Current Issue

Issue 79 – will be distributed in April 2021. Click on the image below to view the table of contents.

FFQ79_rev_cover

The Editorial by Sergio Montes reports on the work of the University of Illinois, under the direction of Prof. Michael Selig on  testing of model aircraft propellers of diameters up to 11 and 12in. The tests were conducted on a special wind tunnel of 3 x 4 ft with state of the art instrumentation and measured their thrust and efficiency,  as well as the typical propeller coefficients of torque and power. The emphasis of the experiments was to determine the effect of Reynolds number on  efficiency and thrust. It was found that these propeller characteristics were seriously affected by the Reynolds number. For example best efficiencies of about 65% were achieved  (and which could decrease to about 40% in some propellers). In comparison the Durand-Lesley tests at Stanford in the 1920’s with larger model propellers of 36″ diameter, recorded efficiencies of 80%, and had a smaller spread of efficiencies compared with the smaller propellers in the Selig tests.

In the second installment of his series on Power Models,  Jim Baguley speaks about optimum proportions of the model to achieve stable vertical climb. He also deals  with wing and stab  airfoils and best wing construction practices for them.  For the high-climbing model Baguley recommends a wing with moderate aspect ratio and moderate polyhedral. Fin area should be large and stab moment arm moderate. To ensure left-rolling tendencies,  a slight wash-in of the right wing panel is advised. Glide turn can be obtained by tail tilt or auto-rudder.  For the wing, he prefers  a moderately cambered airfoil derived from the Goldberg G610, via Dave Kneeland’s 1953 FAI winner. A number of wing construction schemes are suggested.  He reckons here that D-Boxes in balsa wings may suffer more on hard D/T landings than the more elastic multi-spar geodetic structures that he prefers.

In the F1E tailplane and its profile, Gerhard Woebbeking examines in some detail a couple of tailplane airfoils that have proved highly successful.  The first is one of his own design that he calls “Woe 8%”. This airfoil is derived from the much thicker Goettingen 625  by the simple expedient of cutting this  profile longitudinally, retaining the topside and replacing the lower profile with a straight line. The very ample nose radius is retained, but the new airfoil has a maximum thickness of 8% rather than the 20% of the Goettingen 625. The main property of this new profile is that its coefficient of lift CL changes very little between angles of attack -4 deg and +4 deg. For large or smaller angles the airfoil reacts much more vigorously. This means  that the tailplane will be able to damp well large deviations from normal flight trajectory and its indifference to smaller oscillations lets the model return quickly to normal.without “overshooting”. None of the other airfoils tested by the author,  which included the popular Clark Y8% had this useful property. In practice the Woe8% proved spectacularly useful and has been adopted by many designers, especially after its adoption in the F1B category  by the famous “A.A.”,  Alex Andriukov. The second airfoil, as explained in the article, was one that the F1E pioneer,  Hans Gremmer, had found. It was  a 10% symmetrical airfoil with a pointed nose, and it  worked well,  having some of the Woe8%  fast increasing lift characteristics with angle of attack.

Jim Baguley is also the designer of a small rubber model of excellent performance, a true rival of the Gollywock, the Last Resort. It is intended for Open Rubber competitions, with its excellent power to weight ratio.It has a wingspan of 32″ , same as its great American rival, a slimmer fuselage and sports a single bladed propeller of about 14″ diameter. The motor, 16 strands of 1/8″ or 8 of 1/4″ rubber weighs 45 gr and can take about 1,000 turns. The steep and fast climb is spectacular, allowing a calm air performance of 4 minutes, plus. Construction is very simple, with a multi-spar wing that uses the same type of airfoil as Baguley’s power models, 8% thickness and moderate camber. The design was first published in “Model Aircraft” magazine in 1960.

The dynamic behavior of a model is analyzed in an article by Claudio Bognolo. This analysis  considers the forces acting on the model and the model’s response to these forces.  The model’s response to the forces results in acceleration, of which the most important components are considered. The forces are, in turn, related to the orientation and speed of the model relative to the air. A complete statement of the orientation and  speed of the model requires 12 parameters.  This article, which follows closely the work of Tjarko Van Empel,  published 30 years ago in the Vol Libre magazine,   focuses on three parameters; the angle of attack (alpha), the angular velocity, which is  the change of alpha with time and the angular acceleration of alpha.  These three parameters are used to develop a dynamic system of a spring, damper and mass in terms of the motion of a point located at the horizontal tail.
The angular acceleration of a rigid system of masses due a moment (a force acting through a distance) is  J x Angular_Acceleration = Moment  . For a system of masses, J (the angular mass moment of inertia) is the sum of the products of the masses times the square of their radial distance from a reference point.
An equivalent single mass, the mass of the dynamic system M* is  located at the horizontal tail, so the system has the  with same angular moment of inertia. The spring represents the resistance to change of the angle of attack, often expressed as the change in pitching moment with respect to the angle of attack. The lift of the wing and the horizontal tail acting  through their distance to the center of gravity create a moments that acts to assist and resist the change. The rate of change of the combined moment of wing and tail due to their lift forces with alpha is  the  stiffness K* of the system.  If the mass M* is moved a small amount and released it will oscillate as the spring K* resists the motion and in turn causes acceleration of M*.  The motion of M* can be considered to be represented by a sinusoid, for which the natural frequency can be easily calculated in terms of the parameters K* and M*. Bognolo compares the predictions of this highly simplified system with the actual behaviour of one of his F1E models, UGO-2. The calculated damping ratio for this model is slightly less than  unity,  that means the model will oscillate, but  the model will quickly suppress the oscillations after a perturbation; the same behaviour has been found in other F1E models. The author explores the means modellers have in order to make the model more dynamically stable by increasing the stiffness of their models, for example increasing the tail-boom length, increasing the tail area, or alternatively moving the CG forward.

The Rumpler Taube scale model is an design of an unusual and very stable configuration  Rumpler Taube aircraft flew with considerable success in the years previous to the First World War, and although conceived initially for civilian purposes,  they were in fact used as observation aircraft by the German,  Austrian, Italian and Japanese Air Forces. The design was due to Igo Etrich from Austria-Hungary, and the aircraft first flew in 1909. The inventor, Etrich, abandoned the patent of the aircraft,  which came to be known by the name of the factory producing it. There about 10 variants. The wing shape is not one of the dove (Taube in German),  but actually that of the Zanonia seed,  that flies well after being released from the tree. The addition of a tail to the Zanonia-shaped wings improved the actual design . The model suggested here was initially conceived for half-A glow engine power. As these engines are increasingly difficult to find today, possible builders of this 1:18 model of 807 mm span will quite likely use electric power in this model, for which there is a wide combination of motors, batteries and ESC units. The actual construction has been simplified to some extent by the model designer, Ted Enticknap, by omitting the complex external bracing,  while retaining the graceful Zanonia-seed planform. The construction,  trimming and flying are covered in the notes added to the plan.

Adrian Duncan has written a great article on the history of the MK-12S 2.5 cc Diesel engine of about 1957/58. This engine was designed by Oleg J. Gajevski, and described even more fully in a Russian language book on Model Aero engines published by Gajevsky in 1958. The MK-12 engine was a serious and quite successful design, very much competitive with Western engines of the time. It had advanced features of a twin ball-race crankshaft, rear disk valve induction system and fuel delivery by means of single-nozzle metering system without the obstruction of a spraybar. The new engine was not heavy, weighing only 128 gr. In a competition organized by DOSAAF the MK-12K engine  won first prize for engines, achieving an output of 0.357 HP at 16,500 rpm. This level of power exceeded easily that of other 2.5 cc European or American engines of the middle 1950’s. It does seem the light weight of the engine conspired against its practical use and the production units had to be redesigned to some extent. This was done and the production model gained about 12% in weight and some de-tuning  due to increased piston weight and intake diameter reduction. These measures reduced the output to about 0.23 hp at 11,600 rpm, which was still above most Western Diesel engines of the same size. This modified engine was made in fairly large numbers  for several years. In spite of this it was not well-known in USA or the UK and Adrian Duncan careful description and analysis of the engine will be of greater interest to engine aficionados. Adrian obtained a test engine from Peter Valicek, an engine that proved to be in top condition . Under careful test it revealed itself to start almost at the first flick,  hot or cold, have positive response to compression adjustment and needle valve setting. And even more importantly this production engine reached about 0.29 hp at  11,500 rpm, much better than what previous tests by Peter Chinn had found. Other close variants of the initial design appeared after a production run of about 2 to 3 years, variants that went on production in Russia up to the 1990’s. It estimated that more than a million MK-12 engines were made in the Moscow Aviation Repair Centre (DOSAAF/MARZ) plant alone.

The Caproni CH-1 was an exceptionally attractive biplane fighter prototype produced in the then vigorous Italian aircraft industry of the middle 1930’s. It is described here by Sergio Montes. It came at a time when biplane fighters were on the verge of being replaced by faster monoplanes by all major aircraft producers, so they represented the ultimate development of their type. The Caproni CH-1 was a personal design by Antonio Chiodi, a Regia Aeronautica flight officer, but not a professional designer, only 25 years of age. His design must have impressed the Caproni factory,  so they built a single prototype in 1935. It was somewhat smaller and much more graceful in its lines than the FIAT fighters that were used so effectively in the Spanish Civil War and that even had a small role at the beginning of WW2. There was an obvious purity in its lines that remind many of the successful Italian car designers of the post war period, some of which produced  the Ferrari, Maserati, Lamborghinis and other marvels of the car industry.  The Caproni CH-1 had wings of equal span with very slight dihedral,  only on the top wing. This was set flush with the top of the fuselage to improve pilot visibility. The cockpit was closed and well-faired into the fuselage. Landing gear was fixed, but really a model of design and proportion with carefully spatted wheels. A long NACA cowling enclosed the 9-cylinder Piaggio radial engine of 560 hp, which allowed a maximum speed  of nearly 380 kmh, good for the available  engine power, but already surpassed by other biplane fighters. Flight tests revealed delightful characteristics. Performance figures were similar to other front line fighters of the Italian Air Force, the Regia Aeronautica. It was a great misfortune when after a short period of testing the aircraft had a landing accident, flipping on its back and sustaining such damage that it was considered un-repairable. The pilot, Capt. Chiodi emerged unhurt,  but that was the end of this project. It was not adopted by Regia Aeronautica and no other examples of this remarkable design were made by Caproni.

“Adjust for Vertical Climb” is  a short series of three articles by Dave Hipperson on the design, trim and flying of Slow Open Power models, a very popular power model category in the UK. This series appeared first in the AMI magazine for April, May and June 2000. Slow Open Power (SLOP) has designs rules that try to simplify the model  by not allowing variable incidence for wing or stab, require that the engine must have a plain bearing crankshaft, be of maximum size 3.5 cc. No pressure feed is allowed, with  engine run of 10 s. for glow engines and 12 s. for diesels. Following the success of George Fuller’s “Dixielander” design,  many SLOP models have somewhat similar lines, the main characteristics being a high pylon that puts the wing (of moderate aspect ratio) outside the propeller blast, also a rear position of the CG, say 80 /90% of wing chord, with small decalage, a tail moment of slightly over 3 wing chords  and generous fin area. General characteristics of the wig and stab  are considered in this first article of the series, including airfoil and wing construction. In this case  a moderately undercambered airfoil and multi-spar construction  are used  for the wing, together with  flat bottom airfoil and multi-spars for the stab. A most important part of the trim concerns the wing warps, which are discussed in some detail and are absolutely necessary  to transform the basic looping trajectory into a stable steep rotating climb. Two further episodes of this series will be published in the July and October issues of Free Flight Quarterly .

A comprehensive essay on the tailplane, the need for it,  its size, shape, location, effect of the vortex wake and some discussion of suitable airfoils is provided by Gerhard Woebbeking in this informative article. According to the author, the tailplane had been investigated by Englishman Joseph Pline in first half of the 19th century ( Free Flight Quarterly did publish in issue #78 some comments on  the pioneering efforts of Sir George Cayley, who included tailplane and dihedral in his 1810 designs for a glider). Pline’s work was continued by Adolphe Pénaud whose 1863 rubber model. the “Planophore” flew very successfully in Paris. The Planophore had what the author terms “intrinsic stability”, that is the ability of returning to normal flight after a perturbation. This stability was possible when an additional wing was affixed to the airplane. Following nature and birds,  a rear, smaller wing as found in birds. In many cases the tail feathers in birds are quite sketchy, yet they retain stability using their highly developed sense of equilibrium and neuro-muscular coordination. As Woebbeking  asserts so happily: “birds are living gyroscopes with feathers attached”. Yet in the absence of the advantages that Nature bestowed on birds, it has been possible to attain automatic stability in full-size aircraft and models by careful design of the tailplane.  This design has been taken up by the author invoking the theory of the neutral point, a relatively modern theory  that indicates the position of the center of gravity of the aircraft needed to attain automatic stability. Today one finds several computing aids (quoted in the article) that solve this mathematical problem for a variety of wing and stab configurations, not only the “normal” one with stab behind the wing,  but also for  tail-first aircraft, biplanes and triplanes. A suitable position of the CG will make the aircraft stable and also will determine the agility with which the aircraft will respond to perturbations, such as wind gusts.  As the reader will understand, there is even for the normal position of the tailplane a great variety of tailplane geometries: behind the fin,  in front of the fin,  above the top of the fin,  below the fuselage, at each tip of the tailplane, etc., and each may have some advantages for the special conditions of the aircraft or model. A secondary effect of some importance is due to the impingement of the “vortex wake” of the wing on the tailplane, the so-called called “downwash”, a result of the deflection of the airflow due to the production of lift by the wing. The stab position should be one where the stab is not in the path of the vortex wake, and this is not always easy to attain.

Alexandre Cruz has produced an intriguing type of timer for Free Flight models, a hybrid obtained by joining  a RC servo with a modified vintage clockwork timer. In effect, he has replaced the spring mechanism of the timer by the servo. The servo can be controlled by a miniature RC receiver, allowing normal DT operation, and other functions,   as well as the useful RDT function, especially helpful when flying in today’s limited size fields. The clockwork timers are still available,  new or second hand and have simple and reliable release mechanisms for actuating  sequential functions by means of a rotating disk.  In the article the author details the timer software and circuit schematics needed to produce this hybrid timer. If the timer is used in an F1A model,  the duration of the  stages of a zoom launch can be easily preset in the timer, moving from Tow to Rotation, Ballistic phase, Bunt,  Glide and final DT. Cruz notes that this hybrid  timer resulting from the combination of the older clockwork timer with the RC servo will allow the use of older F1A models  that were often limited by the mechanical timers they used. Now they can serve as useful trainers.

 

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