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Under Control
October 6, 2011

F-15B flying with hot film sensorsF-15B performing flight test with Aeroelastic Test Wing 2 test fixture. (NASA Photo)
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Flight Experiments Validated 'Hot Film' Sensors. Now, The Challenge Is To Move The Technology To The Next Level.

A flight on the F-15B Aeroelastic Test Wing 2 test fixture has validated that new "hot film" sensors could one day be used to help aircraft avoid conditions that lead to flutter.

In addition, the sensors could be a part of the solution to a number of aeronautics challenges ranging from laminar flow control and sonic boom suppression to aircraft safety.

"The sensors worked as advertised. They can measure flow angularity and they measure critical aerodynamic parameters like stagnation point, which was predominantly what we wanted to see," said Marty Brenner, Dryden principal investigator for the project.

The hot film sensors measure flow angularity through the stagnation point as measured by angle of attack or sideslip. A stagnation point is a point in the airflow field where the local velocity of the fluid is zero. In this flight condition static pressure is at its maximum value and the streamline at the stagnation point is perpendicular to the surface of the aircraft, Brenner explained.

"Stagnation point is a more critical parameter than angle of attack in controlling performance of the wing. You could improve the performance of the wing by measuring and controlling where these critical aero parameters are, such as [at] stagnation point and flow separation. Supersonically, you could reduce the shockwave coming from the wing and alleviate a sonic boom on the ground," he said.

There could be additional benefits.

"If we can adjust the lift distribution, we can maximize the performance of the wing more to improve maneuverability or fuel efficiency," he added.

The F-15B flight marked the culmination of work on a new system that combines the hot film sensors with advanced signal processing techniques and is the first of its kind to measure unsteady aerodynamic loads, or forcing function, in real time and correlate those data with how the structure responds to the loads.

The system as a whole is called the distributed aerodynamic sensing and processing, or DASP, toolbox. The project was accelerated with a 2007 Innovative Partnerships Program seed fund project. TAO of Systems Integration, Hampton, Va., has been a key partner in development of the DASP Toolbox (see related article).

"With the hot films, from what we have looked at so far, it was very successful. It is another measurement system that, along with accelerometers and strain gauges, can monitor the aero forces that go into the wing. It is a good technology to demonstrate because we never get the forces in the wing; we usually just get a response from the wing. This was the first demonstration in flight of getting these forces from these critical aerodynamic parameters," Brenner said.

The single flight also produced another valuable validation.

"The other aspect of this was the signal processing of the sensors. The toolbox part is on-board signal processing. We want to take this technology to the F/A-18 no. 853 and demonstrate it on full-scale aircraft wings. We want to see the signature of the forcing function going into the wing as well as the response of the structure due to the forces," he said.

The technology has intriguing potential uses.

"Ideally, you could feed this information to a control system and use these parameters you get from the hot film sensors in a feedback mechanism. That's one of our next objectives," he said.

The advantages could be revolutionary for control systems.

"Now you can control where these critical aero parameters are and, by doing that, you can control the forces on the wing going into the structure. You can change the force distribution on the wing so you can reduce loads on the wing, or with a combination of other sensors, adjust the wing twist or the performance of the wing by adjusting the force distribution and alleviating the stresses," he added.

In addition, controlling aerodynamic forces could help with other key technology developments NASA is pursuing. Some of the applications could improve aircraft performance in areas such as load alleviation, wing twist, gusts and maneuvering loads in flight.

The next step is determining whether the F-18 no. 853 Full Scale Advanced Systems Technology, or FAST, aircraft is available and when.

"We are planning on installing theses sensors and system this year. Eventually, we want to get it connected to the flight control computer and then start using it for control. We might use it locally for a wingtip, but somehow we are going to try to put it in a feedback control scheme," he said.

Initially the system will look at the same basic parameters, but on a larger scale and the goal is to put it in a control-oriented context with similar objectives.

"We can even look at a supersonic flight and see if we can monitor the shock. We may be able to control aero parameters the way we want them to, to optimize the performance of the wing in some way and also to alleviate the loads, or suppress vibration," Brenner said.

The DASP Toolbox may piggyback on adaptive controls work being conducted with the FAST aircraft and will not interfere with the aircraft's primary mission for aviation safety, he said.

"We can augment some of their goals with these sensors. If you have some adverse flow condition and you have to accommodate for it, this would be one way to think about doing that. If you detect something you don't expect, you can try to accommodate for it with the controllers. If there is a fault in one of the surfaces, then they try to recover from it with some adaptive rescheduling scheme. That is where the DASP can potentially be helpful," Brenner said.

Installation on the F/A-18 is expected to take a few weeks and then experiments with on-board processors will begin. Distributed sensing is a goal for performance and aviation safety, he stressed, where distributed sensing for distributed controls can happen.

The emerging technology might one day mean safer airplanes capable of avoiding conditions that lead to accidents, maximizing fuel economy and potentially even reducing the impact of sonic booms on the ground.

What is known for sure is the DASP Toolbox and its components have proven through flight research that they work and merit further research and development – a signature of success.

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