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HomeRC & PowerAircraftHelicopterEngines Plugs Mufflers Fuel › Gear ratio vs. blade size
04-01-2018 10:36 AM  5 months agoPost 1
Yeager

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Lisbon, Portugal

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YS96SRX OM states a max gear ratio (g/r) of 9,0 and advises the use of 7,5-8,4.
Standard gear ratio in my heli is 8.89, this can be easily changed to 8.18 (which is in the advised range). I'm using a Hatori 963 F3C pipe.
I'm assuming that 15,000 rpm on the 96SRX is around optimum.
I would like to run larger main blades, so keeping the 8,89 ratio would therefor seem like the best option, when I don't fancy, nor need, high head speeds.
What would be a good/best combination of gear ratio and main blade size, to keep the engine happy at the same time. Example 810mm at 1680rpm (8.89 g/r) or 710mm at 1830rmp (8.18 g/r)?

Robbe Nova Cuatro Gas/Nitro, Futura Nova Nitro, Futura SS Nitro, Vario Scale S300C/1002/21 Gas.

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04-01-2018 12:33 PM  5 months agoPost 2
wjvail

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Meridian, Mississippi

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Yeager
I would like to run larger main blades, so keeping the 8,89 ratio would therefor seem like the best option, when I don't fancy, nor need, high head speeds.
What would be a good/best combination of gear ratio and main blade size, to keep the engine happy at the same time. Example 810mm at 1680rpm (8.89 g/r) or 710mm at 1830rmp
Interesting question. I've never heard of someone trying to run 810s on a glow motor. A few years back, as motors became bigger and more powerful, the trend was to run lager blades. Blades as large as 720 were tried. Currently the trend seems to be the opposite with folks running blades closer to 690 or 700. SAB's Goblin runs 650s. Synergy's new 556 is running a .50 class motor on what would historically have been 30 blades.

I suppose the answer will depend on your flying style. You can certainly run 810s at low head speed and the 8.89 gear ratio with the head speed you quote would certainly be close. I have no practical experience to draw from to say for certain but I think you would find this setup pretty boggy.

Another data point: I routinely set up my acrobatic helicopters with 3 head speeds. Typically that is around 1,400 1,650 and 2,000. With my gear ratio around 8.2 that puts the engine at 11,500 13.500 and 16.400. Each of these head speeds flies differently but none of them "damages" the engine. Clearly the engine (and clutch) is happier at higher RPM but it still performs adequately at well below optimal. It's just a matter of what mood I'm in.

Whatever you decide I think you're looking at it correctly by targeting an engine rpm and calculating a head speed and gear ratio. For performance your going to have to keep the engine @ 16,000+. But if it's performance you're looking for, I have to question your blade choice.

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04-01-2018 02:50 PM  5 months agoPost 3
Richardmid1

rrProfessor

Leeds, England

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Just because the engine produces it's max power at X rpm doesn't mean you need to run it at that. Heli engines are torquey and are designed to run well at medium throttle. 8.18 ratio would still work well with 800 blades at around 1600 rpm for example.

60% of the time, it works every time!

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04-01-2018 03:05 PM  5 months agoPost 4
wjvail

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Meridian, Mississippi

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Richardmid1
Just because the engine produces it's max power at X rpm doesn't mean you need to run it at that. Heli engines are torquey and are designed to run well at medium throttle. 8.18 ratio would still work well with 800 blades at around 1600 rpm for example.
All true but I wonder how a 90 class engine will turn 810s if not kept on a boil? That is a lot of blade and could be quite a load on the engine even at its peak. I wonder how it would perform at 3-4,000rpm below peak power.?.?

As I said, interesting question.

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04-01-2018 06:27 PM  5 months agoPost 5
Richardmid1

rrProfessor

Leeds, England

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1600 is not all that fast for 810's and even with 8.18 ratio the engine is turning 13,000 rpm which isn't exactly slow. The engine will also have more power/torque in reserve. Heli engines have a wide power band, a lot of people seem to forget this.

60% of the time, it works every time!

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04-02-2018 08:07 PM  5 months agoPost 6
Yeager

rrApprentice

Lisbon, Portugal

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Gents, many thanks for your replies.
I guess I'll be running this fly-barred heli at 8.18 gear ratio, with SAB 710s and a head speed of max 1900 rpm - which is not really my cup of tea, but that's what I believe this engine is build for in the first place. Keep in mind that it's a 5.3 kg Robbe Cuatro (dinosaur) heli. I'll keep the lower head speed for the Petrol's I've got. I guess it doesn't make much sense to use a "speedy" YS96SRX engine with large blades, like 800 size.

Robbe Nova Cuatro Gas/Nitro, Futura Nova Nitro, Futura SS Nitro, Vario Scale S300C/1002/21 Gas.

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04-02-2018 08:42 PM  5 months agoPost 7
ticedoff8

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Morgan Hill, CA. USA

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Unlike an electric motor, an IC engine has very well defined power curves.
Torque and RPM related produce the peak power of the motor.

The YS 96SR-X has the best power (around 2600W (about 3 1/2 HP) at 16,000 RPM.
Any suggestions in the owner's manual in gear ratio is based on the engine running within its normal peak power range (between 15k and 16k for the YS 96SR-x).

If you turn it slower, it will load up and will overheat
If you turn it at 16,000 and put too much load on it, it will load up an overheat.

Trying to turn a 810mm at 1650 RPM will require a certain amount of power. The easiest way to figure out how much power that would take is to test it with an E motor where things like Voltage, Amps and efficiencies are easy to log (using the ESC's internal logs).
Once you figure out that, then changing to an IC motor like the YS 96SR-X is easier because you know if the IC motor is going to have the power to turn that blade size.

Using the YS 96SR-X on a "90 size" heli (AKA: "700 size" ) is a well know use case.
I run the YS96SR-X on my Goblin 700 Black Nitro with 697mm VTX blades at 2000 RPM headspeed using a 7.95:1 gear ratio (15,900 motor RPM).
It is a beast.

Your best bet is to run the motor at or near the rated RPM - not slower and never faster.
Running it "3,000 or 4,400" RPM below peak would be a horrible idea.
A 10% change in RPM is 1600 RPM. And that is a big change.

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04-02-2018 10:27 PM  5 months agoPost 8
ICUR1-2

rrElite Veteran

Ottawa, Ontario

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So the short answer is The shorter the blades the lower the gear ratio required.
For an IC engine in this case.
A lower gear ratio also allows for a higher head speed.

spending time, paying attention

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04-02-2018 11:46 PM  5 months agoPost 9
wjvail

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Meridian, Mississippi

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ticedoff8
If you turn it slower, it will load up and will overheat
If you turn it at 16,000 and put too much load on it, it will load up an overheat.
ticedoff8
Your best bet is to run the motor at or near the rated RPM - not slower and never faster.
Running it "3,000 or 4,400" RPM below peak would be a horrible idea.
A 10% change in RPM is 1600 RPM. And that is a big change.
Well. It still depends on how you want to fly. And all of the above is untrue.

I fly my X-Cell Carbon SE with an OS 91HZ. Rotor blades are very heavy RotorTech/ScottGray/F3C 720s. The gear ratio is 7.75 and the head speed is 1,400 1,600 and 1,900. At low head speed the engine is very happy at 11,000. Even at high head speed the engine is only turning 14,500ish. While I admit it is not optimal for maximum performance, it is very smooth and quiet and I'm very happy with this setup. To suggest you can't do this is simply not true. The helicopter is 20 years old and I have no intention of flying at 2,000 head speed and the engine at 16,000+.

Yeager - Because you are flying an older helicopter you might consider something similar. I don't know about 810s - that's outside my experience. But don't be fooled into believing you must run at 17,000 rpm engine and 2,000+ head speed.

"Well, Nothing bad can happen now."

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04-03-2018 01:46 AM  5 months agoPost 10
wjvail

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Meridian, Mississippi

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I flew my Carbon SE a few weeks ago at the Birmingham flyinn. Below is from 2011. It was many years old when the below video was taken.

Very little has changed since the vid was taken. Same car. Same wife. Same helicopter, blades, engine, muffler, servos.... Different transmitter.

Build your helicopter to fly the way you want.

Watch at YouTube

"Well, Nothing bad can happen now."

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04-03-2018 11:42 AM  5 months agoPost 11
Richardmid1

rrProfessor

Leeds, England

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ticedoff8
Your best bet is to run the motor at or near the rated RPM
2000 - 17000 rpm then?!

As said, you don't HAVE to run these engines at the RPM they make their max power. Just because these engines have a max power at 16000 rpm doesn't mean they will have no power and run horrible at anything below that!

The OP has the option of 8.89 ratio and running 810's at 1500 rpm isn't any more taxing than running 710's at 1900 rpm and the engine would be at 13,335 rpm which is absolutely fine for what he is doing with it.

60% of the time, it works every time!

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04-03-2018 12:42 PM  5 months agoPost 12
Yeager

rrApprentice

Lisbon, Portugal

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wjvail
Yeager - Because you are flying an older helicopter you might consider something similar. I don't know about 810s - that's outside my experience. But don't be fooled into believing you must run at 17,000 rpm engine and 2,000+ head speed.
Sure thing, I did run a YS91SRS with 810mm mains, and it appeared to me that when I spun them at 1550-1580 rpm (8,89 g/r) the engine was "alive" as opposed to my Idle1 spinning them at 1400 rpm, which didn't quite sound right and I have a feeling that I overheated the engine, doing scale type aerobatics (with lots of "high" blade pitch and then zero pitch descends). But unfortunately I'm no heli engine expert at all, so not sure.
wjvail
Very little has changed since the vid was taken. Same car. Same wife. Same helicopter, blades, engine, muffler, servos.... Different transmitter.
LoL. Good pick on changing the transmitter
Richardmid1
The OP has the option of 8.89 ratio and running 810's at 1500 rpm isn't any more taxing than running 710's at 1900 rpm and the engine would be at 13,335 rpm which is absolutely fine for what he is doing with it.
Because the OEM states a main rotor gear ratio of 7.5-8.4 (however also states MAX 9,0) I'm inclined to opt for the 8,18 gear ratio and go with shorter blades (SAB 710mm) for this heli. If the head speed feels to overwhelming, I'd probably test some EDGE 753mm and slow it down a bit.

Once again, many thanks Gents for your inputs.

Robbe Nova Cuatro Gas/Nitro, Futura Nova Nitro, Futura SS Nitro, Vario Scale S300C/1002/21 Gas.

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04-03-2018 03:03 PM  5 months agoPost 13
Richardmid1

rrProfessor

Leeds, England

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Overheating the engine is down to tuning mainly but yes 1400 rpm is on the low side even with 8.89 ratio. It sounds like you know your stuff and going for the 8.18 with 710's is the way I would go as well.

60% of the time, it works every time!

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04-04-2018 01:15 AM  5 months agoPost 14
Yeager

rrApprentice

Lisbon, Portugal

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Richardmid1
It sounds like you know your stuff and going for the 8.18 with 710's is the way I would go as well.
Doing the math's are quite easy, right. To go out to the field, tune the engine and make it all work out, in reality, is a bit harder though, in this hobby - or at least it takes time and patience. At least, or so it seems, RC heli flying has come a long way since then, when I used to admire the "freaks" back in the late 80s, with their heli's. A much more approachable hobby now, I'd have to say.

Robbe Nova Cuatro Gas/Nitro, Futura Nova Nitro, Futura SS Nitro, Vario Scale S300C/1002/21 Gas.

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04-12-2018 08:48 PM  5 months agoPost 15
Yeager

rrApprentice

Lisbon, Portugal

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On last question in this regards. I ended up having to install the 23 tooth clutch bell (due the belt being to tight with the 25 tooth clutch bell), so I have to go with the 8,89 gear ratio - which I don't see as a big deal (other than it's outside the recommended range of up to 8,4, but again the defined max is 9,0 gear ratio.

Would you guys run the 710mm main rotor blades all the way up to 1900 rpm (17000/8.89=1912rpm) or would the engine have a "better, as in longer, life" if keeping it running at say 1800-1850 for Id-2 and say 1600 for Id-1? does it make any difference? Thank you.

Robbe Nova Cuatro Gas/Nitro, Futura Nova Nitro, Futura SS Nitro, Vario Scale S300C/1002/21 Gas.

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04-12-2018 09:18 PM  5 months agoPost 16
ICUR1-2

rrElite Veteran

Ottawa, Ontario

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running over the rated rpm and/ or running lean is what will hurt them

Otherwise there is not much to gain in running them slower than the rated rpm. Maybe you'll get 15% more life, I don't know of any proof of longevity.
The only longevity upgrade I know is the center glide bushing if there is one available.

spending time, paying attention

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04-13-2018 02:43 AM  5 months agoPost 17
PaulBowen

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Victoria, Australia.

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Lower rpm provides much lower fuel burn which provides much longer flight times. My Freya runs happily at 1400-1500rpm on a 7.92 ratio for 15 minute flights. Nice quiet and neighbourhood friendly.

I’ll point the obvious out again, not everyone is into 3d.

Fly whichever rpm suits your needs and ignore the high rpm BS brigade.

Futaba T18SZ, JR Propo XG14, Hirobo fanatic!

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04-18-2018 11:22 AM  5 months agoPost 18
Richardmid1

rrProfessor

Leeds, England

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I would not run it at 17,000 if you don't have to and running 1900 rpm on 710's is a little unnecessary unless you want crazy 3D. 1800 rpm should be plenty on 710's and would put the engine at 16,000 meaning it has more 'over head' on the governor and will sound and feel much more consistant.

60% of the time, it works every time!

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04-20-2018 07:43 PM  5 months agoPost 19
Yeager

rrApprentice

Lisbon, Portugal

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Richardmid1
I would not run it at 17,000 if you don't have to and running 1900 rpm on 710's is a little unnecessary unless you want crazy 3D. 1800 rpm should be plenty on 710's and would put the engine at 16,000 meaning it has more 'over head' on the governor and will sound and feel much more consistant.
This is also what I ended up with after flight testing. The GY701 is a good governor, but with fast off loading and the belt driven mechanics, it would run up to a max just below 1900 rmp when pushed. And the engine sounds happy around 1800 rpm with the 710mm (8.89 g/r).
When I next have to disassemble the engine section, I will possible make the effort to change the gear ration to 8.17.

Robbe Nova Cuatro Gas/Nitro, Futura Nova Nitro, Futura SS Nitro, Vario Scale S300C/1002/21 Gas.

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