Excellent stuff here guys! Thanks to Oleg for the original question and thanks to the rest for the really informative discussion.
Rick Van Clief ----- Original Message ----- From: Mark Drela <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Tuesday, December 18, 2001 12:42 PM Subject: Re: [RCSE] Aerodynamic washout > > >Generally speaking any type of washout is not the ideal approach. > >Washout is used to compensate for bad planform design. > > 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. > > 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. > > 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. > > > >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. 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. > > - Mark > > > RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] > RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
