Cable Replacement for H-9 DHC-2 Beaver
Not sure if it bothers anyone else here, but I just couldn't put my Beaver in the air with the marginal, unreliable cable system.
I'm an engineer (retired) who used to design and build production equipment and controls, the sort of stuff where somebody can get hurt (or worse) if things go wrong. This has taught me patience, but also has made me somewhat over-critical of accepted model aircraft methods, So I apologize if I am overthinking some of the Beavers systems.
I often let problems like this set and work through my mind rather than jump right into them and try to rig something custom or exotic on the spot. I've discovered this often results in something worse than the original problem.
I've used pull-pull systems before and hate them. They are a constant adjustment issue, and just plain sloppy and unreliable in my opinion.
I spent some time letting this problem rattle around, and while fixing up my "Nitrostick" (a frankenplane not currently "Alive") it came to me...
I've found a simple, fail-safe solution that isn't a lot of work, and won't cost you a lot.
Note: I don't work for, or sell Sullivan Products. I think they are among the best parts made though (which is why I had them in stock).
With the pull-pull system, Slack or broken cables cause a loss of control. period.
I watched a very sad story on TV a few nights back about a broken 'Elevator up' cable on a Twin Otter that took 20 lives due to cable wear from parked Otters (with the controls locked down - apparently with the airplane version of "The Club" antitheft device? - I never understood that part). Every time a big jet turned around, the Otters were getting hit with jet wash and it was wearing out the Elevator cable. When the pilot took the flaps off, and went to climbout, the stick just fell in his lap as the last few strands broke. I can't imagine the feeling. (Smithsonian Channel if you're interested)
I chose to divorce the wheel controls from the rudder, because I don't believe in parasitic loads on critical systems, but you can do it with a single servo if you wish. Hangar 9 conveniently left an extra Servo pocket which I am using for my rear steering, and the center Servo is now dedicated to the Rudder only. I happen to have 16 channels (twin X8Rs) so my steering gets it's own system, but you can do it any way you like.
The factory system is OK, (level 1)
An improvement would be another Servo for the wheel, and "Y" cable, (Level 2)
or you can go the full isolation like I did (Level 3)
You could even go "Super Cheap" and just replace the Rudder cables with Sullivans, and leave the floppy fishing wire cables for the wheel, but as you've probably gathered by now, that's not how I do things. A little play in the ground steering probably won't bug most people. It will me.
First, purchase Sullivan Gold-N-Rod #S504 control rod set. (Red/Yellow, 48", #2-56) This will replace your wheel wires. It's that simple, cut the wires, and pull em out, unclip the clevisis, and install the Gold-N-Rods, using the factory cable guides. Fit is very good, and the fact that the Sullivan instructions indicate you should not leave anymore than 1 inch of "stickout" is irrelevant, as we are still using these as a pull-pull system. Even with the 3-4" of stickout at the tailwheel, it steers like it's on rails. The beauty of this system is that if it goes out of adjustment, one side breaks or becomes unconnected, the other WILL act as a push-pull rod and get you safely on the ground. (not that it matters with the wheel, but the Rudder is needed to land safely. This IS NOT the case with the cable system, if it fails, you lose all control. (In engineering parlance - you just can't push a rope)
OK, now you've proven the concept - Scott's not crazy (at not about this anyway)
So, foward to the Rudder upgrade. This WILL tighten up the linkage to your rudder, and make your plane fly tight and precise (rudderwise anyway).
(Pet peeve of mine. Worn out or just generally sloppy controls, like cheap servos with worn out bushings, loos or improperly installed arms or horns, and flex in the surface itself all make a plane fly like a 1969 Dodge Truck with original frontend parts and 200,000 miles on it. You "point" it rather than "steer" it.
If you're happy with the functioning of this mod, move on to the Rudder, The Critical control surface, which we REALLY want smooth, precise, and failsafe.
OK, done rambling, Here's the Rudder Mod:
You'll need a Sullivan Gold-N-Rod #S505 control rod set. This is the next size larger, and you'll have to do some test fitting to make sure they do indeed fit properly in the Beaver's factory guides, Mine JUST fit perfect, but if it had been any tighter, I may have had to shave the Sullivan liner a tad, or even drop down to a #504 size. I happened to have a set of #S505s, and thay JUST fit lengthwise. If I were purchasing them, I'd buy the 48" ones (#S506) just for insurance.
The Procedure is the same, other than fitting the inner cables, and it's a little tougher to make the clevis connections at he Rudder arm (on a retrofit, if you do this while you're building, it's easier than the factory method, because you'll be spared the tensioning process)
I had originally intended to use a set of the Big Black ones (#S518) for the rudder, but once i started really looking things over, it became clear that that level of strength isn't needed on this plane, and it would require installation of bracketry through the length of the fuselage. I had fully intended to make 2 full sets of twin tube brackets, one set for the rudder and one for the tailwheel, but patience and thought prevailed, leading to the method I suggest here.
While I have used Sullivan rods on many planes, I would not use 2-56 sized rods on a 30cc plane. At 20-25lbs RTF, it my opinion you need 4-40 sized rods.
I have used combos of pull/pull and control rods on quite a few planes in the past with complete success. My control rods are solid 3/16" aluminum round stock, with 3 mm ends and matching ball links. I like using the 200lb test downrigger cables for the pull/ pull systems. I'll try to find a picture of my system on my 29 1/2 lb 132" Turbo Beaver and include it here. Doug B
You got me thinking, so I ran the numbers.
The rudder on the Hangar 9 30cc Beaver is approximately 5” deep x 12” high (very conservatively)
which is .42 square feet (60 sq inches)
The commonly in use formula for wind loading @ 90degrees incidence is (wind Speed in MPH)squared x .0027.
While this isn't a gnats eyelash formula, it condenses about a full page of Bernoulli etc into a conventionally used formula. I think it was developed by the guys testing how big an atomic bomb it takes to blow down your basic bridge or something like that. If you get into the angle of attack and all the REAL numbers, it is, admittedly well past my math skills. So, like most engineers, I resort to computers and tables.
WORST CASE with a 90 degree rudder angle (it only goes to 30 degrees on my Beaver, but I'm really going worst case here) we end up @100MPH with (100 x 100) or 1000 x .0027=2.7lbs. (I know what you're thinking, and it seemed low to me too, but I did some checking. Imagine a piece of foamboard the size of the Beaver rudder, then put a 3 lb weight on it. How hard would you have to blow on it to tip it over - that puts it back in perspective. Did for me anyway)
This is a 2.7lbs load at the rudder surface, to translate it to load on the pushrod, we need to figure the leverage on the rudder control arm. I call it an inch - It's a tad less, but I can't get my calipers in there easily.
If we do the leverage calculations again assuming worst case with ALL of the load on the tip of the rudder (as opposed to a distribution based on shape) The rudder is 5” from the pivot t the rear tip, so I used that dimension for the rudder moment, to the 1” control arm, we take the load up by a factor of 5, for a net load on the Sullivan rod of 13.5lbs.
I looked through Machinery's Handbook Guide #29 and didn't see any thread strength data for polyethylene (although it can take hours to sort through those tables), so I have a plan just to be sure:
I have a scrap of Sullivan rod, a spare clevis, and piece of Sullivan 2-56 threaded rod. I'm going to do a pull test with a peak recording scale (fishing) and see what the Sullivan is capable of, but I'm betting well over 100lbs to failure.
135lbs would be a 10 : 1 safety factor, and beyond that, we've exaggerated our numbers throughout our calculations that I have no doubt the final number (13.5lbs) is well beyond double what the actual loading would be if you were to use real world numbers.
I'm also going to go out on a limb and predict the failure will be at the clevis, NOT the polyethylene/allthread joint. (I've always looked at the interwoven fingers which form the threaded barrel of the Sullivan (and others) clevis. Now I guess I'll get to see.
I'm hoping to test this weekend, (Got to break in an engine in the shop anyway, I can rig the test while the engine runs in) and will include photos (maybe even film if I can get my camera to put out a windows compatible format)
I too, am a great lakes fishermen and recently changed over from downrigger wire to spectra (Power Pro), so have a LOT of nylon coated 200lb wire. I have no doubt to it's ultimate strength, which is well past the Sullivan pushrods (We've dragged 15lb cannonballs all over lake Ontario and never broke a wire, despite some hard bottom strikes)
A lot of this is convention, and I had no doubt people would jump to the defense of the old pull-pull system. It's time proven, and ingrained in the culture. I'm just looking for a better way.
Most guys put huge servos (guilty here) and pushrods or wires capable of transmitting 200lbs or more of force to control surfaces that would be torn from the plane at 25 or 30lbs of force.
I'm just as guilty, have one plane with 3mm STEEL hiem joints throughout, and steel 3mm rods with carbon fiber jackets. Actually, That was the plane that made me start to rethink this super - strong control gear connected to balsa framework and plastic hinges, supporting thin plastic film........ and just how much force is really on these surfaces? Nobody could give me a straight answer, so I started doing my own math. It's way less than you'd think.
Just to be clear here, I'm NOT looking for a fight, or any nastiness. None intended. I simply saw what I think is a better way and thought other Beaver owners would appreciate it. My pull-pull planes always seem to have loose rudders and I 'd like to try something more precise.
Clear Skies, Scott
United States, VA, Clifton
Joined Nov 2005
Scott, one thing I discovered while designing airplanes is not to rely on static load. Aerodynamics introduces a lot of dynamic load, which we had to test the control surfaces in the wind tunnel with strain gauges. However, a good rule of thumb is to use 5X static load, but you won't see that at our RC airspeeds. Then our aircraft safety factor is 1.5.
After all that, the killer is usually fatigue. The parts strain over cycles, and that is hard to design for. If it doesn't flex or rattle; not to worry. If it does, beef it up.
I like your analysis.
I've got about 22 planes under my belt at this point, since I started up the hobby again. Many are electric, some are scratchbuilds, and a few are ARFs. This is my 3rd giant scale.
I'm disabled (which is why had to close my Engineering firm - had a "simple surgery" go very wrong in 1998) so I have time, although projects often take longer than I'd like, and I don't get to fly nearly as much as I'd like to. I can't do much physical on account of my issues, so I've taken up building, and have done a fair amount. I started flying when in my late teens, and always wanted to get back to it. Still have a 72Mhz car from those days - resistor speed control and 2 ch Futaba and all. Wish I has a plane or 2 from that era.
Even so, I still consider myself a novice. There are many of you folks from I can learn a lot, and I listen to all thoughts.
The wire in the pull-pull planes is from a variety of sources, usually the kit, which is often Du-Bro, or bulk downrigger wire as has been discussed. I have installed and removed wire systems a couple of times now, (all factory kit supplied hardware that came with the plane) just not being happy with the behavior. None of it has any stretch to it (at the pressures we are putting on it) Maybe I'm too fussy, but when the wire is tight enough to give precise control, it beats up the bearing, while this is easily handled by using a ball bearing servo in a larger plane, the tail bearing is often just metal on wood. The tension of the wire, in effect is created by using the planes frame as a compression spring, and the constant tension on it seems to cause framework distortion and thus tension changes. I don't like my Balsa frame under constant tension, and it's long term effects concern me. I often find 1/2 to 1 turn on a 4-40 screws is often too much of a change in tension. Going DOWN in cable size is something I have NOT tried. Maybe 60lb is a little stretchier, and more well behaved than the heavier stuff. 60lb is plenty strong for the job. Probably has better fatigue resistance as well.
My original thought was to add a sliding servo mount, and use a spring to maintain the tension by pulling the servo forward (with enough force it won't be overcome by the control loads)
Adjustment is difficult, as getting the rudder centered and the wire tension where I want it tends to lead me into an hour or more of chasing my tail. Maybe I just need to be shown the correct way by someone who has mastered the system, but it just seems like a stop-gap method, considering the options these days, and I personally suspect it's a holdover from times past. There are the relatively new servo kits, often used by 3d pilots, and sometimes even ganging up to 3 or 4 servos. Some of thes provide adjustment methods that are more covienient, and possibly more reliable, but with the powerful servos available these days reasonably priced as long as you're not too brand concious, I can't imagine the need for 3 servos on a rudder unless you're going to go mega-scale (1/2 or larger), and in that case, I'd want to distribute the force, not concentrate it in one power train.
Tclaridfge, thanks for the positive thoughts, I agree with you that consideration of dynamic forces is very important, one of the things that's kept me in engineering through the last 45yrs is the things that sneak up and surprise you. The pitfalls are rarely obvious. I've got stories you wouldn't believe. Nature sure has a way of reminding you whose in charge.
I envy your wind tunnel experience, I never made it into one of them.
I was in everything from semiconductor process equipment @ IBM to Arc furnace Hydraulics for EMCI steel mills, and seeing all the wild stuff is what I miss most. I'll never forget watching a schoolbus sized block of red aluminum reduced to a mile of 1" sheet by rollers at ALCAN. My heart still runs fast from that sight. They roll that ingot into soda can stock in several stages, it's all surreal to watch. Scary too, you always feel too close to escape if it goes wrong.
I really, really miss working....
I thought I created this saying many years ago, but guess it's a universal truth: The more you know, the more you know you don't know.
Clear Skies, Scott.
|Category||Thread||Thread Starter||Forum||Replies||Last Post|
|Sold||Flyzone DHC-2 Beaver||jbynum1631||Aircraft - Electric - Airplanes (FS/W)||6||Feb 06, 2013 09:28 PM|
|For Sale||Flyzone DHC-2 Beaver||jbynum1631||Aircraft - Electric - Airplanes (FS/W)||9||Jan 20, 2013 09:27 PM|
|For Sale||Flyzone DHC-2 Beaver||jbynum1631||Aircraft - Electric - Airplanes (FS/W)||1||Jan 11, 2013 07:32 PM|
|Discussion||DHC-2 Beaver (Bush Plane) STOL EP ARF from Hobbyking||maestros||Scale ARF/RTF||4||Apr 27, 2010 03:17 PM|
|Sold||E-Flite Beaver DHC-2 Seattle Area Ready-to-Fly||viperjody||Aircraft - Electric - Airplanes (FS/W)||1||Apr 10, 2010 09:39 AM|