AXI Brushless

| The motor is an AXI 2820/10 with a JETI 40-03P speed control, on ten cells. | Can someone tell me if this is normal behavior? I ask because the rig has | been in a crash once, and the last takeoff did seem a little underpowered.

Sounds like your ESC has a brake. It stops the propeller from spinning when no power is applied. This reduces drag.

Completely normal.

If you are not using a folding propellor, then you will want to disable the brake. The Jeti instructions will have info on reprogramming.

| If you are not using a folding propellor, then you will want to disable the | brake. The Jeti instructions will have info on reprogramming.

Why would you want to disable it? I'm told that a windmilling propeller creates more drag than a propeller that's not moving. (I'm not sure I understand why, but I'll take their word for it. I only dabble in aerodynamics :))

Assuming that this true, it seems like a good thing to leave the brake enabled for just about anything that flies, except for a helicopter.

I've read the same thing, Doug. I use the brake on both my electrics (with non-folding props). Neither of my electrics have landing gear, so having the brake also allows me to land without breaking my prop! If the prop isn't in the right attitude for landing, I just blip the throttle and check it again....

Good flying, Bob Scott

I too enjoy reading/listening about things i have no hope for ever understanding. Electronics for example. (Flying an RC airplane is another example) If you know enough not to look like a fool, and dont EVER pretend to actually know as much as who your talking to, you'll have a fascinating conversation.

Hopeless newbie - and mediocre comedian, MikeF

| > I'm told that a windmilling | > propeller creates more drag than a propeller that's not moving. (I'm | > not sure I understand why, but I'll take their word for it. I only | > dabble in aerodynamics :) | | I too enjoy reading/listening about things i have no hope for ever | understanding.

Well, to be fair, I could understand aerodynamics rather thoroughly if I were to put enough time into it -- but it would be a lot of time. :)

Assuming that the motor is producing almost no resistance to the windmilling propeller (which should be sort-of close to the truth if there's no brake) I'd expect a windmilling propeller to create less drag than one that's being stopped entirely.

I suspect that it all hinges on my assumption that the motor produces almost no resistance.

I guess this would be easy enough to measure in a wind tunnel. Alas, my wife likes the kitchen the way it is ...

| Electronics for example. (Flying an RC airplane is another example)

Two more things I'd probably be better at if I spent all kinds of time on learning. Alas, there's so much to do, and not so much time ...

| If you know enough not to look like a fool, and dont EVER pretend to | actually know as much as who your talking to, you'll have a | fascinating conversation.

... of course, the person you're talking to may think that nobody will be able to tell that he doesn't have a clue :)

Yes.

Try something else. Disconnect the motor from the ESC and short its wires together an try and turn it...

The ESC is live the moment you plug teh batery in, and probably braking

the motor by shorting the windings. The switch only powers up teh receiver.

Beware of this. The odd circumsatnce can cause tha motor to fire up - like a dodgy joint in the ESC to receiver wiring. EVEN WITH THE SWITCH OFF.

Only plug packs in when you expect to fly, and treat the model as 'live' after you have.

I had a small model leap off the test bench whien I switched the transmitter off with the throttle closed.

(i) you may want a windmilling prop - brushless motors don't idel that well, and the airbrake effect is useful

(ii) slamming the throttle closed on a braked motor has been known to spin prop nuts off and strip gears.

I belly land with freewheeling props. It's OK as long as they are not braked...they just nudge to 'quarter to three'

No. Which makes more noise? Which is absorbing more power and turning it into noise?

Nothing to do with it. The blade speed, and drag, is higher on a windmilling prop on a frictionless shaft.

>

Well thanks, all, for the input - I am most surprised indeed: firstly, that the brake is that effective, and secondly, that it's active the moment power is connected, regardless of switch position. I think I may disable the brake for a flight or two and see if I enjoy the "airbrake" feature of a windmilling prop (not that I have ever coasted the GeeBee in for a landing).

I was taxiing around in the living room a little bit (another advantage of electric!) and just what you said happened - I closed the throttle and the prop came bouncing right off!

Thanks, Dan.

underpowered.

| > Assuming that the motor is producing almost no resistance to the | > windmilling propeller (which should be sort-of close to the truth if | > there's no brake) I'd expect a windmilling propeller to create less | > drag than one that's being stopped entirely. | | No. Which makes more noise? Which is absorbing more power and turning it | into noise?

The amount of energy lost to noise is infinitesimal. I'm sure much much much more is lost to creating turbulent air behind the prop (which may be noisy too.)

| > I suspect that it all hinges on my assumption that the motor produces | > almost no resistance. | | Nothing to do with it. The blade speed, and drag, is higher on a | windmilling prop on a frictionless shaft.

Well, if the shaft is truly frictionless, there will be no noise generated there.

I'll see if I can find a better answer to this question. But certainly, it's not quite this simple.

In article , Doug McLaren wrote: | In article , The Natural Philosopher wrote: | | | > Assuming that the motor is producing almost no resistance to the | | > windmilling propeller (which should be sort-of close to the truth if | | > there's no brake) I'd expect a windmilling propeller to create less | | > drag than one that's being stopped entirely. ...

| | Nothing to do with it. The blade speed, and drag, is higher on a | | windmilling prop on a frictionless shaft. ... | I'll see if I can find a better answer to this question. But | certainly, it's not quite this simple.

I went to google, looking for `windmilling prop drag'. I've found lots of opinions on the subject, on both sides of the fence. I now believe that, assuming little or resistance from the motor or engine, that a freely windmilling propeller generates less drag than a stopped one under otherwise identical conditions.

Perhaps the most convincing argument was this one --

where he suggest a very simple test where you can see it yourself. I haven't done it myself, but I will soon.

Note that this is only valid if there is very little drag being produced by the motor or engine. If the engine is slowing the engine down, then it's extracting energy from the moving air and therefore increasing drag. It's not certain if it's extracting more energy than the motion is saving, but it certainly complicates matters.

For a full sized plane, with a prop with adjustable pitch, the best solution in a glide is to feather the blades, and if you can't do that, at least pick the highest pitch possible. That will reduce the drag as much as possible. If the prop still moves, and it's moving the engine as well, this will remove energy and cause drag -- so if your plane has a clutch, it would be best to disengage the prop from the engine.

Intersting discourse ... I have a degree in physics, so I understand most of this stuff, though I don't have the in-depth understanding that an aerospace engineer would probably have.

| Beware of this. The odd circumsatnce can cause tha motor to fire up - | like a dodgy joint in the ESC to receiver wiring. EVEN WITH THE SWITCH OFF. | | Only plug packs in when you expect to fly, and treat the model as 'live' | after you have.

Or remove the prop while you're working on it.

You'd be amazed at how hard even a small parkflier prop can bite you ...

True, the air gets hotter. But the noise is a by product of the turbulence,

Actaully, it is. Jusat try with and without a brakke and look at glide angles.

Or get one of those childrens windmills, and wave it around. Resistance incrases as the blades spin up.

Believe what you like. Nature doesn;'t care. nature will let you know by steepening your glide angle.

We aren't talking about feathering. We are talking about a fixed pitch prop spinning an engine. If its a big prop it slows you down more when it windmills,. That is observed fact. Little props it makes piss all difference.

BECAUSE the theory of turbulent flow is so complex, its better to rely on real world tests.

| > I went to google, looking for `windmilling prop drag'. I've found | > lots of opinions on the subject, on both sides of the fence. I now | > believe that, assuming little or resistance from the motor or engine, | > that a freely windmilling propeller generates less drag than a stopped | > one under otherwise identical conditions. | | Believe what you like. Nature doesn;'t care. nature will let you know by | steepening your glide angle.

Nature can be cruel like that. But it's not quite that simple ...

| We are talking about a fixed pitch prop spinning an engine.

An electric motor, actually. Which is actually quite different from an engine in this case, because if there is no brake on it's ESC, it produces very little drag, especially if it's a direct drive motor with no gearbox. Only if there's a electrical load on it does it produce signifigant drag (because it's working as a generator.)

An IC engine, on the other hand, produces lots of drag, all the time. I don't recall ever seeing a windmilling prop on a gas or glow model, though it's certainly possible if you go fast enough.

| If its a big prop it slows you down more when it windmills,. That is | observed fact. Little props it makes piss all difference.

Observed in full sized aircraft with engines producing lots of drag on the prop's rotation, yes.

| BECAUSE the theory of turbulent flow is so complex, its better to rely | on real world tests.

Yes, and I gave it a shot.

I stuck a needle into a straw, and put a GWS electric 10x8 prop (those ugly orange things) on it. The straw didn't bend much, so I used three more straws to make it much taller, and held the straws upright by sticking them between some stuff on the floor.

I then blew a large fan on it from many feet away (trying to reduce the effect of the uneven flow from the fan), and noted how much it bent -- a few inches. I then put a rubber band on the prop so it couldn't spin, and backed off again, leaving everything else alone. It deflected a little more than it did before, indicating more drag. I tried a few different variations of angle and distance from the fan, and got the same general results.

So, there you have it. A real world test. Feel free to repeat it yourself -- you probably already have the equipment you need.

This does *not* mean that a windmilling prop will always have less drag than a stopped one -- only that in the extreme case of a prop that can spin freely with no drag, it seems to have less drag, at least with this prop. In the real world, an unconnected (or unpowered with no ESC brake) electric motor might come pretty close to this ideal (especially if it's a high quality motor with ball bearings and with no gearbox involved) but an IC engine isn't likely to come close at all.

It's not often I go into a thread saying one thing, then end up arguing the exact opposite, but that would appear to be what's happened here :)

On the question of windmilling props generating more resistance, maybe it'd help to think of the prop _disc_ as being what's presented to the airflow. Seems like an unfeathered windmilling prop on a frictionless shaft would present a disc much like a helicopter in autorotation, or an autogyro. Then think of what would happen to the heli or autogyro if the blades were stopped. Bill(oc)

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This is of course why helicopters regularly arrange to stop their rotors when the engine fails, so that the higher drag of a fixed blade rotor will slow them down more than the old fashioned 'autorotation' that you have so conculisvely proved would be inferior to slowing their descent....