Purpose of 727 rudder anti-balance tabs

[JammedStab]

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?

[MarkerInbound]

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

[blackhand]

Servo tabs???

[Rainboe]

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.

[Brian Abraham]

All you want to know SmartCockpit - Airline training guides, Aviation, Operations, Safety (http://www.smartcockpit.com/pdf/plane/boeing/B727/systems/0008)
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.

[BelArgUSA]

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

[hetfield]

And yet easier to kill yourself with, especially if it was your first "jet"...

: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...?
;)

[Bullethead]

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.

[JammedStab]

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. The anti-servo tab serves to artificially increase stability and also make the controls heavier in feel to the pilot

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?

[BelArgUSA]

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 (http://www.boeing-727.com/)
xxx
:ok:
Happy contrails

[JammedStab]

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."

[Old Aero Guy]

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.

[MarkerInbound]

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.

[Brian Abraham]

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?

[TURIN]

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:

[JammedStab]

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?

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.

[AeroTech]

Hi,

Originally Posted by Old Aero Guy
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

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

[truefaith]

The purpose of anti-balance tabs is to:

a) trim aircraft
b) relieve stick forces
c) increase stick forces
d) make control more responsive

Now, which is the approproate answer? All answers (typically b and d) look the same to me.

[rudderrudderrat]

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?

[Old Fella]

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).

[rudderrudderrat]

Hi Old Fella,

My memory was failing when I posted on 17th. You are correct that the rudder boost was selected on most of the time, but Boeing published VMCA figures for Boost Off (about 60 kts faster). I found a copy here:
http://www.vpiaheritage.com/707limitations.pdf
on page 10.
I seem to remember we would turn the boost off in the cruise to dampen the yaw. Happy days!

[aviatorhi]

Boeing found they weren't going to be able to put enough rudder on the airplane for it to be as controllable as they would like. Instead of redesigning the whole tail and increasing the size of the entire rudder them simply cut it in half (vertically) and added the tabs to increase effectiveness without increasing size.

727 Prototype Airplane Historical Photos and Data (http://rbogash.com/727history.html)

You can scroll the the photos of the prototype here. You'll notice that on the first flight the tail and rudder all match in color, but there are photos interspersed of the aircraft at a later point with an all white rudder, which is the rudder used on the production aircraft.

[hetfield]

@aviatorhi

Thx for the link:ok:

Brings back sweet memories...

[Algy]

Boeing stats. (http://www.boeing.com/news/techissues/pdf/statsum.pdf) I've always been vaguely curious about that.

[aviatorhi]

Like i said, he's got the pictures mixed up, that's a later test flight being labelled as the first, if you look at the rollout stills and the in flight photos from the (verified) first flight (and even video if you wanna look it up) you'll see the original (off-brown) rudder, later flights had the white rudder and modified system in response to the need for more yaw control/rudder authority.

[roulishollandais]

Dutch roll is a typical problem with Boeing. :E

Instead of rolling right and left and right and left , Airbus is worth : the tail fin gets broken by the typical strength of the roll. :}

But if the dutch roll begins, it is possible to stop this crazy dynamic, with a little mathematic. I did the demonstration five times in 1992.

At the end the ''dutch roll'' is cause of many incidents and accidents, false qualified. :ugh:

When will pilots get a real formation to recognize a dutch roll, and come out of it ?? :confused:

('roulishollandais' is the french expression for 'dutch roll':ok:)

[TURIN]

I am not sure whether the B707 ever did not have hydraulic boost to the rudder system or not.

According to 'Cats Eyes' Cunningham (I think) in his auto-biography, Boeing only fitted the rudder boost as a consequence of his insistance (to the UK CAA) that an average pilot would not-in certain conditions-be able to control the a/c. I think he meant after an engine failure at T/O, but as i don't have the book anymore I can't check. I'm sure someone will.

[DozyWannabe]

Instead of rolling right and left and right and left , Airbus is worth : the tail fin gets broken by the typical strength of the roll. :}

If that was supposed to be a joke, it's not very funny. :ugh:

[con-pilot]

The -100 aircraft we flew for training was "bent" from a prior mid-air

Are you sure that it had been damaged by a mid-air?

The reason I am asking is that I flew the old 'Hoot' Gibson -100 and it flew just like you related. It always seemed like it wanted to fly sideways and it was impossible to keep in trim.

Fortunately we only leased it for a short period, about a month.

[galaxy flyer]

That's a new story about the old "Boeing". EAL had a -100 that flew pretty strangely, as an F/E, I'd watch the pilots putting in trim, then taking it out, then putting some more in. I'd nervously watch fuel balance, assuming I'd screwed it up. It had been in a snow bank at some time inits life and not under control.

GF

[stilton]

Boeing has never installed a Yaw 'dampener' in any Aircraft.




It is called a Yaw Damper !

[con-pilot]

The aircraft I’m referring to is N27, which was in a mid-air SE of Kingfisher and was recovered safely with a complete hydraulic failure (A & B system) and part of the stab missing. (They were IFR assigned in VMC and hit by a VFR GA aircraft.)

N27 was definitely “bent”.

This airplane was transferred to the Marshal’s Service. It was eventually re-designated N530KF in honor of the wife of one of the USMS pilots. She sadly died as a result the Murrah bombing.

I eventually flew this aircraft again when I transferred over to the USMS (Con-Air, Air Slammer) to escape management madness on the west side of the airfield. At this time, USMS operated two 727-100’s & one 727-200 plus two Hawker 800’s.

I believe the projected cost of a number of required AD’s for the old gals resulted in their eventual demise, and eventually 727-200’s were leased from a couple of operators, prior to converting to 737-400’s. Now they operate Mad Dogs.

I eventually ended up back on the west side in R&D and finally retired. I always highly respected the USMS/JPATS pilots as a group

Yes, you are correct. I did fly that aircraft for ten years. You must have joined JPATS just after I left, we still had the Sabre 80s then. My favorite 72 was 113. I did get my type rating in the aircraft we are discussing, then it had the 'N' number of N2777. In fact there is an in-flight photo of N2777 with the original USMS paint scheme hanging in my bar.

As for the 'Hoot' Gibson 727, we leased it from some airline, that I cannot remember the name of now, which was based at DFW. Let me tell you, after flying that hunk of junk and then got back into 530KF, you thought you were flying a brand new 727. God that thing flew horribly.

In the ten years I was with the Marshal Service I flew over ten thousand hours, 7,000+ was in the 727s. I got to where I really loved the 727-100, my God, the things you could do with those aircraft. I never really understood just why they replaced the 72s, after all, they were bought and paid for and when I was there, in great shape. So why get rid of bought and paid for aircraft and replace them them with leased aircraft, that could not do the mission profile the 727 was capable of.... Well, it just never made sense to me. Of course I heard rumors, but rumors are rumors and I never assign much credit to them.

Anyway, I'm sure we flew with a lot of the same guys there, overall I enjoyed my years with the Marshal Service, but I never looked back after I left.

Oh, the lady killed in the OKC bombing was one of my best friends and she sang at our wedding when my wife and I were married.

And finally;

The aircraft I’m referring to is N27, which was in a mid-air SE of Kingfisher and was recovered safely with a complete hydraulic failure (A & B system) and part of the stab missing. (They were IFR assigned in VMC and hit by a VFR GA aircraft.)



Believe it or not, I was standing on the ramp of Catlin Aviation (an long gone FBO at Will Rogers) and saw N27 land after that mid-air. Never in my wildest imagination would I have believed that years later I would be flying that same aircraft.

[con-pilot]

One to change the bulb and four to talk about how great the old bulb was.


Yeppers, but it was a grand, great old bulb. :p

Did you ever have a chance to fly a -100 with MD-80 type engines?

I did once, and if you closed your eyes at the wrong moment, you'd out climb the wing in a heartbeat.

But progress is progress and life goes on, I miss flying the DC-3 as well.


Okay, not as much. ;)

[stilton]

I remember Catlin Aviation from my check flying days !


The B727 was the First Jet I ever flew. It was a dream come true.



I think it was the best handling narrowbody made.




I was lucky.