How do airplanes go so fast?

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Heat, that's how. Airplanes like the Celera 500L don't have sufficient heat or HP to achieve very high speeds. Nor do electric planes.

We all know that prop airplanes, with a few notable big HP exceptions, do not reach jet like speeds. Exceptions include Reno racers, the Soviet era turboprop bombers and the Piaggio Avanti, all of which can go almost as fast as the slowest jets.

The 3200 HP Avanti can reach 400 Knots, world class fast for a turboprop, but it's normal cruise speed is 350 or less. At jet altitudes, the Avanti is simply in the way. And it has plenty of hot jet thrust from it's two hard working turbine engines.
The swept wing TU-95 turboprop bomber cruises at 380Kts and is said to have a top speed of 500Kts (probably downhill) . It takes 60,000 HP to go this fast, along with the counter rotating props and significant jet thrust from the 4 engines.
The Reno air race planes achieve their speed through raw HP, aerodynamics, and the thinner air of Reno. Even then, they are not jet-fast.

The heat of combustion is used in turbine engines (both jet and turboprop) to create the power to drive the turbines. But the heat is also effectively used to achieve high exhaust discharge velocity. The nozzle's discharge velocity is limited by and to the speed of sound. The speed of sound increases with temperature. So the hotter the exhaust, the faster we can move it through the nozzle.

Electric drive is very cold, with no real free heat. The only real way to get jet like speeds would be to create enough heat to raise the nozzle discharge velocity. One way to do this is to have a multi stage fan. The heat of compression can be used to achieve a high discharge velocity.

F110.jpg
 
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Cujet

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Notice that the afterburner of a fighter's engine is simply a way to increase heat, hence the Brit term "reheat". Mass flow does not increase appreciably with the afterburner operational, as the volume of the afterburner's fuel is minimal compared to the airflow.

But you will notice that the nozzle expands when the afterburner is lit, to accommodate the increased volume due to the increased temperature. The result is that the nozzle maintains a condition where pressure is low and velocity is very high.

That high discharge velocity can push an airplane to remarkable speeds. Something a simple airscrew can never do.

I guess the reason I bring all of this up is to illustrate the fact that aviation remains full of absurd claims, very few of which materialize into reality. The Honda jet is a great example. Lots of claims were made. The reality is that the plane is slow, can't carry much weight and has poor range. Tell me again where the improvements are supposed to be?

Consider the work required to achieve lift. There is no magic here. It takes power to create lift, lots and lots of power. All modern aircraft are very low drag designs. Claims that drag can be reduced by half have little chance of being real. Engineers go bonkers for a 5% reduction in drag.
 
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How to they go so fast? Easy, it's flactuator valves and ball bearings, it's all ball bearings these days.

😁
 
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Astro14

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The pictured exhaust velocity is well above the speed of sound. The “Mach diamonds” are visible in the AB plume.

You can’t get to Mach 2 if the exhaust velocity is less than Mach 2. The net transfer of momentum (air going in v. Air going out) has to be positive, so the exhaust velocity has to be above the velocity of the airplane.
 
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Is it known what the props should have looked like in case of the higher output engines or would some 20 to 30000hp per shaft have meant distribution across the wings?
 

Cujet

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The pictured exhaust velocity is well above the speed of sound. The “Mach diamonds” are visible in the AB plume.
You can’t get to Mach 2 if the exhaust velocity is less than Mach 2. The net transfer of momentum (air going in v. Air going out) has to be positive, so the exhaust velocity has to be above the velocity of the airplane.

Correct, the discharge at a divergent nozzle exit is supersonic. But upstream of that, the airflow has a stagnation pressure. In some engines it's about 22 or 23 psi. This stagnation pressure is the same with AB on or off. This is the pressure the engine feels and is the same with AB on or off. The airflow inside the engine is not supersonic.

Although not common, it's good to know that we can push a craft supersonic without the exhaust being supersonic itself. The exhaust can be sufficiently hot that it's sonic conditions are not reached, yet discharge velocity can be exceptionally high, see example below. Supercruise can be done this way.

At 2550 degrees F, (an achievable temperature) air pushed to Mach 0.99 at a stagnation pressure of about 23 psi will be about 2560 MPH. A divergent nozzle can expand the air (lower the pressure) and achieve even higher velocities. But that divergent nozzle is not always used. Examples of where a divergent nozzle might not be used include disposable propulsion units....

Heat is really the key. Again, I bring this up because I don't want people to think that battery powered airliners or diesel powered craft will compete with today's technology.
 
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Cujet

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Is it known what the props should have looked like in case of the higher output engines or would some 20 to 30000hp per shaft have meant distribution across the wings?

Not sure I understand the question. But it's good to know that modern prop designs are just over 80% efficient. With regard to powerful engines driving props, we can easily get the prop tips supersonic. There is a loss of efficiency when this is done, and a massive increase in noise. Even so, the Russian TU-95 does operate with it's prop tips supersonic.

But as Astro talks about, the "discharge velocity" is key here. So far, I'm unaware of any prop that can achieve a discharge velocity above the speed of sound. Even with multi stage props.

220px-Safran_Open_Rotor.png
 
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I'd only stumbled over the round number of 60,000 actually, as Balandin had planned for up to 30,000 hp each from double-acting two stroke Diesel versions of his engines.
They'd certainly not been after higher speeds, piston engines had soon lost their future with all compounding and other complications, but even so I wondered what the implications regarding props had been then. (The NK-12 also remained one of the largest turboprops, it seems.)
 
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The amazing thing about jet engines is that thrust increases with velocity, for a given (fixed) fuel flow. That means efficiency increases with velocity, which is incredible. At high speeds, the efficiency of a jet can't be beat. But at low speeds, their efficiency sucks. It's not what they're designed for.

However, piston engines on aircraft are highly efficient at slow speeds. Much more efficient than cars. Most props are more efficient than car transmissions (even manual transmissions). A Mooney M20J burns 10 gph at 150 kts (172 mph), which is about 17 statute miles per gallon. Very few cars can go 172 mph, but the ones that can, get less than half that mileage at that speed. And this is comparing 1950s technology (aviation) to modern car engines.
 

Cujet

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Don't need heat to make trust to go fast.


Of course, that's cool for an RC plane, but 290Kph/180 MPH is not fast in the real aviation world (my Cessna Cardinal was flying that fast yesterday). Additionally, RC planes (for the most part) don't have to fly at high altitudes, carry a load or fly for exceptionally long distances. RC planes can have energy dense propulsion systems (battery or fuels) and be otherwise very light weight.
 

ZeeOSix

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^^^ Of course, but the point was that it's thrust that matters, no matter how it's made.
 
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Density is a measure of how much matter occupies a given amount of
space and flying in air offers the least resistance... that is how aircraft
are able to achieve such high speeds relative to water and land...

Density1.JPG
 

Cujet

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Interestingly water injection was used to increase thrust at low altitude by cooling the combustion process and increasing exhaust mass.
That's right, water injection is very effective in turbine engines. It allows an engine to be operated with fueling levels that would otherwise overheat the components, resulting in more mass airflow too. Today's engines are generally designed to prevent water ingestion. The fan blades on the Rolls Royce BR710 engines are designed at the root to direct water (and ice) away from the core engine. In fact a section of the blade actually has a bit of reverse pitch. This is to move water away from the core engine inlet and sling it out into the fan duct.

Here is a pic with the fan blades removed, notice the core engine inlet:

lYwZcw6.jpg
 
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