Are coaxial propellers more efficient?

Description

Title

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The effects of coaxial propellers for the propulsion of multirotor systems

Date Created

2017

Other Date

2017-10 (degree)

Subject

Extent

1 online resource (vi, 53 p. : ill.)

Description

Multirotor vehicles offer access to the skies for users across all walks of life and industry due to their simplicity, availability, and low cost. Although advancements continue, developers are slowed by the limits of available propulsion systems. Coaxial rotors stack propellers over one another to provide more thrust without increasing a vehicle’s footprint nor battery voltage. Previous investigations studied the thrust lost to coaxial rotor systems, wherein downstream propellers produced less thrust than their predecessors. This experimental study examines the effects of propeller spin direction, separation distance, motor speed, and propeller pitch to explore different methods of recuperating thrust losses. During testing, each propeller’s thrust, current draw, and rotational speed was measured. Results show that for 13-inch propellers, reducing the distance between the planes of rotation from 8 to 2 inches produced variations up to 123 grams of thrust, representing a 4.5% improvement. Controlling the motors’ speeds independently confirmed that a coaxial pair will provide thrust most efficiently if the back (downstream) motor is operated at a higher throttle setting than the front (upstream) motor. Similarly, a coaxial pair will provide more thrust if the back propeller’s pitch is higher than the front propeller’s pitch. This was applied to the effect that the back propeller in a coaxial pair provided 119% of the thrust of the front propeller. This allowed for a coaxial pair’s thrust to range between 1790 and 2530 grams, allowing for a 41% increase in thrust from the worst case to the best case. When varying propeller pitch was applied to six different arrangements of four propellers, maximum thrusts ranged between 2960 and 4010 grams. One of these coaxial quadruplets was tested to provide a total of 401% of the thrust of its front propeller.

Note

M.S.

Note

Includes bibliographical references

Note

by Jonathan Elliot Holzsager

Genre

theses, ETD graduate

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Persistent URL

Language

eng

Collection

School of Graduate Studies Electronic Theses and Dissertations

Organization Name

Rutgers, The State University of New Jersey

Rights

The author owns the copyright to this work.

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I think what you'd ideally want to do is to have a fast-rotating "inner" propeller and a slower "outer" one, or actually a propeller whose RPM gradually decrease from the inside to the outside. Although that would look cool, it doesn't work with non-fluid materials for the rotor :o) So, the next best thing is to have two behind each other, where the downstream one spins a bit slower.

How I understand your proposition

Counter-rotating props do have the advantage that the second one can "straighten" out the flow from the first, effectively working similar to (but not entirely like) a rotor/stator couple in a turbojet. This means they can transfer more power per rotor area than single rotors. By putting the slower-spinning one behind, this couple will leave less swirl in the flow than the other way round, which sounds like a good idea to me.

Difficulties with regular counter-rotating rotors

In a counter-rotating rotor couple, the second rotor is exposed not just to the mean swirl coming from its upstream partner (for which it is designed, and which can increase its effectiveness), but also the pressure signatures and wakes coming off each blade. Every time a blade passes through one of these, the stagnation pressure at its leading edge goes down very sharply and then increases again. That makes a lot of noise, and also causes vibration in the blade, which then has to be made strong enough to deal with this. This is also the reason why most such rotors have swept blades like these:
Antonov AN-70 with swept counter-rotating props; image from https://wordlesstech.com/revolutionary-airplane-propeller-action/

This means that at no moment will the entire blade of the downstream rotor be in the wake of an upstream blade, but it will gradually pass through it. Still the blades have to be quite sturdy, which also prevents them from being very long. That, in turn, limits the ability to increase efficiency by making long, slender blades which produce less thrust per rotor area but make up for it in radius (like wind turbines, for example). That's one reason why most such configurations these days are seen in military airplanes where large elegant propellers would reduce maneuverability but lots of thrust is needed, pronto. Also, these machines are built very robustly anyway, so the added vibrations can be dealt with. In a passenger aircraft, the noise alone would be hard to sell to airlines, but developing the mechanical components would also not be easy. At the moment it seems that only Antonov has experience with large configurations of this type.

That said: These concepts keep cropping up, and it seems as if it should be possible and efficient to use counter-rotating rotors with strongly swept blades to replace turbofan engines at speeds that would seem a bit high for regular propellers (but maybe a bit low for turbofans). Here are two sources I just found which give you some idea of what the flow looks like, and what types of aerodynamic and acoustic problems the designers have to content with:

The problem with having a larger downstream prop

You can also see in the references above that usually the downstream blades are actually shorter than the upstream blades. The reason is that otherwise the tip vortices from the upstream propeller would hit the downstream one, and that's acoustically, aerodynamically and structurally not nice. Even without those vortices, it would make sense because the downstream prop gets more vibrations either way, so it has to be sturdier (shorter, thicker blades will help!). I think some of the existing configurations (the AN-70 above for example) do have equal blade lengths, but they are very much known for being loud, and any new civil application cannot have that.

The most recent example of a counter-rotating open fan that has actually been built and tested is the SAFRAN open rotor:
photo by Eric Drouin / SAFRAN SAFRAN claims that the design can fly at Mach 0.8 and is about as loud as a comparable turbofan engine from the previous generation (i.e. still louder than the latest turbofans, but not horrible, either). As you can see, the downstream rotor is also shorter in this configuration.

Noise, by the way, is also one reason why most configurations under discussion (with some exceptions, because there always are...) are rear-fuselage mounted rearward-facing rotors: That puts the main source of noise a good way downstream of the passenger cabin, instead of right next to their windows (which is loud enough with regular props already, as any passenger in a Saab 2000 can confirm). It also has the benefit of preventing accidents where a blade breaks off and hits the passenger cabin ...

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