Notices
Brushed/Brushless motors, speed controls, gear drives Discuss all aspects of brushless motors, brushed motors, speed controls (ESC's), gear drives and propellers in this forum.

Plane Won't Accelerate

Old 09-05-2023, 08:42 PM
  #1  
Flysfloats
Thread Starter
 
Flysfloats's Avatar
 
Join Date: Apr 2008
Location: Whidbey Island, WA
Posts: 34
Likes: 0
Received 1 Like on 1 Post
Default Plane Won't Accelerate

Need Help!! Tried to "maiden" my recently finished Protege kit. During the takeoff roll, it wouldn't go fast enough to lift off, not even close! I was comparing this plane to my Kaos .60 which performs nicely. Both weigh roughly 8.5 lbs. The Protege had a Diamond Dynamics 5065, 420kv motor and a 4,000 mah 35-70 C battery. The Kaos had a Turnigy G60, 5960, 500 kv motor and a 4,000 mah 35-70 C battery. Both have 13x5 propellers.

First thing I did was to switch to a 6S battery and do a static run up/comparison. The Kaos pulled hard and the Protege didn't, sigh.

Next was to replace the motor with a Turnigy AeroDrive 5055, 280 kv, and a 6,200 mah 40 40C battery. Still about the same pull. Next I switched esc's. Static comparison, the same, Kaos pulled quite hard and the Protege not so much.

I know I am trying to compare apples and oranges, but I couldn't find another Turnigy G60, 500kv motor. I am pretty much at a loss to explain this dilemma so I am hoping to get some advice.

Thanks for your help, FF
Old 09-07-2023, 04:17 AM
  #2  
flycbr
 
Join Date: Dec 2013
Posts: 1
Likes: 0
Received 0 Likes on 0 Posts
Default

The essential information missing is the cell count you started with before the "switch to a 6S battery", the cell count you used with the 5055-280, and wattmeter readings for all the setups. If you don't know the expected power and the actual voltage, current and power, you don't know whether you have a fundamental setup problem, or an adequate setup that just isn't working properly, eg a weak or un-charged battery or a mis-calibrated ESC.

But even with limited information, I think you have a fundamental setup problem. At Kv of 420 rpm/V, 13x5 is too small a prop for anything less than 8S.

On the same cell count and prop, the DD motor with Kv of 420 rpm/V will use only 60% of the power the 500 rpm/V Turnigy does - so it's no surprise the performance is a lot less. The 280 rpm/V motor would just be an even bigger step backwards unless you were using at least 1.5 times the cell count, ie 6S vs 4S or 9S vs 6S.

Assuming 6S, the 420 rpm/V setup is never going to adequately fly an 8.5lb model with a 13x5 prop. Max power will be of the order of 500W which is OK for a trainer, but pitch speed will be too low at 40 mph. Less than 6S with a 13x5 will never work, period.
Assuming 6S, 13x8 would get you around 80W/lb and sufficient pitch speed at 65mph, or for more reserve power if there is enough clearance 14x8.5 would work well.

If you wanted to use 4S, you'd need to be able to use a 16x12.

Last edited by flycbr; 09-07-2023 at 04:56 AM.
Old 09-08-2023, 06:09 AM
  #3  
ron_van_sommeren
 
ron_van_sommeren's Avatar
 
Join Date: Sep 2002
Location: Nijmegen / Nederland
Posts: 630
Likes: 0
Received 22 Likes on 17 Posts
Default

Originally Posted by flycbr
The essential information missing is the cell count you started with before the "switch to a 6S battery", the cell count you used with the 5055-280, and wattmeter readings for all the setups. If you don't know the expected power and the actual voltage, current and power ...
RPM numbers can also be useful for troubleshooting.

A very long clearance sale, originally 200+$, versatile, high quality, now literally peanuts:
Hyperion Emeter II wattmeter with local&remote logging, optical&electrical rpm meters, servo tester - RCG
Contents
  • Closing out at
  • Reviews by Bernard Cawley and Ken Myers
  • Tech info & help threads
  • Increasing max.current, double, triple, simple and cheap
  • Software and manuals downloads


Without a watt/volt/current/multi-meter you are in the dark.
Until something starts to glow
Prettig weekend Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC

Old 09-08-2023, 02:07 PM
  #4  
Flysfloats
Thread Starter
 
Flysfloats's Avatar
 
Join Date: Apr 2008
Location: Whidbey Island, WA
Posts: 34
Likes: 0
Received 1 Like on 1 Post
Default

I will try and clarify a little My Protege had a 4,000 mah, 5 S, 35-70C battery.(poor performance). My Kaos had a 4,000 mah, 6 S, 35-70C battery9good (performer). Both with 13x5 props.

I next tried a 6-S battery in the Protege and, again, poor performance. Protege's motor was a 420 kv while the Kaos was a 500 kv, Next attempt for the Protege was a new motor, one with a 280 kv ratimg, No improvement, sadly.

SO, to me, I am questioning the KV rating as the most likely cause for this difference in performance. Really that big of a difference between a 420 and 280 kv motor vs a 500kv motor? Al else is basically the same except for different motor mfg. I HATE to spend money for another motor until I have this figured out.

Thank you for the advice/education! FF
Old 09-08-2023, 05:24 PM
  #5  
ron_van_sommeren
 
ron_van_sommeren's Avatar
 
Join Date: Sep 2002
Location: Nijmegen / Nederland
Posts: 630
Likes: 0
Received 22 Likes on 17 Posts
Default

Originally Posted by Flysfloats
... Really that big of a difference between a Kv=420rpm/volt and 280rpm/volt motor vs a 500rpm/volt motor?
Disclaimer: I know a thing or two about motors but precious little about good plane/prop/rpm combinations.
Warning: I have a tendency to go overboard w.r.t. e-motor info Just say when.

Yes, your motors are very different.

The effects of Kv are massively disproportional, not linear at all:
Both current drawn and power drawn both are proportional to Kv³.
Another surprising effect, current drown is proportional to voltage², power drawn is proportional to voltage³.
The other power system parameters also have a huge disproportional unexpected effect, propdiameter even worse than Kv.
All of the above has led to the fiery demise of many a controller, motor or wiring, or even the complete plane.

Some silly, expensive and hopefully instructive useful examples
Battery life with brushless ESC and motor - post#6.

More later, way past my bed time here.


Prettig weekend Ro

Last edited by ron_van_sommeren; 09-08-2023 at 06:03 PM.
Old 09-21-2023, 03:23 AM
  #6  
ron_van_sommeren
 
ron_van_sommeren's Avatar
 
Join Date: Sep 2002
Location: Nijmegen / Nederland
Posts: 630
Likes: 0
Received 22 Likes on 17 Posts
Default

IC-engines and e-motors have fundamentally different behaviour.
An ic-engine tries to keep a constant torque.
As a result, as propeller load goes up, rpm must go down.

An e-motor tries to keep a constant rpm, no matter prop-pitch and -diameter. It does not want to slow down as you prop up.
As a result, as load goes up, torque goes up, to keep rpm the same.
Mechanical power is proportional to torque, therefore electrical power has to go up.
Electrical power is proportional to voltage and and proportional to current. Voltage has not changed, therefore current has to go up.

Even if this would lead to the fiery demise of motor and/or controller.


Kv is not related to maximum continuous motor power.
Note that the velocity Konstant kv says very little about max.power capacity, nor about max. current a motor can handle, nor about max. torque, nor propsize, nor rpm, nor rpm range.
A 1:1 train motor and the motor in your toothbrush or in a bedroom appliance can have the same kv = 1000pm/volt.
Kv is the velocity Konstant, a physical property (like mass, length and volume), expressed/measured in the physical unit rpm/volt (like kg, meter and m³).


Kv is not a rating, not a figure of merit.
Not something the motor delivers like power and torque (explanation).
A kv=1000pm/volt motor on 10volt and a kv=2000rpm/volt on 5volt will give same no_load speed of 10,000rpm, both motors will draw same power, the second motor will draw double the current.


Kv, what the motor wants to do, versus max. power, what it can do.
Kv matches desired rpm and battery voltage, there's nothing more to it.
  1. Want
    Kv and voltage determine how fast motor wants/tries to run.
    (rpm_noload = voltage × kv, or, in other words, kv = rpm_noload / voltage.)
  2. Current ~ torque
    RPM and prop determine torque needed, which in turn determines current drawn.
    Torque and current are proportional, in the same way that rpm is proportional to voltage.
    (torque = current × kt, where kt = 1/kv (in SI units!), or proportional to kv³ ).
  3. Can
    A motor's max.current and max.power ability determine whether that battery/motor/rpm/prop combo can run without going up in smoke.

Vriendelijke groeten Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC

300

Last edited by ron_van_sommeren; 09-21-2023 at 03:28 AM.
Old 09-30-2023, 08:38 AM
  #7  
ron_van_sommeren
 
ron_van_sommeren's Avatar
 
Join Date: Sep 2002
Location: Nijmegen / Nederland
Posts: 630
Likes: 0
Received 22 Likes on 17 Posts
Default

Selecting power needed by plane type and mass works very well.

Below an excellent quote about motor selection and powersystem misconceptions
From
brushless motors Kv?.
Originally Posted by scirocco
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it:
    Magic numbers for modellers.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as eCalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
The reason I suggest picking Kv last, is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. On the other hand, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, not a rating, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS

Prettig weekend Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC
438

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Manage Preferences - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service -

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.