|Jan 08, 2006, 08:11 PM|
Mark interesting comments on winch launching settings
Very interesting thread ! Mark, are you working on a simulation model
for the winch launch ? Such a tool would allow us to optimize this
very important flight phase in various conditions. In real world the
air is different each time you launch, so it is very hard to know
after a good launch if you did better, or if the wind or a thermal
helped you, even with an alti logger... With a math model you can set
the atmosheric parameters and play with different settings...
About your elevator preset during launch and rotation: i wonder if
additionnal flap deflection would'nt help in this phase. This moves
backward the center of pressure of the wing which help stabilize the
plane,. No matter the drag, it gives you maximum lift which is, for
sure, necessary in this phase. Then, when "critical" altitude is
reached, less flap may help to find a better compromise "lift/drag"
(I mean here "still hudge lift/ but not to much drag") so the glider
climb fast... Maybe both elevator and flap presets are necessary for
the transition phase, what do you think ?
But while saying this i'm going for this question: what is the
optimal setting for the flaps ? For a given airfoil, between launch
at let's say, 10 m/s, and the end of the climb (before zoom) at 40
m/s 50 m/s, or more, the Reynolds number varies a lot ! Appart from
handling qualities, is the optimal flap setting the same between 10
m/s and 50 m/s ?? Do we know if we want the higher Cl, no matter the
drag ? Or a compromise ? If we can identify this compromise, is Xfoil
able to predict the airfoil performance accurately ?
Thanks for your work, wish you many interesting flights for 2006.
--- In firstname.lastname@example.org, Mark Drela <drela@M...> wrote:
> This is rather far aft hook position which requires
> special action during launch. On my Evo program the
> left slider is an elevator (speed) trim with a small gain.
> For the launch throw I push the slider all the way forward
> which feeds in a slight amount of down-elevator.
> Once the glider rotates and settles down in the climb,
> I pull the slider back to its normal center position
> for maximum load just short of stall. I have 100% Ail.Diff
> and lots of Ail->Rud coupling during launch, so I can
> steer with might right thumb, leaving the left thumb
> free to work the slider.
> I've done some sims of the initial pitch dynamics
> immediately after the throw. There is a very significant
> CL overshoot at the end of the initial nose-up rotation.
> So if you trim the glider for maximum pull during the
> climb and zoom, and throw with this trim, then you are
> guaranteed to stall at the top of the nose-up rotation.
> So for maximum launch performance, it is necessary to add
> some initial nose-down trim to safely get past the
> initial pitch transients after the throw.
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