Large-scale powered-lift demonstrations underline the value of flow control for aircraft efficiency
During the past year, two large powered-lift models have been tested in the National Full-Scale Aerodynamics Complex (NFAC) at NASA Ames Research Center in California, evaluating the low-speed benefits of blowing air over high-lift flaps for short takeoff and landing (STOL).
High-speed tests in the National Transonic Facility (NTF) at NASA Langley Research Center in Virginia have investigated the benefits of blowing to reduce drag in the cruise. And while a STOL transport, civil or military, looks unlikely in the next decade or so, the wind-tunnel tests conducted for NASA and the U.S. Air Force Research Laboratory (AFRL) are adding to growing interest in active flow control for energy efficiency.
Over the years, there have been several powered-lift STOL demonstrations, but they focused on low-speed performance. What distinguishes these latest efforts is the desire to combine high lift at low speed with low drag at high speed, to use short runways but cruise at jetliner velocities—what NASA terms cruise-efficient STOL (Cestol) and AFRL calls «speed agility.»
Past efforts included the Air Force’s early-1960s Lockheed NC-130B Hercules boundary-layer control testbed, with its blown flaps and flight controls, and in the 1970s NASA’s de Havilland Canada Buffalo-based augmentor-wing and upper-surface-blowing jet-STOL demonstrators as well as the Boeing YC-14 and McDonnell Douglas YC-15 prototypes for the Air Force’s stillborn Advanced Medium STOL Transport program.
«We have built short-field aircraft before, but they have not been efficient cruisers and so they did not transition [to programs] because, when optimized with an extreme focus on STOL, they lacked multi-mission capability,» says Barth Schenk, air mobility technology lead at AFRL. «So we set out to validate technology to do efficient cruise at higher speed as well as low-speed STOL.»
The YC-14 used upper-surface blowing from overwing engines to increase lift, while the YC-15 lowered externally blown flaps into the exhaust from under-wing engines—an approach used on the Boeing C-17. But cruise efficiency suffered. «You need a lot of power for short field,» says Schenk. «If we can integrate it in a more clean and efficient way, that power can translate into productivity, flexibility and multi-mission capability.»
Speed Agile set out to demonstrate concepts for an Airbus Military A400M-sized tanker/transport able to fly as slow as 70 kt., lifting a 65,000-lb. payload from field lengths less than 2,000 ft., yet cruising above Mach 0.8 with an aerodynamic efficiency (Mach number times lift-to-drag ratio) greater than 13, better than a C-17 and closer to commercial airliners.
«What’s unique is the hybrid lift system,» says Cale Zeune, AFRL program manager. «There are two mechanisms: one inboard on the main engine exhaust and one outboard on the main lifting surface. That’s key to getting efficiency across the speed range.»
Speed Agile designs feature stealthy shaping and embedded engines. «The crux was to build on commercial engine technology to deliver the capability [the Air Force] is looking for,» says Zeune. «There was a big emphasis on finding a way to develop an efficient lift system and clean integration using off-the-shelf engines. Not making a next-generation capability contingent on developing new engines saves years, and billions.»
Both Speed Agile concepts use a circulation-control wing (CCW) system with internally blown flaps on the outboard sections. Inboard, Boeing uses upper-surface blowing while Lockheed Martin has patented reversing/ejecting nozzles. In Boeing’s design the engines exhaust through slot-like nozzles over the wing trailing edges and deflected flaps, to increase lift at low speed.
Lockheed’s nozzle allows engine exhaust to go straight aft to produce thrust, or vectors it downward in an ejector/augmentor arrangement that entrains additional airflow to boost low-speed lift and thrust. Vectoring the exhaust also provides pitch control, avoiding the over-sized tail typical of STOL designs. And the nozzle reverses thrust for short landings, directing engine exhaust upward and forward to minimize debris stirred up on unprepared airstrips, says Ed Di-Girolamo, Lockheed program manager. Powered lift is split between inboard and outboard wing sections in a way that minimizes pitching moment, «so the tail can be smaller,» he says. Engine bypass air is ducted to internally blown flaps on the outboard wing. «We take less than 10% of the fan air via low-loss ducting to the circulation-control lifting surfaces.»
Lockheed’s blowing system is hard-plumbed to the engine—»If the engine is on, it is blowing—there are no valving losses. It is a clean and elegant approach,» says Zeune. This «always-on» approach means the wing flaps are blown in cruise flight. «We are still evaluating cruise blowing. CFD [computational fluid dynamics] predicts a weaker shock, which might be a benefit,» says DiGirolamo.
Boeing’s concept was tested in 2009, at low speed using a 5%-scale model and at high speed in the NTF with a 3% model. Lockheed’s design was tested in 2011, at high speed in the NTF using a 5% semi-span model and at low speed in the NFAC with a 23% model, complete with engines.
Almost 44 ft. in span and 36 ft. in length, the large-scale model is powered by two Williams FJ44 turbofans. The model is close to the preferred concept, except for having two engines instead of four and bellmouth rather than stealthy inlets, to reduce risk. Engines and flight controls were managed dynamically during the tests, which included angle-of-attack (AoA), sideslip and power sweeps. The large-scale model tests were the capstone of Speed Agile, says DiGirolamo. «We had to integrate all the components into a transonic airframe… and go through preliminary and critical design reviews, as well as safety reviews, for a 23%-scale aircraft with real turbofan engines.» Running engines in the tunnel, rather than using pressurized air, made safety a concern, but took the program to a higher technology readiness level (TRL).
The NTF tests, meanwhile, achieved two objectives, Zeune says. «We validated the design tools used to shape the aircraft for efficient cruise performance. We also assessed the benefit of blowing at transonic speed through the flaps. It was a total system assessment of the aerodynamic benefit versus stealing a bit from the main thrust.»
The Speed Agile program is almost complete. «We have been working on the orderly development of technologies for the last 10 years, going from concepts to configurations to a capstone large-scale wind-tunnel test,» says Zeune. Low- and high-speed tests met all objectives, says Schenk. «We have one more step to go, which is to put it all together this fall in a pilot-in-the-loop simulation using the tunnel data.»
There are plans, but not funds, to go further. «Speed Agile validated to TRL 5 that we can get speed agility with acceptable handling. We achieved twice the induced-lift-per-unit-thrust of the C-17. We also got low drag—lift was produced very efficiently,» Schenk says. «We got the data we expected, and validated the CFD, propulsion and optimization tools that are key to developing a configuration in the future,» says DiGirolamo.
Given funding, «the next step would be to turn up the scale, and test the inlet, nozzle and offtake in a realistic structure,» similar to the large-scale propulsion models built to ground-test vertical-lift systems for the Joint Strike Fighter concepts, says Schenk. Integrating the propulsive-lift system with structure and hardware close to flight weight would allow vibro-acoustic and actuator loads to be measured at full scale.
NASA’s Cestol research, meanwhile, is aimed at a 2020-timeframe 100-seat airliner able to take off and land at 85-90 kt, cutting field length by up to a half yet cruising efficiently at Mach 0.8. In NASA’s vision of a future air transport system, this aircraft would increase airport capacity by using short, underutilized runways, while enabling steep, tightly curved approaches and departures that keep noise within the airport boundary.
«There is not a lot of data for CFD validation of circulation-control aircraft,» says Tina Jameson, assistant professor at California Polytechnic University, San Louis Obispo, which was awarded a NASA research contract «to develop a configuration using Cestol to reduce airport noise, then perform a large-scale wind-tunnel test to build an open-source database to be used for CFD validation.»
The Advanced Model for Extreme Lift and Improved Aeroacoustics (Amelia) combines a circulation-control wing (CCW), with leading- and trailing-edge blowing, and over-the-wing engines mounted to provide some upper-surface blowing and noise shielding. Unlike previous low-speed CCWs with rounded trailing edges, including that flown on a Grumman A-6 in 1979, Amelia has a supercritical airfoil for reduced transonic drag. In cruise, blowing slots are closed and dual-radius flaps retracted for a sharp trailing edge.
The 1/11th-scale, 10-ft.-span Amelia model was mounted high in the 40 x 80-ft. section of the NFAC, chosen so that downwash from the blown flaps, which can extend a span’s length below the wing, would not reach the tunnel floor. Air was supplied at high pressure to turbine-engine simulators and low pressure to the blowing slots. «This is the only large-scale model to have leading- and trailing-edge blowing,» Jameson says.
Integrated aerodynamic and aeroacoustic testing included measuring forces and moments, wing surface pressures and skin friction, as well as flow visualization and sideline and flyover noise measurements. Tests were run with engines on short and long (double-height) pylons and with a clean wing.
Preliminary results for the clean wing show leading-edge blowing is critical to CCW performance. Lift increases with trailing-edge blowing, but declines with increasing AoA unless blowing is used to delay leading-edge separation. Adding the engines increases lift at higher AoA, and the higher pylon had a small, but noticeable performance benefit. Noise, like lift, increased with blowing, but rapidly at first, then more slowly.
While the next steps after Amelia and Speed Agile are unclear, NASA is continuing research with the Fundamental Aeronautics Subsonic/Transonic Modular Active Control (FAST-MAC) model in the NTF, focusing on viscous flow separation at full-scale Reynolds numbers (Re). FAST-MAC tests were conducted at Reynolds numbers around 30 million, equivalent in scale to somewhere between a Boeing 737 and 777 in the cruise.
«In the past, there was a lot of 2-D, low-speed, low-Re testing, more a proof of concept than system integration,» says Rich Wahls, project scientist for NASA’s Subsonic Fixed Wing program. «FAST-MAC is a modern supercritical airfoil, a legitimate wing designed to be a good non-blown wing, not a blunt 2-D airfoil.»
The model is a 4-ft. semi-span wing with a 30-deg. sweep, aspect-ratio of 5 and a blowing slot at 85% chord. «Unlike Amelia, which is a configuration, this is a generic research model,» says Wahls. «In addition to high lift, we are looking at blowing in the cruise to move the shock and reduce drag. So at a system level, if we have the piping for high-lift blowing can we take advantage of it in the cruise?»
In circulation-control aerodynamics, blowing for separation control keeps flow attached to the trailing edge of the flap; but higher lift is achieved with supercirculation, where higher blowing moves the jet into the oncoming flow. Low-speed tests of FAST-MAC with flaps at 60 deg. and an 0.06-in. slot height achieved separation control, but failed to get into the supercirculation regime, suggesting the slots were too large.
Cruise tests were conducted at Mach 0.85 with 0-deg. flaps and 0.0375-in. slot height. «Low blowing moves the shock forward, so we lose lift and add drag.» says Greg Jones, a NASA Langley engineer. «With moderate blowing, the shock moves back to baseline and restores lift. Elevated blowing moves the shock aft, increases lift and lowers drag,»
The next tunnel entry this summer will have improved internal flow distribution and uniformity. «We did not get supercirculation at Mach 0.2, but did at 0.1. The second test will have improved parts to make the flow more as intended, and we expect [Mach] 0.2 to look like 0.1,» says Wahls. Cruise-configuration tests will focus on repeatability, but low-speed tests will be conducted with a smaller 0.022-in. slot height.
«At low speed, with 60-deg. flap, we saw a 30% increase in maximum lift coefficient with relatively low blowing, but even a momentum coefficient of 0.1 equals 30% engine bleed. That’s too high for a commercial transport,» says Wahls. «In fiscal 2013-14 we’d like to test different ways of blowing, like sweeping-jet actuators, which work with a quarter of the airflow and require very little engine bleed.»
While an airliner looking like Amelia is unlikely to be built, Wahls admits, «we now have data on 3-D performance and acoustic effects that did not exist for flow control.» The next step could be research into active flow-control high-lift systems, «not so much for increased performance, but for reduced parts count and cost,» he says. «At the same time we will explore how to use flow control transonically to reduce wave drag, all the while trying to minimize bleed.»
The Pentagon’s Joint Future Theater Lift requirement for a future C-130 replacement, at which the Speed Agile demonstration was aimed, was a victim of budget cuts, but AFRL believes the flow-control work will go forward. «If you don’t have the money to build a bridge, then the key is to lay big rocks across the stream,» says Schenk. «The databases, tools and facilities we have developed will help with the energy-efficient focus of the next aircraft.»