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DX8 Throttle control with motors

Old 09-18-2013, 02:20 PM
  #1  
chuckk2
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Default DX8 Throttle control with motors

For some time, I've noticed that certain combinations of motors and speed controls had an undesired and somewhat intermittent symptom.
Usual conditions

Motor not rotating, Throttle at minimum.
Advance throttle rapidly partway, up to about 1/2 or more.
Motor "screeches" and may or may not turn, or "wiggles" the prop"
Naturally, on hearing the screech, the tendency is to return the throttle to low.

The motors involved are E-Flite Power 25, 32, 46, and 52.
Speed controls are E-Flite, and E-RC as well as a Castle.

These motors have a fair amount of both power and rotating mass compared to the much smaller motors.
I had one instance of an in flight catastrophic (to the model) failure.
A potential runway over run into the rough resulted in a very rapid
throttle advance to full throttle from the low position.
The power 32 screeched, no thrust, nose full up, abrupt stall at about ten feet AGL, and the resulting basket case.
The battery was close to acceptable discharge levels, (Between storage and minimum) and LVP in the speed control may also have been a factor.
Ground test of the electronics showed no problems. this included a no propeller motor and speed control test.

When I initially encountered similar symptoms on a 60A E-RC controller, and a Castle 75A, I dinked around with the
speed controls timing and rate a bit. This produced a noticeable improvement, but no Cigar.

In most cases, the propeller is sized such that close to the maximum allowable wattage from the motor is possible at full throttle
on a static test.

One possibility is to slow down the throttle change rate, I suppose, either manually or via the ESC or TX.

Advice, Comments, etc Please!

Last edited by chuckk2; 09-23-2013 at 09:04 AM.
Old 10-10-2013, 08:36 AM
  #2  
chuckk2
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The only suggestions I've been able to find - -
change the timing advance and or reduce the ESC pulse frequency.

What I suspect, but cannot prove - - -
The motor has mass, and a resulting lag in responding
If the pulse rate is too high, the motor and ESC are "out of sync"
Since load (propeller) also slows response, it makes sense that adding the propeller load
increases the motor response time, and make the issue more evident.
Old 10-10-2013, 09:48 AM
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AndyKunz
 
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Yes, that's a good way to look at it. When your timing is too advanced this can get real smelly, real fast.

Andy
Old 10-10-2013, 02:41 PM
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chuckk2
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The interesting part of this is that the E-Flite outrunner motors I use are 12 armature poles, with 14 magnet poles.
So far, I've tried three different mfr's ESC's. E-Flite, E-RC, and CC.
In general, the ESC instructions recommend a faster pulse rate than seems to be the best.

Propeller size/pitch I use is based upon maximum usable diameter for the model, and an efficient pitch for the motor
RPM between about 7500 to 11,000 rpm as a start. Then things get tweaked to obtain the maximum allowable
wattages and currents. Because of battery voltage differences, The 25, 32, and 52 motors can all use the same 12x8x3
prop. Efficiencies vary between 81 and 91%, according to E-Calc. A two blade propeller might be 13x6.5 or so.

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