> >
>

# SIDE SLIP ANGLE

Tech Log The very best in practical technical discussion on the web

# SIDE SLIP ANGLE

6th Aug 2012, 16:23

Join Date: Feb 2001
Location: Milkway Galaxy
Posts: 240
Likes: 0
Received 0 Likes on 0 Posts
SIDE SLIP ANGLE

Question is simple, but I guess answer is difficult

What is the method of estimating/measuring of Angle of Sideslip (AOS-Beta), in modern airliners like Airbus FBW?

I would like the emphasise, I am NOT asking SLIP or SKID, measurment, that is simple.

Last edited by JABBARA; 6th Aug 2012 at 16:24.
6th Aug 2012, 17:59

Join Date: Feb 2001
Location: Milkway Galaxy
Posts: 240
Likes: 0
Received 0 Likes on 0 Posts
Intruder
Thank you for reply but I do not consider it is correct.

If you know heading and track you can derive the wind (this is ususllay calculated by IRS) Or if you know heading and wind, you can calculate track (as conventional way). An airplane can fly with a very strong crosswind with zero-sideslip

But my question is completely different, that is the Realtive Wind which is hitting to airplane not exactly from nose but from sides as increasing drag. This may be because of, either rigging of plane or because of mistrimming of rudder at an engine failure of a twin.

I know on some little planes or gliders there is a whip in the middle of canopy (outside) which is aligned to Relative wind gives an idea about where the Relative wind is hitting the airplane to help trimming. What I am asking is the other method normally used in airliners. I am sure it is measured but I do not understand how?
6th Aug 2012, 19:03

Join Date: May 2000
Location: Seattle
Posts: 3,195
Likes: 0
Received 0 Likes on 0 Posts
I deleted the answer right after I posted it, because it didn't sound right in retrospect.

There is no angular measurement that is available to the pilots. There is only a qualitative measurement given by the slip indicator. The old style indicator is merely a ball inside a curved tube. The electronic indicators are driven, IIRC, by the IRS accelerometers.

The AV-8 Harrier is the only airplane I know of that has an actual "weather vane" on the nose to show sideslip/crosswind angle. I don't know if it drives a cockpit gauge, or is used simply as the yaw string is on a glider or F-14 Tomcat.
6th Aug 2012, 19:38

Join Date: Oct 2009
Location: UK
Posts: 1,270
Likes: 0
Received 0 Likes on 0 Posts
Hi JABBARA,

Intruder is correct. The slip is an indication of the lateral acceleration measured by the IRS.

The Beta Target is a slip indication which is modified to give minimum aileron displacement and roll spoilers. Since following the Beta Target results in a small side slip, it will be necessary to apply a constant bank into the live engine(s) to maintain a constant heading.
6th Aug 2012, 19:50

Join Date: Feb 2001
Location: Milkway Galaxy
Posts: 240
Likes: 0
Received 0 Likes on 0 Posts
Intruder,

Now I agree somehow.
I am an Airbus driver and similiar to Boeing the small index below the sky pointer on EADI shows lateral acceleration, which is equal to skid ball on conventional planes.

Why I am sure somehow Side slip is measured or estimated in Airbus, because that slip skid indicator on Airbus changes to show side slip indicator in Blue Color (They call Beta Target) to show the amount and direction of side slip in the case of Engine Failure at take off. And the pilot is advised to use that side slip indicator to minimize the side slip which also minimizes the overall drag of the plane at that critical situation.

My curiosity, there is no weathervane -as you also mentioned- on the plane to measure the sideslip. On photos of A 380 showing the nose section, I noticed some weathervanes just in front of windshield which is aligned with longitudinal axis, whic I guess is used to measure Sideslip. But not on the other Airbuses. So I guess there is another method to measure sideslip, for example on A 330, which may not be so accurate as A 380 though.

That is I am wondering about.
6th Aug 2012, 20:06

Join Date: Feb 2001
Location: Milkway Galaxy
Posts: 240
Likes: 0
Received 0 Likes on 0 Posts
Hi rudderrudderrat
Thanks

FCOM explains that as minimizing aileron input we can get minimum sideslip, but infact, as way of Flight Mechanics work, reverse is true: As minimizing sideslip we can get minimum aileron deflection. So Beta target information input should be sideslip, not aileron amount.
And also as you mentioned, on asymmetric thrust scenarios (like one engine failure on twins), to maintain a constant heading with zero sideslip, the airplane Definitely (there is no another way) should be banking a few degrees on good engine side, which again definitely requiring some aileron input.

Last edited by JABBARA; 6th Aug 2012 at 20:08.
6th Aug 2012, 21:19

Join Date: Oct 2009
Location: UK
Posts: 1,270
Likes: 0
Received 0 Likes on 0 Posts
Hi JABBARA,
the airplane Definitely... should be banking a few degrees on good engine side, which again definitely requiring some aileron input.
Not necessarily.
The effect of applied rudder is to produce a sideways thrust component (slip).
If the aircraft is moving sideways at a constant velocity and with level wings, there would be no indicated slip sensed by the IRS (or by a ball in U tube). A swept wing would induce a roll which would require aileron input to counteract.
If the aircraft is banked into the live engines, then the side slip produced can balance the rudder induced slip - hence no aileron would be required. It would however produce an indicated side slip, hence the Bus displays a Beta target instead.

Last edited by rudderrudderrat; 7th Aug 2012 at 13:20. Reason: typo
7th Aug 2012, 17:46

Join Date: Feb 2001
Location: Milkway Galaxy
Posts: 240
Likes: 0
Received 0 Likes on 0 Posts
Hi rudderrudderrat

The problem is the confusion of description of Sideslip.
Probably we understand different things from the concept of Sideslip.

The way what I understand is completely different than the way Airbus put to FCOM but completely same what The Boeing describes.

The index below the sky pointer in EADI is working with the same way as conventional ball on small planes: Indicating lateral accelaration, or in another word indicating a coordinated turn (a consistency between bank angle and turn rate) when it is centered. Boeing names this as Slip-Skid indicator (correct), Airbus names it as Side slip indicator; unless it turns to blue color and changes its function (Beta tareget), the naming of Airbus is wrong.

In Flight Mechanics Side slip is irrelevant with lateral accelaration; it is a constant angle of the wind which is hitting the plane because of the Aerodynamic forces not equally distributed on the plane and/or because of asymmetric thrust. However at the end, the total of these forces is still equal to zero which means there is no accelartion in any direction. In this meaning, there is no option to achieve a zero side slip in one engine failure case, unless a slight bank on good engine side. This is difficult to explain as simply writing but there are a lot of documenst in web with quite details in pictured way.

However my question is still away from this discussion.
How this side slip (in the meaning of angle of Relative Wind hitting to plane) is measured or estimated?. It can not be a function of IRS because it is not related to acceleartin in any direction.

Meanwhile, I confirmed for A 380, those vanes in front of windshield is measuring sideslip. Google it.

Thanks
7th Aug 2012, 19:36

Join Date: May 2000
Location: Seattle
Posts: 3,195
Likes: 0
Received 0 Likes on 0 Posts
How this side slip (in the meaning of angle of Relative Wind hitting to plane) is measured or estimated?. It can not be a function of IRS because it is not related to acceleartin in any direction.
Explain to us how you perceive a sideslip without any acceleration.

If you apply rudder to induce yaw only, the airplane will slowly turn, and a lateral (centripetal) acceleration will be perceived.

If you apply rudder and then use aileron to prevent the turn, acceleration due to gravity will change in direction relative to the airplane. The acceleration vector of the airplane to counter that gravitational acceleration will change accordingly.
7th Aug 2012, 20:05

Join Date: Oct 2009
Location: UK
Posts: 1,270
Likes: 0
Received 0 Likes on 0 Posts
Hi Intruder,
Explain to us how you perceive a sideslip without any acceleration.
The resultant of asymmetric engine thrust and rudder side load would produce aerodynamic slip without any acceleration.

Fortunately it is easily felt on a conventional swept wing aircraft, because you'd need aileron to counteract the induced roll.
Unfortunately you can't feel it on FBW Air Bus in Normal Law.
7th Aug 2012, 20:27

Join Date: Feb 2001
Location: Milkway Galaxy
Posts: 240
Likes: 0
Received 0 Likes on 0 Posts
Hi Intruder,

I apologize, it may be may English but, I didn`t understand what you mean with this:
acceleration due to gravity will change in direction relative to the airplane. The acceleration vector of the airplane to counter that gravitational acceleration will change accordingly.
In a level turn the gravity is not a factor because it is perpendicular to plane of turn. If we are turning level, there will be no acceleration due to gravity.

I am not talking about yaw, yes yaw is accelerative, that is the change rate about the vertical axis and this can be sensed by IRS (more technically by seperate rate gyros dedicated for detection of yaw and turn coordination).

However for example in asymmetric thrust case like engine failure, yaw is NOT a continuos maneuver. Even if you do not apply any counter rudder, eventually (after the RW angle to the fuselage reached a certain angle) it will stop, (provided, by aileron, you stop any yaw induced bank tendency).

I am asking about the later phases once yaw stops and a certain side slip angle is reached. After this angle is reached the motion of airplane is not a turn any more just a straight unaccelerated flight. Only the difference, airplane is not moving in the direction where the nose is pointing but sideway. This causes a big drag which is not good if only one of the engines is running.

My question is, how this reached RW angle is measured to drive beta target (airbus) on EADI. Because when you apply enough rudder to center Beta target, means your sideslip is zero, minimum drag which is essential for OEI climb after take off.

Regards

Edit reason is typing

Last edited by JABBARA; 7th Aug 2012 at 20:49.
7th Aug 2012, 20:53
Moderator

Join Date: Apr 2001
Location: various places .....
Posts: 7,179
Received 93 Likes on 62 Posts
It can not be a function of IRS because it is not related to acceleartin in any direction

I have no idea just how the various boxes process sideslip in the real world. However, the above statement certainly is not correct.

If the kit detects an acceleration, it can integrate that observation to figure a velocity. If the acceleration then stops, the velocity doesn't (in the absence of an additional, different acceleration). Knowing the side velocity and forward velocity vectors, it is a trivial calculation to figure slip angle.
7th Aug 2012, 21:19

Join Date: Aug 2005
Location: fl
Posts: 2,525
Likes: 0
Received 0 Likes on 0 Posts
When I was an FO one captain would center the needle and fly all day with one wing down in a side slip. I brought a small screwdriver our next trip and after we landed I adjusted the indicator after he left the cockpit to make him fly right. If the wing are level and you are holding a constant heading you are trimmed properly.
7th Aug 2012, 21:38

Join Date: May 2000
Location: Seattle
Posts: 3,195
Likes: 0
Received 0 Likes on 0 Posts
However for example in asymmetric thrust case like engine failure, yaw is NOT a continuos maneuver. Even if you do not apply any counter rudder, eventually (after the RW angle to the fuselage reached a certain angle) it will stop, (provided, by aileron, you stop any yaw induced bank tendency).
Try it some time. Simply add (or reduce) some thrust on 1 engine. Keep the rudder centered and use aileron to keep bank angle at 0. Tell me if the airplane turns or not.
7th Aug 2012, 21:43

Join Date: Feb 2001
Location: Milkway Galaxy
Posts: 240
Likes: 0
Received 0 Likes on 0 Posts
Hi JT.

No. The last part of your reply is not correct. Or I can say it is correct if there were no actual wind which is the movement of air mass where the plane is flying. But in reallife this is not possible, always there is wind.

Yes The IRS calculates velocity vector (so the position) as you explained, but reference is geographical position, in another word, earth, not airmass. So, as in your explanation, if they are different (heading and track), algebrical subtracting true heading from true track may give you an idea about where the air mass (Relative wind) is hitting the plane.

But this is not the case, the air mass where we are in also moving.

Consider a boat is crossing the river with 90 degrees. Relative to bank airplane is moving sideway (similiar to IRS calcualation); but relative to river airplane is moving straight, no sideslip. Water is hitting from nose.
7th Aug 2012, 21:57

Join Date: Feb 2001
Location: Milkway Galaxy
Posts: 240
Likes: 0
Received 0 Likes on 0 Posts
Intruder

No need to try, You are right, yes it turns,
Well, therotically it should not turn. All the documents and force vector diagrams say so, google it.

I am not an expert but I can explain like this: Therotical explanation of sideslip is expressed with a few forces only: e.g, Trust, Drag, Fuselage Side Force, Rudder force. But in real life each component on the plane like fins strips, winglets and maybe hundreds more have their own drags and moments. So in reallife the overall effect all these may be causing a kind of creeping turn.

Anyway I got away againfrom my question

Regards
7th Aug 2012, 22:35

Join Date: Apr 2012
Location: Southern Jessieland
Posts: 101
Likes: 0
Received 0 Likes on 0 Posts
Measurement of Sideslip

The direction of incident airflow can be determined by air flow direction sensors mounted on either side of the fuselage near the nose.

The airflow direction sensors can be vanes or multi-holed probes.

The average of the 2 air flow angles will indicate angle of attack, the difference between the 2 air flow angles will indicate sideslip angle.
7th Aug 2012, 22:41

Join Date: Feb 2001
Location: Milkway Galaxy
Posts: 240
Likes: 0
Received 0 Likes on 0 Posts
Hi P Bonsai

I also consider some probes/vanes needed for sideslip measurement

he average of the 2 air flow angles will indicate angle of attack, the difference between the 2 air flow angles will indicate sideslip angle.
Can you give more details

thanks
7th Aug 2012, 23:09
Moderator

Join Date: Apr 2001
Location: various places .....
Posts: 7,179
Received 93 Likes on 62 Posts
But in reallife this is not possible, always there is wind.

Are you comfortable that you are not confusing drift and slip ? Chalk and cheese, I suggest .. Just how the box does it all is not in my ambit but it appears not to be a massively difficult exercise albeit that the real installations would be expected to use additional inputs to resolve any conflicts in the sums.

In the real world, however, a yaw vane/string makes it a doddle.

but reference is geographical position, in another word, earth, not airmass.

The accelerometers are looking at inertial references. The box then does whatever sums it is programmed to do to come up with some half plausible story as to what the aeroplane might, or might not be, doing. (Caveat - not my area of expertise so I can only look at things generically)
8th Aug 2012, 00:22

Join Date: Apr 2012
Location: Southern Jessieland
Posts: 101
Likes: 0
Received 0 Likes on 0 Posts
Details

The airflow around a cylindrical fuselage is essentially linear. At any point on the fuselage surface the local incident flow angle that can be measured by a vane (say) and the incident flow deduced.

alpha = K * alpha_vane + offset

K is between 0.5 to 1.0 (the local airflow angle can be up to twice that of of the free-stream angle)

The K factor and offset can be determined by wind tunnel or flight test.

A single vane mounted on the top or bottom of the fuselage centre line should sense the lateral flow of air but that tends not to be used as two vanes either side of the fuselage can determine both angle of attack and sideslip.

AoA = Ka * (vane_left + vane_right)/2 + offset_alpha

Sideslip = Kb *(vane_left - vane_right)

For symmetric airframes there should be no offset for the sideslip calibration. There will be an angle of attack offset due to wing/fuselage setting angle.

The airflow is linear over the normal flight flow angle range near the nose and the vanes are usually sited somewhere near the cockpit.

For increased reliability several vane sets can be used though they tend to be in close proximity to the sweet spot where the airflow is most linear in characteristic.

I'm not sure how common this is on airliners but angle of attack and latterly sideslip sensing has been used on military fast jet for aircrew indication (ask any fast jet pilot how easy it is to fly an approach using AoA), flight control systems and weapon aiming. Some attack helicopters also use airflow sensing in weapon aiming.

Sideslip is a useful indication of lateral departure - significant sideslip when you approach the stall angle of attack will precipitate a spin departure - the slip ball is not always a reliable indication of that in that it senses total aerodynamic, engine forces etc forces acting on the airframe.

In the Harrier's case the sideslip angle is of particular importance in jet-born flight near the ground as the the jet efflux circulating off the ground will move to one side when there is a sideslip and roll the aircraft over with fatal consequences. I don't know what the tolerance is but I'm told Harrier pilots watch that sideslip vane very carefully.