An airplane is a giant physics exercise. I say exercise because it is a series of complex problems. I once read that drag reductions in full scale soaring had come a pound at a time. That means that getting to where we are today took a long time. That being said, we in RC soaring have come on pretty strong, and it continues. The fact is that some full scale ships have a better L/D or glide ratio when fully ballasted. It's a function of the airfoils and configurations used. They also have a higher sinking speed in the ballasted condition, and that's why they have dump valves. They are now optimized for fast cruise, with thermaling a secondary consideration. There have been 300 mile tasks flown where the pilot never circled in a thermal, and achieved better than 100 mph in average speed.
We, on the other hand, and excluding the cross country boys, are interested in thermalling performance, first. Basic physics will tell you that a lightly loaded model will sink slower than a heavier one. It's like a skinny kid and a fat kid on the same hill on bikes. They coast down, with the fat kid getting there first. The hill, however, doesn't change for either of them. A sailplane in a thermal goes up at the rate of the thermal, minus it's sinking speed. This is also proven with bank angle. A lower bank angle will most often have the model sinking slower than a high bank angle, and the model goes up faster...right(?)...not always, because thermals have cores. I've been at 30 degrees bank and barely going up, and increased it to 60 degrees with the stick on the backstop and gone up at over 1000 feet per minute. Again, though, the physics of the thing are there and the numbers do NOT lie. The increased bank got me to the meat of the thermal where, before, I was floating in the fringe at a shallow bank, and not gaining much. I said all that so I could say this...there are too many variables to make a blanket statement that heavier flys better, or that lighter flys better. I will say that a Legionair flys better the lighter you can make it, but the patriotic one I flew at last years Mid-South, though a little heavy, did not dissappoint. Jack Womack Real men scratch build... --- "Don Stackhouse @ DJ Aerotech" <[EMAIL PROTECTED]> wrote: > Jim Monaco asks: > > >Can someone explain the difference between Max L/D > and > >Minimun Sink? > > Assuming still air, "Max L/D" tells how far (i.e.: > the maximum distance) > you can go from a given altitude. > > "Minimum Sink" tells you the maximum time you can > stay aloft from a given > altitude. > > Obviously the two are not the same at all, and they > do not normally occur > at the same airspeed. The speed for min sink is > generally a little faster > (typically a few knots or so) than stall, and the > speed for best L/D is > faster than for min sink. > > Best L/D occurs at the speed where induced drag is > exactly half of the > total drag. Since the weight is approx. equal to the > lift (at least at the > L/D's we're interested in), and therefore more or > less a constant, an > improvement in L/D generally is the result of > achieving the lowest possible > drag. Since induced drag decreases with speed and > all the other drags > increase with speed, the minimum total drag will be > at the point where the > induced drag is exactly equal to the sum of the > other drags, i.e.: exactly > half the total. > > If your span is fixed, then reducing the weight will > reduce induced drag, > and therefore improve L/D. However, if the area is > not changed, the plane > will now be flying at a slower speed for a given > lift coefficient, which > reduces the Reynolds numbers, perhaps enough to > cause problems for the > airfoils used. There are also whetted area issues > that get involved. Making > the airplane heavier could improve performance in > that case. The > improvement would not be due to any blanket "rule of > thumb" like "heavier > airplanes perform better" (actually the opposite is > more likely to be > true), but because the airplane's design is suited > for a particular range > of flying weight, and adding weight brought it more > in tune with that range. > > If the airplane is designed for the weight it's to > be flown at, and that > weight is less than others of the same span, then it > will have less induced > drag than those others at any given airspeed. Since > the airspeeds we fly at > are at least in part a function of the winds we fly > in, the typical > airspeed ranges of a given class of models tend to > be similar. Therefore, > reducing weight, provided that the design is > aerodynamically tuned to > perform well at that weight, will improve > performance. It's only when you > try to fly a plane at a weight well below what its > design demands > (regardless of whether its designer realized that or > not) that its > performance will improve by making it heavier. > > > Don Stackhouse @ DJ Aerotech > [EMAIL PROTECTED] > http://www.djaerotech.com > > RCSE-List facilities provided by Model Airplane > News. Send "subscribe" and "unsubscribe" requests > to [EMAIL PROTECTED] Please note that > subscribe and unsubscribe messages must be sent in > text only format with MIME turned off. __________________________________________________ Do you Yahoo!? Yahoo! Shopping - Send Flowers for Valentine's Day http://shopping.yahoo.com RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off.
