Going faster than the speed of sound or breaking the sound barrier was once the dream of every pilot, and Chuck Yeager, a US Air Force pilot, was the first one to do it.
The speed of sound is commonly termed as Mach 1, and the velocities above Mach 1 are considered to be in the supersonic range. Mach1 is termed as the sound barrier due to the sonic boom created by airplanes when exceeding the speed of sound. As you might know, a sonic boom is very loud, and if an aircraft is flying at low altitudes, it can even shatter the glass windows of buildings on the ground.
Now, Chuck Yeager broke the sound barrier while flying an experimental rocket-powered aircraft, the Bell X-1.
But is breaking the sound barrier possible in propeller planes?
Propeller planes can probably not break the sound barrier since the propeller, for the airplane to go faster than the speed of sound, must go even faster. This will inevitably cause shock waves powerful enough to even break the propeller. In addition, as soon as a propeller approaches supersonic speeds, its efficiency dwindles.
Propeller planes can be categorized into two distinct categories, namely:
- piston-engine aircraft
- turboprop aircraft
Although both types of planes cannot reach supersonic speeds, the turboprop aircraft have greater available thrust, are bigger, and can fly faster than smaller piston-engine aircraft.
With that said, let’s explore this topic a little further!
Table of Contents
Why propeller planes cannot go supersonic
The limitations imposed on the speed of propeller-powered airplanes have much to do with their basic ingredient, the propeller itself.
The propeller is an aerofoil, much like the wing of an airplane, except the lift it generates is not vertical; it is horizontal, in the direction of flight of the airplane.
As you probably already know, the propeller is rotated by the force generated by its engine, and scoops through the air, pushing it forward.
We can safely assume that during level flight, the airspeed experienced by the propeller blades themselves is, therefore, greater than the airspeed on the airplane it is mounted on.
The airspeed of the propeller blade
So, as said, the airspeed of propeller blades, due to their rotational velocity, is much higher than the airspeed of the airplane being flown by them, in level flight.
The airspeed varies for different sections of a propeller as well. For example, the airspeed is highest at the tip of a propeller owing to greater distance traveled by the tip in each rotation.
This is why supersonic airspeeds can be achieved by the tip of propeller blades before the rest of the sections.
Similarly, the propeller blades can achieve supersonic speeds while the airplane is flying in the subsonic velocity range.
So, how are propellers impacted by supersonic speeds?
The airflow around the propeller changes its behavior above the speed of sound, and as sections of a propeller blade reach these speeds, small shockwaves are generated, disturbing the airflow. These shockwaves reduce the amount of lift being generated by the propeller and therefore make it inefficient.
In other words, the lift or propulsion generated by a propeller reduces as it experiences supersonic airflow.
In addition, the shockwaves generated at the sound barrier are quite powerful and on top of the sonic booms, they cause a lot of stress on the propeller itself.
So, as we have read above, a loss of efficiency in producing sufficient thrust, in addition to the loud and powerful sonic booms coming from the propeller, presents significant challenges in making a propeller-powered airplane supersonic.
But that has not stopped the scientists and researchers from trying to find a novel solution to break the sound barrier. Many experimental military propeller airplanes have come quite close to achieving supersonic speeds. Let’s have a closer look at this!
How fast can a propeller airplane go?
Propeller airplanes, even if they are not supersonic, are quite fast and economical. However, we cannot compare the small piston-engine airplanes in this category with the much more powerful turboprop-powered airplanes. It would simply not be fair to compare the two.
With that said, let’s compare them separately!
Piston Engine Powered Propeller Planes
Piston engine-powered propeller airplanes are typically small general aviation aircraft, which are quite economical in terms of their maintenance and operational costs. These aircraft fly well within the subsonic range.
However, some modified piston engine planes used in airplane racing and sports are quite fast. For example, in 2017, a P-51 Mustang aircraft named Vodoo reached a speed of 855km/h, and to put that in perspective, this speed is within the cruising airspeed range of a modern commercial turbo-jet airplane.
Other records of piston engine powered aircraft have also been recorded such as a Grumman F8F Bearcat reached 850km/h in 1989 and P-51C Mustang flew a 2000km circuit at 720km/h in 1948.
Turbo-Prop Engine Powered Planes
A turboprop engine is a jet engine that primarily generates thrust by rotating a large propeller mounted in the front. Jet engines can generate a higher amount of torque as compared to piston engines and therefore, bigger propellers can be mounted on them.
Turboprop engines were widely utilized in military applications but these days, they are used more in regional commercial airliners and some modern military Unmanned Aerial Vehicles like the MQ-9 Reaper.
The fastest turboprop airplane is an experimental XF-84H Thunderscreech aircraft that has reached a maximum speed of 1002km/h (Mach 0.83).
In comparison, a widely used commercial turbo-prop airliner such as the ATR42 has a cruising speed of 550km/h (0.44 Mach).
Propeller powered airplanes, whether piston engine or turboprop, are considerably slower than their jet engine-powered counterparts. On the other hand, they tend to be more economical.
So, what are the key differences between these airplane types?
What are the differences between propeller planes and jet airplanes?
Before diving into the differences between these two types of airplanes, we must acknowledge their commonality which is safety. Both these types of airplanes vary in terms of their design, speed, service ceilings, and cost but they are inherently safe.
In modern commercial operations, propeller-powered airplanes are utilized for short-haul flights typically less than 500km long.
Due to their fuel consumption and maintenance costs, these airplanes become the weapon of choice for airlines operating short-haul flights on airports having shorter runways.
So, why is this?
Cruising Altitude and Economic Viability
Commercial aviation is driven by profitability and economic considerations. One of the biggest expenses borne by airlines is the fuel cost. The flight profile of airplanes is designed to maximize the cruising phase where the maximum fuel economy can be achieved.
Jet planes fly at cruising altitudes greater than 30,000ft to achieve maximum engine efficiency. Climbing above this cruising altitude and maintaining level flight for longer periods is not possible on short-haul flights (less than 500km).
On the other hand, propeller-powered planes tend to have lower service ceilings as well as cruising altitudes.
For example, an ATR-42 airplane has a cruising altitude of 20,000 – 25,000ft. Therefore, airlines find it more economically feasible to operate propeller planes such as the ATR-42 or ATR-72 on short-haul routes.
Another benefit of propeller planes is that they generally require a shorter runway length for takeoffs and landings, which makes them the ideal choice for the small, regional airport.
Speed
In general, jet planes are faster than turboprop airplanes owing to the higher thrust ratings of the jet engines. Additionally, the propeller of turbo-prop planes offers additional drag which contributes to their loss of speed.
We now understand how turbo-prop powered airplanes and jet planes are different and more or less suited for long and short-haul flights.
However, which of the two would be more fuel-efficient if compared side-by-side?
Are Turbo-Prop engines more efficient than Turbofan engines?
Comparing fuel efficiency of propeller planes and jet planes is quick tricky in the sense that they are designed around different mission profiles. As we saw earlier, we can
With that said, let’s instead compare the fuel efficiency of the two engines to each other.
Turboprop engines are generally more fuel-efficient than turbofan engines in similar operating conditions because the propeller can displace a greater amount of air (greater bypass ratio). However, at higher altitudes, as the air gets thinner, turbofan engines are the best option.
Summary and Conclusion
- Propeller powered airplanes have come a long way since their invention by the wright brothers. Although these planes saw significant technological advancements during and after the world wars, they remain subsonic airplanes.
- It is not practically feasible to break the sound barrier in level flight flying a propeller plane, as the propeller blades become less efficient at supersonic speeds. Since sections of a propeller plane reach supersonic airspeed before the airplane itself; it becomes impractical, if not impossible, for a propeller plane to go supersonic.
- Propeller powered airplanes can be categorized into piston engine-powered and turbo-prop airplanes.
- Although commercial propeller planes are smaller, slower, and fly at lower cruising altitudes when compared to jet planes, they are employed as regional airlines for short-haul flights due to lower operating costs.
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