# Viik

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1. ## Tank transmission - how to?

I don't really want to simulate a complete physical interaction of the differential itself. More to understand how torque from wheels is passed back through differential to driveshaft. Let's say that driveshaft turns at 30rad/s, vehicle rolls straight so both wheels spin at 30rad/s * differential ratio. let's assume that differential ratio is 1. When vehicle turns right wheel spins at 20rad/s and left one at 40rad/s. But what if you start to brake both wheels while turning? Let's say that their new velocity is 15 rad/s and 35 rad/s, how do I figure out at which speed driveshaft is suppose to be turning now? is it really just (RightWheelTorque + LeftWheelTorque) / 2, that would be too easy...
2. ## Tank transmission - how to?

Hi,   I'm building rather simple tank chasis simulation, with elements like engine and transmission. Quite a few car physics tutorials where helpfull but it seams that most of them avoid the subject of how differential should properly work. What I mean is this - assumption that drive shaft rotates with the same speed as driving wheels. From what I understand, when car starts to turn and drive wheels starts to turn with a different speed, differential will "cancel" that difference and driving shaft will be spinning just as you drive forward. What I'm trying to understand is how this "canceling" actually happens. in a sense that as rotation of wheels can be "slowed down" by other forces, which can be different on different side. How do I propagate this difference in rotation back into driving shaft? This can't be a simple sum.   In case of tanks it get's interesting. I was looking at this artcile: http://www.gizmology.net/tracked.htm and if I want to model Double Differential Steering I would need to somehow move torque between separate tracks, calculate propulsion and friction for tank itself, calculate torque effecting tracks and then calculate new angular velocity of driving shaft from those effects.
3. ## Simulation of drag in flightsims

You are right. There is indeed a difference but it's very small and hard to see on that graph. I've crafted coefficients manually for now: https://www.dropbox.com/s/hdw58m4hel64cfh/NACA_0012_Cl_smooth.PNG?dl=0 https://www.dropbox.com/s/yu1ww6p8m6cp7ne/NACA_0012_Cd_smooth.PNG?dl=0 https://www.dropbox.com/s/1kk7b1c36zolpsk/NACA_0012_Cm_smooth.PNG?dl=0   Go the book and it looks like it has empirical formulas for all the coefficients. So it might be possible to plug Mach and Reynold numbers as variables as well. That would be nit. I don't need them to be super realistic but more like reasonably - not what JavaFoil provided for 45-180 degrees :)
4. ## Simulation of drag in flightsims

So it should look more like in this article: http://www.aerospaceweb.org/question/airfoils/q0150b.shtml   But isn't it weird that for such aerofoil (NACA 0015) with a thin trailing edge, at angles 90-180, which means trailing edge is going against flow, mirrored magnitude of Cl is similar to 0-90? I mean, the same aerofoil installed 180 flipped shoudn't work in the same way, right?
5. ## Simulation of drag in flightsims

Got coefficients exported from JavaFoil for symmetrical NACA 0012 aerofoil at Re=100000.   Cl looks fine, with just a minor weird slope between 7.5 and 10 degrees: https://www.dropbox.com/s/7uyczcnqgadnxn9/NACA_0012_Cl.PNG?dl=0   But Cd and Cm looks weird, some weird spikes here and there: https://www.dropbox.com/s/xkcp5f4idmue4fq/NACA_0012_Cd.PNG?dl=0 https://www.dropbox.com/s/2vvenin03vchp4l/NACA_0012_Cm.PNG?dl=0   I guess there is something wrong with precision of calculations or it just can't handle +-180 degrees AoA well.
6. ## Simulation of drag in flightsims

Makes sense now.   I was looking into what kind of calculations can be done in JavaFoil, mainly getting Cl/Cd/Cm coefficients. I know that you don't use lookup table anymore and I might take the same approach, for a simple reason that Mach and Reynolds numbers are used as constants during calculation and I can't figure out how to take them out of calculated values. So I've looked into interpolating a set of Cl/Cd/Cm, calculated for different Reynolds numbers at least. Unfortunately linear interpolation won't work so I might have to go with using RBF for this. Main argument against manually building my own Cl/Cd/Cm functions is that I want to run simulation for different than earth atmosphere, which would mean that I couldn't really compare what I'm getting as end result to flight in earth atmosphere.   EDIT: To figure out more details about how I can get coefficients without JavaFoil or taking them from somewhere, I ordered "Aircraft Design: A Conceptual Approach" by Daniel P. Raymer. As a bonus side I hope to figure out the rest of the puzzle - side forces and etc.
7. ## Simulation of drag in flightsims

Sorry, messed it up with angular velocity
8. ## Simulation of drag in flightsims

Cool, thank you!   I thought of adding pitching moment when I get Cl/Cd/Cm for assymetrical aerofoil. So just checked them in JavaFoil and to my surprise, default symetrical aerofoil actually has some Cm values, they are small but still. I guess wikipedia article just makes some generalization. I'll add a pitching moment at the next iteration. Right now physical simulation is very unstable, maybe because I apply each Lift+Drag contribution from aerofoils separately.
9. ## Simulation of drag in flightsims

It does sound counter-intuitive, I've imagined that it would be something like a right part of this illustration: https://www.dropbox.com/s/nsqdfy4xrgry56n/AddTorqueAtPoint.png?dl=0 So the part of the torque originating not at the center of mass would lead to some both linear and angular acceleration.   EDIT: Or you mean that in this specific case of pithing moment, the relative application to center of mass is already accounted in Lift and Drag portion of forces?   EDIT2: Just to be sure, expected value of pitchingMoment = Cm * wingArea * q * wingChord is in radians, right?
10. ## Simulation of drag in flightsims

Oh, I see now - you mean Cm coefficient. I was just reading today about pitching moment and if I understand it correctly it accounts for additonal torque at aerofoil due to the shifting centre of pressure as AoA changes. So far I got from 1 to 8 implemented. But not with all the details that you've mentioned: - "modify Cl, Cd, Cm and AoA depending on control surface deflections" - I take into account only AoA right now. Ailerons are attached to the sides at the back of the wing as separate surfaces and tail consists solely of a single elevator surface. I'll add a rudder and then look into baking ailerons and elevators into Cl/Cd/Cm coefficients of the main and tail wings. Should be trivial using JavaFoil for the start. - "Apply the lift force at the "quarter chord" position (adjust this if the wing is stalled or in reverse flow)" - I guess for the reverse flow I should take a quarter chord from a trailing edge and for stall it will be somewhere between 25-50% of the chord. I definitely should add pitching moment. According to wiki: How do you transfer pitching moment to the centre of mass of airplane? Is it just a cross product of pitching moment at aerofoil with a vector from COM to aerofoil?   Thank you for your help, MrRowl!
11. ## Simulation of drag in flightsims

Just second, does it mean that resulting angular force of the body will depend only on Lift force in relation to center of mass of full system? I thought that process is like this: - we calculate Lift and Drag at aifoil system of coordinates, then we sum them up and after transforming to airplane system of coordinates apply them to center of mass of airplane - to find resulting angular velocity our torque would be T = vector_from_com_to_airfoil X (Lift_airfoil + Drag_airfoil).
12. ## Simulation of drag in flightsims

I see, for torque I relly on physics engine, I'm just applying resulting lift/drag forces at the places of aerofoil elements and physics figures out the rest. I remember seeing one model where center of application of forces slightly shift along the chord as AoA changes, so I thought CM was related to that.
13. ## Simulation of drag in flightsims

Ohh, ok, I thought they are simulated as separate surface. But now it makes sense why in some homebuilding articles they modeled them as a single but shape chnaging surface. CM - is it a point of application of Lift force?   Thank you for your feedback, MrRowl! I'll put this suggestions into plan. It's a bit more clear now of how it suppose to work.
14. ## Simulation of drag in flightsims

Hmmm, I had impression that stall is simulated simply by getting a "downhill going" Cl, which starts at critical AoA and higher degrees. So that only one Cl coefficient lookup table is used and if it's generated for 180 degrees then we kind of cover all cases. With the article you linked it's between 15 to 20 degrees. But if I understand correctly what you are saying is that I need to have a second table for Cl and Cd which describes a behaviour during stall, right? EDIT: Ohh I see now, beyond stall angles different airfoils will behave very similar to flat plate. Now I understand what you mean by blending between different sets. So in principal, a more generic approach is to have a complete +-180 degrees lookup table for each type of airfoil or have a set of 0-20 lookup tables and one for 20-180 for stall. Regarding deflection of control surface, thank you for suggestion. Right now I have ailerons just attached at the back of the wings as separate surface. They are not a part of the wing geometry yet, so calculation there goes independent. In case if they are part of the wing, I could "cut-out" proportional part of the area and Cl coefficient from wing. I'll check it in javafoil, there were a way to simulate control surfaces, I think.
15. ## Simulation of drag in flightsims

Yes, you are right, I should take into account other surfaces as well, like drag from the fuselage and other parts by the crosswind and etc. I'll give it a try. Thank you, Buckeye, for the guidance!   EDIT: Thank you for the link. Now I see that I'm just neglecting Lift and Drag coefficients at high angles of attack.