Monday, September 07, 2009

Yank That Stick Back

A quote from the .org:
I actually did try the demo before I bought it, and I found it sorta arcadish as I took off with a King Air 200 and performed barrel rolls on t/o lol! That kind of tuned me out!
I don't mean to pick on that particular poster - we hear that a lot, particularly from people who use MSFS. Andy Goldstein points to Bob Hoover as a counterpoint.

More generally, the criticism is that since X-Plane will allow you to do things in commercial airliners that you've never seen in real life, X-Plane must not be realistic. And phrased like that, you can see the possible flaws in the reasoning:
  1. How do you know your control inputs match real life?
  2. How do you know the real plane is physically incapable of doing such a thing?
The first is the age-old problem of consumer joysticks - to really maneuver an airliner hard you have to put a lot of pressure on the controls - they put up a lot more resistance than a $20 SideWinder.

The second is an issue of falsifiability again - the absence of evidence is not evidence of absence.

Now in truth there are things that X-Plane models and X-Plane does not model. If you load up the airframe too much with fail-on-over-G on and the G limits are set correctly, X-Plane will start removing parts as you try to fly a 747 like an F-16. But we don't necessarily simulate some of the internal things that might go wrong. For example, accelerations on the fuel system might potentially cause flow to the engines to fail, stalling an engine. I don't know how much of this Austin simulates, but we certainly don't simulate the geometry of the fuel lines and the fuel as a fluid flow. So there may be cases where a maneuver is impossible for logistic but not aerodynamic reasons (e.g. you could do it but you'd lose your engines).

With that in mind, here are a few youtube links:
  • Bob Hoover flies a Commander like an aerobatic glider - thanks Andy!
  • Roll an airliner? It's safe, really!
  • The 757 has a lot of power - thanks Andrew.
  • I have no idea what happened here...I thought the Airbus computer prevents you from doing this kind of thing.
On that last video: the first time I flew with Austin in his Cirrus (this is before he traded it for a Columbia) he pulled the same maneuver: take-off, very slow rate of climb to pick up speed, and then: yank. Our climb rate was well over 2000 fpm in a single-engine prop for a while.

(Logistic note: please don't interpret this post as an invitation to contact me regarding any aspect of the flight model - when it comes to the physics engine I am just another user, with no special insight. Physics is Austin's domain, definitely not mine!)


Michael said...

Right on! People need to understand that in "normal" flying there is an enormous gap between what is actually done and the limits of the airplane's capabilities. The reason for this is because the closer you get to the limits of the airplane's capabilities, the greater your chance of an accident.

Consider a takeoff technique which can be performed successfully 999 times out of 1000. If every airliner used this technique, there would be thousands of airline passengers killed every day in the United States alone. And yet you could sit there all day in front of X-Plane using this maneuver and never see anything go wrong. It will look perfectly safe and easy to you and yet be far beyond the bounds of acceptable safety in real life.

It's very easy in X-Plane to perform a maneuver that is safe most of the time, but in real life the standards are vastly higher.

Peter Cole said...

For another take on the RNZAF 757, see this:

Anonymous said...

There's not much wrong with the flight model or the rendering models considering the complexity and regular addition of features and tweaks. But there's plenty of weaknesses in operations modeling.

Here's an example: you could have an annunciator tied to flaps which could be triggered by a general electrical failure, a Vfe overspeed (flap-off) or a flap actuator failure that goes ON if the event pertains to any of the flaps. There is no easy way to model this.

I'm talking about sequential events in that one failure triggers other events, not something that necessarily works on a per frame cycle. Here's another example: the brake button/annunciator/light. Setting the light ON when Parking brakes aren't completely off is easy. But you might want it to go ON too if you have a left or right brake failure, tire failure, landing gear failure, or even general hydraulic failure.

When I make suggestions as in this post, I get the feeling I'm preaching to an empty hall. Care to comment? Who looks after this side of the sim?