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Turn coordinator ball

Vitus

Vitus

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newzealand
Hi all!

Could you please enlighten me about the turn coordinator ball please? I don't think it works how it should, or I am missing something here.

I put my aircraft on the runway, applied the parking brake and cranked up the throttle. The aircraft sits perfectly still, but all of a sudden the turn coordinator ball wanders left. I thought I was using the variable inn a wrong way, so I loaded up the Grumman Goose (strong brakes!). And it showed the same effect.

Is this yet another example of FSX sucking at physics, or do I have to screw my head back on?

Cheers,
Vitus
 
It's probably the inverse coriolis effect in the Southern hemisphere or the after effects of too much proximity to a whiskey compass..
The ball should stay centered until there is a lateral acceleration.
Taxying straight it should be centered, turning left it should go out to the right.
I have never seen the effect you describe in the sim.
Roy
 
I experimented a bit more. Turns out that it is the wind. :banghead:
When there's 0 knots, the ball stays where it belongs. As soon as I crank up the wind speed, the ball moves left as soon as I crank up the throttle while applying the brakes. Make sense of that! :rolleyes:
 
furthermore: it doesn't matter whether the wind blows from the left or right - the ball always moves left. If the wind comes straight ahead, or from the back, the ball stays centered.
 
Propeller torque? Try adding a rotation=-1 line in the [propeller] section of a single engine piston aircraft.cfg and see if the ball moves to the right when full power is applied. On the Goose, adding rotation=-1,1 counter rotating props, might keep the ball centered.

Bob

[edit] apologies for the very poor video quality ... but, is this what you're seeing?

- The first example is the default C172 and ball moves to the left

- In the second example, I added rotation= -1 in aircraft.cfg [propeller] section. Ball moves to the right

- If rotation= -1,1 is added to the Goose, ball remains centered
 
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Since you are seeing the effect when the throttles are open and there is slipstream over the rudder, you might try looking at Cn delta rudder due to propwash. Depending on your airfile structure it could be in 1544 or in 1101. Try putting it to zero as a check.
Yves document on pages 39 and 40 has some explanation of the effect
Roy
 
The video shows the effect, except that the aircraft is actually moving slightly towards each side as an effect of the prop. And again, as soon as I turn off the wind in the weather menu, the effect stops as well.

To me, nothing of this makes any sense. Shouldn't the ball only move when the aircraft moves?

Roy, I'm not aware of the document you're referring to. My "Cn delta rudder due to propwash" is at 0.02 at the moment.
 
When the wind blows and you are static on the ground you have airspeed. If the wind is from the side and your airplane is restrained you have sldeslip. By this I mean your velocity is not aligned with the airplane fore-aft axis and as far as it is concerned you are in unbalanced flight, so the ball indicates that.
Actually, the ball is deflected by sideways acceleration, but maybe that is tough to simulate and the sim just does its take on the air velocity.
AFSD does not show any sideways velocities though it does show TAS and VX equal to the wind component along the fore-aft axis.
Prop planes real and simulated always need some attention to where the ball is.
As for the effects of wind on prop planes on the ground, try taxying downwind in a tail-dragger with no wheel brakes in a 15 knot wind. not fun.
Roy
EDIT.
I tried Bob's rotation switch and it did affect the magnitude of the ball deflection. I also put the yaw from thrust airfile coefficient to zero and it made no difference.
So the sim appears to treat slip based on velocities relative to the wind, which is a reasonable way to do it when airborne, but not on the ground.
 
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In that case you could simply put a flag on the ball such that it is disabled whenever the aircraft is on the ground.
 
It is rather ridiculous because a ball in real life isn't going to sense wind blowing over the aircraft while it is standing still.

Vitus, you can try some trigonometry using bank angle, lateral acceleration, and G converted to m/s^2 to simulate the ball.
 
The turn coordinator ball essentially follows the same principles we discussed earlier in this thread:
http://www.fsdeveloper.com/forum/threads/variables-for-forces-on-the-airframe.440768/

Unfortunately, the solution for my compass gravity bank is not precise enough for the ball. I think Bill's solution is probably the simplest: check if the aircraft is on the ground and the ground speed is 0, if so ball = 0.

These things really get on my nerves in FSX... :banghead:
 
If I had a ball that did not indicate on the ground and I was flying an old twin prop plane I would be sorely annoyed.
Taxying out is when you check instruments work before you risk your life taking off.
No ball indication when you needed it could be lethal if one engine dies on climbout.
Disabling flight instruments on the ground is a BAD idea.
Roy
 
I agree. But what's wrong with making sure that the ball doesn't move when it's not supposed to move? That's why I wrote that I'd be checking for being on the ground AND ground speed = 0.
 
Why not use a different method?
The ball indicates unbalanced flight, the airplane is not going through the air in the direction/vector where it points in the lateral axis. Bit like a lateral Angle of Attack and called Beta. Usually called sideslip angle.
It has a variable A:INCIDENCE BETA, radians
My point is that the stock method looks a bit dubious so why not do it right?
Roy
 
That was a very good suggestion Roy! I tried it and observed the following: while on the ground the incidence beta stays 0, no matter how much rudder you put. :(
 
My fault,
I forgot you are using FSX and the variable is not in the FSX SDK. Indeed INCIDENCE ALPHA, its brother is not there either.
However there is ANGLE OF ATTACK INDICATOR in the FSX SDK, but no sideslip BETA INCIDENCE.
You probably can get it from VELOCITY BODY X divided by VELOCITY BODY Z. Using small angle approximation and appropriate scaling that should give you a representation of sideslip angle.
Roy
 
FSX SDK has the following:

Code: 
INCIDENCE ALPHA Angle of attack Radians
INCIDENCE BETA Sideslip angle Radians

You got the BETA and INCIDENCE swapped around.

The problem of using VELOCITY BODY X / VELOCITY BODY Z however is that in real world, you can have a different flightpath angle to your heading due to crosswinds however the ball will remain centered in that situation so it is not a reliable slip indicator.

Instead, you want an idea of where your local gravity is relative to the aircraft. If you bank and slip, your gravity vector will be on the inside of your bank. If you bank and skid, your gravity vector will be on the outside of your bank.
 
JB3DG said:
FSX SDK has the following:
INCIDENCE ALPHA Angle of attack Radians
INCIDENCE BETA Sideslip angle Radians
Click to expand...

What is your source for the FSX, not FSXA and not ESP SDK? I have an original copy of the FSX SDK which is what I consulted prior to my above post. The P3D SDK has the ALPHA and BETA variables.
Edit. I apologize for the above error, I found them in the FSX SDK at the opposite end of where I looked. So the sideslip BETA should work at least in the air.

You got the BETA and INCIDENCE swapped around
Click to expand...

Please explain what you mean and state where this occurred.

The problem of using VELOCITY BODY X / VELOCITY BODY Z however is that in real world, you can have a different flightpath angle to your heading due to crosswinds however the ball will remain centered in that situation so it is not a reliable slip indicator.
Click to expand...

A crosswind has no effect on how an airplane flies. It is in and of the air. It effects the track over the ground but has no effect on the aerodynamics.
The body velocities are relative to the air. That is why there are a separate set of velocities relative to the earth.
Sideslip has nothing to do with wind velocity or crosswinds. It occurs when the relative direction of the airflow passing over the airplane is not aligned with where it is pointing. If it is not perfectly aligned the ball will move out to one side. Typically the pilot will compensate by adjusting the rudder trim to center the ball and have the fuselage aligned with the airflow again. Happens all the time in prop planes following power changes unless they have counter rotating props. Sideslip is uncommon in jets unless it is induced by rudder or occurs due to poor directional stability. Sideslip does occur, of course, in any airplane if there is assymetric thrust and the pilot has not applied compensating rudder and/or rudder trim. That is where it gets dangerous because sideslip close to the stall on take-off can lead to loss of control and a spin.

Roy
 
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Roy Holmes said:
A crosswind has no effect on how an airplane flies. It is in and of the air. It effects the track over the ground but has no effect on the aerodynamics.
The body velocities are relative to the air. That is why there are a separate set of velocities relative to the earth.
Sideslip has nothing to do with wind velocity or crosswinds. It occurs when the relative direction of the airflow passing over the airplane is not aligned with where it is pointing. If it is not perfectly aligned the ball will move out to one side. Typically the pilot will compensate by adjusting the rudder trim to center the ball and have the fuselage aligned with the airflow again. Happens all the time in prop planes following power changes unless they have counter rotating props. Sideslip is uncommon in jets unless it is induced by rudder or occurs due to poor directional stability. Sideslip does occur, of course, in any airplane if there is assymetric thrust and the pilot has not applied compensating rudder and/or rudder trim. That is where it gets dangerous because sideslip close to the stall on take-off can lead to loss of control and a spin.
Click to expand...

I'm not saying you don't get a sideslip, I'm saying that the ball system will not detect it. If you have an inertial navigation system or GPS system that can detect a difference between your heading and your track, you can get a sideslip indication. A yaw string such as found on gliders will also indicate sideslip due to the airflow.

But the ball instrument is entirely isolated from airflow. It will not detect it. The only thing that influences it is the gravity vector. It basically provides an indication of the direction of the gravity vector.

As for the BETA variable:

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