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737-800 Rudder Crossover Speeds

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737-800 Rudder Crossover Speeds

Old 27th Jan 2002, 08:34
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ScoobySnacks
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Post 737-800 Rudder Crossover Speeds

Anybody know where I can find some rudder crossover speeds for various flap configurations on the 737-800?

I've scoured the web looking for a table or chart. No luck. All I've come up with is 190kts for flaps 1 for 737-300s.
 
Old 27th Jan 2002, 08:40
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The current minmaneuvering speeds are based on a margin above crossover speed.

What you are looking for is something like Vmu (minimum unstickspeed) which is used for other calculations but not directly tabulated.

Furthermore, Crossover speed is actually directly a relationship with angle of attack, and is indirectly arrived at via maneuvering speed (Eg go faster reduce angle of attack)

Cheers. .Wino
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Old 27th Jan 2002, 09:28
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Wino or anybody else for that matter...

Could you please explain to me why it is that below a certain speed, the roll authority of the ailerons are no longer able to oppose the secondary roll caused by the displaced rudder?

I would have thought that as the airspeed reduces, both the forces produced by the rudder and ailerons/spoilers reduce at the same rate and therefore the balance of forces is maintained.

But obviously that is not the case. So what are the aerodynamics involved? You say it has to do with the angle of attack. How so??

Thanks.
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Old 27th Jan 2002, 09:35
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I cannot speak to the 800, but consider a related problem.

If you do any sim exercises purportedly to look at crossover handling, be aware that some of the older software may not model the 737 characteristics well in this area.

I had the interesting experience on another model of doing some pre and post mod testing when the particular operator upgraded the software for the relevant simulator. The handling pre and post mod was chalk and cheese ... the earlier software being quite unconvincing, and the later exceedingly convincing and quite like the accident reports

Blip,

I suspect that you have omitted consideration of the following ..

(a) the aerodynamic lift forces will be a function of a number of variables including airspeed and incidence as you have observed. There is no reason, though, why these forces will vary in the same way with speed due to, for instance, the different lift curve characteristics of the particular controls.

(b) however, the effect of these forces on the handling depends principally on the moments generated by those forces (rolling and yawing moments) which depend both on the force and the distance of the mean force vector from the aircraft cg.

[ 27 January 2002: Message edited by: john_tullamarine ]</p>
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Old 27th Jan 2002, 10:18
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Blip,. .I have no first hand knowledge of the B737, but if it has a rudder ratio changer system, as speed decreases the rudder throw increases and hence force exerted may stay more or less constant, while the ailerons lose effectiveness.

[ 27 January 2002: Message edited by: mustafagander ]</p>
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Old 28th Jan 2002, 08:48
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No the fitment of rpr (rudder pressure reducer),s actually is part of the fix to resolve the issue of crossover speeds. As i understand it below 1000ft agl only half the flow rate is available to the rudder (same psi) in the event of an engine failure full rate is restored, however the change over is seamless to the pilot thus very little info is published to crews.. .Regards the 800 series there is probably no published figures for crossover speeds as the min man speeds more than cover them. This assumption is made on historical grounds as there were never crossover speeds published for the classic series until the rudder issue arose and it was found that some min man speeds were below crossover speeds. Thus the 700/800 were certified with this knowledge so i think they left those speeds in the don,t need to know basket.. .Blip i,ve pondered the same question, but i can assure you from lot,s of work in the sim that it is definitly the case that more speed will keep you upright, maybe the aielerons and spoilers have a completely different effectiveness curve over the speed range than the rudder does ???
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Old 28th Jan 2002, 13:31
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Domestic,. .A rudder ratio changer as fitted to the B747 is a system whereby the rudder deflection increases/decreases inverse to speed keeping the force produced by the rudder with a certain pedal input essentially constant. There is no modulation of rudder hydraulic pressure.
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Old 28th Jan 2002, 14:38
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domestic,

That is just the problem I referred to above ... how have you validated the particular sim software for such a specific non-routine point in the envelope ? ... if you haven't, then your observations in the sim may have little validity.
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Old 29th Jan 2002, 04:20
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For anyone who has an interest in the practical side of cross-over alpha can I recommend (without reservation) a video produced by American Airlines Training Captain Warren Vanderberg. It is one of a series which he made, essentially dealing with performance issues in a variety of scenarios - unusual attitude recovery, wind shear, micro-burst, etc. One of the series deals with the issue of cross-over alpha and includes a set of video compilations from Boeing's Flight test division demonstrating the onset and both recovery and (initially) failure to recover. It is certainly the best articulated exposition of the phenomenon that I have seen and puts the whole subject in context. . .While the primary area of interest is obviously the 737, it makes the point that this is a generally alpha related condition and could be of concern to any aircraft operators. Beg, borrow or steal a copy, it is exceptional value.
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Old 29th Jan 2002, 08:39
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Sorry Scoob. I can't answer your question. But at least with the convesation that's followed, it'll keep it the subject at the top of the screen and eventually someone will be able to give you the answer you're looking for.

The concept of crossover speed relates to the situation where the rudder has displaced from centre due to a malfuntion in the actuator system and is stuck in a fixed position. I have been led to believe that no mater what the airspeed, the rudders's position is fixed (Is that incorrect?). Therefore rudder ratio devices etc don't come into it.

As the airspeed reduces, you gradually need to use more and more opposing aileron to keep the wings level. Actually you need a little bank angle otherwise the heading will be changing, a flat turn I suppose you'd call it. Eventually you reach the stage where the airspeed has reduced so much that you have full aileron and the thing just begins to roll in the direction of the rudder displacement.. .Of course this is all happening with symetrical thrust. We found that a great deal of control can be restored if you introduce asymmetrical thrust. A bit like a engine failure situation only the rudder deflection came first.

The landing would be interesting. Haven't tried that one yet.

Still wondering why it happens. I hear what you say john_tulla in your point (a) but I'd love to see it (the graph) for myself.

Cheers.
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Old 29th Jan 2002, 17:59
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Cornish Jack,. .I agree wholehartedly! What an exceptional piece of video. Excellently explains the importance of understanding the shortcomings of today`s automated cockpits.. .In our company we dedicated simulator time to let all pilots try the unusual attitude recoveries as explained by mister Warren Vanderberg.

[ 29 January 2002: Message edited by: static ]</p>
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Old 31st Jan 2002, 01:22
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Static. .Nice to see that the video is getting around but nothing like as widespread as it should be. I cannot recall anything in the training arena which has made such an immediate and lasting impression. Having watched the series on at least 30 occasions, I still find new bits of value. One particular area is being able to make note of the height loss incurred in the 'natural' (maintain your height) reaction as against the 'trained' response of gaining airspeed and ignoring the height loss. Fascinating stuff !!
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Old 31st Jan 2002, 05:23
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Cornish Jack,

Not familiar with the videos to which you refer .. can you assist with contact details for further info ?
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Old 31st Jan 2002, 19:44
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John T. .Have a feeling that there may be difficulties in easy access to the vids 'Dunnunda'. We only came upon them by lucky chance. E-mail me at [email protected] and I'll see if I can offer some help. . .What I can say, again, is that they are the most worthwhile training videos I've come across for anyone involved in the aviation business.... and NO, I don't have a commercial interest and I don't know Capt Vanderberg personally.
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