Quote:
Originally Posted by Crumpp
Actually to illustrate my previous point, if you just calculate the direct effects such as being done here, you completely miss the importance.
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No one has denied the effects of the weight nor importance of them, Graham's original point was just that effect of the fuel state is small at high speeds near sea level in this kind of planes. And that has been proven true.
Quote:
Originally Posted by Crumpp
Why? There are quite a few coupled affects we are actually witnessing which are hidden behind the scenes.
At full power in sustained level flight:
When we add weight our drag increases.
When we add weight we need more power to increase our dynamic pressure. If we don't have more power, the wing must increase AoA to a higher coefficient of lift, and coefficient of drag.
This higher CL occurs at a lower maximum velocity.
As the velocity slows, the thrust increases but so does our Power available. It increases at the rate of velocity squared for drag other than lift and to the first power for induced drag. So we have more power available the slower we go until we get to Dmin.
Thrust is increases too as we slow down. We are getting the "double whammy" as our thrust required is reducing and our thrust available is increasing.
The coupled affects manifest themselves in the direct affect of a small change in velocity. It becomes easy to make a false assumption if all you do is parrot formulas to draw a conclusion like Pihl.
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All these changes are accounted in my demonstration ie higher Cl, therefore higher Cdi and Cd and further slower speed and therefore even higher Cl and higher thrust and so on.
At higher weight there is new balance point of drag and thrust at lower speed and at this new balance point all the key parameters (V, Cl, Cd, T and D) are different than at original weight.
This is also why the parametric study can't be used because we don't have a constant key parameter. This was demonstrated well when you tried to use constant Cl for this particular problem; the analysis gave completely wrong picture of the effects in this specific case. Same problem was with drgondog's analysis when he tried to use constant thrust.