I think we are agreeing on the aerodynamics, but disagreeing on the applications.
Mark Drela wrote: GB> Generally speaking any type of washout is not the ideal approach. > >Washout is used to compensate for bad planform design. > MD> I have to disagree here. For a given tip stall margin, > a slight amount of washout (1 degree or so) allows using > a stronger taper. More taper has all sorts of advantages > for structure, inertia, wetted area, and low-speed induced drag. Sorry, but I disagree with you. Strong taper with washout in most applications is a poor substitute for correct taper without washout. Structural considerations in my opinion are negligible and very easy overcome. Infact the larger chord =thicker cross section = stronger. Inertia, once again in most applications is negligible and who cares about a little extra inertia. Wetted area, the slight gain you get from less wetted area is offset by the increased Cd due to reduced Re#'s. Low-speed induced drag is a function of wing span, the amount of lift being generated, and planform efficiency. A good untwisted planform wins here. MD> The tradeoff strongly depends on the type of glider. > On a RES ship the importance of low yaw inertia is overwhelming, > and the significant washout/taper combo is very attractive. > An F3B must do very well in the speed run at near-zero CL, > and little or no washout is more important. A TD aileron glider > is somewhere in between, but closer to the RES case I suspect. Agreed, I personally don't relate to HL or RES. A TD glider in my opinion IS a F3B glider, maybe with a slightly more lifty airfoil. Look at more successful F3J designs, many of them are F3B ships. I personally think of a TD ship as being much much closer to F3B than RES, but maybe that's just me. MD> And washout doesn't automatically kill penetration. It's just > necessary to make sure that the tips stay in the drag bucket > at the highest design speed. This is quite doable, especially > on a poly wing where the tip's dCL/dAlpha is effectively > reduced by 5-10% by the local dihedral angle. It may not kill penetration, but at L/D max, induced drag is about 50% total drag, so a less than ideal lift distribution will hurt. GB> >With any type of washout you are only optimized for 1 condition. > >If you fly faster or slower than that optimum condition you will > >pay a penalty in efficiency. > > Having "optimum efficiency" sounds nice, but it's not the bottom line. > The bottom line is L/D at some given speed, or just sink speed. > With 1 degree of washout, the span efficiency at high speed, > maybe CL=0.05, might be down to 50% or whatever. But induced > drag is less than 1% of the total at this speed, so you're paying > a measly 0.5% overall drag penalty from the "horribly non-optimum" > spanwise loading. True, but what happens when that high speed low Cl wing slams a high speed, high Cl speed turn? All of a sudden induced drag jumps to 80%+ total drag. In this case 'horribly non optimum' is truly such. We have to fly over a large speed and AOA range. Highly tapered twisted wings may be well suited to airliners that spend 99% of their flight time at one condition, but not good for RC gliders. MD> But the washout allows you to taper the wing more, > and the drag reduction from reduced wetted area (i.e. increased loading) > can easily overcome this penalty and then some. And again, the main > requirement is that every spanwise station (the tips in particular) > is in its drag bucket in this high speed condition. I agree that, no matter what the planform (or twist), the airfoils from root to tip need to be designed to perform correctly over the entire speed and AOA range. Once again the reduction is wetted area is usually offset by the increased Cd due to the reduced RE. Washout has been used for years and obviously is effective for improving tip stall characteristics. In my opinion I will restate that this is a poor substitute for good planform (I don't know of any top F3B, F3F, or F3J design that use any significant amount of washout). When it comes to HL, RES, and lightweight poly ships, I will agree that small amounts of washout will not hurt the performance much, and may even be beneficial. Gavin RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
