I wonder why?

[trillian]

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!!

It's not so much being underpowered as 'overweight' because what you really start to fight is inertia in terms of changing direction etc. Then you start to be up against the tensile strength of the materials, which is where you gain a big advantage when you scale down since tensile strength is also constant.

Going back to the UH-1 as an example, one version had an empty weight around 5000 lbs
and 1440 shaft horsepower. That gives a power to weight ratio of 1/3.47 or 3.47 pounds per horsepower. Compare this with a Raptor 30 with an OS32. Weight about 7 lbs and about 1.3 horsepower. That's a ratio of 1/5.38 or 5.38 lbs per horsepower. Yet the Raptor has performance a UH-1 could only dream of because the Raptor benefits from the relative low inertia/air drag and super-high tensile strength, it can change direction without ripping itself apart, not to mention crash with only broken blades and a few bent parts.

The headspeed of full scale helis in general is closer to 1/5th or 1/6th of a model because some are running over 400 and close to 500 RPM (Hughes 500 for example runs up to 497 RPM).

[163phil]

It's not so much being underpowered as 'overweight'

Agreed! In this context, and the relatively simplistic terms in which I was trying to get the point across, the two factors are effectively the same thing, or at least have broadly similar effects

Perhaps I should have put "....underpowered/overweight compared with a model"

My last comment did try to keep it lighthearted!!

[trillian]

Agreed! In this context, and the relatively simplistic terms in which I was trying to get the point across, the two factors are effectively the same thing, or at least have broadly similar effects

Perhaps I should have put "....underpowered/overweight compared with a model"

My last comment did try to keep it lighthearted!!

Yeah, it's a phenomenon of scaling up or down. You have things that change like size, weight, power etc. and things that remain constant like air density and strength of materials. I have always found it fascinating.