Konrad

As odd as it sounds one can even undercut the shaft and still have a stronger pin than the one that offered by the OEM. But generally I just like to use a conical stone and just grind the flat axially until the corner is gone. The other flats have survived my crash landing as the shaft has enough flex. Now the down side of this repair is that the shafts actually bend prior to snapping. This means that I have to cut what is left of the shaft close to the trunnion to remove the broken shaft.

The topic I'd like to discuss is stress risers (force concentrators) in the geometry of metal parts. What has brought this to my attention has been the rash of failed landing gear pins in the retracts of Freewing models within the group of fliers I fly with. All the failed pins I've seen fail at the closest corner to the trunnion block. These pin have flats cut into them to allow the grub screw a flat purchase to seat. These flats also index the strut and tiller arm to the pin. Unfortunately these flats are cut with sharp non-radiused cutters. An unfortunate characteristic of load stress is that it concentrates in features where the cross section changes abruptly. A sharp corner of these flats is such an abrupt change in cross section. These pin fail so easily that folks think there has been a problem with the heat treating (temper) of the pins. Folks are seeing that the pins fracture with little distortion (bend) in the shaft prior to failure. This may be true but as I haven't looked at the fracture face of the failed pins with a 30X glass I can't say what the metal's grain structure looks like. What I can say is that simply grinding the pin to remove the sharp corner the life of my pins has increased from say 3 landings to hundreds of landings. Also when I do have a less than ideal landing my pins now bend rather than shatter. Another common stress riser failure comes from bends in threaded areas of push rods. Don't do bent the threaded area of a push rod! The classic 60° thread has a sharp point at the minor diameter of a threaded shaft. If one bends the thread this sharp point will start a crack that can easily be seen with a 10X glass. I've been surprised to see metal push rods fail in the threaded area and at the same time see no damage to the plastic servo arm or horn. Please lay out your push rod path so that you don't need to bend the push rod in the threaded area of the push rod. This is why all thread rods often make very poor push rods. Now if you need to make a bend adjacent to the threaded area it is generally a good practice to thread the shaft after the shaft has need formed. Please see what not to do with a threaded push rod and how a properly fabricated push looks as the bend radius is away from the threads. Please, don't do bend the threaded area of a push rod! The model you save might be your. The life you save might be yours, or more to the point mine! Please think safety first when setting up and flying your models!

One needs to look at the geometry not only in side view but also in the fore and aft view. In the side view I was trying to show how to place the pushrod/servo to be tangent to the arc of the elevator pin. In the rear view I'm trying to show that we need to also place the push rod as close to tangent on the servo arm as possible. I often I see folks trying to center the spherical joint in the center of the vertical fin. This is in error as it biases the push rod to bind prematurely. I like to place the spherical joint as far as possible to the side that the servo arm is pointing. Now for my gliders I actually like to use a lot more down elevator than up elevator. I use this this kind of differential as I like to get a balanced feel when flying inverted (pushing the elevator stick forward). With this in mind I actually bias the servo arm to be pointing upward a bit when the elevator is in neutral (centered). I've also found that most gliders need this extra downward elevator when using crow in the landing mode. One of the driving criteria in my servo set ups is to use as much of the servo's rotation as possible. This results in much better resolution and power. With most modern computor radios we able to use over 100% (closer to 150%) of the servo's rotational movement. (Most computer radios allow one to over drive the servo. Mike Shellim's templets take advantage of this). This is why my servo arms are so short. Please recall that the elevator servo arm I showed earlier is only 4.7mm long. This allows for the servo to use as much of the output shaft rotation as is practical. But with all that rotation one needs to be aware of binding the push rod with the servo output shaft. Please note that I'm showing and using the required dual spherical joints with this kind of elevator actuation. With longer push rods one might get away with a simple Z bend at the servo arm. But with the short push rod there isn't enough flex in the push rod or clearance in the servo arm to allow for the range of motion that the push rod needs to travel. Please when setting up your servos use proper mechanics with a full understanding of geometry. I like to set up my servos and linkage to get the most from my mechanics. I then fall back on the programing power to fine tune the expected surface responce. P.S. I use distorted aluminum spacers to kep the spherical joint placed on the elevator drive pin.

Over the last few years I’ve noticed a change in the how the tail is being set up. For a long time it was fashionable to see All Moving Tails (AMT), Full Flying Tails (FFT) on performance gliders. These were thought to be better suited in trying to get the most efficiency out of glider as they did away with the drag producing hinge line. Today this might still be true but many glider drivers are now looking for control power, particularly on the slope. As a result we are seeing the camber changing stabilizer making a return to the high performance slope glider. I recently purchased 2 slope gliders that have these camber changing stabilizer set up. I was shocked to see that the design/manual doesn’t adhere to established mechanical practices. I don’t know if these designs were from a clean sheet of paper or if the designer was just modifying old designs to the “new” camber changing stabilizer. The models I’ll be showing are the Aeroic Schwing Corsica 108 (this is an out growth of the Schwing 88) and the Aeroic Alpinebrise 4m ( This looks like a modern take on the Multiplex Alpina). The first problem is that to my way of thinking the push rod should be set tangent to the control pin arc. What the designer seems to have over looked is that a line between the hinge and the drive pin is NOT parallel to the control surface cord line. As a result the designer appears to have placed the servo access window way out of position. With the Schwing Corsa 108 (yellow) you can see that to place the servo with the push rod tangent to the described arc the servo is almost totally out of the window opening. With the Alpinebrise (white) the placement of the servo as shown in the manual would put the servo in the rudder. For a host of reasons, that I might go into in another thread, I swapped places between the rudder and elevator. This now allows the elevator servo to be placed in the lower window keeping the elevator servo out of the rudder. Unfortunately this and the original elevator position do not allow access to half the servo mounting flanges. The other issue is that the manuals totally ignore that with the servo axis and the hinge axis placed 90° to each other that the push rod will need to move with two arcs at each end of the push rod. This will require some kind of spherical joint at both ends of the push rod. See the spherical joint on the Schwing Corsica 108 servo arm. With the Alpinebrise the designer isn’t staying with good mechanical practices as the drive pin is placed far too close to the hinge line. As a result the servo arm has to be less than 5mm long. With this small a servo arm any radial clearance in the drive train becomes a large fraction of the total motion available. This is driving the necessity to use externally supported servo output shafts. Attached is a photo of what I came up with that I had in the bottom of my hardware drawer to address these needs. The short push rod, short servo arm and the need for dual spherical ends is driving the need for some new hardware. As this need isn’t constrained to just one designer I think there might be a market for a bearing supported servo arm that accepts a spherical joint.

Have you guys thought of using a threaded shaft into the servo arm. I do this a lot when I need a servo arm less than 5mm. Here I'm showing how it is done with a 4mm arm. Note that any need for the push rod to go past the servo output shaft is addressed by having the push rod pass over the arm/output shaft. The threads are the retention feature. This works real well with metal arms. (This is shown for a glider I was setting up. But I hope you all get the idea.)

Here is what I received from the custom order with TJIRC. I really liked the clean look of the paint scheme. I set it up to look like a flashing sign as I roll fast going in the vertical. That is the bottom wing is inverse from the top with the black and white being switched. I had TJIRC add the bubble lace to the bottom of the left wing in white. This has worked out great! I was pleasantly surprise as I learned that in the intervening years TJIRC has added some significant changes to the construction of the mini Q. Key amongst these id the addition of carbon over the top of the fuselage bulkhead to hold this arch ridged. Also the bulkhead is now twice as thick ad the 2018 production run (yellow) mini Q I had. This was done to control some wing flapping we saw when we really pushed the mini Q at speed. It also adds a lot of durability against the ground loop. I was impressed to see that the hinge line is now reinforced with carbon on each side. This really stiffens up the trailing edge of the wing ahead of the hinge and stiffens the control surface. Of note it looks like the live hinge id made from durable polypropylene rather than the more delicate Kevlar. This has a slight cost in that the hinge is ever so slightly stiffer than a kevlar hinge (something to think about when sizing the servos). I don't know where the communication breakdown happened but my request for some added carbon on top and bottom of the drag spar as an alternate load path did not materialize. I'm not too concerned with this as the new carbon reinforced hinge might offer this and that there a large carbon servo mounting stiffener adjacent to the drag spar.

I report my findings to their distributor. But historically FrSky doesn't like to hear from me. But the real power to make changes is with our wallets! Flanged bearing are often much more expensive. But yes, that would work after one removes the misplaced lip. The idea of just removing the lip and placing an end plate to the outside of the pillow block is a simple effective and cheep solution, using materials any hobbyist would have in their possession. Finding a flanged bearing might be a problem. P.S. The pillow block might have to be narrowed the dimension of the flange to allow the servo to sit properly in the frame.

All things are possible. But why add another complication when installing IDS/external bearing supported servos. We as the consumer need to hold firms accountable for poor engineering! I'm trying to show the limitation of the FrSky external bearing servo tray. None of this should have happened had they thought through the design rather than copying (reverse engineered) the tray for their servo family. Folks, in the day of the information highway we should not tolerate sub-standard products. I saw a problem with the as manufactured product and the intended application and pointed it out. I love all the ideas offered to address these problem. But the OEMs need to be held accountable for poor engineering of products. I use the forums as a way to hold the firms accountable. I hope that my observations and solutions will make it much more difficult for firms to sell sub standard products. At the same time, I hope that these threads will help them make a much better product. Along with finding the engineering flaws, I hope to offer solutions for the end user of the products we purchased with our hard earned money. It is far too easy to say something is junk and run away. I really like to offer or find solutions to the problems often presented to us as a result of poor engineering. We need to hold management accountable for poor products! I'll step down off my soap box.

Thanks guys. No interference fit. The clearance between the ball bearing and the pillow block and arm journal is zero or more like a slip fit. It is extremely easy to push out the bearing while bringing the servo and forked arm into position. This is very poor engineering as the retention feature (lip) should have been placed on the outside of the bore not on the inside. Servorahmen had this problem with some of their early servo/IDS trays, such as the ones for the MKS HV6100 servo. Later designs placed the lip on the correct side of the bore (outside of the tray) to hold the bearing captive between this lip and the servo arm. The use of Loctite 638 is not recommended adjacent to thermoplastics as the compound will breakdown the plasticizer and make the plastic material very brittle. This servo tray is plastic. There is no access to the stub shaft after the servo tray is installed to manipulate a retention screw. One needs to be able to remove the servo arm as a function of fitting and maintenance. Glueing a retention cover on after the bearing is installed is a fix. But there is the same concern with glue contamination as with the capillary CA. A concern not often mentioned is that these bearing often corrod in the coastal sea air we often fly in. There is also the high probability of very fine sand contaminating the bearing. As such a properly designed external bearing servo tray needs to have a provision to allow the bearing to be serviceable during the life of the airframe. These tray do not, and as a result exhibit this design flaw. While not as glamorous the proper bearing for this application is the simple sleeve bearing from a dry self lubricating material (non-galling). After I've had my cup of tea. I'll see if I can take some photos and annotate them showing the various constraints we have to deal with, when using or designing applications that use the external bearing supported servo. P.S. I like the idea of removing this inner lip and installing a new endplate to the outside of the bearing pillow block. Thanks JD-8!

I'd like to point out what I see as an engineering flaw in the IDS servo trays from FrSky. Here I'll be showing FrSky's IDS (5611) but the issue is the same for the larger FrSky IDS (5101). I'm planing to use these FrSky IDS servo trays for their external bearing support feature on my NoBody build. I'll go into more detail in that thread but thought it might be good to give notice in the FrSky forum. The core issue is that there is no bearing retention feature for the ball bearing. Most external bearing support trays from Servorahmen had the bearing held captive between an outer lip and the servo arm. With these two trays the bearing stop lip is on the inside of the pillow block rather than on the outside. This means the the bearing is free to fall out of the pillow block and get lost in the wing. The only fix I've come up with is to use some thin CA and allow it to wick by capillary action between the bearing and the pillow block. This is very dangerous as there is a high probability that the CA might get inside the bearing and lock up the rolling elements. P.S. While not mentioned in the documents I've seen, the IDS for the 5611 will fit the W5651H servo.

As I was going to go with the cross over style of push rod (servo arm pointed down and the control horn pointed up) and I was going to try to set up the system as an IDS (Internal Drive System) I knew I was going to violate the rear drag spar. As such I asked that there be some extra carbon added the inner wing skins above and under drag spar adjacent to the servo pocket. I also asked for some clean up of the livery. At one time I built professional race cars. One of the axiom in American racing was that nobody rides for free. As such I don't like ads or livery that aren't paying for the privilege of riding on my vehicles. So I asked that these names be removed.

Very nice. Looks like somebody had some aptitude!

New cars just need a different set of tools. Now if the OEM will only sell to his dealer network then we are talking about the right to repair issue. I don't see new owners manuals even mentioning how to replace brake pads! I like modern cars as much of the art of old, is now taken care of by reading the fault codes. If folks aren't comfortable with the basic maintenance there is still the option of the dealership. My main point was just how ridiculous the powers that be are, in thinking that we (their customers) need to cautioned against drinking battery acid!

I'm involved in a restoration of a 1941 Buick. The owners manual actually has useful information on how to maintain the car. Such as how to adjust the valve tappets. My personal car is a 2018 Toyota Mirai. There is an actual warning not to drink the battery acid! But little or nothing about how to change the myriad of filters. Yes, they do go into how to change the air conditioning air filter behind the glove box. Really they need to warn today's consumer not to drink the battery acid!

I recently learned that Jerry at TJIRC is looking for ways to improve and expand his product line. I noticed that he was claiming to have a new lighter weight lay up of the mini Q. I was intrigued as I had a 6 year old mini Q. While I liked the over all model it had some issues that just kept me from finishing it. I contacted Jerry at TJIRC about a special order lay up of the mini Q. Most of the Special Orders he deals with have more to do with the livery and colors. I actually wanted some rather extensive changes to be made. Key was the placement of the control surface hinges on the bottom surfaces. I was keen to set up the model with IDS (cross over linkages). This was to add durability as it eliminates all the linkages hanging down from the wing that often gets snagged in the weeds and rocks. I also felt that this might add a lot of performance as the original set up is rather draggy with all the linkages out in the wind and that the push rods aren't placed in line with the relative air flow. Jerry said he could do the mods to the lay up and we came up with a price. Jerry still supported his dealer network and had me place this special order through his USA importer Aloft Hobbies.