# Aircraft Dynamics

## Just sitting on the runway, my plane turns or slides in the wind

1. This is a known problem with how ground friction is handled, made much worse by computational flutter. See the detailed description on the Theory page. Current X-Plane versions only suffer from this problem at extremely low frame rates or in very high winds. If your plane is uncontrollable on the ground under "reasonable" conditions...

2. A problem has been reported with older aircraft loading up with a tire friction coefficient of zero. You need to set the Cf to a reasonable value (like 0.75 for maximum friction and 0.025 for rolling resistence) in PlaneMaker. See the aircraft format and conversion page for details.

## How do I steer the plane on the ground?

It depends on the plane and what kind of control setup you have. Most X-Plane aircraft are set up with nosewheel steering even if the real plane being modeled doesn't have it. Real planes without steerable wheels are typically steered with differential braking on the main gear, but not all of us have the fancy (and expensive) pedals that support this. BTW, the term "nosewheel steering" is a generic term that applies to tail draggers as well, only in their case it's the tail wheel that steers.

Anyway... If you have a rudder axis assigned - i.e., if you have rudder pedals or a twist grip on your stick or whatever and you've set it up as the Yaw axis in the joystick setup, the rudder axis controls nosewheel steering. If you don't, your stick aileron control (i.e., left-right stick) controls the nosewheel steering. (Only the nosewheel steering - there's no implicit aileron to rudder coupling.)

## When I apply full steering on the ground the brakes come on

This is a feature to help you get around a tight turn. What's happening is X-Plane is applying differential braking - braking force to the wheel on the inside of the turn - when you apply full, or nearly full, steering input. You'd do this with a real plane. X-Plane also applies differential braking in response to any steering input if the nose or tail wheel is free-castering. Assigning control axes to the left and right brakes (e.g., if you have the CH Pro Pedals with toe brakes) overrides this feature.

## Ground handling is lousy - I get squealing tires when I try to turn

First off, this is an airplane, not a sports car! Don't expect the same handling with those little tires as you'd get with the Pirelli 245ZR17/45s you have on your car! Also remember that planes with tricycle gear have lots of angular momentum and relatively little weight on the nose wheel.

The other problem is that with no force feedback, it's easy to overcontrol. Many planes allow extreme nosewheel angles for tight turns, so you really have to ease into the turn slowly and gently.

## I just bought X-Plane and I'm having a hard time just controlling the plane

I can't get the plane into level flight or even get controllable responses to the stick.

A couple of recommendations:
1. Get yourself upgraded to the current version if you haven't already done so. There are many bug fixes and improvements over whatever version is on the DVD.

2. Every time you start up X-Plane, move the stick through its entire operating range. X-Plane saves the stick setup when you exit, but it still auto-calibrates when it sees the extremes of the stick positions.

3. In the Center tab of the stick setup (called Null Zone in earlier versions), give yourself a null zone of about .05, make sure you have the stick center set up right, and pull out the center response sliders a ways so you get a modest bend in the response curve. This reduces stick sensitivity on center and makes it easier to handle. (These settings are new features in 8.15, BTW. You won't see them in older versions.) The main problem is that the feel of any PC joystick is completely different from what you get on a real plane, and you just have to get used to it. It's very easy to overcontrol, and these adjustments help prevent that. Most of flying a real plane is done with pretty small control inputs and X-Plane responds accordingly. The problem is you don't have the opposing forces you'd feel in a real plane.

4. Start with a plane that's relatively easy to fly. My favorite is the Beech King Air. It's a nice stable plane that's not over-sensitive, and being a twin it doesn't have the prop torque problems of a single engine plane. Once you get used to the feel of the stick you can move on to some of the trickier planes. The X-Plane C-172, BTW, is generally regarded as having some pretty serious accuracy problems, especially in the area of ground handling. There are a number of much better C-172 models available on X-Plane.org. The included Piper Malibu is a lot better.

5. Trim the plane as soon as you're off the ground. Keep trimming it as you build up airspeed and climb out. Keep trimming it until you're in stable, level flight. Then keep trimming it anyway. The problem is that real planes, once they're set up properly, are basically in correct trim for level flight. The planes in X-Plane, on the other hand, start out basically out of trim and you have to get them set up right every time you take off. You'll find yourself having to tweak the trim a lot more often than in a real plane. This has been the subject of a lot of debate. I suspect it has to do with secondary aero effects that are not completely modeled by X-Plane.

I put elevator and aileron trim on the stick's hat switch so that it works like the electric trim in a real plane. After several minutes flight I can usually get the King Air trimmed out well enough that it will fly hands off for a minute or two. Even so, it requires the occasional nudge to stay level because you simply can't get the very fine adjustment in a sim that you can get in real life.

## My plane doesn't want to fly level

It keep trying to roll to the left or right.

That's called prop torque. The engine torque gets reflected back into the plane (another manifestation of Newton's third law of motion). You need to compensate with aileron deflection. You'll have to readjust the trim in response to any change in engine power, airspeed, or attitude.

## As I change airspeed with full power, the plane starts to roll the wrong way.

Ordinarily, the roll tendency you feel at takeoff (left for a clockwise prop, right for a counterclockwise prop) should decrease smoothly as you pick up speed and your ailerons become more effective. If there are abrupt changes in the roll tendency or the plane needs opposite aileron trim at certain speeds, it may be suffering from inaccuracies in the modelling of spiral propwash. Increasing the number of wing elements helps to solve this problem.

## My (single engine prop) plane veers to one side during the takeoff roll

This effect results from either P-factor or spiral propwash, or both. See the following...

## Real pilots tell me you need to use rudder trim to keep a single engine plane straight.

There are two effects, P-factor and spiral propwash, that induce a yaw force on the plane during low speed, high power flight. X-Plane prior to version 6.14 does not correctly model either of these effects. Versions 6.21 and later model both. (I am not completely sure when a bug in the modelling of P-factor was fixed, but it is correct in 6.51 and after.) The modelling of spiral propwash works correctly for some planes and not for others - it depends on the wing and stabilizer configuration. This is still under investigation.

## Why does my plane keep bouncing around?

That's atmospheric turbulence.

## But I checked the weather menu and wind and turbulence are at zero.

They may not be. The slider controls in the weather menu are a bit deceptive. The default weather setup (when you start X-Plane with clean preferences) has a pretty significant amount of turbulence set up, even through the summary Wind and Turbulence slide in the lower left may indicate zero. To really set wind and turbulence to zero, move the summary slider off zero (watch the wind and turbulence sliders in the center column wiggle around) and then pull it back to zero.

## Why can't I fly a loop in X-Plane?

Well, first off, you can. Pick the right plane (high power to wing loading ratio) and fly it right (start with enough speed and manage your energy properly) and you can fly a pretty decent loop. Lots of people have done it. That said, it may be harder in X-Plane than in some other sims because other sims may not properly model prop torque or the higher drag that results from a high angle of attack. If you're losing control near the top of the loop it's probably because you've run out of airspeed, not because there's a problem with X-Plane.

As of version 6.14, X-Plane uses full quaternion math for its flight model. Users report that loops and other forms of vertical flight are now correctly handled.
A new airfoil model as of version 6.50 improves the behavior beyond the stall angle, improving the accuracy of spins and stalls.

But... In versions prior to 6.14 there are problems. From the X-Plane manual:
"Rolls and stalls are possible, although loops and spins are not entirely accurate due to the limitations of Eulerian flight propagation and turbulence simulation, respectively. (Eulerian flight propagation, which X-plane uses, has a hard time tracking the heading when you are pointed straight up, and the turbulent airflow of a stalled wing is still not completely mathematically simulateable by any computer)."
The X-Plane flight model pre- version 6.14 does not behave correctly when the plane is very nearly vertical. You can see problems ranging from mild anomalous roll and yaw all the way to loss of control and getting trapped in a vertical attitude.

## I'm flying along and all of a sudden the flight model goes crazy.

Any of the following symptoms:
• Sudden uncommanded, uncontrollable roll or yaw
• Sudden unexplained acceleration
• Uncontrollable bouncing on the ground
You've encountered computational flutter. The aircraft's parameters are changing too fast for the flight model's frame rate to handle correctly. To avoid the problem, slow down or reduce rendering options to increase your frame rate. See the Theory section for a complete explanation of computational flutter.

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