Slip indicators correlate to wind speed direction, not to aircraft lateral acceleration as they ought to

[Venturi]

I have noticed, being very mindful of the slip indicator and its importance to real flight, that it is not behaving as it ought to. It ought to correlate to sideslip, IE, lateral acceleration of the plane due to wind or dynamic rudder yaw forces.

 

However, when the ball is centered in flight, often I find that my heading begins to change, indicating that the aircraft is actually changing direction while in flight.

 

I have further noted that when on the runway, aircraft stationary, and with wind blowing, that the slideslip indicator shows lateral acceleration of the aircraft. Obviously, this cannot be the case, as the aircraft is stationary. 

 

Further, I have noted in the above scenario, that the sideslip indicator veers based on the direction of the wind. If I angle the aircraft one way (say, wind on left beam), and then cease motion and wait, the indicator shows slip in the direction of the wind. If I then turn the aircraft and put the wind on the right beam, and cease motion and wait, the indicator then shows slip on the opposite direction.

 

I have noticed this across multiple aircraft now. I believe I have posted about the Ju87 previously, but that was in flight. Now that I have a bit more time, the photos below show what I mean, but now in the Yak-1, same sortie, same runway, stationary and angled two different directions to the wind.

 

In other words, something is certainly not right with the slip indicator mechanics. As this is a critical part of correct flying, it should be looked at ASAP. I only hope it is not indicative of a larger problem of aerodynamic modelling.

Edited May 10, 2015 by Venturi

[JimTM]

Your thumbnails appears to show a slight tilt to the right and left respectively. I wonder if that explains the offsets on the slip indicator (on the ground anyway).

 

Jim

[6./ZG26_5tuka]

The ball only indicates your attitude around the roll axis. It does not correlate to transitional forces / momentums primarily. My guess for the small offset indication on ground is due to the terrain being uneven, so the aircraft is angled little.

 

I've not yet tested this in the air though so you might have a good point there.

Edited May 10, 2015 by Stab/JG26_5tuka

[Venturi]

The ball only indicates your attitude around the roll axis. It does not correlate to transitional forces / momentums primarily. My guess for the small offset indication on ground is due to the terrain being uneven, so the aircraft is angled little.

 

I've not yet tested this in the air though so you might have a good point there.

Actually, that is the role of the pointer indicator, directly above the slip ball. But I recognize, everybody is an expert..

Edited May 10, 2015 by Venturi

[Hoots]

Actually, that is the role of the pointer indicator, directly above the slip ball. But I recognize, everybody is an expert..

Don't think anyone was claiming they were right? No more than you were anyway, not sure why you needed to be rude to people trying to join in the discussion. Wait, I'm assuming that by posting something on a discussion board that's what you wanted?

[6./ZG26_5tuka]

Thats not correct. The pendulum does indicate your corner speed (relatively) while the ball shows you the direction you need to correct your roll angle. If you turn at constant speed lefthandside for example with too steep roll angle the ball will slide off to the left. As result the aircraft slides and flies uneffeciently.

 

To correct this you either need.to increase your corner speed by pulling the elevator or adjust your roll angle to the right.

 

I don't know the exact word to describe the function of the ball properly, but in german it's called "Wasserwaage". When flying straight it shows the bare roll angle + minor inaccurancy for sideway accelerations, once in a turn it shows the roll angle - the compensation due to negative G force.

 

Maybe it's an language issue but I already made my pilot degree in avionics and have practical expirience in addition.

Edited May 11, 2015 by Stab/JG26_5tuka

[Bullets]

I agree with you Venturi.. The slip ball should be centred when stationary ? I think 5Stuka is just lost in the translation barrier, The Slip ball can move during acceleration in a roll however that is not its primary function. It is used to see if you are slipping to the left or right while in level flight or a turn. Having the ball in the centre means your aircraft is flying through the air straight = more efficiently than if it was slipping.  (Correct me if I am wrong but this is what I have been taught in my experience of Gliding!) 

[6./ZG26_5tuka]

Bank = roll. At least from my side of the "language barrier".

 

Again: Flying straight with ball centered = no banking around roll axis = plane flys level

 

BUT that does not mean the pendulum is straight too. If for example win pushes the nose of the aicraft sideways the pendulum will move indipendant form the ball.

 

The ball is only partly influcrenced by accelerations, mainly by bank angle.

[=LD=Penshoon]

Bank = roll. At least from my side of the "language barrier".

 

Again: Flying straight with ball centered = no banking around roll axis = plane flys level

 

BUT that does not mean the pendulum is straight too. If for example win pushes the nose of the aicraft sideways the pendulum will move indipendant form the ball.

 

The ball is only partly influcrenced by accelerations, mainly by bank angle.

Ball measures slipping or skidding and banking the plane will make the plane slip or skid unless you compensate with rudder. 

http://en.wikipedia.org/wiki/Turn_and_slip_indicator

 

 

[6./ZG26_5tuka]

The last row of pics shows what I tried to describe. the middle one is very confusing though as the aircraft int he picture is actually performing a smooth turn and not flying straight. Otherwise the ball would move to the right (can we at least agree on that?).

 

I never said it's uneffected by sideway acceleration forces. I hope I'm done now with this.

[Bullets]

The last row of pics shows what I tried to describe. the middle one is very confusing though as the aircraft int he picture is actually performing a smooth turn and not flying straight. Otherwise the ball would move to the right (can we at least agree on that?).

 

I never said it's uneffected by sideway acceleration forces. I hope I'm done now with this.

No, If the aircraft had rolled right and not performed a turn (Aka pulled back on the stick) Then yes the ball would "roll" right, BUT if the aircraft was in a stable "in balance" turn the ball should still be in the middle =) 

P.s. Penshoon those are amazing images to describe this situation =P 

[6./ZG26_5tuka]

[Bullets]

That is the exact reaction I am having to you at the moment haha.. 

[Venturi]

I doubt more needs to be said on this.

 

The ball indicates lateral acceleration, that is all it does. 

 

The fact that by inducing and holding bank, without rudder correction, one can create lateral acceleration, is of no importance.

 

The basic principle that I am pointing out is that there is indicated lateral acceleration of the plane along the transverse axis - this is what the ball is measuring (note the gyroscope connected in the picture I attached) - when there should not be any lateral acceleration, at all (since the plane is level and stationary).

 

If you are familiar with gyroscopes (note that the slip indicator is connected to a gyro), that is all that needs saying.

 

Now, I understand what you are saying, that if the plane is tilted in regards to wing position not being parallel with level, that even if stationary, the acceleration of the earth's gavity will pull the ball to one side or another.

 

That is still lateral acceleration (in this case, acceleration of the ball, only - since the ground prevents the plane from slipping downwards, as it does in a turn).

 

However, if this were the case on the aircraft in the thumbnail pictures, then the indicator which detects aircraft wing orientation with regards to being level / the ground (the pointer above the slip indicator) would also show the plane as being tilted in that axis. 

 

You will note, it is perfectly straight.

 

I hope this calms your objection.

 

Back to the topic at hand.

Edited May 12, 2015 by Venturi