I thought I knew how the brake behaved.

[Erfolg]

This weekend has been busy for me no flying.

Although some time to think. My wife and myself were at the Maryport Blues Festival, we were sitting in the Admiral Nelson, Bonnie Mac had finished their first set. My wife mentioned that she would not like to be sitting under a Ceiling Fan that was wobbling as it revolved. Another near by spun smoothly. I said I am not sitting under the one over your head either.

It was in this interlude that I was pondering, that my Firenza with a HK Blue Series ESC, had inadvertently been set to braked propeller. The model behaved as if it had hit a brick wall if the throttle was closed. Accompanied by a massive loss of height, and a strange note from the propeller (this note had previously been allocated to fitting a inappropriate propeller). On the approach to landing the propeller could be seen to rotate

The ESC had been reset to no brake, now the shut throttle resulted in a loss of height, far more controlled as the loss of speed was not as evident. The propeller still rotates. I was not aware of any noise now coming from the model.

I then thought when I first started with RC as a returned lapsed IC RC modeller, that I had set up some models with the brake on. This was done as we fly from a rough field, where UC are not very practical. On the landing approach, I could see if the prop was horizontal. If it was not, i would nudge the propeller to get it horizontal, which it would often naturally do.

Which has had me thinking what determines how the propeller behaves. Is it the ESC and its brake function, or perhaps the propeller relative to the motor, may be Kv, or something else?

Before I had even got to this point Bonnie Mac had returned for their second set ( i almost sound knowledgeable). Then it was off to other venues.

Has anyone any ideas, or better still, knows?

[Erfolg]

I should clarify that on the first braked set up, the propeller did not rotate.

[Simon Chaddock]

Erfolg

I can't comment on your particular ESC but as I understand it the brake effect is caused by the ESC shorting out the motor coils when the throttle is fully closed.

Many power tools use the same effect to stop the motor rapidly for safety.

With the ESC armed you can feel the electro magnetic resistance even when you turn the prop by hand but the faster the motor is turning the stronger the brake effect - I believe as a squared function.

None of my ESCs make any special noise with the brake on although the prop will still spin slowly at speed.

Most of my planes use relatively large diam fine pitch props so the drag from wind milling with the brake off is considerable.

I am not sure about the effect of motor kV.

A low kV motor should produce a greater brake effect at a low speed but then it spins slower anyway and turns a bigger prop as well. I suspect the ability to brake a particular motor/prop combination is not directly influenced by the motor kV.

My V-1 EDF certainly windmills its fan with the brake off (you can hear it) and doesn't with it on but I can't really tell any difference in the glide performance although for scale effect I was rather keen than it went silent when gliding!

[John Tee]

I'm not too clued up with electric flying although I do it, but I thought the brake function was normally used with folding props. will now expect to be corrected.

John

[Erfolg]

Simon

I suspect the noise is propeller related rather than ESC related.

I think what has taken me aback, is that I am certain that some braked ESCs, used in the past, actually stopped the rotation of the propeller.

It seems as certain that my low Kv motor (driven as a 4s), windmills. Perhaps apparently more significantly acts as an airbrake.

A non rotating, braked propeller, seems to have little impact on drag. The braking effect from an braked, yet rotating propeller, can be very significant, so it seems.

The uncertainty of even beginning to understand what matters, is a little disturbing, in that what I thought I knew, I now know is not necessarily predictable. That is by me with my level of knowledge and understanding.

From now on, at least in the foreseeable future, it seems that setting the brake on, is only sensible with folding props.

 

Edited By Erfolg on 29/07/2013 22:24:22

Edited By Erfolg on 29/07/2013 22:26:13

[Erfolg]

John

There has been discussion on this site, with respect to the drag of braked and un-braked propellers.

I personally had read them, and had thought that it was all of no real significance. That was a consequence of my braked, non rotating propellers, having higher drag than braked and folded propellers, yet not enough to get excited about, if flying a sports type model, when the motor is switched of only when landing.

Now I find, if the propeller rotates and is braked, on large propeller diameters, the drag force is massive. Enough, that the only way to get the model down safely was to put it in a steep dive, essentially pointed at the intended landing point, if any hope of accuracy is wanted, in landing. This is broadly similar to bringing down a thermal machine at the end of the slot, using airbarkes. In a similar manner, on reflection, there is no apparent speed build up of the model, the drag apparently increasing in proportion to the speed of the propeller rotation.

Yet when the propeller is stopped, and not folded, I do not remember any issues of the drag significantly increasing disproportionately with airspeed.

[Chris Bott - Moderator]

Erf I recently set up a glider that had a big prop running on two cells, and the ESC could not completely stop the prop.

I ended up changing to a smaller prop that the ESC could stop, and to compensate went up to 3 cells.

The props were folding props. Which won't fold until they are stopped. So stopping them is an absolute necessity on electric gliders.

[John Tee]

I have a few sport electric models but can't say that I have noticed any appreciable drag when motor at low throttle or off. The only model Ihave with a folding prop is a foamy glider that I have not flown yet as one wing tip goes up and the other down which could prove interesting. I'll follow this discussion to see the outcome.

John

[Simon Chaddock]

I would tend to agree with Erfolg a rotating prop has more drag than a fixed blade stationary one. How much more depends on the speed of rotation.

A prop wind milling at high speed creates a drag approaching that of a solid disc of the same area and makes a very effective air brake.

What I don't know is whether the minimum drag occurs with the prop stationary (the blade acting as a flat plate with fully disturbed airflow) or from one braked yet rotating just fast enough to keep the airflow over the prop from stalling.

Now if I had a wind tunnel.....

[TheFlyingCrust]

Maybe I'm missing something here. I'd have thought that a windmilling prop would be producing less drag than a static one - intuitively if the prop is turning then its slightly moving out of the way of the incoming air so allowing some of the pressure to disipate. I accept there will still be significant drag. I've heard the oposite of what I've just suggested being the case but I just can't see it. Has anyone done any definitive tests?

Ian

[Tony K]

I think the way to approach at this is to look at how a gyroplane works. There was a discussion here on that topic in which, I think, BEB proposed that gyroplane lift is the vertical component of the total drag of the spinning rotor.

A gyroplane will not fly if the rotor is not spinning fast enough or not spinning at all. It follows that drag must increase with the speed of rotation.

[Erfolg]

The limitation with this method of communication seems to be a lag in time. Plus a tendency to repetition, as points are highlighted. I will attempt to avoid the repetition.

All I can really say, much of understanding, including my own, comes from observation, then equating what has been seen and experienced to your knowledge, to form a view.

In the case of the braked propeller, there seems to be a fundamental issue, that is a braked propeller does not seem to consistent across a range of set ups.

I have or probably had set ups, where the propeller would stop on closing the throttle, apparently creating very little additional drag. The only time I would close the throttle completely, was on landing, on rough ground.

I had not noticed that I rarely closed the throttle on my Firenza, as I came into land, possibly because of having an undercarriage and judging that the air speed needed some throttle to keep going. It was the dead stick requirement of the "A" cert. that highlighted, that closing the throttle, increased the drag dramatically.

At present I cannot even begin to predict what will happen with a braked propeller.

As to the the very valid point, that braked propellers is primarily intended for gliders, with folding propellers. The Firenza issue has come about, I guess, that previous experience had indicated that brake on or off had little impact on drag. On that basis i was not to concerned, as i would only close the throttle completely when landing on rough ground. Now I can see it could be a real issue, that the propeller could increase the drag to such an extent that it acts as a brake, at least in some cases.

It is the lack of being able to predict or understand the issues, that bothers me. It is easy to get into the folklore scenario that occurs in modelling as well as life in general.

[Steve Hargreaves - Moderator]

Surely though Erf each set up will be different with regard to the prop pitch & diameter, the speed the model is flying at & even the normal magnetic resistance to rotation of the motor.....all will have an effect on determining the speed at which the prop will windmill or indeed stop.

Consider a highly loaded fast sports model in a dive fitted with a large prop.....such a model might build up enough speed to overcome the brake & turn the prop......a lightly loaded glider in normal flight probably wouldn't...

My electric Extra (43" span, 11x7 prop) doesn't have the brake activated.....in the glide the prop will stop turning. Dive more steeply & the prop will start turning again....level out & the prop stops!!

If only the ESC could harness this energy & use it to recharge the flight battery.....true regen braking in the way of automotive hybrids....

[Andy48]

Posted by Steve Hargreaves - Moderator on 30/07/2013 11:16:06:

If only the ESC could harness this energy & use it to recharge the flight battery.....true regen braking in the way of automotive hybrids....

No reason why it can't. Any good at circuit design? There's a real winter project for you.

[Pete B]

If Mark Webber's motor is still giving him KERS problems, with all of Red Bull's best engineers working on it, I'd steer well clear of that, TBH!......

Pete

[Erfolg]

What I had not recognised and I also suspect many others are in a similar position, is that the drag from a stopped propeller may offer more drag than a freely revolving propeller.

What was not discussed is the drag from a propeller that is not revolving freely, due to being braked.

My experience tends to indicate that the difference in drag between a fixed propeller and freely rotating propeller, is the equivalent of arguing over" how many angels can dance on a pin". The difference is not massive, with the revolving propeller winning.

However add a braked electric motor into the discussion, and it can be a different ball game. A braked revolving propeller, can add a massive drag component. Discussing how much, is for me at the moment a little pointless, as at present, no informed furomite has come forward with a electrical concept, and the factors relating to the electrical issues.

Edited By Erfolg on 30/07/2013 13:37:35

[Steve Hargreaves - Moderator]

My understanding Erf was that a spinning propellor (braked or freewheeling) will ALWAYS produce more drag than a stopped one.....a spinning prop offers effectively a circular area roughly equivalent to the diameter of the prop where as a stationary prop only offers the surface area of its blades...much less in other words...

[Peter Beeney]

Is this not just a little bit of a ‘suck - it - and - see’ situation? If the model performs better with the brake off, then just leave it disabled perhaps. I’m not greatly into electric models, although I’ve been doing quite a bit more recently, but I did punt ‘hotliner’ type powered gliders around at one time. These were all dragged along with folders, so they needed a brake. Because they were relatively heavy, powered by nicads, the landing was fast and I used a sort of half-crow configuration to slow down. The brake, as already said, is just a load placed on the motor because it’s now acting solely an electrical generator, that’s how a motor works, oddly enough, and I do seem to recall that a theoretical circuit, if not indeed the practical application, was a straightforward pair of relay contacts to make a short circuit across the terminals. I also seem to remember this can be a bit severe, though, a full short can stop a rapidly turning motor dead and possibly cause some mechanical damage, so a transistor that turned on slowly may have been the next step; a ‘soft’.brake. Nowadays I’m sure it’s possible to make the brake do exactly whatever you want it to.

The electric models we’ve flown recently have been standard trainer types, all with standard propellers and as far as I’m aware no one has used any form of braking. This seems perfectly adequate for learners, very similar to i/c, but perhaps a little more gentle. I’ve also recently been given a much crashed ParkZone Mustang, again reduced to a multitude of component parts, and having resurrected it again it’s proved to be a real little cracker! Quite spritely and accurate, perfect for close in low flying. However, I thought it might be improved slightly, (made even quicker!), and I think the easiest change is to get a better matched propeller, so I’ve been experimenting and I’m nearly there, but against what is generally the perceived wisdom. That’s another story, however, and more to the point, there is no brake, which seems just right. Fly in for a landing with just a whiff of throttle as required, with no undercarriage I prefer it to be close enough to be able to see to land it as slowly as possible and a bit nose up. So far I’ve not broken a prop landing, but an e-prop did break in the air.

So I’d go along with Steve and consider each case on it’s merits. Watch the effects and then work out the issues and reasons why from that. In any case, I can’t think that a prop acting as a windmill is very efficient anyway, so it’s possible the effort required to stop it is not going to be massive. Is just the terminology confusing too, ‘braking’ or ‘stopping’ the prop is not necessarily ‘braking’ or ‘slowing down’ the model?

Now for the most important brake, the T break……

PB

[Andy48]

Posted by Steve Hargreaves - Moderator on 30/07/2013 13:46:08:

My understanding Erf was that a spinning propellor (braked or freewheeling) will ALWAYS produce more drag than a stopped one.....a spinning prop offers effectively a circular area roughly equivalent to the diameter of the prop where as a stationary prop only offers the surface area of its blades...much less in other words...

Yes but.... The spinning prop, due to its shape is cutting through the air rather than acting as a circular barrier.

[Steve Hargreaves - Moderator]

Agreed Martin.....I'm sure its not like sticking a 13" diameter (or whatever) flat disc to the front of the model but then again the "cutting" action you mention is also a form of drag..... But this is all just speculation on my part...I don't know....

What we need is a learned opinion from, say, a Professor of Aerodynamics..if only we knew where to find one.....

[Peter Beeney]

Erfolg - If you wanted to vary the braking effect of the motor you would simply vary the load in terms of resistance you applied to the the spinning motor. Full short = full load, up to open circuit = no load.

PB

[Steve Hargreaves - Moderator]

As I understand it thats exactly what the ESC does Peter....by using the O/P transistors to vary how much the motor is short circuited (a horrible way to describe it but you get my meaning!!) you get soft medium or hard braking.....

[Erfolg]

Peter

I was not seeking a braking effect from the motor. I had thought that the ESC was set to no-brake. For me the best set up has no drag.

It came as a shock that the Firenza did not glide, with the throttle of. Behaving as I had suddenly opened a set of airbrakes. It did take some time to notice that there was a noise element, when in a dive, trying to keep the air speed up. It was only then, did I slowly suspect all was not well, and it was perhaps the motor esc that was the issue.

As many of you and certainly the odd person in my club, we quickly come to recognise that with a folding braked prop, if it was necessary to open the throttle from closed when landing, you did not open it a bit, you opened the throttle a fair bit, otherwise, the model was likely to descend even quicker. It was recognised there was an a braking effect. I do think that not any of us gave any more thought to what was happening, only what was  necessary to gain airspeed. We now do this intuitively. Not a thought of what in detail is happening.

As to the argument to the drag from a fixed prop and freewheeling prop, for most of us, the difference is apparently so little in RC, terms (perhaps other than gliders, which normally have folders), as not worth the argument. I was more concerned with saving props with sports models, landing on rough ground.

Edited By Erfolg on 30/07/2013 15:05:08

[Peter Beeney]

No, sorry Steve, it would have to be separate transistors. If you switched the bridge circuit on again you would connect the battery to the motor and it would just start running again……

A short circuit is to all intents and purposes one that effectively has very little resistance indeed. I suspect that this would be fine to keep the brake applied because it seems to me there is little current flowing here; the propeller is virtually or perhaps actually stationary. I imagine a soft brake would just be a gradual switch on. A partly applied brake might feed the current into a resistor, but I’ve never seen a description of this. There is little information, it seems, so it would all be largely guess work on my part…

Sorry Erfolg, I thought you were querying the operation of the brake and so on.
So in that case, it may pay dividends to actually check the various modus operandi of your particular arrangement first, and then start flying in the certain knowledge that the brake is on or off…

But I do agree that in the general scheme of things this stuff is pretty low priority, questions about propeller braking are very thin on the ground at our patch.

PB

[Erfolg]

Peter

So what is happening when it said that there is a short across the motor?

In practical terms, what does it add up to?