Best Gear ratio for ESC and efficiency

[helimadness]

Hi All.

Just in the process of building a Goblin 700 and working out the gear ratios. But that aside I had a more general question regarding what is better for the setup and I guess ESC.

Im using a 450 kv Align 750mx, have the standard 21T and an extra 24T and Vbar GOV. Performance flight times etc aside say I am happy to run 1800 RPM as an example I have 2 options to achieve the same outcome.

21T with 95% throttle
24T with 83% throttle

So 2 questions.
1) what is going to be the most efficient.
2) what is going to be the kindest on the ESC.

Cheers

[trillian]

This is something I have been researching myself for a couple different helis.

In general I think the answer would be that the option where you're at 95% throttle will usually be the most efficient but I'm not sure if the governor will have enough headroom to hold the speed you want as well as it can.

A rough guide would be setting your gearing so that you get the headspeed you want at 90% of the motor KV multiplied by the lowest LiPo voltage you expect. So usually you'd take 44.4 as the voltage on 12s.

What I have learned with the Castle Ice ESCs is their governor wants quite a lot of headroom. Their guidelines are to have the gearing so that the target headspeed shows around 90% output in the Castle software, but this actually equates to a considerably lower percentage of the theoretical max (more like 83% of the maximum possible using the voltage X the motor KV).

Some ESCs can handle running at lower throttle better than others, namely the Kontronik and YGEs that use active freewheeling.

Just to give you an idea how much headroom the Castle Ice governor wants, I am setting up a Rave ENV and want 1900 for my idle-2. For the castle gov to give me the 'OK' for a governed 1900 headspeed I would need to gear it so that (using the motor KV x 44.4 volts) the max possible speed would be 2300!

I tend to think the Castle gov is asking for too much headroom for the type of flying I do as I'm not slinging in massive quick pitch changes or Tareq style smackdown stuff, so I am going to start at a pinion one tooth smaller than it's asking for because I want maximum flight times.

[ghost2212]

I would use the 24T as I highly doubt you will have enough headroom with the 21T, The 24T will be of very similar efficiency and might even be better

[helimadness]

This is something I have been researching myself for a couple different helis.

In general I think the answer would be that the option where you're at 95% throttle will usually be the most efficient but I'm not sure if the governor will have enough headroom to hold the speed you want as well as it can.

A rough guide would be setting your gearing so that you get the headspeed you want at 90% of the motor KV multiplied by the lowest LiPo voltage you expect. So usually you'd take 44.4 as the voltage on 12s.

What I have learned with the Castle Ice ESCs is their governor wants quite a lot of headroom. Their guidelines are to have the gearing so that the target headspeed shows around 90% output in the Castle software, but this actually equates to a considerably lower percentage of the theoretical max (more like 83% of the maximum possible using the voltage X the motor KV).

Some ESCs can handle running at lower throttle better than others, namely the Kontronik and YGEs that use active freewheeling.

Just to give you an idea how much headroom the Castle Ice governor wants, I am setting up a Rave ENV and want 1900 for my idle-2. For the castle gov to give me the 'OK' for a governed 1900 headspeed I would need to gear it so that (using the motor KV x 44.4 volts) the max possible speed would be 2300!

I tend to think the Castle gov is asking for too much headroom for the type of flying I do as I'm not slinging in massive quick pitch changes or Tareq style smackdown stuff, so I am going to start at a pinion one tooth smaller than it's asking for because I want maximum flight times.

I was working it out on 3.8v a cell which is close to what I fly to on my 500. 3.7 min. Also working on mr mel head speed calc at 90% efficency. So at 100% should get 1900 rpm worst case.
My understanding is ESC work better closer to being maxed out. so surely the higher the better, as long as enough head room for the governed speed you want.

To start with im going to be taking it fairy easy so might start with the 21 and see where that gets me.

Cheers

[trillian]

I was working it out on 3.8v a cell which is close to what I fly to on my 500. 3.7 min. Also working on mr mel head speed calc at 90% efficency. So at 100% should get 1900 rpm worst case.
My understanding is ESC work better closer to being maxed out. so surely the higher the better, as long as enough head room for the governed speed you want.

To start with im going to be taking it fairy easy so might start with the 21 and see where that gets me.

Cheers

Yeah, you've totally got the right idea. I was just surprised by how much headroom the Castle gov asks for but I think it must be accounting for either large sudden loads or lower than 'nominal' pack voltage. I also don't want to go as low as 44.4 so I'm starting out with a 14 tooth pinion even though the Ice 120 'wants' a 15 tooth and hoping it will give me around 1900 without issues.

[helimadness]

Yeah, you've totally got the right idea. I was just surprised by how much headroom the Castle gov asks for but I think it must be accounting for either large sudden loads or lower than 'nominal' pack voltage. I also don't want to go as low as 44.4 so I'm starting out with a 14 tooth pinion even though the Ice 120 'wants' a 15 tooth and hoping it will give me around 1900 without issues.

Ive had the 500 for a year which was my first CP heli, and now this is my first 700 so will be taking it very easy to start with. As long as I can get 1800 min I think will be fine to start with. Certainly for the run in flights. As I get more adventurous I might step up to a bigger pinoin. Goblin actually have a smaller tail pulley to run with low head speeds. Im kinda keen on the lower head speed smoother slower moves myself so not in a hurry to push the bang smack headspeeds.

Cheers

[trillian]

Ive had the 500 for a year which was my first CP heli, and now this is my first 700 so will be taking it very easy to start with. As long as I can get 1800 min I think will be fine to start with. Certainly for the run in flights. As I get more adventurous I might step up to a bigger pinoin. Goblin actually have a smaller tail pulley to run with low head speeds. Im kinda keen on the lower head speed smoother slower moves myself so not in a hurry to push the bang smack headspeeds.

Cheers

My guess would be that considering the high-ish C of G and relatively light weight of the Goblin (plus being flybarless) you should have pretty good response at 1800. With flybarred .90 nitros I'm used to 1900 ish being the sweet spot but that's partly due to the engine needing to be in the powerband and partly due to the flybar paddles getting quicker with more headspeed (plus more weight - my Avro is touching 6kg with fuel! ). With electric and flybarless it's a whole other ballgame. The Rave is still flybarred but if it works great at less than 1900 I'll be going for the longest flight times that still give me the punch I need.

[Planehazza]

I would be tempted to drop to 23T if you can. That will increase the thr % to match the efficiency more, but will also reduce the current draw.

[GravityKills]

With a new heli, I start with the smallest (reasonable) pinion, then I work my way up. The alternative (too big, then reduce) results in boggy headspead, wagging tails and burning ESCs.

The sweet spot is usually if you get adequate performance at the end of the pack.
"Sweet" is somewhat subjective, depending whether you prefer Coke or Pepsi...

[helimadness]

Ok so flying on the weekend I got to the end of my flight and flicked idle up back to normal mode and no head speed change. Flicked to idle one and 2 both higher % and no noticeable change. Battery must have been at the limit of my normal mode with no head room.
So took blades off tonight and ran on the bench with the 21T and a pack at storage voltage 3.85v. With the Vbar Gov set to max 2200 at 100% throttle I was getting about 1950, same at 95% and about 1850 at 92.5%. So at the limit of that pinion. I think those % are right I didnt write down.
Anyway change to the 24T and set the head speed to max 2300 to try and find the upper limit of that pinon. Got to about 2250 at 100% throttle with lipos in storage again. So I think about perfect. Running on the bench now on the Vbar monitor I get
90% = 1830
92.5% = 1950
95% = 2070
100% = 2250.

So 1830 in normal which is about the slowest you can go without it bobbing around due to low head speed with the DFC head.
1950 should be a sweet idle one I would think with 2070 to play with.

The Vbar numbers don't really seem to add up but that what I get so will have a run at that.

Cheers

[Andy from Sandy]

Calculating the headspeed of the v-bar governor requires to take max headspeed and divide by 50. You set 2300 so 1% = 2300 / 50 = 46. So for a 1950 headspeed you need to set your throttle curve to 1950 / 46 = 42.39 + 50 = 92.39. The closest to this is 92.5 which is what you have. You add the 50 as the governor doesn't kick in until you get to 50% throttle openeing.

[helimadness]

Calculating the headspeed of the v-bar governor requires to take max headspeed and divide by 50. You set 2300 so 1% = 2300 / 50 = 46. So for a 1950 headspeed you need to set your throttle curve to 1950 / 46 = 42.39 + 50 = 92.39. The closest to this is 92.5 which is what you have. You add the 50 as the governor doesn't kick in until you get to 50% throttle openeing.

I sort of understand the Vbar way of doing it but never had a good way to work it out. Thanks for the explanation. The 24T seems about spot on. Just seemed strange to go from a 21T to a 24T pinion and only have such a small % difference in the throttle. Before the 2200 was way above what the max achievable would have been. The 2300 is is just above the max set point.

Max would be (204 / 24T = 8.5 Gear ratio) (3.8v X 12s = 45.6v x 450kv = 20,520 / 8.5 = 2414 RPM. Should I take say 90% efficency @ 2172 (actually measured about 2250) and set my max head speed in the Vbar to this? Should the max headspeed be inside the limit of the system?

[cjcj1949]

Don't forget your batteries will be useless at this time of year, unless heated up and insulated. You'll have a lot more power later in the year.

[helimadness]

Don't forget your batteries will be useless at this time of year, unless heated up and insulated. You'll have a lot more power later in the year.

I didn't have them heated but in my bag and had't been outside that long. Got 10 mins one flight, cant wait till they are better :-) Was a little long but still 3.71 cell so not so bad.

Cheers

[discoferret]

I have a my flat throttle line set at 72% and running a 24T which gives me 2200 HS. This is with the Kosmik ESC