Rabe Rudder

Hello, Just installed a Rabe Rudder hookup on my nobler ARF. It looks good,and operates as it should. Have to wait until weather breaks to try it.Any suggestions or personal opinions on this set up. Thanks. George Eckhardt

Sort of depends (in my mind at least) on what you're trying to accomplish. My buddy, Pat Johnston, always installs a wiggly rudder. Usually in conjunction with hard contact so that the rudder can be made to move or just adjustable. But he usually ends up using the wiggly rudder. He believes it really helps with GP in hard outside corners. Usually takes a bit of tweaking over several flights to get it adjusted correctly. I've used one to cure specific issues with a plane's trim (like yaw problems in hard corners). But I don't generally install one.

>
> Hello, Just installed a Rabe Rudder hookup on my nobler ARF.
> Any suggestions or personal opinions on this set up. Thanks.
> George Eckhardt
>

Yeah.

--> Read this thread! <--

If you are going to do one of these, may as well understand how to set it up.

To quote Al Rabe: "Without systematic adjustments, satisfactory operation of the rudder is a matter of luck."

Hello, Thanks for the info. I heard about the rabe rudder from windy urtnowskis dvds. I went to the same site to get info on it.Just an experiment to see how it works. Thanks again George Eckhardt

>
>Hello, Just installed a Rabe Rudder hookup on my nobler ARF.
>It looks good,and operates as it should. Have to wait until
>weather breaks to try it.Any suggestions or personal opinions
>on this set up.

It's conceptually very sound. The problem in practice is that no one seems to be able to adjust it properly. Essentially ALL Rabe rudder setups I have watched in any detail had grossly excessive movement. Not just a little off, way way off. The total motion for most is probably 5-10 times what is necessary. If you adjust it like that, it's doing more harm than good.

Brett

The coolest one I've seen is in Col. Trostle's Bearcat. It is programmed by a removable cam inside the aft fuselage. To change its operation, he replaces the cam.

Howard Rush

>The coolest one I've seen is in Col. Trostle's Bearcat. It
>is programmed by a removable cam inside the aft fuselage. To
>change its operation, he replaces the cam.
>
>Howard Rush


Ron B.
F4Fguy

Howard,

You mean like this? Great minds and all that! Seriously, I think I've been using this as long as Kieth. Mine's slightly different, as it mounts on top of the wing (to keep the weight forward) and actuates via a short pushrod forward from the flap, and an (ugh!) RC "Nyrod" back to the rudder. As pointed out by Brett, the biggest mistake (among others) most make in application, is using faaaaaaarrrrrr too much motion of the rudder.

Ron B.

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> It's conceptually very sound. The problem in practice is
>that no one seems to be able to adjust it properly.
>Essentially ALL Rabe rudder setups I have watched in any
>detail had grossly excessive movement. Not just a little off,
>way way off. The total motion for most is probably 5-10 times
>what is necessary. If you adjust it like that, it's doing more
>harm than good.
>
> Brett

Well, maybe not all. Matt has one on his plane. He set it up first with a fixed position so it was just an adjustable rudder--no motion. He trimmed the plane to its essential best and then hooked up the rudder to move with the elevator. There was an immediate improvement in the plane. He has done additional trim adjustments since--you are never done with your trimming until the plane's last flight--but, at least with larger props, it is a help.

Hello, Could you please explain what you meant about useing a larger prop. Thanks George Eckhardt

>
>Hello, Could you please explain what you meant about useing a
>larger prop. Thanks George Eckhardt


Ron B.
F4Fguy

George,

The need for a "wiggly rudder" is to overcome the yaw due to gyroscopic precession of the propeller. In theory you'd need as much left rudder on "up" elevator as right on "down". In practice, this is not true, as flying on a spherical surface, and the adjustments required to do so, reduce the left requirement and increase the right,usually requiring an assymmetric trim in the rudder. The forces involved are very real and can be calculated. The main thing to keep in mind is that they increase dramatically with the size and/or weight of the rotating mass. This is usually thought of as the prop (larger dia.=more rotating mass and larger precession forces). But extra large or extra heavy spinners and heavy nuts and balance weights on the shaft also contribute.

These forces are not insignificant. With large props, even if turning lower RPM, 30- 40in.oz. can be generated. On the other hand a classic plane with a classic engine* turning low RPM with a 10-5 prop can easily be trimmed to fly well without the complication and extra trimming procedures of a wiggly rudder.

* Nobler with Fox,or Max35S

Some examples:

At the same RPM; a 10-6 Rev-Up EW has a Gyro moment about 50% higher than a normal REV-Up. A 12.5-4.75 Bolly 3 blade has almost 3x the wood 10-6 EW 2 blade. A 13-6 Rev-Up a little more than the 12-5 bolly 3 blade, and a 13-6 Topflite Maple almost 50%more than the Bolly.

Hope this helps, Ron B.

(snip)
>Some examples:
>
>At the same RPM; a 10-6 Rev-Up EW has a Gyro moment about 50%
>higher than a normal REV-Up. A 12.5-4.75 Bolly 3 blade has
>almost 3x the wood 10-6 EW 2 blade. A 13-6 Rev-Up a little
>more than the 12-5 bolly 3 blade, and a 13-6 Topflite Maple
>almost 50%more than the Bolly.
>
>Hope this helps, Ron B.

How about a 14 inch three blade Bolly turning around 10,000 rpm on the ground?

>(snip)
>>Some examples:
>>
>>At the same RPM; a 10-6 Rev-Up EW has a Gyro moment about
>50%
>>higher than a normal REV-Up. A 12.5-4.75 Bolly 3 blade
>has
>>almost 3x the wood 10-6 EW 2 blade. A 13-6 Rev-Up a
>little
>>more than the 12-5 bolly 3 blade, and a 13-6 Topflite
>Maple
>>almost 50%more than the Bolly.
>>
>>Hope this helps, Ron B.
>
>How about a 14 inch three blade Bolly turning around 10,000
>rpm on the ground?


Ron B.

Len,

Without the prop in hand, I can't measure the Ip, and from that the the forces. My "top of the head" guess is that you're looking at (minimum) 40-45 in. oz., and you'll need a lot of "rudder kick*.

* A lot is still probably less than 1/4" right and 1/16-1/8" left, depending on the rudder size and configuration.

Edit for stupidity.

Len: You're right,of course.
F4Fguy

>* A lot is still probably less than 1/4"left and
>i/16-1/8" right, depending on the rudder size and
>configuration.
>F4Fguy

It is a Stuka (Did you guess?), but isn't that backwards. It is mostly kicking right with very little going left. Still, I am thinking it is a bit more than a 1/4 inch, although that would be at full deflection and he doesn't give it full deflection.

Posted this on another forum a while back but maybe some don't visit other forums. Would like to see a picture of how you set yours up.
*********************************************************
Thought it might be a good idea to retrofit my ARF Nobler with a Rabe Rudder. Mainly because it's going to be powered with a ST 46 and be swinging a large prop. If you haven't seen a Rabe Rudder Al Rabe came up with the idea way back when and has published several articles explaining what it does and how to set one up. Basically it counter acts the gyroscope effects of the prop in hard turns. Discovered installing a Rabe Rudder on a Nobler wasn't as straight forward as I first thought. The rudder is really too close to the elevator and you don't have allot of room between the elevator and rudder to work with.

Al says the elevator horn pivot should be on a 45 deg angle. I started by drawing 45 deg lines on a piece of paper and taping it to the fuselage side so I could draw a line on the elevator horn at a 45 deg angle from the hinge line. (Photo #1). I then drilled a couple holes along the line I drew on the horn and tapped them for a 2-56 screw so I could use a ball link at the elevator horn.

At first I was trying to use the real rudder to experiment with different control horns and linkages by taping a control horn to the rudder. This proved to be an exercise in frustration and almost gave up thinking it was impossible to make a Rabe Rudder work on this airplane. The rudder hinge line was just too close to the elevator hinge line and I didn't have enough room between the inside of the elevator and rudder for a control horn long enough to provide the desired rudder movement.

After sleeping on the problem, decided not to give up and made a sacrificial rudder from scrap balsa and started poking holes. After several tries finally found a combination I thought would work. Persistence pays off, as you can see in photo #2 my temporary rudder looked like swiss cheese before I finally settled on a control horn, linkage and location that would provide the proper rudder movement.

3rd photo is a test fit with actual rudder but still using the copper wire linkage from my test setup. Final photo is the actual linkage I will use. Will need to trim about 1/8 inch off the inside rear corner of the elevator to clear the quick link and put a couple washers on the inside of the ball link to space it further out from the control horn.

My reference was from a that Al Rabe posted on Stuka Stunt several years ago I saved to my hard drive. If I thought I could find the thread I would post a link but..........



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>Final photo is the
>actual linkage I will use. Will need to trim about 1/8 inch
>off the inside rear corner of the elevator to clear the quick
>link and put a couple washers on the inside of the ball link
>to space it further out from the control horn.
>

>[photo not recovered: 22104.jpg]
>
Bob,

Have you flown this yet? If so, how did it work?

If you have not taken a flight yet, I would suggest that you work out an alternative trim arrangement so that you increase your rudder offset at full down to be no more than another 1/4 inch or less from when the elevators are at neutral. I would suspect that you will see some very uncomfortable control response with the amount of additional offset you show in the photograph. In fact, as little as another 1/8 inch offset at full down will probably get you a better trim adjustment.

If what you have shown here is satisfactory, it would be interesting to hear about it.

Haven't flown it yet (weather) but what you are seeing in the photo is a little deceiving. Doesn't look it but the elevator is almost at the full down position in the photo, when the elevator is at neutral it only has a tad of rudder offset.

It still has more movement that I was wanting to start with but was going to try it as is for the first few flights. The position of the stab to rudder on the Nobler made it really tough (for me) to come up with horns and linkage that had the ability to adjust everything like I really wanted.

>Haven't flown it yet (weather) but what you are seeing in the
>photo is a little deceiving. Doesn't look it but the elevator
>is almost at the full down position in the photo, when the
>elevator is at neutral it only has a tad of rudder offset.
>
>It still has more movement that I was wanting to start with
>but was going to try it as is for the first few flights. The
>position of the stab to rudder on the Nobler made it really
>tough (for me) to come up with horns and linkage that had the
>ability to adjust everything like I really wanted.

I did not think the photo at all deceiving. I could tell that you had full down or near full down elevator in that photo. What I am suggesting and also what Ron Burn is suggesting is that the rudder offset you show at full down is more than excessive. Ron and I both have more than a few years experience with these things. Again, a suggestion would be to start with considerably less rudder throw at full down, like around 1/8 inch but no more than 1/4 inch.

At least, set up your horn geometry between your elevators and rudder so that you can go to less throw than you are showing before you go out to test this thing. It will save a trip to the field because what you have now will likely not be very satisfactory.

It really does not take much rudder throw on outsides to get the effect you want. Too much rudder throw, which is a problem that many have with a movable rudder, you get all sorts of unfavorable yaw. And depending on the configuration of the model, there will be varying amounts of unfavorable roll. That Nobler may not get too much roll, but there will be some. The Bearcat with a lot of rudder area well above the thrust line, there can be a nasty amount of left roll while the model is yawing right. With excessive rudder throw, the model will do some very unsightly things and you might even see parts of the model from the center of the circle you do not want to see.

>(clip)
>
>>Final photo is the
>>actual linkage I will use. Will need to trim about 1/8
>inch
>>off the inside rear corner of the elevator to clear the
>quick
>>link and put a couple washers on the inside of the ball
>link
>>to space it further out from the control horn.
>>
>
>>[photo not recovered: 22104.jpg]
>>
> Bob,
>
>Have you flown this yet? If so, how did it work?
>
>If you have not taken a flight yet, I would suggest that you
>work out an alternative trim arrangement so that you increase
>your rudder offset at full down to be no more than another 1/4
>inch or less from when the elevators are at neutral. I would
>suspect that you will see some very uncomfortable control
>response with the amount of additional offset you show in the
>photograph. In fact, as little as another 1/8 inch offset at
>full down will probably get you a better trim adjustment.
>
>If what you have shown here is satisfactory, it would be
>interesting to hear about it.


Ron B.
F4Fguy

Listen To da man!

Ron B.

>>(clip)
>Ron B.
>F4Fguy
>
>Listen To da man!
>
>Ron B.

When it comes to making our airplanes do tricks I would be silly not to.

I've put these on quite a few airplanes.

They work. Sometimes they work too well. I've disabled more than I've kept.

My conclusion:

A plane like a Nobler (non-scale, designed to stunt), has so much aft fuse and rudder area, it doesn't need or want a wiggler.

Scale or near-scale planes (with substandard aft area) get huge benefit from this gizmo.

Hello Paul, Can you elaborate on how it disables the plane,or its negative results. Thanks George Eckhardt

>
>Hello Paul, Can you elaborate on how it disables the plane,or
>its negative results. Thanks George Eckhardt

I think he disabled it, not that it disabled the plane. I agree that with small props and large aft fuselage side area it may not be that beneficial or needed. "Needed" is always a question or judgment call. Matt built his plane to allow him to set the rudder fixed and made his first flights this way to do his initial trimming. When he was reasonably satisfied with the trim, he set the rudder to move with the elevator. He saw an immediate positive benefit in doing so.

I would recommend anyone who builds a plane with the Rabe rudder to add a mount for the push rod where it can be set rigid. Do your initial trimming that way and then switch over. You may (probably will) need to adjust the Rabe rudder to get the most benefit, but by this time you will know that you are trimming the rudder and not needing to change other areas of trim in the plane. It also gives you the opportunity to decide whether or not you feel it makes a positive difference for your design.

Matt was using 13 inch three-blade props when he first used the Rabe rudder. He is now using 14 inch three-blade props and climbing (looking at larger props yet.)

>
>Hello Paul, Can you elaborate on how it disables the plane,or
>its negative results. Thanks George Eckhardt

Yes, I disabled it. Bolted the rudder rod to the fuse and made it ground adjustable. In many cases, it made the plane yaw out more than I wanted.

I only have/had one Nobler. I feel that the massive aft area and airfoil fin is plenty.

I used this concept on Slow Combat planes for a while & felt that the ability to bunt at will was a big advantage.

>>
>>Hello Paul, Can you elaborate on how it disables the
>plane,or
>>its negative results. Thanks George Eckhardt
>
>Yes, I disabled it. Bolted the rudder rod to the fuse and
>made it ground adjustable. In many cases, it made the plane
>yaw out more than I wanted.
>
>I only have/had one Nobler. I feel that the massive
>aft area and airfoil fin is plenty.
>
>I used this concept on Slow Combat planes for a while &
>felt that the ability to bunt at will was a big advantage.
>


Ron B.
F4Fguy

Paul,

No matter how much side area, the effect is still there. You can mask it with static rudder (or other offsets), but only by introducing yaw in other flight regimes where you don't want or need it. Even a 10D-5/6P at 9-10K RPM generates about 8-10 in.oz. in a 7'-10' radius corner.

I'll say it again, if you can't make it work, you're probably using a geometry giving too much throw. Also, as Keith said, rudder geometry can make a difference depending on the area distribution, but this will affect roll. It* has to get pretty extreme to have a detrimental yaw effect.

*Rudder area distribution top/bottom.

Ron B.

Edit to add clarification.

I have used a Rabe Rudder setup on a couple models. Personally, I think they are appropriate for only a very select group of people. Mr. Rabe, obviously. Mr. Trostle, certainly. Mr. Burns, quite possibly.

All others would have to prove it, simple as that.

Pat Johnston was named earlier in this thread, and while I will note that Pat is a skilled designer/builder/ flier and I am willing to concede that (maybe) he has made the RR work on a couple models, I have seen others in his stable fly--during contests--where the thing goes herky-jerky on him.

Maybe he can't see it, I don't know. But surely he can feel it at the handle, although with the way he swings his flying arm around, maybe not.

For myself--and y'all can do whatever pleases you--I have taken advice given to me by Brett and trimmed my models thoroughly with other features, leaving the option of hooking up the RR as the last thing to try.

**Use of the RR should certainly not be one of the first things tried. That's just a fact.**

To date, I have never felt the need to use a Rabe Rudder setup on a properly trimmed PA model.

Furthermore, I submit that with above noted exceptions, if one finds an improvement in trim when using an RR, that is merely confirmation that some other aspect of the trim was adjusted incorrectly.

Dan

(snip)
>Furthermore, I submit that with above noted exceptions, if one
>finds an improvement in trim when using an RR, that is merely
>confirmation that some other aspect of the trim was adjusted
>incorrectly.
>
>Dan
>

You thrive on absolutes, do you not, Dan? There are quite a number of people using it quite successfully. If you continue to use small engines and small props, well, maybe you don't need it. Others (Matt included) have found it helps immensely, and, as I have stated earlier, Matt did his trimming without it first. He then added the Rabe rudder and noted an immediate improvement. And, no, his other trim was not incorrectly adjusted. The Rabe rudder helped to correct for procession as it was intended to do.

>To date, I have never felt the need to use a Rabe Rudder setup
>on a properly trimmed PA model.
>
>Furthermore, I submit that with above noted exceptions, if one
>finds an improvement in trim when using an RR, that is merely
>confirmation that some other aspect of the trim was adjusted
>incorrectly.

That's a little over the top. The idea behind the rudder is completely sound and if adjusted correctly (along with everything else) it can be an improvement and take out the small amount of residual yaw that cannot otherwise be removed.

But in less-than-masterful hands it almost always causes far more harm than good. Certainly the vast, vast majority of movable rudders I see are creating borderline catastrophic trim issues, even in the case of most expert-level fliers.

Brett

>
>Hello Paul, Can you elaborate on how it disables the plane,or
>its negative results.

What usually makes the rudder a net detriment (over a fixed-rudder airplane) is that the adjustable rudders are almost always OVERADJUSTED.

The purpose of moving the rudder is to prevent the airplane from yawing in response to the precession of the prop. What usually ends up happening in practice is that the budding moving-rudder enthusiast obsesses over the rudder to the point that it ends up causing a lot more yawing than would otherwise have been there. And, they conclude that every trim problem they have is the result of the adjustable rudder being out of adjustment and thus they obsess on it, and never solve all the other problems they have.

The other issue is that no one seems to be able to see the problem they are trying to correct. I have seen very enthusiastic movable rudder fans brag about how well their airplanes are trimmed, and then you watch and they are wallowing all over the place. What I think is happening is that people can't really see whether their "problem" is yaw, or roll, or a static rudder issue, or a leadout position issue, or... and then just adjust it until it "feels good". Usually, meaning that it has a lot of line tension in certain parts of the pattern. Well, yes, if you kick the rudder over a lot in the corners you will indeed help the line tension - for a few fractions of a second until the inevitable overshoot in the other direction comes.

Bottom line is that the effect you are trying to correct for is so small that you only need a TINY amount of help from a moving rudder. The amount of yaw you get from precession is VERY SMALL in any reasonable case, and that definitely includes large props at low revs.

Most of the yawing that people see is the result of OTHER SOURCES, not precession. You get a very minor theoretical advantage that most people will never ever see, Very very few people trim well enough to get any advantage from having a moving rudder, and that includes everyone at *all* levels. In fact in most cases I see at the field, it's a huge negative.

I have repeatedly strongly discouraged anyone with anything less than world-class trimming skills to avoid adding this additional, very powerful but easily-abused feature. If you absolutely must try it, I urge you to make sure you can put it to a fixed position (0 offset, VERY IMPORTANT) where it doesn't move at all. Then spend a year or so trimming everything else until it is absolutely as good as you can get it using correct trimming principles. Make sure that you can see the residual yaw effects of precession, which will be quite small. If it's not small, then you haven't got it trimmed correctly. Then and only then, hook up the rudder and then adjust it and nothing else until you get rid of the tiny residual yaw. Make sure you linkage can be adjusted so that you can get as little as +- 1/32" with no slop. Start with NO differential, and the smallest possible movement. I am willing to bet that you will find the required rudder motion to be exceptionally small. If it isn't there's almost certainly something else wrong.

Brett

Wow, does everything have to turn into a controversy..

My decision to install a Rabe Rudder on my Nobler was based on a conversation I had with Al Rabe several years ago when I was having a trim problem with a similar sized airplane running a big prop. Al coached me well and I believe I know enough about what to do and what not to do to make it work. If I run into a problem I'll ask the man that invented it not someone that has an opinion based on his perception of others experiences.

George, I would sure like to see a photo of your setup. I am still not real happy with the adjustment range I have available on mine and was wondering if what you came up with was any different. If you are concerned about someone on the forum discounting your ideas you can email me directly.

Thanks
Bob

"But in less-than-masterful hands it almost always causes far more harm than good. Certainly the vast, vast majority of movable rudders I see are creating borderline catastrophic trim issues, even in the case of most expert-level fliers."

Brett

Well, my earlier comments being over the top or not--and as always Len seems to protest a smidgen too loudly--the above is what I have seen with RR installations.

Even did it myself once upon a time.

Yes, I know they can work. I understand this quite well, if not perfectly.

The issue is effective application of the technology. And how very unusual it is to see the desired results.

Ah, but I was tempted a few times this last year. The Impact had been built with the RR setup; all I had to do was to install a few parts and get 'em hooked up.

And the siren song of "Boy, you be running a beeeg prop, best to at least act as if you know how to counter the resultant forces," would sometimes waft across the practice field.

I resisted, and truth be told think my trim combination was the better for it.

Absolutely? Yeah, Len, absolutely.

Sigh...

Dan

One thing I like about these little airplanes is that you can try anything you want and see what happens. There's no boss to arbitrarily veto it.

Howard Rush

>One thing I like about these little airplanes is that you can
>try anything you want and see what happens. There's no boss
>to arbitrarily veto it.
>
>Howard Rush

I had heard there are some frustrations in being a creative engineer in the employ of the Boeing Co.

Dan

>One thing I like about these little airplanes is that you can
>try anything you want and see what happens. There's no boss
>to arbitrarily veto it.
>
>Howard Rush

Yes, we have done an lot of experimenting with different size engines, different size props, and yes, even Rabe rudders. And if it works better, well, that becomes square one and we use it.

>Wow, does everything have to turn into a controversy..

It's only a controversy if someone makes it one.

I was answering a specific question and giving my best advice. If you want to say I don't know what I am talking about then by all means disgree and tell me how I am wrong. But I don't appreciate the cheap shot.

And for the record, I have had 7 airplanes of my own with Rabe Rudders, including a Nobler with an ST46. And seen 100s more. Given that even most top 5 fliers can't get it within a country mile, most people can't manage it.

Brett