View Full Version : Rudder turns in an airliner...

5th May 2015, 11:51
This is a purely theoretical / flight dynamics oriented question by someone who IS NOT a 744 ( or other than gliders ) driver, but who is always interested in almost everything about Aviation ( mostly Civil ) and flight dynamics modeling in flight simulation.

Of course I am aware that such a maneuver, even for testing purposes, would never be executed IRL, if not for other reasons, because it would cause dangerous stress to the fin and rudders and even probably cause their partial or total tail separation. The reason is that I would like to evaluate a given B744 flight dynamics model for a flight simulator I use, and find if some maneuvers possible on that sim make sense / are plausible.

Ok, here it goes... If at cruising levels a Boeing 747-400 AP is disengaged and the aircraft flown manually, can a full 360º "flat" ( meaning wings level ) rudder turn be performed ? I mean, the pilot initiates rudder deflection towards max rudder ( left or right ), and then uses the yoke to try to keep wings as level as possible, thus setting a continuous sideslip situation.

1) Will there be significant yaw rate ? Can you give a guess of how many degrees per second ?

2) Since rudder ratio will be effective ( given no malfunctions ) I believe the outcome will be different if, say, the aircraft is being flown at 4000', and initial approach speed, or even partially configured for landing ( say flaps 5... ) ?

My guess is that the roll induced yaw as a consequence of using aileron cross-control will probably compensate the yaw initiated by the full rudder deflection, plus drag will be considerable, and sustained rudder turns would take "ages" to complete a full 360º ... but at the same time, and given the tail and rudder are designed to be really efficient, specially if you have to cope with a ( even possibly bi- ) engine failure after V1, I wonder if indeed such a maneuver could be performed and what the actual yaw rate would be, depending on speed and altitude...

Thanks for any answers :)

5th May 2015, 13:19
Technically yes, but at what rate? And how comfortable would it be? Those are other issues. And as you are positing that all your other control surfaces are functioning, it would be much much more efficient to use coordinated controls to initiate a balanced turn.

So an equally notable question is, why would you? Or is this purely a curiosity thing? ( if it is, cool, I approve of people seeking answers! That's science!!)

5th May 2015, 13:52
Good question :-)

Pure curiosity from someone who would like to validate a given flight dynamics model for a Boeing 744 :-)

5th May 2015, 14:50
You will also need to factor in the fact a B744 goes a lot quicker than a glider, and the load imposed on the rudder would be strong enough to rip it off. Boeing introduced a Rudder reducer to the B737 for this very reason.

5th May 2015, 15:27
Bear in mind also that in a 747, unlike a glider, there are people sitting a long way back behind you and any rudder waggling you undertake will spill their drinks.
Not to mention over-stressing the fin.
So it is just not done.

5th May 2015, 15:55
I am aware performing such a maneuver in a 744 wouldn't make much sense, but the question is more that of it being possible, and achieving a full 360º turn ( flat turn ) in around 1' 15''...

In a glider, as in most powered GA aircraft, the turn rate would be minimal, and normally requiring one of the wings to be down, because, for instance, under most circumstances if you can add opposite aileron to keep wings level, than the opposite yaw ( roll-induced in this case ) will compensate the "rudder-induced" yaw...

Add to this that at cruise, and with the yaw limiters ( rudder ratio ) operational, a full deflection of the rudder towards one of the sides wouldn't be as effective as if the aircraft was flying lower and a lot slower, right ?

5th May 2015, 16:45
The rudder probably won't come off. Only a Boeing test pilot can tell us if full side slip maneuvers are tested at high altitude.

Do a 360 in 75 seconds at that altitude? That's laughable. We don't do 3 degrees/sec at altitude so there's no way it's doing 4.8 degrees per second in a flat turn. And I wouldn't be surprised if spoiler drag makes the a/c turn in a direction opposite to the rudder deflection.

6th May 2015, 06:03
Are you alluding applying the 767 Gimli glider technique to a 744? I'm sure you can but not at high level - the fin load would be too high I think.

6th May 2015, 09:06
What you are discussing is purely theoretical and imaginary and if you are fond of physics. There is no question of any one performing such uncertified manoeuvres in commercial airplane. Use of full rudder can stall the fin just as wing stalls leading to rudder ineffectiveness, in trying to recover too much opposite rudder can result in tail separation. The engines may surge and stall. It's futile discussion.

6th May 2015, 09:40
It's a purely theoretical question, and of course I am aware that such a maneuver wouldn't even be tested IRL.

It is, as I somehow explained above, related to the evaluation of the flight dynamics in a simulated model of the B744.

I have edited the OP to make sure people reading this can better understand the scope of the question being asked.

@mikedreamer787: Mike ( I believe :-) ) not really, although I am well aware of that legendary use of the forward slip maneuver on an emergency landing in a 767. IRL I execute it quite often ( typical in gliders, specially when you have to land out.... ). But indeed my question is also related to the way sideslip / fwd slip physics are being modeled on the sim I allude to.

FE Hoppy
6th May 2015, 10:54
I think you would run out of thrust long before you completed 360degs

6th May 2015, 11:44
Yes Hoppy, I also believe, and others have suggested, engines might surge under such circumstances...

The actual scenario that gave rise to this question was an emergency situation in the simulator with a 747-400 with all 4 engines "off", and all of the associated malfunctions.

Rudder limiters will be unavailable under such circumstances.

The starting situation had the aircraft placed at around 6000', near an airport, and the task was that of trying to land it safely....

My first thought, since one of the rws available for the emergency landing was right ahead of me, but I was too high, was to act like the PF in the "Gimli glider event", and enter a forward slip to loose as much altitude as possible without gaining much speed, while lowering the flaps and gear ( using the the corresponding alternates )... But, as I started to fwd slip I found that I could add full rudder, then compensate with opposite aileron, and enter a "flat" turn, loosing altitude and some speed.... As the 360º turn completed, I was again pointing towards the rw, and ready to perform a "normal" glider-style approach :-) I managed to land the aircraft uneventfully ....

But... was this anyhow plausible for a real 744 ? I really don't think so... and although it is certainly not meaningful in terms of RL operations of the 744, it may well suggest some problems with the flight dynamics model. Of course, not being an aeronautical engineer, and even less a 747 driver... I couldn't resist to ask, as I did in the OP :)

6th May 2015, 23:55
Rudder limiters will be unavailable under such circumstances.

The upper rudder actuators incorporate hydraulic relief valves which I believe, allow a blow down function. i.e. If the rudder actuators are subject to too much force as a result of air loads, pressure relief should take place and the upper rudder should not break (in theory).

7th May 2015, 07:19
That fin seems pretty strong.

The old Northwest had an incident with a 744 in cruise flight where the rudder jammed hardover and they had to land like that.

Used differential thrust I think, did a good job, not sure if it was the lower or upper rudder but the fin stayed on !

7th May 2015, 08:46
Just a word on the 747 rudder ratio system - as IAS increases the rudder ratio system reduces the travel on upper and lower rudder.Both systems are independently operated and monitored by a comparator which will give a warning in the cockpit if the two rudders are not synchronised. At speeds greater than 428 kts rudder movement is restricted to 5deg in each direction. As far as blow back valves are concerned I have never heard of them being fitted to 747 rudder hydraulics - there are hydraulic fuses in each hydraulic supply line which will close in event of a leak downstream to prevent loss of system fluid.

Plumb Bob
7th May 2015, 10:55
Hello jcomm,

as a retired airline ops engineering guy and with 40+ years of glider and (TMG) power experience I am happy to confess to not just using sideslip techniques on final, but also at other times. Even climbing, during a (short) pull-up over the beginning of the airfield, to kill airspeed in and then land halfway near the hangars.

Already before flying for many years from a coastal airfield, I have always been keen on knowing how cross-controlled one can fly during crosswind operations. Because some gliders and TMGs are rather restricted in the slip angle that can be maintained in a straight line with full rudder and appropriate aileron input.

The German Schleicher Ka-8 from the 1960’s (now a vintage glider) is at the limit of rudder authority in a really effective sideslip.
The original Grob 109 TMG, often named the 109A, is worse, and will maintain only a small slip angle. It is very challenging to handle in a right-crosswind take-off.

I once had the joy of flying one of the few remaining Reinhardt Cumulus IIIf gliders. A kind of Grunau Baby, but with only a very narrow tail boom and hardly any fin surface, but a very large rudder. It seemed to want to rotate freely around the centre of pressures when I gave it some sideslip input!
The rather more modern Dimona and Super Dimona TMGs seem tot be happy in any cross-controlled situation. I guess the Katana and other Diamond types may well behave similarly.

So I have always been curious how far a particular airframe can be steered to withstand the normal weathervaning tendencies. Because you don’t want an invisible but powerful hand turning the landing strip away from you when practicing sideslips on short final.

To correct for wind variations and turbulence, and to point the flight path to exactly where you want to touch down, it may be necessary to diminish the bank angle in a slipping approach while maintaining full rudder. This then commences a form of rudder turn that you are curious about.

If planes can be made to fly straight in a steady sideslip, and pilots shall show proficiency in that technique, it follows that gradual turning flight in such a cross-controlled attitude must be feasible as well, and not just towards the low wing!

So I found out that my Standard Cirrus glider could be lazily turned, wings level, to kill altitude when it was time for my mate to fly. This turned out to be a fairly stable manoeuvre at low speed, with the advantage that nobody around you would be fooled to think that you were in a thermal, as happens when you circle down with airbrakes out.

Having experienced the slow rate of turn together with the considerable sink rate that comes with it, I would expect a real all-engines-out 747 (I can’t really say ‘dead stick’ when supposing the use of full rudder!) to require more than 6000 feet to complete one 360!

7th May 2015, 13:04
Going back to basic physics, in a flat turn the centripetal force must be derived from the lateral "lift" of the fuselage with the sideslip angle standing in for the angle of attack. I don't imagine the CL will be particularly large, but the area can be - especially on a 747 (or an A380!), which may actually be better at this trick than a glider with a thin tail boom.

7th May 2015, 13:28
Also worth remembering that airliners with 35 degrees of sweep back will have an awful lot of adverse roll secondary issues.

Boeing recommends landing in strong crosswinds with crab- basically landing sideways then correcting after main gear touchdown.

İt takes almost full roll control input to put a 737-800W down in gusty 34kt cross winds.... plenty of rudder available, the limit is roll control, keeping the wings level.

Airliner rudders are not there to side slip but to keep it straight under max take off power asymmetry. Outer engine in the case of the 747....

7th May 2015, 17:55
Again, thank you all so much for your posts. Each one has added interesting / important information !

Plumb Bob, I am in the process of converting my ICAO GPL license to an EASA SPL. I might consider adding TMG to it someday in the future, but there's only one TMG in Portugal ( HOFFMANN H36 DIMONA Mk.II :CS-ASJ )

Yes, each glider I have flown has it's own sideslip / fwd slip behaviour, and rudder / aileron authorities !

Chu Chu
8th May 2015, 01:38
In a glider, the centripetal force would come only from lateral "lift." Once a 747 is in a slide slip, though, the thrust from the engines will have a component toward the center of the turn. I'm not suggesting, of course, that this means rudder turns are easier in a 747.

8th May 2015, 09:59
Interesting observation Chu Chu, thx!, but actually in the situation I tested in the sim, the 4 engines were shutoff, thus creating only a lot of yaw and roll inertia... and drag too :-)

8th May 2015, 13:06
Quote by vilas: "The engines may surge and stall. It's futile discussion."

It's t'other way round. The airflow through the engine will be disrupted in severe side slip causing compressor stall closely followed by a surge, or multiple surges. :ok:

8th May 2015, 15:54
"Boeing recommends landing in strong crosswinds with crab- basically landing sideways then correcting after main gear touchdown."

Boeing recommends side slips for crosswind landings except for the highest crosswinds. Beyond X kts(typically 30(+/-) kts XW) Boeing says additional crosswind should be corrected by landing in a crab. So in a 35 kt XW the majority of the crosswind will be corrected by the side slip and only a couple of knots will be corrected by the crab at touchdown.

8th May 2015, 16:57
jcomm - I hope you get your answer sometime despite all the irrelevance here.

From where I sit, I ask where your normal axis stability factors came from as it would appear the directional moment from yaw is too low while the moment from the rudder deflection is too high, and I question the value of any spoiler yaw factor used. Wings level with full rudder would require a large spoiler deployment producing a significant degrading yaw rate.

As misd said a while back, any resulting turn rate would be extremely slow and I suspect you would hit the ground well before completing a 360.

8th May 2015, 17:06

thx for your comment, and yes, that's exactly how I see it and the flaw in the mentioned simulation model of the 744.

I have no air or stability data for that model - I am a simple user - but I questioned the accuracy of the flight dynamics as they are right now because I also think that...

1) The rate would be marginal

2) The loss of altitude huge!

among other factors...

8th May 2015, 23:21
As far as blow back valves are concerned I have never heard of them being fitted to 747 rudder hydraulics

From the Boeing maintenence training notes:

"The [rudder]Actuators are snubbed at the end of the stroke and contain Load Limiting Relief Valves set at 2800 psi."

Regarding the rudder ratio system, the actuator has an AC electric servo. If main bus power is lost, the actuator will remain in the last commanded position. If power was lost at high speed, it would be interesting to see how much control you would have at lower speeds with crosswind landings.

9th May 2015, 02:50
The QRH limits landings to 10 kt with a dual rudder ratio failure in the high-speed mode, so authority is reduced by 2/3 or more.