New to the forum and scale sailplanes...and towing

[Mark9]

Sorry, the pictures were not easy to understand how it was constructed. Mine has no solder on the cross bar too. It is also a press fit into the holes where the area is double thickness brass K&S tubing. However, if the holes are a press fit for the cross bar and 1.5 hole diameter minimum from the edge of the part you should be good. The surrounding structure should hold the pin in place. The loads in the expected use are in a direction that will not press out the cross bar, only applying lateral loads to the method by which the bar is installed. It is then effectively a single piece of metal. The only way the cross bar could come out is if it was bent. That would be highly unlikely with a seven pound aircraft on the piano wire of the length and diameter being used.

By the way 1.5 minimum edge distance is the standard engineering practice for safe hole design we used in aircraft structures. Just some of the knowledge I picked up working around aircraft all my career. My first job at the firm I was with was to plan the operations required to build a part or assembly from an engineering drawing. This included what engineering specifications applied to the requirement. The plan was then sent to the shop floor with the words I wrote, the materials, the basic steps/work stations and or machine operations required to produce it. Feel free to ask any question you're not sure of I might have an answer then again I might not. Nobody knows everything, nor can they.

[SteveNeill]

My first attempt wasn't the double thickness which I'll get today. I did think about that. The tube in the nose is 1/4 inch aluminium as far as I can tell. The model came with it already installed.

The quarter inch brass fits nicely inside. And Mark when you say 1.5 you do mean 1.5 mm? What is great about my first try is once and example was made I could see and understand exactly how it works, how strong it is and the way to improve it for a usable mechanism. Thanks once again Mark.

Fridays at my studio are an all airplane day for me. Stopping at the local HS on the way in. Fun day.

Steve

[Mark9]

Great Steve, I'm still not sure you understand the concept of edge distance, so humor me for a minute or two. Looking at the attached .jpg if a hole in a support structure such as the circle A, in the rough sketch needs to be X in diameter, then the distance Y to the edge of the support structure needs to be X multiplied by the factor 1.5. For our use that is independent of the support structure material thickness.

Some examples would be if you install a 1" diameter rod in a part the hole needs to be 1.5" from the edge the part is installed on. If the hole being used is 1/2" in diameter the hole can be 3/4" from the edge. This is to prevent tear out from the supporting material.

The commonly accepted edge distance for man carrying aircraft is now roughly 2, including a few other engineering factors. However, since many of the aircraft we maintained and modified were older airframes that could get below the 2 factor. But, it required approval from the stress guys before the aircraft flew. They had to account for many other factors such as metal fatigue, thicknesses, where the component was, the expected loads, ETC.

For our aircraft and the material thicknesses we typically use, 1.5 is a pretty good factor to use with the expected loads for a typical RC model. Also, they do not hold the same level of safety of flight as a man carrying aircraft. Plus, many people would have difficulty holding the tolerances for man carrying aircraft with normal shop hand hobby tools. Those tolerances are not really necessary unless you are trying to push the envelope of design performance. Then one could possibly expect at times undesirable results trying to find the limit.

[SteveNeill]

Mark thanks that helps explain. I pretty sure I'm in the comfort zone. If the cross bar is too far back I found it is a huge problem at this small diameter on this 3 meter to thread the loop. Upcoming post will tell more.

[SteveNeill]

Thanks to Mark and Michael I managed to do another release mechanism based on their instruction. The local hobby shop did not have the thick walled brass stock I needed so I used two pieces of stock one inside the other and drilled through them both the 1.5 back from the leading edge.

I then cut some steel rod and press fitted it into the whole. The raining overhang I ground down with a Dremel until it fit inside the preinstalled tube in the K8. Keep in mind the K8 I have is a 3 meter so the release tube is a smaller diameter then the larger 5-5 meter ships most of you are flying. Give me time. I'll catch up.

Once I established it fit well the next trick was to use and small guide tube of brass and some piano wired for the release.

First up was to tin the small brass tube and flux the inside of the main brass tube with the bar. I oriented it so the wire came out over the cross bar and using a torch soldered it in place. All this was pretty easy the second time.

Now I have the mechanism and it's time to mount it. Last thing I wanted to do was epoxy it in. It might not hold and what if I need to service it for any reason in the future?

The fit is snug and being that it is there's little chance of it sliding back into the fuse. But coming forward under load I quested if soldering would be good enough. So I used a large wheel collar and that did the trick. It's super strong and there's no way it will move under tow.

Next I had to increase the arm on the servo so that it could pull the release wire easily as was suggested by Michael. As it turned out I didn't need to change the servo's location. Using a E-Z connector made it easy to adjust and lessened the load on the servo.

The hardest part was getting the 65 pound test fishing loop threaded. What I did was to grind and contour from underneath the cross bar so that when the loop is thread it naturally ends up in front of the release wire. That worked we have a success.

Using the radio I set the travel in the gear channel just enough so that when I hit the gear switch the servo retracts and easily releases the loop.

All this took about 3 hours to do. I put a lot of load on the loop and with all my might I could get it to move or break until I release it. I must have 50 easy 50 pound of pull on it.

The K8 is 4 pounds as it sits with everything in it but I have yet to balance the plane. It is as always it would seem...tail heavy. So we might get up to a whole 4.4 pounds I'll know today.

Thanks for all the help it is greatly appreciated.

[Mark9]

Yep, that's exactly why I designed one like I put in my Ka-8. The release in my little Fox is a pain to thread the loop. It's way back in there.

I was at my first big meet in Kansas two years ago and ended up having to fly the Fox, don't ask why. Anyhow I was using heavy weight monofilament line for my loops. I was only one that was and by the end of the second day I didn't have any. That evening in the club house having an adult beverage one my flying buddies took me to an old hand.

He quickly made me up three loops from coated stainless steel fishing leader. Real simple to do. Cut off a suitable length and tie a simple overhand knot in it, then trim the tails off. Being stiff wire they insert much more easily into deep tow release catches. It also is a great source of pull-pull cable. Here's a link to the leader. 20# would probably be plenty heavy enough for your Kate.

https://www.basspro.com/shop/en/Malin-Malon-7-Nylon-Coated-Stainless-Steel-Wire

[SteveNeill]

Mark this 65 lb test fishing line I believe is Kevlar based because it's hard to cut with a fresh Exacto blade. I have a huge roll of it and I posted pictures of the brand a few posts back. I got it at West Marine for rigging my 1850's RC schooner.

[Mark9]

Just saw your post and pictures. The one in my Ka-8 is quite similar. Mine is just two pieces of telescoping K&S tubing also. I might add a flat for the wheel collar to bite into under heavy load and prevent it from pulling the collar loose, if you haven't done that. Just my belt and suspenders approach.

Also, the wood dowels that set the position of my pull rod guide tube and holds it are only glued in with thin CA. Naturally it was all skuffed and degreased before I just dripped the CA into the assembly. OOH, the blocks were glued on the tube before I glued that assembly in as a subassembly. CA in shear only loadings is amazingly strong, if the joint is tight. Not so much if there are gaps in the joint. That is where most people fall short when using CA.

An interesting story about CA. I was working while in college at a local ring manufacturer, back in the 70's. I ran a small bench mounted mill that had a rotating fixture which allowed us to remove the excess gold and silver under the stone mount of a ring. One day the depth of cut stop broke. The tool and die manager came out with this "new glue"...CA. He put a few drops on it and told me to let is set up for about 10 minutes. It held for about two more days! By then their maintenance crew had fabricated another stop.

[SteveNeill]

Mark I have done that for sure. I usually do that with wheel collars when possible. I could find any balsa doweling in my balsa reservoir. So I figured the brass would work well too and it did.

Now that the plane is all working and control throws set I started in on the CG. I found I need a pound to get the right CG. But what I wondering about is it possible to cut through the forward bulkhead into the fiberglass nose piece. It must be ho;ow in there and it I move the lead in there and possibly the battery I might be able to shed some of that weight off. I could not believe it needed that much nose weight. but then again most ARF's I have come to know including my Habicht was the same. Even my scratch built Aquila need a bunch.

[Mark9]

Yes, it is disappointing to have to add all that lead in the nose, but one must. I have not actually seen the structure of the nose area in that airplane. Only how well the one flies that my friend has.

If the nose is hollow one could drill a hole and pour slow setting very thin epoxy in it mixed with lead shot. The best size to use is # 7 1/2 or 8 it may be hard to find now though, the powers that be don't want us to harm ourselves. You want to use very slow setting epoxy so the thixotropic reaction does not occur too fast and melt the plastic.

Average hobby resins set too fast. A reasonable source of small quantities of epoxy and different hardeners is West Marine. Their 105 resin is quite good, not overly expensive and they have four different hardeners with different working and cure times that use the same resin. I have a quart and their 206 hardener that I purchased years ago and it is still good. Also, just about anywhere there are boats you will find a West Marine. The 206 hardener gives a cure to solid time of 10-15 hours. The problem with many of the high quality resins is that they are usually sold in quantities that are too large for the hobbiest.

My Ventus when I get ready will have the West and shot poured in the nose. Back years ago when CG'ing a new TD ship with a fiberglass fuselage I poured lead in the nose. I would collect tire weights, ETC. suspend the nose of the airplane in a bath of ice water. I then would working from up wind (almost is never calm here) heat the lead in a can over a propane torch until it just melts. Quickly pour it in the nose and let it cool. Never even damaged a painted fuselage. Hey, I had a tin soldier casting set when I was a kid too.

[ARUP]

Steve, can you just turn the servo around within its bay to make the pin fit to it? Adding the extension to the arm actually is more work for the servo! All of those screws are just more reasons for something to fail. KISS(illy). You want the servo arm as short as possible to increase the power. The pin only needs to move enough to clear the hoop which is only about 1/4" or so. That's why I always have the servo arm pointing toward the nose of the sailplane when it is in the closed position. The servo gets to 'ramp up' torque before the pin moves very much. Also, can you fit a 'Z' bend on the end of the pin? Those E-Z connectors have a habit of not working at the worst time! Ask me a story about them whenever we fly together. Because of the debacle that ensued I never use those anymore and threw all of the ones I had in stock away!!! 'Z' bends never fail! I can imagine you are getting a little tired fiddling with the release but the release must be the best part of your sailplane for aerotow! Open up the hole in the wood where the pin is located. The hole isn't doing anything except possibly causing the pin to bind. Don't use 65lb test for your loops. Get something on the order of 10-20lb. You want the loop to break if the sailplane has a mishap on tow. I've seen wreckage get dragged halfway down a runway before anyone releases! Have an 'itchy' finger always at the ready on the tow release switch which most fliers have located on the gear retract switch. 'Itchy' trigger finger has saved my bacon a few times. I still broke the wing of the Balestruccio at Winamc last outing but I was off the tow in a fraction of a second when it started looking bad! I was happy with my performance even though the poor Balestruccio took yet another hit! It is repaired and flying!

Bear with us and you will have knowledge that insures success!

[SteveNeill]

I can do all those things Michael. The servo is on the retract switch. The servo is programed to move just enough to release the tow line. It hardly moves. I can move the servo close and use the shorter arm and Z bend it. Will do and many thanks!

[Mark9]

Michael, good points I do have a little more to add though. Steve, sorry I missed those and just saw it awhile ago. Also, sorry if this is a little wordy too since it took a while to type it to explain it, which hope I did, if not please ask. It is really hard to catch every potential failure point with 2 D photos from miles away. Also, having never looked “under the hood” of that particular model adds to the difficulty. It's always better to see the object in person.

Anyway, another way to think about the KISS method is ask yourself is this part really needed to do the job I want? Can I relocate something, can I use something else cost/labor effectively to do the same task, or worst case usually from experience or what one has learned from others what will fail, "this won't work at all"!

I personally try as best as possible to need to use 70% or more of the available servo travel for best resolution and power. Before computer radios we had to calculate that or make an example of our idea and see if it did. Also, servo technology was nowhere near what it is today. However, the shorter the arm is the tighter the parts fit needs to be and your construction techniques applying them. That is where tolerance build up really comes into play. I’m discussing thousandths of an inch here. A simple way to think about that is proportionality. As the part gets smaller the size of the gap to allow them to move, yet obtain the desired movement (preventing mushy controls at the same time) has to decrease as well.

On a related note I dislike "Z" bends. In order to insert them in the arm the bend radius has to go through the hole in the horn. For that to be done the radius of the bend, which is required so the metal does not fracture, plus the material diameter has to go through the hole and will at some point of insertion be in the hole at the same time. To do that the hole must be bigger than what you would want to reduce the mentioned tolerance build up, inducing unnecessary slop in your control linkage or a wallowed out hole simply put. Not desirable for crisp control response. A 90 degree bend would be better affixed with a wheel collar, as long as the collar always stays put.

Often a simple threaded clevis with a straight stiff control rod is the best inexpensive option when the room is available and the design allows. Even better still, reducing parts count, would be a properly silver soldered clevis on an appropriately sized straight control rod, effectively a single part at that point. All this assumes no binding in the system, so no additional hindrances and friction exist. I say straight because a curve or flex in a control system again reduces control feel, tolerances required to allow the desired movement.

Unfortunately, most of the hardware I've seen in the lower level ARF's IMO is terrible. I usually will throw it all away. In my experience the control rods are soft and flimsy in those kits. My Ka-8 from Phoenix Model had a Z bend in the two soft push rods outside the fuselage for the elevator to hook it to each separate control surface as designed, YIKES! I solved it with a different DuBro control horn set, two ball links and eliminated the bend for a little over $6. The push rod is now very straight. However, now even some of the hardware from American manufacturers seems to be sub-par, again just my experience and opinion which everybody has. That is another concern all its own.

Saw an interesting Nova a while ago about wooden bridges relating to good engineering. They are disappearing, many existing until recently were in excess of 100 years old here in the US and are/were still functioning. The methods required to maintain and repair them are disappearing. Only a few people know how to do either here now. The Chinese are having a similar problem, but some of theirs are over 400 years old.

The ones in China use no fasteners at all and have survived multiple earthquakes, with little or no damage. Just the weight of the structure and precise wood joinery holds them together and carries the required design loads! The construction method is also used in the temples of the Forbidden City, BTW. Good engineering is based on what has been learned to work and it takes time, with experience to know what that is. Even the best engineers keep learning as new answers to engineering questions are solved. So, as Michael said bear with us.

[SteveNeill]

Mark as always a very good read. Thank you.

I'm not a big fan of Z bends either for the reasons stated. But a right angle and a wheel collar is a good suggestion. And Michael's urging of me to move the servo closer to the movement will allow me to use a servo arm more directly. This should answer all the concerns expressed here in this now very interesting thread.

For any beginners here this will be quite enlightening at best. After all the hard work, money and time we spend on these models, it all ends depending on that thin line connected to the tow release. It is most important we get it right.

I will make the changes tomorrow. This will now be the third pass. I sure wish I lived closer to you. I haven't so far met anyone in southern California that has this experience. I joined the CVRC club in Visalia California. But I haven't yet heard back from them. I plan to go out there for an aeroetow on the 14th and 15th of December. The filed there is a great one and only a 3 hour drive for me.

And Mark as for the West Systems Epoxy I have worked with it for years now. I use to master and mold EDF jet kits a few years back and all my hard molds and fuses were made from it. I still have a stock of it at my studio.0

Here's to of the WW2 German jets I mastered and use to sell.

[ARUP]

Yeah... 'Z' bends aren't perfect, a 90 degree set up is about as good, ball links are nice and my big sailplanes use a clevis. On the smaller ones, like yours, I just drill an interference fit into the nylon horn. The nylon is somewhat 'slippery' which is nice. The 'Z' bend has to be 'massaged' with needle nose pliers to make sure it aligns the pin to hoop properly. A million dollars to the person who can get 5-10 thous movement detected in my set ups at the 'Z' bend end! The servo will have more gear lash. Remember, we just want the loop to release. We want it to release every time, with minimal battery drain and without stalling the servo. It makes no difference if your sailplanes flies or not although we do want your sailplane to fly but if it doesn't at least the release works! So long as you can release and not cause the tow pilot to have to let you go or, worse yet, cause the tow pilot's bird to get damaged that is all that is important!

The Me P1101 and FW Ta 183 are really cool! I started drawing a Blom und Voss P210 to fit a Jets Munt 100X turbine but then decided to trade the turbine for a LET Rheirer! The BV was to span 75" so the turbine would have been plenty with 22lbs thrust.