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[Andy Shutt]

Hello,


I have been setting a new Jeti Spin 99 and I find the the default cut off voltage is 3.2 V.


Can I set this lower, or say 2.8 V


What is a safe lower voltage to set this at.


Thank you for your assistance


Andy

[Bob Wasson]

Hello again Andy,


Practices differ.


Some people set 3.2v per cell or thereabouts in order to protect the 10s LIPO.


I normally set a lower voltage which effectively disarms the feature (see Wind S/Jet 99 spreadsheets).  I choose to do this on the basis that if it came to the crunch I would rather have a little bit of residual power to avoid a dead stick. In other words I would rather risk damaging a LIPO than damaging a model.


Needless to say, if that philosophy is adopted it is vital to work strictly to a timer setting which avoids the volts per cell dropping below 3.7v, or a little higher if possible.


Bear in mind also, than when pulling higher amps (for example on an extended upline), due to the internal resistance of the battery, the volts/cell will drop significantly below the resting voltage of each cell. This effect is particularly relevant as a battery ages and the internal resistance increases.  Even at 5 mOhms per cell, not unusual for an older battery, the on load voltage at 80 A would be around 0.4V less than the resting voltage.  Enough to risk triggering a higher cut off point towards the end of a longer schedule or practice session.


Regards


Bob

[AW2]

I think the recommended cut-off voltage is indeed 3.2V.per cell as you say.
I think I remember that, in theory, if you use the slow cut off option, you can stagger round and land on the patch---but when it happened to me, I couldn't make it back to the landing zone, and landing out with no power wrote my airframe off.
Since then, I've followed what was Jason Shulman's practice (at the time at least,a few years back), of setting the cut-off voltage at 2.0 V per cell.
If by any silly mistake( for example re-flying a used pack, or missing your set flying time by quite a bit), in my experience, you'll notice the severe drop in power and realise something is wrong.
But you'll still have enough capacity left, to fly gently round and land safely.
Obviously , you're likely to ruin your flight pack, but that's less financially painful than wrecking your model!
Just my take on the subject of course!

[Andy Shutt]

Bob / AW2,


Thank you for your coments, yes, I agree with your logic and will be dropping mine from 3.2V. I thought that the default would be in the safe zone.


Regards,


Andy

[Mike Wood]

Hi Andy

One thing I like to do is put the batteries in, strap them up, then as part of my pre flight checklist,
check the battery voltages on the flight pack and receiver packs. I've seen several
people at our field pick up the wrong pack and get a nasty short flight.

Mike

[Peter Jenkins]

An interesting discussion.  I have to say that I've set mine at 3.4 v/cell. 


Thinking back, I once forgot to change the pack between flights due to distraction (isn't that always the precursor to trouble!) and ran out of grunt on the 2nd manoeuvre.  The voltage was now so low that the limp home function was locked out.  While still able to land on the strip, I chose to dither around looking to see if I'd accidentally switched in the THRO CUT switch before returning to the matter of landing by which time reaching the strip was not possible!  The resulting out landing ripped out the port undercarriage mounting and part of the fuselage and snapped the 3 blade prop! 


I generally find that after landing my packs are in the range 3.68 to 3.75 v/cell.  The lower figure for when I've repeated a manoeuvre or just flown too big.  Despite having used full throttle on the last manoeuvre I cannot recall a time when the LVC operated - perhaps I'm lucky!


This discussion has certainly flagged up a question as to whether I should reduce the Spin 99 setup to follow Bob's and Alan's recommendations.  As they say, a wrecked pack is preferrable to a wrecked model!  On the other hand, there is the other good engineering maxim, if it ain't broke don't fix it.  Clearly, on-board telemetry of pack voltage would help to avoid these situations arising.  However, my on-board telemetry, provided by a 3rd party supplier, is not particularly reliable - unlike those of you with Jeti kit!

[Bob Wasson]

JETI 99 SETTINGS

Further to the above comments, I thought it might be helpful, especially to pilots newer to 2 metre F3A electric models, if some additional guidance was provided on some of the other settings on a Jeti 99.  The Jeti 99 Spin Opto has stood the test of time and is still one of the most popular ESC's for F3A purposes.  Its continuing popularity is not without good reason.  It has proved to be versatile, highly reliable and is also very easy to programme via the separate inexpensive Jeti Box.


What follows is a mix of personal experience and early advice provided by Alan Wild and Malcolm Harris.  I am deeply indebted to both Alan and Malcolm for providing me with excellent guidance and support when I was dragged kicking and screaming from my much loved YS 4-Strokes into the brave new world of electric flight some 10 years or so ago.  It seems like only yesterday!


Settings are very much model and power plant dependent.  They can be schedule and pilot dependent too.  For these reasons, there is no such thing as a universal answer.  What follows is mainly based on personal experience with 2 bladed single propeller set ups.  More modern 3-bladed and contra set ups will be different.  I would encourage others, especially those with more recent hours and more recent models under their belt, to share their findings.  It is only by so doing that we will learn as a community.  As with trimming in general, a better understanding of the correct set up for an ESC can only help in reducing pilot workload and making a model easier and more pleasant to fly.


BRAKE SETTINGS

There are 5 different parameters that can be set via the Jeti Box.  Taking them in turn:


Manual or Auto

Auto is the easy option.  Having said that, I have always chosen manual as it gives the user the option of tailoring the remaining 4 parameters to the specific requirements of each model.


Dead Time

Dead time is the time between throttling back and the initial application of the brake.  On a car, you would probably want the dead time to be zero. Aeroplanes are different.  Take the case of a vertical downline.  If the brake came on as soon as the motor was throttled back, it could result in a 'mushy' end to the preceding radius.  Better to leave a short delay to help ensure that the model is cleanly established on the downline.  Around 0.2 or 0.3 seconds seems to work well for 2-bladed single prop set ups. A slightly longer delay might work better for a 3-blader or a contra.  But I'm no expert on either, so I'll not speculate on the detail.


Initial Brake


This setting is adjustable in % terms.  As the name implies it defines the initial setting of the brake following the expiry of the Dead Time.  The best setting is something of a compromise.  Too low and it will enable the model to build up too much early downline speed.  Too high, and the sudden application of heavy brake will compromise the required impression of constant speed performance.  Between 10% and 20% seems to work well for 2-bladers.  Draggier 3-bladers and contras may need less.


End Brake


Also adjustable in % terms.  Again, as the name implies, this is the final setting after the "Brake Speed" period (see below) has expired.  I have found between 30% and 60% to work well for most 2-bladers.  Again, draggier 3-bladers and contras may need less.


(Note that on schedules which include point rolls or other more 'time consuming' manoeuvres on a downline - for example Figure No 2 Et on P19 - higher Initial Brake and End Brake settings may be called for.  Otherwise a lot will seem to be happening in short order and the manoeuvre can end up appearing rushed.)


Brake Speed

This is the time in seconds that the ESC takes to ramp up the braking from the 'Initial Brake Setting' to the 'End Brake Setting'.  Too short a period may compromise the constant speed effect.  Too long a period may mean that the model has reached the bottom of the downline before the "End Brake" setting comes into play.  Around 0.5 seconds seems to work well for most 2-blade set ups.


ACCELERATION


This is the time taken, following the application of full throttle, for the ESC to ramp up from 0% to 100% output.  At first glance a low value may seem the better bet.  However a somewhat higher setting brings the twin advantages of a) smoothing out the response to increasing throttle, and, b) reducing the likelihood of the motor dropping out of synch.  Around 1.0 sec seems to work well but a slightly higher setting may be appropriate for heavier propeller loads.


TIMING AND FREQUENCY

In both cases, the best starting point is to follow the recommendations of the motor manufacturer.


CUT OFF


Slow Down (But see posts above)


THROTTLE CURVE

An individual throttle curve can be set up through the Jeti Box.  However I have always set the Jeti Box to "Linear" and chosen to set "Throttle Curves" via the Transmitter.  Not only have I found this to be easier, but it is also more versatile and gives the option of setting up more than one throttle curve – of which more later.  I have heard that Throttle Curves may be less of a necessity with contras but I have always found them to be advantageous with 2-bladed mono-props. The power in I/C engines comes in quickly at the bottom end and dies out progressively nearer to full throttle. My specific experience with AXI outrunner electric motors has shown them to have the opposite characteristics i.e. slower to respond at the bottom end with most of the power coming in nearer to full throttle.  A Throttle Curve which smooths this out helps with good control.  The following setting have worked well on my AXI powered models.  Other motor sets ups in other models will almost certainly be different.  Remember that what we are looking for is not a linear relationship between throttle stick position and RPM, but a linear relationship between throttle position and the perceived response of the model during flight.  The following figures refer to IN and OUT percentages respectively – 0/0, 25/46, 50/69, 65/80, 80/90, 100/100.  On my JR PCM 12 Tx I have opted to apply exponential smoothing between the points.  I have also chosen to set up a second Throttle Curve selectable via a 2 position switch.  This is used as a safety feature which prevents the ESC from arming when the throttle stick is fully back.  It differs from the first curve only in that the initial IN/OUT setting are 0/3.  This second curve can also be used to provide a slightly higher idle speed in windier conditions.


My sincere apologies for the length of the above post. To experienced pilots it will all appear old hat.  However if it is even of minor assistance to newer members then all may not be wasted.


Bob

[AW2]

All very true, Bob!
Just to remind (or possibly inform?), when using two throttle curves---the one with the lowest starting point will arm the ESC, and start the brake when reached during flight.
The second curve--there will be no brake whenever it is in use ,if selected during flight.

[Bob Wasson]

True Alan and well worth pointing out.


I have rarely used Curve No 2 in flight.  Only reserved for really windy conditions when braking was much less of an issue, or, more usually, for creating an actual idle (i.e. other than zero) during windy landings.


Thank you for setting me off on the right path all those years ago.  Hope you're keeping well and still getting great fun out of practicing all those P and F schedules.


Regards


Bob

[Ralph Arrow]

Thank you Bob


That is the best explanation of Jeti spin settings I have read. So much clearer to me now.


Kind regards


Ralph

[Bob Wasson]

Really pleased that it has helped Ralph.


Hope that it gains you some extra points along the way!


Very best wishes


Bob

[Alan Williams]

Hi Bob,
one thing that I have wondered about for a while is exactly when the brake becomes active. Is it ONLY when the throttle is low enough to arm the motor, or will it come on when the throttle is low enough to stop the prop turning. The reason I ask is that I find there is a large amount of difference between the two points in terms of stick position. I have a number of flight conditions I use one throttle curve but have the trim leaver different for each condition so that in Landing mode the trim is advanced to give a slight idle, Normal mode the trim is pulled back enough to stop the motor and in start mode the trim is low enough to arm the motor. I think the brake is coming on in normal mode but that could be because my downlines arn't long enough for the plane to speed up!


thanks
Al.W

[Bob Wasson]

Hello Alan,

The throttle function on the Jeti ESC is controlled by the width of the pulses in the output signal from the Rx.  Fortunately, if the Jeti Programming Box is plugged into the throttle channel it can be used to measure pulse width as well as to programme the ESC (via the ESC's Throttle lead).


Nominally, a pulse width of 1.0 ms equates to the zero throttle position and is also the point at which the ESC will a) arm and b) activate the brake.  (Note that, should you wish, the "Initial Setting"of 1.0 ms could be adjusted as one of the main set up parameter when programming your ESC.)


A pulse width of 2.0 ms would normally equate to 100% throttle although this figure can also be adjusted as one of the ESC's set up parameters via the Jeti Box.  (This can be used to fine tune the maximum amps draw from the flight batteries when the Throttle stick is at full travel.  This could be useful, for example, when using a heavier load propeller with a high RPM/KV motor.  Throttle Travel adjust on your Tx could be used for the same purpose)


To be sure that the brake is activating properly I would make sure that the output pulse width from the Throttle channel on your Rx does not exceed 1.0 ms when your Throttle stick is fully back and your Throttle trim is in the normal flying position. 


As I mentioned in an earlier post, I use 2 Throttle Curves on my JR PCM 12.  They only differ at the bottom end.  The one I normally fly with has 0% In/0% Out at the bottom and the other 0% In and 3% Out.


If I select the latter, it acts as a safeguard to prevent the ESC from arming, but it also has the same effect on the brake.  Once the ESC has been armed using the former curve it can occasionally be useful to switch to Curve 2 if the weather is sufficiently windy that it would justify flying with no brake.


Hope this helps.


Bob

[Alan Williams]

Thanks Bob,
I use manual settings in the jeti box set up by measuring the pulse width. I can easily set up the arming position to match the normal flight mode. I will give it a try when I get back in the air
Thanks


Al

[AW2]

I do very much the same as Bob, but my second throttle curve has a little bit more added to the lowest setting---I use whatever extra percentage that will just hold the lowest possible idle rpm when selected---on my JR Txs it's usually in the+ 7-8% region from memory.