Originally Posted by
ShyTorque
A disadvantage of battery powered aircraft is that they don’t burn off fuel in flight. The aircraft weighs the same at destination as on departure and the performance remains the same throughout.
Not really a problem for a small helicopter remaining at low altitudes, but definitely so for something larger, a utility aircraft or an aeromedical/ SAR one.
Originally Posted by
ShyTorque
A disadvantage of battery powered aircraft is that they don’t burn off fuel in flight. The aircraft weighs the same at destination as on departure and the performance remains the same throughout.
Not really a problem for a small helicopter remaining at low altitudes, but definitely so for something larger, a utility aircraft or an aeromedical/ SAR one.
unless they can be jettisoned...
Good point on efficiency, though. Fuel fractions are a major driver of excess power available.
Nick P had commented on the mass of electric motors vs engines, and his figures are correct for most motors, but there are some stunning motors available. A long time back, a certain French inventor came up with a nice little design that was plonked into the back end of a B206 for testing, and it was lighter than the RGB that it replaced. The power cables were about a wash. The motor design was pretty neat, but the tech owner was particularly sensitive to IP issues and barely gave information to the engineers to do the test. The size form was neat, and the output was scalable. The rotor efficiency can be improved by an incremental margin, but not enough to make up for the battery power density. What would be intriguing would be to have a hybrid design using the same novel motor as a generator to reduce mass, and using same for drives. staying with a lightweight generator with a reduced fuel load and using high density batteries... maybe there is a future. The novel motors were light, really really light. Haven't spoken to the guy for many years, his paranoia may have had justification, but Lordy it made it hard to get anything done.
As to what can be done on the rotor, about 25 years ago (? 97? could have been around 2003...) there was a neat little engineering note in the AIAA JoA that covered some interesting CFD done with Wayne Johnsons CAMRAD II on autorotation of a disk (+ dangly bits) applying lift enhancing tabs, LET's to a rotor. There was no methodology of the structure which has been a pain for implementation. I had been doing tabs on propellers for about 10 years by then, and had transferred that to flight testing on a 12":1' scale helicopter. The CFD gave an improvement of 20% in autorotation, but used a design that was known to be far from optimal from the prop and rotor testing, but was at least interesting, as LETs alter the L/D in such a way that it is conceivable that they preclude autorotation in a fixed geometry. In the end, auto worked OK, but it was a very uncomfortable flight getting out there to try it the first time. Parachutes from a helo have some obvious technical drawbacks. Glad to never have to see if the technique would work to remove the blades without removing the top of the cabin.
The curious outcome suggested that the variation of the coefficients of CL and CD were effective as follows...
- Baseline 63105 blade, NACA smooth, rotor stall at 82% Nr, measured fuel flow, 62 units( fuel flow and engine power were quite linear)
- MOD 1, (leading edge of blade, certified later) rotor stall at 77% Nr, 58 unit fuel flow... pitch link loads reduced
- MOD 1 & 2 (adding an LET...) rotor stall at 68%, 39 unit fuel flow. loads per MOD 1. Acoustic spectrum attenuated, but added a 1600hz line
- collective loads measured and no significant change from baseline, other than SR was reduced, as was vibration.
The MOD 2 change appeared to be considerable, and raised questions about the accuracy of the measurements, but the Nr was strobed and was valid, and the stunning control effect was that the baseline chopper gets to LTE about the same time it runs out of rotor, Mod 1 recovers some pedal at the stall, indicating the torque demand has definitely improved, and then with the tab, approaching the stall, the pedal demand was 50% of the left pedal of the baseline case. The last check before going into translational flight and autos was to go to idle, set full up collective and see if the rotor could accelerate, and it did, was able to accelerate and lift off and hover with a full up collective, Unmodified, the rotor would not accelerate to flight speed. This little project languished as it became secondary to modification to jet aircraft, and my helicopter had one bit of design that made high speed testing of a tab uncomfortable from a risk aspect, it had a TRB that was not confidence building for dropping bits off. I did some failure tests, and videoed the tabs departing from the rotor, and at low speed they had departed merrily away from. the TR disk, but in forward flight that may not have been the case. Was working up to testing NOTAR or fenestron before going back to a light structure TRB, but a UH-60 TR would have worked... The tabbing model CFD'd in JoA I also flew, and it was not effective at improving CL/CD, it did definitely increase CL, but that is never in doubt, only whether the rotor AOA puts the CL/CD into the position where stunning changes occur. Tabs shift CL by around 0.4 - 0.6 CL for the same AoA, dependent on multiple parameters but that is indicative. How it is done on a rotor is somewhat more interesting than the CFD model, in the real world that config made almost no improvement.. hardly a surprise. The CD however is altered stunningly at mid AOA levels, like those associated with props and rotors and fans, sections that are constrained from approximations of ideal blades. We did the tabs as well on the CART 2 test rig of NREL, and that was effective, but then the CART 2 section is a pretty awful analogue of optimised wind turbine blades. Torsion loads were no factor which was similar to the rotor. The tabs do mitigate a bit the hysteresis of the coefficients from pitching effects, the stuff that Boeing Vertol was trying to sort out on the CH47s VR7 blade section by using VGs. John C Lin and co of NASA showed how wild the CD shift could be, however the implementation on a propeller, rotor or fan blade has some constraints that make the hover value that I got closer to the practical limit than Lin, Storms, Carranto and co's theoretical limits. If anyone has a test vehicle that is NOTAR or has a fenestron, (or a UH-60...) the LET is worth the candle IMHO.