Correct Head Speed

[Granpappy]

Is there some sort of scientific method of determining the correct head speed for a particular helicopter?

If the head speed is too low then the heli becomes unresponsive and loses tail authority. If the head speed is higher then you sacrifice flight time.

I've had a look at how disk loading and tip speed can effect how a heli performs and I've tried to come up with a magic formula that would apply to different sized helis, but it doesn't really work.

My 7HV flies with an ideal head speed of 1650 rpm and it flies just the way I want it. If I applied the tip speed/disk loading equation to my Trex 500L then I should be flying with a head speed of 2800 rpm, where as in reality it feels just right at 2200 rpm.

I'm guessing that most people trial and error it to get it just right, but with my 7HV I'm running 60% flat line in governor mode to get my 1650 rpm and I would rather run it higher then that so that I don't stress out the esc/motor.

I think that the main point of this is that when buying a new heli, I would like to make an informed choice of what Kv motor to buy with out having to trial and error the head speed. As it stands at the moment, my 7HV is running a 470kv motor on a 13T pinion and I would probably have been a lot better off on a 440kv motor and running 75% in governor mode.

Any thoughts on this would be appreciated.

Cheers.

[waveydavey]

I think it really depends on what you want to get from the heli. The 700n for instance was geared to put the 91 in the right area for power production and geared accordingly, this being around the 1950 to 2050 mark and based around 3D. For E's much the same, the gearing should be based around giving you the headspeed you're after or feel comfortable with and put the ESC in it's ideal range, IE with a bit of headroom. Of course there are silly headspeeds on E's looking for that ballistic missile type of machine but I suppose horses for courses. 1650 seems really low for a 700

[milly0812]

I think its just a personal choice and how you fly. The 7hv does work really well on low headspeed if thats your thing. Obviously if your into smacking it then low headspeed is just a waste of time. I tend to set my helis where I like them. Ive tried low when i had a 7hv but soon got bored with 10 min flights so think i ended up around 1900 and it suited the way I fly but obviously halved the flight time. I have a Tach to check headspeed for rule of thumb but its really down to how you like it .

[trillian]

I came from the nitro and flybarred mindset about it where you'd really need something like 1900 ish on a .90 / 700 size heli and 2000 on a .50. But with flybarless and electric you can really throw out the proverbial rulebook.

These days I generally go for the lowest headspeed where the tail holds well enough and I get the subjective feel that I like. If it seems to lack 'punch' in the collective, instead of upping the headspeed like I would have done before, I just up the pitch range.

On the TDR and the Faifa as they're both very light, this ends up being about 1300 for my idle-1 and around 1100 for hovering.

When I had a Rave ENV it worked best at around 1500. The Triabolo, even though it has three blades it still seems to need between 1300-1400 as it's quite a bit heavier than the TDR and Faifa.

In my recent 6s experiment with the Dune It's working really well at about 1550.

Regarding the issue of throttle % at the ESC vs motor kv. If the ESC is running cool that's at least some kind of guide of how stressed it is. The Kontroniks handle low throttle really well and most others will as well IF they are not also being pushed really hard at the same time. I ran the Hobbywing 70HV ESCs on a couple 600 helis on 12s that were really geared too short but the gear ratio couldn't be changed and in both cases the Hobbywing coped brilliantly at low throttle.

In general the best option if you want to run at a higher throttle is to go to a smaller pinion if possible, but this is not always possible.

In the case of my Dune on 6s, now that I have determined that it all works quite well at this headspeed I will try to get the gearing so that the ESC is closer to its ideal range for the governor (currently running at 48% on the Jive where about 63-65% is optimum). Since there's only one pinion for the Dune what I will do is swap motors with another heli that is currently fitted with a 740kv motor but it has different pinions to accomodate the 850kv motor that's in the Dune now, so they should both work really well swapping motors.

[Grumpy]

As the others have said already, i think it's very much subjective. What 'feels right' to you 'is right'

Personally on the 7HV i thought the sweet spot was around 1800RPM, but that's just me. For lower headspeeds the KBDD blue dampers are the way to go otherwise you tend to get nodding.

[Granpappy]

Cheers guys. Appreciate the comments. I'm a sort of slow lazy flier really so I prefer the lower head speeds. If I get excited and turn my pacemaker up then I can always go to idle2. But what I was after was some sort of scientific method of deciding what motor kv to get say if I wanted to buy an HD500 or a Core 700 or a Synergy 766 or anything where I don't really know what would be the best head speed for me to fly at. Motors are expensive and getting the wrong one is something to be avoided if you can help it. If it all boils down to trial and error and advice from other flyers who fly as you do and have the same heli then that would be ideal, but sometimes you are just not lucky enough to be able to draw on that knowledge.

Edit: Sorry. It took me so long to type this that I didn't anticipate other replies coming in.

[waveydavey]

A good clue to the manufacturers/designers idea of headspeed is to look at the gearing and pinion size, some even suggest the Kv. As above though, that's often the intended flight type and becomes subjective once it becomes personal

[trillian]

Cheers guys. Appreciate the comments. I'm a sort of slow lazy flier really so I prefer the lower head speeds. If I get excited and turn my pacemaker up then I can always go to idle2. But what I was after was some sort of scientific method of deciding what motor kv to get say if I wanted to buy an HD500 or a Core 700 or a Synergy 766 or anything where I don't really know what would be the best head speed for me to fly at. Motors are expensive and getting the wrong one is something to be avoided if you can help it.

I think there are some fairly universal rules of thumb you can use in that you can begin to get a feel for what certain headspeeds are like and this will apply to anything of the same size and close to the same weight. But the variables will be things like the tail ratio and head damping etc. So unfortunately a specific headspeed that works great on one heli may not work as well on another but you should be able to get something close that works. For choosing a motor on a new heli I supposed the thing to do is to see what headspeeds it is being successfully flown at and what setups are working well.

From my experience I can predict, for example, that if I'm looking at a new 700 size heli and it has a takeoff weight of under 5kg and uses 710mm blades (give or take a bit) and assuming it allows a healthy pitch range of around +/- 14, I can be pretty sure that it will do what I want at between 1300-1500 rpm (assuming it has a fairly conventional tail ratio and / or allows up to 110s on the tail). There would have to be something very odd about it for that to not work.

---Just to add, on small helis I just go for ballistic performance because I'm not really concerned about flight times on a 450, I only try to optimise the bigger ones for flight times and low amps etc. And on IC powered helis it's really all about whatever gets the engine in the powerband.

[Grumpy]

i guess the obvious starting point would be what the manufacturer recommends. You can then adjust that to suit what you know of your own personal preference. The manufacturer's recommendations for their helis will usually be for maximum performance and so quite high headspeed. If you have found you generally prefer a milder setup go for about 90% of that headspeed or whatever 'de-rating factor' you generally arrive at on your current helis.

[Granpappy]

A good clue to the manufacturers/designers idea of headspeed is to look at the gearing and pinion size, some even suggest the Kv. As above though, that's often the intended flight type and becomes subjective once it becomes personal

True. I think all manufacturers promote their products on 3D capability and therefore their recommended setup would reflect that. But that is not what I'm looking for.

[SaneAdam]

Correct headspeed for me on a 700 is 2050 to 2100. Anything less feels dull and lifeless to me.

[waveydavey]

Correct headspeed for me on a 700 is 2050 to 2100. Anything less feels dull and lifeless to me.

I think that's exactly the point in question Adam, to a 3D skygod like yourself as you say you want a heli to feel light and quick but others are happy to pootle around with the heli feeling heavier and lazy. I'm a bit in the middle I run 1980ish on my 700, it's all it needs and I can manage my wild stirrings with plenty of feel. It's a subjective thing the feel of a heli and how it flys

[Granpappy]

i guess the obvious starting point would be what the manufacturer recommends. You can then adjust that to suit what you know of your own personal preference. The manufacturer's recommendations for their helis will usually be for maximum performance and so quite high headspeed. If you have found you generally prefer a milder setup go for about 90% of that headspeed or whatever 'de-rating factor' you generally arrive at on your current helis.

Again true. But if you take the 7hv as an example, the manufacturers recommended motor is 510kv which would take me well off the mark. I think that the 470kv I'm running with is a little high.

I suppose there is no magic formula really. Just recommendation and a bit of trial and error.

Cheers everyone.

[Granpappy]

I think that's exactly the point in question Adam, to a 3D skygod like yourself as you say you want a heli to feel light and quick but others are happy to pootle around with the heli feeling heavier and lazy. I'm a bit in the middle I run 1980ish on my 700, it's all it needs and I can manage my wild stirrings with plenty of feel. It's a subjective thing the feel of a heli and how it flys

I suppose with nitro you must be a lot more limited on what you can adjust having to keep the engine within a certain rev range.

[gazmk2]

Yeah, nitro engines have a power band where they only make optimum power at a certain RPM range. Unlike an electric motor that makes 100% torque the minute it starts turning.