I wonder why?

[David Drew]

I wonder why, when you watch a real heli it's rotors go around so slowly that you can almost see them turn - or actually turn on a Chinook - but an RC heli needs the blades turning much faster.

Is it something to do with torque & pitch? I guess there must be some relationship between weight,rotor blade area, pitch and rotor speed.

Experts please chip in

Cheers

David

[cambs flyer]

I think it's just the simple case of shear size mate.

I remember seeing a Rappy 30 tail in hover about 12" off the deck and as you say they seem to be going almost too slow to create lift.

Compared to my 450 that goes like a nutter to create the same lift

It's a bit like if there was for example an 8" rota head compared to a 10" rota head, the 10" would have something in the region of 33% more lift, as the very outer end of the bades even though it's only 2" longer adds a massive difference to the overall extra lift capability.

It's all about the shear volume of air larger blades can shift at lower revs compared to smaller rotas which have to work twice as hard as they haven't the extra surface area.

Can you see what I'm getting at here, and not making a good job of it

Sorry mate

[jamiesawyer28041978]

I may be miles out here david but eg . look at a cars wheels when it accelarates they appear to speed up then slowly stop and almost appear to go in the oposite direction . This may have something to do with it . Or just weight to OH f;;k knows. out of my deapth here me thinks.LOL. Prime example why i'm a Plumber

[David Drew]

Hear what you're saying, but look at the blade width of an RC heli & that of a real one. They are reletively bigger and real heli blades look shorter too. Unless those are both optical illusions.

I'm waitng for some egghead to post a formula of speed, lift, rpm etc. There must be one out there.

David

[DemonJim]

For one thing you need to ensure the tipspeed isn't above the speed of sound, as you don't want a sonic boom to hit the blade, or probably even worse for one to keep whacking the leading side in forward flight (if a heli moves forward at 100mph there is a 200mph difference in airspeed at each tip as it goes round, so if when static the tipspeed was 700mph it would keep going above and below the speed of sound, ie rapidly from 600 to 800mph).

So the smaller the diameter the faster it can spin. I imagine it's mostly down to lift efficiency though, as a larger blade has more lift and has to displace more air.

Or something.

[David Drew]

Or something.

LOL - for a moment I thought you knew what you were talking about. LOL

[solutech]

Its all to do with your eyes and how the brain processes the images . Its a sort of stroboscopic effect .

Then again it could be some secret military design to stop us looking for ufos instead

[David Drew]

I know some members have worked on Chinnoks or other helis. They will know the rotor RPM of them. Then we can compare to say 2000 rpm on an RC

[Boggy]

i know from my navy days that the rotor speed of a wessex mk5 was 230rpm. not quite upto trex 450 standards!

[machasm]

Perhaps they all have the same blade tip speed(or close to it).
A larger helicopter will have a larger diameter rotor so will turn slower to have the same tip speed as a smaller heli (450).
Just a thought.
I am off to research this a bit more.
Adrian.

[andylinney]

I think some of it has to do with our expectations of performance too, the comparative power and agility of a Wessex or Chinook is positively stationary compared to what we expect of an RC heli. You can imagine a Chinook climbing out at the same scale rate as a Trex 450 pulling 12 degrees at 3000 rpm - it'd rip itself apart.

If I remember rightly from cadets, the increase in lift with the increase in surface area is square so it's a lot more lift from a much bigger blade - I can't imagine the blades would be symmetrical either - more likely to be high lift airfoils designed to give maximum shove at an optimum speed. I could be very wrong however....

Equally, we can get a RC heli to fly at silly low headspeeds and massive pitch angles but the stability suffers and the motor is labouring like crazy....

[163phil]

It's all to do with the fact that air behaves as a fluid, albeit a very "thin" one. It's the reason why bees buzz and eagles don't!

andylinney's absolutely right, to expound:

In order for a helicopter to hover the blades must produce exactly the same amount of lift as its weight. If it were to climb, it must produce more lift. There are fundamentally two ways in which it can produce this lift: by angle of attack (the angle at which the airflow meets the wing (blade), or "pitch"), and the speed at which the wing moves through the air. (We can ignore the effect of height as the air pressure decreases, as we are only really concerned with low levels).

There is a mathematical formula for this which states that "the amount of lift produced is proportional to the coefficient of lift (a "fudge factor" peculiar to each different wing section), multiplied by half the air density, multiplied by the square of the speed, multiplied by the wing area."

Therefore if we increase the pitch of the blades, the headspeed will have to reduce in order to continue in the hover, or else the helicopter will climb.

Ron is correct in stating that the headspeed of a full size heli is about one tenth the speed of a model - it is a) alot bigger and so displaces alot more air, and b) is hugely underpowered compared with a model. Its blades are operating at a higher angle of attack than a model (I'm guessing generally at somewhere around 10 degrees, and will stall at somewhere around 15 degrees or so). In the hover a model's angle of attack is probably only 3 or 4 degrees, unless you are running at a very low headspeed.

Going back to the maths, because the speed is a squared function, a relatively small decrease in the wing area will lead to a massive increase in the speed required in order to provide the lift needed.

Generally the smaller the body passing through the air, the more that air takes on fluid properties relative to that body.

...but really........it's MAGIC!!

[Nige321]

Here you go...

http://en.wikipedia.org/wiki/Reynolds_number

Scroll down to:-
Example of the importance of the Reynolds number

Nige

[David Drew]

You guys are good. Now I know.


David

[trillian]

Yeah, as was stated above, the air we all have to fly in stays the same. So when you scale up your wing area grows exponentially.

The air density stays the same so the impact this has on scaling down is that you have a constant balance to strike between wing area, weight and power.

A UH-1 'Huey' has a 48 foot diameter main rotor that turns about 300 RPM. (Incidentally the tip speed of the advancing blades in forward flight can break the sound barrier but the sonic boom generated is very small).

So between the huge relative wing area against the air density, it gets plenty of lift at only 300 RPM. But they had variations of the UH-1 ranging from 770 to over 1000 horsepower. Compare that with a .50 size glow engine that produces close to 2 horsepower.