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

[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!

[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.

[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]

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 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]

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]

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.

[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.

[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]

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]

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]

Mark just to be sure then I will use the method Michael shows which eliminates the the risks. BTW the cross bar is not soldered but pressed through the holes and burnished like a revit. It is too close to the edge for sure as I got a bit selise with the house of balsa tuff grind and the dremel tool. I planed to make another tomorrow. Yes it is amazing how fast a glider can go without and engine. Again thanks for the education it is important to me!

[Mark9]

Steve, I have noticed one concern on my part when looking at your pictures as best I can, not being able to see the joint well enough. If the cross pin is held in only by the solder you are relying on the strength of only the solder to carry the weight of the entire aircraft and dynamic loads generated during towing. These could approach easily 10-20 times the weight of the aircraft, may be more, for short periods. Solder is not very strong. That is why in my design the cross pin is set back about 1.5 times the diameter of the pin, minimum, from the front edge of the tube. More would even be better for strength alone, but concessions were made for functionality.

Brass is significantly stronger than lead, tin or sliver which is what solder is most usually comprised of. In this case solder may work, but I would not want to trust it. Never the less at some point the solder joint will fail much sooner than the brass. Brazing is an acceptable method to join the two parts for this purpose, but many people do not have the equipment or skills to perform that process. I by the way have brazed parts in school, but do not have the equipment nor the place to perform that process either.

To give you a reference about possible RC sailplane aero loads a flier in California this year set a new world speed record for RC aircraft, a sailplane, of 545 MPH. In a symposium he told the attendees that they put a G sensor in the airplane that was capable of reading to 120 G's. When they looked at the data after the run which was in the low 540 range the sensor was maxed out. They do not know what the actual G loads are. They can only be calculated at this time because no sensor small enough to carry is available. When I began flying gliders in the 70's no one knew or even imagined it was possible to fly that fast with a model no less one powered only by the wind!

[SteveNeill]

You guys are so helpful and thank you Michael and Mark for your assistance and time...oh and cartoons! I built this based on both concepts and I can't believe how strong it is and once installed it will have a backplate as per Mark's so the tube can't be pulled through under load.
In addition I used all brass and a torch and solier. There is some square stock the music wire passes through. It went together easy and I have you both to thank.

[Mark9]

Good point Michael if the loads are high enough the tube can be removed at an undesired time or manner from the aircraft. That is why I incorporated the brass plate on the one for my Kate. Also, in the nose we almost always need to add weight, so why not have it do something? The old belt and suspenders concept.