The concept of active flight controls is not new. In fact, active controls were certified on the Lockheed L1011-500 wide body jet transport many years ago. Now Tamarack Aerospace is working to certify a version of active controls on the Cirrus SR22.
Active flight controls are attractive because, in general, a longer wing span adds to the performance and efficiency of an airplane. But longer wings also add structural weight because the length of the wing increases the bending moment—leverage, if you will—of loads. Just like using a longer pry bar, a longer wing when loaded by turbulence or maneuvering applies more force to the wing root. If the wing is not structurally strong enough the increased bending moment can snap it.
An active flight control senses increased loading and bending of the wing and moves a control surface to relieve the load. For example, if turbulence adds positive G pushing the wing tip up, the active flight control rises to alleviate the load. To relieve negative g, the control surface moves trailing edge down which, of course, pushes the outer wing up to lessen the bending loads.
Lockheed wanted to add wing span to increase efficiency, climb rate and range when it developed the larger “500” version of the L1011 but didn’t want to totally redesign the wing structure to carry the increased bending loads of the longer span wing. The active aileron system solved the problem. The active system also saved weight because less wing structure was required.
Winglets have become common on all types of airplanes because a correctly designed winglet causes the wing to behave as though the wing span was longer. A winglet is usually a good tradeoff because it does not add as much bending load as the equivalent increase in actual wing span. But winglets do add at least some structural load. If they didn’t, they wouldn’t be adding any performance benefit.
Typically when a winglet is added to the tip of an existing airplane wing some structural modification is required to carry the extra load. Those modifications add weight and cost. But Tamarack Aerospace has developed winglets for the SR22 piston single that they believe will require no wing structural changes because active controls relieve the bending loads.
The Tamarack winglets have a small aileron style control surface just inboard of the winglet. This surface is “active” moving counter to turbulence or maneuvering loads to relieve bending of the wing. Accelerometers mounted in the SR22 sense a change in G load during turbulence or abrupt maneuvering and command near instant deflection of the active surface to relieve bending loads.
The active winglet concept is new, as far as I know, and what’s also new in the Tamarack approach is that the system is “single string.” There is only one actuator and sensor/computer package, and one set of wires to the surface. If any one item fails, so does the active winglet surface.
The active flight controls in the Lockheed are totally redundant so no single failure of any component can cause loss of the entire system. That’s what you would expect from something as critical as a system that protects aircraft structure.
Tamarack proposes a warning light that alerts the SR22 pilots to any failure of the active system and the pilot then must slow to a new airspeed limit of 119 knots indicated airspeed. At that airspeed the SR22 wing and winglets meet the structural turbulence and maneuvering load requirements so that effectively becomes the “red line” airspeed with the active controls failed.
At first, my airplane design nerdyness wants the Tamarack system to be redundant. It just seems that flight controls, even minor ones such as this, should meet a high standard. But then I think of how much margin is included in structural certification requirements. Unless you are penetrating a thunderstorm when the active winglet system fails, you won’t be close to exceeding any limits while you slow to the 119 knot limit speed. And, if I think even a little longer, it’s apparent there is only one engine on an SR22 so we have all opted to fly without complete redundancy anyway.
If we are going to make progress in bringing novel design ideas into piston airplanes systems like the Tamarack winglet need to be acceptable. It doesn’t meet the same standard Lockheed did, but the SR22 is a piston single designed for personal travel, not a wide body jet. I hope the FAA agrees and approves the Tamarack system so SR22 owners can gain some performance with minimal weight increase.