Purpose of 727 rudder anti-balance tabs
 

Curious why the 727 has these tabs on both the upper and lower trailing edges of the rudders and move in the same direction as the rudder but at a greater deflection. Other Boeings don't seem to. Because of the T-tail?

They're there to fair the rudders when hydraulic power to the rudders is lost.

Servo tabs???

It would be to increase rudder effctiveness. I may be wrong, but I think the DC10 has it as well because the centre engine restricts rudder height. The rudder height of the 727 is restricted by having the tailplane mechanism perched on top.

All you want to know SmartCockpit - Airline training guides, Aviation, Operations, Safety
An anti balance tabs deploys in the same direction as the control surface, making the movement of the control surface more difficult and requires more force applied to the controls by the pilot. This may seem counter-productive, but it is commonly used on aircraft where the controls are too light or the aircraft requires additional stability in that axis of movement. The anti-servo tab serves to artificially increase stability and also make the controls heavier in feel to the pilot (From wiki) Nothing to do with fairing rudder on loss of hydraulics.

About the 727
 

Reminds me of my old 727s, the 100 and 200s.
Otherwise known to me as my "sports car". I loved the 100s...
The 200s were ok, provided if with high "dash number" JT8D power.
Some of the 200s had high MTOW limit, over 200,000 lbs.
xxx
As long as we had hydraulics (A&B for the rudders) we were in fat city.
But no hydraulics, we had all kinds of altitude and speed restrictions.
This was due to the nasty "Dutch Roll" habits of the 727.
Anti-balance tabs were obviously fitted to assist the yaw dampers.
xxx
My description of the 727, was "an easy plane to fly".
And yet easier to kill yourself with, especially if it was your first "jet"...
xxx
:}
Happy contrails

:uhoh: It was and I'm still alive...

But I had the great opportunity to start on the F/E seat for some years before I became a First Officer on this lovely plane. So I could ask the guys up front a lot about handling that ship.

I would spend some money doing traffic patterns. Any chance somewhere...?
;)

Dredging the memory, I seem to recall that the B707 I flew had an arrangement whereby the rudder tab was an anti-balance tab when hydraulic power was available and a servo tab if hydraulic power failed. Maybe the B727 had a similar arrangement.

Regards,
BH.

Seeing as the rudder is hydraulically powered, the heavier feel to the pilot does not make sense to me. Perhaps the stability issue is the reason. Without hydraulics there is no manual control of any sort for the 727 rudders.

Are there any other jets with these anti-balance tabs along the entire trailing edge of the rudder, as well as taking up such a large percentage of the rudder chord length?

Hydraulic A and B failure - All flight controls operate in Manual Reversion.
Hydr. System A controls lower rudder. Syst B controls upper rudder.
Stanby hydraulic pump will operate rudder.
See below -
boeing-727.com
xxx
:ok:
Happy contrails

True, in that a loss of A and B is considered to be manual reversion and the standby hydraulic system will operate the lower rudder through a separate actuator. However, as previously stated that "Without hydraulics there is no manual control of any sort for the 727 rudders."

The tabs are there to increase rudder camber then the rudder is deflected, thereby increasing rudder effectiveness.

Several other Boeing airplanes use double hinged rudders. Look at the lower rudder on the 777.

Brand "E"s flight manual had the line about fairing the rudder after a loss of power. The maintenance fam guide talks about increasing rudder effectiveness, same as having a longer (more chord) rudder.

JammedStab, sorry, the provided link was meant to provide the 727 information, the verbiage was the general roll of tabs. The link on the 727 says "to increase rudder effectiveness". As for fairing the rudder, would that not be accomplished by the feel and centering mechanism?

B777, DC10, B747SP etc all have double hinged rudders. They are there, as has been mentioned, to increase the contol surfaces effectiveness.

In the case of the DC10 it's because there is an engine stuffed half way up the fin, result is a very small control surface.

The B747SP has (I think) the same size fin as the standard B747 but as the moment arm is much shorter (fuselage chopped) then a more effective rudder was required.

B777 is all to do with engine out at T/O power. A hugely effective rudder is required when you lose 100,000 lbs of thrust on one side. I think it's deflected automatically too if I recall.

Don't know much about the 727 but I assume it is as described for similar reasons as the DC10.

Hope this helps. :ok:

Thanks for all the answers. It appears that it is to increase the effectiveness of the rudder and used in the case of the 727 because the space for the rudder is quite small. At first glance it appears to be much smaller than a 737 rudder.

The anti-servo portion of the rudders deflect almost 40° more than the 27.5° deflection of the forward portion of the rudder. I guess a lot of rudder space is taken up by the horizontal stab. It is interesting to see the view of the rudders from the side and the angled top and bottom of the rudders. I don't know the reason why.

As for fairing the rudder after a power loss, if that means keeping the rudder in a position with no deflection, I would think the airstream would do that with actuators acting as gust dampers. I believe the feel and centering mechanism will provide a new neutral position which could be somewhat of a control surface deflection requiring hydraulic power. Not 100% sure though.

Hi,

Is B 777 fitted with double hinged rudders?

Besides B 747, which Boeing airplanes use double hinged rudders? (may be you included DC or MD airplanes).

Feedback appreciated
Regards

anti-balance tabs
 

Hi truefaith,

Old Aero Guy (and others) is correct.

In the old days, the B707 had servo tab flight controls (no hydraulic power). The pilots' imput simply displaced the control tab on the trailing edge of the control surface. The tab's movement changed the pressure over the control surface which then moved to give the correct response. Hence the control tab always moved in the opposite direction to the main control surface. The side effect was that some effectiveness was lost due to the tab's movement.

In order to improve VMCA, a hydraulic control system was installed (rudder boost) which not only moved the rudder in the correct sense, but it moved the control tab even further in the same direction as the control surface - thus improving the rudder's effectiveness (responsiveness). The tab was then called an anti balance tab.

Which answer would you now choose?

Anti-balance tabs & pilot feel.
 

I am not sure whether the B707 ever did not have hydraulic boost to the rudder system or not. What I do know is that the rudder is the only flight control surface on the B707 which is hydraulically assisted. It normally has 3000 psi available at low speed, this being reduced to 2250 psi at speeds above 250 kts. The pilot feel, when hydraulic assistance is available, is provided by a "Q" feel unit and spring at speeds above 130 Kts. The purpose of the anti-balance tab is to make the rudder more effective. For what it is worth, rudder displacement from neutral available when no hydraulic assistance is available is about half that available with hydraulic assistance. (about 13 degrees without, 26 degrees with).