Why not brake the propeller

[Chris Bott - Moderator]

Erfolg I've had a cheap folding prop explode in the air, the vibration ripped the front off the model before I could react so I think you're right to go for quality ones.

I'm also of the opinion that when using a normal prop, stopping it with a brake reduces the drag compared to leaving it freewheeling. Perhaps when it's freewheeling, work is being done against a cogging motor?

[Simon Chaddock]

BEB

I am very surprised at your proposition that a free wheeling prop creates less drag than a stationary one.

In my experience, both models and full size, it creates more drag and quite a bit more at that.

The huge coarse pitch props on rubber powered FF probably do create less drag windmilling but there is also the issue of the torque generated when it is held still.

The relatively fine pitch props common on direct drive electric definately create more drag windmilling and I have several planes that easily show it.

Even a humble Cessna 172 glides about 15% better with the prop stopped than with the engine idleing.

Erfolg

I always set the brake on any plane that glides even passably well simply to improve its flight endurance. As far as I am aware the ESC only shorts the motor winding and consummes no power.

The prop will windmilI slowly at speed even with the brake on and I suppose it follows that at extreme speeds the motor might actually generate enough current to damage the ESC.

[Dave Bran]

Posted by Martyn Johnston on 18/10/2012 16:54:21:

I've oft heard people saying that a windmilling prop causes higher drag than a stationary one, but I've never seen any convincing arguement to back it up.

Well, I have proved it to my own satisfaction and frequent use, and that is all that matters

[Daithi O Buitigh]

What I'd love to find out is just HOW to stop the prop on a 'leccy motor at quarter to three (the same as we did on infernal confusion engines) so that it's horizontal when it meets terra firma

[Chris Bott - Moderator]

Posted by Daithi O Buitigh on 18/10/2012 19:15:46:

What I'd love to find out is just HOW to stop the prop on a 'leccy motor at quarter to three (the same as we did on infernal confusion engines) so that it's horizontal when it meets terra firma

Hear hear!

[Dave Bran]

Posted by Simon Chaddock on 18/10/2012 18:16:27:As far as I am aware the ESC only shorts the motor winding and consummes no power.

The prop will windmilI slowly at speed even with the brake on

The brake on most of my ESCs appears to be a low power application which holds the prop steady even when at zero throttle. You can test whether your ESC does this by going to zero throttle so applying the brake and turning the prop very very slowly by hand, too slow for back EMF generation, then while doing so unplug the battery. It will usually then immediately rotate more freely.

Whether the prop freewheels or not with the brake applied depends on motor magnet cogging levels, prop pitch, etc, it is not as far as I have found a WILL situation by rote. Some of my ESC brakes are quite strong!!

[Myron Beaumont]

Windmilling at any constant airspeed equals very little drag -It's obvious .Feathering (which we can't do) equals even less drag I suspect . Prove me wrong !

Edited By Myron Beaumont on 18/10/2012 19:49:50

[Erfolg]

I am personally less interested in what generates more drag, than if there are issues with braking my ESC.

It has been said that a braked propeller will still rotate when braked. I know that in one example that it did not happen. However I do not know this is always true.

As for the issue, can you guarantee or ensure that the propeller will be horizontal, I can say, that in my albeit limited experience, it is doable, without great skill. I have done it many times. You just blip the throttle. The work load in doing this is not great.

What I do not know is this always the case. I am getting the impression that no one else has tried the same. In my case it was a consequence from going from a folder to fixed propeller for practical reasons.

I am asking for personal experiences and knowledge.

[Erfolg]

Unfortunately (for me) I did not sleep very well last night, a trivial set of dreams and thoughts found me half awake at 4:30. What is depressing, is what seems to woke me is something which does not really concern me nor is it relevant to resolving my issues.

I woke up thinking that the reason why a feathered propeller causes little drag, is that it can be set to the airflow, such that the drag value for the section is minimum, probably -ve a few degrees from the nominal datum line.

I was also thinking about a fixed propeller. I thought that this would be set an angle which was close to the stall angle, probably at the max lift to drag value. The root could well be stalled. There would probably be a strong turning action.

I was also thinking about a freewheeling propeller, this would be at an effective AoA of less than the stall, due to forward and rotational velocities. The drag for the same total velocity would be less than the fixed value.

Then I was also thinking, but the velocity is not the same. Now that the formula for drag has a V^2 in it, all the other terms being the same. Now this complicates the comparison, as the drag increases by this ^2. So a lot depend on the revs of the propeller to determine for any set up whether a fixed prop or rotating prop generates more drag. What does a wind milling prop rotate at I wonder, 500, maybe 1,000 revs or less?

I was then thinking, what does this do to the ESC, is it different to the effects of braking the propeller?

I was then thinking why can you not sleep, how much power does a wind turbine extract from the air.

Then I was reliving walking down the sea front at Lytham, my granddaughter asking, what is that? An old windmill, I said. What is a windmill, was the next question. They were used for grinding corn, cutting wood, pumping water, a little like a wind turbine today. Why is it not working was asked next. They were not as efficient and reliable as electric motors, particularly if there was no wind, I said. So why do we have Wind turbines, are they different?

I continued like this until 08:00hrs. Why, oh, why, do people raise issues that work on the brain, preventing sleep. All i want to know is will I damage my ESC, using the brake function.

[Martyn Johnston]

Erflog, I was also dreaming last night. I woke up at 4ish to tell the missus that she and I had been on an absolutely enormous great big boat, and we had been talking to each other in a quant old English dialect 'so that the Americans and the robots wouldn't understand us'. Whatever we'd been saying must have been really funny, 'cos I woke up laughing out loud.

I then had another dream where I wrote to the writers of 'The Big Bang Theory' to see if they wanted to use this story in one of their scripts.

And I wonder why she thinks I'm strange !

[Pete B]

I'm out of my normal environment at present, using unfamiliar equipment, so my posts will be a bit more cursory than normal - a good thing, some might think........

Anyway, in a nutshell, here's my findings:

Folding prop - brake on - least amount of drag. Normal mode for obtaining maximum glide.

Folding prop - brake off - lots of drag. Wouldn't have any reason to use it.

Fixed prop - brake on - least amount of drag - try it at higher airspeed than normal and the prop may start to turn, albeit slowly*.

Fixed prop - brake off - normal mode for power model. The additional drag generated (and it's much higher than a stationary prop!) acts as a useful brake in 'downhill' manoeuvres.

Now, I know that is not an answer to Erfolg's question but, in practice, I tend to find that belly-landing tends to push the down-pointing fixed prop aside rather than cause any damge such as ripping out the motor mount, providing the prop isn't disproportionately large in relation to the fuselage x-section. My E-Flite Sea Fury, with a 2-blade 10" or 11" prop, broke its mount several times on landing when it 'pole-vaulted'. Changing to a 10" 3-blade prop eliminated the problem totally.

Blipping the motor to move the prop more horizontally is rather hit-and-miss, in my experience.

* I think the issue of the prop turning with the airflow with the brake on depends more on the nature of the motor and the strength (?) of the magnets, rather than anything to do with the ESC. We have all, I'm sure, some motors which are 'notchier' than others when turned by hand and, invariably, those that are easier to turn manually will tend to turn in flight, even if the brake is set 'on', if the airspeed generates enough force on the prop to overcome the resistance of the magnets.

Pete

[Simon Chambers]

Posted by Erfolg on 19/10/2012 12:54:14:

I continued like this until 08:00hrs. Why, oh, why, do people raise issues that work on the brain, preventing sleep. All i want to know is will I damage my ESC, using the brake function.

If the ESC is within the rated specs for driving the motor, then No it won't.

As others have mentioned, our ESC's short the windings to stop the motor. When braking our motors, it won't charge the batteries as the energy from the spinning propeller is going to be dumped into the MOSFETs as heat - rather than dumping that energy back into the battery. This is because to short the terminals is easier than regenerating the power back into the motor. To dump the energy back into the battery, it'd have to monitor the current levels going back into the battery (LiPo's don't appreciate large peaky current flows back into them) - however the vast majority of RC ESC have no current level monitoring hence unable to do it.

The FET's on ESC's (even the cheaper ones) are rated to a higher current rating than the actual ESC is rated for. So large peaks of power shouldn't damage them. In fact their peak current ratings is significantly higher than their continuos rating anyway. Also the main current from the braking is a short period, so the additional heat generated would be dissipated pretty quickly. Once the prop is stationary, there is little/no energy left in the spinning prop to be dumped - thus no heat being generated across the transistors.

If this wasn't true, we would have many burnt out speed controllers and many complaints on these forums about how the brake, breaks controllers!

Also to note, is that some ESC's have varying brake strengths. This is basically pulsing the transistors on and off very quickly to reduce the braking effect. Obviously this would dump less heat into the controller and put less stress through the drive train.

Finally, as a point of interest, due to the design of some controllers, it is possible to have the motor in a braking state with no battery and power connected! This is most likely on the lower current controllers - but still can be applicable to the higher current units.

Si.

[Simon Chaddock]

Erfolg

I think a prop can windmill at any revs depending on the airspeed however the aerofoil section is not that efficient as it is effectively operating inverted.

With a pusher installation you have the advantage of sound (propellor slap) and in a fast dive without the brake on the motor was turning at revs equivalent to the sound at full power static - 12000ish rpm. It obviously takes quite a bit of energy to do this

As an aside I have found that a pusher mounted exactly on the trialing edge does tend to stop inline with the wing trailing edge. I assume because this is the most disturbed airflow position for the prop.

[Martin Harris - Moderator]

Only one way to settle this (short of a fight!)

Take 2 kid's windmills. Blob cyano on one to stop it rotating.

Hold them out of the car window on a quiet road.

Get driver to accellerate progressively.

See which one is pulled out of your hand /stick breaks first...

(or try the same with motors/props screwed to some bits of 2 x 1

[Simon Chambers]

In regards to the OP. I prefer to have the brake on for my belly landers. My experience is with it off, the spinning props tend to more likely loose their tips and snap than with the brake on.

I do try to flair as much as possible when landing to avoid the prop being the first part of the aircraft to strike the ground.

Also I find that the brake on, my aircraft speed up in a dive, where as brake off, the speed stays relatively constant. But this is only a seat of the pants measurement - I've never actually taken any readings to see (such as aircraft speed, etc).

Si.

Edited By Simon Chambers on 19/10/2012 14:38:34

[Simon Chambers]

Posted by Martin Harris on 19/10/2012 14:37:43:

Only one way to settle this (short of a fight!)

Take 2 kid's windmills. Blob cyano on one to stop it rotating.

Hold them out of the car window on a quiet road.

Get driver to accellerate progressively.

See which one is pulled out of your hand /stick breaks first...

(or try the same with motors/props screwed to some bits of 2 x 1

What about having a twin (twinstar?) and having one side with ESC brake on, and the other with the ESC brake off?

If one side noticeably slows down in a dive, then that is causing more drag?

Si.

[Peter Beeney]

Erfolg, - As always this is just another guess, but I’d say the brake is a switch across the motor drive wires, this will (can) only be switched on when there is no input to the motor, otherwise it would short the motor supply. If it’s switched on when the motor is turning, such as might happen if the throttle is fully closed with the model in a dive it presents a considerable load to the motor, so much so that it would have to exert a lot of effort to drive it. I would think this is very unlikely to be obtainable from a windmilling prop. If the switch is a full short the load will always be at a maximum. If the switch is operated when the motor is turning fast the motor will stop very quickly; such as might happen it the throttle is moved swiftly from fully open to fully closed. I suspect this is not exactly what is wanted, so I’m thinking a resistor is maybe wired in series with the switch, and possibly others then wired in parallel with that to make it adjustable. To make it rather more ‘medium’ or ‘softer’. As the motor runs down the braking effect is diminished and when the motor is stationary there is no braking effect at all; the motor is not locked in any way and so may be able to move slightly. I think it will be due to any intrinsic friction within the motor as to whether the motor will turn or not; and if it does manage to turn the brake starts to operate slightly, limiting the prop to slow revolutions, so this seems to be in keeping with your observations. I’d tend to consider the a windmilling prop is not a good source of power anyway, a bit like flying a wing backwards to generate lift, I think that might work, but not very well. So all in all, I don’t think you are likely to damage the ESC, but the motor might come to a quick full stop it it’s applied in a hurry!

I’ve always been very impressed by the power of magnetic fields. It’s possible for a 3 phase generator the size of a 45 gallon oil barrel that’s bolted onto the back end of a diesel engine that is big enough to drive a ship, that under the right conditions, (overloaded!), the concentrated magnetic field within the generator is strong enough to bring said engine to a shuddering stop in a cloud of smoke and steam. Although I have to say, this is a bit specialised, it’s generating mains electricity and therefore operating to a mains spec., the control unit is monitoring everything, including the frequency, and if for whatever reason this goes below a certain small percentage of 50 Hz then it just shuts the engine down. So perhaps it’s not quite that spectacular, but it’s still a whole lot of power in a small space!

With regard to any dreaming I suspect that like the old fashioned windmills the electricity generating types will come and go. Nice to be able to dream about the extremely bountiful advantages of either making them or owning the land on which they are erected and operating them. I understand that in some circumstances the remuneration for not operating them, i.e. keeping them turned off, is as good as actually running them.
I believe that in the past, when the corn grinding windmills first appeared, there was something of a general outcry against them, for very much the same reasons we have today about changing the national scenery. But now what is a relatively short time later we are slapping preservation orders on them, and so I’m now wondering when the last remaining electricity pylon and electric wind turbine will be so preserved?

Good Luck!

PB

[Erfolg]

Martyn

I had a similar idea,though I was thinking of mounting on a pivoted stick, with scales underneath. Measuring the force on the scale (due to the lever), with the propeller free to spin and the second constrained.

I then thought it seems a lot of effort and it may seem that I am more interested in making some sort of point. Also i thought I would need to do the experiment in a range of wind strengths to be able to make a plot..

As I have said, the propeller thing was not my interest, but like many observations, you do get sucked in, wanting to know. The trouble with wanting to know, your life starts getting taken over, investigating all the aspect of "if" and "why".