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Re: Wright Stuff C
Posted: October 13th, 2019, 7:10 pm
by xiangyu
bjt4888 wrote: ↑October 13th, 2019, 6:55 pm
If, for example, your loop length is about 30.5” (.0415 g/in), and your stretch 5x relaxed length to wind (after break in, properly lubed) an 85% breaking turns number would be 4,100. Probably need more back off (the large number of backoff turns needed is another indicator of needed changes (thinner rubber, more pitch or flaring prop).
Brian T
Hi Brian,
Sorry been finishing up my piled up weekend homework like any other high school kid do Sunday night (
)
Just measured the loop length and the loop is just a tad bit more than 30 inches.
I'm always scared to step back more during winding since there seems to be so much tension in the rubber. I think it may be beneficial for me and my team to dedicated a day where we just wind rubber bands to their limits and get a feel on what that is like.
Thanks for your help!
Xiangyu
Re: Wright Stuff C
Posted: October 13th, 2019, 7:12 pm
by xiangyu
lechassin wrote: ↑October 13th, 2019, 4:37 pm
The steep dive may have been from the sudden movement of your team mate ducking. We noticed that too so now we anticipate getting hit and move away slowly ahead of time. I'm curious to see if you're using up most of your knots in 1'08". You can see we use all of our knots in 1'30" running three blades.
The main thing that surprised me in your video is how well the plane does with one wing! Apparently the decrease in drag of your design offsets at least some of the increased wing loading. Your times are equal to our first-day times and it'll be interesting to see how much more you can get.
We're getting 3750 turns into a fully broken-in 2.9 gram motor and we break them in about 10 flights, so I'm curious to see how many more winds you can reliably get into a 2.5 gram motor. I wonder at 3600 turns you might be close to 100%.
Yeah I was surprised too that single wing worked so well. I decided to do that for our first flight since I got the kits Friday night and wanted to test this week so single-wing was a little quicker to build.
I will keep you updated on our progress.
Re: Wright Stuff C
Posted: October 13th, 2019, 7:24 pm
by bjt4888
Xiangyu,
Sorry to hear about all the homework. Yes, very good idea to practice winding to destruction. Tan SS should stretch 10x relaxed length before breaking. A nondestructive breaking can be accomplished by hooking over a doorknob (lubricate first and pat most lubricant off) and stretching 7x and holding for 4 minutes. You will probably see that the motor is permanently extended about 1” after this.
Wind about 60% of projected max turns fully stretched while monitoring torque and torque increase rate. Wind remaining turns while walking in slowly. This “walking in” phase is the difficult part. We monitor torque and have a ceiling “walk-in torque” that we try to stay below during this phase. The idea is to walk in very slowly and only come in when the rate of torque increase is problematic. To wind a motor to 90%+ breaking turns requires lots of practice attempting to get the feel. Breaking torque for this motor is probably about 1.0 to 1.4 in-oz.
Brian T
Re: Wright Stuff C
Posted: October 13th, 2019, 7:25 pm
by bjt4888
“Non destructive break in not “breaking”. Bad autocorrect.
Re: Wright Stuff C
Posted: October 13th, 2019, 7:32 pm
by xiangyu
bjt4888 wrote: ↑October 13th, 2019, 7:24 pm
Xiangyu,
Sorry to hear about all the homework. Yes, very good idea to practice winding to destruction. Tan SS should stretch 10x relaxed length before breaking. A nondestructive breaking can be accomplished by hooking over a doorknob (lubricate first and pat most lubricant off) and stretching 7x and holding for 4 minutes. You will probably see that the motor is permanently extended about 1” after this.
Wind about 60% of projected max turns fully stretched while monitoring torque and torque increase rate. Wind remaining turns while walking in slowly. This “walking in” phase is the difficult part. We monitor torque and have a ceiling “walk-in torque” that we try to stay below during this phase. The idea is to walk in very slowly and only come in when the rate of torque increase is problematic. To wind a motor to 90%+ breaking turns requires lots of practice attempting to get the feel. Breaking torque for this motor is probably about 1.0 to 1.4 in-oz.
Brian T
Huh, interesting, I will give that a try next weekend. Thanks for your help!
How do you properly lubricate your rubber? We use the son of a gun lubricant and spray some into a sandwich bag then put the rubber in it to mix, is that a good technique?
Also, what times do you think my team should aim for in our gym? (30 ft)
Re: Wright Stuff C
Posted: October 13th, 2019, 7:35 pm
by lechassin
Brian, based on your advice, we have work to do. If I understand correctly we should be able to launch our 32" motors at 4400 turns!
Re: Wright Stuff C
Posted: October 14th, 2019, 4:51 am
by bjt4888
Xiangyu,
As for possible improvements in flight duration under a 30 ft ceiling, there are many variables to test. Matching rubber to propeller, propeller design, propeller pitch and learning to wind close to 90% of breaking turns are a few of the most important. Actual projections of flight time I need to keep as private team information as you will understand.
Keep up the good work and keep up the research and testing.
Brian T
Re: Wright Stuff C
Posted: October 14th, 2019, 4:53 am
by bjt4888
The formula for projected maximum turns for a particular length and density motor that we use is a variant of John Barker’s. His formula, and interesting discussion about it, is posted in Hip Pocket Aero.
Brian T
Re: Wright Stuff C
Posted: October 14th, 2019, 5:43 am
by bjt4888
Eric,
If your 32” motor is 2.9 grams with two rubber o-rings, then the rubber weight would be 2.9-.09=2.81. The rubber length would be 32*2=64+.25=64.25 (counting the .25” approx in the knot) and the density would be .0437 g/in.
Using .083 g/in as a standard for 1/8” rubber strip (this used to be typical and is still close to the norm although we find the range to be .079g/in to .0845 g/in) you can back in to a “width” for the purposes of One version of John Barker’s equation. So, this particular rubber loop can be thought of as 0437/.083*.125=.0658” “wide”. However, I would say that all serious indoor flyers refer to motors in density only (either g/in or g/meter). The reason for this is to increase precision of data recording and analysis.
Brian T
Re: Wright Stuff C
Posted: October 14th, 2019, 6:33 am
by lechassin
Brian, do you have a link to the article? We'd like to read it but we could only find a piece on braided motors.
Also, when we couldn't find the information, we just decided to sacrifice a full length motor prepped and wound using best practices as gleaned from here. The motor broke at 4500 turns, 90% of that puts us just over 4000 turns. We'll try that today and see how far we have to back off to get launch our launch torque, and post the results. We also need to calibrate our torque meter so we get actual numbers, not relative.
How does the formula actually help in the "Real World"? It seems that it would only serve to alert a flyer of improper winding technique, i.e.: the motor breaks before predicted turns are achieved. Am I missing something?