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Floppy Rudder Cables

L

lewy

Well Known Member
Page 32-13. We hooked up the rudder cables
tonite and was wandering how much sag in the cables is normal . In the neutral position or no pressure applied to the pedals the cables almost touch the bulkhead inside the tailcone, is this normal, can a to loose condition cause flutter? The cables and rudder move smoothly when pressure is applied to the pedals. Thanks Don Lewis
 
The cables will be held secure with you feet during taxi & flight.
 
O.K. so at "rest" the cable are saggy but in flight they are tight. Why is this? It seems like they should be tight at rest also.
 
All the RV's have slack....

...in the cables when there are no feet on them....including my -10....not a problem. Some guys have added springs to hold the pedals back but it's not really necessary.

Best,
 
How do you know?

O.K. so at "rest" the cable are saggy but in flight they are tight.
Click to expand...
The cables are also loose in flight unless feet are resting on the rudder pedals.
Joe
 
In the "early days" return springs were specified.
My -6 has them.
 
Floppy cables

Thanks for the info, I fly r/c also and my 40 pound piper cub has a similar set up and if the cables are loose to the rudder you get the f word (flutter) and if you do get flutter it is followed by the other f word. So, a little saggage is ok, stabilator is next on the list, thanks again, Don Lewis
 
Mich48041 said:
The cables are also loose in flight unless feet are resting on the rudder pedals.
Joe
Click to expand...

So the tab on my rudder has no purpose (at least that it accomplished. Seems like if the rudder cables were loose in flight the rubber, even with the rudder trim tab, would simply be centered by the passing air.

I guess I still do not under stand why it is good to have loose cables to the rudder. I wonder how much rudder authority is lost.
 
Sunriver Ken said:
So the tab on my rudder has no purpose (at least that it accomplished. Seems like if the rudder cables were loose in flight the rubber, even with the rudder trim tab, would simply be centered by the passing air.

I guess I still do not under stand why it is good to have loose cables to the rudder. I wonder how much rudder authority is lost.
Click to expand...

The trim tab works fine while the rudder is free to find its own position aerodynamically. Some large aircraft are controlled entirely by the tab and the control itself just moves freely.
 
Sunriver Ken said:
So the tab on my rudder has no purpose (at least that it accomplished. Seems like if the rudder cables were loose in flight the rubber, even with the rudder trim tab, would simply be centered by the passing air.

I guess I still do not under stand why it is good to have loose cables to the rudder. I wonder how much rudder authority is lost.
Click to expand...

The rudder will attempt to self-center if the rudder cables are free and not applying a bias to the rudder. With no trim tab, a centered rudder is fine if the plane is perfectly rigged for the particular airspeed. But RVs fly within such a wide envelop the airframe will not fly perfectly straight at all airspeeds if the rudder is centered. We add a trim tab so the rudder will fly "off-center" at cruise speed which will then compensate for the forces that are trying to push the ball off-center. No rudder authority is lost because as soon as you push on both rudder pedals the cables are tight. I fly my RV-6 with both feet resting on the pedals since it damps out slight oscillations of the rudder in turbulence.

It isn't a matter of loose cables being "good", it's just that they aren't bad and are very simple to install and maintain. The rudder couldn't care less since it will seek its centered position that is dictated by airspeed and the trim tab as long as we don't frustrate it by stepping on one pedal.

Aerodymanicists can offer a much better explanation but maybe this will work for us shadetree engineers. ;)
 
Not good but not necessarily bad either

With no feet on the rudder pedals, the rudder is free to move just like the flaperons are free to move when the control stick is released. I agree that tight cables are better. But Van's thinks that loose rudder cables are OK. They designed it that way for simplicity and lighter weight. The pilot keeps the cables tight by holding his feet on the pedals.
Joe
 
Sunriver Ken said:
So the tab on my rudder has no purpose (at least that it accomplished. Seems like if the rudder cables were loose in flight the rubber, even with the rudder trim tab, would simply be centered by the passing air.

I guess I still do not under stand why it is good to have loose cables to the rudder. I wonder how much rudder authority is lost.
Click to expand...

Look at it this way...... During flight, the rudder is always in trail. If you have a trim tab installed on one side, the air flow on that side is resisted by the tab, and it pushes the rudder away from the tab.

The rudder would be in trail (during flight) even if you had no cables connected. And the trim would work as above even if you had no cables connected. The main problem I see with loose cables is your rudder will flop around in the breeze when parked outside.

Van has provided almost friction free controls that make the RV soo much fun to fly. When people get to the installation of the rudder blocks they think that the rudder should be as friction free as the other surfaces. This is not the case with the rudder. Friction here is a good thing. Your legs will over power the friction of snug nylon blocks within reason.

My 6A has no tab on the rudder. I have a spring on the right pedal. My system has just enough friction that just a little tap will center and hold the ball except in steep climbs. I would install springs so the rudder does not flop around when parked.

The Warrior rudder did not flop around because the rudder was tied to the nose wheel.
 
Rudder Cables

When I was building my RV6A several years ago a friend brought his A&P friend in to look at the construction. When the A&P saw no rudder springs he got absolutely livid when I said they were not necessary, and he red-faced insisted they were mandatory. When I pointed out that there were over 1,000 RV6A's (at the time) flying without springs, he refused to believe me.

A little knowledge is a dangerous thing!
 
JohnF said:
When I was building my RV6A several years ago a friend brought his A&P friend in to look at the construction. When the A&P saw no rudder springs he got absolutely livid when I said they were not necessary, and he red-faced insisted they were mandatory. When I pointed out that there were over 1,000 RV6A's (at the time) flying without springs, he refused to believe me.

A little knowledge is a dangerous thing!
Click to expand...

The reason for springs on floor-mounted rudder pedals is to keep them from falling backwards. Our planes (except for the very early RV-4 and RV-6) have hanging pedals so no springs are needed to keep them in proper position.
 
Sunriver Ken said:
So the tab on my rudder has no purpose (at least that it accomplished. Seems like if the rudder cables were loose in flight the rubber, even with the rudder trim tab, would simply be centered by the passing air.

I guess I still do not under stand why it is good to have loose cables to the rudder. I wonder how much rudder authority is lost.
Click to expand...

The tab is servo. The small tab pushes the rudder over which deflects the air to center the ball. Just like your elevator trim tab except it's fixed.

The cables are not loose when you have your feet on them so there is no loss of authority.

While cruising in the -4 it's common to use your toes to pull the rudder pedals back and put your feet behind them. No flutter scare but if it gets really rough you can get a dutch roll like a Bonanza. From what I understand that is more fuselage design than rudder.
 
Floppy cables and yaw

My RV9a has the no-tension rudder cables. (no springs). When I encounter turbulence the aircraft yaws quite a bit. I was thinking of adding TruTracks standalone yaw damper. A friend (RV10 pilot) felt that adding springs to the pedals would help.

Anybody have experience with yaw and adding the springs??

Mark
N1075. RV9A
 
The rudder does not know

The rudder will not know if springs are pulling on both pedals or if your feet are pushing on both pedals. Other than reducing the effort required to push equally on both pedals simultaneously, I do not see why springs would help.
Springs will add weight and complexity.
Joe
 
Yaw in turbulene

Mark 960 mentioned yaw in turbulence...on my RV6a I installed vortex generators and while I didn't expect it, it greatly reduced yaw in rough air.
Must have something to do with air flow over the tail ? I never did figure it out, but other RV's with vortex generators have made the same comment.

FWIW
 
Why springs might help yaw in turbulence

The theory of how springs could help reduce yaw in turbulence, is that they would help keep the rudder from freely moving. If the rudder is freely moving, it is doing nothing to reduce yaw.

Of course, as a number of people have pointed out, pressing equally on both pedals has the same effect.
 
Mich48041 said:
The rudder will not know if springs are pulling on both pedals or if your feet are pushing on both pedals. Other than reducing the effort required to push equally on both pedals simultaneously, I do not see why springs would help.
Springs will add weight and complexity.
Joe
Click to expand...

OK Burt.............:D

If you like flying along for three hours with your feet pushing on the rudder pedals then go for it. Springs are NOT heavy and they are NOT complex.

But they do the job.
 
If you like flying along for three hours with your feet pushing on the rudder pedals then go for it.
Click to expand...
It is not necessary to push on the rudder pedals.
Springs are NOT heavy and they are NOT complex.
Click to expand...
I never said they were heavy. Springs might not be complex, but figuring out how to install them is.
But they do the job.
Click to expand...
Not on the RV-12. None of the RV-12s have rudder springs.
Joe
 
gasman said:
OK Burt.............:D

If you like flying along for three hours with your feet pushing on the rudder pedals then go for it. Springs are NOT heavy and they are NOT complex.

But they do the job.
Click to expand...

Flying with the feet on the rudder pedals is no problem. It takes no pressure, just your feets' presence.
 
No Way

would I fly a plane with slack rudder cables. Fitting a couple of springs to take up the slack is not complex and the weight addition is miniscule compared additional after market parts added by builders in order to give their plane a stamp of their character. Just because Vans does not fit the springs to some models does not mean they cannot be added by builders who want them. If you don't want them, then leave them off but accept the slack rudder cables, after all it is your plane and you can build it how you want it to a certain extent, as long as it is safe.
 
islandmonkey said:
No way would I fly a plane with slack rudder cables. Fitting a couple of springs to take up the slack is not complex and the weight addition is miniscule compared additional after market parts added by builders in order to give their plane a stamp of their character. Just because Vans does not fit the springs to some models does not mean they cannot be added by builders who want them. If you don't want them, then leave them off but accept the slack rudder cables, after all it is your plane and you can build it how you want it to a certain extent, as long as it is safe.
Click to expand...

Curious minds would like to know.....what is the problem with the "slack" (really a misnomer since most RV pilots rest their feet on the rudder pedals...gotta put your feet somewhere...) rudder cables on Vans aircraft?
 
Sam Buchanan said:
Curious minds would like to know.....what is the problem with the "slack" (really a misnomer since most RV pilots rest their feet on the rudder pedals...gotta put your feet somewhere...) rudder cables on Vans aircraft?
Click to expand...

I second the motion...

We don't need the springs with top mounted pedals........period. In flight, the rudder remains in trail and is rather stiff with the air loads. It actually takes some reasonable force to push from side to side. Certainly more force than what light springs would provide.......... to counteract.

My 6A has the short tail, and no offset in the rudder. Before I installed the fixed trim tab, I was surprised about the amount of force to keep the ball centered. With a small tab, no force is required. It's also the reason I don't like bias springs for rudder trim.

L.Adamson --- RV6A
 
One of the few aircraft I know of to have had rudder flutter was the Swiss Diamant sailplane. They had gone to heroic measures to reduce slop and friction in the rudder circuit, implementing a relatively complicated system of push-pull tubes and levers from nose to tail. When the prototype demonstrated flutter, they had to make the system even more complicated by implementing an in-line hydraulic damper.

In the majority of contemporary sailplanes, they just use cables inside nylon tubes glassed to the fuselage wall, and the friction of the cables provides plenty of damping. Problem avoided!

I think that there is some credence to the idea that loose rudder cables can allow rudder flutter. There are many reports in the soaring world where rudder flutter initiated while the pilot had their feet off of the pedals, and stopped once pressure was applied to both pedals.

However, the kind of pressure we're talking about is more than you'd get from any but the stiffest return spring. So as far as I'm concerned small airplane rudder pedal return springs are primarily there to keep the cables out of trouble while the airplane is at rest.

Thanks, Bob K.
 
Sam Buchanan said:
Curious minds would like to know.....what is the problem with the "slack" (really a misnomer since most RV pilots rest their feet on the rudder pedals...gotta put your feet somewhere...) rudder cables on Vans aircraft?
Click to expand...

Sam, for me it seems that slack rudder cables and no method of taking that slack up is sloppy engineering. This is a personal view and one I am not willing to change. Maybe there is no problem with slack rudder cables but I just do not like the idea of anything slack or sloppy in my plane.

I can imagine a scenario of slack rudder cables wearing away at the skins or a slack rudder cable getting caught around a protrusion and binding. My curious mind can imagine that this scenario is less likely if the cables are not slack.
 
islandmonkey said:
Sam, for me it seems that slack rudder cables and no method of taking that slack up is sloppy engineering. This is a personal view and one I am not willing to change. Maybe there is no problem with slack rudder cables but I just do not like the idea of anything slack or sloppy in my plane.
Click to expand...

It's probably best to own one, & fly one.................and then you'll see that the springs are just a waste of added weight.

After climbing out of the plane, the cables will usually have no slack. The rudder must be moved back & forth to get it. In the meantime, install a simple rudder lock on the rudder itself for winds.

L.Adamson -- RV6A
 
islandmonkey said:
Sam, for me it seems that slack rudder cables and no method of taking that slack up is sloppy engineering. This is a personal view and one I am not willing to change. Maybe there is no problem with slack rudder cables but I just do not like the idea of anything slack or sloppy in my plane.

I can imagine a scenario of slack rudder cables wearing away at the skins or a slack rudder cable getting caught around a protrusion and binding. My curious mind can imagine that this scenario is less likely if the cables are not slack.
Click to expand...

Anthony,

Your concerns just don't reflect reality in RV aircraft, and this has been proven by over 7000 aircraft in the fleet. If you wish to install springs in your RV that is certainly your prerogative (especially since you aren't going to change your mind ;) ), but doing so will add no safety or functionality to your plane.

The point concerning sloppy engineering could certainly be debated, but in my opinion engineering with the minimal number of components and achieving full functionality is....elegant.

Rudder springs are an answer to a question that doesn't apply to RV aircraft (the earliest RV-6's with floor pedals excepted).
 
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