[Crumpp]

Franek,

I think the cancer analogy is perfectly valid. One cannot judge the significance of a cancer tumor based on the size ratio to the body. It brings one back to the days of leeching and humors when “doctors” used simple cause and effect.

One cannot deem weight as insignificant simply because we only perceive a small change in level flight velocity. You will find plenty of aerodynamic textbooks which will tell of the importance of weight.

You won’t find any that tell you it is insignificant because we only loose a tiny bit of speed. That is a fact.

Everyone who calculated performances will know.

Actually to illustrate my previous point, if you just calculate the direct effects such as being done here, you completely miss the importance.

You can't see the forest for the trees.

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.

That is why one must do a parametric study to eliminate all the coupled affects if you want to see just the affect of weight.

All things must be held constant while we change the weight and note the affects.

Of course, what do I know?



All the best,

Crumpp