PREDIMRC

Manual

Update of 08.04.07, valid for PredimRC_2.1

Index

1. INTRODUCTION 2

2. BLOCK-DIAGRAM PREDIMRC 3

3. SOME DEFINITIONS 4

3.1. GEOMETRICAL MAGNITUDES 4 3.2. AERODYNAMIC SIZES 4 3.3. CONSTANT PROFILE 5

4. KEY STAGES OF THE DESIGN 6

4.1. TO DEFINE ITS NEED 6 4.2. TO ESTIMATE THE RE OF REFERS FLIGHT (RE_REF) 6 4.3. TO CHOOSE THE PROFILES 6 4.4. TO CALCULATE POLAR 7 4.5. TO INTEGRATE THE POLAR ONES IN PREDIMRC 11 4.6. TO OPTIMIZE LENGTHENING 14 4.7. TO DIMENSION AND REGULATE THE MODEL 15 4.8. TO OPTIMIZE the GEOMETRY Of WING 17 4.9. TO EVALUATE THE PERFORMANCES OF ITS MODEL 18 4.10. TO DIMENSION THE PROPULSION 20 4.11. TO DIMENSION THE CONTROL SURFACES 21 4.12. TO DIMENSION THE SERVOS 21

5. CONCLUSION 22

1. Introduction

PredimRC is a software of design of flying models functioning on Microsoft Excel. Conceived initially for predimensioning, it became with the wire of the evolutions a tool of aerodynamic design complete and powerful. The majority of the flying models can thus be conceived with a maximum of effectiveness and in a minimum of time.

PredimRC is based on the following outstanding points:

- polar of profiles numerical exits of blowers - original method of the calculation of optimal lengthening for the machines of performances - (60pouces, F3J, F3F, F3B, F3D,…) - calculations with variable Re, on the globality of the model (wing, stab, fuselage) - complete equations of the taking into account of the wake of the wings for the problems of - stability, of centering and chock of stab (Mr. Sherrer/T. Plato) - original method of calculation of the point of operation of an electric model

PredimRC makes it possible to obtain the following results:

- optimization of lengthening - optimization of the geometry by VLM - curves of performances of the model, for 3 profiles X 2 masses - performances with the engine - adjustments (centerings, chocks) and stability - dimensioning of the servos

In practice, the results given by PredimRC were checked on many models - micro model with the sailplane of competition F3F while passing by electric racers - and showed the great robustness and reliability of this software.

REMARKS:

PredimRC can be used in two radically opposite ways: direct design, starting from a white sheet. retro-design, starting from a model existing more or less advanced.

PredimRC is compartmentalized, which makes it possible to separate two aspects from the design: the performance part, which uses the polar profiles exits of numerical blower. the adjustments part, which uses the data profiles Cm0 and Alpha0.

The adjustments parts and performances are of course complementary, but one can thus perfectly use one without the other according to what one seeks. It is a very important aspect: useless to seize polar profiles if one does not seek to calculate the performances, but simply to regulate the model for a healthy flight.

Do not hesitate to use the integrated assistance of PredimRC: the majority of the cells are commented on, with target values or explanations. If the comments are not posted, go in small Outils/Option, Affichage miter, then to only choose “indicating”.

PredimRC uses macros VisualBasic. By defect, Excel asks for the confirmation of activation of these macros, which thus should be accepted. It is also possible to configure Excel to accept these macros automatically, in small Outils/Macros/Sécurité. On certain nonFrench versions of Excel, if the macros do not function, it is necessary to replace the “Graphic” word by “Chart” in the macros.

PredimRC is reserved for a private use, and can thus be neither copied, neither distributed, nor used within the framework of a commercial application without the opinion of the author. In the same way, this software is delivered in the state, the responsibility for the author could not be committed in some manner that it is in the case of an accident implying a model designed thanks to this software.

1. PredimRC block-diagram 2. Some definitions

3.1. Geometrical magnitudes

Lengthening ( scale ²

defines mathematically by:

A=

Neutral line: line characteristic of a fuselage, which corresponds to its incidence of minimal drag. It is comparable with the line of vol.

Chock: angle enters the wing or the stabilizer compared to the neutral line of the fuselage. That it is for the wing or the stab, this angle is positive when the leading edge is higher than the trailing edge.

Volume of stab: a dimensional value which reflects the capacity of the stab “to hold” the wing in the various configurations of vol. the volume of stab must be all the more large as the apparatus is likely evolutions with the great angles, which often goes against the research of the minimal drag. It is also the case if Cm0 of the profile is important.

Hearth of the complete model: not neutral stability of a model, where balance the variations of the forces of bearing pressure (wing, stab and fuselage) during a variation of incidence (wanted, after an action with the depth, or undergone because of a turbulence). In practice, centering must always be in front of this point to ensure the stability of vol.

3.2. Aerodynamic sizes

Alpha (

Cx: model or wing, profile, coefficient of drag. The Cx characterize resistance to advance.

Cz: coefficient of bearing pressure, a profile or a wing. Cz evolves/moves about linearly according to the angle of incidence of the profile.

Cm: coefficient of moment, a profile or a wing. Cm reflects the couple of swivelling around the hearth of the profile (25% of the cord) generated by the air flow. It is positive for a hang-glider (stable), and negative for a standard profile (unstable). In the last case, it is the stabilizer which must counter this couple, all the more important as the absolute Cm value is high.

Trail (Fx): force parallel with the advance of an object. The trail is expressed in NR (newton, 10N ρ

1kg), and is calculated by the relation: Fx =

1

…S.Cx.V ² , with. = 1.292kg/m3² (standard density of the air), S =

2 surface (in m ²), and V rate of advance (in m/s).

Bearing pressure (Fz): force perpendicular to the advance of an object. The bearing pressure is

expressed in NR and is calculated by the relation: 1

Fx =… S.Cz.V ²

induced Trail: trail related to the lengthening of the wing. Null for Cz = 0, it increase with Cz, but of as much less than lengthening is important. Physically, the induced trail is generated on the level of salmon by a circulation of air of the under-surface (overpressure) towards the suction face (depression). The more important lengthening is, the less this circulation affects the remainder of the wing. Is calculated by

Cz ² relation: Cxi = z.A

Reynolds number (Re): a dimensional coefficient which includes the speed of evolution and the dimension (cord) of a profile. Postulate: a profile of cord X evolving/moving at the speed Y behaves

): characterize the importance of the scale in front of the cords of wings or stab.

): angle of incidence of the profile compared to the air.

manner identical to this same profile of X/2 cord evolving/moving at the Y*2 speed, because they evolve/move with the same Reynolds number. Re = 70.Corde (m). V (m/s)

Polar (S): curve (S) characteristic (S) of the performances or the behavior of a profile, a wing or a model. Polar the most traditional is:

- Polar profiles: Cz according to Cx, Cz according to Alpha, Cm according to Cz. One generally traces these polar for different Re.

- Polar model: rate of fall according to the speed of flight, smoothness according to the speed of vol. One generally traces these polar for various masses.

Twist average aerodynamics (CMA): virtual cord, equivalent from an aerodynamic point of view to the whole of the cords of a wing. It is at the base of the calculation of the centering and the arms of lever of an aircraft. Mathematically, this cord is the average of the cords balanced by elementary surfaces. More the math students will recognize the definition of an integral, in fact:

env /2

2 with S = surface wing, C (there) = cord and C (there) .dy = elementary surface.

CMA =. C ² (there). Dy

- S 0

Wake of wing: it is the flow of downward air generated by a wing. According to its arm of lever and its height, a stabilizer undergoes this flow more or less, and its chock must be corrected consequently to obtain the good line of vol.

Interaction: qualify the trail generated by the intersection of two surfaces. Typically, one finds the trail of interaction to the junction of the fuselage with the wings or the stab. By defect, one estimates that it accounts for 10% of the total trail of a model.

Stroke static: percentage which indicates the degree of stability of a model, defined by the report/ratio of distance CG/hearth in the average cord. This value is also valid for the hang-gliders. What it is necessary to retain:

- If the static margin is negative: the model is divergent, the least disturbance of trajectory (action with the depth or movement of air) is amplified.

- If the static margin is null: the model is neutral. - If the static margin is positive: the model all the more takes again its natural trajectory quickly

that the static margin is high. Concretely, this value can go from 0 for an apparatus speed or stunt-flying to 10% for a calm apparatus where stability is privileged. Attention, for a model with stabilizer, these values are valid only for one aft C G limit taking account of the contribution of the fuselage.

3.3. Constant profile

Cm0: coefficient of moment of the profile with bearing pressure (Cz) null.

Alpha0: angle of incidence of the profile with bearing pressure (Cz) null.

4. Key stages of the design Before starting, most important to understand is that the design of a model is an iterative step. Each stage can call into question the initial assumptions, and it is thus necessary to rehabilitate them then Re-to unroll the process of design. It is then the experiment of the originator who will allow to arrive as fast as possible at the best compromise.

In order to facilitate comprehension of it, all the method is unrolled through an example, in fact the design of a sailplane of relaxation 3 axes with stab in T. 4.1. To define its need That amounts raising the question: which is the model which I wish to design, and for which flight envelope? If the question is commonplace, the answer is less because it poses the bases of all that will follow. Here the elements minimum to be defined, roughly for the moment: - which scale? - which wing surface? - which beach of mass? - which nominal speed (or “average”) of flight? This stage of the design, it simply acts of very broad orders of magnitude, that the design will make it possible to refine, to even call into question if they are not adapted. For the example which interests us, here starting assumptions: - scale: approximately 1,80 m - surface: approximately 30 DM ² - masses: from 700 to 1000 G - mean velocity: 50 km/h

4.2. To estimate the Re of reference of flight (Re_ref)

This Re will use to calculate polar the profile (S) with nearest to the flight envelope of your model. Working PredimRC with variable Re, the polar ones calculated for Re_ref will be extrapolated with all the Re of vol.

The important thing here is not to find a value precise, but simply an about logical base of work compared to the use of the model. For example, a racer F3D flying to more than 300 km/h will have great chances to have Re_ref definitely more raised than that of a launch-hand optimized less quickly to evolve/move almost 10 times!

A priori definite data in stage 3.1 make it possible to calculate this Reynolds number of reference. Nothing good wizard: Re_ref = 70 X Speed (m/s) X average chord (mm) With: Speed (m/s) = Speed (km/h)/3.6 and Average chord (mm) ρ 10 * Surface (DM ²)/Scale (m) If this part appears too indigestible to you, it is possible to retain Re_ref of 200000, it is enough pass key for many models.

For the example: Re_ref = 162000, rounded to 200000.

4.3. To choose the profiles

This stage is vital for the model and its performances, even if it is necessary to be conscious that the profile does not do all: the geometry of the wings and the stab, the fuselage, the chocks, the trail of the orders, play a part quite as important in the final performances.

How to choose a profile? This question is not so innocent that, because the number of profiles available has what to drown more informed of the model maker. In fact, the choice of a profile is directly dependant on the use of its model, and a comparison with other profiles. And of course, one compares the profiles for a Re representative of the flight envelope chosen, which corresponds to Re_ref defined into 4.2.

Here some broad outlines: them profiles of beginning: the trail should not be too weak, and Cz_max must be high to improve the behavior at low speed. them profiles of scrapes: the principal criterion is to have a weak trail in rather important Cz, about 0,3 to 0,5, as well as high Cz_max. them profiles speed: the trail must be minimal in weak Cz. For categories F3F, F3B, F3I, F3D, which requires in addition to turning tight, it is necessary to have raised Cz_max, which is often contradictory with the mini criterion of trail. To circumvent the problem, more and more profiles are optimized for the use of shutters which shift Cz_max and Cz de Cx_mini. them profiles of stunt-flying: with or without shutters, it must present broad a enough negative beach of Cz for evolving/moving well on the back. Then, if it is about stunt-flying sailplane, it is also necessary to seek the smoothness and scrapes it, which requires a compromise often difficult to find. them profiles of hang-gliders: their main feature is the car-stability, defined by positive Cm0.

Other considerations that the Cx and Cz can also return in account: It Cm: if it is weak, that makes it possible to reduce the size of the stab, and thus the associated trail. Without counting the more important neutrality of the model whatever the speed. it answer to the control surfaces: certain profiles have more or less effective answers, which can be important for stunt-flying. It critical Re: it is minimal Re in lower part of which a profile functions badly. For our profiles of reduced models, that can go from 20000 to more than 100000 following the profiles.

For the example: three profiles are selected a priori: - FAD05S-9%, a rather general-purpose profile perso and teasler - Clark there, famous for its beach of high bearing pressure - Eppler 195, which had its hour of glory

FOOT-NOTE: one can also select only one profile, and decline his operation with various values of flap deflection. It is typically the case for a sailplane of competition. 4.4. To calculate the polar ones After having thus preselected some profiles, they should be passed out of numerical blower to evaluate their performances and to retain most interesting (PredimRC manages 3 of them simultaneously). In fact, the reference is XFOIL, used on line (but it is rather not very ergonomic) or by the means of interface like XFLR5. The method is below detailed for this software, but the majority of the numerical blowers function in a similar way. Of course, such a software is not limited to some functions presented here, with you to discover all the power of it! First of all, throw XFLR5, then to choose the workshop of analysis of profile: To use the button then “to open” to go to seek a file of profile to the format .dat. The profile is then drawn by the software. One can of course open profiles as many as necessary.

To go then in the menu of polar to define the type of polar to calculate, which will apply to the profile in progress:

To then put the Reynolds number adequate, here Re_ref calculated into 4.2: The bar of menu then shows on the left the name of the profile and on the right the polar in progress one:

And to launch the calculation of polar, it is enough to fill the beach with incidence for the analysis (typically -6 with

13°), then to click on “analysis”:

Polar that one can export in textual file for then recovering it in PredimRC: Before passing to another profile, it remains to measure Cm0 and Alpha0 of the profile in progress.

For Alpha0, a right simple click on the bottom of screen makes it possible to rock on the posting of the polar one

One can then measure the sought value graphically, while zoomant with the scroll of the mouse on the zone of Cz= 0. Here, Alpha0 = - 2°.

For Cm0, it is complicated a little more, it is necessary to create a graph personalized having for Cz ordinate and X-

coordinate Cm, always with the click on the bottom of screen (finely “variable”). While zoomant, one reads in Easy way: to facilitate the search for Alpha0 and Cm0, to use small “the Settings” to post a more precise squaring.

FOOT-NOTE: if one seeks only Alpha0 and Cm0, without being concerned with polar profiles, there exists other simpler manners to obtain these values: it can be simply given in the file of certain profiles. - grace a small software like ProfilKonverter (http://members.aon.at/p-51/download.html). The results relatively right are compared with Xfoil: the approximation seldom exceeds 0,2° for Alpha0 and 0.01 for Cm0, which is largely sufficient for trimming a project well.

4.5. To integrate the polar ones in PredimRC

For having launched PredimRC, it is necessary to position on miter 1 (Profiles). For more convenience, it is to better

remove the existing data, but it is not obligatory. Initially, to fill the names of the profiles chosen, then Re_ref

calculated into 4.2. Then, in small “the file” of Excel, to click on “opening”. In the window which opens, to choose it

To click directly on finishing, the separation of the data being well made by Excel for the files resulting from XFLR5.

All the data are thus imported in an Excel sheet. It is then enough to copy the Cz data (= CL)/Cx (= CD), for the zone of positive Cz, within the limit of 33 lines (maximum envisaged in the zone of importation of PredimRC):

In PredimRC, to stick the data with a special joining:

To choose the option “values”, then OK. The imported points appear in the graph. It may be that does not function in the case or the decimal separator is the comma. In this case,

Then remain to regulate the parameters Cxmin and Czopt (with being read in the imported values, adapting to the need), then the coefficient K and Cz of unhooking. The goal is to make as well as possible stick the curve calculated on the imported points. Not to forget to defer Cm0 and Alpha0 of the profile. The advantage of this method of

interpolation is that PredimRC is freed thus from the small errors or To reproduce the same thing for the other profiles:

Important FOOT-NOTE: if a profile is not used (or if one uses only Alpha0/Cm0 and not his polar), it is necessary to put a nondecimal character (by ex: -) in Cxmin corresponding so that PredimRC does not take

it into account in the curves of performances and search for lengthening. 4.6. To optimize lengthening

This stage is optional if one does not seek the performance in term of smoothness. This zone of PredimRC is thus voluntarily independent of the remainder of the software (except the polar profiles). It should be noted that the smoothness is not the prerogative of the sailplanes: on a plane, a good smoothness results in a less power necessary to fly at a given speed.

The research of optimal lengthening rests on two antagonistic phenomena: - To reduce lengthening increases the cord profile, therefore its Re, and the trail profile decreases. - To increase lengthening decreases the induced trail. This calculation depends only on the wing, the contributions of the stab or the spindle in the polar ones creating only

one offset without influence on optimal lengthening.

Then, one seeks Cz of flight (curve in top on the left) corresponding at the speed estimated into 4.1, and optimal lengthening corresponding (curve in bottom in the medium). By deferring these values in the adapted cells, the cursor (in blue) makes it possible to graphically check the selected value. The value of lengthening thus found will give the best performances of smoothness for desired speed.

To help with the choice of Cz of flight (typically, 0.1 to 0.2 for a model speed, and 0.3 to 0.4 for a calmer model) and ideal lengthening corresponding, the two curves of smoothness on the right give following information:

- Potential smoothness max (in top): each point of this curve gives the maximum smoothness possible for the optimal lengthening of this point. This curve is thus a curve with variable lengthening, a kind of ideal theoretical.

- Smoothness wing for selected lengthening (in bottom): it is the curve of smoothness for the lengthening fixed by the user (cell: Cz of flight to be optimized). Compared with the curve maximum smoothness, it makes it possible to visualize the loss of smoothness due to selected lengthening for all other Cz that optimized.

Then, to facilitate the future drawing of the model (miter 3), this lengthening is represented into scale and cord of root of an elliptic wing (optimal output for selected lengthening).

The use of the originators of certain models of competition (60pouce, F3F, F3B, F3D), which must at the same time go quickly in straight line and turn to tighten without lose speed, PredimRC makes it possible to quantify the smoothness in turn, as well as the distances covered during this phase (to be compared with the distance covered in straight line). The graph “smoothness wing in tight turn” thus gives the evolution of the smoothness according to lengthening, for the conditions of turn seized just audessus (speed and Cz wing). The blue cursor indicates the smoothness selected graphically previously. These indications make it possible to the originator to balance optimal lengthening, in order to find the best compromise between smoothness on the level and smoothness in turn.

Caution: the optimization of lengthening can result in wanting to use important lengthenings. It will then be necessary to make sure of the mechanical resistance of the wings with the technique of suitable construction. For this purpose, PredimRC indicates the effort which the wings in tight turn must support (or in resource, with the weight of the model near).

In our example, Cz of flight of 0.3 is selected for the optimization of lengthening. According to the mass (700 or 1000 G), the smoothness will be thus optimized for speeds of flight from approximately 40 to 50 km/h, for the profile FADS05-9% which has a better smoothness for this Cz than the other profiles. Optimal lengthening correspondent will be 12, with a cord of root of approximately 200 mm for a scale of approximately 1,90 Mr.

4.7. To dimension and regulate the model

The creativity can be expressed here freely, on the condition of complying with some simple rules: - If the smoothness is a criterion of dimensioning, it is necessary that lengthening approaches to

elliptic lengthening (calculated starting from the scale and of the cord of root) and the lengthening optimized into 4.6. See also the coefficient of Oswald into 4.8 which gives a finer indication of the output of the wing. the dimensions and the shape of the fuselage have a considerable impact on the performances and centering.

- For the models with stabilizer, the volume of stab is a very important data, it conditions the capacity of the model to evolve/move with the great angles.

- Attention with the geometries too alambiqués. Often, a simple and elegant design is an good indicator of correct design…

Foot-note: starting from version 1.9, PredimRC integrates the formulation of T. Plato for the taking into account of the contribution of the fuselage in the calculation of the aft C G limit. This limit is more representative of reality, and one can note that it is often more before without the fuselage, and this more especially as the fuselage is bulky. This formulation has also the enormous advantage of making homogeneous the concept of margin static for the hang-gliders and the apparatuses to stabilizers.

Here the interface of seizure of the geometry: For the adjustments, here the logic used in PredimRC: it stability is function of the choice of the static margin, which must be selected between 0 (perfectly neutral model) and 10% (very stable model), and which positions the center of gravity compared to the aft C G limit (= hearth of the complete model). Generally, whatever the model, a value of 5% is a good approach for the first vol. it Cz wing for neutral trim of stab: this value is given as an indication, it acts of Cz for which the center of bearing pressure coincides with the CG chosen, which gives a neutral stab for this point of vol. It is used as value of control of design, particularly while underlining (when it too high, i.e. >1) a stab too slightly dimensioned or a too large spindle. It Cz of flight: it conditions the chocks of wing and stab, which will be thus adapted to a type of flight in particular. Generally, one selected Cz of flight of about 0.1 to 0.2 for a machine speed, up to 0.4 even 0.5 for an apparatus of beginning or of scrapes. PredimRC alerts the user in the event of incoherent choice (if Cz<0 or >0.5). Attention, it is an obviousness, it is necessary also that the profile is adapted to Cz of selected flight. It Cz of stab: so that the stab of course functions all the beach speed of flight, it should not exceed 0.3 in absolute value, under penalty of being likely to take down before the wing.

One can thus note that centering and the chocks (by the means of Cz of flight) are uncoupled. That can shock, but

this approach is however very close to the “true life”. Indeed, a final tuning of a model during in-flight tests shows that one can modify a centering without having a significant impact on the choice of the chocks, and vice versa. What is confirmed by the theory, which shows that centering is conditioned by purely geometrical considerations (hearth of the complete model), whereas the chocks are it by purely aerodynamic problems.

Thus: one centers for the model is stable, and one fixes it so that it is pleasant (and powerful) at the mean velocities of flight (thus Cz) to which it is intended.

The graph hereafter represents the model seen of top, which makes it possible to control coherence (and

esthetics…) of its work:

4.8. To optimize the geometry of wing

PredimRC integrates a calculation of distribution of Cz and bearing pressure, using method VLM (Vortex Lattice Method, developed on Excel by John Hazel). The principle consists in segmenting the wing in several small pieces (40 here), and to analyze the interactions between each piece. It is rather close to the principle of calculation by finite elements used by the industrial computation software (RDM, thermics, flow,…). For reading the continuation, it will be necessary well to make the distinction between Cz and bearing pressure: Cz is a coefficient, the bearing pressure is a force (see paragraph 3.2).

One uses these graphs in the following way, while varying Cz_aile by the means of the angle of attack of the wing (button with arrows): - Distribution of Cz: by putting a value of Cz_aile close to unhooking, the ideal is to have a decreasing distribution

of the root towards salmon, so that unhooking occurs initially with the root. It is a pledge of healthy behavior at low speed or at the time of one started.

- Distribution of bearing pressure: ideally, it must follow an elliptic distribution of bearing pressure, which gives the weakest trail induced for a given lengthening. Of course, it is desirable to optimize the distribution of bearing pressure for Cz of flight for which one sought best lengthening.

- Factor of Owald: indicate the output of the wing compared to an elliptic wing of the same surface and lengthening. Its maximum value (to be sought) is thus of 1.

Two lines of action are possible to optimize the distributions:

- modifying the cords: the only precaution to be taken is to prevent choosing a too small salmon cord if the selected profile supports weak Re badly.

- modifying the twists: to handle with many precautions, particularly in the case of machines being able to go very quickly. Indeed, an evolution of twist can be perfect for Cz of flight, but induce a nonhomogeneous evolution of bearing pressure in other Cz, which can go until inducing too great efforts for the wing. A traditional case consists in putting too much twist at salmon of a large sailplane with strong lengthening, as it is the case on certain models commercial: at high speed, one can see the end of the wings taking an anhedral because of the negative efforts of bearing pressures in end of wing, which sometimes can go until a rupture of the wings.

Foot-note: if the worksheet LiftRoll of John Hazel (sheet who was thus used as a basis for these calculations) is limited to strictly 4 panels, method VLM used by PredimRC functions whatever the number of panels per half-wing (1 to 5), which is nevertheless definitely more flexible.

4.9. To evaluate the performances of its model

PredimRC makes it possible to post the curves of performance in two different ways: - curves of reference with an elliptic wing “ideal” of the same surface and lengthening that in the course of design. - performances by the method VLM, which takes into account the complete geometry of the wings. Foot-note: to forget to click only the button “Calculations” if the geometry changes.

The aerological conditions (altitude, temperature) of the site of flight can also be taken into account, which shows the impact of these parameters in the performances of a model.

These performances are given in the following graphs: The first graph gives polar speeds of the complete model, One reads there the rate of fall according to the speed of practical vol. Of manner, in the case of a sailplane, one can interpret these curves like giving best speeds (stabilized) possible for a given bearing pressure.

Always according to the speed of flight, the curve of smoothness is revealing performances of an aircraft. This curve translates the distance covered report/ratio/starting height. Here, it is seen that the maximum smoothness is given by Clark_Y, but that the FAD05S-9% are shown more powerful apart from the maximum smoothness. One notes there also the important effect of the wing load: the more one charges, the more the smoothness is obtained at a raised speed, and better is the maximum smoothness. On the other hand, more one charges and more mini rate of fall is high…

The last curve makes it possible to find Cz of flight according to speed and the mass, as well as Cz of stab. One finds there also the evolution of the factor of Oswald according to Cz_aile.

Foot-note: thanks to small the stage coaches, one can decontaminate the taking into account the contributions of the stab (also including/understanding the drift), of the fuselage, the interaction between the elements (10% of the total trail), or of calculation VLM. That makes it possible to better include/understand the contribution of each element. As for the buttons +, they act on the scale factor to improve legibility.

In our example, if one considers the speed of flight of 50 km/h, one can read the following things on the curves: - According to the mass and the profile, the rate of fall goes from 0.9 to 1,5 m/s. To fly in a stable way, the

model must then steal in a mass of ascending air of the same rate, or a possible engine must provide an equivalent power (=masse * revolved * rate of fall).

- Always according to the mass and the profile, the smoothness goes from 9,8 to 14,8. That wants to say

that, in planed, the model will advance of 9,8 with 14,8m for 1 m of descent in the mass of air.

- According to the mass, Cz of corresponding flight goes from 0,2 to almost 0,3

Another manner of approaching the things: in the example, Cz of flight chosen for the adjustments is of 0,3. While referring to the Cz/V graph, one from of deduced a speed of flight of approximately 41 km/h to the minimal mass (700 G), and of 49 km/h to the maximum mass (1000 G). These speeds are those towards which the sailplane will naturally once tighten the released handles, one can thus compare them to speeds of better comfort of piloting. That is very interesting for a sailplane of performance that one ballaste according to the conditions weather: while ballastant, the smoothness increases as well as the speed of flight, without there needing to change the adjustments.

4.10. To dimension the propulsion

This dimensioning requires only traditional values of entries, to seize in the interface cidessous:

On the other hand, the research the speed of flight to the engine (on the stabilized level) is more original: PredimRC puts in glance (see graphic above) the power absorptive by the model with the power transmitted to the mass of air by the motorization, the whole according to speed. The latter is maximum on the ground (null speed) and is null when the speed of the model corresponds to

mode with vacuum of the engine. The point of intersection of the curves gives this speed of level flight. This method is particularly relevant, in the condition of knowing Kv of its engine well. To note, that, as for the other curves of performances, one can post the powers in the case of an elliptic wing of reference or with calculation VLM (just).

In the example, the speed of level flight is of approximately 60 km/h. One can notice that, on this level of rather modest power, the profile has relatively little importance in the result.

4.11. To dimension the control surfaces

PredimRC does not manage this aspect of the design of a model, but it is good to point out some basic principles: - twist of a control surface depends on the profile to obtain the best trailed effectiveness ratio/. In general, it is

about 25% of the airfoil chord, but that can vary from 20 to 30%. This choice is particularly sensitive on the sailplanes of performance whose shutters and ailerons are used out of camber flaps, with often a coupling to the depth to improve the performances in tight turn.

- For the ailerons not full-span (very traditional case): their length must be close to half of that of the wing, and the aileron must be located at nearest to salmon.

4.12. To dimension the servos

The interface was desired ultra-simple, but complete. So seemingly the levers of swing bar or puppet seem to miss, it of it is nothing because these arms of levers are included in servo clearances and controls.

It should be noted that one can as calculate the couple necessary to control a pendular stab, with the standard assumption as the axis of rotation is 5% front the hearth of the stab (located at 25% of the average cord).

The case of a system of control per integral incidence was also taken into account, on the basis of formalization developed by the author. There too, few values to be seized, with a great reliability of result. The arms of lever are to

be respected as well as possible…

For both type of control surface, a graph gives the curve of evolution of the couple according to the race of servo, of 0 until maximum clearance (100%). In the case of the control surfaces traditional, a whole of buttons to notching makes it possible to choose for which control surface the graph must be posted. It should be noted that, for the system with integral incidence, the neutral position of the control surface does not correspond inevitably to a null couple on the servos.

Foot-note: the speed seized for calculation of the couple requires some tweezers: it is a question of considering a speed easily reached by the model, noted here “standard speed”, and not of taking a too high value which will oversize the servos unnecessarily. For this reason, some usual values are given in the comment of assistance of PredimRC. In the same way, the values of couple are calculated with the most important clearance, which gives a certain safety margin. Indeed, particularly at high-speed, the control surfaces are very seldom used with their full clearance. Reason moreover not to overestimate the standard speed of its model.

For the model of the example, standard speed selected is of 80km/h. the calculated couples are traditional enough for this type of model: they correspond to microphone-servos traditional of 5g with 9g for the ailerons and depth, and to servo of 15g for the drift.

5. Conclusion

Here, it is finished! For those which will have traversed this document length into broad, you will have included/understood it: beyond the tool, it is a true design method of model which I propose to you here, complete and powerful. With you to play…

F.A.