The quarterly newsletter for JSBSim, an open source flight dynamics model in C++ VOLUME 3, ISSUE 2 SUMMER 2006 Building an Aerobatics Aircraft for JSBSim : The Su-26 Enrique Laso Leon What's the point? aesthetic judgment! Pure unlimited aerobatic aircraft have seldom been Modeling the Beast modeled in any mainstream simulator. Many will argue that the Extra 300 introduced in Microsoft® Aerobatic aircraft have plenty of features that make Flight simulator-98 is a clear proof of the contrary, them easy to model (which is good since this is my yet the FS flight model was (and still is) too limited first add-on aircraft for FlightGear): to provide a decent sensation of flying. The most recent aerobatics planes correctly modeled are the • Straight wing with little taper ratio (this allows SF260 and Spitfire MkXIV from Real Air® but those are not dedicated “stunt airplanes”. See Page 12: The last serious attempt to model this category of aircraft was the first Flight Unlimited simulator back in 1996! It was nicely packaged with a fine tutorial (remember, this was at a time when games came with a manual) and allowed one to fly machines ranging from an aerobatic sailplane to the Su- 31 unlimited aerobatic aircraft. Many reasons can be found for the lack of interest in this cate- gory. The most obvious is that aerobatic aircraft are limited to Inside this issue: Visual Flight Rules since in- struments are heavy and sensi- tive to the kind of flying in- Aerobatics Aircraft: Su-26 1 volved. What's worse, they have a very short range using the lifting line theory for 3D wing deri- because a routine of 10 minutes of high-g maneu- Scripting Multiple Runs in 3 vations (Prandtl) vers will bring the pilot to his (or her) knees, and • Symmetric Airfoils JSBSim the less fuel, the better for maneuverability. Finally, • Low Mach number (incompressible flow) performing a correct maneuver requires a lot of • Scripting Changes in 6 training (first loops look awful when replayed!). Reciprocating engines JSBSim • No high lift devices (lift is obtained through All this being said, it is an extremely rewarding thick airfoils) JSBSim and MSFS 9 way to fly as it gives a sense of the third dimension • Simple avionics News 10 with fine management of potential vs. kinetic en- ergy being the key. Nevertheless as I wrote before, the post-stall enve- Modeling Aerodynamic 11 lope must be modeled for the magic to begin and Moments Flight modeling is the most important part when this is were it gets tough. recreating the feeling of aerobatic flight. Most of OpenEaagles Simulation 12 the maneuvers are performed inside or near a stall. I did not have explicit references for the SU-26 air- Framework Asymmetric stalls are also a key ingredient of ma- foil. The only data I could get was a relative thick- neuvers such as the snap roll. JSBSim already pro- ness (see The Incomplete Guide to Airfoil Usage, The 2006 AIAA Modeling 14 vides a fine experience of flying and, being open http://www.ae.uiuc.edu/m-selig/ads/aircraft.html). and Simulation Conference source, has the potential to evolve to give the whole So, I started with a typical symmetric airfoil and envelope. scaled it to match a relative thickness of 15%. Simulators Aboard the 16 While this may look like an appalling approxima- U.S.S. Lexington Museum Why did I choose the Su-26? Let us say that it is a tion, thin airfoil theory indicate that the thickness matter of personal taste. Sure enough, there are bet- law (which is the only variable for a symmetrical at Corpus Christi, Texas ter aircraft around (Cap-232 or the superlative Su- Simulate This! End 31 come to my mind), but one can hardly fight his (Continued on page 2) Page 2 (Continued from page 1) function of the aspect ratio). For post-stall behav- airfoil) only influences pressure repartition, not ior the article mentioned above provides a correc- Clift vs. alpha slope (2*π for incompressible tion for finite span. The combination of both pro- flow). This in turn will change the stall character- vides the following coefficient curves : istics (abruptness, critical angle, etc.), but I thought the difference would be too small for anybody to notice, provided the airfoil had a rounded leading edge: The software used to compute the stall character- istics was xfoil (http://web.mit.edu/drela/Public/ web/xfoil) which gave polar curves up to 30 de- grees of angle of attack. It is interesting to notice that the values given by Aeromatic (blue curve on the lift diagram, see www. jsbsim.org for Aeromatic) are not that far from the complete deriva- tion. They would even be better if the airfoil camber effects could be taken into account (at zero camber, i.e. symmetrical airfoil lift at zero incidence is zero). Post stall behavior derivations was found from an Drag at zero incidence was increased by impos- article written for the blades of wind turbines ing a minimum value in order to model fuselage (Stall coefficient, aerodynamic airfoil coeffi- drag (very very crude). cients at large angle of attack, C. Lindenburg réf. ECN-RX-001-04 Energy research center of the The result of all this is an aircraft accomplishing About this Netherlands). simple aerobatics neatly (barrel rolls, loops, Cu- ban eights) and even complex ones newsletter ... such as the spin with a nice feel. The latter came as a surprise since JSBSim comes without asymmetric Edited by Jon Berndt stall effects so far. Snap rolls, while being possible, are somewhat slow “Back of the Envelope” is a and induce a large loss of altitude. communication tool written Stall on the other hand is extremely generally for a wider audience than brisk with g-loading dropping sud- core JSBSim developers, including denly as elevator authority is huge instructors, students, and other (purposely in order to get adequate users. The articles featured will These computations were made using Open Of- control all through the envelope) likely tend to address questions and fice Calc v2.0 (any spreadsheet would do for that comments raised in the mailing matter). It should be noted that the curve from 0 Future Improvements lists and via email. If you would to 30 degrees (xfoil) matches quite well with the like to suggest (or even author) an curve for AOA higher than 30 degrees (post The beta version of the Su-26 will be released article for a future issue, please stall). sometime soon in order to get a first feedback email the editor at: (hopefully from real life pilots, too!). Derivations for the finite span wing was obtained [email protected] using the Prandtl theory of the (vortex) lifting The model can stand a lot of improvement with line (the one that gives the induced drag as a (Continued on page 3) (Continued from page 2) of JSBSim, itself. In order to model post stall be- Page 3 the current definition of JSBSim. The next step is havior, asymmetric effects could be taken into ac- to correctly model the moment coefficient of the count, especially since propeller airplanes will tend whole aircraft (that is, taking the wing and tail to depart in asymmetric stall due to engine torque separately) for the whole AOA range. This does and P-factor. not require a lot of additional theory, just to take into account the direction of motion (the “focus” is Furthermore variations of lift along span would at a quarter of the chord for low incidence, but add an extra feeling, since loss of aileron effi- measured in the direction of the flow !). ciency at low speed is related to wingtips being more loaded than roots for tapered or swept wings. Fine tuning of the propeller is really lacking for But in turn that would have little influence on now. Particularly thrust in the low speed envelope aerobatics airplanes as they often have full span is abundant, making landings quite difficult. ailerons to handle this problem! ▲ Other improvements could come from evolutions Scripting Multiple Runs in JSBSim Agostino De Marco A couple of months ago I began to think over the A first problem arises here. After each run this following idea: One may have the need to run approach should incorporate a "post-run" JSBSim multiple times and compare the results; step to save the output (if specified), e.g. would it be worthwhile to implement a new capa- B737_datalog.csv, in a number of unique files, i.e. bility into the scripting language supported by B737_datalog_1.csv, B737_datalog_2.csv, ... , JSBSim's FGScript class that would make it possi- B737_datalog_N.csv. ble to launch one or more successive simulations with one single script? From the coders point of view the development of advanced scripting features would of course After all, in each of the many scripting languages become quite demanding, as this job would be like available today we always find the following ele- implementing a sort of interpreter on top of ments: conditions (if-then-else clauses), selection the main JSBSim structure. (switch clauses), iterations (for clauses), inclusion of blocks of code (include/import clause) etc. Actually, looking more closely at the kind of code Then, what if we had something vaguely similar in that JSBSim is and how it is intended by the JSBSim scripts? developers, even if the above "capability" would be nice to have, one can comment that (quoting As someone said in the development-issues mail- Jon Berndt) this is the kind of thing that, in spite of ing list, there are many reasons for wanting multi- being explicitly provided for in the scripting lan- ple runs from a simulation.