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Author Topic: Living room model underpowered  (Read 902 times)
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fitnezz
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« on: November 12, 2015, 05:01:49 AM »

Recently I wanted to use thinner rubber for one of my living room models(0.34g), so I split 0.0625" rubber (the only I have) to 0.031" (half) and wanted to use one strand of it. Normaly for my models I use 0.0625" strand (way overpowered probably). The model flies good on a strand of 0.0625", giving times about 3++ minutes in my room, but does not want to fly almost at all on one strand of 0.031" (tried diferent strand lenghts). Even using very low pitch prop (P = 7.2, P/D = 1.2), the model does not want to climb using that power. I have a feeling it should be enough power for my model so I give parameters:

My model is 6" wingspan, 9" long, 0.22g weight (without propeller), propeller weight: 0.12g, propeller diameter: 6", diferent pitches tested.
I even tested 5" diameter propellers, also smaller blades, still poor results.

So why the model is underpowered using 0.031" strand?
Is my model having too heavy propellers? (0.12g out of 0.34g feels like a lot of weight sits in the prop),
Can a reason be a prop bearing? (I make my own pigtails using 0.11 MW, prop shaft 0.09MW)
I dont grind the front pigtail loop, just squeeze it a bit with a pliers. Fells very smooth movement, but somehow I have a nasty feeling, that it can be a prop bearing, who is guilty Smiley

I am totaly stuck and dont know where to go. Either keep on flying on a strand of 0.0625" or do something about it and go down with a power. Please help Smiley
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Maxout
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« Reply #1 on: November 12, 2015, 08:03:55 AM »

I think the operative question is, what's your reason for wanting to fly it on so much less power? 3+ minutes isn't half bad. I can definitely see where cutting your available torque in half might prevent the model from climbing. That's way too much of a power reduction.
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fitnezz
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« Reply #2 on: November 12, 2015, 10:52:56 AM »

Hey, thank you for the answer. Well, I had an airplane which was about 0.8g and it was giving 3 minutes under same conditions and same motor. Again, 1/16" strand. It was 7" ws in place of 6" I fly now. I thought, if I reduced weight more than twice, the motor should be also thinner, so I could get slower prop speed and slower, longer flights. Basically I gained a lot of weight advantage here, but did not extend my flight times a lot. So feels like no progress being done...
I also read in the micro stick topic on this forum, that the guy was using 0.018" strand for 5" dia prop. I thought 0.03" should be ~ok for my model. I also read your opinion on the mini stick rubber on this forum saying 0.025 loop is commonly used. Theese are heavier, bigger models with bigger props 7". And again they use less power than I do at the moment for lighter airplane:(
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Maxout
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« Reply #3 on: November 12, 2015, 01:07:04 PM »

Well you're talking a strand of .062, and that's equivalent to a loop of .031, which is reasonable for a ministick. Yes, I run thinner, but I also have a very highly developed airframe with tricky airfoil, rolled tube motorstick, and fancy prop. CG is right on the edge of stability and I'm running the least incidence I can get away with. That'll get you 7+ minutes in a living room...it's a pain in the neck to get there, too!

You reduced your airframe weight, but also your wingspan, so it's hard to compare to the older model.
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ykleetx
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« Reply #4 on: November 12, 2015, 03:34:39 PM »

Fitnezz,

Be sure that your model is trimmed properly. I am guessing that the CG of your model is relatively forward, which is fine; this means that the model needs a lot of incidence between the wing and the stab. Be sure that this is the case.

Your .34 g model should fly about at about 65% of the speed as your .8 g model, if both models are well trimmed. Try to confirm this.

As for your bearing, if you feel that it is smooth, I think it's okay. Are you using a teflon washer between the prop and bearing?

-Kang
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fitnezz
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« Reply #5 on: November 13, 2015, 03:51:46 AM »

@Maxout, thanks for the construction details! Would be awesome to see the picture of your ministick framework. A lot of plans online, but not too many professional videos and pictures of ministcks around Sad
I think I have never seen video of a pro ministick flying on the contest, esp first stage of the flight. They drown in the sea of F1Ds, EZBs, and A6s Smiley Btw. why 90% of pro ministicks dont have rolled motor tubes? Is ministick having same problem that EZB and its motor stick has? (You need to go through many to find that golden one?).

@Kang,

Thanks, I was also thinking that it can be trimming problems.  My newb guts feeling was that thinner motors are less forgiving for errors. Thanks for the directions to go!
Unfortunately I do not have teflon washers. I use, as someone on this forum suggested plastic washers I make myself from plastic soda caps, like cocacola. This red plastic seems very hard and be ok. Do you think it can affect
performance a lot? Also can you tell me if my 0.12g prop is not too heavy for 0.22g airplane (0.34g total). Feels like this thin rubber has a lot of weight to work with.
« Last Edit: November 13, 2015, 04:18:25 AM by fitnezz » Logged

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Maxout
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« Reply #6 on: November 13, 2015, 07:50:36 AM »

@Maxout, thanks for the construction details! Would be awesome to see the picture of your ministick framework. A lot of plans online, but not too many professional videos and pictures of ministcks around Sad
I think I have never seen video of a pro ministick flying on the contest, esp first stage of the flight. They drown in the sea of F1Ds, EZBs, and A6s Smiley Btw. why 90% of pro ministicks dont have rolled motor tubes? Is ministick having same problem that EZB and its motor stick has? (You need to go through many to find that golden one?).

I'll have to video mine sometime. Each is different, though. Mine races around in level circles for a while before climbing away. I haven't flown it much this year; last year at Lakehurst it did 11:00 without ever going above 60'. I seem to remember it deadsticked about 20' up. That prop is the best I've ever made for a mini and is a major component of the high performance. I went to a rolled motorstick after finding that I couldn't build a light enough solid stick that could handle the torque required--solid ones flex too much, require excess incidence and other such nonsense. I also use a thick airfoil...somewhere around 6% as I recollect. Stab is flat--no camber.

The biggest thing you can do to increase the power capacity of a ministick is to use a large, bottom mounted vertical stab. Top mounted fins and tip fins cruise great, but if the model stalls or hits and obstacle, it'll fall into an unrecoverable deep stall in the higher torque ranges. Mysteriously it'll fly just great at cruise torque. Top fins and tip fins work great on high ceiling minis which don't have to use flaring props or worry with banging around in the rafters.

Nick Ray is really your man in the ministick world. I stopped working with mine when I got past 9 minutes in Cat I last year...that was good enough for me and other events warranted more attention. Nick finally cracked 10 in the same site a month ago. It was a properly impressive thing to behold. He'll be going for 11 next weekend and I've little doubt he'll hit it very soon.
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fitnezz
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« Reply #7 on: November 14, 2015, 05:07:39 AM »

Thanks for this info Maxout, nice stuff! I read a lot about bottom fin vs tip plates, interesting stuff. What is the +/- weight of your rolled motor tube? I think no bracing is allowed, right? I know Nick Ray's model from INAV, the one with curly stick. Unfortunately no videos online (at least I could not find them). 11 magic number, fingers crossed for Nick Smiley
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