Is It Time For Active Controls?

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.

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28 Responses to Is It Time For Active Controls?

  1. Kayak Jack says:

    Redundancy is not as large a concern of mine as is a porpoising oscillation due to the system activating a fraction of a second behind the turbulence force. Maybe that type of a problem has already been designed out of the system, I don’t know. But I would want to be very well assured that the mini-ailerons work as advertised. Otherwise, a pilot may find him/herself rolling back and forth in an increasingly larger amplitude. Or, if they worked in concert, the plane could be pitching increasingly. IE: I’m a bit goosey about the idea, similar to a runaway electric trim or flap system.

    Related to winglets – I read that if they are below the wing it improves top end performance, and if above the wing they improve lower landing speeds. (Did I get that right??) Has anyone tried a winglet that extends BOTH above and below the wing to gain performance on both ends of the airspeed scale??

  2. Cary Alburn says:

    It strikes me that the Cirrus is already too high performance for many of its pilots, from the accident reports I’ve read. I’ve seen reports of TKS equipped Cirrus purposely being flown into serious known icing situations, failures due to excessive reliance on the factor-equipped bells and whistles, flight into hazardous situations because of the “get out of jail free” card of the CAPS (Cirrus’ acronym for a BRS), etc. Perhaps Tamarack should concentrate on fundamentally more reliable airframes which really could use a boost in cruise performance, like the Cessna 100 series.

    Jack, I don’t know about winglets per se, but I have extreme droopy tips on my Cessna P172D. They add a lot of low speed control, but they detract from cruise speed because they also add drag.

    • Kayak Jack says:

      Sounds like I got my “uppin and downin” cornfused (As Lil Abner used to say). Explained many of my landings. ;=))

    • Kayak Jack says:

      Sounds like I got my “uppin and downin” cornfused (As Lil Abner used to say). Explaining many of my landings. ;=))

  3. Dave Parsley says:

    “Winglets have become common on all types of airplanes”

    First introduced by the noted “failure” Burt Rutan.

    • Mac says:

      Actually, Dave, the creation of the modern winglet is generally credited to Dr. Richard Whitcomb of NACA/NASA fame. Who thought of a winglet first is impossible to know, but Dr. Whitcomb working at NASA Langley in the early 1970s developed the tall, nearly vertical winglet that has become common on so many airplanes. Certainly Burt used winglets on many of his designs, and used them in more ways than any designer I can think of. He even had some different names, including calling them “tip sails” on the Beech Starship.
      More recently Joe Clark and the engineers at Aviation Partners Inc. (API) developed and patented the “blended winglet” that has added very signficant gains in efficiency to airline and business jets. So, the winglet, as with most aviation technology, has many fathers each who has contributed to advances.
      Mac Mc

      • Derek Kuhl says:

        NASA does credit Burt Rutan for being the first aircraft designer to introduce winglets. The same can be said for Apple using Siri in their latest phones, even though it was created at SRI International (Standford Research Institute). Often research programs create most of the tech we see in business, but it is business adoption that creates the innovations for the every man.

        Winglets are not tip sails because winglets do not have rudder control. This was a unique design that added control, stability, and efficiency in one blended form.

  4. Tom Miller says:

    Interesting stuff. Thanks, Mac!

    This is just the kind of thing this EAA’er gets excited about. Real experimentation and innovative design! That’s the essence of our hobby.

    I wish we saw more homebuilders willing to try some stuff like this instead of just building “by the book” RVs. That’s not experimenting, it’s merely assembling.

  5. Thomas A. Muller says:

    I would thing these would provide increased load bearing, reduced drag and most importantly a better ride in turbulence in *any* aircraft. Wish they offered an experimental version; I’d consider them for my existing RV-9A.

  6. Don Stephens says:

    How much crosswind component loss do these winglets have on the aircraft?

    • Mac says:

      Cross wind capability is not a limitation so I doubt that it will be a factor in certification of the active winglets on the SR22. As for the many airplanes that have had conventional winglets approved for installation I am not aware of any significant change in crosswind capability.
      Mac Mc

  7. DEL says:

    On the face of it, active control for alleviating wing root stresses can fail in a mode more problematic then just stop working. A failure of an electronic component may, in principle, cause an active control system to go berserk and work in a sense opposite to that intended, aggravating stresses rather than alleviating them.

    Don’t understand me wrong, I am not, by any means, opposed to such innovations. But I’d say that replacing the safety provided by dumb solid metal with that promised by smart-design electronics requires more assurance than just the existence of an option to reduce speed.

  8. Brent says:

    Interesting concept! I would image This could be carried forward to be able to fly the airplane unconventionally in the event of a catastrophic event, such as a structural failure.

  9. Kayak Jack says:

    That’s interesting. What kinds of limited, structural failures are you envisioning? And, what kinds of recovery actions would an autopilot do?

    • Brent says:

      I was thinking about that Israeli F-15 back in ’83 that landed with most of the right wing missing. The ‘active controls’ along with the massive control authority and differential thrust brought him home. This is obviously an extreme case, but it illustrates the point. Although I’m not sure how comfortably I would be with the computer controlling all my surfaces with deflections set way beyond tested limits with out an ejection seat. Oh yeah, Cirrus has a parachute…

      • DEL says:

        In modern aircraft, combat aircraft in particular, fly-by-wire is now standard. It had some disasterous beginnings, but so did aviation in general. The industry has learned how to do it right. But I’m sure redundancy and fail-safe are part and parcel of the current fly-by-wire technology.

  10. Mike O says:

    If the active winglets were controlled by two lines anchored at the root and going to a bellcrank they shouldn’t have much lag time in response to wing loading and unloading stresses.

  11. Kayak Jack says:

    That’s also dependent upon whatever actuates that control linkage. First, it has to sense a change in condition. Then decide what to do about that change, then make an appropriate adjustment. All the while – even if that while is only a fraction of a second – the condition out there is probably still changing. Maybe, it’s already stopped – or even reversed.

    It’s that pesky time lag that is inherent in the solution; it becomes part of the problem itself.

    • DEL says:

      Automatic (feedback) control is a mature technology that knows how to deal with the issue you just raised for many decades now. Active winglets may effectively respond to fast-changing dynamic stress disturbances as short as 1/20 of a second. (In technical lingo, with a 20Hz bandwidth.) My concern, expressed above, addressed a system malfunction situation, in which the system no longer feels obliged to obey the smart engineers that have designed it.

  12. Kayak Jack says:

    I’m glad to be wrong about the time delay situation. A “failure to comply” issue could be just as bad, I would expect. The problem with both “automatic systems” and “moving parts” is that sometimes – neither one is. But, leak proof tanks do.

    It’s probably a good thing I’m flying a 50 year old Skyhawk. KISS.

  13. Gordon Arnaut says:

    Active control is what hang gliders and weight shift control aircraft have…it’s built in…due to the flexibility of the wing…

    It is almost impossible to break a hang glider wing because as you add angle of attack and increase lift…you also increase the pitching moment…as with any wing…but unlike a rigid wing that does not give way…the flex wing twists leading edge down…especially at the tips…thereby reducing alpha and shedding the load on the wing…

    This wing flexibility is also like a shock absorber in rough air…gusts and turbulence are absorbed by the flexing outer wing structure…and less force is passed to the fuselage…that is why weight shift aircraft have a better ride in slow and lightly loaded aircraft like ULs and LSAs…

    Some people have looked into making a rigid HG wing that is similarly flexible and able to shed load…with some interesting ideas about flexing spars etc…but nothing commercial yet as far as I know…

    The problem with trying to do this electronically is that you need an electronic brain that is going to try to mimic what a flexible wing does naturally…any piece of electrics can go bad…and it can never be as smart as we would like…

    • DEL says:

      There are many physical systems inherently stable in one respect or another. For pilots, the most obvious example is the static longitudinal stability of an aircraft with its CG within limits, the stable feature being the angle of pitch. The anglular position of a pendulum at its lowest point is also statically stable. In your example, the stable quantity is the wing load. Yet we do not say these stabilities are due to built-in active control. The term active control is reserved for the technology for, among other things, artificially stabilizing a system in a state which is naturally unstable, using electronics, servo actuators, etc.

  14. Kayak Jack says:

    For some of us, there seems to be an inherent lack of total confidence in machines and electronics. Otherwise, why else pre-flight the bird? I feel a bit more confidence if a system is designed to go to some “neutral” position in case of failure. Having a system/sub-system/component get stuck, or still actuating out of phase with what it is supposed to be, can be dangerous for us. No matter how nice it is to have around when it works right.

    Maybe I’m dwelling on that too much, but it looms real to me. Maybe others too, maybe not. As in driving a car, where the most dangerous part is the nut behind the wheel, I prefer to maximize my chances of success.

    • DEL says:

      Your worries are understandable if you fly experimentals or other uncertified designs. For “normal” aircraft, isn’t it the FAA’s job, and its FAR Part 23, to ensure the airworthiness of the bird you fly, whatever technology it employs?

  15. Kayak Jack says:

    Well, Del, I fly the least experimental aircraft in the world – a Cessna 172. FAA rules over the design, yes. But, as I remember, confirmation of continued airworthiness is the pilot’s responsibility.
    I’m probably fussing overly much. But I’ve seen high tech stuff fail in sneaky and insidious manners. I’ll hush up and just go back to sleep here.

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