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Cheap LiFePO4 BMS?


jetzi

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22 minutes ago, Keeping Up said:

No idea, I had a spare hole to use when I had the exhaust moved from one side to the other, and the bilge blower fitted with some trunking was perfect for it. It comes on with the ignition so I totally ignore it, and it made more than 10-15 degrees difference. Probably even better because it's directed at the alternator. Somehow I just don't instinctively feel that sucking is as effective as blowing - but I take yuor point about fumes etc; even with the cruiser stern there is a slight leakage through the electrical ducts etc into the rear cabin.

If this were thunderboat I’d make a comment. Unfortunately it isnt!

 

But that aside, yes I might need a second small blower to pump cooler air at the bottom of the engine bay, to the proximity of the alternator. But I’ll try just getting the engine bay temperature down first.

Edited by nicknorman
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27 minutes ago, nicknorman said:

If this were thunderboat I’d make a comment. Unfortunately it isnt!

 

But that aside, yes I might need a second small blower to pump cooler air at the bottom of the engine bay, to the proximity of the alternator. But I’ll try just getting the engine bay temperature down first.

thanks for the temperature info Nick.

I did what Peter suggested which was to get an axial bilge blower and use aluminium ducting to pull cold air from the bottom of the engine hole and flow it over the alternator. I just rigged up a plastic box so any air being drawn in at the bottom was flowing within half an inch of the steel at the bottom of the bilge. The blower was £30 (IIRC and took a couple of amps). My engine hole didnt seem to have any vents (in or out!!!) but the blower seemed to drop the temp by 10°C by just circulating the cooler air.

My new boat does seem to have a large air vent in.

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25 minutes ago, Dr Bob said:

thanks for the temperature info Nick.

I did what Peter suggested which was to get an axial bilge blower and use aluminium ducting to pull cold air from the bottom of the engine hole and flow it over the alternator. I just rigged up a plastic box so any air being drawn in at the bottom was flowing within half an inch of the steel at the bottom of the bilge. The blower was £30 (IIRC and took a couple of amps). My engine hole didnt seem to have any vents (in or out!!!) but the blower seemed to drop the temp by 10°C by just circulating the cooler air.

My new boat does seem to have a large air vent in.

Thanks, yes I‘m sure pulling in the cold air from the bottom would help a lot, but I also want to try to get the engine bay temperature down because I don’t like how hot the batteries get in summer. And sucking in hot air into the engine can’t help efficiency and power.

 

So I’ll try the external venting first, and maybe the localised alternator blower subsequently, if necessary.

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1 hour ago, nicknorman said:

Yes I need to do something like that, however I think mine needs to pull air from the engine bay into the outside. This will of course suck outside air in from the other vents. I think if I pumped outside air in, there would be some leakage of the slightly pressurised air that could result in engine bay air (and fumes, oily smells etc) getting into the engine room which is living space.

 

1 hour ago, Keeping Up said:

it made more than 10-15 degrees difference. Probably even better because it's directed at the alternator. Somehow I just don't instinctively feel that sucking is as effective as blowing - but I take yuor point about fumes etc; even with the cruiser stern there is a slight leakage through the electrical ducts etc into the rear cabin.

 

I think I'd be tempted to put passive ducting directed straight at the alternator from an external air intake if you go the blowing hot air out route.  This would both remove the hot and direct the cold into the right places with only the one fan, and shouldn't send the fumes into the cabin from overpressure.

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6 minutes ago, TheBiscuits said:

 

 

I think I'd be tempted to put passive ducting directed straight at the alternator from an external air intake if you go the blowing hot air out route.  This would both remove the hot and direct the cold into the right places with only the one fan, and shouldn't send the fumes into the cabin from overpressure.

Yes that’s certainly a possibility. I suppose the only problem is there are multiple air inlets to the engine bay at the moment, even if one becomes a forced exit, which would reduce the flow from any one inlet.

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  • 3 weeks later...

I finally have my paws on my equipment! Oh happy days. 

 

Jeremy connected the 16x 160Ah Winston Thundersky LiFeYPO4 cells in 4 blocks of 4 - so 4p4s for a 640Ah battery at 12v (13.2V). This is what I asked for but I'm now second guessing myself. 

 

I've read in a number of places that it is important to go parallel-series rather than series-parallel, including @nicknorman on another thread "the important thing is to connect the cells in parallel first, then in series. So that electrically-speaking, you just have the 4 cell groups to monitor. After that I don’t think it matters too much"

 

and Will Prowse on this video says "a lot of people make the mistake of putting the cells into series first and then into parallel". In the comments someone asked him the reason and he replied "The likelihood of matched cells is higher in this arrangement than if you had multiple separate cells in series, then parallel. I think I need to make a video to explain this. Has to do with the dynamics of cell aging when in parallel." but unfortunately he hasn't made that video yet and I don't really understand this explanation. 

 

Why is series-parallel connection of your LiFePO4 battery a bad thing? (as opposed to parallel-series).

 

One obvious advantage to series-parallel is that you can monitor each individual cell rather than the parallel cell blocks. I have the GWL cell performance monitor (much better name than "BMS", since this is just one component of my planned complete BMS). It does in fact support up to 16 cells, so I'm wondering if it's not a better plan to series-parallel so that I can monitor each cell individually? Or is it a case that the cell monitor can't "understand" this configuration, since it's now 4 batteries rather than one?

 

 

Or is it something to do with the robustness of having the cells in parallel able to help balance each other?

Edited by ivan&alice
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1 hour ago, ivan&alice said:

I finally have my paws on my equipment! Oh happy days. 

 

Jeremy connected the 16x 160Ah Winston Thundersky LiFeYPO4 cells in 4 blocks of 4 - so 4p4s for a 640Ah battery at 12v (13.2V). This is what I asked for but I'm now second guessing myself. 

 

I've read in a number of places that it is important to go parallel-series rather than series-parallel, including @nicknorman on another thread "the important thing is to connect the cells in parallel first, then in series. So that electrically-speaking, you just have the 4 cell groups to monitor. After that I don’t think it matters too much"

 

and Will Prowse on this video says "a lot of people make the mistake of putting the cells into series first and then into parallel". In the comments someone asked him the reason and he replied "The likelihood of matched cells is higher in this arrangement than if you had multiple separate cells in series, then parallel. I think I need to make a video to explain this. Has to do with the dynamics of cell aging when in parallel." but unfortunately he hasn't made that video yet and I don't really understand this explanation. 

 

Why is series-parallel connection of your LiFePO4 battery a bad thing? (as opposed to parallel-series).

 

One obvious advantage to series-parallel is that you can monitor each individual cell rather than the parallel cell blocks. I have the GWL cell performance monitor (much better name than "BMS", since this is just one component of my planned complete BMS). It does in fact support up to 16 cells, so I'm wondering if it's not a better plan to series-parallel so that I can monitor each cell individually? Or is it a case that the cell monitor can't "understand" this configuration, since it's now 4 batteries rather than one?

 

 

Or is it something to do with the robustness of having the cells in parallel able to help balance each other?

Parallel and series is what I have done

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3 hours ago, ivan&alice said:

I finally have my paws on my equipment! Oh happy days. 

 

Jeremy connected the 16x 160Ah Winston Thundersky LiFeYPO4 cells in 4 blocks of 4 - so 4p4s for a 640Ah battery at 12v (13.2V). This is what I asked for but I'm now second guessing myself. 

 

I've read in a number of places that it is important to go parallel-series rather than series-parallel, including @nicknorman on another thread "the important thing is to connect the cells in parallel first, then in series. So that electrically-speaking, you just have the 4 cell groups to monitor. After that I don’t think it matters too much"

 

and Will Prowse on this video says "a lot of people make the mistake of putting the cells into series first and then into parallel". In the comments someone asked him the reason and he replied "The likelihood of matched cells is higher in this arrangement than if you had multiple separate cells in series, then parallel. I think I need to make a video to explain this. Has to do with the dynamics of cell aging when in parallel." but unfortunately he hasn't made that video yet and I don't really understand this explanation. 

 

Why is series-parallel connection of your LiFePO4 battery a bad thing? (as opposed to parallel-series).

 

One obvious advantage to series-parallel is that you can monitor each individual cell rather than the parallel cell blocks. I have the GWL cell performance monitor (much better name than "BMS", since this is just one component of my planned complete BMS). It does in fact support up to 16 cells, so I'm wondering if it's not a better plan to series-parallel so that I can monitor each cell individually? Or is it a case that the cell monitor can't "understand" this configuration, since it's now 4 batteries rather than one?

 

 

Or is it something to do with the robustness of having the cells in parallel able to help balance each other?

Cells that are in parallel must by definition and the laws of physics, be at the same voltage. Therefore if you have say 16 cells and put them into parallel first, you only have to monitor 4 cells. If you put them into series first, then parallel, you have to monitor 16 cells.

 

There is nothing to be gained by monitoring 16 individual cells except a lot of complexity and hassle!
 

I also suspect (but can’t be bothered to work it out properly) that as you suggest a parallel then series configuration would be more tolerant of one cell at slightly lower capacity than the others, because the other 3 cells in parallel would make the % loss of capacity 1/4 of what it otherwise would be.

 

You also have to balance 16 cells, as opposed to 4 cell groups.

 

Also you should check whether your super BMS is happy with cells at any relative voltage to each other, or whether it is expecting 16 cells in a series chain. I suspect the latter. Especially if there are only 17 connections. Most likely it is designed for a 50v string.

Edited by nicknorman
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1 minute ago, nicknorman said:

Cells that are in parallel must by definition and the laws of physics, be at the same voltage

Under normal circumstances sure that makes sense, but what if one cell of the 16 is failing?

 

If one cell has a lower capacity than its 3 mates, don't I risk over/undercharging it when I charge that block?

 

Thanks for your response. I'm not disagreeing - I just really want to understand this point.

 

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10 minutes ago, ivan&alice said:

Under normal circumstances sure that makes sense, but what if one cell of the 16 is failing?

 

If one cell has a lower capacity than its 3 mates, don't I risk over/undercharging it when I charge that block?

 

Thanks for your response. I'm not disagreeing - I just really want to understand this point.

 

The point is that under or over charging are defined by voltage. And that with things in parallel, the current is shared according to the “path of least resistance”. So if you have 4 cells in parallel, 3 of which are 100Ah and one sick one is 80Ah, and let’s say you charge them at 100A, then firstly the voltage of the cell group will and must rise in unison, and secondly there will be a bit less current flowing into the 80Ah cell compared to the 100Ah cells.

 

Expanding on this slightly: because the sick cell has lower capacity its voltage would (left to its own devices) rise more quickly than the others. But it can’t, because they are tied together, and consequently more current is diverted to the good cells. It all balances out such that the cells all reach fully charged at the same time. The same applies during discharge, more current will flow out of the good cells, so that all cells remain at the same SoC relative to their actual capacity (not their badged capacity).

 

Of course none of this applies to cells in series where a weak cell will reach an overcharge or over discharge state whilst the other cells in series remain happy. This of course being because current through a series circuit does not vary throughout the circuit - current can’t be “dropped”, destroyed, created etc.

Edited by nicknorman
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2 minutes ago, nicknorman said:

The point is that under or over charging are defined by voltage. And that with things in parallel, the current is shared according to the “path of least resistance”. So if you have 4 cells in parallel, 3 of which are 100Ah and one sick one is 80Ah, and let’s say you charge them at 100A, then firstly the voltage of the cell group will and must rise in unison, and secondly there will be a bit less current flowing into the 80Ah cell compared to the 100Ah cells.

 

Expanding on this slightly: because the sick cell has lower capacity its voltage would (left to its own devices) rise more quickly than the others. But it can’t, because they are tied together, and consequently more current is diverted to the good cells. It all balances out such that the cells all reach fully charged at the same time. The same applies during discharge, more current will flow out of the good cells, so that all cells remain at the same SoC relative to their actual capacity (not their badged capacity).

Head slap moment. Yes I understand, thank you for spelling it out for me Nick!

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Hey clever people I'm back! 

 

Little update. 

I've now installed 3x 340w panels via a 100a MPPT (I thought I'd go big as I might want to add another panel later on. 

 

The Sterling B2B has been working fine but it's been unused since the solar install. 

 

I still haven't got to high charge with the B2B and the cells stayed balanced within 0.05v but now with solar I get to 100% charge easily and past the 14v mark they go out of balance by 0.3v so I decided to build myself a nice power supply to top balance the pack which I never did.

At the moment I'm running 4 cells as I have lots of solar and I have the other 4 in parallel on the power supply set at 3.6v and 6a (about 20w. I guess it will take a while from 3.34v!

 

Out of curiosity what do you guys use to top balance? 

 

Also the bms has been working flawlessly and the Bluetooth app gives access to more parameters and monitoring than I need. Of course the balancing turns on as set but as expected it can't keep up with this kind of power. 

So far I've only stretched to a 1kw chop saw (bms is rated at 120a continuous) via the 2kw inverter and don't plan to run much more than that. If I ever need to run more power I have another exact bms which I might try and run in parallel.

 

This setup has been a joy to build and a joy to use so far and I'm hoping the balancing will work and last for a while. 

 

Dre

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6 hours ago, nicknorman said:

 

Also you should check whether your super BMS is happy with cells at any relative voltage to each other, or whether it is expecting 16 cells in a series chain. I suspect the latter. Especially if there are only 17 connections. Most likely it is designed for a 50v string.

Parallel then series is the only way to go as all the BMs units I have seen are expecting the 8 or 16 cells in series. It is so much easier to just monitor and balance 4 cells. It would be a nightmare to have to balance 16 cells.

Not sure how you would use a 16 cell monitor to look at a 4 series 4 parallel set up.

 

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4 hours ago, Dre said:

 

Out of curiosity what do you guys use to top balance? 

 

 

I made up a number of high wattage wire wound resitors which draws 10A at 3.3V. I wire it across the high cell that needs to be reduced in capacty. Resistors are mounted on a heat sink and I check the Amps out via a clamp meter. Very cheap.

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Quick relay control wiring sense check please!

 

The BDS-A relays I have have two pairs of controls - ON and OFF - each with a positive and negative.

 

My cell monitor "bms" board supports 3 bistable relay outputs - overvoltage (Umax), undervoltage (Umin) and emergency (Uemergency).

 

The documentation for each of these looks like this:


Connection 1: 150 ms GND pulse for bi-stable relay after switching on or resetting the board.
Connection 2: 150ms impulse GND for bistable relay if the Umax or higher is maintained for 35 seconds

 


My understanding of all this:

 

1. All 6 positives (2 per relay) need to be connected to the battery positive (13.2V)

2. The ON relay negatives will be connected to the (Connection 1) outputs of either Umax, Umin and Uemergency.

3. The OFF relay negatives will be connected to the (Connection 2) outputs.

4. When the cell monitor board starts up, it is going to turn all three relays ON by grounding output 1 for Umax, Umin and Uemergency (i.e. GND pulse implies connecting to the battery negative).

5. When the appropriate voltage situation occurs, it turns the correct relay OFF by grounding output 2 for the correct situation.

 

 

Have I got my negatives and positives around the right way? i.e. relay control negatives to BMS and positives to battery?

Edited by ivan&alice
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12 minutes ago, ivan&alice said:

Quick relay control wiring sense check please!

 

The BDS-A relays I have have two pairs of controls - ON and OFF - each with a positive and negative.

 

My cell monitor "bms" board supports 3 bistable relay outputs - overvoltage (Umax), undervoltage (Umin) and emergency (Uemergency).

 

The documentation for each of these looks like this:


Connection 1: 150 ms GND pulse for bi-stable relay after switching on or resetting the board.
Connection 2: 150ms impulse GND for bistable relay if the Umax or higher is maintained for 35 seconds

 


My understanding of all this:

 

1. All 6 positives (2 per relay) need to be connected to the battery positive (13.2V)

2. The ON relay negatives will be connected to the (Connection 1) outputs of either Umax, Umin and Uemergency.

3. The OFF relay negatives will be connected to the (Connection 2) outputs.

4. When the cell monitor board starts up, it is going to turn all three relays ON by grounding output 1 for Umax, Umin and Uemergency (i.e. GND pulse implies connecting to the battery negative).

5. When the appropriate voltage situation occurs, it turns the correct relay OFF by grounding output 2 for the correct situation.

 

 

Have I got my negatives and positives around the right way? i.e. relay control negatives to BMS and positives to battery?

Looks good to me. Presumably Umax goes to a relay which disconnects the charge circuit and Umin goes to one which disconnects the loads. What does Uemergency do?

 

MP.

 

ETA note that the BDS-A contacts are not symetrical. They have much greater brealing capacity with the current in one direction. Check the datasheet for which way round it is.

 

 

Edited by MoominPapa
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53 minutes ago, ivan&alice said:

 

Have I got my negatives and positives around the right way? i.e. relay control negatives to BMS and positives to battery?

Yes that’s correct. Relay control positives to battery via a fuse, of course.

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Thank you!

1 hour ago, MoominPapa said:

Looks good to me. Presumably Umax goes to a relay which disconnects the charge circuit and Umin goes to one which disconnects the loads. What does Uemergency do?

Yes that's correct. You can set Umin to 2.8V, 2.9V, 3.0V or 3.1V an Umax to 3.5V, 3.6V, 3.7V or 3.8V. Uemergency controls a third backup BDS-A relay in case one of the primary two fail. This one kicks in at Umin minus 0.3 V or Umax plus 0.3 V. I've put this third relay on the battery negative directly after my BMV shunt.

 

 

1 hour ago, MoominPapa said:

ETA note that the BDS-A contacts are not symetrical. They have much greater brealing capacity with the current in one direction. Check the datasheet for which way round it is.

Thanks for the tip! The datasheet states the load current from terminal B(+) to A(-), and specifies the terminal on the left (if the control connector is pointing downwards) as A- and the terminal on the right as B+. This implies the current flows right to left, correct? So I have arranged the relays as follows:

* the load disconnect on Umin: load on the A- side and the battery positive on the B+ side
* the charge disconnect on Umax: charger on B+ side and battery positive on A- side

* the emergency disconnect: load/charger on B+ side and battery negative on A- side (Ok so this one is by necessity bidirectional, but I figure the load is likely to be more current than the charge in most cases, and this relay should never activate - I should think that as long as the relay stays on it shouldn't matter much which direction the current flow is)

 

My brain has melted from this install - I can't tell my +ve from my -ve anymore... Does the above make sense?

 

 

 

1 hour ago, nicknorman said:

Yes that’s correct. Relay control positives to battery via a fuse, of course.

Another good point. The datasheet doesn't say the current the coil takes but specifies it at 19.9 Ω @ 12V, which is 0.6A - would a 1A fuse inline in each of these relays controls (6x fuses) be appropriate?

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8 hours ago, ivan&alice said:

Thank you!

Yes that's correct. You can set Umin to 2.8V, 2.9V, 3.0V or 3.1V an Umax to 3.5V, 3.6V, 3.7V or 3.8V. Uemergency controls a third backup BDS-A relay in case one of the primary two fail. This one kicks in at Umin minus 0.3 V or Umax plus 0.3 V. I've put this third relay on the battery negative directly after my BMV shunt.

 

 

Thanks for the tip! The datasheet states the load current from terminal B(+) to A(-), and specifies the terminal on the left (if the control connector is pointing downwards) as A- and the terminal on the right as B+. This implies the current flows right to left, correct? So I have arranged the relays as follows:

* the load disconnect on Umin: load on the A- side and the battery positive on the B+ side
* the charge disconnect on Umax: charger on B+ side and battery positive on A- side

* the emergency disconnect: load/charger on B+ side and battery negative on A- side (Ok so this one is by necessity bidirectional, but I figure the load is likely to be more current than the charge in most cases, and this relay should never activate - I should think that as long as the relay stays on it shouldn't matter much which direction the current flow is)

 

My brain has melted from this install - I can't tell my +ve from my -ve anymore... Does the above make sense?

I found the datasheet difficult/unclear on this, but that makes sense to me.

 

8 hours ago, ivan&alice said:

 

 

Another good point. The datasheet doesn't say the current the coil takes but specifies it at 19.9 Ω @ 12V, which is 0.6A - would a 1A fuse inline in each of these relays controls (6x fuses) be appropriate?

Six fuses seems overkill. I'd probably have one supplying the four coils  of the load and charge relays, and another supplying the two coils of the emergency relay, given that one of it's functions is protection if the first fuse blows. You're not going to have multiple coils energised at the same time, so maybe 2A fuses, to give a bit more headroom against blowing on surges? The fuses are to protect the wiring, don't forget.

 

MP.

 

 

 

 

 

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On 10/06/2020 at 17:14, nicknorman said:

Well some sort of engine failure anyway!  The jet ranger had metal blades and a huge amount of rotor inertia so it was very forgiving in situations like that. It was said that you could do an engine off landing, lift back into the hover and yaw 90degress, settle back on the ground all before running out of rotor rpm.

 

I did my training at Redhill in 1980, on the Bell 47.

I often hear the term cyclic in reference to helicopter flight, stupid question but what is it and what does it do?

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7 minutes ago, tree monkey said:

I often hear the term cyclic in reference to helicopter flight, stupid question but what is it and what does it do?

In its simplest form it’s the big lever between the seats that looks like a handbrake. Pull it up and it increases the pitch of the blades to make the aircraft rise, but that will load the engine so it also incorporates a twist grip throttle. 

 

Watch a helicopter pilot take off and you’ll see them gently pull up on the cyclic whilst stirring the pot with their other hand and simultaneously waggling their feet. It’s a bit like patting your head and rubbing your stomach at the same time... or drumming. 
 

I’m sure Nick can expand on that ;)
 

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1 hour ago, WotEver said:

In its simplest form it’s the big lever between the seats that looks like a handbrake. Pull it up and it increases the pitch of the blades to make the aircraft rise, but that will load the engine so it also incorporates a twist grip throttle. 

 

Watch a helicopter pilot take off and you’ll see them gently pull up on the cyclic whilst stirring the pot with their other hand and simultaneously waggling their feet. It’s a bit like patting your head and rubbing your stomach at the same time... or drumming. 
 

I’m sure Nick can expand on that ;)
 

No sorry you are talking about the collective. So called because it changes the pitch on the blades collectively (ie all at once by the same amount).

 

The cyclic is the “joystick” that controls the attitude of the helicopter (pitch and bank). The attitude is affected by the attitude of the rotor disc (ie the imaginary disc around which the rotor tips fly). The lift from the rotors is perpendicular to the disc so if the disc is tilted forward, the helicopter pitches nose down and moves forwards. If the disc is tiled lateral, the helicopter banks and moves sideways (in forward flight this makes it turn).

 

So how to tilt the disc relative to the fuselage? Well it can’t just be forced because there is a lot of gyroscopic force and the consequence would that the fuselage tilted the wrong way whilst the disc remained fixed. So the disc has to be “flown” into a  new attitude. This is done by varying the pitch on each rotor blade as it goes through one revolution. So for example (and let’s take a clockwise- rotating disc) when you move the cyclic forwards to pitch nose down / move forwards, a rotor blade as it passes beyond the forward position will be twisted progressively to increase its angle, which makes it start to fly up. As it passes the 3 o’clock position this twist will be at a maximum and then decrease so that the angle is nominal at the 6 o’clock position but by now the blade is higher than it was. As it moves forward again the blade is progressively twisted the other way to decrease its angle so it starts to fly down, reaching a maximum reduced pitch at the 9 o’clock position before arriving at the 12 o’clock position at nominal angle but at a lower height. So the blade is flying high at the back and low at the front, ie the Rotor disc is now tilted forwards, the lifting force is inclined forwards and the helicopter pitches nose down and moves forwards.

 

So with the rotor blade pitch angle varying as it goes round in a circle, or a cycle if you will, what should we call the control? How about the cyclic?!

 

To spell it out, the cyclic controls blade pitch cyclically whereas the collective controls blade pitch collectively!

 

The is another issue which is that in forward flight (well, flight in any direction really) a rotor blade has different airspeed as it goes around a complete rotation. When the blade is moving into the relative wind (“wind” arising from both the actual wind and the motion of the helicopter) the speed of the air over the blade is greater compared to the when it is moving away from the relative wind. An aerofoil produces more lift as the air speed over it increases, so we have a situation where as the blades moves forward into the wind it produces more lift and starts to fly up, and as it moves away from the wind it produces less lift and starts to fly down. Thus the disc is tilted away from the direction of travel and the helicopter will pitch or roll away from it and want to go in the opposite direction. Not ideal!

 

So in forward flight the cyclic has to be moved forward somewhat so that the increased airflow over the rotor blade moving into wind is countered by it having a reduced angle, to give the same overall lift. Ditto the blade moving down wind has lsss airflow and so has to have more  pItch angle to produce the same lift. So as a rotor blade completes a circle in fast forward flight, it is twisted first one way then the other, to maintain constant lift all the way round so that the helicopter attitude remains constant.

Edited by nicknorman
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