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HelicopterMain Discussion › Head speed question
01-28-2013 02:57 AM  4 years agoPost 1
Roger Rocket

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Morgan Hill,Ca-United States

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I was out flying my Heli today and a spectator asked a question that I could not answer. Why do the blades have to be turning so much faster than on a full size Helicopter ? Most of our RC Heli's turn at about 2000 or higher RPM, but full size they turn in the hundreds of RPM. The proportion of blade size to Heli size seems to be close and I know our power to weight is much better than full size. So I told him smaller blades just have to turn faster, kinda like the wings of a humming bird, which flap at a very high rate. I don't think he bought it, don't think I do either. Just something I came up with quick. But heck, maybe I'm right, but really don't know why. Does anyone know the answer to this ?? I would think the very first people building the very first RC Heli's had to struggle with this, to find the right head speed.

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01-28-2013 03:51 AM  4 years agoPost 2
Tbird

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Indiana

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I am sure you are going to get a bunch of scientific answers here really soon, but I think its pretty basic.. long the blade slower it needs to turn to create equal lift as a shorter one going faster....

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01-28-2013 04:09 AM  4 years agoPost 3
BobOD

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New York- USA

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Run around a coffee table and then run around the block. If you're running the same speed, which can you get around faster.

Team POP Secret

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01-28-2013 04:23 AM  4 years agoPost 4
AWittleWabbit

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O.C., CA

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I am sure you are going to get a bunch of scientific answers here really soon,
I agree

That said,
Reynolds numbers and some crap about things squared or cubed

Heli-itis sufferer.

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01-28-2013 04:48 AM  4 years agoPost 5
Ravenhyper50

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Canada's Capital

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That said,
Reynolds numbers and some crap about things squared or cubed
Im not going there! Ill sum it up laymen's

Its all about Blade tip speed

Some Rc helicopters can have about the same blade tip speed as a full size heli.

Smaller blades need to rotate faster to get the same tip speed as larger blades and vise versa. Bigger blades can achieve the same blade tip speed spinning much slower.

BObOD said it in his riddle

Its the tips of our helicopter blades that provide most of the lift.

Cheers,
Raven

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01-28-2013 04:51 AM  4 years agoPost 6
Roger Rocket

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Morgan Hill,Ca-United States

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Reynolds numbers,I had heard that many years ago but never investigated that. It's probly part of the so called 'Scale Effect '. It may be pretty basic Aaron, but I do know for a fact that there has been many talented people over the years that have designed rc model aircraft that had very good flying characteristics, and scaled them up, only to find thier full size version flew terrible. And spending tens of thousands dollars to learn about scale effect. I don't understand it, but now will have to go on line and try to learn about Reynolds numbers. I THINK this has to be the big part of it.

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01-28-2013 05:08 AM  4 years agoPost 7
Solmanbandit

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Tucson , AZ

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Think about this. Do full scale heli's fly the same way as do rc's? To actually a 450, I could easily fly it around at about 60% throttle. But what kind of performance would I have? Also if full scale spun their blades at our speeds, they would not last very long and would be slinging off in no time.

Trex 700E / Trex 500 ESP - Ikon/ HD 500 - Ikon 2/ Goblin 500 Ikon 2

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01-28-2013 06:13 AM  4 years agoPost 8
AWittleWabbit

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O.C., CA

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Don't forget loads and forces that will not go up linearly whil material strength remains the same.

Cubed or squared? Hehe

Heli-itis sufferer.

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01-28-2013 06:35 AM  4 years agoPost 9
Santiago P

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South West, Ohio

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I am sure you are going to get a bunch of scientific answers here really soon,
dang, I get slaped before I can type anything.

Yes, is a Reynolds number thang

Santiago

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01-28-2013 12:04 PM  4 years agoPost 10
abwomack

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Houston, TX - U.S.

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My 2 cents

Being a full scale pilot I know that passenger and cargo can be very critical.

Usually this is only a small part of a air craft weight.

I onced lifted 20 lbs with a 810 mm blade camera Heli with no problem. This was more than the Heli weighed.

I would say in many cases we have and demand much more power than a full scale craft.

Have you ever seen a rc pilot checking density altitude before flying?

Allen Bryan

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01-28-2013 02:22 PM  4 years agoPost 11
Santiago P

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South West, Ohio

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Have you ever seen a rc pilot checking density altitude before flying?
I have, years ago when I had to fly heavy cameras for a job, using my XCell 60. The load was 18 lbs. I could barely hover, but could climb slowly once I pick up 20kts of fwd speed. The approach from 200ft was fun too, but uneventfull. Transitional lift baby!

Had to do the same flying some 28lbs of stuff on a hot day with a Bergen twin. It got worst as the day warmed up. That was one hairy.

Santiago

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01-28-2013 02:50 PM  4 years agoPost 12
gwright

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Champaign Il

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I think maybe tip velocity should be the true comparison. A longranger for instance, spinning at 394rpm with 33'4" blades would have a tip velocity of 468mph. A raptor G4 with 715mm blades at 2000 rpms, which is a pretty common range for a 700 class machine, is only 376mph tip velocity, so if anything the model heli rpms are a bit low, not high, in comparison to fullscale.

Gary Wright

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01-28-2013 02:54 PM  4 years agoPost 13
Ronald Thomas

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Gainesville, Fl, USA

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Think about this. Do full scale heli's fly the same way as do rc's? To actually a 450, I could easily fly it around at about 60% throttle. But what kind of performance would I have? Also if full scale spun their blades at our speeds, they would not last very long and would be slinging off in no time.
+1!!!
Our toys need that extra HS to be "fun" We could easily turn the HS down and reduce the pitch range (and eliminate negative pitch) if we wanted to turn the same super low HS but the performance would completely suck. Full size helis simply do not do what we do with our helis so the needs are very different. Of course you need a higher HS to accommodate the size difference but it is not too far off a FS heli to get off the ground and do basic maneuvers.

Team MikadoUSA 480XXTreme, 550SX, 600SX, 700XXTreme, 800XXTreme!!

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01-28-2013 04:14 PM  4 years agoPost 14
Solmanbandit

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Tucson , AZ

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I think maybe tip velocity should be the true comparison. A longranger for instance, spinning at 394rpm with 33'4" blades would have a tip velocity of 468mph. A raptor G4 with 715mm blades at 2000 rpms, which is a pretty common range for a 700 class machine, is only 376mph tip velocity, so if anything the model heli rpms are a bit low, not high, in comparison to fullscale.
That is pretty wild, good comment.

Trex 700E / Trex 500 ESP - Ikon/ HD 500 - Ikon 2/ Goblin 500 Ikon 2

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01-28-2013 05:28 PM  4 years agoPost 15
Ravenhyper50

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Canada's Capital

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I think maybe tip velocity should be the true comparison.
Bingo!!

Thanks for the doing the math

Cheers,
Raven

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01-28-2013 05:34 PM  4 years agoPost 16
BladeScrape

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Warren OH. US

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BECAUSE

Team JR

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01-28-2013 06:20 PM  4 years agoPost 17
BobOD

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New York- USA

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Right, RPM isn't the measure to look at. Velocity over the blade is what is important. Shorter way around and you need more RPM. RC helis don't really stand out in this at all. If you consider RPM and diameter, as you should, they are not so different.
What does stand out is power to weight. Pitch range.

Team POP Secret

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01-28-2013 07:15 PM  4 years agoPost 18
darren heaver

rrApprentice

rochester, kent, england.

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BobOD can you explain what you mean I'm getting lost on this one not that it takes much

Still can't see why or blades need to go so fast

Cheers

" one day I will learn to fly till then I will just hover"

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01-28-2013 07:41 PM  4 years agoPost 19
chris6414

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Sneads Ferry, NC USA

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http://www.mionome.com/Uni/Helicopter%20Aerodynamic.pdf

Good read.

Century Hawk Sport, OS .32, Futaba 7C, GY-601 9251
Predator Gasser SE G-23, Fut 3010 servos, JR gyro/servo

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01-28-2013 07:43 PM  4 years agoPost 20
BobOD

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New York- USA

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It's the blade velocity that is important. RPM is simply going up as the rotor diameter goes down...to keep the velocity in the ballpark.
RPM isn't important at all if you fail to consider how much distance is traveled in each revolution.
With that in mind, there is nothing significant about the RPM of an RC helicopter. This doesn't make them stand out at all. In fact, as Gwright pointed out, the blade velocity is often higher on full scale.

The power to weight is what makes the RC model more agile.

Team POP Secret

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