Dedicated to the Promotion of Electric Propulsion in all types of Aeromodeling

European Version of DBF


This translation by Carl W Murphy (former and future member of The San Diego Silent Fliers,  Civil Structural Engineer and flying enthusiast) was done as a hobby for the members of the club and,  in particular Steve Neu (of Neumotors) and the students he participated with,  for a similar event in The USA.  A complete conversion into a form indistinguishable from American English was avoided,  it takes longer (this isn’t a novel) and in the translators opinion detracts from the content.  The articles foreign origin,  including the,  for English,  non standard format,  should be clear.  This translation was far more difficult then others from the same magazine.

An Der Redaktion von FMT
Es ist der Übersetzer (selbst als Bauingenieur ausgebildet) bekannt das English immer öfter an Deutschen Unis befördert wird.  Leider werden für Lehrzwecke meistens Texte aus der Literatur verwendet,  die nicht fürs Beruflichemlebend maßgebend sind.  Einen Bitten daher,  die mitgefühlte an den Mitglieder der Verein weiter zu geben.  Bitte sei aufmerksam,  es ist in den Amerikanischem Form von English gesetzt,  und doch wurde die germanische Herkunft gehalten.  Auch,  die Ingenieurwesen war deutlich zu bemerken,  die Übersetzungen fordert mehr nachdenken als üblich.

To the Editors of FMT
It is known to the translator (himself educated as a Civil Engineer) that English is being encouraged at German Universities.  Unfortunately,  most of the time texts from literature are used that are hardly representative of what will be required in professional life.  Therefore;  A request that the attached be forwarded to the members of the club.  Please be aware that the translation was done into American English.  The engineering background was apparent,  the translation required more thought then usual.

From the section FMT Magazine;

Heavy Load–Lightweight Construction

Akamodell Stuttgart wins at the Air-Cargo-Challenge 2009

It was in fall of 2006 that the notice about the bi-annual Air-Cargo-Challenge came in (see the Internet  This competition was called into life 2003 by the APAE (Portuguese Association for Air and Space Travel).  (Get our former Portuguese Airline pilot turned Radio Controlled expert to lend a hand with the description)  The enthusiasts of the Akamodell quickly agreed and registered a team for the European Competition to take place in September 2007 in Lisbon.  With the “ACC Akut” the team was able on it’s first try to take second place,  they transported 6,2 kilos into the air.

The Challenge,…
…to do better the next time,  was of course already given!  2009 the winning team organized the next competition.  The new rules were slightly altered.  Instead of a maximum wing length of 1.6 meters the new requirement was a maximum wing area of 0.7 square meters.  In addition,  this time the aircraft was required to transport a box enclosing the dimensions 1.100 x 500 x 400 mm.  The other parameters of the competition were in principle the same.  A cargo load in the form of lead plates has to be transported after a take off roll of a maximum 60 meters from an asphalt runway.  Heavily loaded,  the airplane is required to fly at least one lap and land on a stretch of 120 m.  The required propulsion is an AXI 2820/10 electric motor,  powered by a maximum of 3 in series LiPo-,  LiFe-,  or Lilon-Cells.  To avoid torturing the motors the amperage is limited to 45 A,  that also evens out the odds.
In addition to transporting the cargo load there is an additional list of evaluation criteria.  For example:  Bonus points for quick loading.  Very important are the terminology and exact description of the development process and technical drawings that must be presented in a Design-Report.  A Fifteen minute presentation of the teams work is equally evaluated.

Laying out of the Model
After the first thinking it out only a standard single wing came into question.  Other variations such as flying wing or canard carry with them disadvantages for the required maximum lift.
The above already mentioned point of unlimited wingspan with a limited wing area lead to a relative demanding optimization problem.  The bigger the selected wing span,  the lower resistance,  which reduces the required performance for the flight.  At the same time the width of the wing is also reduced.  At the first look that only appears to be a disadvantage for the required structure assembly.
Considerable more critical is the required adaptation of the layout to the required high lift profile.  Since the achievable lift off velocity is low,  and so too the Reynolds Number,  which,  calculated from the wing width and speed (Re = U x t/v) is very low.  Underneath a particular Reynolds Number the lift drastically drops off while at the same time the drag increases dramatically.
In the end we dared to use a wingspan of 2.6 meters – a very wise compromise as it was demonstrated.

Selection of the Wing,  Airfoil and:  Fowler-Flaps
At the competition 2007 and currently in most airplanes the airfoil Selig S1223 is used.  It achieves a high lift coefficient of about 2.2 at low Reynolds Numbers and has proven itself.  To gain an advantage over the competition we needed to think through (dream up) something new.
Split flaps on the trailing edge (also known as Fowler-Flaps) are known from the commercial airplane sector. They are used during take off and landing to increase the lift.
The basis for their functioning is that as the air blows through the gap it recreates the boundary layer.  In this way a stall is prevented and the lift can be increased.  Of course it’s not that easy to define the airfoil and gap geometry.  The layout was made using the computer program MSES with repetitive iterations.  With the initial data an estimate could be made that by using the Fowler-Flaps an extra kilo of load could be carried.  The increase seemed to us to be rewarding and so we decided to venture into unknown territory.
There was some question if the calculations were not partially too optimistic or if special phenomena such as Hysterese-Schleifen (This term exceeds the translators knowledge,  it may mean an oscillating condition such as vortex shedding.  Although Schleifen most commonly means a ramp,  it can also mean a loop in programming terminology.  Hysterisis is also used in english,  most commonly for the physics of the cycle of magnetizing and demagnetizing a material,  it’s use is now becoming common in the description of why better motors are more efficient.) would make the airfoil useless for flying.  As part of University course work the practicality and achievability were thorough researched.  In numerous hours at the Institute for Aero and Gas Dynamic (IAG) Wind Canal for Models,  Andre` Zöbisch verified the airfoil combination.  The most important consideration was to find the optimum flap position.  Varying the location in the tenths of a millimeter range was enough to cause either a very hard stall,  or a soft breaking one.
The expectations of the airfoil were eventually completely realized,  it was capable of a Ca,max of 2.9 at Re=1.5 ex 5.  (Scientific Notation exceeds the translator’s word processor’s capability.)  In comparison with the S1223 that is an increase of 30%!

The Propulsion
Although the motor was specified,  the choice of propeller was any commercially available type.  To get out the most thrust we tested over 30 propellers in the middle IAG wind canal.  Thrust,  voltage,  current draw and RPM were tested at various approach speeds.  (The terminology exceeds the translators technical vocabulary.  The German language word describes the relative speed of the propeller to the incoming air.)  Favorites for our target amperage and speed range crystallized out as the Graupner G-Sonic 12 X 6 and APC Electro 13 X 4.

Completion of the “Heavy Heron”
In order to be able to build the desired airfoil to the required contour,  we decided to build using foam core (laminate over foam) construction.  Especially the sharply curved flaps would have otherwise been difficult to realize.  Since the typically used mould material would have exploded our budget the forms were realized in MDF.  In the end for the wing 12 different forms were required.  After a (unknown word that may mean foam cutting) treatment the MDF was sealed with glue and after hardening smoothed out.  After cleaning off the roughest parts filler was applied and then sanded.  By this means we achieved a surface sufficiently smooth for our application.
The material used for construction depended on load,  either carbon-  or fiberglass with 1 mm RHC reinforcement.  The fuselage,  that was also built in a form,  was made from carbon fiber cloth and (unknown word describing the nature of the carbon fiber layer).  Due to weight considerations the tail surfaces were built with normal balsa rib construction.
Just-in-time before the last possible first flight appointment adjustment stencils (this may be an industrial word for a positioning link construction with for which the translator has no reference,  the Germans often use phenolic for lightweight construction in radio controlled airplanes,  the parts look about like a stencil) for the flaps were milled according to the latest research from the wind canal.
The translator wishes to again note that this is a hobby translation and that while he is able to competently translate a hobby text,  that the engineering back ground of this article exceeds his experience,  sometimes in German and sometimes in English.  I built aircraft hydraulics out of metal,  composites and their industrial terminology are beyond my experience and are not included in easily available references.  If it’s that important contact the club for further explanation of the magazines content.

The First Fight
Only two days remained for us to tune the Heavy Heron and explore it’s potential.  Since a too low wing loading for such a heavy lift flier is not without danger,  we decided to load 3.8 kilos right from the start.  After a pair of roll tests the Heavy Heron lifted off after a few meters and climbed steeply into the Heavens!  Still somewhat soft on the controls,  but controllable,  the box flew like a big HLG.  You didn’t see the overall weight of 6 kilos,  not at all.  On approach the Flier fell through somewhat and the left landing strut broke on landing,  which hurried up further tests.  But,  at least we had made one fight and knew,  there was more to be had!  On the next day the payload was raised to 8 kilos,  before a too long take off roll ended in long grass.  But,  the capabilities were proven and the repairs quickly done.

The Competition
And so the team – consisting of Ruben Bühler,  Alexander Rautenberg,  Anton Streit,  Etienne Pudel,  Jonas Illg and Michael Abel – made their way Wednesday to Covilha (Portugal).  Thursday was completely used for completion of the presentation.  With that the team was able to point well on Friday and found itself in 3rd place out of 22 contestants,  a good starting position.
On Saturday it was off to the flying field.  The first test flight was exiting.  Several of the teams made the poker game stake too high and seriously damaged their airplanes by crashing them.  We started with what was,  for us,  a harmless 6.0 kilo load,  at which we realized that with the lower air density (a result of the altitude and hot outside temperature of 40 C) and the rough asphalt demanded their tribute.  For the first flight for points we went closer to the limit.  For 8.0 kilos the runway was about 10 cm too short.  But,  Alex landed the flier once again and on the second try inside the time limit he completed the flight with the Heavy Heron.  And with that the bar for the competitors was hung very high.
Sunday on the second try were a little more cautious at 8.2 kilos,  the flight was successful.  For the last try we were daring at 9 kilo.  The flight went perfect,  only the touch down was a few meters too early.  But since no other team came anywhere near 8 kilos the victory for us was safe.  In second place was The Akamodell Team München (6.09 kilo) leading ESTG Cargo 2 from Portugal (5.95 kilo).  And so a long project was successfully completed and the countless hours paid off.
Future prospects:  In 2001 The Stuttgart Akamodell and Euroavia will be the hosts of the Air-Cargo-Challenge.  We may be curious what innovations the teams prepare.

A Thanks To
The Akamodell-Team extends a heart felt thanks to the supporting institutions (Institute for Aerodynamik and Gasdynamik,  Institute for Raumfahrtsysteme) of The University Stuttgart.  The sponsors High-End-Engineering (HEE),  DMFV,  Lange & Ritter and Dremel are also expressly thanked.

Captions to the photos:

The inner construction of the wing middle parts.  Spar,  xxx (unknown), ribs and web pockets (webs) are clearly visible,  the carbon fiber fabric shins through.

The Heavy Heron being assembled for the competition in Portugal.

The Heavy Heron on approach.  Touching down must be done gently to avoid overloading the landing gear.

The in the medium wind canal measured propellers.  27 pictured

The wind canal model of the Fowler Airfoil for use in the Model-Wind-Canal of the Institute for Flight and Gas Dynamics (IAG).

The attachment of the linkages of the fowler flaps (in yellow) on the underside of the wings are clearly visible.  On the outside wing panel in front the flap is not yet mounted.

Extreme light weight construction fairing of the cargo bay.  The opening for the ballast is found at the rear (in the photo on the right).