Can someone explain the difference between Max L/D and Minimun Sink?
From : Don Stackhouse
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.
A question, sir. Can you give us an idea of what magnitude of performance change we would be looking at if the weight of an airplane varied say plus or minus 10% (20% total weight change.) Are we talking sink rate improvements of 1/2 of 1%? Or more on the order of 20%?
That depends, but it could be quite significant. In our own experiments it certainly was when done properly. Part of the problem is that "doing it properly" usually involves redesigning the entire airplane in response to a given weight change. That's why so many of the parametric analyses we've seen tend to fall short; they fix some parameter, such as wing area, and then claim to find an optimum. The optimum they've found is for that particular set of assumptions, and if you change those assumptions, or better yet don't limit the analysis with assumptions at all, you'll most likely find a totally different answer. Unfortunately that tends to also make the analysis a lot more difficult.
For the moment let's consider two models, both designed for the same set of weather conditions (and therefore the same operating airspeeds), and for the same span, but one is 20% heavier than the other, and both are aerodynamically optimized equally well for their respective flying weights.
The induced drag component of the overall L/D is linearly inversely proportional to weight. The induced drag component of the total L/D of the 20% heavier model will be 20% worse than the lighter model's. If that is the only difference, then the total L/Dmax of the lighter model will be 110% of the L/Dmax of the heavier model, since induced drag is half the total at L/Dmax. However, there are ripple effects throughout the rest of the design that can impact the other drags as well, so that by the time it's all added up, the other types of drag could see significant benefits as well. For example, the lighter model will also most likely have significantly less whetted area, which will help its high speed performance in particular.
At thermalling speeds, the induced drag is more than half the total, so the benefits of the reduced weight would be even greater. At high speeds the induced drag becomes less significant, but is still present as long as the plane is flying at a nonzero lift condition, so the benefits of reduced induced drag would be present to some small degree even at high speeds, in addition to the "ripple effects" I mentioned above. Properly done, the lighter model can end up with better performance over the entire speed range, depending on the designer's priorities and on how well they did their homework.
Don Stackhouse
DJ Aerotech
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