If you asked most of your friends which was
faster. The fastest production car in the world or
the fastest helicopter, I think most of them would guess the helicopter. Intuitively we expect anything flying in the
air to have a higher top speed than anything on the ground could achieve, but physics is
a cruel mistress and conventional helicopters are doomed to a max speed of just 400 km/h,
with that record being set over 30 years ago by John Trevor Egginton in a Westland Lynx,
while just this year the Koenigsegg Agera RS demolished that record driving at 445 km/h,
and sure you don’t have to worry about traffic in your helicopter or those pesky speed suggestions
on the road, but the bragging rights for top speed will always go to cars for the rest
of time. So how can this be, what quirks of physics
are limiting helicopters from flying faster? First let’s look at what limits a cars top
speed. To determine top speed, we first need to identify
the forces attempting to slow the vehicle down. In space, with no resistance, even a small
force can continually accelerate an object until it reaches close to the speed of light,
if it is maintained for long enough, which would require a silly amount of energy. On earth though, every time we provide energy
to our vehicle, resistance in the form of air resistance and rolling resistance in the
wheels are sapping it away. Eventually we get to a point where the energy
we are providing the vehicle equals the energy being taken away, and the vehicle cannot travel
any faster. In the case of cars the top speed is predominantly
determined by air resistance, for now we will ignore rolling resistance as it’s negligible
in comparison. The equation for drag force is given by this
equation, and the equation for power is simply force times velocity. Rearranging these variables we get an equation
for top speed. Applying this to the Agera RS specs, we find
it’s top speed almost perfectly with a decent degree of accuracy, considering we ignored
rolling resistance. Decreasing our drag coefficient and frontal
area also increases top speed, but to increase power we need to increase airflow to cool
the engine, so this is a difficult balancing act. We also discovered in a previous video that
designing rubber tires capable of withstanding these rotational speeds is an incredibly difficult
task, with the Bloodhound SSC opting for aluminium wheels to break the land speed record. These are the limiting factors for a car,
so what are the limiting factors for a helicopter. Helicopters have to deal with all the same
problems as cars in counteracting aerodynamic drag, but first a helicopter needs to overcome
the force of gravity, using the same rotor that will need to provide forward thrust. So is this just an issue of needing engine
power? Partially yes, The Westland Lynx was powered by not one but
two Rolls-Royce Gem turboshaft engines, each equaling the power of a single Agera RS twin-turbo
V8 engine. Yet, even with twice the power, it still can’t
beat it in a straight line race. How can this be? Let’s go on a journey as the helicopter
takes off and transitions to forward flight, and see why it can’t go any faster even
with stronger material for blades or more powerful engines. As the Lynx powers up the blades begin to
rotate faster, providing more lift according to this equation. Where A is the swept area of the blades, Cl
is the coefficient of lift of the blades and v is the velocity of the blades. The coefficient of lift depends on a lot of
things, like the geometry of the blades and angle of attack, but for the sake of simplicity
we will assume it is constant. Once the lift is greater than the weight of
the helicopter it begins to rise, and when it equals the weight the helicopter will enter
a hover state. Now to go forward, the pilot will need to
transition some of the lift to thrust by angling the rotor disk forward. But here we meet our first problem. Looking downwards, we can see that left-side
of our blade is moving backwards relative to the the direction of travel, and our right
hand side to moving forward relative to the direction of travel. Similar to planes, the aerodynamic surface
of the blade will generate more or less lift depending how quickly it is moving through
the air. So our right side generates more lift that
our left. To counteract this the rotor integrates an
ingenious little mechanism, where the blade can change its angle of attack as it rotates. Here the advancing blade will have a lower
its angle of attack, thereby lowering it’s lift, and the retreating blade will increase
it’s angle of attack, increasing it’s lift. This helps equalise the lift across the rotor
disk, but this solution has it’s limit. As we increase an aerofoils angle of attack
the lift increases, but eventually he hit a point of where flow separation occurs and
the aerofoil begins to generate less lift. This is our first speed limit. The helicopter will eventually hit a speed
where it cannot adjust the angle attack any further to compensate to dissymmetry of lift. However there are solutions to this problem. If we have two blades rotating in opposite
directions, we no longer have to compensate for this dissymmetry in lift, as the two blades
will have the opposite dissymmetry of lift and cancel each other out. This is what allows the Chinook to cruise
along faster than any other military helicopter at 315 km/h. Now we have overcome one speed limit, but
even now the Agera RS is still speeding ahead .
The next speed limit we reach is the sound barrier, if we continue to increase forward
velocity, the tips of the advancing blade will eventually break the sound barrier. And while planes can handle breaking through
the sound barrier with the correct design, they don’t need to pass through it several
hundreds of times a minute. This adds to the problems of dissymmetry of
lift, but also causes problems with varying stress that fatigue the material of the blades
and ultimately lead to failure. This speed limit poses a more difficult challenge
to overcome in our current configuration. To travel faster, we need to increase thrust,
to increase thrust we need to increase lift. Let’s take a look at the equation from before
to see how we can do that. We can either increase our rotor speed, or
we can increase our blade diameter. Increasing the blade velocity will obviously
make us more likely to break the sound barrier, but so will increasing our blade diameter,
as the velocity of the blade increases as we travel down the blade. Designers have hit an optimum balance between
these two variable already, so that’s not an option for increasing speed. So how else can we increase helicopter speed? By converting more of that vertical lift to
horizontal thrust, but this poses a new problem, at some point were are going to hit a point
where the helicopter is not generating enough lift to keep itself aloft. The solutions to this problem are hitting
a point where the we can scarcely call the aircraft a helicopter. Enter the Eurocopter X cubed, which holds
the unofficial helicopter speed record, unofficial exactly because it is not strictly a helicopter,
as it generates a large portion of it’s forward thrust from vertical propellers. This decreases the burden of the horizontal
rotor to generate forward thrust allowing it slow its rotational speed, minimising the
impact of our previous speed limits, the rotor can also decrease it’s rotational velocity
as the helicopter gains speed as more lift is generated from two small wings on either
side of the helicopter. These design choices allowed the x cubed to
reach top speed of 472 km/h, beating the previous record held by the Sikorsky X2, and demolishing
anything ever achieved by a production car. Pushing this design ideology even further
we completely blur the lines of what a helicopter is, with tiltrotors, like the Osprey. This aircraft has a top speed of 565 km/h,
this combined with it’s incredible vertical take-off capabilities, that are not hindered
by limited weight issues like jet engine powered aircraft like the Harrier, has made it an
incredibly versatile tool for the US military. Another incredibly versatile tool that could
give you some additional lift, is Skillshare, all the animations and illustrations you saw
in todays episode were created by me, but just 3 years ago I knew nothing about animation. I taught myself the basics during one Chinese
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skill to gain. As usual, thanks for watching and thank you
to my incredible Patreon supporters. I just opened a Discord server for my Patreon
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Funfact: Elon Musk's cars 0-60 times are quicker than his rocket's 0-60 times. (Of course the 0-1000 times may be different)