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

[SteveNeill]

I was starting to put a tow release in my big Sinbad when I was warned that if it didn't have ailerons it would be unsuitable for towing. Shes only rudder and elevator.

[Mark9]

Stay-Brite is great stuff. Off to the field.

[SteveNeill]

Mark I'm OK with the single full size Futaba 3003 because I took the precaution of using Stay Brite silver solder to join the two control rods which BTW are dead on to the servos. One of the things that Seagull got right. But the release servo I am going to change as recommended. Thanks for the pictures and very clean and nice install Mark. Bravo.

[Mark9]

Steve, had time this morning to post a few pictures on my Ka-8 radio installation. Look them over and note how it looks. I had to put blocks under the servos because there would be too much friction on the rod into the housing otherwise. I had a thought when looking at the installation one could eliminate more parts (2 by my count) by using two smaller servos for the elevators instead of the domino joiner as Pheonix calls it. That might be a tight squeeze, but there are some mighty strong small servos out there. Also, the nice thing about using another servo is you also eliminate all the tiny grub screws which have a propensity to loosen.

[SteveNeill]

My friend Bob Martin came up with this for RC Submarines which I also enjoy. It could work well with the release or has this already been done? Here's one of my subs. 66 inches long scale model of the Nautilus.

[SteveNeill]

Thanks Mark no problem. Thanks for the link.

[Mark9]

Steve, please understand I'm not ignoring you. My club and I have offered to help Reto Fiolka a club member and other people from the USA decide who will go to the next RC Sailplane FAI F3B Worlds Competition representing the USA next year. I will be very busy all day for the next four days. It requires the airplane to fly three different tasks essentially unchanged. The only thing the pilot can alter is the weight and wing camber which must be done via the radio of the aircraft, actually a very complex series of tasks. I posted a link to the US site below.

http://www.teamusaf3b.com/what-is-f3b

[SteveNeill]

OK. Stop that. You're being silly. LOL!

[ARUP]

What did the French diver vacationing in America, who had decompression sickness, tell his friends he had who were visiting him in the hospital?

'Zee bends!' Sacré bleu!

[SteveNeill]

Mark to this day I have never had a E-Z connector fail. But there's always a first time. No doubt when I build the Cherokee RM 3rd scale next year I won't be using E-Z connectors at those loads from the large air frame. But I could start practicing the discipline now on my current K8. She's a 118 inch span and 5.5 pounds.

So what you are suggesting I'd like to see. One or two or maybe even three pictures can speak volumes. From what I'm reading you don't use the long arms but rather the wheels? It would just help me to see it. I don't air tow for the first time until Dec. 14th (weather permitting) which gives me time yet.

BTW I never forget. Three years ago I went to Normandy, a sobering affair that brought tears. I was so glad to go there and pay my respects. It was on my bucket list.

[Mark9]

True the additional clearance to insert a Z bend in the hole would not matter that much on a tow release since either it does or doesn't. But when I see them being used on aircraft for controls like the elevator and ailerons I cringe. Probably my OCD kicking in again. I just saw it as a good segway to discuss control linkages and a little about servo set-up.

OOH, another good point about horns on servos. I try to have the rod to hole lateral lines form as close to a 90 degree angle as possible when at center. This prevents mechanical differential to be introduced in the linkage system. Many know this, but I have seen many aircraft with it on controls like the rudder, not good IMO.

Sometimes I intentionally set up the linkage with mechanical differential though such as flaps that are used for glidepath control with inboard aileron mixing where one needs more down than up. Just all good setup tips that should be used on every airplane. However, computer radios have allowed many to get lazy by punching in some numbers and not take that care in their installations.

Most of what I mentioned applies to what we have learned flying our very thin wing competition gliders on monofilament lines. F3J is a class that has a 100 meters to the turnaround pulley line length , here half of which is mono on a winch, at the World Championships it is all mono towed by two strong quick people. F3B is a class where the line is 100 meters all mono on a winch. Monofilament when under tension can store a great deal of energy!

For those that fly in these classes many try to have all of the push rod linkage internal to the wing, which results in a very short horn set-up both on the servos and very short ones on the surfaces. This requires precise sizing of the pivot point holes. I use a #53 number drill for this and after the clevis pin is inserted and the horn rotated by hand a few times it is a snug rotational fit. If there is any slop in the control system then flutter can occur in the dive and pull out when releasing from the line to gain that extra energy. Not pretty as many airplanes have been destroyed that way... VNE reached rapidly on that airframe!

An amusing comment I remembered last night relating to trying to transpose 2D into 3D in one's mind. Shortly after hiring in at the company where we modified aircraft I was on the manufacturing floor supporting one of the assemblers about a plan I had composed. He mention he had to stop work on the assembly and wait for an engineering change as the parts would not fit together. The engineer came down and saw the assembly for the first time in 3D and said when looking at it, "well it worked in CAD." Remember to honor our Veterans today.

[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.

[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.

[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]

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!