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Author Topic: Are large propellers inherently more energy efficient?  (Read 931 times)
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aeronautics
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« on: May 16, 2018, 07:14:34 PM »

If the goal was to lengthen the endurance of an existing aircraft and you had the luxury of using a propeller twice in diameter, would you to improve efficiency? Roughly how much greater efficiency (i.e. air time) would you have? The following conditions would apply:

1) You don't have to worry about physical interference, i.e. blades won't hit the ground or any part of the aircraft. No problem with take off or landing with the 2X larger propeller.

2) You can have a redesigned engine or electric motor to optimize efficiency to have higher torque at lower RPM. Along with the 2X diameter propeller you would also have the ideal engine or motor to match.

3) Aircraft speed remains the same.

4) There are no other issues due to the 2X diameter propellers, i.e. climb rate, instability, etc.
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flydean1
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« Reply #1 on: May 16, 2018, 09:34:37 PM »

In a word:  Yes
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Tapio Linkosalo
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« Reply #2 on: May 16, 2018, 11:43:53 PM »


Freude's (sp? ref: Simons: Model AIrcraft Aerodynamics) prop theory says that the theoretical efficiency of the prop is related to the increase of velocity in the prop wash: if dv is the increase of airflow velocity that the prop causes to the slipstream, and V is the flight velocity, then efficiency is 1 / (1 + dv/V). In other words, efficiency is the better the smaller the propwash speed increase. On the other hand, each airplane requires a certain amount of thrust to fly, and the thrust equals to the increase in the kinetic energy to the propwash (which by Newtons third is equal but opposite to the increase of kinetic energy of the plane). This increase of kimetic energy is m*dv. dv as above, and the m is the mass of air in the slipstream flowing through the prop disk. Here we home to the prop diameter: the mass depends on prop diameter: larger prop = more air mass = required velocity increase is smaller.

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strat-o
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« Reply #3 on: May 17, 2018, 12:40:49 AM »

Tapio is correct.  What I might add is helicopter rotors are propellers.  Their large size enables them to accelerate a big disk of air but that disc doesn't need to be accelerated much in order to generate the lift needed for flight.  And that is how helicopters are able to be useful and relatively efficient.  The efficiency formula also applies to jet engines.  Fan jets produce a larger, slower, more efficient disc of air than older turbojets.  A nice side effect is the more efficient jets are quieter when you don't have to accelerate the air to such high velocities.

Here is one cool thing: Two 5" propellers creating a combined thrust of X is more efficient than one 5" propeller producing a total thrust of X.
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aeronautics
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« Reply #4 on: May 17, 2018, 11:06:59 AM »

It seems like the magnitude of the efficiency gain of a larger propeller is an obscure topic. Data exists for variations where the propeller blade diameter is the same but I have yet to find data comparing energy efficiency of different diameter blades.
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davidjohnnorman
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« Reply #5 on: June 14, 2018, 02:34:13 AM »

I was once involved in a man-powered aircraft project, and an aerodynamics expert used to say that "Theoretically the most efficient propeller is of infinite diameter rotating at zero revs"
 In other words, the bigger the propeller the higher it's potential maximum efficiency.
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OZPAF
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« Reply #6 on: June 14, 2018, 10:42:39 AM »

The basic equation for thrust efficiency at low speeds dictates that efficiency increases as the amount of air moves with the slowest velocity increase.
This is what is achieved when the  diameter of the propeller is increased as for a given volume of air to be moved( thrust) the velocity reduces.
This is a very basic explanation but a check on propeller design will add more detail.

John
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Hepcat
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« Reply #7 on: June 14, 2018, 12:25:21 PM »

Response to 4#.
Aeronautics, I really can’t agree with you that propeller information is obscure.  When Froude started to look at the idea of using a propeller to drive a boat he recorded his reasoning in a very few pages and clearly explained how it was necessary to accelerate the water backwards to achieve the forward thrust and that this process would be most efficient if a large mass of air was accelerated slowly and gave clear arguments as to the efficiency that could be attained.  He also was clear that he had depended upon the valuable work of earlier workers in fluid flow, particularly Bernoulli. Later, when using propellers to power aeroplanes became practical, the aeronautical engineers realized that they could treat the propeller blades as small wings which they could investigate with a similar approach to what was needed on the actual wing of an aeroplane. This is often referred to as the blade element approach.  All these early developments and much later stuff is available in libraries and, of course, with a diligent look on the Internet.  You can ignore the maths and still get a good idea of the subject.
John 
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lincoln
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« Reply #8 on: June 16, 2018, 10:00:08 PM »

snip  the thrust equals to the increase in the kinetic energy to the propwash (which by Newtons third is equal but opposite to the increase of kinetic energy of the plane). This increase of kimetic energy is m*dv. dv as above, and the m is the mass of air in the slipstream flowing through the prop disk.

That's the increase in momentum. The increase in kinetic energy is 1/2m(v22 -v12)
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