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Author Topic: VP Hubs - for the techies among us  (Read 19685 times)
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_shadow_
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« Reply #75 on: September 17, 2013, 03:40:36 AM »

This one  Cool

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Olbill
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« Reply #76 on: September 17, 2013, 11:54:02 AM »

That would be it! Pretty crude but it works (when I can find all the parts).

And for those who might not already know - you always measure the angles when you're reducing the torque. Taking measurements while increasing the torque doesn't give you much of an idea of what's happening during a flight unless you can build a frictionless VP.

I'm sure there's a way to do this with a digital torque meter but I haven't spent enough time trying to figure it out. The basic problem is that you can't twirl the digital meter with your finger like you can a spring meter.
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Tapio Linkosalo
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« Reply #77 on: September 23, 2013, 02:52:08 AM »

1. Use thicker rubber and increase low pitch instead of lowering low pitch.  Your P/D for low pitch is already too low, in my opinion. 

If I use thicker rubber, what other adjustments should I then make? For one thing, the high pitch should be increased as the max torque will increase. But then, the model would fly the first half at higher pitch, so should the spring then be also tightened, to avoid the model landing before pitch change as it will be at higher pitch in the start?

 
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« Reply #78 on: September 25, 2013, 10:15:49 AM »

If I use thicker rubber, what other adjustments should I then make? For one thing, the high pitch should be increased as the max torque will increase. But then, the model would fly the first half at higher pitch, so should the spring then be also tightened, to avoid the model landing before pitch change as it will be at higher pitch in the start?

This is where it always gets interesting. Upon review, I don't think I would go to much lower of a pitch after all. I've gone as low as 20" on F1D's in low ceiling using props in the 17.5-18" range, but definitely wouldn't go any lower than that as the efficiency really starts to drop off. I would, however, try to stay with that big prop you've got, as there is much to be gained, and it doesn't look like your model is struggling with it. Your launch is definitely less shaky than Bill's, which is almost as scary as launching my F1D.

So yes, everything will change when you go to a thicker motor. I'd recommend staying at that same 550mm low pitch, maybe dropping it a little if you keep landing with unused turns, aiming to use a motor with 1600 turns or so maximum. You'll have to increase the high pitch to stay out of the ceiling, but because of the higher torque baseline, you'll probably find that you have to increase the preload. This setup is likely to force you into bumping the ceiling if you're to use up all of the turns. As mentioned before, I'd set the high pitch so that you bump the ceiling for 30 seconds or so on the first climb, then descend as far as necessary to avoid an extremely long bumping phase at the top of the second climb, setting low pitch as needed to use up as many turns as possible (meaning land with no more than 200).
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Olbill
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« Reply #79 on: September 25, 2013, 11:49:19 AM »

The only think shaky about my F1M launches is when my hand sticks to the model. The scary part is loading a motor with over an inch-ounce of torque onto the model.
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Tapio Linkosalo
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« Reply #80 on: September 26, 2013, 02:39:40 AM »

Ok. I'll try that next Saturday (thicker motor, higher high pitch and turn in (increase) pre-tension). I have no intention to go to a smaller prop, as this one (440mm, almost 18") seems to be working good. Funny thing, the prop has the same layout as the first I built for this model, but I never got it work, as it seemed to need very thick motors and still the model could not sustain level flight after initial torque peak. I built this new prop using the same outline and pitch distribution, but with the "Kagan" method of sparless prop, and all of a sudden the model started to work much better than before. I wonder if the old one was not stiff enough torsionally; I doubt that the spar could be such efficiency killer... I recently also built another similar prop, but this has harder spring on the VP. I intend to keep this first one for Cat 1 flying as it seems to work ok for that, and try to set the new one for higher ceilings.

I did not use as high torque as Bill did, last time I would my motors to 45 g*cm max (that is about 0.63 oz*in). I guess those long motors could take up to 60g*cm (which would be almost 0.9 oz in), but I doubt they would tolerate anything more than that. Shorter motors possibly could, but then I am afraid that my practice site might get a bit tight during the initial turn...
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Olbill
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« Reply #81 on: September 26, 2013, 11:23:07 AM »

Tapio
I just looked at the motor lengths you've posted and they look way too long for Cat 1. At my last flying session I was using 12.5" or 318mm loops. When I've tried loops about the length of yours I drop into the 12 to 13 minute range. Also according to my info Aki set his Cat 2 record of 19:39 on a 12.3" or 312mm loop.
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Tapio Linkosalo
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« Reply #82 on: September 26, 2013, 02:20:10 PM »

Ok, good. This means that (at least the bottom pitch) settings of the prop are about right.Will test shorter motors on Saturday!

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Tapio Linkosalo
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« Reply #83 on: September 29, 2013, 11:39:34 AM »

Not so successful session yesterday.... I tried a thicker motor, but had problems for the initial climb. With motor wound to 45 g*cm (used 40 with the thinner motors) the opening of the initial turn was more pronounced, and with the narrow flying site I tended to hit the wall. If I cheated here, picked the model from the wall it had collided and turned it by hand (practically impossible to turn the model by 90 degrees with proper steering), I still barely climbed to the ceiling. Even though I eventually set the top pitch back the where it was for the thinner motor. And then, in the descent, the model landed (even though just barely) before the VP switched. I then cheated again, increased spring pre-tension and re-launched, to make a really nice climb to the ceiling and land at 14 mins (with all the "cheat" times excluded). So the model shows good promise, if I just get the VP change earlier. I wonder if it is a deal of friction; I'll need to sand the carbon rods and add some epoxy on them, then add some graphite for lubricant...

I also tried another prop (similar layout but harder spring) on half-motors, intended for higher ceilings, but could not get that work; prop rpm to fly was way too highand the model landed at 6+ mins after exhausting all turns. Needs more work...
 
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Olbill
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« Reply #84 on: September 29, 2013, 12:50:16 PM »

You could make a simple rig to check the VP action by using a stationary "motorstick" with a torque meter for the rear hook. Then some close up pics or videos of the hub and/or the blades while a full wind is unwinding should give you a good indication of what's happening in the air. It wouldn't be exactly the same as in flight but it should be close enough.
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mkirda
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« Reply #85 on: September 29, 2013, 02:35:54 PM »

Hi Tapio.

I've been using silicon grease made for camera housing o-rings as a lubricant.
It doesn't seem to bind at all and works pretty well.

Regards.
Mike Kirda
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Tapio Linkosalo
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« Reply #86 on: September 30, 2013, 02:33:06 AM »

I took a closer look at my two new VP props. The older, with the softer spring seems to move quite freely, no binding in the hinges, so when I use the torque meter to see at what torque it meets the top stop or reduces pitch from it, there is minimal difference (indicating little hysteresis = no binding). Whereas for the newer one there is a considerable difference. I recall that I sanded the carbon pins smoother on the older one, and also recall reading the pultruded carbon tends to be quite rough on the outside, so maybe this sanding is indeed needed. Will have to sand also the newer one. And, if there is no binding problem, then I just need to wind a new, harder spring for the old prop, so that the pitch change will start a bit earlier. Looking at some torque graphs, it seems that the motor will reach 17 g*cm after exhausting some 400 to 500 turns; with rpm ~60+ this is in the ballpark of 6 to 8 mins, and this figure matches the flight pattern.

About lubrication; I hesitate a little to use any liquid lubricants, as I fear that they could pick up dust and get more sticky over time. So if I can manage without lubricant or with the graphite only I would be happier.

 
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« Reply #87 on: September 30, 2013, 05:20:15 AM »

Understand your reluctance re oil/lubes for these systems Tapio.
I imagine having to find a way to clean them periodically - and their performance becoming slower over time, between cleaning.

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Tim
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« Reply #88 on: October 01, 2013, 12:49:11 AM »

Understand your reluctance re oil/lubes for these systems Tapio.
I imagine having to find a way to clean them periodically - and their performance becoming slower over time, between cleaning.

Far be it from me to discourage experimentation. Please try graphite and let us know how it works out for you.
The silicon grease I use is about the consistency of Vaseline. Only a touch is applied and it hasn't seemed to dry out or capture dust/dirt.
It would be pretty trivial to disassemble and clean if necessary, though I admit I wouldn't want to do that during a flying session.
My next carbon hub will be sanded smooth and I imagine I will still use the silicon grease as I have it on hand and I know it works very well.

Regards.
Mike Kirda

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Tapio Linkosalo
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« Reply #89 on: October 01, 2013, 01:09:36 AM »

I reworked my new hub last night, it seemed to bind at the high pitch so that might be the reason for ill behaviour (after initial burst the model leveled, and when the climb eventually restarted raced to the roof). I ended up sanding the carbon rods smoother, then covering with some CA (made the surface shinier). The major modification, however, was to enlarge the plastic tubes for hinges. Now the binding seems gone/less, but it will almost two weeks until I get to test that....   

Meanwhile, I think I need another spring with 1/2 turns less for my older (Cat 1) hub, to start the second climb a bit earlier.
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THB
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« Reply #90 on: October 03, 2013, 02:43:01 AM »

Hi Mike - thanks for your thoughts on this - I've never used a lubricant - or even carbon tubes - on a VP, so I'm interested that silicon works well and might give that a go. My own VPs (and props) are pretty old school and maybe time I updated a bit  Smiley
Following the topic with interest Tapio
cheers
Tim
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Tapio Linkosalo
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« Reply #91 on: October 12, 2013, 04:00:36 PM »

I reworked my new hub last night, it seemed to bind at the high pitch so that might be the reason for ill behaviour (after initial burst the model leveled, and when the climb eventually restarted raced to the roof). I ended up sanding the carbon rods smoother, then covering with some CA (made the surface shinier). The major modification, however, was to enlarge the plastic tubes for hinges. Now the binding seems gone/less, but it will almost two weeks until I get to test that....   

Meanwhile, I think I need another spring with 1/2 turns less for my older (Cat 1) hub, to start the second climb a bit earlier.

Another Saturday night trimming session done. Indeed it was binding; with the same settings I was racing to the roof, and had to open the high pitch a little and the spring quite a lot to stop that excessive climb on a half motor (7 meter ceiling). When I eventually managed to tame the climb I was doing that "climb on burst, then some descent until the spring jumped in and the model climbed again" -stuff. SO looks like I need to wind even harder spring on this hub (too), to get onto spring sooner (at higher torque). But anyway, best flights no-touch at 7 meter hall were 7 1/2 mins on half motor, so there is potential. The next session in two weeks will not be practice but contest, in a 18meter football hall (quite a cold one), so it will be interesting to see how this prop goes on full motors!
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« Reply #92 on: October 15, 2013, 05:51:58 PM »

How about the dry-spray Teflon (PTFE) stuff that the Heli guys are using on their mainshafts (and all other close-tolerance surfaces)?  Doesn't attract dirt so maybe it would be more suitable (but it's pricey)?

I also got my latest issue of Thermiksense, and saw that Z. Sukosd is offering VP hubs for sale.  They look quite nice (but then, I wouldn't know a good one from a shoe Roll Eyes).

Pete
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« Reply #93 on: October 15, 2013, 08:03:28 PM »


SO looks like I need to wind even harder spring on this hub (too), to get onto spring sooner (at higher torque).


Tapio,

Why do you need an even harder spring instead of just adding more pre-load to this spring.  More pre-load would start the pitch change at a higher torque.

-Kang
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Tapio Linkosalo
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« Reply #94 on: October 16, 2013, 02:00:14 AM »

Quote from: pit
How about the dry-spray Teflon (PTFE) stuff that the Heli guys are using on their mainshafts (and all other close-tolerance surfaces)?  Doesn't attract dirt so maybe it would be more suitable (but it's pricey)?

Might work; however seems like sanding the carbon smooth and hardening with a little CA, then making the plastic (PVC I suppose) tubing hinges large enough makes the hinges sufficiently smooth, without binding, without using any lubricant. After all, the issue is a little different from that of the heli guys: they need to reduce motion friction, we need to have the static friction (from static to moving) as low as possible to minimize binding.

Quote from: ykleetx
Why do you need an even harder spring instead of just adding more pre-load to this spring.  More pre-load would start the pitch change at a higher torque.

Even the new hinge is too soft = when movement starts, it progresses too rapidly. The symptom is that when the pitch change starts, the climbing speed increases. In order to get the climb speed (on spring) low enough, I need to reduce pre-tension until the model stops climbing or descents a bit before the change starts. If I increase pre-tension, then the movements starts earlier, and the model rushes to the roof as the pitch is reduced too early related to the motor torque -> too high rpm -> too much power -> too fast climb.

What I would need is a spring, whose torque as a function of angular movement is bigger. Thus I could add pre-tension, and have the pitch reduce at higher power, and avoid the stop/descent in the climb. But, with a harder spring, a smaller change in pitch would reduce spring force and find a new balance of torque and pitch, so that the initial movement of pitch would be less, hence rpm would not increase that much, and climb would not accelerate too much. Consequently, the range of torques (and therefore range of time) where the pitch changed would be longer, and I would not get a rollercoaster ride (climb-descent-climb-descent), but a flatter flight profile.




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ykleetx
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« Reply #95 on: October 16, 2013, 12:45:43 PM »

Tapio,

I was under the mistaken impression that your second climb didn't go high enough, and hence you had too many turns left over, and your time wasn't as good as you wanted.  If this were the case, I would just recommend increasing the pre-load so that the second climb starts a little earlier and goes higher, allowing you to use more turns and increase flight time.

I personally would not change the spring based only on a few flights, unless the stiffness is way off, which doesn't appear to be the case.  I would first spend time fine tuning high/low pitch, pre-load, and rubber cross-section to maximize flight time.   Only after many many flights would I start thinking about changes.  Just my opinion.

In addition, it is my opinion that a softer spring is easier to trim.  When I first flew VP, I used a softer spring, and it was very easy to see visually and measure when the pitch starts to change.   I find this to be a benefit to the non-expert because from flight to flight, as VP adjustments are made, other variables aren't always constant -- namely, the rubber, which changes properties from flight to flight.  I found it beneficial that changes I made to the VP was readily observable from flight to flight.  Later, when I stiffened the spring, I found it more difficult to observe changes I made to the VP.  Again, just my opinion.

I'm a big fan of flying low-ceiling VP, and I will fly more low-ceiling next year.  Good luck with your upcoming contest.

-Kang
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Tapio Linkosalo
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« Reply #96 on: October 16, 2013, 02:50:53 PM »

Quote from: ykleetx
I was under the mistaken impression that your second climb didn't go high enough, and hence you had too many turns left over, and your time wasn't as good as you wanted.  If this were the case, I would just recommend increasing the pre-load so that the second climb starts a little earlier and goes higher, allowing you to use more turns and increase flight time.

I personally would not change the spring based only on a few flights, unless the stiffness is way off, which doesn't appear to be the case.  I would first spend time fine tuning high/low pitch, pre-load, and rubber cross-section to maximize flight time.   Only after many many flights would I start thinking about changes.  Just my opinion.

Just in case I did not make myself clear enough: I have 2 of these new props now. The first showed such promise for low ceiling, that I decided to put it aside for such work and built another one. For the second I made a little harder spring, but not hard enough it seems. The latest session was trimming the second prop on half motors. Again in the 7 meter (22 foot) ceiling, so the aim was to trim model for a 15+ meter hall. With this prop I had problems of the second climb going too hot, until opening the pretension to a point where the model came down a bit before starting the second climb. Too soft.


Quote from: ykleetx
In addition, it is my opinion that a softer spring is easier to trim.  When I first flew VP, I used a softer spring, and it was very easy to see visually and measure when the pitch starts to change.   I find this to be a benefit to the non-expert because from flight to flight, as VP adjustments are made, other variables aren't always constant -- namely, the rubber, which changes properties from flight to flight.  I found it beneficial that changes I made to the VP was readily observable from flight to flight.  Later, when I stiffened the spring, I found it more difficult to observe changes I made to the VP.  Again, just my opinion.

I agree that soft springs are easier to get to decent performance. And then, you should get more time by not wasting energy for the climb (on higher rpm) but keeping constant height. So looking for to bend a new spring....

Quote from: ykleetx
I'm a big fan of flying low-ceiling VP, and I will fly more low-ceiling next year.  Good luck with your upcoming contest.

Thanks. Too bad there will not be the IIFI contest, that would have been nice. The site for the next contest is a bit challenge; tends to turn cold in winter and heating is via air conditioning, so the air indoors may not be that good all day. Hopefully for some periods though.
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Tapio Linkosalo
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« Reply #97 on: January 08, 2014, 07:18:38 AM »

Following up the F1M hub in message #51 I used the same construction for F1D. Here the cross bar is 0.5mm dia carbon rod, and the hinges are short lengths of teflon tubing (from cyano bottle add-on). I still cannot understand how Kang builts his hubs to 70mg, this one is almost double: currently 125mg. Ok, I should shorten the rubber loop on the shaft, and for final versions discard the paper tubes and glue to hinges directly on prop spars. That should save some 10 to 15mg.

I do have more carbon on this, for a reason: I want to have the prop attachment more rigid, have the hinge points further apart. This should reduce the play on the blades and also reduce catching of the hinges, as torsional loads on the bearings would be smaller.

Had to do some thinking with the adjustment screws. 00-90 sized nylon, the heads are terribly heavy. So I cut them off, and tried to cut a slot for screwdriver in the end. No luck, the resulting slot is too weak and fails quite quickly. So after a little thinking I recalled the way to make the line release on silicon damper viscous timer: a cross har out of carbon rod. Tried the same on the setscrew, and voila! it works. Just had to make and adjustment tool from a piece of aluminum tubing, which I cut square and dremeled a slot to the end.

Did some test hops today, and the hub withstood maximum rubber torque. There is one modification I need to make: now the adjustment screw holder is attached to the 0.5mm carbon rod only, and this is a bit flimsy to torsion. So I need to lengthen the 0.5/1.0mm tubing in the center slightly, and attach the adjustment fork directly to that piece    
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mkirda
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« Reply #98 on: January 08, 2014, 08:17:49 AM »

I think nylon will melt safely. You should be able to use some metal tubing that will slip over the 00-90 screws. Squeeze that about 2-3 mm from the end all the way flat. Then heat the end and push a 00-90 screw into it. The nylon should deform to the inside shape of this tool. Remove and cool. The end should be shaped kind of flat and the tool can be used to turn it.

I haven't tried the above, but it should work.

Regards.
Mike Kirda
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Tapio Linkosalo
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« Reply #99 on: January 08, 2014, 08:58:01 AM »

Ahh, neat idea. Did not come to think of that, even though I have melted nylon before. Anyway, added a third picture above to show the screws with crossbar. these are work nice with the tool made of tubing.

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