Why aren't there two tangent lines drawn from the origin rather than a single line from some mysterious value (3.x m/s)?
Sailplanes live and die over this stuff. Where that straight line E-48 is tangent to the curves is the best Lift/Drag (best glide) speed at two different weights.
The true origin (0,0) is not on the graph, the x range starts at 60 m/s.
The speed increases with weight because 'best glide' (L/Dmax) *is* a "determined" by AOA. The 1g trim speed at that AOA will be higher at higher weight, just as you described, but the AOA for best glide doesn't change with weight.
Both curves are tangent at purd near exactly the same Lift/Drag ratio (glide angle) of about 48. And as Mr. Nauga pointed out, the origin is somewhere off to the left of the graph.
Doh!
It seems to me that as weight goes up the nose needs to go down in order to gain more thrust component from gravity to offset the increase in drag caused by the additional speed needed to lift the weight. Add enough weight and you'd be pointed straight down just to avoid stalling. So what am I missing?
Yes. As you can see in the graph, the higher weight shifts the lift / drag (speed / sink) curve to the right. I'm sure that if you pushed it far enough, the tangent line (best L/D) would move, but for practical ranges of weights, it remains about the same. That's why gliders carry ballast - increase their "best" airspeed (at the expense of climb rates in a thermal).
As weight goes up so does the thrust component from gravity, because weight is where it comes from.It seems to me that as weight goes up the nose needs to go down in order to gain more thrust component from gravity to offset the increase in drag caused by the additional speed needed to lift the weight. Add enough weight and you'd be pointed straight down just to avoid stalling. So what am I missing?
But as speed goes up, drag goes too, so you need even more help from gravity, no?
Best glide *is* determined by angle of attack. The *speed* for best glide is a function of angle of attack and weight.
Yes. It's all proportional. Say, for example, you double the aircraft weight. If you're gliding at the same angle, you now have double the "thrust" from gravity, no? Since weight equals lift and lift is proportional to the square of the airspeed, the airspeed now has to be sqrt(2) or 1.414 times the original speed. But drag is also proportional to the square of the airspeed, so it's doubled too, equal to the doubled "thrust".
What is interesting with this is that as the weight of the 182 approaches the max gross for a C150 (1600) the best glide speed approaches the best glide for the C150 (57 KIAS) as well.
Yep, minimum sink rate and best glide are two different targets.
Of course not, I was just using that as an extreme example.
Right, and there's an easy way to demo this. We trim the plane not to a specific airspeed, but to a specific angle of attack (with some minor caveats that don't matter here).
Well, both use the NACA 2412 airfoil, the 182 wing is slightly larger 174 sq ft to the 150's 160 sq ft. Other than an extra 17" span on each wing, what else is completely different as you say?
Ok, yes, I see that, and it is the same for all 182 (and 172) and 182RG post 1972. But that would make it slightly different than the 150 wing which did not have the Camber-lift wing modification added at the same time.
