Electric running with no prop

[Ken Culbertson]

I think I know the answer, but I wanted to hear if from someone that actually knows.  Is it safe to run an electric w/o the prop for at least a minute.  I have a new timer and I want to test it's accuracy before flying it.

Thanks- Ken

[Dan McEntee]

  There is no load, so I don't see why not. Set your RPM a bit lower, just to check the timer. The load doesn't affect the timer. . In the early days with brushed motors, it was common to run them just on a batter with no load in a glass of water to seat the brushes. you don't have to do the wet part!!
  Type at you later,
   Dan McEntee

[Ken Culbertson]

 

  There is no load, so I don't see why not. Set your RPM a bit lower, just to check the timer. The load doesn't affect the timer. . In the early days with brushed motors, it was common to run them just on a batter with no load in a glass of water to seat the brushes. you don't have to do the wet part!!
  Type at you later,
   Dan McEntee

Thanks Dan.  I just ran it through a cycle.  Timer was dead on.  They are so quiet without a prop but they don't move much. 

Ken

[John Rist]

It's strange but true.  A gasser will destroy itself if you run too small of a prop (over rev).  An electric motor will fry if you use too big of a prop.  Electric motors want to turn a fixed rpm and will keep pulling more current until this is achieved.  So if you install an oversized prop and the current goes ballistic and the dreaded smoke comes out.  So yes  - no prop is wonderful!          

[bill bischoff]

From a safety standpoint, you should always remove the prop if you are powering up the system for servicing or adjusting. An unexpectedly spinning prop is very dangerous.

[Tim Wescott]

Dan is right.

Just a bit of backfill:

Our motors are permanent-magnet motors.  Such a motor generates torque that's pretty much proportional to current (and current that's pretty proportional to torque).  They generate a little bit of heat in their bearings, a little bit in "iron losses" (from switching the coil voltage), and if they're drawing current, a lot of heat in their coils.

They also generate a voltage that opposes the applied voltage, and is proportional to speed (that proportionality constant is the "Kv" rating).  Because of this opposing voltage (it's called "back EMF" for historical reasons), the motor "tries" to go at a speed proportional to the voltage -- and succeeds pretty well with the prop off.

When you run it unloaded, the only torque it needs to generate is just enough to overcome bearing friction and that little bit of wind drag from the case spinning around.  That means there's very little current, and very little heat generated.  Because of the back EMF, it'll only run up to the speed determined by the battery voltage, the throttle setting, and the motor's Kv constant.  So -- let it run.

This behavior is different from our slime engines.  Because of the whole back-EMF thing, a motor's torque drops off rapidly as it approaches its 'natural' speed.  Contrast this to a slime engine, where the engine produces useful torque at RPMs much higher than where you'd want to fly it.  This is why a shaft run is such special fun on a slimer, and a "don't care" on one of our motors.

[Tim Wescott]

From a safety standpoint, you should always remove the prop if you are powering up the system for servicing or adjusting. An unexpectedly spinning prop is very dangerous.

I do run mine on the ground with the prop on.  But rarely, and briefly, to make sure that the prop is turning the right way after I wire it up.  When I do this I'm behind the plane, holding it so it can't go forward or back, and I make sure I'm in a position to shut the thing off promptly.

[Ken Culbertson]

Thanks gentlemen.  I was pretty sure it was OK, but this stuff is just to expensive for me to do something stupid.   I was about to order a new Jetti box because my timer accelerator setting wouldn't change.  Then I remembered to say "Mother May I" and it changed.  It has been 2 years since I have had to change that setting on a Fiorotti and I totally forgot that it is different than the other settings.  So much to remember, so little memory....getting old sucks.

Ken

[Dan McEntee]

Dan is right.

Just a bit of backfill:

Our motors are permanent-magnet motors.  Such a motor generates torque that's pretty much proportional to current (and current that's pretty proportional to torque).  They generate a little bit of heat in their bearings, a little bit in "iron losses" (from switching the coil voltage), and if they're drawing current, a lot of heat in their coils.

They also generate a voltage that opposes the applied voltage, and is proportional to speed (that proportionality constant is the "Kv" rating).  Because of this opposing voltage (it's called "back EMF" for historical reasons), the motor "tries" to go at a speed proportional to the voltage -- and succeeds pretty well with the prop off.

When you run it unloaded, the only torque it needs to generate is just enough to overcome bearing friction and that little bit of wind drag from the case spinning around.  That means there's very little current, and very little heat generated.  Because of the back EMF, it'll only run up to the speed determined by the battery voltage, the throttle setting, and the motor's Kv constant.  So -- let it run.

This behavior is different from our slime engines.  Because of the whole back-EMF thing, a motor's torque drops off rapidly as it approaches its 'natural' speed.  Contrast this to a slime engine, where the engine produces useful torque at RPMs much higher than where you'd want to fly it.  This is why a shaft run is such special fun on a slimer, and a "don't care" on one of our motors.

   I knew all of that, just didn't have time to type it all out!! 
  Type at you later,
  Dan McEntee

[Howard Rush]

Plettenberg says not to do it with their motors, but I don't know why.

[Frank Imbriaco]

I recall that my dear friend, Dean Pappas, advised me against running without a prop- even for brief blasts.
I never run electric aircraft indoors. Besides, running them (outdoors) mounted (aircraft or bench) with a prop helps to determine proper prop rotation. As you know, motor wires can be reversed or set up in the ESC for clockwise or counter- clockwise rotation.
And ,some guys switch back and forth between tractor and pusher- on the same aircraft. We all brainfart and that's when accidents happen.

[Ken Culbertson]

I recall that my dear friend, Dean Pappas, advised me against running without a prop- even for brief blasts.
I never run electric aircraft indoors. Besides, running them (outdoors) mounted (aircraft or bench) with a prop helps to determine proper prop rotation. As you know, motor wires can be reversed or set up in the ESC for clockwise or counter- clockwise rotation.
And ,some guys switch back and forth between tractor and pusher- on the same aircraft. We all brainfart and that's when accidents happen.

Do you remember the reason?  If it is only the prop rotation, then the reason I "need to"/"have done it" only relates to motor run duration and timing of when things happen in which case, I don't care which way it turns.  Once I have the ESC and Timer programmed, I rarely change anything other than timer settings.  I learned from my military service that most equipment failures came from preventative maintenance, so I do little other than observe and check connections.     

Ken

[Sean McEntee]

    Ah, the fun and follies of electrics...

    I'm working on building an E-36 FF model, and I got all of the powertrain parts in the mail from Texas Timers a couple weeks ago.  Same components basically:  Motor, ESC, timer, plus servo and pigtail for a remote dethermalizer receiver.  The timer is a neat setup: two switches used to change motor run time and automatic DT deployment.  When one hits the button on the DT while the motor is running (in the event of, for example, a bad launch)  The motor is supposed to shut off and 1.4 sec later, pop the DT.  However mine wasn't working.  I was texting Hank at Texas Timers and he had me test it by closing the circuit on two of the RDT receiver pins.  Now I was just holding the motor in my hand, and when I tested the circuit, the motor shot out of my hand and went halfway across the room!  Not sure what happened but not wanting to just hold onto the motor anymore, I used a pair of vice grips to hold the motor mount and tried it again, and the motor still gave me a pretty good "jerk".  When the DT signal is received, that motor stops RIGHT NOW! Incredible amount of torque from such a small motor!

    For those FFers out there, yes, I had the wrong RDT Rx.  Called Ken Bauer and he fixed me up with what I needed.

[dave siegler]

I can think of a few reasons not to run motors without a prop

1) cooling  Motors need a stream of air, and unlike industrial motors they expect the prop to be given some air. 
2) Controller tuning /instability during start-up.  The control of the ESC is tuned to be ready for some level of inertia during start-up.  The currents could get pretty high for an inertia mismatch,  With a high number of motor poles, I have seen industrial PM motors like a little inertia ti run smoothly   
3) Maybe the bearings don't like the big jump in speed.  No thrust load may make an issue.  But I don't think so. 


 

[Ken Culbertson]

There is a whole bunch of chatter on the RC sites about this issue.  From what I read, as long as you are not exceeding the ratings for the motor and watch the temperature there are no problems.  One poster had tested over 300 motors at no load for 10 minutes at their maximum voltage rating with no failures.

So, for my purposes, a one minute run with RPM being controlled by the ESC and timer to test the timer functions is far better than running it for that length on the ground with a prop, and safer!

Thanks for all the input - Ken

One thing that has me curious is how does propwash reach the motor of a full fuselage plane with a spinner larger than the motor.  When I first got into this, I was told that the outrunner cooled itself by drawing air through the motor via the spinner gap and it was our job to vent the air coming out by suction from other vents/scoops, etc.  Am I wrong?

Ken

[Tim Wescott]

... When I first got into this, I was told that the outrunner cooled itself by drawing air through the motor via the spinner gap and it was our job to vent the air coming out by suction from other vents/scoops, etc.  Am I wrong?

I would really like to see some serious studies done about this -- like some maniac mechanical engineer with 50 itty bitty glue-on temperature probes, a wind tunnel, and a mock up fuselage kind of serious.

Because I suspect that when we do manage to cool things adequately, we're overcooling something.  It probably doesn't matter for stunt, because a bit of extra drag isn't going to hurt, but it'd be nice to have something better than decades-old rules of thumb that one just applies and hopes.

[Dan McEntee]

One thing that has me curious is how does propwash reach the motor of a full fuselage plane with a spinner larger than the motor.  When I first got into this, I was told that the outrunner cooled itself by drawing air through the motor via the spinner gap and it was our job to vent the air coming out by suction from other vents/scoops, etc.  Am I wrong?

Ken

   I don't even run them, and I'm curious about this also!! One of my flying buddies here has one of Walt Brownell's old electric models that he has been flying a lot over the last couple of seasons. One day the motor acted a little rough, then stopped a few seconds after start up. Pulled the plug on it, and the motor wouldn't hardly turn. Looked in through the slots on the from of the motor and saw black stuff, and thought it had just burned up. he had a spare motor just like it, changed it out and is still flying it. I got the old one from him and brought it home for a post mortem tear down, and once I got everything apart, I couldn't believe all the crap I found in there!! What we thought was a burned winding was the remains of a cooked bug!! All sorts of grit, dirt and rocks in there also. All that interference knock on or two of the magnets loose. I cleaned everything up, set the magnets back in place, and reassembled the motor and it turned over very smoothly. If I knew what glue to attach the magnets back in place, I would have glued them back in and run the thing. I checked the windings out with a meter as best as I knew how, and they seemed clear. This made me curious as to whether r not any of you electric guys run any kind of intake screen or filter on the front of the motors?? It made sense after thinking about it. The nose is right on the ground when spooling up and on a take off run. anything that is on the ground is going to get kicked up and thrown into the nose of the airplane. If all that debris and a bug can get in there, I would surely think air is getting through there also!!
  Type at you later,
   Dan McEntee
   Type at you later,
   Dan McEtee

[spare_parts]

I can think of a few reasons not to run motors without a prop

1) cooling  Motors need a stream of air, and unlike industrial motors they expect the prop to be given some air. 
2) Controller tuning /instability during start-up.  The control of the ESC is tuned to be ready for some level of inertia during start-up.  The currents could get pretty high for an inertia mismatch,  With a high number of motor poles, I have seen industrial PM motors like a little inertia ti run smoothly   
3) Maybe the bearings don't like the big jump in speed.  No thrust load may make an issue.  But I don't think so. 

These are non issues. idle current with no load is about the lowest heat production there is in a motor. The ESC should have no trouble at all stating the motor regardless of load. if the ESC stumbles, the software is poor. I would have expected to experience it (and did) 20 years ago, but today it just shouldn't happen. Bearings don't like an instant ramp to speed ( the balls have inertia), but car motors generally run at much greater RPM and continuous rapid speed fluctuation, bearing life suffers compared to other applications, but typically is still much longer than the first figure 9 landing. Most hobby motors do not properly preload bearing either, but oh well.

Cooling really depends on air flow and need for cooling. How much heat is lost in the motor due to load, efficiency, runtime? Outrunners do a pretty good job whipping the air around and keeping cool. Most magnets will die before the copper insulation fails, but there are many factors involved, so burned coils are just as likely. If you're within spec for current draw, the motor should never fail unless the manufacturer pushed the rating, you have totally insulated the motor, or are running it with very large batteries. Many High power RC setups use no cooling at all. High efficiency means little heat, and it can all be absorbed in the mass of the motor without overheating.

A little gap at the spinner can certainly flow air, especially at angles of attack other than zero. Back in the day, I would flow air through brush motors with a flush inlet on the fuselage somewhere and exhaust it at the spinner gap, it was very effective.

Debris in motors is something I've thought about since the 90's, but never experienced. We'll except those times I tried planting them. Flying off grass, vs dirt or gravel would be a big factor. If you ever have to remove debris from magnets, adhesive tape is your best bet.

[Fred Cronenwett]

If the brushless motor has power and signal either from the timer or a reciever it will want to turn. Electric will still want to turn after hitting something. to test a motor without a prop is normal. Since I fly with throttle control I use a reciever and 2.4 Ghz to control the motor speed. I will plug the ESC into the reciever, turn on the transmitter (set to idle) then plug the battery into the ESC. Since the ESC has BEC (battery elimanater circuit).

When a glow engine strikes the ground, the prop breaks and the engine stops. If you throw a rag into a running engine the glow engine will stop.

Electric will keep going regardless of what happens to the prop

[spare_parts]

If you stall a brushless. most controllers will give up trying to restart after a couple attempts. This is  dependent on how the mfg programmed it.  If it doesn't stall or the ESC doesn't sense such an event, the motor spins and twists wires until some connection fails, or the ESC fails due to excessive current.

[John Rist]

The KR timer, after initial spinup, turns off the motor if you get a prop strike.  If you hand launch or launce with a stuge you will not burn out a motor or esc during a prop strike..   If you walk out to the handle and let the airplane launch with initial start up a prop strike could be a problem.  Spin up time is short so you may survive (or not) ----------------