Drag in a vertical has been discussed here before but the discussion is usually limited and the idea of upwind cycle blade drag is actually of great interest to several of us. Not many of us are actually trying that hard to innovate verticals but I can assure you that for those of us who are we're not lost on drag we just don't all agree on methodology. That said I can nicely seguay into my response that I don't agree that a thin profile is the only appropriate profile for a vertical.
The flutter your machine was experiencing wasn't neccessarily due to excessive parasitic drag. You could have been simply seeing the consiquences of lift on a blade that wasn't strong enough. As your blades come into the wind reaching the AoA's that produce the most lift you could have seen a deflection of the blade that caused a lift stall, lift stall type oscillation. It's also possible that the flutter was due to vortex drag at the tips causing deflections if you blades were attached to center mount struts.
Thin and thick airfoils both experience parasitic drag in all it's incarnations because other than thickness they both share features that create drag. Both have tips that cause vortex or "tip" drag, both have a pressure differential between the upper and lower surfaces and boundry layer transition areas that cause induced drag, both will experience pressure drag and both will experience viscous drag. I almost forgot to mention that attachment points will of course cause interference drag equally with both thicknesses.
I'm no aerodynamicist and I have no wind tunnel but I can speculate on the drag consiquences of various designs based on others research. The design you've come up with is interesting but I can't see how it reduces drag simply as a result of the low thickness ratio of your blades. I think the best part of the design is the doubled blades which essentially are opperating with a wing/flap or slot relationship. This is likely to reduce some of the lift induced drag or pressure drag because of the delayed seperation at higher AoA's although as rod pointed out above, your leading edge on your lead blade is likely doing you harm by causing premature separation at the lead edge. I would also question the harm that may be occuring at the tips especially in the open area between blades
In my opinion a vertical requires a blade that performs over a very large AoA range. If you utilize a blade with a low thickness ratio you are like to experience severely limit AoA ranges and significant drag as a result of more time spend in heavy stall. At the reynold numbers that smaller machines are operating at, thinner profiles can actually experience more drag in relation lift because of the abrupt leading edge. The air may not have enough energy to make an abrupt transition which can lead to premature separation.
Anyway, I'm curious if perhaps I've somehow mislead myself because I have actually come to the opposite conclusion as you. I agree that the slotted wing is a good approch but it's not new.
Could you provide a section view of your profile?
Mike
They say that even a microsecond of flutter on an aircraft is enough to destroy it, so careful is everyone to avoid such problems. It is the first thing I always look for on this rotator, my head swinging back and forth like a bobblehead. It was gratifying the day that it began to accelerate with stronger winds rather than showing signs of stall. As you say, these things are quite complicated, more so than many realize.
I honestly think, having seen what I have seen, that the horizontals, with the exception of some of the Danish mid-range turbines like the Windmatics and the Nordtanks, are in need of improving their thickness-to-chord ratios. Those narrow blades need to be made either thinner or wider if they are not already pushing the envelope. I am not afraid to suggest also that doubling blades even there would be an acceptable approach.
Anthony "The Knucks" Chessick www.integener.com [ Parent ]
The project identified in these postings has been submitted to a state grant program as a pre-proposal abstract and word was just received that it has passed this first hurdle. The deadline for the full proposal submittal is the end of the month of September. Competition in these is quite high especially now with increased focus on energy issues. It does not matter where you are located as long as benefits from the work accrue to this state, my location. It is very good news that this project is in the running and you are invited to provide help with this insofar as your interests and time allow.
Incidentally, both verticals and horizontals are to be covered and a rotor for a horizontals rotator that makes use of these same concepts is to be made today. There is no time to put all this out on this discussion list and discretion is advised in keeping the good name of the "OtherPower" board intact.
Anthony "Knucks" Chessick www.integener.com [ Parent ]
In addition, the horizontals rotator also mentioned has been constructed and has run for over a month now. It has the same type of blades as the verticals rotator, thin aluminum sheets with a folded (rounded) leading edge and doubled blades with the offset.
Everyone agrees hereabouts who has seen it that it performs remarkably well (albeit just a free wheeling, nonpowered device), rotating at high rates of speed in light winds. The trailing edge pitch angle of all the blades is also a feature of it - zero degrees. Yes, it performs noticeably better than the verticals rotator for approximately the same blade swept area - one half of a square meter.
Doubled, sheet thin blades. A low thickness-to-chord ratio. This is a serious attempt to reduce parasitic drag and so far it has provided excellent results. It is important to understand that my approach has a name - the Newtonian Principle of Aerodynamics, a term coined by others. This is unlike the more inductive approaches typically seen in most flight-oriented aerodynamics work until just a few years ago. A look at what is on the www.integener.com website explains. Some nice photos are also to be viewed there of both rotators in action.
Thin blades. Thin blades. Thin blades. I have seen the difference in the actual reality of what has resulted with real hardware. Get rid of all flutter and flow separation off the leading edges and make the blades thin. The one and only function of the blades in producing energy is to shovel the wind from one vector direction to another. I will accept no disagreements or other viewpoints on this.
Anthony Chessick IntegEner-W Tehachapi, CA www.integener.com [ Parent ]
