Aces High Bulletin Board
General Forums => Aces High General Discussion => Topic started by: 28sweep on November 01, 2002, 07:22:44 AM
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What does it mean when sombody says that a Flight Model is based on "Physics?" I have heard people on these boards say that AH does not have a Flight Model based on "Physics" but rather has a model based on "Tabels." What does this mean? What about the full-motion simulators the military uses? I would think they would be very very accurate...how do those flight models work?
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Instead of doing calculations in real time, data is looked up in tables. The tables themself don't really make a difference to real time calculations.
It all depends on the quality of data the tables are filled with.
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It's a meaningless statment. Physics and tables are not exclusive items. Milatary sims tend to be much more table based then games do. The choosing between an equation or look up table when coding has more to do with performance than it does with the outcome of the model. Either way you get the same numbers out. For instance you can run equations one time to populate a table. Then during the real time run you just use the table for speed of lookup.
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When I say something like this I am usually refering to the fact that they are made by data. This will never be the same as RL, thats why it is called a simulator ;) Because of this, sometimes planes can do things in a simulator that they could never do in RL because of physics. Yet other times in a sim the very same plane may be limited by the data.
It is not any programers fault unless they enter bad data. It is just the fact that the way models are designed, by military and civilian, they will never be 100% accurate.
IMHO AH has some of the best that I have seen. Ofcourse I haven't flown military WWII wims, just military modern jets, so my view could be way off as well:rolleyes:
I guess untill someone who flew the aircraft gets in here and says, its right on, or it is way off, then I think I will take HT models as the way it was. All except that damn la7 anyway :D
Booky
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I think I know what they mean.
A FM based on tables uses known information from historic data in order to make the planes fly like they should.
In a Physics based FM, the planes are modeled using accurate blueprints modeling every shape of the wing and fuselage, and then the FM interprets what the plane is supposed to fly like.
When WWIIOL was made (Physics based), they modeled planes and put them in the Physics Engine and they had a tendancy to fly like they're supposed to. In a physics model, some tweeking based on tables would probably be necesary.
A Physics model has numerous advantages because where data is not available, the FM can extrapolate/interpolate where as a table based FM depends entirely on data (or speculated data). A Physics FM would be much harder to code IMO.
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Originally posted by AtmkRstr
When WWIIOL was made (Physics based), they modeled planes and put them in the Physics Engine and they had a tendancy to fly like they're supposed to.
Hehe I still get a chuckle out of the 110 helicopter ;)
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An accurate flight model comprises a set of linked, non-linear differential equations. Such equations can be solved by a variety of numerical means (e.g., Runga-Kutta method) on a computer, but the computations can be very time intensive. One can make it easier for the computer by using simpler models (i.e., less complex with linearized and easier-to-solve equations), but the fidelity of the simulation may suffer. I assume that so-called "physics" models use a differential equation model and solve the equations in real-time to continuously compute the state variables incorporated in the model (e.g., linear and angular position, velocity, and acceleration in three dimensions).
I assume that what makes a "table-based" model fundamentally different from a "physics" model is that the table-based model does not involve solving differential equations in real time (i.e., as the simulation is running). I can imagine that a table-based model might include entries that would specify that, for example, at a particular position (e.g., altitude and orientation), velocity (3d), and acceleration (3d), a set of inputs (e.g., thrust, control surface deflections) results in a pre-defined change in position (3d), velocity (3d), and accerlation (3d). Obviously, the more entries the table has, the more accurately it can simulate a "physics" model.
I can see a couple of advantages to a table-based model. First, and perhaps most importantly, the CPU overhead needed to implement it is far below that necessary to chug through a complex physics model. Second, it may be difficult to construct accurate differential state equations to model a particular plane--these are generally complex and, as stated in an earlier post, depend critically on all of the plane's physical characteristics. A table-based model allows the designer to more readily incorporate heuristic information (e.g., climb rate and speed charts) about a plane's performance into its flight model.
In the end, I don't think that one approach is "better" than the other in every situation. If you have "perfect" equations for your physics model and ample CPU overhead with which to implement it, then that is probably the way to go. My guess, however, is that HTC had neither when designing AH.
- JNOV
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What HT said. Usage of physics and LUT's (look up tables) are not mutually exclusive. LUT's are quite common in engineering.
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Also, despite what some have said here, you can most definitely solve differential equations in real time using LUT's. The state variable derivatives are equal to nonlinear functions of the state variables and control inputs. You can use LUT's in the calculation of the derivative functions and then integrate normally.
And don't forget that many of functions are curve fits of discrete data points. In that case there may not be a significant difference in accuracy between using the curve fit equation (which is just an approximation of the actual data points) or using a table based on that curve fit.
I'd wager that for a program like AH, the error introduced by the neccesity for numerical integration exceeds any error introduced by using an LUT instead of a calculation. In fact, using LUT's can make each integration cycle process faster, allowing a smaller time step size, and therefore increasing accuracy AND graphics performance.
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Lets not forget blade element theory.... :D
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Funked:
I didn't mean to imply that using a table would eliminate the need to "integrate." Ineed, I was referring to what I think you describe when I mentioned changing the state variables by a pre-defined amount. (I wasn't precise and was trying to leave out the math.)
I didn't read HT's post carefully before posting (that's a cardinal sin -- sorry). He mentions "running the equations to populate the tables." I wonder if that's really what they did? If so, perhaps they do have the equations and it's really all about performance. Maybe WWII era planes are sufficiently similar that a set of parameterized equations is sufficient to model the lot.
The only sort of table-based simulation I've ever created invovled linearizing a very non-linear system about a number of operating points in the state space, effectively dividing the state space into a set of linear systems. I didn't think about it at the time, but I suppose that could be considered a table-based simulation. (We didn't linearize because of performance concerns; we were concerned with creating control systems and, in that context, linear systems are much easier to deal with than non-linear systems.) In any case, I doubt that this is the sort of approximation that AH uses.
For a general time-invariant, non-linear system of the form
dx/dt = f(x,u)
y = g(x,u)
what does "populating the tables" involve? I would guess that it involves defining f and g at a sufficient number of discrete points-- basically discretizing the functions so that they are indexable at discrete points in the state and input spaces. With such tables, numerically integrating would be reduced to look-ups and simple arithmetic. It would be quite fast, I imagine. Do you know if that's how AH does it? If it is, it would be interesting to know how many "index points" and "dimensions" the tables have.
- JNOV
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*reads thread*
*head explodes*
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X-plane
If you want the most "accurate" flight model for your computer at home. Get it.
http://www.x-plane.com
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Unfortunately, no calculated physics model that can run on a present day pc will ever work very accurately. Even NASA's computers fail to model compressibility and transonic flight regime accurately enough to eliminate the need for wind tunnel and full scale flight testing.
X-Plane seems to work ok for Cessnas and Pipers (I say that having flown both in real life). But try taking up the F-4 Phantom and telling me that X-Plane is the most realistic sim. Jane's Fighters Anthology was based on tables that were based on the standard height-Mach energy-maneuverability graphs. Some of the player made libraries had very accurate tables that produced results far superior to X-Plane.
I like some aspects of X-Plane, but by no means would I consider it the most realistc flight sim.
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X-plane can model supersonic flight, as well. :)
Current version is 6.40 take a look.
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Does 6.40 have blade elemental theory in it, Jbroey?
In no more than 1000 words prepare a treatise that discusses the major points of computer based flight modelling and quantitively explain why you might think X-plane is more accurate.
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Dowding its not "blade elemental theory" its blade element theory.
Straight from the man himself,
X-Plane goes through the following steps to propagate the flight:
1: Element Break-Down
Done only once during initialization, X-Plane breaks the wing(s), horizontal stabilizer, vertical stabilizer(s), and propeller(s) (if equipped) down into a finite number of elements. The number of elements is decided by the user in Plane-Maker. Eight elements is the maximum, and studies have shown that this provides roll rates and accelerations that are very close to the values that would be found with a much larger number of elements.
2: Velocity Determination
This is done twice per cycle. The aircraft linear and angular velocities, along with the longitudinal, lateral, and vertical arms of each element are considered to find the velocity vector of each element. Downwash, propwash, and induced angle of attack from lift-augmentation devices are all considered when finding the velocity vector of each element.
Propwash is found by looking at the area of each propeller disk, and the thrust of each propeller. Using local air density, X-Plane determines the propwash required for momentum to be conserved.
Downwash is found by looking at the aspect ratio, taper ratio, and sweep of the wing, and the horizontal and vertical distance of the "washed surface" (normally the horizontal stabilizer) from the "washing surface" (normally the wing), and then going to an empirical look-up table to get the degrees of downwash generated per coefficient of lift.
3: Coefficient Determination
The airfoil data entered in Part-Maker is 2-dimensional, so X-Plane applies finite wing lift-slope reduction, finite-wing CLmax reduction, finite-wing induced drag, and finite-wing moment reduction appropriate to the aspect ratio, taper ratio, and sweep of the wing, horizontal stabilizer, vertical stabilizer, or propeller blade in question. Compressible flow effects are considered using Prandtl-Glauert, but transonic effects are not simulated other than an empirical mach-divergent drag increase. In supersonic flight, the airfoil is considered to be a diamond shape with the appropriate thickness ratio... pressures behind the shock waves are found on each of the plates in the diamond-shaped airfoil and summed to give the total pressures on the foil element.
4: Force Build-Up
Using the coefficients just determined in step 3, areas determined during step 1, and dynamic pressures (determined separately for each element based on aircraft speed, altitude, temperature, propwash and wing sweep), the forces are found and summed for the entire aircraft. Forces are then divided by the aircraft mass for linear accelerations, and moments of inertia for angular accelerations.
5: Get Back to Work
Go back to step 2 and do the whole thing over again at least 15 times per second. Aren't computers great?
The newest enchancments/fixes are now.
New engine type: tip rocket! These are the rockets on the tips of helo rotors that turn the rotors without any need for a tail-rotor!
They look pretty funky at night with the rotor spinning round and round with flame shooting out of the tips!
New VTOL control option: Differential collective with pitch input: This is used in VTOLs with engines fore and aft of the CG
to control the atitude of the aircraft in hover. Design your own heavy-lift 4-engine Osprey now with very precise pitch control
by increasing or decreasing collective pitch on the front or aft rotors in hover. No more need for cyclic control!
New VTOL option: LIFT FAN! Here is how it works: A drive-shaft is connected from the JET ENGINE to the FAN.
When engaged, the jet engine sacrifices some part of it's thrust to drive this driveshaft which is connected to the fan.
In this mode, you get less thrust from the JET engine (because it is turning the driveshaft hooked to it's turbine), but
the shaft is turning the lift fan, which uses up the power to produce thrust itself! This is how the (real) F-35 works!
(the jet engine is in the back, the lift fan in the front... they engage the lift fan for hovering flight only)
Now you can simulate it in X-Plane! Here is how it works: You specify what percentage of the jet engine's output is sent to the
fan, and designe the fan just like you design any propeller in Plane-Maker. In the sim, when you transition to vertical flight,
you can wtch the prop visibly spin up as it starts producing lift for you! Onbly in X-Plane, that's for sure!!!
Example: If the jet engine puts out 20,000 lb of thrust, and you duct HALF of the output to the drive-shaft, then the jet engine will now
put out only 10,000 lb of thrust, and the fan will absorb the rest of the power. Does that mean the fan will put out the remaining
10,000 lb??? NO WAY!!!! The fan accelerates air at a much different rate, over a much different area, with a much different
efiiciency and peed range than the jet, so the fan will be no means put out 10,000 lb! In fact, if you design the fan right,
you may get that fan putting out 20,000 lb of thrust! Voila! A 20,000 lb het engine is now putting out a total of 30,000 lb of thrust!
You are now hovering in a 30,000 lb airplane with a 20,000 lb thrust engine! "Impossible!", you say? NOT AT ALL! The jet engine
accelerates a LITTLE BIT OF AIR a LOT (small engine face, exhaust at near Mach-1, which is efficient at high speed) but the lift
fan accelerates a LOT OF AIR A LITTLE (big fan face, exhaust at maybe 100 knots, which is much more efficient at low speed).
So, since the fan need only run at LOW SPEED (you don't need to HOVER fast!!!) you get a lot of thrust-augmentation by ducting
the jet's output to the fan!!
Now the challenge: Huch more lift YOU can get by ducting power OUT of your jet engine(s) to the fan that you design????
You design the fan in Plane-Maker like you deisgn props. If you play your cards right with the fan radius and size and
design speed and RPM and fan airfoils, HOW MUCH MORE THRUST CAN YOU GET??? Find out with X-Plane!
Reverse-flow over wings, and props improved.
This helps in those backslides, helos, and high-sped autogyros.
New blown flap improvement: Blowing speed foud for you based on the throttle reduction you decide...
and also you can blow air over ALL the control surfaces to keep flight control effectiveness at low speed.
Flap-deflection with pitch and roll for high-lift manuevering.
Engine idle thrust and tire rolling friction coefficient tweaked to get that taxiing just right in the heavies.
Improved wing-tilt modelling: All element locations now found when the wing tilts for steering.
Transmission drag now taken out of helos and VTOLs.
Thrust-reverser lockout on the props if props not full forwards, as in reality.
Auto thrust-reverse on touchdown, as used by Swedish Viggen fighter.
Lateral cyclic with heading input, for helos like the CH-46 Chinook, and other new prop and rotor control in Plane-Maker.
So dowding, I would love to hear what portions of this you think that Aces High does/doesnt do.
Xplane simply has the better system of "simulating" reality, (interaction of an object through a fluid mass "air", as opposed to that of Aces High).
Aces high does very well in many areas, but because it is firstly a game its demands of simulating reality are spread out in more directions then x-plane: guns/damage modeling/multiplayer/graphics etc...
Not that it means I am an expert in the laws of aerodynamics, but I am a real life pilot, have flown wwii fighter trainers, as well as multiple civil aircraft. Reality is Very fluid, xplane catches all the little dynamic occurences faster/more accuately then ANY other PC based simulator/game thus far and it is improving with each passing update.
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The question I always had about xplane was "is 8 enough?" and assuming that a physics model can totaly simulate the forces on those 8 point and that is a high fidelity model of how the plane would act in the real world..home many dicrete pointes were used to model the real world? 12. Nature is complex does the game have a 'Nature' model to go with its 'Physics' model?
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Lonestar, roger, we are on the same page. :)
For a general time-invariant, non-linear system of the form
dx/dt = f(x,u)
y = g(x,u)
what does "populating the tables" involve? I would guess that it involves defining f and g at a sufficient number of discrete points-- basically discretizing the functions so that they are indexable at discrete points in the state and input spaces.
I think, from what HT has said over the years, and from what I know for sure about the code in other very similar products to AH, that your guess is correct.