Max Bee—

1 Stunt News 8It was good old times of Jozef Gabriswere gone and we Slovak F2B fliers, copying whatothers have done, and were doing, were not able tokeep pace in the world stunt arena. Havingexperience with many models from our side of theAtlantic, like Gabris s Supermaster, Cani s Zralok,and others like the Juno, Stiletto, Dreadnought, andCardinal, I told myself there must be way to collectall their strong points and concentrate them tosome good .. Did you ever try averaging? It ispretty simple: take all those good models, take theaverage of all you see there, and you will certainlyget the best model in the world.

2 Unfortunately, itdoes not work. Do not ask me how I know! Averaging adopts all the weak points, ratherthan the strong points, so it is not the way. To get agood result one needs to explore those strongpoints and extend them. This means that the resultcertainly cannot be the average; it will besomething like letting the good things grow toextremes. However, they need to be found Line Stunt has undergone many years ofdevelopment. It is not so easy to push it furthersimply by trial and error. Once I saw Lou Crane sstunt analyzer (thanks, Lou), I told myself that thisis the way.

3 I built myself a larger analyzer whichgave me a lot of numbers which explained what isgoing on during tethered flight, what the flaps andelevator are for, what the facts and the fictions areof so many rules we have, and much other usefulinformation. That was the initial point of mydevelopment, which actually ends in my latestmodel called Max Bee. (I hope not for long.)In the first half of this two-part article I willdescribe the aerodynamics which I first used on my2002 model. I flew it at the World Championshipsin Sebnitz with a piped OS Max.

4 46LA (I placed10th). It survived for a long time, and in 2008 Iconverted it to electric power and I flew it in theWorld Championships in Landres (I placed 2nd).In 2011 I made a newer version, builtspecifically for electric and with almost the sameaerodynamic configuration, just with a little largertail and with a new fuselage shape. Yes, I wantedsomething different, so the look of the fuselage islittle bit unusual, but it works well. With it I placed first in the EuropeanChampionships in Czestochowa, Poland, and justrecently placed first in the 2012 World Champs inPazardzhik, Bulgaria.

5 Let s take a look at thetechnical details of the first thing I tried to play with was the wingairfoil. It is not so easy to do a full aerodynamicanalysis of an airfoil with an amateur program ifthe airfoil changes its properties with changingangle of attack (AoA, or alpha). Additionally, it isalso very bad for the pilot if the airfoil changesproperties. So the task was to design an airfoil which cansafely fulfill everything necessary for easycalculation and for predictable flying. In otherwords, it was necessary to find an airfoil which canMax Bee Max Beedesignconcepts the Slovak WayJanuary/February 2013 9by Igor Burger Stunt News 10make lot of lift in the linear segment of the lift vs.

6 Alpha curve. I think this needs a little explanation. Every airfoil has a rangeof angle of attack (AoA) in which the lift coefficient changeslinearly by per 1 deg of angle of attack, independently ofairfoil shape. If we know the maximum lift coefficient of thatlinear segment, then we can very easily calculate how theairplane flies at any lift coefficient up to thatmaximum (knowing the area, wing loading,etc.). That is one point. Besides that, if wekeep the wing in the linear segment, thenalso its responses to control inputs are verypredictable, so flying such a model gives amuch better feeling compared to a modelwith an airfoil that is going to stall, or has some bumps on the liftcurve.

7 Lastly, such a model is easier to trim, as we do not need toavoid some unstable is example airfoil (NACA 0012) (Fig. 1). The lift curveshows clearly that the linear segment at positiveAoA is from 0 to 10 degrees. The lift is linearwith AoA, and a program can very easilycalculate the AoA for that wanted lift. It is asymmetrical airfoil, so we can use that airfoil inthe range from -10 to 10 degrees AoA. An AoAgreater than 10 degrees will not only makecomplications for any calculations, but alsoflying will be difficult. As you can see in the illustrations from Martin Hepperle sJavaFoil program, we are in the time of computers, and since wehave several airfoil analyses available and design tools like thisone, the work is not so hard.

8 I found that the best way to proceedfor me was modifying the NACA 0018 airfoil, known for its goodproperties, for our use with flaps. Unfortunately, flaps are verytricky. They extend the lift of an airfoil, but they also do one not-so-good thing. Let s take this slowly. The top surface of the airfoil shouldbe a smooth curve. The curvature of the upper side shouldchange from a small radius at the leading edge to a large radiusat the trailing edge, because air flow stability is good at thefront of the airfoil, but weak at the back. But a deflected flap causes a small radius at the hinge line,allowing the air flow to separate from the flap upper surface,and the worst thing is that it happens abruptly at someparticular AoA.

9 Flow separation does not progress slowly withangle of attack from the trailing edge (TE) to the wing leadingedge (LE); the flow just simply separates abruptly at the hingeline. So while a smoothly curved airfoil makes more and morelift with AoA to the point where it starts to stall (called criticalangle of attack), a flapped airfoil does it only to the point whenflow on the flap separates. Then, as the angle of attackincreases further, the lift falls down a little bit, and then itcontinues to rise again up to the stall point. This means that aflap makes a kind of bump on the lift curve slope.

10 That makesthe flight characteristics hard to calculate and the airplane notso easy to fly and trim. Such a model must be trimmed to fly without getting to sucha place on the lift curve slope. For example, it will fly wellonly tail heavy, or only nose heavy, or it will need someparticular flap-to-elevatorratio or such, while a modelwith a well-working airfoilis easy to adapt to thepilot s preferences, becauseit will allow any regime is an airfoil clearly showing that illness. It is flapped andthe flap is deflected 30 degrees (Fig. 2).The lift curve shows what is happening.