I thought I knew how the brake behaved.

[Peter Beeney]

Erfolg - In a word, and strictly in my opinion of course, a short circuit across the terminals of the motor is simply joining them together, so completing the circuit and not adding any resistance. Thus the maximum current can flow through the motor, which is now a generator being driven from the windmilling prop; but I think not in a particularly positive way. This current makes it more difficult for the motor to turn, so it simply stops. If the circuit is not completed, the motor is free to turn as it will. A variation could be a circuit with some resistance, this might just enable the prop to turn, but slowly.

In practical terms, it merely gives the operator a choice as to whether he would prefer the propeller turning or stationary when the model is on the glide.

We could delve into the exact electrical principles as to why this happens if you wish, I’d be more than happy to do that, but the last thing I would want to to do is to describe in detail something which you already know…

Hope this is of some interest.

PB

[Erfolg]

Peter

The propeller seems to move more than just a little, spining apparently quickly, with the brake on.

If that is the case, does that mean it is taking a lot of energy from the forward movement?

Would it also suggest that the motor should increase in temperture?

[Shaunie]

I fly mainly I/C but I am quite certain a plane floats on much longer dead stick than at idle.

I believe this is due to the fact that a stationary prop is just a "stick" out in the airflow with no effective aerodynamic properties at all, two lolly sticks of equivalent area would have a similar drag. It is totally stalled aerodynamically.

If the prop is allowed to rotate however it starts to behave as an aerofoil (at negative AoA I suspect) with the associated induced drag etc. which brakes the aircraft. If the prop spins freely the drag will be low but I still think it may be higher than when stationary. If the prop is allowed to rotate but has increasing braking applied to its spindle then the aerodynamic drag will increase, until the point that the AoA is so negative that the prop aerofoil stalls again. At this point the aerodynamic braking effect would fall surely? Obviously when braking the camber of the section on the prop is unfavourable and will give increased drag for this reason along with an increased noise level.

I do think the disc analogy has some value, when turned through 90deg the term "rotor disc" is usually considered pretty valid.

Sorry PB, why would you need external transistors for braking, what's wrong with with switching on all three low side arms of the bridge simultaneously (or all three high side makes no difference, just not both together!). The circulating current will give dynamic braking with most of the heat going in the windings, some in the transistors.This braking could be controlled by PWM switching of the transistors rather than just switching them hard on. Don't forget MOSFETs will conduct in either direction when switched on. I do suspect that maybe they just change the phase angle and lightly drive the motor in reverse though. Bit like stopping a car by selecting reverse and then slipping the clutch. Harsh, but it would work.

I'm no aerodynamicist, this is just what I think so feel free to berate me if you wish .

Shaunie

[Steve Hargreaves - Moderator]

Well your thinking certainly ties in with mine Shaunie so we can be berated together.....

[Martyn Johnston]

It's not just us that keeps having this debate.This boating site (link here) says;

Does a sailboat propeller cause more drag under sail when it’s fixed or when it’s allowed to rotate? The answer seems to be that there’s so little difference, nobody can really tell.Dave Gerr, a respected New York naval architect and author, asserts: “The simple answer is that a propeller creates less drag when free to rotate.” But, he adds, if you can hide a two-bladed propeller from the flow of water by locking it vertically behind a skeg or keel, it produces less drag than when it’s rotating freely.On the other hand, Francis Kinney, an equally well-respected naval architect, maintained in Skene’s Elements of Yacht Design that “the shaft should be locked so that the propeller cannot revolve. It has been found that a revolving propeller causes more drag . . . ”Eric Hiscock, a vastly experienced British sailor and author, wrote: “Experiments made by P. Newall Petticrow Ltd. have shown that a two- or three-bladed propeller offers less drag when it is locked than when it is free to spin, and that the drag of a spinning propeller is greatest at about 100 rpm.”Some sailors don’t have a choice in this matter; on certain boats, the propeller must be locked under sail because the transmissions will not be lubricated when the engine is not running.On ocean passages, most sailors lock the shaft for the simple reason that the noise of the racing screw is unacceptable.

[Erfolg]

I think it has been demonstrated that the drag force increases by a significant amount, when a rotating prop is braked, relative to either a free wheeling prop or a stationary prop.

The amount of increase, is where there is no conclusive model of how the system is operating.

By observation, diving the model does not increase the airspeed, by a significant amount. My impression is that there is a "power" relationship at work, something X^2 type relationship perhaps.

Anyway, from my perspective, I feel a little happier, in that I now believe it can be very important to have the brake switched off, other than with folders, as if that prop starts to revolve, your model does not fly well, almost certainly degrading the glide dramatically.

[Martyn Johnston]

Just a little light reading (link here) for anyone with a little time to spare. (start at about page 130).

[TheFlyingCrust]

I think I'm getting a better handle on this now. Thanks to all for their knowledge. An analogy. 2 kites the same size (I have so I know), one is a simple diamond kite and their MO is they are permanently stalled and one is a quality stunt kite which isn't stalled. In a given wind my single line kite will pull under 10kg. In the same wind I have to fight with the stunt kite to stay standing and I need 70kg breaking strain lines. 2 of them. It has snapped lesser strength ones.

I'd not thought of applying that analogy before.

Ian

[Erfolg]

Martyn

The general conclusion that the drag force from freely rotating propeller compared to a fixed propeller is not worth the debate, fits with my own and probably many others observations.

It does appear that a braked propeller when compared to either of the two arrangements generates far more drag force.

The link you have found appears to be excellent, I was going to print it out, that is until I realised i is a book. So I have book marked it and will read it, at least the immediately interesting and relevant sections.

[Peter Beeney]

Erfolg , I think I would certainly agree with you that an electric model is best flown with the brake off. Unless you are using a folding propeller or there is another very specific reason to have the brake on, that is.

Shaunie, Regarding the transistors you're right; I stand corrected; and if this is done for a folding propeller application I suspect the motor would soon be stationary, so very little energy would need to be dispersed anyway. The theoretical circuit I once saw used a transistor for the brake, but that was a brushed motor, a long time ago. I’ve never seen anything related to brushless ESCs, so a while back I sketched a bridge and eventually got the switching sequence right to get a motor running; at least on paper. Unfortunately it didn’t occur to me at the time to consider the brake, so I didn’t give it a thought! And from what seems to be the general consensus of opinion, perhaps it’s not that important; do all ESCs have a brake facility? From what I’ve seen of recent ARTF trainer type models, such as the Riot, there is virtually no information on the ESC at all, I think it’s assumed that the operator will simply connect a battery to it and it will perform. Which it does, more often than not.

So maybe this is one of the least of the electric modellers’ worries, getting the motor to start in the first instance is sometimes difficult enough……

PB

[Chris Bott - Moderator]

OK so here's the bridge.

Which transistors do we switch on together to short accross the motor, without shorting accross the battery?

A, B and C are the motor connections.

It's my belief that separate transistors are used for the brake, but I'm happy to be corrected.

[Chris Bott - Moderator]

Aha I may have thought of a mechanism myself

If all three "low" mosfets were turned on for braking then all three windings would be connected to the same place. However at any instant, any particular winding will be "shorted" by two mosfets, but one will pass current in the forward direction while the other will be in reverse, so this may not work. Except, there are flywheel diodes in there, which would pass any "reverse" current necessary. Wouldn't it?

Edited for typo.

Edited By Chris Bott - Moderator on 31/07/2013 17:21:38

[Shaunie]

Chris,

MOSFETs are inherently bidirectional when switched on. When switched off the parasitic diode conducts when the device is reverse polarised but the device blocks when forward polarised. In low loss rectifier applications (large, high efficiency power supplies and now in some cases high power car alternators) MOSFETs are used as synchronous rectifiers where they are switched on in sequence with the A.C. they are to rectify, in these cases they have to be used "in reverse" as it were because otherwise the parasitic diode gets in the way. This is just a way of getting round the 0.6V (or more) voltage drop of a conventional diode rectifier and the losses involved.

What this means is that in our application if all three lower transistors are switched on you might just as well have soldered the motor windings together .

Source: Worked for twelve years for a little company you may have heard of "International Rectifier". At it's peak we made nearly 12 million MOSFETs a month at their factory in Oxted, Surrey!

Shaunie.

[Simon Chaddock]

I am a little surprised at the comment that there is little difference in aerodynamic drag between stationary and wind milling props because there is.

How significant the difference is to the planes performance will depend on the diameter of the prop relative to the total airframe drag but it is still there.

An extreme case but consider a helicopter during engine off vertical auto rotation. What is limiting its vertical rate of descent and would it be the same if the rotor was stationary?

Folding is of course the minimum drag arrangement, followed by feathering, but stationary is still better than wind milling.

Every ESC that I have come across has a brake facility even if some are hard to programme!

[Erfolg]

Simon

I certainly have made the observation that you make reference to.

I think that it is possible to put the issue into context, using your own description of the issues. The propeller on many models is quite small say in the region of 9" dia. In the case of the auto rotating helicopter, I get the impression you have to get the rotor into a specific condition, to auto rotate, or the helicopter free falls.

A few examples from the world of rubber power would suggest that many FF belive that a free to rotate propeller generates less drag force than a fixed prop.

As some have suggested the variables are numerous enough to be absolutely certain, whic condition does generate the most drag force.

I essence the difference either way, is not worth dying in a ditch over.

On the other hand, brake the propeller and let it rotate, then the forces will be substantially greater than either of the other conditions. As yet, we do not seem to have identified the factors that are relevant, or the conditions, which make any sort of prediction sensibly possible.

Regards

Carl

[Chris Bott - Moderator]

Posted by Shaunie on 31/07/2013 21:08:25:

Chris,

MOSFETs are inherently bidirectional when switched on.

What this means is that in our application if all three lower transistors are switched on you might just as well have soldered the motor windings together .

Thanks Shaunie

In which case there's absolutely no need for extra transistors in our brushless ESC for the brake, like we used to have in an ESC for a brushed motor.