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HelicopterEngines Plugs Mufflers Fuel › Headspeed vs Gear Ratio vs Optimum Engine RPM question
01-31-2018 08:21 PM  83 days agoPost 1
GeezerJoe

rrNewbie

Kernersville, NC USA

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Hello,

This is my first post here.
I'm kind of an old hand at RC helis (mostly electric).
I have three nitro helis as well but I have never really got the hang of knowing how to set em' up and tune em' (gearing vs headspeed vs optimum engine rpm).
I'm pretty fair at setting everything else up.
The nitro helis I have now were setup and tuned by a friend who knew what he was doing but he liked to keep his tricks to himself, he did teach me how to make fine adjustments for various weather conditions. Unfortunately I didn't really push myself to learn more and my friend is no longer with us.

I have a 90 size machine with an OS91SZ Hring (pumped and regulated) that has logged just over 53 hours according to the governor (about 40 gallons of CP 30%), it was showing signs of serious wear and power loss so I rebuilt it. Now I need to break it in again and fine tune it. I have watched other guys at the field and I have watch some good videos on proper tuning procedure so I think I'm good there, it may take some time and practice but I do know what it should sound like when it's happy.
I thought that the videos with Tim Jones explaining tuning was good info, what he says makes sense to me.

The real problem is that I have built a new 700 machine with a slightly used OS91SZ (pumped as well).

I'm wondering about the gear ratio for that engine.
According to OS's recommended rpm for max power, it needs to run at 15,000. So I need to gear it so I'll be flying with the engine turning at 15,000 rpm for max power output when it's at the headspeed I want. Is that close to right?
If I run it with a 10T pinion and a 95T main, the max headspeed would only be 1578 but If I set the governor for 1800 which is what I want, I would never be able to get it tuned right since it would never reach 1800, it would constantly be trying to reach the set headspeed rpm in the governor and never really allow me to tune the mid needle. Does that make sense or am I way off base?

I'm thinking that I need to use a 12T pinion with the 95T main so the max headspeed would be at about 1900 or a little less, this would give me a little headroom with the top headspeed and what the the engine can deliver at max power.
OS says that the engine's operating range is from 2,000 to 16,000

Or...should I use an 11T pinion that would give me about 1750 at 15,000 engine rpm, (1850 at 16,000)?

Does it make sense to figure out what headspeed I want first, then find out what the max power rpms are for the engine I want to use, then gear it so there will be a few hundred rpms headroom above my target headspeed? Or should I try to match the max power rpm with the target headspeed?

Is there a rule of thumb for max engine power rpm vs the desired headspeed?

I would really appreciate any information and advice you can offer!!

Joe

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01-31-2018 09:58 PM  83 days agoPost 2
Rafael23cc

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Junction City, KS

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GeezerJoe
If I run it with a 10T pinion and a 95T main, the max headspeed would only be 1578 but If I set the governor for 1800 which is what I want, I would never be able to get it tuned right since it would never reach 1800, it would constantly be trying to reach the set headspeed rpm in the governor and never really allow me to tune the mid needle. Does that make sense or am I way off base?
Everything that you said seems correct to me with the exception of this statement. Here you may be confusing a governor with a Rev-Limiter. A rev-limiter will prevent you from reaching the programmed RPM while the governor will keep the RPM within a reasonable band around the programmed RPM.

My suggestion would be of course to tune the engine with the governor / rev-limiter disabled. Once you are comfortable with the tuning, then enable the throttle managing devices.

hope that steers you in the right direction.
Rafael

Keep your feet on the ground, but your eyes on the sky.
Team Heliproz.com

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01-31-2018 10:35 PM  83 days agoPost 3
GeezerJoe

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Kernersville, NC USA

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Rafael23cc
Everything that you said seems correct to me with the exception of this statement. Here you may be confusing a governor with a Rev-Limiter. A rev-limiter will prevent you from reaching the programmed RPM while the governor will keep the RPM within a reasonable band around the programmed RPM.

My suggestion would be of course to tune the engine with the governor / rev-limiter disabled. Once you are comfortable with the tuning, then enable the throttle managing devices.

hope that steers you in the right direction.
Rafael
That sounds like good advice to me and I'll take it.

I guess what I was thinking when I wrote that statement was that if the headspeed was set at 1800, the engine would have to reach 17,100 rpms which is about 1100 rpms above what OS says the operating range is, meaning that it would over rev the engine to reach that headspeed. I would assume that the power would reduce when above and below a certain "Sweet Spot" rpm?

So the operating rpm range doesn't necessarily mean that it won't run faster than that without flaring after a dive? (even above 17,000 rpm?)

I would also assume that running it much over the recommended speed could either damage it or at least shorten it's life.
I've still got a lot to learn but knowing that I'm a step closer to being on the right track with how to choose the right gearing setup is a step in the right direction.

I'm still curious if I should shoot for a gearing setup that will allow for some headroom or if I need to try to match the target headspeed to the engine's best power speed.

I've setup a bunch of electrics, that stuff is more clear to me than combustion engines but I have always liked running nitro.
I know I've bought at least 80 gallons of Cool Power 30% over the years.
You'd think that I would know more about them other than just flying them and making small adjustments here and there. Lol

Thank you ever so much, Rafael!!

Joe

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02-01-2018 02:32 AM  82 days agoPost 4
Rafael23cc

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Junction City, KS

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I see what you are asking a little better now. Myself, being a stick banger (not that smooth of a flyer) subscribe to the idea of setting my RPM on the upper range of the engine optimal RPM. By doing this, when i stick-bang, allowing the headspeed to drop, will bring the engine "into the power band" of the optimal RPM.

I have friends that fly "low headspeed" in this case, again, you have to match the gearing to the desired headspeed but you will have to be smooth in the controls to prevent bogging the head and allowing the headspeed and engine rpm to drop too low.

Rafael

Keep your feet on the ground, but your eyes on the sky.
Team Heliproz.com

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02-01-2018 12:39 PM  82 days agoPost 5
GeezerJoe

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Kernersville, NC USA

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Rafael23cc
I see what you are asking a little better now. Myself, being a stick banger (not that smooth of a flyer) subscribe to the idea of setting my RPM on the upper range of the engine optimal RPM. By doing this, when i stick-bang, allowing the headspeed to drop, will bring the engine "into the power band" of the optimal RPM.

I have friends that fly "low headspeed" in this case, again, you have to match the gearing to the desired headspeed but you will have to be smooth in the controls to prevent bogging the head and allowing the headspeed and engine rpm to drop too low.

Rafael
Your response reminds me of my first used nitro heli that I know was geared too high, the engine never reached the power band range, the headspeed was there but the engine was probably at least 1,500 rpms lower than it should have been.
When I pulled up inverted from a small loop close to the ground, the thing bogged down from adding too much negative collective pitch and it smacked the ground, not too hard though thankfully.
I think it cost about $150 for parts, I was lucky on that one!
That was my first nitro, I bought it from someone else and it was already setup.
I know I didn't go full negative so I wondered why it bogged so easily.
My friend looked it over and pointed out that the gearing was wrong, that's when I let him take it home so he could set it up right.
BTW, I setup my friend's electric helis so the trade was him setting up my nitros.

I try to manage collective pitch with all my helis but that doesn't always work the way I plan, I've had to abort hurricanes cause of HS bogging :-)
I don't fly too close to the ground anymore :-)

I'm thinking that I should set it up with an 11T pinion for a ratio of 8.63, that would allow for 1,800 HS with the engine running at 15,500 which is just above the magic power band or "Sweet Spot" as they say.
It makes sense to be on the upper side of the power curve so it can drop into it if I go a little too far with collective pitch (as you mentioned) so thanks for adding some insight to this for me.

I feel much better about what gearing to use now, thanks again!! :-)

Joe

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02-23-2018 02:37 PM  60 days agoPost 6
Richardmid1

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Leeds, England

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Just remember different pipes and tuning will determine at what rpm the engine makes the most power so it may not be 15,000, it might be 13,000, it might be 17,000.

Also when unloaded (zero pitch or decent) the engine can be at near max rpm but still at mid throttle/needle, so you can still tune the mid with high engine speeds but remember that with the governor on and with a rich mid the governor will open the carb upto the high needle to get to the programmed head speed, so it can be tricky.

60% of the time, it works every time!

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02-23-2018 06:09 PM  60 days agoPost 7
GeezerJoe

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Kernersville, NC USA

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Richardmid1
Just remember different pipes and tuning will determine at what rpm the engine makes the most power so it may not be 15,000, it might be 13,000, it might be 17,000.

Also when unloaded (zero pitch or decent) the engine can be at near max rpm but still at mid throttle/needle, so you can still tune the mid with high engine speeds but remember that with the governor on and with a rich mid the governor will open the carb upto the high needle to get to the programmed head speed, so it can be tricky.
Thanks Richard!

Great information!

I was wondering about how far the carb rotor would open when not under full load but still at high rpm.

I was also curious about max power at different engine speeds.
Problem is I can't really find any specific information on the Revolution pipe I have now.
The Revolution Pipe measures 12 inches long by 1- 5/8 diameter.
I compared it to other pipes recommended by OS but only by size, the pipe I have doesn't have a model number on it.
I'm sure that the way it's made inside has a lot to do with how it performs.
I'm not sure if mine is double coned or not and it may be just a basic "Silencer" pipe rather than any form of "Tuned" pipe.

If I can avoid it, I prefer not to buy another pipe since they are so expensive, it's easy to spend more on the pipe than the engine.

I haven't had a chance to do much tuning yet, it's been raining a lot and work has been unusually demanding these past few weeks.
Hopefully I can get to the field in the next few days and see what I can do with it.

I disabled the governor and plan use throttle curves to get the high and mid needles close.
I have the curves set with the throttle rotor 50% open at midstick.

I've read where lots of folks recommend tuning with the governor off, is that your recommendation as well or would it be OK to enabled the governor and use hard climbouts for the high needle, then sudden transition to 0 collective to monitor smoke and sound when it unloads for basic mid needle?
I feel like tics tocs are good for mid needle settings because of the constant transition between loaded and unloaded, then fly full collective hurricanes for high/main needle settings, do you agree with these methods?

I may have to bite the bullet and buy a good pipe.
I'm considering the Hatori 939, it can be had for around $90.
Any thoughts on using that pipe for an OS.91SZ Hyper Pumped engine?

Thanks a bunch for the very informative response!!

Joe

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02-24-2018 06:53 PM  59 days agoPost 8
Richardmid1

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Leeds, England

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GeezerJoe
I've read where lots of folks recommend tuning with the governor off, is that your recommendation as well or would it be OK to enabled the governor and use hard climbouts for the high needle, then sudden transition to 0 collective to monitor smoke and sound when it unloads for basic mid needle?
I feel like tics tocs are good for mid needle settings because of the constant transition between loaded and unloaded, then fly full collective hurricanes for high/main needle settings, do you agree with these methods?
I tune with governor on as that is how I would be flying the heli anyway. Your flying techniques for tuning are spot on but remember to throw in some throttle holds to listen for any 'hanging on the pipe', slight hanging is ok if you've been hammering but it should be at idle in a couple of seconds.

60% of the time, it works every time!

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02-25-2018 12:53 AM  59 days agoPost 9
GeezerJoe

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Kernersville, NC USA

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Richardmid1
I tune with governor on as that is how I would be flying the heli anyway. Your flying techniques for tuning are spot on but remember to throw in some throttle holds to listen for any 'hanging on the pipe', slight hanging is ok if you've been hammering but it should be at idle in a couple of seconds.
Thanks for the tips!

I'm familiar with "hanging on the pipe" but I had forgotten to use that trick to make sure it's not running too lean while it's in the air.
Thanks for the reminder!!

I like the idea of tuning the engine while the governor is on rather than using the throttle curves and I don't see why it can't be tuned reliably as long as I use the flying techniques mentioned.

Once I get the needles set close enough I should be able to tell to some degree where the best power rpm is. I think I have finally got my head around this.
I've been fine tuning for years but have never done the initial tuning or gearing setups on nitros but after tons of research and tips like yours, I feel comfortable with it now.

Thanks a lot, Richard!!

Joe

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02-25-2018 02:26 PM  58 days agoPost 10
GyroFreak

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Orlando Florida ...28N 81W

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Here is a link to a post I made about how I figure gear/belt ratios when modifying. Probably not needed for you but here it is anyway. It also gives me a way to go back later and see what I was doing.
https://rc.runryder.com/helicopter/t824265p1/

I think about the hereafter. I go somewhere to get something, then wonder what I'm here after ?

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02-25-2018 07:29 PM  58 days agoPost 11
GeezerJoe

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Kernersville, NC USA

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GyroFreak
Here is a link to a post I made about how I figure gear/belt ratios when modifying. Probably not needed for you but here it is anyway. It also gives me a way to go back later and see what I was doing.
https://rc.runryder.com/helicopter/t824265p1/
I always have a hard time remembering what gear ratio and what tooth count I have for every machine I have, so it's a good idea to write them down somewhere easy to locate.
But, making notes to help remember things only works if you can remember where you put the dang note!
Good idea to sketch it out like this as well! It helps a lot when changing pinion or other gear tooth count to get the target HS!
Thanks for the tip!
Joe

Making notes to help remember things only works if you can remember where you put the dang note!

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