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Old Aug 07, 2010, 09:53 AM
The Lone Blue Plaid Flyer
Bob Cook's Avatar
Seattle
Joined Jan 2003
7,586 Posts
Hi Mike,

For me it's just a personal preference. I have NO experience rigging a pull pull system. I know how to do push rods. I'm sure they work fine, if you know what your doing on the set up and rig.

Bob in Seattle
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Old Aug 07, 2010, 10:39 AM
Making wood fly since 2007
Windependence's Avatar
USA, MN, Rochester
Joined Mar 2008
2,512 Posts
Quote:
Originally Posted by Mike Smart View Post
The pull-pull rudder linkage works absolutely fine. At the end of the day the Raven 3m is not a F3F model. I have never had a cable failure, but if you did have a rudder failure it wouldn't be the end of the world on a model with ailerons.
I use pull-pull on 2500W electric 3D models without any problems, so I can't see there being any risk of flutter on a glider as long as it is installed properly. I certainly never had any issues on my Raven 3m.

Mike
Pull pull rudder setups are used successfully in many different types of airplanes. Many composite DLG's use them and the entire Genie line of planes use them and they easily handle the loads of a full pedal gorilla style winch launch. But back to my question: What is the better material to use? I have seen kevlar line, braided no stretch line, i.e. firewire, and nylon covered cables used. Is one significantly better than the other?

Wayne
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Old Aug 07, 2010, 11:06 AM
life long racing nut & modeler
granada don's Avatar
Granada Hills Ca.
Joined Nov 2009
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Iwould not wan't to take a chance of loosing my plane because i used some new fangled stuff, braided steel cables are the best !!

Don
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Old Aug 07, 2010, 03:02 PM
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Mike Smart's Avatar
Aylesbury, Buckinghamshire, Great Britain (UK)
Joined Jan 2000
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Quote:
Originally Posted by Windependence View Post
Pull pull rudder setups are used successfully in many different types of airplanes. Many composite DLG's use them and the entire Genie line of planes use them and they easily handle the loads of a full pedal gorilla style winch launch. But back to my question: What is the better material to use? I have seen kevlar line, braided no stretch line, i.e. firewire, and nylon covered cables used. Is one significantly better than the other?

Wayne
I don't know what you call it in the US but I use nylon covered fishing trace wire on gliders.

Mike
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Old Aug 07, 2010, 03:05 PM
The Lone Blue Plaid Flyer
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Seattle
Joined Jan 2003
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Hi Mike,

It's called Fishing Leader.

Bob in Seattle
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Old Aug 07, 2010, 03:08 PM
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mhodgson's Avatar
UK, Coventry
Joined Aug 2005
551 Posts
I have used stranded control line wire and fishing trace for years, never a problem. It has always been my prefered method for rudder linkage.
If it is good enough for those big aerobatic power planes then it is good enough for me.
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Old Aug 07, 2010, 06:14 PM
If it floats....sail it!
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Elk Grove, CA
Joined Sep 2002
4,162 Posts
I have used small diameter wire (the Sullivan packaged one with all the connections) on both the rudder and elevators of a GWS Tiger Moth size aileron balsa biplane. It was a bear to get all of them tensioned correctly, but once set up properly I never had any issues with it. As Don says, it requires a lot of preload.

I also use Kevlar in place of wire on RC sailboats for standing rigging without any failures. However it is critical that the thread runs free without rubbing on anything and the boats are designed with preload in mind. I believe Kevlar is lower in stretch than steel and lighter for the same strength, but it doesn't tolerate abrasion very well.

I think I will be using the Sullivan yellow centers with CF rod (if the LHS has the longer rods in stock) on the Raven. It has worked well in other planes and is much simpler to set up.

Mike....how long a rod will be necessary? 36" - 48"?

FC
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Old Aug 07, 2010, 08:20 PM
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United States, OH, Bradford
Joined Jun 2005
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Avoid Fireline, Dyneema, etc.. It's a polyethylene-based advanced composite fiber that has extremely high tensile strength and stiffness (very good), but extremely poor creep characteristics (extremely bad). Whatever preload you rig into it will be all gone by the next morning, and in general keeping it properly rigged will be like trying to nail jelly to a tree. (Guess how I know!)
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Old Aug 08, 2010, 08:54 AM
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KickAce's Avatar
Watertown, WI 53098
Joined Aug 2007
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I've noticed that the full flying stab uses a fairly flat bottomed airfoil...

Would it be best to have a symmetrical airfoil...? Just thinking about possible issues where a different speed ranges that the Raven my encounter and what the possible effects would be... Does it really matter with a full flying stab...?
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Old Aug 08, 2010, 10:20 AM
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United States, OH, Bradford
Joined Jun 2005
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Quote:
Originally Posted by Mike Smart View Post
The pull-pull rudder linkage works absolutely fine....
I use pull-pull on 2500W electric 3D models without any problems, so I can't see there being any risk of flutter on a glider as long as it is installed properly. I certainly never had any issues on my Raven 3m.

Mike
The question is not whether it can be made to work. Obviously it can. The question I usually try to ask is "Is it better?"

It wastes half the stiffness of the linkage material, so it takes twice as much to achieve the same level of stiffness. It has a number of additional failure modes and a greater parts count. It is sensitive to rigging properly.

If the plane is really weight sensitive in the aft end AND is not stiffness sensitive (so something light and slow, like an indoor model, where the only thing that matters is the actual milligrams)AND it's difficult to get a conventional pushrod that is thin enough that it isn't overkill for the task, then a pull-pull might be the best choice.

As far as something faster, where loads and stiffnesses are substantial issues, then it certainly is possible to make a very workable pull-pull system, as you and others have amply demonstrated.

However, the idea that it's lighter for an equivalent level of stiffness, strength and reliability than an equally well-designed and optimised conventional pushrod system is an illusion.

Workable? Definitely. Better? Doubtful in most cases.
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Old Aug 10, 2010, 04:56 PM
The Lone Blue Plaid Flyer
Bob Cook's Avatar
Seattle
Joined Jan 2003
7,586 Posts
Hi Guys,

I got my .030 CF push rods from Goodwinds LLC. I ordered 12 of them. They are not expensive. I only need 2. But now I have 10 of them for AOG Spares.

Bob in Seattle
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Old Aug 10, 2010, 06:00 PM
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United States, OH, Bradford
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Quote:
Originally Posted by KickAce View Post
I've noticed that the full flying stab uses a fairly flat bottomed airfoil...

Would it be best to have a symmetrical airfoil...? Just thinking about possible issues where a different speed ranges that the Raven my encounter and what the possible effects would be... Does it really matter with a full flying stab...?
Cambered stab airfoils typically show up for three issues:

1. Old-time free-flights with (upward) lifting tails. I was asked about this a while back, did a bunch of research on it, found no sound aerodynamic advantage to it. Finally found an old timer who actually had the answer. Some of the old free-flight classes had a limit on wing area, that forced them into a smaller than optimum wing. The rule did not limit stab size, so they made the stab much larger and ran the C/G way aft, so that in effect the plane started acting somewhat like a tandem-wing aircraft. It was not as efficient as a properly-sized single wing would have been, but it got around the limitations of the rule.

However, somehow the idea got entrenched in model design lore, and a bunch of other folks started applying it to general designs, where it had no advantage. It was just tradition, "cultural baggage".

2. Downward-cambered stabs are used on some aircraft (including some of the Schleicher ASW series full-scale competition sailplanes) to overcome the powerful nose-down pitching moments of large flaps without having to go to a larger tail.

3. On some models, particularly with all-flying horizontal tails, the stab's airfoil can have problems with aerodynamic hysteresis near the zero-lift angle of attack. The very popular fully-symmetrical Selig 8020 tail airfoil was the one that probably did the most to educate the model sailplane community about the problem. Selig came out with the 8025, essentially the same airfoil but with a little bit of downward-lifting (concave-upwards) camber added. It still had aerodynamic hysteresis problems, but the camber moved the zone where the hysteresis occurred off to one side of the lift curve, just clear of where the tail normally operated, over to an area of lift coefficients where the hysteresis would not matter. Worked great, except that you had to be careful which way you put the stab on.

Edit:

Regarding the rest of your question, for a full-flying stab you do need to be a little careful about the amount of camber in the stab. Camber causes aerodynamic pitching moment, and more camber means more moment. If you have too much, at higher speeds the moment loads could overload the servo or overstress the linkage. The amount of camber in the Selig 8025 arfoil is not enough to be a problem, but the airfoils you posted definitely could cause trouble.

Also, it's very important to have the pivot for the stab at the aerodynamic center of the stab, i.e.: 25% back from the leading edge at the mean aerodynamic chord (NOT the root chord!). There are articles in the Ask Joe and Don section of our website that discuss how to find the MAC and AC.
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Last edited by Don Stackhouse; Aug 11, 2010 at 07:49 PM.
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Old Aug 11, 2010, 05:52 PM
The Lone Blue Plaid Flyer
Bob Cook's Avatar
Seattle
Joined Jan 2003
7,586 Posts
HI Guys,

I got the bell cranks and horns from Don at Hilaunch. Just servos and reciever to go.

Bob in Seattle
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Old Aug 11, 2010, 10:51 PM
Making wood fly since 2007
Windependence's Avatar
USA, MN, Rochester
Joined Mar 2008
2,512 Posts
Bell Crank comments

Hi everyone,

Well my second bell crank from Don at Hi-launch arrived, where the first one ended up is known only by the US post office. So here is my basic review.

Specs: weight is 4.3 grams, hub is 3/16th thick, arms are 1/16 thick, very rigid. Overall the arms are slightly longer than the bell crank shown on the Raven plans but it is nothing a little sand paper can't handle. Overall I say very nice, get yourself one. You can get the bell crank, two aileron horns and two flap horns for for $10 plus $1.50 shipping. hard to beat it I say.

One of the things I am trying to do is find items from fellow modelers. The bell crank and horns can be purchased from either Don at hilaunch.com or Mark Miller at Isthmus models. I saw a reference a few posts back from Don at DJ AeroTech talking about carbon rods for control rods. Jim at vintagesailplaner.com has .07" diameter carbon rods for sale at a very affordable price and he will cut any length you want up to 300'.

Wayne
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Old Aug 12, 2010, 07:13 PM
The Lone Blue Plaid Flyer
Bob Cook's Avatar
Seattle
Joined Jan 2003
7,586 Posts
Hi Wayne,

My servos arrived today. All I need is the reciever. When the rain starts, I'll be ready.

Bob in Seattle
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