Hello Glenn

ok i got the point! :-)


you have no fear! :-)))))))
i would like to understand if these tweaks are "endorsed" by
the ligeti-stratos people. what do they think about?

from my own experience and some interpretation of theory:

control surfaces on fwd wing only

- Pitch issue
did you check the lift vectors on the top (rear) wing in
x-plane while you are
pitching up (deflecting forward elevator down)? they should
show a reduction in lift, should be due to the downwash of
the forward wing (increased circulation, turns the velocity
vector field downwards behind the wing: intensity of
downwash no matters you are upper or lower, it is just
proportional (among other things) to radius distance
(aerodynamic center of airfoil 1 versus airfoil 2) on the
vertical plane))
so what should actually happen is that the rear wing is
getting a lower alpha than the forward wing, therefore less
lift. therefore you have a better pitching up moment for a
pitch up rotation, but you don't have a better overall lift
coefficient on the whole configuration. i think that's why
Mr. Ligeti had flaps on the rear wing: by lowering them the
actual angle of attack of the upper wing increases (or you
get a higher Cl at lower alpha, whichever fits for you ;-),
therefore the wing "lines up" with the vector field from the
front wing and produces a better overall Cl. BUT he will
have less pitch up rotation if he deflects both forward
elevators down and rear flaps down... as a matter of fact
they say (on the website) that the thing climbs up without
changing attitude... like a B52 in some of its flight
envelope :-)

- Roll issue
i think that you should be getting a very annoying feature.
do a roll and pause the sim while rolling and check the lift
vectors on the rear wing: they should be opposite to the
roll (or at least detrimental) again due to the induced
field
of the forward wing and therefore reduce the roll rate, as a
first issue;
then the next one is really nice: say you roll right, you
should have this sort of lift distribution (+++ means more
lift on the wing than in leveled flight, --- means less)
according to the logic just exposed:

top view of the thing

       ++++|----
   ++++    |    ----
++++        |        ----
           |
------------|++++++++++++

the thing gets a cross loading lift forward/backward on the
plane not to mention drag asymmetries (due to induced drag)
on the vertical and front plane, and the less one can expect
from such a 3D pattern is that your roll will crank the nose
somewhere else in sky than straight in front of you. in my
opinion the issue should be studied thoroughly to determine
its magnitude.

moreover, expecially at lower speeds, where inertia forces
get some ground against aerodynamic forces, you should see a
phase lag between the top and
bottom wing lift vector rising and descending if you apply a
"step" input roll... this is not exactly good as you begin
to develop yaw motion in again another phase lag, so you
actually should have two laggings motion going on....
:-((((( check this sensitivity by reducing the vertical fin
area and by checking the inertia moments X-Plane gives to
your model.


-the twist and dihedral issue
things gets quite complicated here giving it is a a
sweptback affair mixed with a staggered unswept
business..... isolated sweptback flying wings with no twist
at all are nice because have minimum drag at high speed but
at low speed (high alpha) they just quit working on the
wingtips unless you have some airfoil issue there (e.g.
thickened, or max thickness moved forward) or slots (like
the Me 163 Komet just to mention one)... quit working on
wingtips means a pitch up and a spin departure.... looking
at dihedral and sweptback and twist mixed effects, there are
dutch roll versus spiral instability issues waiting behind
the corner and for single sweptback wings there are some
formulas giving what to avoid in order to stay clear. i
really never worried too much about this, maybe i should
:-)))))

in the bible-book "tailless aircrafts in theory and
practice" by Karl Nickel et al, you should find all these
things explained. the book is not any more available at
Amazon unless some charitable person sells it back... of
course i don't have it otherwise i would send the relevant
part to you. i think you know of the existence of
www.twitt.org, they may be able to provide a copy to you.
i have some single papers on the issue, if you cannot get
hold of anything on the web, i would search and send to you.


so, to summarize the tweaks you plan to do, i would better
check some issues with the Ligeti people and know what has
already been tested and written off.

did you input the foil data on the airfoil maker by tweaking
the curves there or by replacing the airfoil file data
itself with stuff you calculated e.g. in XFoil or took from
published data? and by the way, are you using the proper
Reynolds? because if i remember correctly, X-Airfoil Maker
reynolds are "quite high", i imagine you have a mac of about
0.70 m (2.3') and sailing at 180 km/h you should be at
Re=2.4million..., not to mention at stall speed,
Re=0.8million, which begins to make some difference in
keeping the flow attached at high alpha.

those negative pitching moments.... maybe you have a Clzero
alpha on some airfoil somewhere that is not nailing exactly
the high speeds alpha, maybe due to the tweaks you have done
on the wings (e.g. no twist, etc) with respect to the
original one. more practically, maybe the Wortmann and the
23012 have this Clzero alpha difference and that's it, so it
would be an issue of zeroing the alphas (by working on
geometrical incidence) on the Wortmann rather than changing
the airfoil with another one.
and again: if they say it is a Wortmann xyz and with this
airfoil you get this abc behaviour, you should try to
reproduce this in X-Plane (reverse engineering)... if you
cannot match the numbers they give you, in my opinion you
should find the bug in your model rather than fixing it with
another airfoil (just hypothizing)


i am under linux and i have had some problems in the past in
fixing the X-Plane in this machine. i really don't use
Xplane since a couple of years.
i crashed my super-mega windows based PC, companion of
thousand battles, a few months ago, with an old working
x-plane on it (V8.1 or something).
send your model over and i will see if i can do something
with it :-))))))

concerning X-plane flight behaviour, i have often modeled my
own R/C models there (right polars and all the stuff) and
done the true one. X-Plane dampens a lot of things
sometimes, while others not. it is usually more friendly on
the loss of control issues, somehow it smooths things (but
not like any other flightsim which are just crap), but is
absolutely strict on the phugoid motions and long period
stuff, i guess reality smooth things on this other side of
affairs... something to do with time constants :-)))
on the CG centering, if proper airfoils are in, you get it
to the millimiter precision :-)

now that i remember, a trick i used to play in xplane was
adding some turbulence, just a little, to avoid flying in
the perfectly viscid fluid :-). when you add little
turbulence, you see your model wobbling a little and you get
a better grasp of the real thing and behaviour.

it would be important for you, in my opinion, to strictly
model (aerodynamically) an aircraft you fly currently in
reality and of which you know  all the tricks. then you fly
it in xplane and make your own evaluation of the difference.
with this background, you will surely know better where to
check issues on your Ligeti synthetic model. absolutely
steer out of any existing x-plane models, they are 99% eye
candy stuff. build it yourself from scratch.

did i mention a limit weight for the engine? now i
understand why you insist on that light engine! actually i
planned to stuff in something up to ~80 kg complete max, so
the little mega bomb you suggest is a little overkill.
i would steer away from a "overclocked" engine, i am not
technician and i just need something i put there and makes
that for a while (200 hrs?), i will have plenty of issues to
set on the aerodynamics and structural stuff during the
testing, i cannot be grounded for engine troubles too.

just for my two cents, i buy all the R/C stuff here
www.unitedhobbies.com
go and see the prices of the LiPO batteries there :-)))

a final remark, i'm new and i don't know if all the people
here are interested in reading about aerodynamics issues. if
this bothers, maybe Glenn and me can switch to private
emailing

mrk