Pike Precision F3F/F3B
Pike Precision spare parts can still be ordered. Please contact us for delivery time and more information.
It has been 5 years since we released the Pike perfect, one of the first real ‘standard F3J-planes’. In cooperation with Philip Kolb we managed to push the frontiers in F3J plane design. Five years later we are happy to publish another great soaring machine out of our ‘Pike-series’. This time we were able to acquire the help of Johannes Dillinger, Philip Kolb and Benjamin Rodax to add their experience and expertise in designing, calculating and computing to the creation of the Pike precision, our new F3F / F3B competition sailplane. Here is an insight into the design of our new F3F / F3B model airplane by the design-trio.
Tooling of molds was made on HSC tooling machine Röders RFM 600/2.to reach the top quality. Machine is working with accuracy of 0,002 mm. Construction job was made in system Pro/ENGINEER Wildfire 4.0. Model is produced with newest known technologies and with usage of newest high tech materials. Model will be produced in 2 basic versions + other versions on customers requests. Basic version is F3B with weight up to 2000 g, second version is F3F with flying weight about 2200g and will be more solid for harder manipulation on the slope. Maximum ballast is 2kg and CG (center of gravity) will remain the same.
Models will be delivered with RDS system, everything installed in the wing during production including wire harness. One can choose servos JR DS 171 MG (JR DS 378 MG, Graupner DS 3288 BB MG) or MKS 6125 mini. Wing surface will not be disturbed by servo hatches and means the torsion stiffness is not decreased and the wing will be maximally stiff and surface will be absolutely aerodynamically clean.
Over the last five years we able to accumulate lots of knowledge and information on sailplane design due to several projects we have been working on. The Pike perfect was the initial kick-off point towards that. Many model airplane pilots and builders have asked for help on their specific projects, which sometimes successfully came to life but at other times were sadly only taking lots of our effort and time and were not successful to our satisfaction. To justify all the work we had put into the various designs, we decided to go for our own F3F / F3B design without any trade offs using the latest of our design and development capabilities. Designing the plane to be produced by Samba-model, our partner and producer, who we considered capable of achieving all the optimization results in the best possible way, just reflects the logical conclusion. To accomplish superior solutions we chose a multidisciplinary, iterative design process to search for the ideal compromise between aerodynamic/structural performance combined with easy flying characteristics to achieve the optimum performance for competitions. Prior to starting with the actual design we wanted to analyze the flight envelope and the typical lift coefficients flown in the most important segments of the F3F and F3B flight tasks.
Our experience with inflight measurements and data collected from F3J flight-testing set a base to work from for this specific analysis. On the one hand it is important to check and calibrate the analysis tools and on the other hand to define the flight segments for F3F and F3B tasks in a variety of weather conditions. Therefore inflight tests were performed during one summer in all F3B and F3F training flights with an Ascot F3B model and the Eagle Tree System on board. Measuring altitude, airspeed and acceleration in lift and drag direction the aircraft's lift coefficients were evaluated with the aircraft's mass. The statistical distribution of lift coefficients helped to define the requirements for the aerodynamic design. Furthermore the acceleration measurements were used to identify typical load cases needed in the structural design process. After collecting and analyzing the flight data we tried to separate the topics to work on into three basic fields:
The aerodynamic design of Pike precision
The general arrangements and flight mechanics of Pike precision
The structural design and aeroelastic tailoring of Pike precision
Although these subdivisions interact strongly with each other it proved to be very helpful in organizing the development and design process of Pike precision, separating the design work into specific topics. This way we could use our abilities and skills in the most efficient way to finally study and understand these interactions, finding the best compromises for the final solution.
Aerodynamic design of Pike precision
It is certainly not the easiest task to design a plane for both classes, F3B and F3F. But with the recently noted higher valued emphasis on speed and fast distance flight performance in F3B as well as the new, smooth flying style in F3F , it seemed possible to combine all requirements in one design. This new smooth flying style asks for more efficient turning performance - as does the launch in F3B. To combine agility and performance we determined to make the wingspan as large as possible to reduce the induced drag and as small as necessary to offer agile handling qualities even on small slopes. The principles and basics of the aerodynamic design, especially of the wing and the tail, are already well explained in the Pike perfect development article Please click on the link to get closer insight into these basics. Of course, these principles are still elementary and were therefore adapted for the design of the Pike precision. After undergoing some sizing iterations derived from these optimization criteria, the wingspan was finally set at 2.94m. As the induced drag is reduced with the square of linearly increased wingspan, wingspan consequently is one driving factor for the planes’ performance. Thus the Pike precision was given a larger wingspan than many current F3F planes. To keep this advantage in performance and still ensure the high agility of smaller planes, two measures were taken:
A rather ‘’aggressive’’ (by means of taper ratio) planform was designed. Using LLT methods the lift distribution was calculated to be as close to the elliptical shape as reasonably possible. This measure reduces the tip volume and thereby its mass as well as inertia helping to increase the roll rate. The planform was designed to run the ailerons all the way out to the wingtip, thereby maximizing both aileron span and lever arm. This measure minimizes the aileron throw for a given roll rate or, vice versa, maximizes the roll rate for a given amount of aileron throw.
The wing sweep was designed to achieve a constant chord for the flaps and ailerons of 23% of the local chord depths. This also helps to reduce induced drag by keeping the design load distribution (no steps, bumps or kinks) while cambering or reflexing the trailing edge surfaces. For the two-piece V-tail we adopted the same considerations while designing the planform. the only exception is the chord width of the control surfaces which are slightly wider at 30% constant chord.
One of the most critical questions in the design process is the development of adequate airfoils and their optimization. With today's available design tools, it seems that you have endless possibilities to tweak. Therefore one can very easily get caught out somewhere in the design loop trying hard to improve the ‘’off design’’ areas and not stepping ahead ‘’on design’’. Data collected from the various flight tests was a huge asset in gaining reliability for our calculations as well as validation of the results.
Based on these results we determined a weighted function of importance of the different flight envelopes such as ‘’Speed’’, ‘’Distance’’, ‘’Turns’’, ‘’Endurance’’ and ‘’Launch’’. By using this weighting function, the associated CL values the plane will be flown at most frequently or in performance critical situations could be optimized. For the Pike precision we deduced the CL-regimes to optimize the airfoils for with following importance:
By utilizing this weighting function we could balance trade-offs and performance gains in a very elegant way, saving one or the other dragcount in important areas while not giving up too much in less important areas of the flight envelope, or at least only in the designated ‘’off design’’ areas.
.Equally important as developing a high performance main airfoil was the design of the tip airfoil and the airfoil transition along the span. Especially in the case of the Pike precision with its high taper ratio and thereby rather narrow tips the local airfoils needed to be designed to work flawlessly at lower local Reynolds numbers. To enable the handling advantage of the high taper ratio wing, we paid highest attention to boost the more than ‘’sufficient’’ Cl_max performance of the tip airfoil. Using washout in the wing tips to prevent tip stalls was not considered because of its negative effects on the planes’ high-speed performance. Therefore the tip airfoil was designed to reach comparatively high Cl_max values. Secondly the tip airfoils should at least come up with performance characteristics as close as possible to the main airfoils’ characteristics, adapted of course to the local Reynolds numbers. Finally we paid special attention to the geometric shape of the tip airfoil and its local thickness around the hingeline where subspars are located. As the ailerons run all the way out to the tip, subspar height is critical since it strongly affects the torsional stiffness of the ailerons.
Designing the wing-fuselage junction, the airfoil was adapted to the presence of the fuselage. Completing the wing airfoil series for Pike precision a non-linear transition was developed.
Symmetrical airfoils are used for the V tail. The design criteria for the tail airfoils basically follow demands for low drag. Nevertheless the tail airfoils for the Pike precision are not tweaked to the limit of performance in the low drag regime. At some point concessions in terms of handling quality need to be made. To offer sufficient yaw damping factors, the tail needs to be able to supply high lift, and so the tail airfoils need to show reasonable Cl_max performance. In this case a wide laminar bucket is to be favored even at the cost of minimizing drag.
General arrangement & flight mechanics of Pike precision
Practical experience has shown that high wing loading helps to maximize performance in strong conditions, both in F3F and F3B speed task. Therefore the FAI wing loading limit of 75g/dm^2 and the maximum takeoff weight of 5kg call for less wing area. In the final configuration of the Pike precision the FAI wing loading limit is reached at 4.889kg. The wing is designed to take ~2kg of (non toxic and relatively inexpensive) brass ballast without unwanted changes in C.G. The wing span was chosen rather conservatively with 2.94m leading to an aspect ratio of 14.5. This was done in favor of higher Reynolds numbers on the wing to increase performance in the regime of low lift coefficients as well as enhanced CL_max for superior launch heights. In contrast to aspect ratio the wing taper is chosen quite aggressively as previously explained. The low chords towards the tip result in less inertia and hence high roll rates at moderate aileron deflections, which saves drag. A two piece V-tail configuration was chosen mainly for simple construction. The sizing of the tail plane was done after careful analysis and evaluation of handling characteristics in flight of the Highlander ( http://www.jw-air.de/HIGHLANDER.htm ) and the Spline ( http://www.spline.dk/ ). Design and analysis were carried out with Mark Drela's AVL code as well as handbook methods. Main focus of the tail design was to guarantee excellent directional stability and yaw damping.
Structural investigations and calculations were performed to optimize structural mass and realize the aerodynamic potential of the aircraft. These relatively new methods and possibilities are called aeroelastic tailoring and display completely new possibilities in aircraft design. To our knowledge these steps were not taken in model airplane design before. We were very happy to fall back on the knowledge and virtuosity of Johannes Dillinger, one of the designers on the new Open Class Sailplane ‘’Concordia’’ of Dick Butler.
Johannes performed all the structural calculations and optimizations of the Pike precision along with verifying the methods used on previously produced and flying wing structures.
Structural Design and Aeroelastic Tailoring of Pike precision
The structural design of a new high performance F3B/F model is equally important for superior performance as the aerodynamic design. The complex shape optimization for minimum induced and profile drag needs to be supported by a structure that preserves the aerodynamic shape as it was initially computed. The combination of structural and aerodynamic considerations is treated in the field of aeroelasticity. For the Pike precision, besides detailed structural computations regarding the required spar and wing skin layups with the help of finite element (FE) shell models (Fig. 7), aeroelastic computations were performed by coupling the structural model to an aerodynamic model. By doing so, realistic flight loads (Fig. 8, blue arrows distributed loads, red arrows chordwise summarized loads) and wing deformation calculations gave detailed information on the structural behavior for a whole set of relevant flight conditions. These covered the entire range of important flight tasks in F3B/F competition (winch start, sharp turns with different acceleration levels, max. V). Important information on accelerations during these flight phases came from inflight measurements performed by Benjamin.
The goal in structural design is therefore twofold. The structure is designed to cope with all loadings without failure by virtue of surpassing exceeding maximum allowable strains/stresses in every single layer (Fig. 9, stress in spar layer).
As already mentioned, the second fundamental task is to maintain the aerodynamic shape. This not only relates to the airfoil shape (of course), but equally important to the zero-wing-twist distribution, being essential for minimum induced drag. Especially in high-G turn maneuvers and during the launch phase with temporary high lift-coefficients, it is enormously important for superior aircraft performance to maintain an optimal lift distribution, preserving the lowest possible induced drag and therefore the high flight velocity. For both flight conditions, the aircraft's normal acceleration is a multiple of the usual 1g steady flight condition, meaning that the lift will have to be a multiple of the aircraft's weight. Aeroelastic tailoring aims at retaining a nearly untwisted wing throughout the entire range of important load cases.
The structure of the Pike precision wing is aeroelastically tailored with respect to these load-cases. In a parameter study, several combinations of spar positions and wing skin/spar layup angles were tested to check their effect on wings' twisting in several steady 1G turn and start cases. Two mass cases (with and without full ballast) were considered. The strong influence of spar position and fiber alignment within the spar was detected (Fig. 10, spar positions in green).
The results of aeroelastically tailoring the structure are demonstrated by the optimized twist distributions in Fig. 11.
Philip Kolb, Benjamin Rodax and Johannes Dillinger
Here is what Philip Kolb says about the new fuse:
"As we are always interested in areas of improvement on our planes we are especially intrigued to save drag - especially on fast flying aircraft like the Pike precision.
Honestly it is very difficult to reduce drag on the wings' sections as the wing needs to operate within a wide variety of lift coefficients. Reducing airfoil drag at low lift coefficients and high Reynolds numbers - so to say at high speed - will almost certainly result in worse performance in other flight phases than speed.
One easy area of improvement though is to reduce the surface of the fuselage. The latest state of the art equipment (batteries, receivers and servos) even became smaller than before and thereby allow to reduce the length and diameter of the fuselage nose. This reduction will mainly result in a reduction of inertia, as the nose can be shortened to some extend.
Higher modulus carbon fiber and spreadtow technology allow for a reduction in diameter, especially on the tail boom, where the airflow is fully turbulent and thereby very draggy compared to all the other parts of the glider.
We tried to implement this considerations in the new fuselage design making it even more slim and sleek than its predecessor. The structural buildup is certainly a challenge, but manageable with modern fiber technology. The aerodynamic gains will be noticeable and result in better highspeed - and turning-performance."
Philip Kolb - 2014
We have not found reliable, accurate, long life LDS with easy and quick exchange of servo.
No compromises should be made so we have worked together with the MPJ company and improved the current LDS systems.
currently available for JRDS 181/189 HV servos
LDS for Futaba & MKS servos will be available in 3-4 weeks
Advantages of new LDS system:
1. High accuracy and long life-span thanks to mounting of steel shaft rods on both sides with brass tube.
Diameter of the shaft at servo is 2 mm and in the aileron and flap 1,6 mm
2. Very easy and quick servo replacement.
Remove screws that fasten servos to the frame and then loosen bolt (position no 14), that tighten the dural segment on the servo output.
The Pike Precision also comes with an optional electro fuse.
Alternatives for motors:
Motor : Leomotion L3038 – 3500
ESC: IGE 160 FAI
Battery: TP 1800 2 x 2s = 4S max 110 A
Prop: GM 17 x 13
Motor:Kontronik . KIRA 480 – 31/5,2
ESC:Phoenic Castle EDGE 100
Battery:Turnigy Nanotech 2 x 2250mAh 2S 65 – 130 C or Thunder Power TP 2250 2S G8 70C
Prop: RFM 14 x 8
Motor:Neu 1112G 1112/1Y S/6.7:1
ESC:Phoenic Castle EDGE 100
Battery:Turnigy Nanotech 2 x 2250mAh 2S 65 – 130 C or Thunder Power TP 2250 2S G8 70C
Prop: RFM 17 x 13