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Author Topic: VP Hubs - for the techies among us  (Read 20055 times)
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Tapio Linkosalo
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« Reply #125 on: May 07, 2015, 04:45:44 AM »

Thanks!

A few comments: I landed with 50 turns remaining and the model seemed to start sinking quite rapidly, so I think it is about the end of the available turns/torque. With 79 average rpm, 50 turns would be good for 30+ seconds, but to be able to drain them I would have needed much more altitude. Actually that was the reason I tried to increase the bottom pitch, to lower the RPM and make the turns last longer. But alas it did not seem to have much effect on the flight.

I might try adjusting the high pitch, but on the other hand, the adjustment seems very sensitive. I think I increased it a little (half turn or quarter) to stop serious ceiling scraping into this 6m climb, so I fear that if I open it up much, I will stop all the climb. Even now the flight pattern is such that the model climbs to 5 meters in 30 seconds, and the levels off, gaining very little extra altitude after that initial burst, so I need to be careful with that adjustment, to get enough altitude but not too much.

Indeed I should open the spring to let the model sink to lower altitudes before the prop reduces pitch and model climbs again. Remains to be seen if that gains much more duration, while of course it would reduce the risks when avoiding ceiling contact.

This was actually the first motors I cut from a batch of 99/7, and the rubber seems to be quite good. I feel that I got a bit more torque than from 99/5 of similar loop length, and yet the number of turns was comparable. I had used 160mm half motors of the later before, they took about the same turns, but landed with more turns on. I might even be able to squeeze in a bit more torque, as I have would two half-motors to that 60g*cm torque a couple of times now, and no breakage so far. On the other hand, that seems to be at about the upper limit of torque that the model is able to handle, as letting the prop turn just a few turns before launch makes the launch pattern much nicer compered to launching right away, which makes the model bank to the left and make a wide open initial turn (bad thing in such a small gym as I fly in!) before the initial torque burns off and model resumes more normal flying attitude. The launch on the video is with the higher torque, and it barely avoids colliding the basketball hoop on the left wall... :-)
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Maxout
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« Reply #126 on: May 07, 2015, 07:29:18 AM »

A few comments: I landed with 50 turns remaining and the model seemed to start sinking quite rapidly, so I think it is about the end of the available turns/torque. With 79 average rpm, 50 turns would be good for 30+ seconds, but to be able to drain them I would have needed much more altitude.

For cat II, 50 turns is an admirable amount to land with. I wouldn't try to use up that last bit. Save it for a warm day (the hotter it is, the more of those turns you'll use up).

Actually that was the reason I tried to increase the bottom pitch, to lower the RPM and make the turns last longer. But alas it did not seem to have much effect on the flight.

Yeah, unless you go below the optimum low pitch, you don't gain anything from increasing the pitch in hopes of slowing down the prop (actually you usually lose performance).

I might try adjusting the high pitch, but on the other hand, the adjustment seems very sensitive. I think I increased it a little (half turn or quarter) to stop serious ceiling scraping into this 6m climb, so I fear that if I open it up much, I will stop all the climb.

Try another half turn. As fast as it's climbing, you're still not terribly close to stalling the prop. If it does start showing signs of stalling (the prop, not the wing), reduce your bracing tension ever so slightly to get the nose to stay lower on launch (and only a tiny bit lower). Like I said, my own F1M only climbs a few feet at full torque, then just hangs out there at the same altitude for several minutes before starting a slight descent and then climbing back away. You're ahead of me on performance, but my prop is old and pitiful. Wink I do typically find that peak performance occurs when you climb to roughly 2/3 of target height on high pitch, then descend back and climb all the way up with a couple light bumps on low pitch. The stiffer your spring, the lower the optimum becomes for your first climb because you need to start "climbing on the spring".

Indeed I should open the spring to let the model sink to lower altitudes before the prop reduces pitch and model climbs again. Remains to be seen if that gains much more duration, while of course it would reduce the risks when avoiding ceiling contact.

It always gains you time to descend more if you're bumping the ceiling. Anytime the model is bumping the ceiling, the prop is spinning faster than it needs to. The amount of bumping that you were doing indicates that the model is losing several minutes of flight potential (in full motor terms--you're losing about 1 minute on your half motors).

On the other hand, that seems to be at about the upper limit of torque that the model is able to handle, as letting the prop turn just a few turns before launch makes the launch pattern much nicer compered to launching right away, which makes the model bank to the left and make a wide open initial turn (bad thing in such a small gym as I fly in!) before the initial torque burns off and model resumes more normal flying attitude. The launch on the video is with the higher torque, and it barely avoids colliding the basketball hoop on the left wall... :-)

Here's the solution: wind your motor and let it sit for a moment while you carry your steering pole out to your launch site. Remove a small segment of the pole so it will be manageable (like to top two sections if it's a put-over). Now go back and load up the plane. As soon as the model leaves your hand, reach down and grab the short pole segment and follow the model, steering by the tailboom as required. Usually after the first circle or two the model will stabilize, but you need the rest of the pole present to quickly extend those top segments as the model climbs.

Any power you leave off because the model won't turn at that power, that's wasted energy and lost seconds (or even minutes) on the clock.
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Tapio Linkosalo
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« Reply #127 on: May 07, 2015, 08:11:44 AM »


Um, that was 50 turns on the half motor, so it is 100 for the full motor, and that would be a minute... My mistake. Anyway, thanks for the comments, I'll try to open up the top stop and also slightly the spring. Next session will actually be the contest day, 2 weeks and 3 days from today! :-)

 
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Tapio Linkosalo
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« Reply #128 on: May 08, 2015, 07:40:32 AM »

It always gains you time to descend more if you're bumping the ceiling. Anytime the model is bumping the ceiling, the prop is spinning faster than it needs to. The amount of bumping that you were doing indicates that the model is losing several minutes of flight potential (in full motor terms--you're losing about 1 minute on your half motors).

I have been thinking about merits of two climbs vs. changing the prop pitch continuously to sustain altitude, and came to think that actually when a model is climbing it is converting rubber energy into potential energy (altitude), which it can then later exchange to duration, when (in descending flight) less energy is required for the flight. From this perspective it is obvious why ceiling bumping is bad - the model is burning the energy, but as the ceiling limits the ascend, the excess energy is wasted.

Still I could not figure out whether it would be better to make two climbs or not. I guess it eventually boils down the to the variation of prop efficiency at different pitch; it would be most efficient to fly as long as possible with the best efficiency.
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Maxout
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« Reply #129 on: May 08, 2015, 08:42:19 AM »

Still I could not figure out whether it would be better to make two climbs or not. I guess it eventually boils down the to the variation of prop efficiency at different pitch; it would be most efficient to fly as long as possible with the best efficiency.

Bear in mind this is theoretical and based largely on empirical data from F1D flying, but I'm going to throw it out as a possibility.

My experience is that all of my attempts at flying a single climb in low ceiling result in extremely high (90-100") high pitch settings, which for most blade designs stalls out the blade roots (this typically increases rather than decreasing launch RPM, so it's blowing power out the window for no return). Also, the spring required has to be fairly stiff to bring the prop pitch down early on, and since it's a linear curve, but the rubber torque is nonlinear, it's almost impossible to get the prop onto the low pitch stop prior to reaching minimum-torque-to-climb without excessive ceiling bumping. More to the point, it's proven completely impossible for me. Folks like Kornichuk have solved that by going to two springs, but I'm resisting as long as possible. The single climb, for F1D at least, starts to become a possibility in high Cat II ceilings on up, which interestingly is where Brett Sanborn has dominated. It's interesting to note that in conversations with him, Brett has described Cat I as a total mystery to him.

So at the end of the day, it would appear that the dominant efficiency factor is to just stay off the ceiling while still beingat the ceiling when the model reaches minimum-torque-to-climb. If that requires the model to climb all the way up, descend within 6" of the floor, and then climb all the way back up, so be it.

I will say that the pitch settings for high pitch cruise and low pitch climb/descent are drastically different. If I could have a VIT on my model to add 1.5 degrees incidence just as the pitch starts dropping, I could add 2 minutes or more to my flight times. I found that I could get a model to stay at the ceiling on high pitch for more than 10 minutes when running 1 degree incidence, but that was too little incidence for it to climb at all on low pitch. I even tried grabbing the model out of the air and adding in another degree of incidence after the pitch change started, and the model climbed all the way back up to the ceiling when it normally would not have been able to climb halfway back up.

Perhaps it's time for me to cook up a gadget...great...like I need another gizmo with a torque adjuster screw!
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Olbill
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« Reply #130 on: May 08, 2015, 08:57:31 AM »

I thought that was one of the functions of having slack in the bracing wire.
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Maxout
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« Reply #131 on: May 08, 2015, 11:24:32 AM »

I thought that was one of the functions of having slack in the bracing wire.

I tried that. The stick bends back too quickly. Causes the model to go flat at full torque, then the prop stalls 90 seconds into the flight, the model falls back to the floor, and oh, if you have the preload high enough, it climbs happily away just before it hits the floor. And you end up 3 minutes behind everybody else. Wink
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Tapio Linkosalo
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« Reply #132 on: May 08, 2015, 01:25:00 PM »

Interesting discussion.

Now, first of all, I assume that above when Joshua says "pitch" referring to the different settings needed for high pitch cruise and low pitch climb, he means decalage? Or is that also called pitch.

Anyway  find it curious that different decalage would be needed. I would assume, that the optimal decalage for given trim speed (i.e. prop thrust) would be constant. I cannot see why prop settings would affect the trim settings. Except maybe one feature might affect: if the motor stick twists, it adds angle inboard and reduces it outboard. And as (if) the wing has offset, then these two wing halves are different. So adding wing twist will also increase the angle. Maybe this is the reason the model becomes under-elevated at low pitch setting (it is actually low torque condition)?

Short of that, I find the CG location and decalage the most important feature impacting the model trims at different phases of flight. I used to have long (400mm, 16in) peg distance on my F1M's. They are ok for low ceilings, but tend to go flat on high torque launches. No problem if the ceiling is low, but bad if you aim for a high ceiling. It was not a matter of bracing, as much I tightened it or added new thread, the model still went flat. Moving the back hook forward (hook distance is now 300mm, 12in) changed the model behaviour completely. Positive climb at high torque, no problem of tucking in. I conclude that CG more forward and more decalage was the cure. Then for F1D the CG is still further forward, and the problem (as in Slanic salt mine) is the opposite, the model tends to pitch up too much on launch. In 2014 I tried to loosen the rigging, but had the same experience as Josh mentioned, it helps for the initial launch, but in 20 to 30 seconds the model pitches up and stalls. I assumed that this is because the motor looses the torque peak, and the motor tube straightens. Too early for a safe steep take-off.
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Maxout
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« Reply #133 on: May 08, 2015, 02:44:20 PM »

Now, first of all, I assume that above when Joshua says "pitch" referring to the different settings needed for high pitch cruise and low pitch climb, he means decalage? Or is that also called pitch.

Notice I've been talking in terms of pitch and incidence. Pitch means prop pitch. High prop pitch for first climb, low prop pitch for second climb. Ok, we're past that bit...

You need the model to fly faster than normal with the prop at high pitch. There's an optimum forward speed at which the model can climb at a higher prop pitch, and hence lower rpm, than the speed needed to achieve a climb at the minimum-torque-to-climb. If you can hit both of these, you can achieve a major improvement in flight times. Now, there are some tricks to this. Big, low camber stabs are particularly useful in low ceiling flying and produce a model whose pitch trim (pitch attitude trim, not prop pitch), and hence equilibrium airspeed, changes according prop pitch setting. My models fly at greatly elevated speed at high torque, and they slow down as the torque backs off and the prop pitch starts changing over. Unfortunately, some of that change is torque dependent, so the model starts pitching its nose up as the torque backs off. When this happens (around 5 degrees nose up), the root of the descending blade starts to stall. So the blades end up stalled during an ever increasing portion of their rotation. This causes the characteristic shuddering or "nose wag" of the model as the prop rotates from horizontal to vertical. As soon as this blade stall and nose wag start to become significant, the model starts descending, often at a torque setting high enough that it's still capable of a climb were the stab incidence lower. Problem of course, is that this lower stab incidence corresponds to a setting at which the most simply will not climb at low pitch. And thus this unending cycle continues...

None of this is a major factor in high ceiling flying. You can just add downthrust to prevent the stalling that tends to occur 2-3 minutes into the flight, and the cruise doesn't suffer that much. Still further, you can set the stab to zero incidence and put all the incidence in the wing, and off you go. Cailliau and Kang Lee both use this trim setup with results that speak for themselves.
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Tapio Linkosalo
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« Reply #134 on: May 08, 2015, 03:08:41 PM »

When you move the incidence to the wing you are actually increasing prop downthrust relative to the model flying attitude.... Which would increase the pitch down moment of the motor thrust at high torque.
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ykleetx
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« Reply #135 on: May 09, 2015, 11:51:04 AM »

Tapio,

Adding wing incidence isn't the same thing as adding down thrust.  When you actually add downthrust at the bearing, for example, the thrust line changes relative to the center of gravity and center of drag.  Adding wing incidence does not change the thrust line relative to the center of gravity.

I think it's better to think of downthrust as decreasing the amount of up moment.  Too much up moment is the common problem we face in a high-ceiling launch.

-Kang
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Olbill
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« Reply #136 on: May 09, 2015, 01:49:23 PM »

And of course adding down thrust to the bearing pinches the prop shaft causing an unbearable* increase in friction losses as does adding side thrust at the bearing.

*(to me any increase in friction loss is unbearable)
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ykleetx
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« Reply #137 on: May 09, 2015, 03:59:43 PM »

Having incidence in the wing (instead of only in the stab) changes the thrust line relative to the direction of flight.  That is, you can fly less "nose up" trim, or you can have "nose level" or "nose down".   I'm not sure how exactly this helps with high-torque launches, but it does.
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Tapio Linkosalo
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« Reply #138 on: March 01, 2016, 08:23:20 AM »

As this thread is now more about adjusting the VP prop than constructing it, I'll carry on. WIthin a year I have had a few good sessions at my local 7 meter (sub Cat I) gym and full motor with a prop dedicated for Cat I, so I called for an option to try a Finnish record. My test flights had shown that I should aim for a 310mm motor. On the first outing I broke my only ones at that size, so had to try a 285mm (11.4") motor. Only succeeded 13 1/2 minutes. The interesting observation was that with a shorter motor, the model descended a longer time after the first climb. I had noticed this before - use thinner motor than the prop is trimmed for, and the second climb starts earlier & higher! With the short motor I was struggling to get the second climb to start before the model landed at high pitch.

So for the second session two weeks later I cut some new 310mm (12.4") motors. With the first motor had some trim problems, and second try on the same motor did not climb, so I took another one. Tan II 99/7 (seems like as good rubber as 99/5) took 1360 turns to max torque of 49g*cm (0.7 oz in - pathetically low!). No backoff. The model climbed too hot to hit the ceiling in less than two minutes. Almost hung up and tail slided to half the hall height. After that it came down a bit and started another climb at 7 minutes. The second climb was a metre or two short of the ceiling, so without the tailslide the flight would have been longer (as second climb would have gone all the way to the ceiling). While descending it also hit a basketball ring and lost another meter of height. Also the flight needed a lot of steering, probably due to heating the air in the gym was drifting a lot. Despite all problems I got 14'14", a new Finnish record. I think there would be potential to make 15 minutes even in that hall. I shot a video of the flight (to be able to count the prop turns and thus analyze the VP functions later): https://www.youtube.com/watch?v=whrPWIzs98U

Landed with 280 turns, average RPM 72, so there would have been room for a slightly longer flight, if there were not the collisions with the ceiling and the basket ball ring. Also worth noting the the altitude loss and climb after the tailslide was less than half the site height, so with this 310mm motor the flight was level for a good part of it. As said, with 285mm motor (taking 1200 turns) the 7 meter height is barely sufficient for the descent after touching the ceiling!

« Last Edit: March 01, 2016, 08:41:05 AM by Tapio Linkosalo » Logged
Maxout
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« Reply #139 on: March 01, 2016, 10:46:00 AM »

Wow. Tough conditions. If you could get smoother air it would add a lot of time. The amount of energy being wasted on just maintaining altitude in that turbulence is significant. I do think you can do over 15 if you can just get smoother air.
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Tapio Linkosalo
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« Reply #140 on: March 01, 2016, 02:14:11 PM »

That would be in May. While the heating is on, there will be turbulence in such a small hall. On the other hand, I'm looking forward to IIFI as I suppose that would be an excellent Cat I site...
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ykleetx
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« Reply #141 on: April 30, 2016, 12:13:18 PM »

From Russian, with Love.
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ykleetx
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« Reply #142 on: April 30, 2016, 04:56:21 PM »

My mistake. Properly sourced, the tagline should say

"From Serbia, with Love."

(VP by Slobodan Midic)
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jakepF1D
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« Reply #143 on: June 29, 2016, 12:40:14 PM »

I thought I would add a couple pictures of my new VP hub.  I used this hub for the first time at Kibbie, and my best time was 26:18.  I'm planning to build a couple more with minor refinements as I prepare for the team finals next year.
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Olbill
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« Reply #144 on: June 29, 2016, 08:18:34 PM »

Very nice!
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« Reply #145 on: June 30, 2016, 05:43:40 AM »

Interesting design, Jake! 

What are the dimensions of carbon tubing in centerpiece and spars? Weight? Apparently there was no frictional sticking issues as I look at you great times in Kibbie.

Simo
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jakepF1D
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« Reply #146 on: June 30, 2016, 11:57:52 AM »

The center spar is off the shelf 1mm OD carbon pultrusion with a .013" ID hypodermic tube for the shaft pivot.  The prop spars are 1mm ID pultrusion sanded down to 1.2mm OD.  The finished hub minus spars only weighs about 90mg, but the finished prop is heavy due to the carbon spars and prop blades.  This particular prop is 343mg, but it shouldn't be difficult for me to shave 15-20mg off of future props and bring the total weight down to 320-325mg.  In the end I still needed 100mg worth of spacers and 15mg of ballast to bring the model up to 1.4g, so weight wasn't a big issue

I spent some time working Teflon dust into each hinge to reduce friction.  Although I do believe they may have been sticking a little, the prop shifted consistently and my half motors lined up well with my full motors.  It also didn't shift after steers which would indicate it wasn't sticking too much.
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« Reply #147 on: December 20, 2016, 05:52:42 PM »

The center spar is off the shelf 1mm OD carbon pultrusion with a .013" ID hypodermic tube for the shaft pivot.  The prop spars are 1mm ID pultrusion sanded down to 1.2mm OD.  The finished hub minus spars only weighs about 90mg, but the finished prop is heavy due to the carbon spars and prop blades.  This particular prop is 343mg, but it shouldn't be difficult for me to shave 15-20mg off of future props and bring the total weight down to 320-325mg.  In the end I still needed 100mg worth of spacers and 15mg of ballast to bring the model up to 1.4g, so weight wasn't a big issue

I spent some time working Teflon dust into each hinge to reduce friction.  Although I do believe they may have been sticking a little, the prop shifted consistently and my half motors lined up well with my full motors.  It also didn't shift after steers which would indicate it wasn't sticking too much.

I just got a 0.7mm carbon fibre tube and a 0.28mm carbon fibre rod in. I'd really love to have a go at a vp myself. I've got several Tregers and they are very nice indeed if at times a bit delicate. I'd like to try to make a vp with a very supple hinge and perhaps a bit stiffer and lighter than music wire, which is why I'm thinking of carbon inside a carbon tube. I also like the idea of carbon drive pins. I just find that when you load the motor all kinds of things can go wrong with driving pins popping out etc. As Jake says a drill press is handy and anything Dremel seems dubious I've been looking at buying a Proxxon, but I can see that drilling a hole through a tiny carbon tube is going to take not just a drill press but quite a precise small vice.
Hans
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jakepF1D
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« Reply #148 on: December 20, 2016, 06:41:27 PM »

Hans,

I can take a picture of my setup this evening if you want to see it.  I have a full Proxxon kit that allows me to do pretty much anything I want.  I'm using the Proxxon MB 200 drill stand with a 12 volt rotary tool.  I added the KT 70 compound table, and it allows for very precise positioning.  The final piece I added is the MF 70 precision vise.  It's a bit spendy to get it all, but using these items I've been able to do all the precision drilling and milling required for my VP.  I've also started using my rotary tool at it's lowest speed to sand the carbon tubes down to size, and it's much faster than using a hand drill.  

Jake
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« Reply #149 on: December 25, 2016, 07:18:32 PM »

Hi Jake,
I got the Proxxon drill press as you suggested. I also got a set of circuit board drills (the ones with a fat chuck) and promptly broke a .011 tip by accidentally dropping it down the hole in the base. I immediately ordered a half a dozen drill bits knowing the inevitable.

I tried making a Kang style vp with two carbon tabs for the screws on one side. It's a fair bit of effort to make one that's well overweight at .116g, but it looks respectable and the action is nice with a bit of .013" wire either side going through a carbon tube, underneath which I lashed the blade tubes with a small dab of zap a gap and tying thread. I've also had a go at my first ever spring. I suspect it's too strong compared to the Treger' with 5 turns of .009" wire.
Hans
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