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HomeAircraftHelicopterSafety - RC Helis are not toys › Reality and rotor rpm
06-09-2012 03:16 AM  6 years agoPost 1
icanfly

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ontario

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Saw a full size Bell 222 or later improved version fly overhead this aft and noticed how slow the rotor was in comparison to anything small scale. Wikipedia reports the Bell rotor at 100% was 348rpm.

It seems to me that a lot of scale heli damage occurs because of high rpm rotor speeds, blade shape is a given on inflicting harm, thin and knife like.

I would challenge anyone to match the Bell full scale rpm 222 (b), Huey Cobra, whatever, on a scale heli.

Is it actually possible?

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06-09-2012 07:39 AM  6 years agoPost 2
ShuRugal

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Killeen, TX

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the reason our models turn such high RPM is that the rotor diameter is considerably lower, and thus a higher rotational rate is needed to get the correct tip speed to produce lift.

I've seen some projects where people had their 700 size swinging 800mm or 900mm blades and making lift around 1200 rpm, but watching the video, you could tell they were barely flying.

AMA 700159

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06-09-2012 07:53 AM  6 years agoPost 3
BladeStrikes

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Shelby TWP,Mi

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Watch some of Matt Botos videos.He runs 1000RPM flying 3D so they will do more than barely fly https://rc.runryder.com/t697493p1/ <--- Thats with 696MM mains..People have also run 500ish RPM with 800MM mains..

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06-09-2012 01:54 PM  6 years agoPost 4
icanfly

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ontario

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As everyone knows a full size rotor will chop a soft tissue creature (human) in two no problem, scale blades will do an amount of slicing before losing momentum.

Time after time rc heli guys keep reminding everyone how lethal their chopper is, in truth maybe not lethal but absolutely to be kept away from due to high tip speeds and slash conducive shape.

After a little thought, it might be reasoned that matching the tip speed of a 20ft blade at 348rpm to a scale heli would imply increasing rpm as the rotor blade length is decreased, 10ft =696rpm, 5ft = 1392rpm, half, quarter, etc.

This puts a person in the same hazardous area as a full size though the blade of a scale heli will lose momentum very quickly on impact due to it's lack of mass.

I believe 3d flying has introduced high rotor rpm because 2 stroke motors produce high torque at high rpm, and high rotor rpm is more reactive plus stable in wind.

As the Bell heli flew past I noticed the tail wag left and right, it was very windy and they flew across the wind direction at 90 degrees. I often wonder why full size heli pilots often go up on very windy days.

I live on a small plane and heli general flight path, an Augusta Westland AW139 just passed overhead.

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06-09-2012 05:22 PM  6 years agoPost 5
icanfly

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ontario

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here's something to consider

http://www.shephardmedia.com/news/r...ng-low-noise-b/

I've run an experiment where the rotor tip shape was altered (not the same as the one you saw in the link) resulting in improved efficiency by up to 20%.

Hypothetically then a more efficient blade equals reduced headspeed and or span/root with equal results to present strap style blades.

The thing to look at here is that metal blades are attractive to the "scale" and turbine flyer but require a lot of hazard prevention due to high head rpms and potential failure. Metal blade use/life would benefit from lowered headspeed imho.

My little backyard flyer darted off into a lawnchair from wind and obstruction from view (second time a heli I flew tried to take out a chair), thank the lord no one was in it but it serves as a warning, and it has flown into me on several occasions (nose in) because a slight gust of wind, had to pluck it from the sky a few times too.

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06-11-2012 03:04 AM  6 years agoPost 6
icanfly

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ontario

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here's a good example of low rotor rpm heli flight,

Watch at YouTube

I find the low rpm flying much more relaxing and less fearsome, Matt Botos included of course.

In the "Battle Axe" thread we saw what a 900 can do to a tree , assuming the head rpm was over 1500 rpm putting the tipspeed somewhere around 400mph. 400mph seems to be a magic number regarding tip speed on most 3d style helis today.

To go the high rpm route borders a heli on becoming a horizontal mounted airplane prop with a central power and control unit on its shaft.

Has the rc heli hobby been selling the public Top Fuel Dragsters when it should be selling something much more tame yet high tech looking and entertaining without worries of sending a person/pilot to the ho for stitches (should they get too close to the rotor by accident)?

(got my Hirobo Shuttle and am making a plan of action to get it in the air again, pics to come)

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06-11-2012 05:15 AM  6 years agoPost 7
ShuRugal

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Killeen, TX

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700mm-800mm blades will cut to the bone at 900 rpm just as easily as at 1800. There's more than one injury thread starting off with "while the rotors were spooling down" or "after it had lost most of its headspeed".

the problem isn't that the heads are being operated at some unsafe speed and the problem cannot be fixed by reducing to a "safe" RPM.

The reason these are dangerous is because a rotor with the properties that allow it to efficiently cut through air and rapidly provide large amounts of force to the main shaft without failing also allow it to transfer large amounts of energy and efficiently cut through living tissue.

really, the problem is we are dealing with high-energy machines because to get them to perform the way people want them to perform requires high amounts of energy.

a little math:

using the values on the EDGE website for their 800 mm rotor, we have a mass of 270 grams. center of mass position is not indicated, but i'm going to assume it's around 2/3 (if someone knows better, feel free to tell me) at 600mm. this means that at 900 rpm, the center of mass will have a linear velocity of roughly 30 m/s.

if we assume worst case scenario, blade hits limb and is stopped dead, we can assume roughly 1/10 second to stop, which give us a deceleration rate of 300 m/s/s. using F=ma we have 0.27kg*300 = 81 newtons of force.

if we assume frontal area of the blade as it makes contact at 1/10=in wide (and this is a generous estimate, i would not be surprised if the effective contact was much thinner) along, say, 3" wide piece of forearm, that gives us a total contact area of 0.3 inches, for an impact pressure of roughly 270 psi.

Now, i was not able to find a solid number, but most sources indicate that as little as 100 psi can break the skin, and the tissues beneath require less force once skin has been broken. so the initial impact give us almost triple the skin-penetration requirement, and then the second blade come swinging around and adds another 80 lbs of impact to the mix... and this is at the "low" headspeed of 900 rpm as in the above video, and using very conservative figures for the math.

basically, it boils down to what i mentioned above, if you want to toss a 2-3kg machine around at 3-4Gs of acceleration, you are going to necessarily involve a large amount of force to do it. the interface which transfers that force into the air will just as happily transfer it into a person if a person ends up in the way.

AMA 700159

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06-11-2012 01:13 PM  6 years agoPost 8
icanfly

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ontario

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thank you shuRugal

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11-12-2012 01:44 AM  5 years agoPost 9
cannibal440

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cookeville, tennessee USA

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theres a constant involved with size of blades and RPM. The tips should remain sub sonic I think? smaller the blade; faster it can turn to remain in such state.

only way ill ever make the jack is if i work at a cheese factory

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01-04-2013 03:03 AM  5 years agoPost 10
Stephen Born

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USA

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I run 800mm blades that weigh 250 grams each. My headspeed varies from 1300-1450, respectively.

Heavier, longer blades with lower HS is the same as short, light blades running high HS.

Lethal.

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01-04-2013 03:23 AM  5 years agoPost 11
Santiago P

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South West, Ohio

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low RPM?
been there, bought the T shirt.

1300-1500 with 750 blades:

Watch at YouTube

and even lower, 900-1000rpm, 6S

Watch at YouTube

Watch at YouTube

First few flights at 1000rpm and 6S were with 700mm blades. I flew 12 minutes and only used 80% of the battery. (cheap 25C 6S 5000ma)

How's that for efficiency?

Santiago

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01-04-2013 04:54 AM  5 years agoPost 12
BladeStrikes

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Shelby TWP,Mi

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Very nice MoneyPitVictim ...

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01-05-2013 06:23 PM  5 years agoPost 13
Rafael23cc

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Junction City, KS

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Some of you need to read up on physics, and the scaling down/scaling up factors involved. Remember that air molecules remain the same size but the machine is either bigger or smaller.

If you find that interesting, you can read up on Reynold's Number, fluid mechanics and other similar subjects. Yes, i'm an engineer nerd.

Rafael

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

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01-06-2013 12:32 AM  5 years agoPost 14
Santiago P

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South West, Ohio

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@BladeStrikes

Thanks pal

@Rafael23cc

Very well explained!

Santiago

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