Presentation Transcript

1. Reconfigurable “Morphing” Rotors – The Next Frontier Farhan Gandhi Professor of Aerospace Engineering Deputy Director, Vertical Lift Research Center of Excellence (VLRCOE) The Pennsylvania State University Keynote Lecture presented at the NATO Symposium on Morphing Aircraft April 20-23, 2009, Evora, Portugal
2. Snake/eel robots, fish robots etc. show large shape change In the aerospace field the Wright Flyer was controlled by wing morphing What sets Modern Morphing Aircraft apart from other Shape-Changing Systems? Snake/eel/fish robot type shape- changing structures are non load-bearing For early aircraft like the Wright Flyer, wing loading was very low Modern aircraft, with high-wing loading designed to be v. stiff. Energy required to change the shape of such a rigid structure would be very high. Making it flexible would reduce load-bearing capability. Need designs that are rigid enough to be load bearing, yet flexible enough so morphing energy required is reasonable
3. Rotors – Centrifugal Force is MUCH larger than aerodynamic loads – about 1000 g’s at the blade tips. So we are trying to deform a structure that has to be rigid enough to carry the aerodynamic and the CF loads What sets Morphing Rotors apart from Morphing Fixed-Wing Aircraft? Fixed-Wing Aircraft – ability to morph while being rigid enough to carry the aerodynamic loads. Terms frequently used in fixed-wing morphing gross morphing: span, chord, sweep, twist change; big things! fine morphing: camber, leading-edge droop, subtler changes! In the rotary-wing world Span, chord, twist-change, maybe anhederal – gross morphing camber, leading-edge droop – smaller changes made per rotor revolution – rotor active control. Same technologies are used for noise/vibration reduction can provide some (modest) performance improvement, stall alleviation, etc. (Quasi-steady)
4. Morphing Rotors As in the fixed-wing world, gross-morphing is a game-changer Helicopter rotors – morphing a structures that in additional to lift and drag, has to carry CF loads (many times larger than the aero forces). Can you USE the forces present to morph the rotor, rather than OVERCOMING them? What Will Morphing Rotors Buy Us? -- Performance improvement over the flight regime -- Envelope Expansion -- Operational Flexibility … and, at what cost? Types of Morphing Discussed Span Morphing Chord Morphing Twist Morphing
5. Rotor Span Morphing Previous Work (Early 1990’s) Sikorsky Variable Diameter Tiltrotor (VDTR) Optimal Rotor – large, “lightly” twisted, higher RPM Propeller – smaller, highly twisted, lower RPM Actuation using motor at the hub or differential gears Blade moving on a jackscrew; tension strap to carry CF Design was complex and heavy Recent Work (2005 – 2007) Centrifugally Actuated Variable Span Morphing Rotor
6. Retracted or short configuration Low CF Force Little extension L - Position of center of mass of sliding section + end cap Low Ω u -Deformation Sliding outer section Fixed inner section of blade Restraining Spring High Ω High CF Force Large extension Extended or long configuration Centrifugal Force Actuated Variable Span Rotor SIMPLICITY – Compare to VDTR
7. Centrifugal Force Actuated Variable Span Rotor Extension Extension This figure (of a rotor test) in lieu of a movie Movie showing this structure span morphing available with Gandhi
8. Span–RPM Dependence, Is it a Problem? Normally small rotors spin fast and large rotors spin slow But not always! Application 1 – Tiltrotors Propeller Mode – low RPM, compact Rotor Mode – increase RPM, expand. Lower disk loading, induced power, downwash, etc. Application 2 – Slowed Rotor Compound for High Speed Flight Aux Lift and Propulsion available in H/S flight Rotor slowed to avoid compressibility Ideally, you’d like it to disappear (it’s a source of drag) Unloaded rotor susceptible to gust and aeroelastic instability Slow down the rotor and automatically have it contract. A lot of the problems diminish
9. CF Actuated Variable Span Rotor – Applications (contd.) Application 3 – High-Speed Coaxial Rotors (Sikorsky X-2) Higher the max cruise speed, the more the rotor needs to be slowed down. Beyond a certain point, Variable Speed Transmission required (heavy and complex). It is the TIP SPEED that really needs to be controlled, if a small reduction in RPM simultaneously reduces span, a much larger reduction in tip speed realized without a VST Application 4 – Operation in Confined Spaces Shipboard Operations, Urban Canyons, etc. Rotor compact and operated at high angle of attack. Not the most efficient rotor, but it CAN operate. It gives you operational flexibility. Expand to a more efficient rotor when you’re not space constrained.
10. RPM-Span Dependence not that Restrictive Nonlinear Springs give you ENORMOUS Flexibility Extremely stiff to a critical force, then displays soft linear behavior Virtually no extension up to a certain RPM, followed by a large extension over a very small increase in RPM
11. Variable Span Rotors with Locking Mechanism CF actuated Var Span Rotor – equipped with locking mechanism RPM change used as the actuator; once the rotor is locked, then the RPM can be changed as desired and span cannot change – so RPM and span decoupled. This is very empowering. Now the small rotor can spin faster, not at extreme pitch near stall margin; large rotor can spin slower, not at too high tip speeds – of much greater interest for conventional rotors Main Rotor Power (HP) ~10% increase in lift at 700 HP ~10% reduction in power at 6,500 lbs Lift (lbs)
12. The Case for CHORD MORPHINGExtra Chord for High Payload, High-Altitude Ops 4th Generation Blade Shape with Anhedral Tip End 4000lbs of additional payload just through blade design US Marine Core CH-53K 4th Generation Blades In Afghanistan, Chinooks doing the work of Blackhawks MV-22 Potential Rotor Improvements Bonded Tab Rotor – Increased solidity, Hover payload improvement Increase chord AS NEEDED – for high altitude, high payload, very high-speed operation (anytime rotor is close to stall boundary). No need to pay drag penalty when extra solidity not required
13. Effective Chord Increase through Extendable TE Plate Modified shape doesn’t present significant aerodynamic penalty
14. Aerodynamic Benefits of Extendable TE Plate In terms of lift increment per unit drag increment, Extendable TE plate does better than other high-lift devices such as Trailing-Edge Flaps (Variable Camber) or Gurney Flaps Actuation force required is low (no pressure differential, “hinge moment” to overcome)
15. Envelope Expansion with Extendable TE Plate 24,000 lbs Gross Weight (installed 4000 HP SL) Max Altitude and Speed both increased significantly
16. Power Reduction at Envelope Boundary with Extendable TE Plate 24,000 lbs Gross Weight 8,000 ft. Possible to trade-off power reduction instead of increase in max speed or altitude
17. Rotor Power v/s Gross Weight with Extendable TE Plate (Stall Alleviation) 8,000 ft. 112 kts. Payload (or fuel/range) can be increased for given power, Power can be reduced significantly at a given high gross weight
18. Rotor Power v/s Gross Weight with Extendable TE Plate (Stall Alleviation) +10% chord +30% chord
19. Extendable TE Plate within the Flight Envelope All the benefits seen, so far, with extendable TE plate were at the envelope boundaries. Max Altitude, Cruise Speed, Gross Weight could be increased Alternatively Power could be reduced – at envelope boundary What if envelope expansion was not a requirement? Does the extendable TE help within the envelope? With the extendable TE, the fixed chord could be reduced. This implies lower profile drag, increased range, endurance, etc. The lower rotor solidity would shrink the envelope – now the extendable TE plate can push the boundary back to where it was.
20. Extendable TE Plate – Deployed using a Morphing Truss Slit trailing edge over section Stowed Configuration Bounded by ribs. Morphing Truss anchored to the aft of the LE D-spar. Chordwise extension of the morphing truss leads to deployment of the TE plate aft of the “normal” blade trailing-edge Morphing Truss actuated mechanically in the current model. Deployed Configuration Can TE plate deployment be achieved by small change in RPM?
21. Extendable Trailing-Edge Plate – Hardware Plate retracted or stowed, and extended through slit TE Upper Skin Removed, showing morphing truss in various states.
22. Extendable Trailing-Edge Plate deployed using Bi-Stable Arc Bi-stable arc Spar Hub SMA wires (actuators) Arc Plate Roller
23. Continuously Extendable Chord using Cellular Structures Flexible face-sheet over extendable chord section not depicted Movie showing this blade section chord morphing available with Gandhi
24. Twist Morphing of Rotor Blades LOT of work done on active-twist rotors by various groups. Most focuses on piezoelectric actuation. Typically twist of the order of +/-2 deg, at frequencies in the range of ~20 Hz Shown to be quite effective for vibration/noise reduction Also possible to actuate at 5-10 Hz (1P, 2P) – can potentially result in small reductions in rotor power. This is NOT morphing, this is rotor active control Rotor twist morphing implies large, quasi-static twist change for optimal performance at different conditions. 10-15 deg twist change for conventional rotor (hover v/s H/S) 30-45 deg change for a tiltrotor (rotor v/s propeller mode)
25. Twist Morphing of Rotor Blades All prior rotor twist morphing attempts have focused on the use of Shape-Memory Alloys. Most recent was ONR funded effort by Boeing – used SMAs to change the twist of a V-22 blade. Twist change obtained was… Baseline structure was incredibly stiff. If the compliance isn’t increased at all, the force and power required to actuate are going to be enormous!! Assuming such force and power are available, someone should try and calculate the strains in the stiff metallic skin!! So what do you do? If you make the structure compliant, you do so at some peril. Low torsional stiffness can increase susceptibility to flutter, high loads and vibration, etc.
26. Large Twist Actuation of Rotor Blades Is it possible to design a blade whose torsion stiffness under aerodynamic loads is high, but actuation requirements to twist the blade are low? One such concept exploits the idea of warp-induced twist, facilitated by a threaded rod actuated by a motor. Trailing-edge is slit, a threaded rod runs Through threaded/unthreaded housings Alternately connected to the inside Of the upper and lower skins. Rod rotation causes Very low torsion top skin to warp stiffness during relative to bottom morphing (low actuation) and induces twist High torsion-stiffness otherwise, for aeroelastic stability and loads.
27. Twist Morphing of Rotor Blades Movie showing this blade section twist morphing available with Gandhi
28. Twist Morphing of Rotor Blades
29. Twist Morphing of Rotor Blades Could be implemented over entire blade, or over outer section of the rotor blade. Is it worth the trouble? When considering a morphing solution you need to look at the benefits of morphing over a sub-optimal compromise solution Conventional (edgewise) rotors -18 deg twist might be optimal for hover, and -6 deg optimal for high-speed forward flight. What if blade has compromise -12 or -14 deg twist. What is the power penalty at hover, and at high-speed. Turns out – not that much! There may be an advantage for vibration reduction (that’s a different story) Tiltrotors – different ballgame, advantages are unquestionable
30. Rotor Morphing – Span, Chord or Twist For each of these concepts there is IMPACT and Risk/Complexity (RICO) to be considered. Each Concept ranked from 1 (lowest) – 3 (highest), in both categories IMPACTRICOIMPACT/RICO Span 3 3 1 Chord 2 1 2 Twist 1 2 1/2 If just ONE technology had to be chosen – CHORD MORPHING, appears to give maximum bang for the buck. A blade whose chord could increase 20-30% over some spanwise section
31. Some Additional Thoughts on Rotor Morphing Real-estate in the rotor blade aft of the LE spar is very limited Transfer of power is an issue – hydraulics not likely to be used, even for electric power transferring KVolts through an electric slipring is challenging Cannot use classical structures – some increase in compliance is inevitable. We haven’t called RPM change morphing, but lot of opportunity for synergy here. CF force is the 10,000 lb gorilla in the room. If change in CF force can be used to morph – USE IT!! Don’t fight it! Think martial arts….. CF actuated chord variation is likely possible What about anhedral?
32. Some Additional Thoughts on Rotor Morphing Flexible Skins – maybe nice, but not critical. Discrete shape change (rather than continuous) may be adequate. Promising Technologies: Bi-Stable Structures and Cellular Structures have very interesting possibilities Do not get too attached to a specific way to implement a certain type of morphing (ex. SMAs for twist morphing) Do not get too attached to a specific morphing solution for a system level objective (ex. Leading-edge droop for stall alleviation).
33. Through Configuration Change (Morphing)Can we expand the flight envelope(higher max speed, altitude, payload), Improve operational flexibility(operate in tight spaces)Improve performance at multiple points(compared to a sub-optimal compromise) Thank you for your attention! Questions ??