Does a Rabe Rudder reduce line tension?

[Paul Van Dort]

I am a big fan of a Rabe rudder. It is on all my acro machines.
In the process of adjustment this rudder, I was wondering whether this elimination of yaw is not affecting the line tension itself in the turn.
We want to compensate the moment created by precession in outside turns by having an opposite moment created by the rudder.
In my opinion both moments create s force towards the center of the circle . I tried to make it clear in the atteched drawing.
Perhaps my knowledge about statica has degraded over the years, but it appears that there is a resultng force on the c.g., reducing linetension.
Perhaps this force is neglectable. I did not do the math.

[Howard Rush]

The gyroscopic moment is just a moment: no force. You get a little side force from the rudder.

[Ken Culbertson]

My perspective on yaw is a bit different than most and purely anecdotal since I am not much on the math.  To me, the whole point of trimming for yaw is to keep the plane tangent to the flight circle, not necessarily to maximize line tension.  Rudder offset used until the late 70's as the primary source of line tension has been replaced by leadout position.  For me a combination of both is what I like.  That is why I use an offshoot of Keith's CAM version of the Rabe concept to give me control over rudder position in both inside and outside turns.  My favorite trim is to have medium tension with zero rudder in level flight that increases in turns.  We spend half of the pattern flying level and I don't like the plane tugging at me there.  Some strive for equal tension, I don't.  My rudder movement gives me approximately 1/16" outboard travel on inside and 1/8" on outsides.  It is progressive with the outside moving faster than the inside.   The addition of the Fiorotti timer has altered this some.  Since momentum is now controllable overhead, I am less in need of adding overhead tension via rudder. 

Ken

[Trostle]

My best explanation is to use rudder movement to maintain essentially equal line tension throughout the maneuvers, not necessarily to increase line tension any more in any maneuver throughout the pattern.

Regardless of the process how the rudder is moved for inside and then for outside maneuvers, I want the line tension to be the same, not more tension in some maneuvers and less line tension in others.  It will take some experimentation to find that "happy" condition.

For me, once that program/adjustment is determined, it is assuring to know that when the high outside maneuvers are flown, the line tension will be there where without the correct rudder program/adjustment, line tension is reduced, sometimes by an alarming amount on the high maneuvers (like the outside loops of the vertical eights, the top corners of the hourglass, and the upper left loop of the four leaf clover.  A properly programmed/adjusted rudder makes for a very "comfortable" easy to fly airplane.   People who have flown my Rabe Bearcat (the Gulfhawk) seem to all agree.

Keith

[Brett Buck]

Keith has an excellent explanation, but I would add that what Al said/thought he was doing might have been different from what he was actually doing. Not that he was trying to deceive anyone, of course, but after reading back through various things he wrote and knowing how he ended up setting it, I think he was (inadvertently) doing something different. The giveaway was the large amount of static rudder offset. I think he was setting himself for a significant non-zero yaw angle, and using the rudder motion to compensate for the resulting roll/yaw motion due to inertial coupling. This *would* have the effect of increasing the line tension, and you might handle the problems it causes with the rudder motion.

   I think Al needed this extra tension to accomplish his goal, that is, running the CG forward to "get more lift" in the days of power limitations. Far forward CG increases the control loads dramatically, so you need more tension to be able to overcome these loads.

     Brett

   

[Air Ministry .]

" Does a Rabe Rudder reduce line tension? "

Yes & No .

Was getting hysterical with this bloke , last saturday ,as the RUDDER is Not Connected .



EXPLAINED IT , as , Mr Rabe put it . In a P-51 , with 1650 Cu. In. or Horsepower or suchlike . When it traslates from tailwheel to level , or V.c.V. ,
He Found ( and one or two other people ) that it skived off into the sceneary ,UNLESS you kicked Rudder In , at the Transition !

THIS is where his ' P ' effect - one prop blade near vertical - the other at double pitch - ( shall we say ) has things askew .

2/.

If theres MUSH ( Vertical yaw ) you get 'P' effect in manouvres .

3/. GYRO . try ' sopwith Camel; ' https://vintageaviationecho.com/sopwith-camel/

This is why LIGHT propellors are GOOD , not to say there arnt other reasons . And Nobody uses a Rotary engine for Control Line aerobatics .

4/. It Should get the LINE TENSION - EVEN .

The Ten Ton STREGA withoout the Rudder Connected , Yanked your arm in the top of the insides in Sq 8's , and floated across the tops of outsides . More or Less .
Connecting the Rudder got the Tension pretty much even . In the wind its arm stretching , but 70 or 80 ounces does that . But still as a scriber is excellent .
IF it has enough power .

RIGHT ,

So Mr Mustang swings on rotation . Kick the rudder and she dont .
this might be regarded as YAW .

So our widdle wudder stops the wobble , yaw - dutch roll , weave , wobble, pitching, and bobble . We Hope 

PARTICULARLY on something like the Sea fury etc , with Thrust line , side are , wing Center , and most other things OFFSET , as compared to the renowned ' in Line ' efforts .

SO ,



Like a TRIUMPH or NORTON ,  At The Limit , would drift , a jap. job would flex wobble weave & bite unfortunate inept opperators . 
A thouroughly offset contraption like above , may benifit enourmously , stopping it going free flight at the top of the hourglass & into the clover ,
If your HAMMERING IT , ' Hitting ' the Corner . The plane set up so you can ' HIT ' Full controll without it stalling or other misbehaviors . 

YOUR AVERAGE , not so scale device might not benifit to such a degree . But , As a Windy Bloke says " Over 12 inch of propellor ' its a neccesity . tickly weighty ones .

As they Swing the nose ,

ALL the rotateing weighty stuff , prop, spinner , prop driver , crankshaft , is IT . Wich is WHY Triumphs with CENTRAL FLYWHEELS outmanouvre  inline displaced flywheel / counterweight jap fours ! . 



WHEN the plane falls in a hole in the air , and throttles up , as its getting blown to eterity & back *, The amplitude of the forces can vary significatly . In MULTIPLES even . as in 2, 3, 4 times the load . 
* ( with an ' N' . )
This is when you have to hang on , dig in , and dont falter . Helps if the aeroplane does too . The rudder can help there , proportionately .

Havnt used em a lot at all , but the one i did followed the straight , 1/8 T.E. out , 3/8 t.E. out - up level, down , set up . Fine Tuning - Wots That . 

Throw it in the ballpark'n live'n'lean . So you dont back into a hole .

[Air Ministry .]

THIS is about how you can get it where it needs to be . http://www.machineconcepts.co.uk/aeromodelling/rabe_rudder/rabe_rudder.html

[Brett Buck]

THIS is about how you can get it where it needs to be . http://www.machineconcepts.co.uk/aeromodelling/rabe_rudder/rabe_rudder.html

   My point was that if you adjust it that way, you might not wind up with zero yaw angle, and in fact there is an entire range of potential yaw angles you could optimize for.

      Brett

p.s. I would also note that if all that was going on was gyroscopic precession, and you were trimming for zero yaw angle, you would end up with the rudder centered in level flight, and tiny amounts of symmetrical movement. By tiny, I mean, on, say, Als airplane, maybe 1/16". His wound up with lots of differential, with way more right than left, and was usually offset by 1/2" or more at neutral. That means that there is more going on than just correcting for precession.

[Paul Van Dort]

THIS is about how you can get it where it needs to be . http://www.machineconcepts.co.uk/aeromodelling/rabe_rudder/rabe_rudder.html

I am using much more throw than Al, but my rudder is much smaller, so that makes sense (The maxbees show the same) . I have noticed the hard way, that having it move to the left is NOT a good idea. Although in theory it should help :-)

[Brett Buck]

I am using much more throw than Al, but my rudder is much smaller, so that makes sense (The maxbees show the same) . I have noticed the hard way, that having it move to the left is NOT a good idea. Although in theory it should help :-)

  Exactly. There is more going on in most cases than just compensating for precession (which would result in symmetrical movement). At least one reason it might be asymmetrical is because the restoring force from line tension is asymmetrical - you get a lot when you yaw outboard and way less when you yaw inboard. Meaning it is dependent on using line tension for restoring force.

        Brett

[Ken Culbertson]

  Exactly. There is more going on in most cases than just compensating for precession (which would result in symmetrical movement). At least one reason it might be asymmetrical is because the restoring force from line tension is asymmetrical - you get a lot when you yaw outboard and way less when you yaw inboard. Meaning it is dependent on using line tension for restoring force.

        Brett

Exactly(2), and that is the reason that I was never happy with Al's method on my planes and stopped using it in the early 80's and why I love Keith's version.   I have all but given up on trying to engineer changes and gone back to my early days and use the "if I do this, the plane does that" method of trimming.   IMHO, equal line tension is an impossible dream and I would rather learn to predict and control it vs. equalizing it but I have ultimate respect for those that chase it.
 
Ken

[John Carrodus]

Air Ministry,
Please, I must be thick , I like your NZ/Aus ideas and input from obviously wide building and flying experience but I feel like I,m having a conversation of riddles with Gollum  and have a hell of a job getting a handle on what you're saying. I realise it can be a real strain for those who breathe a thinner atmosphere to waste time even bothering to hand out pearls of wisdom to the mere mortals amongst us, but I suspect if you slowed down and wrote as if you were addressing a well mannered but rather slow 10 yr old boy ( me) mad on CL , I for one ( and suspect many others ) would gain far more insight from your excellent contributions and certainly appreciate the effort on your part. Cheers mate - take this in the ANZAC spirit intended. No more no less.
PS Thank you for the photos of your twin motors you sent me a while back- In spiration al.
John 

[Craig Beswick]

John, he's one of yours mate. Lives here but isn't a great fan of us.
You are flogging a dead horse mate. He has replied to questions I have asked and I had not one clue what he was talking about!

As was said to Liam Neeson, "Good Luck"!
Go ANZACs.

Craig

[Howard Rush]

I enjoy AM's posts, although I don't understand them.  They're sorta like Finnegans Wake.  Keep up the good work, Air Ministry.

[Ken Culbertson]

I enjoy AM's posts, although I don't understand them.  They're sorta like Finnegans Wake.  Keep up the good work, Air Ministry.

We are two peoples separated by a common language!

Ken

[Air Ministry .]

Was someone asking what time it was ?


there may well be the odd elbow in the ribs , included . After all , we were in the stone age down here a few hundred years ago .
Er , one wonders , or dosnt , if the bloke who came up with the Boomerang , understood the aerodynamics , VISUALISED the Airflow
and Gyro Forces . dispite no knowing the current aerodynamic terms . Or if its development was the result of a train of coincidance !

Theres also devine guidance , of course .

As to the RUDDER , Kieths obviously the Finest Fettle of it ! The BEARCAT and more assymetric devices can benifit hugely . See Caption to Sea Fury pic.

The Typhoon Id like to fit one , but weight aft hinders . Early flight outside Triangle Hard it turned in , but the assymetric ( 1 1/4 ? ) wing , picked up the lines
and flew it OUT before I blinked , inside 1/4 lower leg . Wot Was That . Gone before your hair could stand on end . FRIGHT - Relief before it registered .
Tho perhaps it DID . ( Doing the tank Now . Got stung with duff fake Enya 3's !  )  On this type of low wing scale thing I think the YAW , with its roll coulple
from the accelerated wing lifting , makes the rudder highly advisable .The High Load Inside ' triangle ' into the top leg of the hourglass , with the ' top wind '
onto the upperside ( if you look at it horizontally ) as it goes across - So the Turn is INTO a ' down wind ' or blowing onto the upperside , so vector needs compensation .
RIGHT , we've all got that ?
AND
In a gale ( over 15 mph wind ) how it was , going from half way up , into the first clover loop , the combined loadings are perhaps at the maximum quantity .
the Back ( tail ) can skid out . aka - the Nose aims over your left shoulder . A AL Rudder is a good idea - to prevent this occurance . And saves some footwork in prevention .

A more Nobleresque type ship , Id think a more rudimentary installation would be adequate . More the ' Cut the Primary Error ' to a manageable Level ' , fly , observe .

KIETH Obviously has done a lot of flying and observeing - so his advise on the finer points of the contraption are most valid . For Optimum Results .

Throwing the rudder on a big thing with a big prop & big forces , to stop it biting you , a rough and ready set up STILL will be a IMPROVEMENT on most planes.
You shouldnt be fighting and loosing line tension with it installed . Still any big plane inna da wind is gunna give you a workout . Takes a lot of care to build em LIGHT .

Mine I do to take a few knocks , the increased loading probably makes apparent ' misbehavior ' Earlier than a Low Loading thing . Then someve found 15 per Sq Foot ' about right '
Though a fully functioning Pipe set Up , gives a more continuous interrelation of power / load requirements . Thus a more evenly ' stressed ' perfoprmance . 

But the Fred Flintstone ' peddal harder ' approach is apt for casual & less expensive ' multi plane ' efforts . Just imagine if all you were allowed to fly was Noblers  ! .
But above that Tech. Level it gets more technical . 

ANYONE fitted a Rabe rudder to a NOBLER ?

and I just use the computulator at thelibrary , so its often rushed . Abreviated , and has a few jibes included , to makeyer fink .

[wwwarbird]

Was someone asking what time it was ?

 He must be an engineer. 

[John Carrodus]

Air Ministry
Much better. Much clearer. I salute you sir. 

[Dennis Moritz]

I consult a shaman about trim. He does a dance, says something unintelligible. Or writes cuneiform messages. It's ok. I don't aspire to be more than mid level advanced. At times messages here on this topic are identical.