficient codes and faster computers. In terms of modeling, today’s CFD codes have their limits when it comes to flow separation. There is still a need for improvement here. When we start to talk about the prediction of vibration, we have to admit that we are not able to produce results with any engineering accuracy. The low progress in this area was, in the past, due to the fact that our aerodynamic predictions were of insufficient accuracy. Today, it is more due to insufficient accuracy in our structural modeling in combination with fluid-structure coupling. I think that we have to think of basic test cases that can be implemented in well-controlled test environments to make progress in this field.”
Klausdieter Pahlke was born near Aachen by the Dutch/Belgian border, but grew up in northern Germany, about 50 km from the North Sea. He found an early connection between aviation and modeling. “My father taught me how to build small, free-flying gliders out of balsa wood, paper, dope and, of course, some small lead balls for trimming. There were a lot of crashes which required a huge amount of repair work after each flight. Repairing gliders in such a way that they were able to fly straight was a good lesson in doing things accurately.
“I was highly interested in mathematics from my first class in school. I always liked the clear structure, the clear distinction between true and false, and the power of mathematical equations, which allowed people to build impressive structures and machines.” At the University of Hanover, the undergraduate math-and-physics major initially aimed at a teaching career. “I finally switched to a diploma in mathematics, as there were no jobs for teachers at that time,” recalls Dr. Pahlke. “My diploma thesis was about a special numerical method for solving partial differential equations.”
The thesis work proved especially relevant when a friend called about an opening at DLR for a mathematician with skills in numerical methods for partial differential equations. “I started in the numerical methods branch of the Institute of Design Aerodynamics reworking fixed-wing CFD codes for parallel computers, which were new at that time,” explains Dr. Pahlke. “In parallel with this project work, I took courses at the University of Braunschweig in aeronautics. During my studies, helicopters didn’t play any role.”
More complex math nevertheless led to a rotary-wing concentration. “After three years at DLR, my branch head, Prof. Arabindo Das, encouraged me to go for a PhD in parallel with my project work. After some basic work on oscillating airfoils, I moved to a PhD on a CFD method for solving the so-called Euler equations for rotors in forward flight. The next steps were about trimming the rotor, considering fluid-structure coupling and viscous effects (solving the Reynolds-averaged Navier-Stokes equations), which brought me in direct contact with all main rotor effects.
“As a scientist, I finally achieved a CFD-based, trimmed-rotor simulation, including fluid-structure coupling and viscous effects. Together with several other DLR colleagues, the features of my research code were implemented into the structured DLR production code FLOWer. The FLOWer code was also extended by developments of the University of Stuttgart and is today the CFD workhorse at Airbus Helicopters Deutschland.”
DLR’s helicopter work involved many players. “Over the years, I became leader of the rotorcraft aerodynamics group and deputy leader of the numerical methods branch of the DLR institute of aerodynamics and flow technology. I managed the European Union Project GOAHEAD (Generation Of Advanced Helicopter Experimental Aerodynamic Database) group for CFD code validation from 2004 to 2009 with more than 10 European partners ranging from helicopter OEMs via research centers to universities.
“The dataset generated in this campaign in terms of experimental and numerical data is one of the most complete datasets for a main rotor- fuselage- tail rotor configuration. This project really was a challenge, and to see how the data are used and that the GOAHEAD configuration is today called the common platform within the CleanSky project is definitely very rewarding.”
DLR today flies a BO105 test aircraft for basic research in helicopter aerodynamics and acoustics, slung load stability, and other basic research. The EC135 ACT/FHS (Active Control Technology / Flying Helicopter Simulator) is a variable stability fly-by-light/fly-by-wire test aircraft equipped with two active side-sticks; infrared, laser, TV and radar sensors; and a helmet-mounted display for flight control and pilot assistance studies. “The aircraft itself is part of the ACT/FHS system, which includes a ground-based system simulator using the same hardware as the flying aircraft,” explains Dr. Pahlke. DLR installed a new motion-based simulator for fixed- and rotary-wing studies, and an EC135 cockpit that can be plugged in to the motion- and fixed-based simulators for rotorcraft research.
In addition to flight test aircraft, DLR has several rotor test stands of different levels of complexity, ranging from a basic blade whirl tower over a basic rotor test stand to a fully-equipped rotor test stand for blades up to 2 m (6.6 ft) radius with a rotor balance, and collective and cyclic control. This test stand is shipped to the DNW (German-Dutch Wind Tunnels) Large Low-speed Facility for isolated rotor or full-configuration wind tunnel tests.
DLR rotorcraft researchers work with many international partners. The German Aerospace Center has a long-standing cooperation with the US Army and NASA on topics ranging from pilot-assisting flight controls to flow-field measurement techniques. “Many efforts directed at military rotorcraft have a dual-use character,” explains Dr. Pahlke. “Low-noise rotors and low-noise flight procedures have civil and military relevance. Many pilot-assistance studies have a dual-use character. Some are focusing on heavy helicopters with complex systems, which we find only in military rotorcraft.”
Dr. Pahlke observes, “The closest cooperation is with the French ONERA covering aerodynamics, acoustics, handling qualities, crashworthiness and bird strike. In addition to this bilateral cooperation, DLR is active within the Group for Aeronautical Research and Technology in Europe (GARTEUR) context in several projects, such as basic acoustics, simulator fidelity, rotor-induced forces on obstacles and flight in wind turbine wakes.
DLR also participates with ONERA in the European Union CleanSky program on the new CleanSky 2 projects contributing to “Fast Rotorcraft” technology. “Higher speed can be a military mission requirement like that for the V-22,” says Dr. Pahlke. “In this case, operating costs play only a secondary role. For a civil rotorcraft, the owner must make money. Therefore, higher speed and greater range pay off only if they are achieved at acceptable recurring and non-recurring costs. Clearly, the tiltrotor has impressive cruise speeds, but at the cost of a very high complexity, which makes the aircraft expensive to buy and to operate. The compound configurations under investigation we see today in the US JMR contest and in Europe CleanSky 2 try to find a compromise between speed, range and complexity.”
Klausdieter Pahlke was recently elected to the AHS International Board of Directors to represent Europe (and Africa). “I hope that I will be able to bring some useful service to the AHS in this new role,” he says. “The AHS Forum is a must in my yearly calendar, just like the European Rotorcraft Forum. Being a member of the International Committee of the European Rotorcraft Forum, I believe that top-level research needs international contacts. AHS as an organization and Mike Hirschberg as Executive Director are really making an effort to bring international experts together. I really appreciate this.”