Author Topic: Best General Aviation Sim?  (Read 1255 times)

Offline Chairboy

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« Reply #15 on: July 24, 2003, 11:26:18 AM »
They use 'em as part of their design process.  Go to x-plane.com and see for yourself.  Also, as good as lookup tables can be, you still end up either needing to develop one for each plane or risk having a general one that doesn't accurately reflect the performance of the plane.

BTW, it doesn't really matter if you believe it or not.  The FAA says you can use X-plane towards a commercial pilot certification, plane manufacturers use it to test their designs, and your skepticism isn't really important because smarter people then you or I have decided that X-Plane is the right tool for the job.

(edit: Changed FCC to FAA, whoops!)
« Last Edit: July 24, 2003, 02:01:09 PM by Chairboy »
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Offline Fishu

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« Reply #16 on: July 24, 2003, 11:42:06 AM »
kids kids.. lets chill

Offline Chairboy

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« Reply #17 on: July 24, 2003, 12:00:15 PM »
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Offline gofaster

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« Reply #18 on: July 24, 2003, 01:22:06 PM »
The ELITE system, with the hardware.

Offline AKS\/\/ulfe

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« Reply #19 on: July 24, 2003, 01:39:00 PM »
The Federal Communications Commission is telling people they can use X-Plane for a commercial pilot's license?

I'm pretty sure that at some point, X-Plane uses a lookup table to reproduce some effects.

Even with the simplified airfoil model, there are so many other forces acting on a plane, it would bring any home PC to it's knees.
-SW

Offline Chairboy

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« Reply #20 on: July 24, 2003, 01:49:23 PM »
Once again, you could clear your misconceptions up if you just visited x-plane.com.  I know it is fun to speculate, but when the actual answer is just a click or two away, it's kind of silly.

If you don't want to find the answer on the x-plane site, then I suggest this article from popular science.  It is easier to digest:

http://www.popsci.com/popsci/aviation/article/0,12543,463052,00.html

(fixed URL)
« Last Edit: July 24, 2003, 02:00:30 PM by Chairboy »
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Offline AKS\/\/ulfe

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« Reply #21 on: July 24, 2003, 01:56:37 PM »
That link isn't complete.

Of course, I'm not speculating that there are either a) extremely simplified forces beyond just the airfoil, or b) using look up tables to relieve some stress on the CPU.

It's not misconception that any home PC could not simulate flight without a lot of short cuts, beyond just the airfoil, and therefore use a look up table for several of the parts of flight.

Il2/Forgotten Battles lays claim to one of those "new-fangled physics engines" to simulate flight, what many people don't realise is that it's still using look up tables to derive much of the input into those equations.

Boeing simulators, the ones for the passenger jets, have a whole load of computers to compute the flight model. Reducing the airfoil to just 8 sections won't relieve that much of the CPU's stresses, since there are so many other factors that go into replicating flight. Therefore, more short cuts must be had to get the process to run at acceptable speeds on home PCs.
-SW

Offline Chairboy

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« Reply #22 on: July 24, 2003, 01:59:23 PM »
For more information about using X-Plane as credited training for FAA (whoops on the FCC part, my bad!), click here:

http://x-plane.com/FTD.html

You'll find that X-Plane can be used towards instrument rating as well as Commercial Certificate and Airline Transport Certificate.

Elite is cool, btw, but can only be used towards instrument rating.
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Offline Chairboy

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« Reply #23 on: July 24, 2003, 02:03:05 PM »
Here's some info from X-Plane about how their engine works:

How it Works:

X-Plane reads in the geometric shape of any aircraft and then figures out how that aircraft will fly. It does this by an engineering process called "blade element theory", which involves breaking the aircraft down into many small elements and then finding the forces on each little element many times per second. These forces are then converted into accelerations which are then integrated to velocities and positions... of course, all of this technical theory is completely transparent to you... you just fly! It's fun!

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?
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Offline Vulcan

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« Reply #24 on: July 24, 2003, 02:57:17 PM »
Quote
Originally posted by Chairboy
They use 'em as part of their design process.  Go to x-plane.com and see for yourself.  Also, as good as lookup tables can be, you still end up either needing to develop one for each plane or risk having a general one that doesn't accurately reflect the performance of the plane.

BTW, it doesn't really matter if you believe it or not.  The FAA says you can use X-plane towards a commercial pilot certification, plane manufacturers use it to test their designs, and your skepticism isn't really important because smarter people then you or I have decided that X-Plane is the right tool for the job.

(edit: Changed FCC to FAA, whoops!)


PC memory could allow for fairly large lookup tables. So precalculation is not that bad and could be fairly complex. Precalculation can add in many aerodymanic factors a PC could not handle.

I believe MSFS is allowed for use towards instrument rating is allowed as well (and has been for some time)?

I'm know aerodynamic genius but I do know what real (big) flight sims require for horse power and what a PC has for horse power. X-Plane fanbois are like the boy-racers (I believe yanks call em ricers?) of the sim world :)

Offline MrCoffee

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« Reply #25 on: July 24, 2003, 03:05:34 PM »
Quote
Originally posted by Vulcan
PC memory could allow for fairly large lookup tables. So precalculation is not that bad and could be fairly complex. Precalculation can add in many aerodymanic factors a PC could not handle.

I believe MSFS is allowed for use towards instrument rating is allowed as well (and has been for some time)?

I'm know aerodynamic genius but I do know what real (big) flight sims require for horse power and what a PC has for horse power. X-Plane fanbois are like the boy-racers (I believe yanks call em ricers?) of the sim world :)


Theres alot more to designing an aircraft than just using lookup tables to see if the airframe will fly or not.

Offline Chairboy

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« Reply #26 on: July 24, 2003, 03:44:59 PM »
Also, MSFS cannot be used towards instrument rating.  The FAA does not allow it because the flight behavior is not representative.

Out of curiousity, how does pointing out a mistaken assumption (Namely, that MSFS has a more realistic flight model than X-P) make me analogous to a 'ricer' or fan boy?

I think I'd be a fan boy if I was posting 'MS Flight Sim sucks!' and 'you loozers are lame for using microsoft krap, x-p rulz!', but I'm not.  I like MSFS, it has a better interface and prettier graphics, but it is apparent that X-Plane has better realism.  The FAA, plane manufacturers, and a bunch of commercial pilots agree.
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Offline udet

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« Reply #27 on: July 24, 2003, 04:55:41 PM »
earlier versions of x-plane had problems simulating stall. From what you guys said iy sounds like they are doing a very crude numerical modelling. I'd rather have tables.

Offline Vulcan

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« Reply #28 on: July 24, 2003, 05:16:03 PM »
Quote
Originally posted by Chairboy
Also, MSFS cannot be used towards instrument rating.  The FAA does not allow it because the flight behavior is not representative.

Out of curiousity, how does pointing out a mistaken assumption (Namely, that MSFS has a more realistic flight model than X-P) make me analogous to a 'ricer' or fan boy?

I think I'd be a fan boy if I was posting 'MS Flight Sim sucks!' and 'you loozers are lame for using microsoft krap, x-p rulz!', but I'm not.  I like MSFS, it has a better interface and prettier graphics, but it is apparent that X-Plane has better realism.  The FAA, plane manufacturers, and a bunch of commercial pilots agree.


I was comparing X-Plane to real aerodynamic sims, ie big uns like NASA uses, not an MSFS-XPlane comparison, as the ricer comparison.

As far as the intrument rating goes, I remember this from years  and years ago. I'm not an MSFS fan (in fact I haven't touched it for years).

All I'm pointing out is that a simplyfied aerodynamic blade model is not necessarily more accurate than a table based model. And (mrcoffee) that neither compares to the larger supercomputer based simulations as far as design goes (ie I wouldn't set foot on an aircraft designed in MSFS :D )

The disadvantage of Xplane is it HAS to be simplified AND exclude certain aerodynamic forces.

For example from your own posting:
"then going to an empirical look-up table"
"but transonic effects are not simulated "
"In supersonic flight, the airfoil is considered to be a diamond shape"

The advantage of a lookup table is that the generating engine can be extremely complex and add nuances or unusual characteristics (such as changes in shape to the wing) without having to sacrifice accuracy to be realtime.

And, as we can see above, X-Plane uses lookup tables which to me is an admission that these are still superior on lower powered computer systems (ie not mainframes).

Offline Chairboy

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« Reply #29 on: July 24, 2003, 05:45:10 PM »
Your selective quoting is misleading.  Regarding the lookup table, here's the rest of the sentence:

"empirical look-up table to get the degrees of downwash generated per coefficient of lift"

Eg, it uses a table to get the downwash based on a certain lift coefficient, NOT as the flight model.  MSFS uses lookup tables for every aspect of flight, and the realism suffers greatly.

It doesn't model transonic flight, but then again, neither does MSFS.
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