Ever wonder why some helicopters have a little fan in the back?

[Beetlegeuse]

It's there to keep this from happening:



The engine applies force (in the form of torque) to the rotor shaft, which turns the rotor head, which turns the rotor blades. Turning rotors generate lift which makes the whole contraption fly.

The hitch is that once the 'copter gets in the air, that torque works both ways. Newton's third law states that the torque doesn't just make the rotors turn in one direction, it also makes the fuselage want to rotate in the opposite direction and with the same amount of force.

The tail rotor counteracts that tendency by blowing in the direction opposite to what the fuselage is trying to turn in.

Until it doesn't.

This particular incident was a forest firefighting operation in China using a French-designed helicopter flown by Chi-Com army pilots. The gearbox didn't freeze up (you can see in the video that the tail rotor is still turning) but it could have just been freewheeling. It might have been that the tail rotor drive shaft failed or the pitch control mechanism on the rotors failed or froze up.

This illustrates why "long line" sling loading is one of the highest paying jobs in helicopter aviation. Because it's severely dangerous to be hovering that high off the ground with a basket of heavy goodies dangling below you on a hundred foot cable.



There's a lot of call for this kind of work in tall timber logging operations, like in the western US and Canada, because the 'copter can extract trees that have been felled while still hovering above the tops of the remaining trees. But you're not long for that line of work unless you have ace flying skills and balls of steel. And a little good luck wouldn't hurt.

[Fluidic Kimbo]

This particular incident was a forest firefighting operation in China using a French-designed helicopter flown by Chi-Com army pilots.

China isn't of consequence. Chinese people don't matter.

I lived in Asia for two years, and even the Asians see China as a fucking dump.

Human sewerage.

[Honkey_Kong]

China isn't of consequence. Chinese people don't matter.

I lived in Asia for two years, and even the Asians see China as a fucking dump.

Human sewerage.

Come on, let's not be bigoted. There are some good people in China and some hot Chinese women. They have no say in what their government does. They really just want the same things in life that everybody else wants. Financial security, friends, professional success, family, love, hobbies, sports.

[Fluidic Kimbo]

Come on, let's not be bigoted. There are some good people in China and some hot Chinese women. They have no say in what their government does. They really just want the same things in life that everybody else wants. Financial security, friends, professional success, family, love, hobbies, sports.

Some things are pervasive among particular communities.

The Chinese as a community of people with a culture don't have much of a sense of justice or dignity. Remember a few years ago the video that came out of China of the child run over on a street by three vehicles in a row while bystanders did nothing. How many places in the world would that happen?

They're the worst country for not brushing their teeth -- worse than all of Africa.

I don't deny that a select few people from any backgroud can turn out well, but still some behavioural traits are pervasive among some communities.

China = Sewer
The Chinese = Human Sewerage

[almostgone]

That's the beauty of the Sikorsky Raider. Coaxial main rotors with a pusher prop. Top speed over 260 knots.

SB-1 Defiant looks promising as well!!!


Edit: The tradeoff is the increased complexity of the rotor hub and God forbid the rotors cross paths. It tends to exhibit the aerodynamic characteristics of a falling bank safe.

[Beetlegeuse]

Helicopters have a serious problem with the speed of sound because the tips of the rotors are going so damn fast, even if the helicopter isn't. A UH-1 Huey at a stationary hover has a rotor tip speed of about 0.8 Mach. Which also explains why 'copters are so much less efficient than airplanes, because the "wings" are encountering the drag of flying 600 mph+ and you're not even going anywhere yet.

Before the tips of the rotor blades ever get to Mach 1.0, they're certain to cross the lower critical Mach number and a shock wave will set up. You might have noticed that all airplanes designed to exceed the speed of sound have short, stubby wings, whereas a helicopter's wings are long and spindly. Long and spindly doesn't do well in the presence of a shock wave and might even come apart. But if your rotors manage to hold up under the strain, the next thing you've got to worry about is Mach tuck, which is certain to create major control problems (unless you throttle back).

But most conventional helicopters will never get that fast because they'll encounter retreating blade stall before the rotor tip speed ever reaches the lower critical Mach number. Yes, helicopters can stall, too, but for the opposite reason as an airplane. Airplanes stall from going too slow. Actually, they stall from the wings exceeding the critical angle of attack, but the slower you go, the less lift the wings produce, so the more you have to increase the angle of attack in order to maintain altitude. Go slow enough and eventually you'll be forced to exceed the critical angle of attack in order to prevent descending. And when you do, the boundary layer, that lovely, impossibly thin layer of air in a state of laminar flow will separate from the surface of the wing, inducing what is known as "a stall."

Conventional (single rotor) helicopters stall because the rotor blade (which is a sort of wing) on one side is flying into the wind (advancing) while the wing on the other side is flying away from the wind (retreating). Lift varies directly with wind velocity over the airfoil so in forward flight, the advancing wing will tend to produce more lift than the retreating wing. This creates an imbalance (dissymetry) of lift and if nothing counteracted this imbalance, it would cause the rotor disk to tilt downwards toward the side with the retreating wing, resulting in the wings banking and causing the a/c to turn in the direction of the retreating wing.

To correct that imbalance, the rotor head is engineered to force the retreating wing to have a greater angle of attack than the advancing wing. Greater angle of attack means more lift and the imbalance is corrected. The 'copter will now fly straight without the pilot having to wrestle with it.

Problem solved, right? Except if you keep speeding up, that retreating wing eventually will be forced to exceed its critical angle of attack in order to produce as much lift while flying away from the wind as the advancing rotor is creating flying into the wind. And a wing that exceeds its critical angle stalls.

That is a thumbnail sketch of the engineering design challenges rotary wing aircraft pose. And that's precisely why older design helicopters, like the Huey, with a 2-bladed rotor and a "teetering" (as in teeter-totter) rotor head, can only go about 140 mph, it doesn't matter how big an engine you put on them. Because even if the a/c is light-loaded and the performance altitude is perfect so you can sneak past retreating blade stall, there is no sneaking past the speed of sound and the transonic shock wave.

In fact the only way to avoid the shock wave is to keep the rotor tips as far below the speed of sound as possible.

The beauty of contra-rotating coaxial rotors, like on the Sikorsky Raider, is that there's always an advancing blade and a retreating blade on both sides of the airplane. No dissymetry of lift, ever, so you don't need to let the rotor head teeter, and it's immune to retreating blade stall to boot.

But there's still the problem of rotor tip speed and that pesky critical Mach number thing, but the Raider delays the onset of that problem by means of variable gearing in the transmission that lets the rotors turn more slowly as the a/c goes faster. On a conventional helicopter, the main rotors have to provide both lift and thrust, but the Raider has a pusher prop for thrust. So the pusher prop and the variable gearbox lets them slow the Raider's rotor RPMs so its rotor tips don't reach the lower critical Mach number until the a/c reaches a much higher airspeed. Which is why the Raider likely will become the fastest production (non-experimental) helicopter in the world, and is damn near as fast as the tilt-wing V-22 Osprey.

But there's no getting around that efficiency problem (yet) and the Raider is an absolute PIG, just 240 knots on 2600 shp. A Mooney Acclaim will do 240 knots on its teenie little 280-bhp recip engine. Granted, Mooneys admittedly are uncommonly fast but that's near as makes no difference the same speed on 1/10th the horsepower. Because when the Mooney is making 275 mph, the airspeed over its wings is ... wait for it ... 275 mph. At the same airspeed, the Raider's (advancing) rotor tip speed probably is getting close to 700 mph, and aerodynamic drag changes with the square of the change in airspeed. So when they're both running at full chat, the Sikorsky's wing drag is about 6x as high as the Mooney's.

And it runs in the family. A Huey only gets about 130 mph from 1100 shp. Because helicopter efficiency is teh suck.

But nobody ever said VERTOL was going to be cheap (and if they did, they were lying).


Ain't helicopter flight physics some weird shit?