Problems with rudder horn
 

I am currently flying a Phoenix Extra 330S 62" with a DLE20. I am having problems with the rudder horn

Basically, the rudder horn consists of two triangular nylon bases with provision or a threaded bolt to go through (one on each side of the rudder). A nylon bit screws onto the screw tip and the same clips on to a clevis which is crimped to the rudder cable. Its a pull-pull system. See the pic for a better understanding

Now the problem is, the screws being in two parts, on tightening the rudder cable, the screw simply deflects fron the nylon base so that it is no longer perpendicular to the surface of the rudder. Seeing this problem, I replaced the nylon base but it has happened again. To prevent the screw from bending over I didnt tighten the cable as much as before- its not slack but not very taunt either. I noticed today that the airplane still tends to wag slightly when flying KE or banked over. The club trainer told me that this is because the rudder still has some play.

Now I cant replace with a standard horn because the hinge point of the horn will not be in line with the rudder hinge line (see pic). The other option is to get one of these

http://shop.singahobby.com/?q=node/24832

Basically it makes the screw one piece so that it doesnt bend over as long as both the cables are taunt. I could also obtain the threaded stud locally so i only need to replace that one. Both are a bit difficult. Please suggest what I should do?

Ameyam

RE: Problems with rudder horn
 

Missed pics

RE: Problems with rudder horn
 

First, I would get the one-piece rudder horn.

Second, with a pull-pull setup, you want the horn to be slightly behind the hingeline to give you some Ackerman effect (Google that term)

RE: Problems with rudder horn
 

I am googling it right now. Somehow I feel I have heard that term from my automobile engineering days. How much aft of the hinge line would that be? Case in point, I am trying to get heavy duty t-style Dubro horns as replacement, dont know how large they are as yet

Ameyam

RE: Problems with rudder horn
 

Yes, it is an automobile term as well. you only need to go about 1/4" behind the hinge line

RE: Problems with rudder horn
 

Well, I saw the image and understood a bit of it. Basically, we should try for fig-3.

Like I said, I am thinking of getting Dubro heavy duty but standard nylon T-type horns from my LHS. This will eliminate any play due to the wood wearing or the nynon bases of the current setup. Alternatively, I can somply get a 100mm long M3 stud and use that. The rudder wood is soft balsa, not a hardwood so I am concerned about drilling through in that the cable tension will cause the MS screw to somply squeeze the wood even with the bases installed. Wouldnt the Nylon horns be a better idea?

Ameyam

RE: Problems with rudder horn
 

Just add a few drops of thin CA to the wood in that area and it will harden up (put a few pinholes first to allow it to seep in)

RE: Problems with rudder horn
 

So I should'nt use nylon horns then?

Ameyam

RE: Problems with rudder horn
 

I would use this one

http://www3.towerhobbies.com/cgi-bin...?&I=LXEMF5&P=7

RE: Problems with rudder horn
 

What I have is basically the same except that I have nylon triangular bases instead of the cupped MS washers. Also, the screw is in mm rather than inches. I will try and get a M3 x 100mm bolt tomorrow and cutoff the head, then use that stud as the threaded rod. The nylon bases will screw on to both sides of the rudder after I screw the stud through

Ameyam

RE: Problems with rudder horn
 

In reference to your drawing in post 6, #1 is ideal as long as your servo arm and rudder horns are the same length. Fig 2 is what you would do if the servo horns are shorter then the rudder horn and fig 3 is when the servo arms are longer then the rudder horns and should be avoided at all times.</p>

RE: Problems with rudder horn
 

Uh... no

RE: Problems with rudder horn
 

Well then enlighten me. If i'm wrong then show me.


Ok now that I am home, I was able to take a few pics of my pattern airplane. It has pull pull on rudder and elevator. Look closely and you can see that the hole in the control horn is just slightly forward of the hinge line. This is because the control horn width is 3" and the servo arm is 2", that is your Ackerman theory. One of the pictures shows the elevator at full diflection, both cables still good and tight.

RE: Problems with rudder horn
 

The test that was with that image said that the fig 3 is what you should go for because it reduces tension in the non pulling arm. You could google it... I will let Minnflyer give a technical explaination

Ameyam

RE: Problems with rudder horn
 

You want equal tension on both cables otherwise you run the risk of flutter. I have literally set up 100+ pull pull setups, I can assure you Fig 3 is wrong.

RE: Problems with rudder horn
 

And I can assure you that 90% of the people who use pull-pull set it up that way (#3).

Now, I will start by saying that I don't totally AGREE with it, but they set it up so that as one side pulls, the other side goes slack. This is to PREVENT the opposite from happening - That is, they don't want both sides to tighten which puts extra pressure on the servo.

And having one side go slack will NOT allow flutter because the surface is being pulled into the airstream. So it has positive pressure on one surface only. Flutter occurs when positive pressure fluctuates from one side to the other.

Also, having the servo arm and the control horn different lengths makes no difference other than getting more or less throw.

RE: Problems with rudder horn
 

Hey, I found these very good large horns at my LHS

http://www3.towerhobbies.com/cgi-bin...&I=LXD937&P=ML

Based on an eyball lineup, I think they will fit. Should I use them?

Ameyam

RE: Problems with rudder horn
 

Sure, those will work

RE: Problems with rudder horn
 

Heck yea! Now your talking control horn! I highly doubt you will ever have another control horn issue with your rudder.

MinnFlyer is absolutely correct and I would take his advice.



Pete

RE: Problems with rudder horn
 

Then I would have to say that 90% of the pull pull setups are not done correctly. And yes having one cable going slack can induce flutter. Yes the rudder is going into the slipstream but it is not constant pressure. As the elevator is deflected the air turbulates and that spills off to the rudder. I have been out at IMAC contests where someone will get a bad osccilation on their rudder. We simply bend the rudder horn forward ( 10-32 bolt ) then retighten the cables and problem goes away.

I really do not understand why you resist accepting my experience. I have competitvly flown IMAC for 13 years, mostly with 40% airplanes all with pull pull rudder setups with multiple servos. After 2006 I moved up to the unlimited class but closed that season with 3 wins, 2 second place and one 3rd. 2nd overall points in my region and National points leader going into the application phase for the Tuscon Shootout. You don't get to that point with poorly set up airplanes!

RE: Problems with rudder horn
 

I am not resisting your experience, and I commend you for your achievements. Also notice that I said "I don't totally AGREE with it".

The reason people prefer the other method is that having the connection forward of the hinge line TIGHTENS the wires as the control surface moves. IF your servos are mounted to withstand the extra tension, then all the power to you. If they are not, you could rip a servo from its mount.

And I maintain that you will not get flutter with one slack cable as long as you have positive air pressure on the tight side - which you do.

RE: Problems with rudder horn
 

Ok, lets not turn this into an argument between tech-heads. We agree that #1 is ideal, I will try for it. My question is, how critical is it? I intend to be methodical and measure the distance between the holes on the horn and the mounting holes of the baseplate, then mark that on the rudder and take if from there. I just hope that the holes come in the flat part of the rudder and not the bevel. Still there could be some adjustment I will need to do to make sure the horn sits properly

The main issue I am going to face is removing the balsa piece I epoxyed below the replacement rudder horn you can see in the pic. I had covered it with fibreglass and then coated over with epoxy to seal it. Now I will have to sand it of with a dremel and sanding roll. Hope I can do a clean job

This horn is designed to have screws screw through to the backplate. In a pull-pull system with the horns on both sides I wont be able to use the backplates. Instead, I will screw 3mm screws through and use nylock nuts to secure them. Wont use spring washers, the wood iteslf will do that job

Ameyam

RE: Problems with rudder horn
 

No arguments, just a difference of opinion. It's what makes the world interesting. With that said, the term flutter may not be 100% correct, it's more like an osccilation.

Mike, yes the cables do have the potential to tighten with travel but that is while on the ground. With flight loads things change. While the working cable is under load, the non working cable would want to relax a bit if the geommetry were perfect. This pretty much takes care of the cables getting tight unless it is drastically off. What I have found is no matter how good the math is, I still have to do some fine tuning once all said and done. Thats why for the most part I prefer to use the screw type horns as they can be bent slightly for adjustment.

RE: Problems with rudder horn
 

OK, but you didnt answer my question, how critical is it to get ib accurate. Will a difference of 1mm or so between rudder hinge line and horn hole make a difference

Ameyam

RE: Problems with rudder horn
 

No problem