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No Split Rudder On B787?  
User currently offlineCaptainKramer From United Kingdom, joined Feb 2012, 225 posts, RR: 0
Posted (2 years 3 months 2 days 19 hours ago) and read 5183 times:

I carried out a search and no answer was found.

I was at LHR on 27th of April 2012 to see the arrival of B787 from Gatwick as part of Boeing World Tour. The aircraft was towed to BA maintenaince Base. It was only then when seeing it in the flesh that I noticed for the first time a detail I had not seen in all the B787 photo's thus far. The B787 has no split rudder.

I understand basically that a split rudder is used to offset yaw in an engine out scenario. What I don't understand is why a spilt rudder on a B777 twin, but no split rudder on a B787 or a B757 or B767 twin for that matter?

Thanks in advance for any elaboration.

19 replies: All unread, jump to last
 
User currently offlineferpe From France, joined Nov 2010, 2800 posts, RR: 59
Reply 1, posted (2 years 3 months 2 days 19 hours ago) and read 5173 times:

I can venture a guess, the Vertical tail works in principle the same as a wing but with a symmetrical profile and a low load in the normal case. You need all it's force generating capability in the engine out situation. The lower part of the fin has the advantage that the body blocks the normal blow around of the end thus you can load it comparatively harder then the top part. For the shorter models of the 777, i.e. the -200 where your moment arm is the shortest (and perhaps for the powerfull 77W), you need that extra moment that the split rudder can give, for the 787 the yaw moment for the 788 is fine with a single rudder, the size of the fin being dictated by other criteria as well.


Non French in France
User currently offlineCaptainKramer From United Kingdom, joined Feb 2012, 225 posts, RR: 0
Reply 2, posted (2 years 3 months 2 days 19 hours ago) and read 5169 times:

Thanks ferpe,

I havebeen reading the tech ops for a while now and enjoying the in depth disccussions you guys have had re B787 and A350 wing aero dynamics and have seen moment arm mentioned before so having googled it I have a very basic understanding of what you said above.

So presumably in a high altitude engine out scenario the rudder on a B787 would only need to deftect a small degree in order to cancel out any adverse yaw and only experience higher loads, initially, until the Captain can get clearance to fly at a lower altitude or are you saying the fuselage moment arm would be sufficient to offset yaw alone.

Cheers Frank

[Edited 2012-04-28 06:42:47]

[Edited 2012-04-28 07:11:22]

User currently offlineferpe From France, joined Nov 2010, 2800 posts, RR: 59
Reply 3, posted (2 years 3 months 2 days 13 hours ago) and read 4995 times:

Quoting CaptainKramer (Reply 2):
So presumably in a high altitude engine out scenario the rudder on a B787 would only need to deftect a small degree in order to cancel out any adverse yaw and only experience higher loads, initially, until the Captain can get clearance to fly at a lower altitude or are you saying the fuselage moment arm would be sufficient to offset yaw alone.

It is more the engine out at start which is the critical case for the fin+rudder, you have low speed so the force generated by the combo is at it's lowest that is why you need to deflect the rudder to the max still leaving a little travel for corrections. The longer the combo sits from the engines causing the yaw moment the better, this is why you put those long tails on your model planes, you need moment (force*arm) not force alone to correct in pitch and yaw.



Non French in France
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (2 years 3 months 2 days 11 hours ago) and read 4919 times:

Quoting CaptainKramer (Reply 2):
So presumably in a high altitude engine out scenario the rudder on a B787 would only need to deftect a small degree in order to cancel out any adverse yaw and only experience higher loads

At high altitude the engines are putting out nowhere close to their full rated thrust so it doesn't take much force to counter the dead engine. Coupled with high airspeed, only a small deflection is required.

As ferpe noted, the worst case is when you're at highest thrust and lowest speed: takeoff. That's what sizes the fin and rudder in almost all twins.

Quoting CaptainKramer (Reply 2):
initially, until the Captain can get clearance to fly at a lower altitude

He doesn't need clearance; if he was at normal cruise, as soon as the engine goes he has no choice but to slow down or descend. He will normally notify ATC of what he's doing but you've lost the ability to hold altitude and the speed band is usually relatively tight so you're going to be defending soon whether you want to or not, whether ATC has cleared you or not.

Tom.


User currently offlineCaptainKramer From United Kingdom, joined Feb 2012, 225 posts, RR: 0
Reply 5, posted (2 years 3 months 2 days 10 hours ago) and read 4897 times:

Thanks Tom,

What would the danger be, apart from traffic behind catching up, if the Captain opted for a slower speed over losing altitude, having first trimmed out the adverse yaw of course, IIRC a Captain of an Air China B747SP failed to do this having lost an engine at altitude which resulted in some high altitude acrobatics.

I was also wondering in an engine out scenario on a Boeing at altitude, would inboard aileron deflection to initiate a roll be greater because of asymetric thrust and at lower altitude would the same hold with outboard aileron activated, would deflections need to be greater, or if rudder pressure is trimmed out would aileron deflection forces remain the same. How would the afermentioned scenario play out on an Airbus.

[Edited 2012-04-28 15:02:31]

User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 6, posted (2 years 3 months 1 day 22 hours ago) and read 4744 times:

The difference between Mmo and Vs[tall] is usually rather small at cruise altitudes. You have a rather good chance of being unable to keep the speed above stall with engine out, especially on a twin.
And if you ever let it fall below Vs, assymetric thrust will be quite problematic in that the plane could easily develop some kind of urrecoverable spin or spiral.

Regarding control forces - logic tells me they should be more or less the same, unless hydraulic power is affected, but I am not totally sure.
There would be no force difference on an Airbus, but there might be response difference, I dont know.



The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 7, posted (2 years 3 months 1 day 16 hours ago) and read 4646 times:

Quoting CaptainKramer (Reply 5):
What would the danger be, apart from traffic behind catching up, if the Captain opted for a slower speed over losing altitude

You will stall rather quickly; you can only trade speed for altitude for a brief period before you're going so slow that you're either going to stall or have to descend. A modern twin doesn't have enough thrust to hold normal cruise altitude after an engine failure, regardless of speed. You are going to descend to the engine-out altitude in pretty short order, whether you want to or not.

Quoting CaptainKramer (Reply 5):
I was also wondering in an engine out scenario on a Boeing at altitude, would inboard aileron deflection to initiate a roll be greater because of asymetric thrust and at lower altitude would the same hold with outboard aileron activated, would deflections need to be greater, or if rudder pressure is trimmed out would aileron deflection forces remain the same.

Aileron deflection would change. In order to counter the asymmetric yaw, the rudder has to come in. This causes roll-due-to-rudder (all modern airliners have rudders only above the fuselage). The ailerons need to deflect to counter that roll. This biases the ailerons so, to first order, the amount of deflection to initiate a roll is about the same but the aileron is starting from a different point so the total deflection is different.

Aileron deflection *forces* may be the same or different, depending if the aileron deflection to counter the rudder is coming from the flight control computers (FBW)/aileron trim (same forces) or if the pilot is just holding it (different forces).

Quoting CaptainKramer (Reply 5):
How would the afermentioned scenario play out on an Airbus.

Airbus uses roll-rate control; the ailerons will deflection automatically to counter the roll due to rudder. Control forces and sidestick deflections will remain the same. Aileron deflections will be biased to counter the rudder roll.

Quoting Fabo (Reply 6):
And if you ever let it fall below Vs, assymetric thrust will be quite problematic in that the plane could easily develop some kind of urrecoverable spin or spiral.

It shouldn't. Unless you drop below Vmca (usually below Vs except at low weights and high thrusts) the vertical fin should still be able to keep you going basically straight ahead.

Quoting Fabo (Reply 6):
Regarding control forces - logic tells me they should be more or less the same, unless hydraulic power is affected, but I am not totally sure.

On any modern irreversible hydraulic control system, the forces relative to stick/yoke deflection should not change. The forces are all artificial anyway.

Tom.


User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 8, posted (2 years 3 months 1 day 14 hours ago) and read 4603 times:

Quoting tdscanuck (Reply 7):
It shouldn't. Unless you drop below Vmca (usually below Vs except at low weights and high thrusts) the vertical fin should still be able to keep you going basically straight ahead.

Good to know. I did not look at it that way.



The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
User currently offlineCaptainKramer From United Kingdom, joined Feb 2012, 225 posts, RR: 0
Reply 9, posted (2 years 3 months 1 day 12 hours ago) and read 4534 times:

Thanks for all your responses.

I have one more query. How does a tailess aircraft, such as a B2 bomber, combat adverse yaw in an engine out scenario?


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 10, posted (2 years 3 months 1 day 8 hours ago) and read 4460 times:

Quoting CaptainKramer (Reply 9):
How does a tailess aircraft, such as a B2 bomber, combat adverse yaw in an engine out scenario?

Same way it controls yaw normally...split ailerons/speedbrakes. Just more input required when an engine is out. Keep in mind that the B-2 has four engines relatively close to centerline, so adverse yaw isn't as bad for them as it is for most twin airliners.

You can see the split ailerons in action here:


Tom.


User currently offlinejetpilot From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 11, posted (2 years 3 months 23 hours ago) and read 4349 times:

A split rudder is powered by two different hydraulic systems for redundancy. On the 727 for example the upper rudder is powered by B system electric pumps and the lower rudder is powered by A system pumps which gets their power from the engine driven pumps. If you lose either A or B system you still have rudder control. The 737 doesn't have a split rudder either. It's the only other Boeing without a split rudder. Why? I have no idea. Maybe the 787's single rudder is powered by multiple hydraulic systems PCU's.

User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 12, posted (2 years 3 months 19 hours ago) and read 4265 times:

Quoting jetpilot (Reply 11):
The 737 doesn't have a split rudder either. It's the only other Boeing without a split rudder. Why? I have no idea.

Mechanical backup?



The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
User currently offlinejetpilot From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 13, posted (2 years 3 months 14 hours ago) and read 4182 times:

The 737 rudder doesn't have manual reversion. If the PCU fails or you lose hydraulic pressure you have no rudder.

User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 14, posted (2 years 3 months 14 hours ago) and read 4177 times:

Quoting jetpilot (Reply 11):
Maybe the 787's single rudder is powered by multiple hydraulic systems PCU's.

It does. Loss of any one hydraulic system won't lose the rudder (it might have all three but I'm not sure).

Tom.


User currently offlineFabo From Slovakia, joined Aug 2005, 1219 posts, RR: 1
Reply 15, posted (2 years 3 months 10 hours ago) and read 4087 times:

Quoting jetpilot (Reply 13):
The 737 rudder doesn't have manual reversion. If the PCU fails or you lose hydraulic pressure you have no rudder.

My bad.



The light at the end of tunnel turn out to be a lighted sing saying NO EXIT
User currently offline737tdi From United States of America, joined Sep 2007, 790 posts, RR: 1
Reply 16, posted (2 years 2 months 4 weeks 1 day 7 hours ago) and read 3878 times:
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Quoting jetpilot (Reply 13):
The 737 rudder doesn't have manual reversion. If the PCU fails or you lose hydraulic pressure you have no rudder.




That's not true. Sort of. You are right it has no manual reversion but it does have the standby pump and pcu. This is a totally separate actuator and hydraulic system. So with a total failure of Sys. A and B you still have rudder control with the standby system.


User currently offlinejetpilot From United States of America, joined May 1999, 3130 posts, RR: 29
Reply 17, posted (2 years 2 months 4 weeks 1 day ago) and read 3787 times:

That's true... I forgot about the STBY Rudder Pump .

User currently offlineflashmeister From United States of America, joined Apr 2000, 2900 posts, RR: 6
Reply 18, posted (2 years 2 months 3 weeks 2 days 14 hours ago) and read 3376 times:
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Quoting CaptainKramer (Reply 5):
IIRC a Captain of an Air China B747SP failed to do this having lost an engine at altitude which resulted in some high altitude acrobatics.

This was actually China Airlines, not Air China.
http://en.wikipedia.org/wiki/China_Airlines_Flight_006


User currently offline787atPAE From United States of America, joined Oct 2006, 143 posts, RR: 4
Reply 19, posted (2 years 2 months 3 weeks 2 days 8 hours ago) and read 3304 times:

Quoting tdscanuck (Reply 14):
(it might have all three but I'm not sure

There are three rudder PCUs. Each PCU is tied to a specific hydraulic system (L, C, R). That's gotta make for one helluva force fight situation. I'm really liking the EHSA / EBHA combination for two actuators on the current airplane I'm working on, but that's a different can of worms...

Also, if you're on RAT power, you still have a single rudder actuator for the 787.


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