<span style="font-family: Times; font-size: 16px; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; background-image: initial; background-repeat: initial; background-attachment: initial; -webkit-background-clip: initial; -webkit-background-origin: initial; background-color: rgb(255, 255, 255); font-family: Arial; font-size: 12px; background-position: initial initial; "><div>Yes, I think people get confused by the datum line because it really means nothing aerodynamically, but people tend to see downthrust, etc. in relation to the datum line or the appearance of the fuse.  <div>
</div><div>I often see people reducing wing incidence to eliminate "ballooning".  What they are really doing is adjusting the trim between the wing and the stab without considering the impact on downthrust. Best thing is to imagine just the prop, the wing and the stab, and consider the relationships between them without reference to a line or a fuse.  Then it becomes obvious that reducing the wing incidence is exactly the same as reducing downthrust, which is not usually what they should be doing.  That's because reducing the wing incidence means you have to reduce the stab incidence correspondingly, which usually means changing the elevator trim.</div><div>
</div><div>Since downthrust functions by causing propwash to strike the underside of the stab, changing the trim can reduce that effect.</div><div>
</div><div>A plane that looks like it has no downthrust at all may actually have significant downthrust.  Mess with the wing incidence and you can unknowingly eliminate it.  People do this with "old timers" a lot.</div><div>
</div><div>Jim
<div>
</div></div></div></div></span>