SEN 2755 Tip for launching your milli-amps

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Category: Archive 2020

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SEN 2755
Tip for launching your milli-amps

1. Tip Launch Glider Experiments

2. So how many milli-amp hours you really have?

Tip Launch Glider Experiments

From:Ken Bauer

Recently Paul Love started studying his TLG launches by making slow motion
videos.  This started an email discussion between Paul, Stan Buddenbohm,
Kurt Krempetz and myself.  I have also done many TLG experiments over the
years and I jumped back into it and started making more videos to
understand how various tail and tail boom configurations can affect the
launch.  The whole idea here is to reduce the energy lost to the crazy
yawing and rolling contortions that happen just as the glider is released
and maybe get higher.  Some of us are flying RC DLGs which can get MUCH
higher than our FF gliders so we are wondering what we can learn from them
and possibly find a FF launch that has more in common with the RC launch.

Lots of good info and stories have come out of our email conversations and
videos and we may organize it all into a symposium paper a year from now.
But for now here is a snippet of a story I shared and I have also compiled
some of my videos and put them up on YouTube.

Quote:
"New discovery today!  I captured videos of my inverted Y-tail glider and
it flew WELL for the first time!

A little background.  I've tried the inverted Y tail a few times over the
last 10 years all met with failure.  One case was an indoor glider with a
flat bottomed airfoil.  Went up much steeper with the inv Y but never got
it to transition.  Years later I discovered that a big problem with my
undercambered TLGs was that the negative pitching moment of the airfoil at
high speed made them launch straight out without pitching up enough and I
was trying to compensate by throwing them UP which screwed up my throw for
a long time.  Then a few years ago I had the idea that the inv Y tail might
be a good match for the undercambered airfoil, the pitchup characteristic
of the inv Y compensating for the undercamber.  So I took my 1 meter carbon
wing model with a heavy undercambered F1A airfoil which never launched well
but glided great and flipped the tail around 180 degrees to make an
inverted Y.  But launches were very strange and unsuccessful so I put it
away.  So Paul's videos inspired me to at least get some videos of what
that inv Y tail was doing at launch.  I took it out a couple days ago to
grab a video but it broke on the first test flight.  So I crudely glued it
back together with the idea that it would only live long enough for a video
or two and put a fast response RDT receiver in it to increase the odds that
I could save it from crazy launches and get some data.  To my surprise it
flew very well this morning!  The inv Y tail was doing exactly what I had
hoped.  I could throw the model flat and straight and it would
noticeably pitch up immediately, went up steep and had many good
transitions and glided away in thermals several times.

Here's some videos for you guys to study.  To me the glider is going up at
a steeper angle much sooner than a normal Y-tail glider.  The boom is
twisting the opposite direction as expected and the right stab half becomes
a large rudder on the BOTTOM of the model instead of the top.  And when you
think about it having the rudder area below the centerline of the model is
better than on top because the yaw correction will also cause a roll
correction in the RIGHT direction.  The normal Y tail with the stab/rudder
area on top is actually creating a roll force in the wrong direction."

TLG YouTube video here:  https://youtu.be/bA-8V-j4v50

I already got asked why I didn't put music to this video.  Maybe someday
I'll write an Opus for TLG, but for now here's my latest music offering
which any Pink Floyd fans should enjoy:  https://youtu.be/C999mFsxftg

-Ken

So how many milli-amp hours you really have?

From: Roger Morrell

Recently a friend found a new source for LiPo batteries for his F1B. His F1B has the full works, electronic timer, RDT, flasher and GPS. For this configuration it usually recommended a battery of at least 250 mah. This one was 250 and it had an auto low voltage cut off to protect the cell from a dangerously low discharge. In the past we did a test of a number of LiPos and as one might expect a number were sub standard and this proved useful. I use a West Mountain Radio CBA IV. This is a Computerized Battery Analyser. It has a “box” which you plug into your PC via a USB port and has sensors and power control electronic in the box. Your battery is connected to the box. On the PC you configure the type of battery number of cells, discharge rate and end voltage.. It plots a graph on the screen and generates a results table

When I did the test I also tested a 180 mh Hyperion LiPo battery that I have been using on some projects. This does not have a voltage protection circuit.
This the results table for the Hyperion

Description: 1 Li-poly cell, 0.2 Ah @ 0.15A (PASSED, 0.18 Ah, 100.2%)
Started At: 7/25/2020 6:46 PM
Discharge Rate: 0.15 A
Starting Voltage: 4.24 V
Ending Voltage: 3.00 V
Total Time (hh:mm:ss): 01:12:12
Tested Capacity: 0.180 Ah
You can see that it started at 4.24 volts , went down to 3 volts and the discharge rate used was .15 amps.
It gave a capacity of 180mah

The I did the test of the new 250 mah battery and got this result table

Description: 1 Li-poly cell, 0.250 Ah @ 0.10A
Started At: 7/26/2020 9:30 AM
Discharge Rate: 0.10 A
Starting Voltage: 4.18 V
Ending Voltage: 3.67 V
Total Time (hh:mm:ss): 01:28:12
Tested Capacity: 0.146 Ah

You can see the capacity was only 146 mah. BUT note that the ending voltage was not 3 volts as in the first case but 3.67 volts ! This is because the battery protection circuit cut in and disconnect the battery, stopping thetest before the voltage dropped to 3 volts.

Now probably the capacity was really about 250 if I took it down to 3 volts, as it was not my battery I did not by pass the electronic safety cut out.

So what do we conclude?

That probably the full capacity of the battery is not really useful because what it can do at 3 volts is borderline, see below.
But probably cutting off that 3.67 volts is a little too high.
In the case of this F1B flyer because he has a GPS the startup can sometime draw a higher than “normal” current and larger battery would help there as it can satisfy the current draw. While the capacity numbers of a battery are a good guide when picking the battery it is very useful to know the cutout voltage if you use safety cut out as it can significantly affect the usable capacity.

3 volts or what

I have also done tests is my timers (a single LiPo Magic timer) because I was curious about this. I already knew that always the timer would keep working after the voltage dropped below a level where the servo would stop working, not that is does you much good.

I did a test with my single LiPo timer (that I use on my F1Bs) and a Dymond D47 servo. I connected this to a bench power supply and a digital volt meter on the RDT power output connection. Nothing extra – no GPS, flasher, RDT or tracker was powered by the timer . Both the timer and servo continued to run with a voltage less than 3 volts. This shows that in this case the timer and servo will keep running to below the safe voltage for the battery. Note that this does not mean that other timers will behave the same nor may other connected extra items such as GPS, Flasher, RDT or tracker. My timers are available with different buzzers. This particular timer was equipped with the most noisy buzzer that draws the most current and there was a voltage drop of .1 volt if the buzzer was sounding. When the voltage got down to 3.1 volts the servo gave some small twitches but still appear to move to the same point. Note that not all servos behave like a D47 some stop way before 3 volts.

We are now connecting more bits of electronics to our timers, GPS, RDT, Bluetooth, etc. Most of these run at 3 volts (actually 3.2 volts) and the timer typically has a votage regulator to supply this voltage level . But one interesting word of warning is that I found when working with Bluetooth modules, from more than one manufacturer that while the work well at 3.2 volts, that they don’t like 3.0 or 2.9 volts. This is because modules such as GPS, Bluetooth and the radios used for RDT are very sophisticated with many options and typically the micro controller in them is bigger that it is in the timer. Because there many options that are configured, by or for the timer, to fulfill it’s particular requirement this configuration is stored in the non-volatile memory in the radio module. This non-volatile memory can be very susceptible to voltage drops so the cleverer the system the more careful you need to be about voltage levels and protect against voltage drops, even momentary ones.

One F1A comment

Now some of the above refers to a F1B (or C or Q) where typically the surfaces are moved by releasing a lever. To do this the servo has to move only a small amount with not much force. But on an F1A the servos move the control surface and during the launch phase the timing is very critical and flight loads require stronger servos. The is getting even more important as launch speeds get higher with aerodynamic changes such as LDA and flappers, launch techniques such as diving and rolling on the ground and increased physical fitness and stronger models. What is also very important is to have the model behave consistently throughout the day and to reduce the risk in change of performance because of the charge level of batteries. This has lead to better power management in the glider. One of the simpler ways of doing this is to use a 2 cell LiPo regulated down so there is headroom to accommodate a variance in battery charge and still provide constant voltage to the timer.