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  • 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.

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  • As usual, thanks for watching and thank you to my incredible Patreon supporters.

  • I just opened a Discord server for my Patreon supporters where we can chat and share ideas

  • for videos.

  • The links to my twitter, instagram and facebook pages are below too.

If you asked most of your friends which was faster.

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B1 US helicopter speed lift top speed blade rotor

Fastest Car vs. Fastest Helicopter - Which is Faster?

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    joey joey posted on 2021/06/07
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