Conference vmszoo::rc

Since last years contest in Noerdlingen, solar flying has become more popular
and advanced. So I decided to start a new note to keep you updated ratehr than 
cluttering up the beginner's electric. Solar flight really has nothing to do
with beginners. I would like to try it some day, but don't feel prepared yet
with my current building skills, flying skills and purse thickness...

For those of you who want to become famous: There are just a few new records
set that want to be beaten: Dr. Wolfgang Schaeper from Immenstaad (Lake 
Konstanz) set the following records on June 17:

	    Class		old record		new record
	---------------------------------------------------------------
       71 (duration)	     4h, 4min, 34sec	     8h, 20 min, 48 sec
     73 (max. flying height)     ----		          405 m
     75 (distance closed circle)  43.5 km	    190.0 km (=118.1 mi)
     76 (speed, closed circle)   ----		    62.15 km/h (38.6 mi/h)      

All these records are recognized as German records and are currently being 
applied for as world records at FAI.

If I find the time next week, I'll try to type in some details of this years
models and maybe some rules of thumb where to start with solar planes (both
classes, with buffer nicad or pure solar planes).

Best regards, and have a great weekend,

Hartmut

    
    
    Hi there,
    Excuse my ignorance but when you say solar plane, do you mean powered
    by solar cells. If that is the case, I have found in the Edmund
    Scientific catalog solar cells that would give 0.45 volt and 500 mA
    I have a Mirage 550 electric from Goldberg and was planning to  buy
    51 of those cells to try to put three parallel rows of 17 cells in series
     to try to get 7.6 volts and 1500 mA. I could then take the battery off
     and fly continuouslly. Do you think that would work? 
                                                                           

    The Mirage is not nearly efficient enough to manage on only 1500ma.
    
    At full throttle, it probably draws 20000 ma from the battery.

    re .1
    
    Take a look in the Beginners Electrics notes, for the descriptions
    Harmut gave of the buffered and pure solar planes. Also Model Builder
    magazine had an article on the same planes in the last 8-10 months.
    
    A pure solar powered plane is very sophisticated, variable pitch
    prop, carbon fiber structure etc. But battery buffered solar power
    can be used on many electric gliders and is used to recharge the
    motor battery during the unpowered portion of the flight. For a
    typical 7 cell, 900-1200 mah pack you would want a 24 solar cell
    array giving ~12 volts at ~ 2 amps charge current. 
    
    I don't recall the catalog number, but the Edmunds cells that would
    do this would cost $175 for 24 cells. This fact has cooled my
    enthusiasm somewhat, but will eventually try it.
    
    A Mirage is too small, IMHO, for such a project. You need a very
    light wing loading, and don't worry about slow climb rates.
    
    20 minutes of good sunshine will put back +75 % of the charge, so
    you only need to thermal for that length of time to be capable of
    another good climb out.
    I have in mind a design of ~100-110", 1000 sq. in. wing area and
    ~50-60 oz. all up.
    
    Terry
    

To answer a question a few notes ago: Yes, this is a note to discuss electric
planes powered by solar cells. 

Sorry for the delay, I finally managed to hack in the details of this years
European contest ships from Prato/Italy. You'll find them in the next reply 
(W132).

There are different classes: most common is to have a buffer battery (nicad). 
This allows for high currents as they are  common with current electric designs,
and the buffer battery is charged by the solar cells during unpowered flight.
This requires the unpowered flight to be considerably longer than the powered
flight, so current solar planes are more or less glider types.

Good point to start at would probably be

	- 3.5 plycristalline half cells per NiCd cell, for example 28 cells
	  (14 per wing panel) with 8 NiCd

	- NiCd cells (cut off type) with 700-900 mAh

	- lightweight 20 A power controller, maybe with BEC

	- cheap motor (Mabuchi style) with gearing 3:1 to 5:1

	- propeller, not too small


A successful pure solar plane could be made of wood (with carbon spar) and
should have:

	- 16 V or 24 V Schindelstring solar generator

	- MPP Solar controller

	- bell type armature geared motor

	- high efficient composite propeller


Details of real models to be found in the next reply. Sources for parts are
included in the article. I'll keep a copy and send the mag off to Al Ryder.

Best regards, 
               Hartmut

Speaking of solar Cells - I just stumbled across in interesting picture
and paragraph about a full scale solar plane.

It is a boom and pod style glider with the principle difference that the fin
goes down from the tail and also has a wheel on the bottom.  This is necessary
to give ground clearance aft for the prop to clear the ground.  It is
a pusher.  Looks like it would be interesting to model?

I'll quote...

==============================================================================
The Sun Seeker, an ultralight aircraft powered by Sanyo Electric Company 
light-weight silicon solar cells, completed a 2,523-mile transcontinental flight
in September, setting a new world record for solar-powered flight.  Aircraft
designer and pilot Eric Raymond logged 125 hours 1 minute on the flight from 
Desert Center, California, to Spot, North Carolina, a spot not far from
Kitty Hawk.  Sanyo sponsored the flight to demonstrate the efficiency of its
solar cells, which are intended to be commercially feasible, environmentally
safe, and pollution-free sources of alternative energy.
==============================================================================

End quote.

Anybody have any more details?

Bye          --+--
Kay R. Fisher  |
---------------O---------------
################################################################################

Unfortunately, I didn't make it to watch the third European contest on Solar
Flight in Noerdlingen that took place from May 30 until June 2. But it was 
reported in the latest issue of the FMT magazine, and here are the main solar
news from Europe:

Some numbers: The contest had three classes for the first time: Pure Solar 
planes, solar planes with buffer battery and solar pattern (!). The 50 (!) 
models attending had an accumulated flight time of about 100 hours, 
corresponding to a distance of about 3000 km (>1800 mi). This is about a 
factor of three to the first contest in 1989 (see earlier replies).

One impressive plane was brought by Jean-Pierre Schildknecht. He was champion
in both pure solar and buffered class. His pure solar plane with a high-lift 
airfoil B7406f even had solar cells in front of the spar! He tested the wing
with and without the cells and didn't find too much a difference. So much for 
airfoil accuracy at low flying speed (lowest sink rate at an airspeed of ~ 6
m/s = 13 mph). At this speed, the plane only needs 8 W input to keep his height
(with common 7 cell planes you're usually way over 100 W). This way, it was
able to fly at 7:00 a.m. as well as at 7:30 p.m.

Pattern planes were brought by Sommerauer and Fredrik v.d. Lancken. Fredrik
won with his CLOUD DANCER with 10-cell battery, Sommerauer obviously didn't
have a buffer battery. With only 16 solar cells with maximum power of 24 Watt,
the plane was able to keep flying at 50% solar intensity. He electronically 
increased the voltage of the solar generator to keep his motor close to maximum
efficiency. Of course you can't do all figures (extended inverted flight will
probably not be very extended...), but it starts to become possible!

The group of the buffered planes was so big that they had to split it, and they
think about modified rules for planes with smaller/bigger solar generator.
Interesting to note that Werner Dettweiler (designer of the later GRAUPNER
electrics as Elektro-UHU, RACE RAT, CHERRY, CHILI etc. and the SOLAR-UHU)
managed to take 4th place with an unmodified Solar-UHU!

After the contest, future projects were shown. Franz Weissgerber showed how far
he already got with his RENO project. A glider of 4.5 m wingspan (177 in) to
be equipped with solar cells and a Nickel-Hybrid buffer battery. This is 
supposed to be the ideal combination for buffered solar flight (if money 
doesn't count). They will try to achieve new records somewhere close to Reno.
I assume this is for the next season, and I'm pretty sure you will hear and
maybe see more about it next year.

Best regards,
              Hartmut

    
    Hey did anyone notice that there is a SOLAR UHU listed in the Hobby Lobby
    catalog. It looks real neat. Big Bucks $390 for the plane with 20 solar
    cells. You can buy the solar cells seperate for 14.75 a cell or $278 for
    20. 20 cells puts out 1 Amp 10 volts and weighs only 4 ounces. 20 cells is
    designed for 6 cells which you still have to have. But you could
    probably get away with much lower capacity (less weight) since you
    can literally charge on the fly.
    

    
    When I last visited our local hobby shop, I saw the solar UHU kit on
    display. It costs DM 498,- here (1 $ = 1.66 DM currently). It is
    supposed to be a pretty good plane since it's designer, Werner
    Dettweiler, took 4th place on the European solar contest this year with
    a stock solar UHU. There were other versions of it as well (one 'tuned'
    solar UHU with 30 solar cells etc.).
    
    The price is horrible, though. I recently read that the kit goes for
    around DM 150.- and the very same solar cells are available at other
    places for DM 180.-. GRAUPNER charges DM 350.- for the set of solar
    cells. So - you might consider buying the kit and looking for the cells
    elsewhere.
    
    Maybe I'll try to build one of these, but certainly not before next
    building season (this season will see the ElectriCub and some
    modifications to the existing fleet.
    
    Best regards,
                  Hartmut
    
    P.S.: If you are interested in more details about the solar UHU kit -
    feel free to ask. Next time I'm in the hobby shop with a little time to
    spare, I'd like to have a closer look at the kit anyhow.

    Edmonds Scientific has solar cells in their catalog that have the
    same specs as the Graupner ones sold by Hobby Lobby.
    Edmunds are ~$5.75, are square, 95 X 95 mm, rather than rectangle
    like the Graupners. 
    
    I'm thinking of ordering a few and see what the performance is on
    the bench. I'm interested in seeing how much the current flow is
    degraded when the cells are covered by clear Monokote, at various
    sun angles/intensities.
    
    If the numbers look feasable I'll think about the solar recharged
    plane again, thats been on the back burners of my mind for awhile.
    
    Mine would be larger than the solar Uhu for a lighter wing loading,
    pod and boom, V-tail. 
    
    <60 oz. seems possible, meaning a Speed 500 with ~10 X 7 prop geared
    3:1 on 7 cells would give adequate climb performance, not a major
    consideration on this type of plane anyway.
    
    Terry

    The Solar UHU is bigger than standard UHU and only weighs 47 oz.
    Also if your going to spend that much money for solar you should
    consider buying a better motor.

    I'm thinking in the ~100" span range, with >900 sq. in. 
    
    The Speed 600, ~20 watts less than a Speed 500, hauled a 113", 68
    oz. glider ok, although the climb rate was hardly inspiring.
    
    Actually, a simple ferrite motor has proven to be the best way to
    go with solar recharging, the Germans have found this to be the
    case and Hartmut had a note a while back explaining the reasons.
    Don't remember the details.
    
    I don't like the brush holders sticking out the sides of the Astro
    cobalts, and as long as I'm only using 7-8 cells, the price/performance
    ratio doesn't seem justified.
    
    Terry

    RE.: .12
    As far as I know, covering the solar cells even with clear film makes
    the cells pretty much worthless. Correct me if I'm wrong, but I think
    what solar cells need most is UV radiation, and this is filtered pretty
    much by any covering.
    
    What people are doing is either use airfoils with as much flat top
    (behind highest point) as possible and put the cells right on there,
    or, if necessary, two (or more) rows of cells at different angles. The
    performance will suffer some (from the ideal airfoil), it depends on
    the amount of work and ease of rework you desire whether you decide to
    simply put the cells on top or if you provide cavities in the upper
    wing surface.
    
    The high-tech way of the SOLARIANE put the cells first in the negative
    mold of the upper wing and pressed them such that they pick up the
    correct form of the foil and just didn't break. Then glass the wing and
    you've got a wonderful piece of art without any chance of exchanging
    failing solar cells at all...
    
    As far as I know, clear covering is only in rare cases used for the
    area in front of the spar when people decide to put cells there as
    well. Not sure what the Solar-UHU instructions suggest, but from the
    pictures I've seen I'm pretty sure that the cells are supposed to be
    uncovered.
    
    Regarding motors: From what I recall, I only said that ferrite motors
    are preferred for high rpm applications. As the statistic from last
    years contest models shows (a few replies back), there are a lot more
    rare-earth magnet motors used than cheap ferrites. Nevertheless,
    ferrites do the job, and the correct choice and adjustment of motor, 
    prop, generator, battery and plane make a working solar plane.
    Experimenting and optimizing this is a lot cheaper with ferrites...
    
    Don't get me into these dreams too deep, gotta save some money first!
    
    Best regards,
                  Hartmut

    Money ? Ah yes, I remember I had some of that once.
    
    Thanks for the observations on covering solar cells, Hartmut.
    It confirms my suspicions about killing the efficiency, which is
    small enough to begin with.
    
    I may examine my airfoil choice again; had thought of using the
    S4061 but the S3021 has a virtually straight line on top behind
    the midpoint.
    
    I will use a built up wing for simplicity. The cells will be mounted
    in a thin tray which will sit in a recess cut in the top of the
    ribs.
    
    The fuselage design suddenly came to me last night while examining
    what I had on hand.
    A 1/2" diameter fiberglass shaft, which will use a pillow block
    and beam made of spruce/balsa/?, from which the motor , batt, speed
    control can be hung. Then a balsa/glass fairing pod to streamline
    everthing ahead of the wing.
    
    Hmmm.. now where did I put that checkbook ?
    
    Terry

>    Thanks for the observations on covering solar cells, Hartmut.

I agree that some UV will probably be reflected by the clear covering
but unless you are able to point the cells directly at the sun at
all times there might be some gain to putting the cells inside the
wing.  Imagine that for the sake of argument they were placed
on the bottom of the fairly flat bottom wing - facing up of course
and you had clear MonoKote on the entire top.  You could either make
ribs of clear plastic or aluminum coated balsa and line the spars
and shear webs with aluminum.  You could allow quite a bit of light
into the wing (much more entering than the area of just the solar cell)
and reflect much of this normally lost light onto the cells.

Here's another variable - they will probably be warmer inside the
wing.  So - are they more efficient at say 90 Degrees F or at 70 Degrees F?

Bye          --+--
Kay R. Fisher  |
---------------O---------------
################################################################################

    These are some of the questions I'm going to examine with a few
    test cells.
    I think that the most efficient location would be on top of the
    wing surface, the direct sun rays deliver more energy than reflections.
    
    With the attitude of the wing constantly changing in relation to
    the sun angle, it's impractical to worry about optimum angles,
    just try to keep the cells out where they can receive the greatest
    average energy, and any covering will affect this but I'm still
    hoping that the losses through clear Monokote will be small enough
    to allow for covering the cells.
    
    FWIW, roof mounted solar water heaters are angled to the optimum
    for the geographic latitude, but the cost/efficiency trade offs
    in tracking the sun east to west are not worth it for the typical
    home unit.
    
    Terry

    
    My EYE doctor put my clear perscription glasses in a UV tester. They
    removed 99.9% of the UV. If the cells run on UV then there is a good
    chance you will not get anything for output. But I strongly suspect
    that they do not depend on UV. 
    

    Yes, stuff that is transparent to visible light is not necessarily
    transparent to UV, or other wavelengths that the solar cells may work
    on.  It seems like a simple experiment to measure the output of a cell
    covered with nothing and various different coverings.  I am sure many
    people would be interested in the results. 

    Solar Cell's can be made more efficient by directing magnified light to
    them.  The common glass used in the magnifying lenses I used to play
    with block a good amount of UV-A, but the output of my Solar Cells
    always went up when the lense was in front of them.  Ergo, Solar Cells
    don't particularly care about UV.

    How about if you place the Cells in the wing, and bought a couple of
    Fresnel lenses to use as the top of the wing surface, in place of Clear
    Monokote?  You'de get strength (the plastic a Fresnel is made from has
    got to be a lot stronger than the equivalent weight of Balsa), and some
    slight increase (the Cells would be well inside the focal length of the
    lense) in light intensity, plus some additional tolerance to less then
    90 degree angles of incidence between the Sun and the Cells.  I've seen
    Fresnel lenses from Edmund Scientific in sizes close to 2 Ft. square.
    They're only a .050 or so thick.


    Randy

    I've got 7 solar cells on order from Edmund, for experimentation
    purposes. 
    The fresnel lens idea sounds like it may be worth investigating.
    What it all comes down to is charge rate during motor-off time.
    
    If a ~ 2 amp charge rate can be maintained for 15-20 minutes while
    gliding/soaring then you've got a worthwhile charge to use to climb
    out again or look for more lift.
    
    To increase this by 5-10% may not be worthwhile. Of course a 50%
    increase would be, but I doubt there is any practical way to do
    that with a given set of solar cells.
    
    Terry

    I've got a battery charging type question...
    
    Do nicads mind being "pulse" charged? As you circle the sun will change
    orientation and the nicads will see a pulsing. A capacitor would level
    this out. Is it worth it? (and diodes to keep the charge heading to the
    battery)
    
    Terry,
    
    Have cells come far enough to get ~2 amps from so few cells or is this
    just a sample quantity?
    
    If you can charge between every motor run you only need enough for a
    single climb so you can save weight with a 900mah pack instead of the
    typical 1700mah
    
    I've always been interested in this and my club had a grad student from
    Worcester Polytechnical Institute (WPI) learn to fly in order to pilot
    a grad student project along these lines. Haven't heard back from him
    about their progress.

    A single solar cell of the type I'll be using puts out 2 amps at
    .45 volts, ideal rating.
    
    Therefore you need an array of ~24 cells to get a charge voltage
    in the 10-11 volt range which is adequate for a 7 cell pack.
    
    Nicads used as motor batteries such as Sanyo SCR certainly don't
    mind pulse charging such as would occur through sun orientation.
    This would be a very small delta compared to typical fast charge
    rates which can be over 5 amps.
    
    Terry

>    Do nicads mind being "pulse" charged? As you circle the sun will change
>    orientation and the nicads will see a pulsing. A capacitor would level
>    this out. Is it worth it? (and diodes to keep the charge heading to the
>    battery)

    I would hardly describe what would happen to the Cell output volatge as
    "pulsing".  I think "fluctuate" is a better descriptive phrase.  The
    rate of change would be quite slow.
    
>    Have cells come far enough to get ~2 amps from so few cells or is this
>    just a sample quantity?

    I believe the way it works is, given enough light, the voltage a Cell
    outputs is primarily a function of its chemical makup.  The amperage it
    delivers is primarily a function of surface area.  If you double a given
    Cell size, you get twice the current, but the same voltage.
    

>    Do nicads mind being "pulse" charged? 

    They prefer it.   If I ever catch up with my project list, I'll build a
    bench charger that chops the charge current.

    But I agree with Randy, this is "variable DC".


    Alton who, as a physicist, still gags at the phrase, "vacuum reservoir"

    No, don't go back to read Randy's comments.  He did not say "variable DC".

    If the circuit is regulated you can probably charge while the
    motor is ON too :-).

    Since you're talking regulating....
    
    There is an interesting issue around controllers. Solar flyers tend to
    use a special kind of controller (so called solar controllers) for the
    motors. They are not exactly like those of conventional electric
    planes.
    
    I am not really sure about the reason, so those of you that know more
    about solar cells, please correct me. From what I understood, the
    efficiency of a solar cell goes down with the voltage. So - if the
    intensity of the sun goes down, the voltage drops. If the load of the
    motor is too high, the voltage drops even further and the efficiency
    goes down the tube. So what the controller does, is keeping the voltage
    over a critical level. If the energy from the sun becomes lower, it
    tends to reduce the current to the motor, and with this keeping the
    voltage and the efficiency of the cells as high as possible.
    
    The best setup for this was implemented in the SOLARIANE. In this
    plane, not the current to the motor was limited, but the variable pitch
    prop was adjusted automatically by a controller driving a servo such
    that the overall efficiency was always best for a given level of
    sunlight. Of course, one of the most expensive vendors for controllers,
    Hans Jakob Sommerauer (Switzerland) has a solar controller to offer. I
    haven't seen it in the mags yet, so the price probably is to be
    negotiated.
    
    BTW, the duration contests at the last solar contest in Noerdlingen
    were flown early in the morning and late in the afternoon. Progress of
    technology has advanced so far that anybody can fly a solar plane for
    hours during lunchtime - it does not make sense to hold a contest then.
    They used the lunchtime for solar POWER (?) planes.
    
    Best regards,
                  Hartmut

    Hartmut,
    If I understand correctly, the solar controllers are being used
    on planes that have the solar cells driving the motor directly.
    
    What about the type that simply charge the motor battery pack from
    the solar cells ? 
    
    I can't see where the latter type would benefit from having a special
    type of controller.
    
    Terry

    Oh yeah,
    
    you're right! So this is only for step two - eliminating the batteries
    altogether. You should keep a small buffer battery for the receiver,
    though.
    
    BTW - talking about radios - Franz Weissgerber, creator and pilot of
    the SOLARIANE, now offers a set of solar cells to assemble directly to
    a GRAUPNER/JR mc-18 transmitter. So even the transmitter battery
    doesn't limit the possible flighttime any more.
    
    What kind of controllers is used in buffered solar planes, I don't
    know. I assume, it's pretty much conventional controllers (or even
    switches to save a little weight) and maybe some diodes to make sure
    the current doesn't under any circumstances flow back into the solar
    generator.
    
    Maybe I can tell a little more on Monday. I am expectig Chad Leigh (now
    in Munich) to visit us tomorrow, and since the weather is awful, we
    might go see our local hobby shop and for example have a look at the
    Solar-UHU.
    
    Best regards,
                  Hartmut

    Well, today I'm not so sure any more that solar controllers are for
    pure solar planes only. Chad was here last Saturday, and we had a good
    time together. We went to the local hobby shop and got his JR X-347
    radio working at 35 MHz with GRAUPNER/JR tx module and receiver. No
    problem at all. 
    
    I had a look at the Solar-UHU kit. There seems to be no special thing
    regarding the wiring. It seems as if the solar string is just parallel
    to the battery. I must admit, we didn't have time to have a very close
    look, and the guy that usually runs the shop and is pretty
    knowledgeable, wasn't there on Saturday.
    
    But the plans show a 'solar-MOS something' controller. So - there is a
    special controller available even from GRAUPNER, but there were no
    details in the plan, and I couldn't find it in the available
    catalogues. The SOLAR-UHU is definately not a plane that can easily be
    made a pure solar, so this can't be the reason for the controller.
    Maybe it just contains some additional wiring to make sure that the
    current does not flow from the battery to the solar cells or something
    like that. I don't know. I'll put anything in here that I might find
    out. I like this note becoming a little active finally.
    
    Best regards, 
                  hartmut

    You're probably right about the solar controller containing some extra
    diodes. These could be added externally to a regular controller.
    
    Solar cell/motor batt. isolation requirements is one of the first
    things I'm going to check out when I get my cells from Edmond Sci.
    
    Terry