Rabe rudder how to

Just how do you get one of these to work without binding or interfering with the elevator? All the plans I have seen show the location of the ball on the elevator, but dont show the actual linkage.
Are the very effective, and are they worth the effort to install them?
Thanks for your input.

Dan;)

Also curious. Thanks for any feedback.

Dan,

The movable rudder is theoretically correct and many stunt flyers have found their use advantageous. The need for such a device is largely determined by the side area daft of the CG. Airplanes with lots of longitudinal stability can get along just fine without the device. Problems occur when the rudder movement is overly sensitive or poorly trimmed. It is not a device simply installed and expected to perform miracles. It needs to be used intelligently and adjusted for optimum operation.

The theory and operation are explained, in detail, in the November/December 2001 Stunt News.

In general, I use a small horn installed flush with the inboard end of the outboard elevator. The pushrod is shaped to clear the end of the elevator in operation. The location of the elevator end horn determines the degree of asymmetry of the rudder movement. That horn should be adjusted far enough forward that the rudder asymmetry is no more than twice the outside movement as inside movement.

Here are a few photos.

> That horn should be adjusted far
>enough forward that the rudder asymmetry is no more than
>twice the outside movement as inside movement.

Quiz question for the physics fans in the group. The inside turn rate, and the outside turn rate are the same, and the engine is presumably spinning as fast as well. Hence, the precessional torque is the same magnitude either way. How come it wants to, (or can...) move outboard more than it moves inboard?

Howard, Al, and Igor are not allowed to answer.

I am back home already and I see the Slovak minority is discriminate here

So at least I add another quiz question. My R/C gyro unit, controlling rudder on base of pitching rate (very similar to Rabe rudder functionality) needed SYMETRICAL deflection both sides … why?

Howard, Al, and Igor are not allowed to answer.

… no, wait, I will not disqualify myself …

Brett is not allowed to answer.

… hmm also wrong, we could miss some good thoughts …

Brett is not allowed to post reply earlier than #10000

That last seems best to me

igor

p.s. some “0” more or less does not play role I think.

OK, I am going to reply so that we can get up to post number 10000 quicker and Brett can answer Igor's question (and, perhaps, his own.) I always assumed that a little yaw didn't hurt, so if it is yawing a bit out, well, that is a whole lot better than yawing in. So we tolerate the extra yaw on the inside turns and counter the inboard yaw on the outboard turns.

However,

Perhaps there is more involved here. When we are kicking the rudder out to counter the inboard yaw, we are also pushing the airplane (or part of it) inward. So we are needing more outward thrust to compensate. (Does that make sense?) Sometimes I don't worry about making sense, just accept the fact that it works.

OK, only 9999 more replies needed.

Len, I hoped you will change the message number in database to 10000

OK, this is message number 10,000. I don't care what it says elsewhere, this is message 10,000.

Brett, you can come in now.

LAST EDITED ON Dec-15-03 AT 09:40 AM (CST)
 
How come it wants to, (or can...) move outboard more
>than it moves inboard?
>
> Howard, Al, and Igor are not allowed to answer.

Because outboard yaw "hurts" half as much, and because inducing inboard yaw is very, very, very bad.

Too much inboard rudder, turns out, was the flaw in the Mr. Hyde at the Nats. I recreated the same condition in the Shear Panic when I got home. Since going to the 13" prop over the 14" prop I have disconnected the wiggle rudder as it turned out to be unnecassary at that diameter.

Brett,

Would it have something to do with the leadouts being stationed behind the CG therefore having some slight damping effect on outboard yawing??

LAST EDITED ON Dec-15-03 AT 01:11 PM (CST)
 
Besides precessional torque we have to deal with the P-factor. This is asymetrical thrust which occurs when the propeller disk is not perpendicular to the line of flight (note that I didn't say ground). Example: When the propeller disk is inclined to the direction of flight the downgoing blade has a slightly greater angle of attack (compared to the upgoing blade). This produces a larger force on "that side of the plane". Our engines rotate CCW looking at the prop so the result is that the plane wants to yaw left or in toward the pilot so we must put more right rudder in. When this P-factor happens on a nose down manuever the plane wants to yaw outward which is a lot less "traumatic". So from a practical sense not as much need to apply left rudder as right rudder.

This is common knowledge for anyone who has ever flown a small plane. You need to crank right rudder in at take-off. This is true for most american planes where the engine rotates clockwise from the pilot's seat.

This is the short answer.

Another quiz - Do the clockwise flyers have an advantage?

PSS - In a full size plane it should be called the "Pee" Factor - first time I experienced it - that's what I almost did!

>Besides precessional torque we have to deal with the
>P-factor. This is asymetrical thrust which occurs when the
>propeller disk is not perpendicular to the line of flight
>(note that I didn't say ground). Example: When the propeller
>disk is inclined to the direction of flight the downgoing
>blade has a slightly greater angle of attack (compared to
>the upgoing blade). This produces a larger force on "that
>side of the plane". Our engines rotate CCW looking at the
>prop so the result is that the plane wants to yaw left or in
>toward the pilot so we must put more right rudder in. When
>this P-factor happens on a nose down manuever the plane
>wants to yaw outward which is a lot less "traumatic". So
>from a practical sense not as much need to apply left rudder
>as right rudder.

That's not it - in the case of small plane, you hardly ever get P-factor the other direction. It's almost always to the left, or at most 0. On a stunt plane, assume that you get to the same AoA either way, so that it's really symmetrical.

BTW, I am sure Ted and Al are chomping at the bit, given that you have (inadvertently) mentioend the magical topic!

I always figured "P" factor was a balancing thing, it is a force in the opposite direction as precession, so one or the other gets canceled out. Of course, if you have a major angle of attack without rotation (as in a tail dragger with a low tail sitting on the ground) then "P" factor can very much enter in.

Now, Brett, how about adding torque to the picture?

>I always figured "P" factor was a balancing thing, it is a
>force in the opposite direction as precession, so one or the
>other gets canceled out. Of course, if you have a major
>angle of attack without rotation (as in a tail dragger with
>a low tail sitting on the ground) then "P" factor can very
>much enter in.
>
>Now, Brett, how about adding torque to the picture?

I assume you mean engine torque - but that operates in roll, not yaw. They are coupled, and the answer to the original question also answers why a little bit too little tipweight is a lot worse and more obvious than a lot too much.

LAST EDITED ON Dec-15-03 AT 09:42 PM (CST)
 
> That's not it - in the case of small plane, you hardly
>ever get P-factor the other direction. It's almost always to
>the left, or at most 0. On a stunt plane, assume that you
>get to the same AoA either way, so that it's really
>symmetrical.

Help me understand what you are saying in the above statement.

The term "small plane". Are you refering to a light full size aircraft or a model?

Are you saying that you can get P-factor induced yaw only if the model is climbing and not when it's descending? If so please explain because I don't agree. The effects are not as noticable during descent because the plane accelerates quickly and the amount of time during which the P-factor is influencing the model is less but still exists.

I'm not sure what you are trying to say in the last sentence of your statement. Could use please elaborate.

Len.... in your example of a tail dragger at rest. One of the conditions for the P-factor to exist is when income air is entering the propellor at some angle that is not normal to the rotational plane of the propellor. This is what creates the difference in AoA of the ascending versus descending propellor blade. So unless the wind is blowing pretty hard, the prop is just a big fan.

My understanding (and experience) is that gyroscopic precession creates a left turning tendency (in a climb)and torque reaction would cause a counterclockwise rotation of the aircraft.

I nearly forgot the spiral or corkscrew airflow of the propellor. It acts predominately on the left side of the plane. Another left turning (yaw) force.

I know I'm applying full size aircraft behavior to our smaller models but if the above isn't true... please let me know.

Edited portion:

After looking closely at the pictures, it appears that up elevator produces left rudder deflection which is counter to my argument. Every force I talked about produces left turning tendencies in a climb situation which requires right rudder to compensate. So what gives?

>My understanding (and experience) is that gyroscopic
>precession creates a left turning tendency (in a climb)and
>torque reaction would cause a counterclockwise rotation of
>the aircraft.

Alright, I know this is not what you are asking but:

A good way to get a direct understanding of precession at work is to sit in the instructor's seat of a 450 Stearman with a big Hamilton Standard prop as a low time student does a wheel landing. About the time the control surfaces loose effect, the tail will come down inducing a pitching moment into the prop disk.

Which WILL come out 90 degrees later as a swing to the right just as you are rolling out.

Which might startle a student who forgot the briefing.

Whose reaction can make the remainder of your afternoon very interesting.

Don't ask me how I know...

Cy.

>The term "small plane". Are you refering to a light full
>size aircraft or a model?
>
>Are you saying that you can get P-factor induced yaw only if
>the model is climbing and not when it's descending? If so
>please explain because I don't agree. The effects are not as
>noticable during descent because the plane accelerates
>quickly and the amount of time during which the P-factor is
>influencing the model is less but still exists.
>
>I'm not sure what you are trying to say in the last sentence
>of your statement. Could use please elaborate.

Light aircraft, not models. The p-factor is a result of the thrust of the propeller, and the angle of attack of the relative wind into the prop disk. It's common to get 8-9 degrees of off-axis relative wind velocity in climbs. In this condition, and assuming conventional rotation, this yaws you to the left. It's almost impossible for your average light plane to get anything like that in descents - in fact, most of the time the relative wind vector probably still *below* the x-z plane, and thus still yaws you to the left, just a lot less. You can only get significant angles where the relative wind come from *above* the x-z plane in negative-g maneuvers. That, combined with the fact that most propellor-driven planes have significant rudder trim built-in to compensate for p-factor in some condition (usually cruise), it seems assymetrical, even though the effect itself is not really so. You just never get into the situation where it wants to yaw you to the right.

This is definitely not true with stunt planes. To first approximation, you get just as much in one direction as the other, just like precession. So I contend the forcing function (a combination of P and precession) is pretty accurately symmetrical. But, the adjustable rudder can be quite assymetrical.

A hint is that you have to look at *everything* that controls yaw, not just some parts.

O.K. Brett,

No one has replied to this for a while - how about giving up the "Scientific" answer? We all know the modelers answer "well, it just works best like that!

>Len.... in your example of a tail dragger at rest. One of
>the conditions for the P-factor to exist is when income air
>is entering the propellor at some angle that is not normal
>to the rotational plane of the propellor. This is what
>creates the difference in AoA of the ascending versus
>descending propellor blade. So unless the wind is blowing
>pretty hard, the prop is just a big fan.

What I meant was a plane with all wheels on the ground but rolling out. The plane is in motion, but with the angle of attack as you describe.

>My understanding (and experience) is that gyroscopic
>precession creates a left turning tendency (in a climb)and
>torque reaction would cause a counterclockwise rotation of
>the aircraft.

That would be "P" factor. Precession would be a factor only when the plane is turning, as in a loop or a square corner. And then precession would be the oppposite.

>I nearly forgot the spiral or corkscrew airflow of the
>propellor. It acts predominately on the left side of the
>plane. Another left turning (yaw) force.
>

This can be misleading since it is actually going to be hitting the plane in waves at different portions of the fuselage.

LAST EDITED ON Dec-16-03 AT 05:03 PM (CST)
 
How about this. It is a concept recgonizing the effect of gyroscopic precession and its effect on the maneuvering flight of a stunt ship. It is designed to compensate for gyroscopic precesion by moving the rudder to the outside on outside maneuvers and left (inside) rudder on inside maneuvers to reduce yaw resulting from high pitch rates and having a gyroscope on the front of the airplane. It's just a practical solution to unwanted yaw during maneuvers which would cause loss of tension on outside maneuvers and unnecessary tension on inside maneuvers.

Brett you sly fox.... precession maybe equal in magnitude but not direction.

LAST EDITED ON Dec-16-03 AT 07:07 PM (CST)
 
Dick,

You got the theory correct, now, read the article in Nov/Dec 2001 Stunt news to find out why introducing asymmetry into the movement improves the PRACTICAL result.

As for torque, a stunt ship with dihedral flying in a normal slight yawed attitude presents the inboard wing to the relative wind at a slightly higher angle of attack and therefore produces a bit of extra lift to offset the equal and opposite force of propeller rotation.

As for "P" effect, we have it any time the relative wind isn't perpendicular to the propeller disk. Because we operate normally with full span flaps this minimizes any increased pitch (angle of attack) due to maneuvering load factor. Not much pitch, not much pee, but as somebody, Brett, I think, pointed out, that while making an inside corner, Precession is partly offset by pee. Partly, not completely. Precession is still the stronger force.

As for general aviation "P" effect and gyroscopic precession, a tail dragger will experience "P" as it rolls nose high with the downward moving blade having a greater angle of attack and more lift due to the nose high attitude of the airplane. So, while rolling tail low, the airplane tries to turn left. As the airplane gains speed, the tail lifts, and "P" mostly goes away to be replaced by gyroscopic precession. As we rotate, the propeller disk is pushed forward forward at the top of the disk. This results in precession in the form of a push forward 90 degrees later, on the right side of the propeller disk. We started with a left turning tendency from "P" effect which is replaced by a left turning tendency from precession. Once the airplane is through rotating, both "P" and precession are effectively gone until we lift off and beginning to climb with a small angle of attack and a constant small left turning tendency from "P".

Did you know the cyclic control control on a helicopter sends signals to the rotor head 90 degrees ahead of the point where we need to tilt the rotor disk for a direction change? Any helicopter commercial and flight instructor knows this, and I are one. In fact, it was this helicopter background which gave me the clues for minimizing the effects of gyroscopic precession in a stunt ship. I might also point out that a Mustang has very strong "P" and precessional effects on takeoff until the airplane completes its pitch to a level attitude. Mustang takeoffs are begun with 6 degrees of right runner rim. Subsequent smaller "P" on the climb is taken care of with reduced rudder trim.

Thanks Al... I'm going to borrow that issue from a friend and read it.

Buried in your post was a little nugget that I think is interesting.

"As for torque, a stunt ship with dihedral flying in a normal slight yawed attitude presents the inboard wing to the relative wind at a slightly higher angle of attack and therefore produces a bit of extra lift to offset the equal and opposite force of propeller rotation."

The part regarding dihedral. From what I read most of your designs have dihedral. The RC guys usually use dihedral (particularly trainers). I think we all know why. Yet from what I can see, most current designed pattern ships do not have dihedral. I would think that given the current trend of bigger is better and the larger engine/prop combo's being used, the additional stability provided by dihedral would be beneficial. (Actually it would seem that it would still be benefical regardless of size).Is there a downside to using dihedral? I can't find one.

Anyone care to comment?

LAST EDITED ON Dec-17-03 AT 08:46 AM (CST)
 
Dick,

The RC guys are flying so much 3D they need equal stability upright and inverted for inside and outside maneuvers. Dihedral can be stabilizing or destabilizing depending on the airplanes attitude.
In this case an offset fin will provide some correction in the correct direction all the time (actually, when under power).

I use dihedral on my stunt ships, not for torque, but because I fly semi-scale. Dihedral doesn't seem to have any noticeable negative effect on my airplanes as long as the leadouts exit the wing tip at the vertical center of gravity. I just thought it neat that dihedraled wings might also offer a small torque correction.

Al