The Unexpected Genius of Contra-Rotating Propellers

Play the incredible War Thunder game and click my link and claim your extra bonuses: playwt.link/ziroth Will these new innovations in the world of contra-rotating propellers bring them back to more applications?

One way to reduce noise with contrarotating propellers is to have one odd and one even numbered prop so they don't pass each other at the same time. Also, the rearmost propeller needs to be slightly smaller so that its tips aren't passing through the tip vortices of the foremost propeller.

You could've put a huge ad read at the front of the video and I wouldn't have cared... I was too distracted trying to spin my fingers in opposite directions 😂

In our research there are two major sources of noise, blade passage noise and tip vortex interaction. Blade passage noise happens when the wake off of a blade in the front prop strikes a blade in the rear prop. That wake causes an unsteady flow around the rear blade, which causes vortices to collapse and the blade to vibrate, both generating a broadband noise. When the number of blades in the front and rear prop are equal, like the Tu-114, all of the blades pass each other at the same moment and so all emit the noise at once. So the noise comes in two major bands, the high frequency broadband noise from vortex excitation frequency of the prop vibrating and the high intensity tone at the blade passage frequency. The solution is pretty straight forward and that is to make the number of blade on each prop a different number. Usually they only need to be one different, with one more blade on the front prop when the blade count is low, but two different when blade count on the front prop approaches ten blades. Then with unequal numbers of blades, only one set of front and rear blades are interacting at one time. This increases the blade passage frequency and dramatically lowers the amount of acoustic energy in each pulse. Higher frequencies being easier to attenuate and less energy in each sound pulse requiring attenuation make it substantially easier to reduce the amount of noise that penetrates into the cabin. A second mitigation strategy is to add sweep to the blades. In the Tu-114, the blades were perfectly straight, which means that the vortices off the front blade strike the leading edge of each of the rear blades along the entire leading edge at the exact same moment maximizing the amount of energy in that blade passage acoustic pulse. If the rear rotor blades had a bit of scimitar shape to them, then the vortex from the front blades would strike the leading edge of the rear prop at slightly different times and spread the acoustic energy generate over a broader and lower peak energy pulse. Both front and rear blades can have some sweep to them, which is especially helpful when applied to high subsonic propeller aircraft (usually called "open rotor" when the flight Mach number approaches 0.8). To maintain the acoustic reduction benefits, the rear blades just need to have a little more sweep than the front blades. The second major source of noise is when the tip vortex shed by the front prop strikes the tip of the rear prop. This interaction is so strong that it causes the entire rear blade to "ring". The obvious solution is to make the blades of the rear prop just slightly shorter so that the tip vortex of the front blades just passes over the rear blades with no tip interaction noise being generated. This solution does slightly reduce the efficiency of the combined propeller system since the vorticity at the tip of the front blade is not countered by the opposite vortex at the tip of the rear prop blades. But the loss in efficiency is minor and the reduction in noise is substantial. The combination of unequal blade numbers per disk, swept leading edges on the rear prop blades (or higher sweep than the front blades), and the reduction in blade length in the rear rotor combine to substantially reduces the noise from contra-rotating propellers.

When I joined the Navy back in the '70's there were old torpedoes all over the place. Most that I recall had contra rotating propellers that still turned. The clearance between them was tight enough to do damage to an errant finger in the wrong place, a lesson most only had to learn once before knowing enough to not play with them.

Sharrow: "Our revolutionary Toroidal propellers greatly increase efficiency and reduce noise" Everyone: "Cool, can we have some figures please?" Sharrow: "No, go away. Here's a promotional video" Color me sceptical, but I smell a monorail salesman.

Geared reduction drives have proven notoriously difficult on small piston-engine aircraft. Placed between an engine going bang-bang-bang and a propeller with relatively huge rotational inertia, the gearbox walks a fine line between being strong enough to withstand the shocks and light enough to be practical. On electric motors and turbines they may prove much more useful.

now that everything is going electric, i can totally see just 2 motors behind each other turning opposite directions, making the gears unnecessary. 15% higher efficiency is worth quite a bit.

Candela hydrofoiling boats have small, low noise, highly efficient, zero maintenance sea water cooled contra rotating propellors operating right now on their hydrofoils in their C-Pods. Each pod contains two motors, one for each propeller. This seems to me the best and most obvious example of the use of the tech.

a pair of 2 contra rotating propellers would need different aero design each. or the rear one must have lower RPM speed. this is due to how the air get carried into the first propeller's momentum and rotating towards the first propeller direction so it will hit the second propeller with more energy, making the second propeller, albeit it running at the same RPM speed as the first, to seem to have more velocity in relation to the flow coming towards it. and if both propeller have identical design, if the first propeller works the aero perfectly it will be off efficient when the flow hits the second. so the second propeller must have an individual design for its purpose that to deal with the higher velocity coming, and produce efficient flow and thrust. remember, the flow of the air from the first propeller not only faster towards the rear than what the first propeller is feed into, but also rotate the flow in OPPOSITE to the second propeller rotation. i prefer to have a pair of 2 uniformly rotating propeller with each has different aero design purpose. the first propeller act as the "setup" to condition the flow for the second propeller. not to produce power. then the second is acting as the main propulsion which i believe would works better. and the goal design is to produce ultra high RPM without causing any turbulence breaks between the blades. counter rotating propeller in tandem is tricky to make it fully efficient.

I saw a video recently about a new open rotor jet engine being designed by GE and Safran where they have decided to use a hybrid design. Instead of a second counter rotating propeller they use a set of fixed, adjustable blades that act to straighten out the airflow without creating all the noise.

Thank God! The voice of a real person using human diction. Guard your health, please.

The CFM Rise engine solves the mechanical complexity problem by using static blades with a spinning blade behind it. Possibly interesting for a future video?

I'm always fascinated by new (and not-so-new reused) technology. Your enthusiasm for the obscure is palpable. One personal criticism refers to the "soundtrack" - I'm not sure if I'm the only one who hates the use of "interrupted cadence" "music" (deliberately put inverted commas) as it is hugely distracting and harmonically unbalanced, almost to the extent in my case of wanting to close the video. This is, I know irrelevant to the content😢.

The new open fan engine designs for aircraft do have a fixed second row of "propellers", which have some of the same benefits without the complexity of two counter-rotating props.

@ 10:41- I was an electrician for Duckworth Steel Boats in Fl, USA. Wired as 154' Casino boat. But that isn't Fla background. The issue with this prop design is, it is constrained. It's use could be advantageous to trawlers and ferries. This due to inefficiencies elsewhere in the performance curves. That is a lot of rotational mass that eats energy when accelerating.

The dual-motor Dornier Do 335 was an attempt for a practical implementation with one motor at the front and the other, counter rotating, at the end. Being a development of WW2, it was never sent to the battle theater, but test flights showed its benefits and disadvantages.

I would like to see some study in applying contra-rotating propellers in a ducted fan configuration. The ducting might reduce tip vortices or perhaps damage the enclosure and prove disastrous. Either way, it would be an interesting study.

For EV, one might be able to just have two smaller motors on one controller which would keep them synchronized. The caveat being if one prop required more torque the motors may need to be different sizes to account for this which would make balancing everything an extra step at the design stage.

Props👍🏼