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Author Topic: Electric Wright Stuff trial event for Div. B or C?  (Read 1315 times)
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ceandra
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« Reply #25 on: May 07, 2018, 03:56:40 PM »

I agree with Lee! These kids know how to use arduino's for data collection! The camera, though, is a nice intermediate touch.

I would consider adding a simple measurement of thrust as well...

Chuck
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Little-Acorn
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« Reply #26 on: May 07, 2018, 07:08:35 PM »

Lee, that would indeed be a good project. It would work for everything but the current-sense resistor, which would need 1mV precision to be useful. The analog-to-digital inputs on an Arduino board can give you 5mV precision at best (actually 10mV according to Mr. Nyquist). Maybe they would have to learn how to put together a ladder-type comparator tree? Does anyone happen to know of a high-precision A/D converter board for the Arduino?

Chuck, yep, the camera is kind of Rube Goldbergish, but it's what I had to hand. The kids got very bored (and careless) when I had them cranking a WS rubber motor on a torque meter last season, I figured they wouldn't do any better writing down values from digital meters. Now at least they can go back and check a reading again, and keep the files on disk etc.
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SyLa-20871
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« Reply #27 on: May 08, 2018, 08:08:41 AM »

Maybe what you (we) are looking for can be found on www.vernier.com?  They sell a large number of sensors for many science (Biology, Physics, Chemistry) related experiments/projects for Middle School and High School students. The sensors are relatively cheap (about $60). The software is free and serves as a data logger.

I agree that the kids get bored winding on a torque meter, but when they win First by winding hard and dewinding (THANK YOU to all the experienced flyers hammering on this concept), it finally hits home.  My daughter's team scored first with no hit on a 22 ft ceiling after the correct launch torque and maximum number of winds/dewinds was worked out during a week long session of test flying.  I can see the same type of experimentation going on with EWS, but with a computer or iPhone set up to collect the data and subsequent analysis.  I think the kids will take to this very quickly.  "But Dad! I am doing science work on my phone, not playing games!"

Lee
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frash
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« Reply #28 on: May 09, 2018, 08:02:33 PM »

I had an old note about the data logger below, but never bought one nor followed up. Maybe it will give someone a lead or a hint. List price was about $60 (USD).

https://www.dataq.com/products/di-1100

Fred Rash
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Little-Acorn
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« Reply #29 on: May 10, 2018, 08:03:31 PM »

Picked up a baker's dozen of supercaps, from both Mouser and Digi-key, all by AVX, all advertised as 5F 3V. AVX has two such supercaps, slightly different in size and shape, slightly different in weight, slightly different in ESR (Equivalent Series Resistance).

The long skinny ones weigh slightly less (1.77g vs. 2.17g), and have slightly higher internal resistance (0.085 Ohms vs. 0.070 Ohms). The difference in weight doesn't sound like much. But when you're trying to minimize weight in a plane that can weigh as little as 7 grams, a 0.4g difference starts becoming more significant. I wonder if there are any long skinny capacitors with actual capacitances around 6.2F to 6.5F?

I've tested three of each so far. It takes a while. Charged one to around 3.1V, then let it discharge through a 10 Ohm 10 Watt resistor while the little digital-meter displays did their thing. Did this six times for six supercaps, three of each size. Data collection with this Rube Goldberg setup (Take a movie with a digital camera while the supercap discharges, then step through the movie and type the data into a spreadsheet) is time intensive. And being a lazy SOB, I've only done six so far.

Here's the results so far, from this small sample. Here's a data list from one capacitor, the pink area is raw data and the green is derived results. Yep, the actual capacitance varies quite a lot from the advertised 5F, but all (so far) are within the -10% to +30% tolerance called out in the datasheet. And here's also a summary of all six supercaps, discharging (one at a time) through the same 10 Ohm resistor. Plus some pics of each size of supercap, with labels.

Interestingly, in this tiny sample of 3 each, the short fat ones had significantly higher capacitance than the long skinny ones. But that might not hold true when larger quantities are tested. Especially if one batch was produced by AVX this month and the next batch was produced in a different lot the following month etc.

I also noted down the number of seconds it takes for each capacitor to discharge from 2.5V to 2.0 volts, as nearly as I could get with one data sample per second. The ones with higher measured capacitance, took longer as expected.

Hopefully these measurements and calculations will prove to be accurate, if/when I ever get a way to measure the caps directly. Until then, this info is worth what you paid for it. ;-)
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Re: Electric Wright Stuff trial event for Div. B or C?
Re: Electric Wright Stuff trial event for Div. B or C?
Re: Electric Wright Stuff trial event for Div. B or C?
Re: Electric Wright Stuff trial event for Div. B or C?
« Last Edit: May 10, 2018, 09:55:26 PM by Little-Acorn » Logged
Little-Acorn
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« Reply #30 on: May 10, 2018, 10:20:24 PM »

The difference in weight (0.4g) between the two capacitors doesn't sound like much. But when you're trying to minimize weight in a plane that can weigh as little as 7 grams, a 0.4g difference starts becoming more significant. I wonder if there are any long skinny capacitors with actual capacitances around 6.2F to 6.5F?

The difference in internal resistance (0.015 Ohms) seems tiny in a system driving around 250mA. An additional 0.015 Ohms would reduce the "effective voltage" by about 0.004 Volts, slightly more than one-tenth of one percent at the highest power (at launch). Every little bit helps, of course, but this is a teeny little bit.

Between two caps (long and short) that have the same measured capacitance, would the slightly lower internal resistance of the short one make up for its higher weight? I sort of doubt it.

If it turns out that the short, fat caps are always higher capacity than the long, skinny ones... meaning that we can get 6.3F fat caps but can't get anything higher than, say, 5.8 skinny caps... then the higher capacity of the fat ones might justify their higher weight. That's a pretty big IF, though. Though it seems true in the small sample measured so far, there might be higher-capacity skinny ones waiting in the wings.
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Olbill
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« Reply #31 on: May 11, 2018, 12:59:16 AM »


We do not cut our own rubber,  I get it pre cut and am at the mercy of whoever supplies the "widths" (and lot dates) I order.  I do however use a caliper to check sizes.  When balancing a plane for trim/rubber/prop, we do use different loop sizes (widths) to get the best times. 

Jerry


I apologize for meandering off the electric discussion but this is an important point.

Jerry
One of the things I frequently rail about is that you should measure rubber by length and weight. For an event that has a maximum motor weight you need to always use the entire weight allowance. The motor length is the variable that you need to control. When you specify your rubber by the cut width then you are not going to be in control of the motor length.

Your rubber supplier is probably not going to be happy about trying to hit a grams/inch target. Because the rubber thickness varies, the grams/inch will also vary for any given stripper setting. The way I deal with this problem is make sample cuts and adjust the stripper to give the result you're looking for. Then instead of running off 100 feet at that setting, you need to cut shorter pieces, like some multiple of what a motor requires. (I cut enough for one motor). In between these cuts you can do short samples and readjust the stripper if needed.

It is a fairly labor intensive procedure if you want the best results.
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SyLa-20871
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« Reply #32 on: May 11, 2018, 08:08:51 AM »

Olbill,

You are correct that it is labor intensive.  However, your advice paid off.  We cut about 6 motor lengths from the same commercial rubber width and came up with three sets of grams/in linear density.  As a result my daughter's team found the optimal wind and launch torque to win first at State.  It was not close.  My son now has his science project all lined up with the data he helped to collect as well.

Our results support what you have been saying all along.  Thank you.

Lee
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SyLa-20871
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« Reply #33 on: May 11, 2018, 08:20:30 AM »

Little Acorn, Fred..

I found a Capacitance Meter DIY kit for 13.95.  This may be useful? https://www.sparkfun.com/products/9485
I am not sure what else needs to be obtained, but these kits are relatively cheap.  I can see many school programs using them as educational tools so it would be easy for some creative students to "appropriate" some items from their computer science class to put these together and do the investigative science.  It would incorporate "STEAM" with the plane and flight itself being the "Artistic" component.

Lee
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ceandra
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« Reply #34 on: May 11, 2018, 09:42:26 AM »

The notes on the meter say 500uF to 1pF.

I wonder how simple it would be to modify it for higher C?

Chuck
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Little-Acorn
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« Reply #35 on: May 11, 2018, 11:16:50 AM »

Lee, that's a great find!

Taking your cue, I dug around and found a number of capacitance-measuring kits on Amazon.com, including one that looks just like that one.

Also looked up the URL that was on the PC board of the one you found and ran across a company in China. They advertise only one capacitance meter kit, again looks like that one, I'd guess that's it. The schematic is here:

https://www.jyetech.com/Products/CapMeter/105-06000-00b.pdf

Basically it's a microcontroller connected to a 4-digit LED display. Almost no external components that might determine its limit of 500uF. The group o resistors in the middle, that I'd hoped were a ladder-type digital voltage tree, turned out to be simply a bunch of current limiters for the LED display's segments. So I'd guess the limit might be set by programming, not by changing resistors or whatever.

Many such devices operate by feeding a controlled current into the capacitor, and waiting for it to achieve a certain voltage. The bigger a capacitor is, the longer it takes to reach that voltage. Since our 5F 3V supercaps are 10,000 times bigger than the ones this circuit seems designed for, we might not get a reading for minutes, or even hours. And it might not know how to display a 5F value on its LEDs.

That's all guesswork on my part, of course, I could be very wrong.

A very interesting gadget. You had suggested earlier, using an Arduino, at least for displaying voltages. Looks like someone has already done it! And for capacitances this time. (The ATmega48 microcontroller this circuit uses, can be programmed to be an Arduino.)
« Last Edit: May 11, 2018, 11:52:28 AM by Little-Acorn » Logged
frash
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« Reply #36 on: May 11, 2018, 11:49:15 AM »

Little-Acorn asked about 6F caps for Science Olympiad. Last year I ordered from Digi-Key some Eaton HV0830-2R7605-R supercapacitors and used them successfully for two indoor models. One of the planes was Chuck Markos' design for the original proposal for this SO event.

These caps are 8mm * 30mm so are skinny and long, and green in color. 2R760 means 2.7V and 6.0F. I cannot break the remaining code and do not know how to judge the internal resistance but Little-Acorn understands this.
 
The last set of SO rules that I saw limited capacitors to 5.0F. Maybe 6.0F rating is a preferred rule.

Fred Rash
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Little-Acorn
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« Reply #37 on: May 11, 2018, 12:18:53 PM »

Fred, I was trying to refer to capacitors marked 5F, 3V, but whose factory tolerances might result in some of those caps being 6F to 6.5F. Sorry if I wasn't clear.

(Oops, paragraph deleted. What I thought was a 3V capacitor, (HV1020-2R7505-R) is actually a 2.7V capacitor, sorry. I cannot find any 3V 5F caps in Eaton's inventory, though other manufacturers have them.)

One interesting thing about the Eaton caps is that the datasheet (http://www.cooperindustries.com/content/dam/public/bussmann/Electronics/Resources/product-datasheets/Bus_Elx_DS_4376_HV_Series.pdf)
for the 2.7V ones calls out a "surge voltage" rating of 3.0V. I wonder if they are a little lighter than the (new?) 3V ones they are making? Might be advantageous to use a 5F 2.7V cap for an EWS plane if it's lighter than the 5F 3V ones, and can still be used at 3V without harm.

I've charged the AVX 5F 3V caps to 3.15V several times so far, and not seen any apparent problems. Though if I keep doing it that might change.

The Eaton datasheet does say "Do not overvoltage".
« Last Edit: May 11, 2018, 12:47:37 PM by Little-Acorn » Logged
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« Reply #38 on: May 11, 2018, 12:47:10 PM »


We do not cut our own rubber,  I get it pre cut and am at the mercy of whoever supplies the "widths" (and lot dates) I order.  I do however use a caliper to check sizes.  When balancing a plane for trim/rubber/prop, we do use different loop sizes (widths) to get the best times. 

Jerry


I apologize for meandering off the electric discussion but this is an important point.

Jerry
One of the things I frequently rail about is that you should measure rubber by length and weight. For an event that has a maximum motor weight you need to always use the entire weight allowance. The motor length is the variable that you need to control. When you specify your rubber by the cut width then you are not going to be in control of the motor length.

Your rubber supplier is probably not going to be happy about trying to hit a grams/inch target. Because the rubber thickness varies, the grams/inch will also vary for any given stripper setting. The way I deal with this problem is make sample cuts and adjust the stripper to give the result you're looking for. Then instead of running off 100 feet at that setting, you need to cut shorter pieces, like some multiple of what a motor requires. (I cut enough for one motor). In between these cuts you can do short samples and readjust the stripper if needed.

It is a fairly labor intensive procedure if you want the best results.

Bill,

I mean no disrespect to you or the more expert mentors/modelers on this forum.  I do not have the equipment or time to become an expert in this sport/mentoring.  As I stated in earlier posts, I am a "wet finger" flier who goes by the seat of his pants after modeling and flying model airplanes for 60+ years.  I do read all your posts as well as the others on the SO forums to gain knowledge in place of my inexperience.  As you will note, I do believe your assertion that weight/length of a rubber motor is the key to performance.  I use the spreadsheet that has the formulas for TanII rubber I got from this forum that only uses weight and loop length to guide my students.  So in effect, I am following your advice and using width only as a general guideline to select rubber.  Since I cannot strip myself, I buy various "widths" since that is the only way I can purchase it.  We do use various lengths that have been cut to 1.5g and match that to the prop to give the best climb/duration for each plane.  It involves a lot of experimentation and fun to get to the goals the kids set for themselves.

Sorry to be off topic.  I think I was replying to a similar post on this thread in April. 

Jerry

P.S.  I am volunteering at the CSU nationals tournament and am looking forward to placing a face with a name for some of you who may attend.
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ceandra
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« Reply #39 on: May 11, 2018, 08:35:15 PM »

Jerry:

Volunteering for WS, or another event?

I'll probably hang out at WS most of the day, but my HS team flies Heli mid-day.

Chuck
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« Reply #40 on: May 12, 2018, 10:58:29 AM »

Jerry:

Volunteering for WS, or another event?

I'll probably hang out at WS most of the day, but my HS team flies Heli mid-day.

Chuck

My assignment is for WS in the Aux Gym.  Never been to a Nationals tournament, I will have to mind my manners.
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ceandra
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« Reply #41 on: May 12, 2018, 02:09:28 PM »

Well, I cannot go in the student area, but maybe I'll get your attention prior to start. I will have their stuff on a grey 4-wheel cart (dolley). I am bald, 6'1".

Chuck
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Little-Acorn
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« Reply #42 on: May 12, 2018, 06:47:09 PM »

Summary of measurements on all 13 capacitors. Sure enough, all the long, skinny ones were in the range 5.0-5.5 Farads; and all the short, fat ones were in the range 6.0-6.5 Farads.

That's with a very small sample, of course. If I keep measuring more I may find exceptions, maybe even lots of exceptions.

But if the present trend keeps up, this becomes a major reason to use the fat ones instead of the skinny ones. The extra 20% capacity could well be worth the extra 0.4g they weigh, which adds 6% to a 7-gram plane's weight. Though I've built (rubber-powered) Wright Stuff planes weighing 4.8g without the rubber motor, which needed ballast, I've never been able to build an EWS plane lighter than 7g with the capacitor, motor and prop installed as last season's rules required. Best so far is 7.8g. So installing a heavier capacitor wouldn't mean I can subtract ballast, there isn't any to subtract, in my plane anyway.

Bad news is, the rules committee might wind up specifying the skinny ones by brand and part number (AVX SCCR25E505SRB) just to keep things from getting too uneven. Unless it turns out that there are skinny ones that have a 6.5F capacity under their 5F-labeled surface, as the fat ones do.
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ceandra
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« Reply #43 on: May 12, 2018, 07:16:49 PM »

I would think if you are in the 4.8g range on rubber WS, you should be able to reach 7g EWS. Though I have not built one. Our kids are building right around 4g, and the EWS would not require the beefy motor stick we currently use.
 
The data is definitely compelling. Thanks for sharing.

Chuck
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Little-Acorn
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« Reply #44 on: May 12, 2018, 08:25:10 PM »

Well, I've been using a Parkzone Vapor 6mm motor, gearbox, and prop, which together weigh 3.87 grams on my scale (with connectors that push directly onto capacitor leads for a contest). My experimental EWS airframe comes in right around 3.9 grams or so, mostly due as you pointed out to the lighter motor stick I'm sure.

Right now I just can't see my way clear to building an airframe that weighs 3.1 grams without the motor, prop, and gearbox, even with 0.8mm and 0.5mm carbon fiber, and Ultrafilm. And if I use a fat capacitor, the EWS airframe would have to weigh in at 2.7 grams. Is it legal to attach a helium balloon?

What motors, props, and gearboxes (if any) do you folks use? Enquiring minds want to know!   Grin  I'm still very much a beginner at EWS, trying to find out all I can.

P.S. I ran into one guy at the SoCal State tournament at Caltech, who claimed he had covered the top and bottom of his child's EWS plane's wing (a Clark-Y-ish airfoil) and injected helium into it! Quite a conversation piece...  Shocked

« Last Edit: May 12, 2018, 08:53:43 PM by Little-Acorn » Logged
ceandra
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« Reply #45 on: May 12, 2018, 09:20:16 PM »

I was just surmising, have not built one yet. However, did not recognize the motor system was that heavy. That does not include cap?

Seems to me the weight at last year's nationals was higher, like 9g or something, to account for the rubber (we are actually 8.5g this year with rubber). My HS team did not make Nats last year, so we did not get to try it out. I don't see a copy of the 2017 EWS rules on my computer.

Is EWS certain for 2019? Or is it still possibly rubber?

Chuck
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ceandra
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« Reply #46 on: May 13, 2018, 12:08:29 AM »

Nope, I found rules online, both this year and last. You are correct, 7g all up flying weight.

Well, it can be done, but they basically need to be building a Penny Plane. I suppose you could dump the gearbox and run direct drive (small prop), but I suspect a larger prop turning through a gearbox will be more efficient.

Chuck
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Little-Acorn
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« Reply #47 on: May 17, 2018, 09:15:51 PM »

Changing the test set a little, moving it to a narrower board and adding a way to measure thrust. Eventually it will have a shroud around it, and a low-speed fan(s) and airflow straightener, and be an EWS-speed wind tunnel.

The motor mount arm is taller, and pivots at the bottom. It has a lever that will press against the top scale. Forward thrust will show up as downward force on the top scale.

That assembly (motor mount and top thrust-measuring scale) will sit on another scale (the bottom one), which simply measures weight of the entire assembly and everything mounted to the arm. Not much use when just a motor and prop are being tested. But someday I'll put a larger shroud around it (3 ft. wide and tall), large enough to mount an entire WS or EWS plane inside with ample wing/tip clearance, so we can measure thrust/drag at flying speed, and lift/weight. That might prove useful when it's all done.

The motor arm can stand straight up for testing, or can lean back to help mount or remove stuff on the arm. Currently it has an Ember gearbox with a Vapor 6mm motor and prop. There's still some wiring to do, obviously. The arm might eventually be a hollow streamlined-strut piece so wires can be inside, with all controls at the front of the test set facing the user. The controls might even be movable so when the wind tunnel gets wider it can expand to suit.

It will also have all the meters etc. I had on the previous test set, measuring battery/capacitor voltage, current flow, and motor voltage. Variable-voltage DC power supply will arrive tomorrow from China.

When it's time to make a wind tunnel out of it, I'll put a glass (actually plexiglass or lexan) floor above the scales, meters etc., with just the mounting arm sticking up through it, so the scales etc. won't be in the wind stream. That will be the floor of the wind tunnel.

Who knows, it might even work. Stay tuned.
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Little-Acorn
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« Reply #48 on: May 17, 2018, 09:29:23 PM »

Will probably have to measure changes in thrust, weight/lift, etc., not simply the raw scale value. First test with prop not turning, then test with prop speed gradually increasing and/or a capacitor for a power supply. Ditto for lift when it's a wind tunnel - test with no wind, then test with wind at various speeds to see how it changes. With wind, should be able to measure the drag of an entire WS or EWS plane too, find L/D ratios etc.

Might also put a small motor that gradually tilts the model up and down to test changing lift coefficients (angle of attack).

Right now the mounting arm is exactly four times as long from the bottom pivot to the propeller thrust line, as it is from the pivot to the little know that presses down on the scale. Hopefully tiny changes in thrust/drag will be more evident. A 1-gram change in thrust will show up as a 4-gram change in the reading on the scale.

Lots of cool things you can do with a wind tunnel, if it's designed right (i.e. if it measures similar to a plane flying freely in a gym, and doesn't tear the wings off the plane). For a 16" span plane, the tunnel probably needs to be at least 3 feet wide and tall. I hope next season's rules (if we have a next season) doesn't call for planes that are a lot bigger.  Sad
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« Reply #49 on: May 18, 2018, 08:12:08 AM »

Little-Acorn,

That is a lot of work and effort you are putting in to determine the variables of flying an electric WS plane.  I get the feeling you are enjoying this (and I can totally relate to that feeling!)  What is the probability that there will be an EWS?  I would use the summer to work myself up to speed to mentor some potential HS students, as well as some MS school for what seems to be elastic launched gliders.  I need to use the summer to teach myself so I can sound somewhat knowledgeable and not totally irrational. Grin


Lee
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