Push rod material.

[Neddy]

My plane does not have a rudder it relies on aelerons and elevator to turn. I want to fit a working rudder. I need to use a push rod of about 650 mm. long. I have measured the diameter of existing stainless steel elevator push rod at 0.8mm. I don't seem to be able to find one available from a supplier so I think I will have find an alternative. I see there is mig welding stainless steel wire of 0.8 mm so I will give that a go and run it inside a small plastic tube. I also have to get a 9g mini servo to suit. 

[Paul De Tourtoulon]

Size, weight and what sort of power has it got would be nice to know.

[John Rickett 102]

SLEC will almost certainly have what you are looking for if you want to use a snake.  https://www.slecuk.com/index.php?route=product/search&search=snake&category_id=0

 

If its a straight run from the servo to the control horn, you could use a carbon rod, these won't result in the trim change often associated with plastic snakes due to ambient temperature variation.  https://www.bucks-composites.com/products?search_api_fulltext=carbon+rod

 

On a rudder you'd probably not notice any trim change, its more likely to be noticeable on an elevator.

 

If using a carbon rod, its preferable to support the rod at intervals to prevent whip.  

[Ron Gray]

Hyperflight

[FlyinFlynn]

7 hours ago, Neddy said:

I need to use a push rod of about 650 mm. long.

Have you considered a pull-pull arrangement using 2 lengths of flexible steel wire connected to either side of the rudder servo? It makes a (arguably) more secure and accurate connection between the servo and the rudder.

  

[Geoff S]

3mm carbon tube with 2mm threaded s/s rod at each end. Surprisingly stiff without any intermediate support.

[James Green2]

7 hours ago, Neddy said:

My plane does not have a rudder it relies on aelerons and elevator to turn. I want to fit a working rudder. I need to use a push rod of about 650 mm. long. I have measured the diameter of existing stainless steel elevator push rod at 0.8mm. I don't seem to be able to find one available from a supplier so I think I will have find an alternative. I see there is mig welding stainless steel wire of 0.8 mm so I will give that a go and run it inside a small plastic tube. I also have to get a 9g mini servo to suit. 

Probably a sound idea to search for an alternative, it will certainly work out much cheaper. Consider also mig welding rod available in straight lengths, I will probably give it a try, cutting a thread on the end for a clevis.

[Neddy]

This is the set up for the elevator with single servo and stainless steel rod. The provision for the rudder will be the same on the other side of the fuselage with single servo and push rod.

[PDB]

37 minutes ago, Geoff S said:

3mm carbon tube with 2mm threaded s/s rod at each end. Surprisingly stiff without any intermediate support.

 

 

What lengths of 3mm carbon rod would you say are OK without intermediate support?

[Geoff S]

I've found they're fine up to at least 600mm and would probably work over even longer distances.  I also use closed loop a lot and often use snake outer as guides for the wire so they can easily be replaced if necessary.  Using the snake outers also means the route between servo and control surface horn doesn't need to be absolutely direct; it can be slightly curved to avoid obstacles.

[Neddy]

This fitting goes at end of rod attacheshttps://www.aliexpress.com/item/1005006133318381.html?src=google&src=google&albch=shopping&acnt=631-313-3945&slnk=&plac=&mtctp=&albbt=Google_7_shopping&albagn=888888&isSmbActive=false&isSmbAutoCall=false&needSmbHouyi=false&albcp=19821179470&albag=&trgt=&crea=en1005006133318381&netw=x&device=m&albpg=&albpd=en1005006133318381&gad_source=1&gclid=CjwKCAiAloavBhBOEiwAbtAJO13PFY-N-uLyUYaQySuwjSYTXLXV8t2AKhng1VyjaxfQU8wj-y87UxoC0eAQAvD_BwE&gclsrc=aw.ds&aff_fcid=7e3a96519c954bf3bd196eccf5e448d5-1709296698399-05985-UneMJZVf&aff_fsk=UneMJZVf&aff_platform=aaf&sk=UneMJZVf&aff_trace_key=7e3a96519c954bf3bd196eccf5e448d5-1709296698399-05985-UneMJZVf&terminal_id=a4eeb528630c42b3ba43256855829d40&afSmartRedirect=y

to horn

[Robin Colbourne]

A 3mm carbon tube would be my choice.  With a metal pushrod there is a lot of momentum if the model stops suddenly and this is more likely to break servo arms and strip gears than a snake will.  A 650mm 0.8mm steel wire weighs 25.8 grams (0.9oz).  The same length in 3mm carbon tube is 3.9 grams.  Of course you need to add the end fittings to the carbon, whereas you could have a 'Z' bend on the steel wire.   If you used a 3mm carbon tube it has a 2mm bore, so you could epoxy a short length of studding into the ends to attach the clevises.

Hyperflight 3mm carbon fibre tube

Edited March 1 by Robin Colbourne

[extra slim]

Model shop Leeds has kavan 1x1000mm

 

https://www.modelshopleeds.co.uk/catalog/product_info.php?products_id=55691&gad_source=1&gclid=EAIaIQobChMIuL-u7fPThAMVWo9QBh3NPw2xEAQYAyABEgI1p_D_BwE

[Geoff S]

7 hours ago, Robin Colbourne said:

A 3mm carbon tube would be my choice.  With a metal pushrod there is a lot of momentum if the model stops suddenly and this is more likely to break servo arms and strip gears than a snake will.  A 650mm 0.8mm steel wire weighs 25.8 grams (0.9oz).  The same length in 3mm carbon tube is 3.9 grams.  Of course you need to add the end fittings to the carbon, whereas you could have a 'Z' bend on the steel wire.   If you used a 3mm carbon tube it has a 2mm bore, so you could epoxy a short length of studding into the ends to attach the clevises.

Hyperflight 3mm carbon fibre tube

 

That's what I do.  If you want a Z bend then use 2mm s/s rod at one end, but I don't think it's as secure epoxied in the rod as threaded (no evidence - just a feeling).  I favour a ball joint at the servo end - they don't come unscrewed and have almost zero play.  At the control surface, I use s/s clevis with a locknut - it's where you can adjust it most easily.

[Engine Doctor]

On 01/03/2024 at 03:16, Neddy said:

My plane does not have a rudder it relies on ailerons and elevator to turn. I want to fit a working rudder. I need to use a push rod of about 650 mm. long. I have measured the diameter of existing stainless steel elevator push rod at 0.8mm. I don't seem to be able to find one available from a supplier so I think I will have find an alternative. I see there is mig welding stainless steel wire of 0.8 mm so I will give that a go and run it inside a small plastic tube. I also have to get a 9g mini servo to suit. 

Hi Neddy what size is the model ; presumably its quite small if you are using a 9g servo ?  For light weight models I use 28g or 26g piano wire run inside a snake inner . The hollow white ones are ideal if available . Ive never used the 0.8 Mig wire as its not as springy as the piano wire and bends comparatively easy and already has a bend from the roll . Stainless may be better but if its like the stainless filler rod used for Tig then it also bends with little spring back.  Ive just converted a Lidle big glider to RC and used 26WSG through a snake inner foe elevator and its more than adequate . Just don't leave too much unsupported wire between the end of the tube and the control horn  don't forget the more weight at the rear will affect the CG 

[Don Fry]

I’ve used

17 hours ago, Geoff S said:

 

That's what I do.  If you want a Z bend then use 2mm s/s rod at one end, but I don't think it's as secure epoxied in the rod as threaded (no evidence - just a feeling).  I favour a ball joint at the servo end - they don't come unscrewed and have almost zero play.  At the control surface, I use s/s clevis with a locknut - it's where you can adjust it most easily.

I use this system (cheap quick, good). I abrade the inside of the tube, so it is grease free. Use a longer set epoxy, mixed very carefully, fill the rod hole with epoxy, (a pin to push apply)and fill the masked threads (same pin). 

I did a test once. Built a test piece . Nothing fancy for units used in the test piece. When cured, hung a 50 kilo lump, no fails, no fails when I tried push pull shock loading. Got bored. 
Word of warning. Destroy the test piece. I saw it much later, a half finished rudder control arm. COO goes I. Finished it, installed it, and the penny dropped, why a half finished rudder rod. Chastened, made a new one.

[Bruce Collinson]

It’s possible to tap the inside of carbon tube, to a degree, and it certainly improves adhesion with epoxy even though there’s insufficient thread to rely on it alone.

 

Warnings: beware the dust

Bind the outside of the tube before tapping (Kevlar thread is excellent, from fly tying suppliers) to avoid splitting it

Heatshrink might do it but is better over the Kevlar to keep it intact

Probably coat the Kevlar with cyanogen in any event.

BTC

[Bruce Collinson]

Cyano.   Blasted autospeel.

[Neddy]

The reason I was looking at 0.8mm stainless this the size of the rod being that is already being used on the elevator. Piano wire I guess you would get it from music shop. 28g is very thin wire surely it would bend very easily. 

[Engine Doctor]

Most model shops should stock piano wire. 26 or 28 gauge is thin however its very springy compared to filler rod . Music shops probably wont know what your talking about these days 😶. Ive used 26g on  Frog 45 and a Taylorcraft both powered with Frog 1.5cc diesel engines. Used for elevator and rudder control its fine even when engjne is maxed out . The Frog 45 is quite quick for a smallish high winger when maxed out but is usualy flown at more sedate speeds and the 26g wire has been fine .  The glider pictured below which I have just converted to rc uses 26g wire running in snake inner for elevator . Its a bit over powered and is stupidly quick for what it is as it was the only motor yhst would fit in the box of bit i had.

If in doubt try a slightly thicker gauge, say 24g , provided it will slide easily in the outer tubing. I always smear the wire with silicone oil on assembly as it deters rust and reduces any friction.

Carbon rod is also very stiff and the "z"bends can be bound on using thead and cyano to lock the join . Dont use epoxy with carbon as small quantities mixed can go rubbery in time and let go.

Edited March 3 by Engine Doctor

[Caveman]

Neddy, your plane uses a pushrod system very similar to that used by Multiplex, i.e. a thin piano wire pushrod running in a plastic outer tube.

 

My Multiplex Easyglider uses a linkage stopper, as you describe, at the control horn end (with apologies for the poor photo) -

 

 

With a Z bend at the servo end -

 

 

The piano wire pushrod is 0.8mm diameter, inserted into a close fitting plastic tube that in turn runs in a larger diameter plastic tube - you should be able to see this in the above photo. A piano wire of SWG 20 (0.914mm dia. ) would fit into the smaller tube.

 

If you have retained the plastic tube then I would keep it simple and use a length of piano wire. Check what is the largest diameter which will fit with a good sliding fit in the tube and which will also fit into the linkage stopper, and use that with a Z bend at the servo.

 

SWG 21 is 0.814mm dia and would be a direct replacement for the original.

 

GDB

 

 

[Caveman]

Neddy, I think that you are in Australia. It would appear that piano wire is also known an Spring Wire and Music wire in Oz.

 

Have a look here - they sell 0.813mm dia. Spring Wire in 900mm lengths, which should be OK for your requirements.

 

Spring Wire (Size Table) (hobbyparts.com.au)

 

GDB

[Neddy]

Thanks yes I am in South Australia. Yesterday I went to our local welding gear supply shop I asked about getting a metre of 0 .8 mm  stainless steel MIG welding wire. He went out the back of the store then came back with a metre of it and gave it to me at no charge. He said he is always getting people in asking for it for model making push rods and also fishermen who use it for wire traces. It's got a slight bend due to coming off of coil of about a foot in diameter but it will easily be straightened. I have ordered a 9 gram servo which should arrive in a few days I have also got a horn assembly and some tubing.  Then the fun part will be setting it up to allow equal left and right travel with rudder. I have also ordered a A2 V3 stabiliser it will be interesting mounting that due to tight space in the cockpit.

[Neddy]

Stabiliser should read A3 V2  got the six axis model.