Sideslip in Cruise - Jets

[Shawn Coyle]

I've often wondered if there is a way to know if a small amount of sideslip is present in the cruise on jet aircraft.
Given that the ball appears to be the only way to know, and it's not particularly sensitive at very small angles, is there some other way that a crew would know if they had a small amount of sideslip on the airplane?

[zerozero]

Yaw string?

:}

[Tinstaafl]

A US$13,683.32 optional kit from Boeing (and equivalent price for Airbus). Quite a bargain - you can choose the colour and it even comes with a precision cut flexible bond attachment device**




*Piece of sticky tape

[FCeng84]

A swept wing airplane in a sideslip will tend to require non-zero roll trim to keep from rolling. The need to insert roll trim would be an indication of sideslip. Procedure to minimize drag is to insert rudder trim as needed to zero out roll trim.

A difference between heading and track angles as displayed on the PFD would be an indication of sideslip in the absence of any crosswind.

The need to hold non-zero bank angle in order to fly a constant heading is also and indication of sideslip. Wings level plus sideslip yields a flat turn (i.e., heading will be changing).

[Shawn Coyle]

The issue is that in a twin engine jet (for simplicity), it's unlikely that the two engines would be producing exactly the same amount of thrust at the same N1 or EPR.
Some aerodynamic surface(s) would thus have to be deployed to keep the sideforce zero, which would mean extra drag.
How does sideslip get measured (the yaw string is a good idea, but needs to be lit at night...), and what can be done to ensure minimum drag?

[Pub User]

it's unlikely that the two engines would be producing exactly the same amount of thrust at the same N1 or EPR

Mr Boeing's machines very commonly have the same EGT, to the nearest single degree, so I imagine the thrust is fairly close to equal too.

[de facto]

As long as there is no lateral acceleration there is no side slip on my aircraft.
There is no ball in the 737 NG,a slip/skid digital indicator only.

[bubbers44]

Had a captain that insisted on flying in a side slip on a 737 200 to keep the ball centered. It drove me crazy so I used my screw driver to re align the ball so it was centered when the wings were level when he wasn't looking. If the wings are level on course you should have a centered ball. Just don't tell him what you did.

[Shawn Coyle]

Having matched EGT (or N1 or EPR or any other parameter) does not mean you'll have matched thrust.
And having no lateral acceleration does not mean there is no sideslip. (Helicopters have inherent sideslip with the ball centered).

[de facto]

And having two legs doesnt mean one can walk:E
Please explain ...i would like to know the hidden side slip that my instruments can not detect.
Thanks:O

[barit1]

Pub User:Mr Boeing's machines very commonly have the same EGT, to the nearest single degree, so I imagine the thrust is fairly close to equal too.

Having worked engine performance issues for over 35 years, I find this a truly amazing statement.

Typically pilots will choose to match N1's in order to reduce airframe vibration (this may be more true with tail-mounted engines), and this tends to equalize thrusts. But to expect matched EGT's is hardly reasonable.

(Besides, I never realized Mr Boeing made his own engines...) :ooh:

[john_tullamarine]

i would like to know the hidden side slip that my instruments can not detect.

Unless you have a vane (or the poor man's alternative - a yaw string) you will only be able to make some nominal and approximate inferences regarding sideslip - significant sideslip is easy - but when you get to small angles, pilot assessment is going to tend towards imagination.

The Concorde folk (http://www.pprune.org/1085835-post15.html), on the other hand, had it all laid out before them ...

[Denti]

The A380 has three vanes right in front of the flight deck on top of the nose which should be able to measure sideslip. No idea if it is indicated.

The skid/slip indicator on modern PFDs however is pretty sensitive, but alas has no scale to show degrees of sideslip. In normal cruise with a correctly rigged airplane there is no indicated sideslip and no control surface deflection. While thrust is not directly displayed (again, it is in the A380) the electronics are there to know and set it regardless and therefore equal thrust is nowadays a reality as well.

A yaw string of course is quite nice and extremely accurate. Having flown with strings but without engines it was a direct performance instrument and and became second nature to keep centered in those, hmm, 15 or 20 years i used it.

[Grum]

I am afraid to ask, but what does a 'Yaw string' look like? Is it like a plumb line? And what would I be looking for? I can see that if the aircraft was banked the line would be off centre, but would there be a displacement if there were yaw? I suppose there would be if the IRS is sensing it, and displaying it on the PDU.

I always have to trim left rudder as I accelerate, and there it remains for the flight, but it needs to be reduced in the descent eventhough speed is still high. So am I experiencing asymmetric thrust? FADEC controlled engines, N1 matched. Pretty new airframe too.

[FullWings]

...what does a 'Yaw string' look like?
http://www.alexisglynnlatner.com/blog/wp-content/uploads/2011/06/DSCN0290.jpg

Is it like a plumb line?Not as such...

And what would I be looking for? I can see that if the aircraft was banked the line would be off centre, but would there be a displacement if there were yaw?
It's the other way round. A well coordinated turn would see the yawstring stay close to the centreline of the aircraft; too much/little yaw would show up as a displacement to either side.

On the 777 I bring up the flight control synoptic and see if there is any bias in the roll controls, e.g. consistently more 'up' on one side than the other, then tweak the rudder trim until the control surfaces are neutral (in smooth air) or averaging around neutral (turbulent). This should produce the least drag configuration.

This is with the autopilot in and not too close to a waypoint, so the aeroplane is trying to fly straight to start with.

I'd prefer a yawstring but Boeing don't fit them, for some reason. Maybe transonic wool is hard to come by?

[de facto]

This is with the autopilot in and not too close to a waypoint, so the aeroplane is trying to fly straight to start with.

....and this is why the Boeing recommended method (fctm i believe)to do so is using the heading select mode for 30 secs then trim ...

Thanks JT.

[Daniel_11000]

A difference between heading and track angles as displayed on the PFD would be an indication of sideslip in the absence of any crosswind

… not at all ! ! ! When a symmetric a/c is flying with a correctly installed sideslip indicator , and zero bank angle , the difference between track and heading is only the wind correction angle ! no reference to sideslip at all ( which is zero) !
You can see this during an approach to landing with strong side wind component : if you approach with a sideslip, your sideslip indicator will show , obviously, off-centre,(though track and heading will be the same), but if you approach with a crab angle (zero roll angle), track and heading will be different , but no sideslip at all. Just change the phase of flight from ‘approach to landing’ to ‘cruise’ and you will surely better figure this .
D

[172_driver]

When a symmetric a/c is flying with a correctly installed sideslip indicator , and zero bank angle , the difference between track and heading is only the wind correction angle ! no reference to sideslip at all ( which is zero) !

Not correct really. Remember back to initial ME-training and engine failure practice. If you are flying wings level with an engine failure you WILL have a drift (..or side slip if you will) towards the dead engine. The use of rudder to counteract the engine failure will cancel the turning moments but there will be a net force pushing the aircraft slightly sideways through the air. This cannot be detected on the sideslip indicator (which will show zero slip) but the yaw string will tell you the truth.


You can see this during an approach to landing with strong side wind component : if you approach with a sideslip, your sideslip indicator will show , obviously, off-centre,(though track and heading will be the same)

Never done a sideslip/forward slip approach in a C172? Full rudder in one direction and opposite aileron to maintain runway centerline? Heading and track is different.

[FullWings]

....and this is why the Boeing recommended method (fctm i believe)to do so is using the heading select mode for 30 secs then trim ...
That would seem logical, for obvious reasons. Interestingly, the 777 in HDG or HDG HOLD turns a fair bit, often quite noticeably... Answers on a postcard as to why that should be. :)

[Daniel_11000]

<<wings level with an engine failure you..>>

This configuration is not what I consider for ‘symmetric aircraft’


<<.. obviously, off-centre,(though track and heading will be the same )>>

You are right, I intended ‘track and heading will be constant’, meaning
‘ aircraft not turning’ .

D

[FLEXPWR]

On the A320,

MCDU -> AIDS -> ALPHA CALL UP -> ALPHA DISPLAY, then type SLP1 (or SLP2). it will show the estimated sideslip with a good resolution (100th of a degree if I remember). You could also look at the drift angle to cross check if any doubts, by typing DA1.

Flex

[Daniel_11000]

<<You could also look at the drift angle to cross check if any doubts, by typing DA1.>>

This can be helpful to visualize drift angle, but drift angle has nothing to do with a sideslip.
D

[FLEXPWR]

Right, I should have added "in no wind condition", so to have an extra reference. Althought I concede it is hard to have zero wind in cruise.

If you still read a drift with a known no xwind condition, then the measuring or calibration may not be accurate.

[john_tullamarine]

This cannot be detected on the sideslip indicator (which will show zero slip) but the yaw string will tell you the truth.

Which sideslip indicator did you have in your initial ME training ? I was limited to simpler aircraft which didn't have such gadgets .. nothing even on the 737/727 told me much about slip ? ... on the other hand, love B's Concorde slip gadget ..

If you are referring to the ball, this should be viewed as a lump of stuff on the end of a bit of string and sensitive to forces - it has no way of knowing what the air outside is doing ? Indeed, one can make it show any indication one might find attractive by manipulation of the controls.

Likewise, wings level after the initial things are done and the aeroplane has settled into steady state flight, it will just show the net force direction in its plane of operation .. ie gravity going straight down. I can't really see how one can deduce much at all about slip angles from a balance ball ?

I am quite uncomfortable with the apparent views of folks that the ball is a slip indicator ... albeit that it generally will give gross indications.

The yawstring, once "calibrated" (if I may misuse that term) will, indeed, tell you a lot about what the wind outside is doing.

[Shawn Coyle]

Thanks to all for the inputs - exactly the very high level of discussion I've come to expect from pprune!!!
Keep it coming!
The slip ball measures something, but what exactly it measures is hardly ever discussed technically. It certainly does not measure sideslip!!