Discussion sur le site OZ Report
(http://ozreport.com/)
OZ Report
Volume 7, Number 79
Sunday, March 23rd, 2003
This topic is
in: <- Sep.3 Jul.21 Mar.23 Mar.21 -> )
Bill Berle < auster5 > writes
I took a few photos of my Platz glider model.
The model was built of curved laminated balsa keel and mainspar, covered
with Japanese tissue held to the frame by glue stick. Thread formed
the trailing edges of main wing and canard/jib surfaces.
An electric motor
platform was added at the front of the keel. I
used a rubber block with the canard strut wires held in place by friction,
so I could easily adjust the decalage and control inputs.
As I mentioned
before, I could not get the model to turn in a reasonable circle with
the canard jibs alone, either in a glide or under electric power. I added a vertical fin, and was all set to put on a rudder
(which I was and am confident would have added proper control authority). But
the model was not able to sustain level flight with the electric motor
weight, and the repairs were already too numerous, and I simply retired
the model.
I had gotten the information I needed, which was that some
form of yaw control was needed. As an alternative to the fin and rudder, drag
spoilers on the wingtips that "pulled" one wing back would more than
likely provide plenty of yaw control without changing the basic shape
of the Platz design. The moment from the CG to the rudder would
have been even less than the moment arm from the CG to the tip drag
spoiler, so yaw control would have been very effective.
Brett Snellgrove < Snelly14 > writes:
It was with total amazement that I saw Dan Gravange's posting regarding
the Platz wing one issue before mine. Although it appeared otherwise,
my posting was not in response to his and quite an amazing coincidence
that shows the ongoing interest in this design.
Although I must bow to
Bill Berle's experience with models (vastly superior to any hypothetical
musing) my attachment to this design makes me reluctant to give up on
it so easily.
Firstly, I see an inconsistency regarding Bill's comments
on roll control. From
the photos I sent you (not the ones posted thus far) it is quite clear
Platz is freely and easily soaring a long dune ridge -- this would be
difficult without adequate roll control and impossible with none at all
as Bill noted. Secondly, Bart had found documentation describing
excellent roll control up to and through stall in both models and the
final design and clear documentation that the rudder Bill considers necessary
was not needed. Something's amiss somewhere.
Bill surmised that the absence of roll control in his model was due
to drag created in closing of the slot on the down going canard. In the
photos it appears the canard was well forward and up of the main wing
-- too far in my view to form an effective slot. If this was indeed
the case I see 2 solutions.
One would simply to be to use upward motion
only on one canard, no downward on the other. If indeed the down
going canard did cause excessive drag then perhaps downgoing motion only,
similar to a paraglider brake would be effective?
Bill also described
difficulties with trimming the model. If as
I, suspect he copied earlier designs shown in the photos, and the canards
were hinged at the LE, this would be expected. Hinging at the 1/3
rd chordline as Platz later did might solve the problems Bill encountered. I
suspect Bill rigged his model hinged from the canard LE and with equal
action via RC servos. In reality the Platz glider was flown with
the hands, hinged at the 1/3rd chord and a good degree of differential
action allowed such that the pilot could manipulate the controls independently
to effect roll.
Bart also notes sailplane clubs were considering usng
Platz's wing as a trainer. I find it difficult to believe they
would do this if the problems Bill notes in his models weren't somehow
solved (without a rudder).
Brett Snellgrove writes:
I built a model of the Platz glider today and believe I solved the riddle
of how Platz achieved sufficient roll control to soar dunes. Firstly,
the glider flies very nicely and is remarkably stable considering how
close to the mainwing the canards are -- yet it seems to fly with the
stability of a tailed design without a long tail lever. This alone
makes it an attractive option.
The glider is quite responsive in pitch
to uniform motion of the canards. I
noted that same thing Bill did in that aileron type deflection produces
little change in direction. I suspect the drag of the downgoing
canard produces significant adverse yaw. I then tried unilateral
canard deflections and as expected, one canard deflected downward produces
a diving spiral to that side.
However a unilateral deflection upward produces
a nice rapid flat turn to the same side!! This may be due to the closing of the slot as
Bill notes but I think it is simply due to the extensive drag associated
with a large upgoing surface deflection associated with mainwing dihedral. There
appears to be no roll away from the upward deflection and significant,
now proverse, yaw associated with the turn. I suspect the dihedral
of the mainwing cancels what would now be adverse roll? There does
not appear to be a significant pitch up associated with the turn. Possibly
the loss of lift on one side compensates for any upward pitching motion.
Platz
would have most certainly experimented with rolling the glider during
the extensive tethering tests and not attempted free flight without effective
roll control. Given Bill's and my experience with models
this seems the most likely solution. So it conceivable Platz discovered
paraglider type control years before MacCready did the same thing with
the Gossamer Condor. In fact the wing would be flown exactly like
a paraglider for pitch and turning and should make for a very simple
transition.
Bill Berle writes:
Brett, I think either you or I have something backwards. When
you describe a unilaterally upward or downward "aileron' deflection I
think we have it opposite.
On the fixed wing aircraft designs that I am
familiar with, an "upward" motion
means that the trailing edge of the control surface moves up. In
the case of the Platz, I would expect an "upward" motion to mean that
the control handle at the rear of the canard boom was raised up above
the pilot's head. In this case "upward" motion of the canards would
be making the glider dive, the same way as upward movement on the canards
of a Vari-Eze canard power plane makes it dive.
A downward motion would
mean that the trailing edge of the canard moves down, making that canard
produce more lift in theory.
What do you mean by a "unilateral" upward motion? Do you mean
that the pilot would raise or lower the handle? Do you mean that
he would move it all the way full travel up or down?
I think I understand
your results after reading it a few times, but I still cannot get how
using full "up-elevator" control on one side (to
cause a drag-related turn) would not ALSO act like an elevator and raise
the nose at the same time.
Although you may be able to trim the glider
into a level flight turn using the method you discovered today, I still
strongly suspect that using the canards as a drag device would have some
negative effect on pitch control especially if you needed it at the same
time.
Brett, please leave room in your model to try the tip spoilers I
suggested. I
think that little tip spoilers or drag plates would allow you instant,
smooth turning control that more importantly did not interfere or affect
the use of the canards together for pitch. I would bet that at
the end of the day, this is the control setup that would allow a safe
man-carrying version to be built.
When the time came for it to be flown
in "big air" as the HG pilots
say, I would bet what's left of my balls that a fixed or movable vertical
fin would be the only thing that would make it safe to fly. Dihedral
is not a substitute for yaw stability dihedral only rolls the airplane
the right way for any given yaw. So when a gust blows you 45 degrees
off course into a hillside, at least your yaw string will be in the center
on impact. |
Vers
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