Helium bubbles used to show bird aerodynamics

Martin Gregorie wrote on 3/21/2020 7:04 AM:
On Sat, 21 Mar 2020 06:40:45 -0700, Eric Greenwell wrote:

Curiously, that is not (we're told) how the model gliders are trimmed:
the tail is lifting at minimum sink. That seems inefficient to have a
small wing producing lift instead the big wing, with it's lower drag
from a larger aspect ratio.

Its not as inefficient as you might think: several people, as well as
myself, have found that a well designed model with a long tail moment has
a fairly low coefficient of lift on the tail. At a normal glide trim the
parasitic drag of the tail is greater then its drag due to lift. The
tailplane is working at a Cl of around 0.05, which puts it pretty much in
the centre of its minimum drag bucket, while the wing will be operating
at a Cl of 1.1 - 1.2.

We don't care what the glide slope of a gliding model is like since its
not going anyplace, just circling in the thermal it was launched into.
All we care about trimming it to glide at min. sink speed. Contests are
won and lost on total airtime recorded during the event.

It's called a "lifting tail" even though it is producing very little lift, and is
producing that lift with a high drag penalty from the parasitic drag? Confusing...

- What is the advantage for trimming it with a small positive lift instead of zero
lift?
- How about using a smaller horizontal trimmed for a higher L/D? That would lower
the parasitic drag and the overall drag of the horizontal, while still producing
the lift needed for stability.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1