SIDE SLIP ANGLE

[Intruder]

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.

Yes, but its track is significantly different from the 90 deg heading. The current is the same as the wind. Assuming constant current, if the heading is changed, the track will also change, and the calculation of the current will remain constant.

However, if a small amount of rudder angle is applied to the boat, the boat will turn and the track will continuously change. An IRS would sense the rotational acceleration (maybe you forgot to give credit for this?) as well as the heading change. If sensitive enough, it would also calculate an instantaneous slip angle as the boat turns, based on the previous calculation of the current.

If the current velocity changes, it would also be detected by a sudden sideways acceleration. Transient calculations would be iterative until a steady state is achieved.

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.

Please post a couple of the URLs that you found to support that theory. I suspect you are oversimplifying the vector math.

[bubbers44]

Once stabilized in the stream no rudder is required either in boat or airplanes because once stabilized no rudder is required. Just hold the heading because you are flowing with the wind or current.

[JABBARA]

P. Bonsai and IGh
Thank you for your explanations; this was the answer that I was seeking for. So Now, I know the sideslip can be related to difference between AOAs, measured by symmetrical positioned vanes on both sides of fuselage. That makes sense. I believe, for Airbus FBW s, this is the case because there are 3 AOA vanes and 2 of them are symmetrically positioned on sides of fuselage. In airliner, normally this information is not needed to be indicated to pilot, except for the case of engine failure. A twin, with wing mounted engines, is prone to a large side slip angle (so high drag) in the case of engine failure. At critical phases of flight like takeoff phase (which has already lots of drag. I.e. Flaps etc.), the added side slip drag with a deficiency of 50% of thrust may not be acceptable. Now exactly at this moment (namely when needed), Airbus Electronic Slip Skid Indicator turns to show Sideslip angle which is known as Beta Target. This function is named aftersymbol, because (I am sure you are well aware than me) it is used to show side slip angle in Flight Mechanic diagrams.

In fact, after your replies, I found this document for GOODRICH Multi Function -Smart Probes (http://www.goodrich.com/gr-ext-templ...tot-Static.pdf), which is also explaining how to find local sideslip angle with static pressure differences from the side located ports.

John T.
I fully respect your vast knowledge, but I guess as mentioned above, finding Side Slip Angle by IRS, at least, is not a preferred method. I have also basic but limited knowledge on IRS; IMHO, only the way, if IRS can detect instantaneous sideway acceleration, this may be converted to side slip angle; however the problem is –as you mentioned- IRS works in inertial reference systems and Air mass may also be accelerating and decelerating and cannot be assumed as Inertial reference system. In addition to this, due to cumulated errors (Although somehow eliminated by Kalman filters) may not be precise and accurate enough to drive Beta Target indication on EADI of Airbus FBW.

Intruder
I would like state that I also respect your vast knowledge. From your last reply, if I understand correctly, you are also mentioning theoretically, IRS may be calculating Sideslip from instantaneous lateral acceleration. I am not saying “not possible”, but my idea is as I wrote to J.T.

Away from this subject here are some URLs for OEI flight characteristics.

http://www.cast-safety.org/pdf/5_asymmetric_flight.pdf

http://www.avioconsult.com/downloads...d%20Flight.pdf

Department of Aviation Animation, Click one engine inoperative aerodynamics

Engine inoperative principles in a twin airplane | pilotscafe.com Animation


Again thanks to all replies

[Intruder]

Read my Post #21 once again. I think you are still dismissing the IRS ability to sense rotational (or angular) acceleration.