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Right now I am working on a UAV design which will use ducted fan. But the performance calculations are a bit troublesome. For some calculations jet and propeller results are almost same but for the parameters like takeoff, landing, endurance and much more T/W and P/W results doesn't make any sense.

For example, T/W takeoff (according to Sadraey's approach) with jet assumption is higher than my max T/W but P/W takeoff (according to Sadraey's approach)with propeller assumption is one third of my max P/W (When calculating P/W, I directly summed engine powers and divided by weight, I am not sure about is either because I didn't involved any efficiency or altitude effect) and in previous studies, I found that I would takeoff with %86 of my max power not one third of my max.

Should ducted fan considered as jet or propeller for the performance parameters?

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    $\begingroup$ They are most definitely propellers. $\endgroup$
    – JZYL
    Sep 8, 2021 at 0:56
  • $\begingroup$ Sir, thank you for the information. I know that propeller efficiency changes from one to another but do you have any suggestion for 12.5 cm diameter, 30 N thrust generating and 1665 W power ducted fan ? $\endgroup$
    – J. Brown
    Sep 8, 2021 at 4:08
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    $\begingroup$ @J.Brown, could you perhaps show the exact calculations you performed? It would also be helpful if you showed the exact reference of the book you used. $\endgroup$
    – U_flow
    Oct 20, 2022 at 8:35

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Performance textbooks classify prop vs. jet based on whether the propulsion system is considered 'thrust producing' or 'power producing'.

Whether a propulsor is thrust or power producing is based on two factors.

Is the thrust (or power) available roughly constant with velocity?

Is the fuel consumption roughly proportional to thrust (or power)?

An idealized jet is a thrust producing engine. The textbook assumptions say that a jet's thrust available is constant with velocity and the fuel consumption is $\dot{W}=\mathrm{TSFC}\,F_n$, where $\mathrm{TSFC}$ is the thrust specific fuel consumption.

An idealized prop is a power producing engine. The textbook assumptions say that a prop's power available is constant with velocity and the fuel consumption is $\dot{W}=\mathrm{BSFC}\,P_{shaft}$, where $\mathrm{BSFC}$ is the brake power specific fuel consumption.

In reality, these idealizations are never true. Serious aircraft performance modeling requires a more sophisticated propulsion model and these concepts of 'thrust producing' and 'power producing' are not very useful. The approximations that result (such as best climb, endurance, range -- at best CL/CD, CL^1/2/CD, or CL^3/2/CD) are not very accurate. Similarly for the Breguet Range equations for prop and jet aircraft. Unfortunately, most of the textbooks that present these approximations never discuss the reality of propulsion and the deficiencies required to write these simplifications.

The design of a duct for a fan has profound impact on the performance of the fan. High speed aircraft (cruising at M=0.8 and above) will have a duct shaped such that when the flow reaches the fan front face, it is substantially slower than freestream (say M=0.6). This helps keep the fan tip speed down and allows the engine to operate at high speeds. This kind of fan actually reduces static thrust. These ducts typically have an exit area smaller than the fan area.

Ducts can also be designed to maximize static thrust. These fans actually accelerate the flow from freestream to the fan face. Though they are unsuitable for high speed flight, they are great at static thrust. These ducts typically have an exit area equal to or greater than the fan area.

Duct performance behavior also depends on how you're driving it -- a gas turbine, a piston engine, or an electric motor. And how the limits on that device are enforced. For example, an electric motor and drive will have certain RPM (bus voltage and throttle) and torque limits (current).

In a sophisticated system, these will be monitored and kept within safe limits by an active control system. In a hobby-grade system, the battery bus voltage drops without control, the speed controller has a 100% throttle, and the controller may limit current -- or perhaps a fuse is the only limit you have. Sometimes, the only limit is by choosing a prop that won't overload or allow the system to over-speed.

My recommendation is to take a step deeper in your calculations and develop an off-design engine deck for your propulsor.

If you are designing a high-speed fan like that used on a transport aircraft, you will start with a traditional turbomachinery fan map.

If you are designing a low-speed fan that will not operate into compressibility, then I would recommend you use Prof. Drela's DFDC. Esotec is a company that claims to have an improved version.

Next, you are going to need a model of the device producing shaft power -- turbine, piston, or electric. Importantly, what are the limits -- max torque, max power, max speed -- these will form an envelope of what is allowed. Ideally, you will also have some measure of efficiency or fuel consumption within those limits.

Finally, you need to couple the two together -- match the torque and speed required to spin the propulsor with the operating torque and speed of the driving machine. Keep the machine within safe operating limits, move the propulsor around in the flight envelope (speed, altitude, throttle).

This paper gives an example of this for a high speed fan driven by an electric motor.

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As a rule of thumb, for a ducted propeller you can consider a thrust increase of 30% in respect to an open propeller, the power being equal. This should give you a reasonable value to start with.

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  • $\begingroup$ That depends very much on the duct in question. If the duct design is bad, you will loose thrust. $\endgroup$
    – U_flow
    Oct 20, 2022 at 8:33
  • $\begingroup$ @U_flow: oh well, also the propeller if it's bad designed is going to generate drag instead of thrust ;-). But I think this is not the point here... $\endgroup$
    – sophit
    Oct 20, 2022 at 8:50
  • $\begingroup$ that is true, but for duct design it is apparently especially easy $\endgroup$
    – U_flow
    Oct 20, 2022 at 9:03
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is a ducted fan considered a jet or a propeller?

Well, is a jet considered a reciprocating engine or a rocket?

Does it matter at all what you call it?

First, the commonality is that both reciprocating engines and turbines produce mechanical rotational force to do work: that is to run their compressors, oil pumps, fuel pumps, create electricity, what ever needs to be done to keep the engine and aircraft systems running.

Early jets relied on expulsion of hot gasses to create thrust, using the mechanical force of the turbine to turn the air compressor.

Reciprocating engines create thrust by turning a propeller, and use the mechanical force of the pistons to also intake air (compresser optional).

The job of the turbine is the same as the job of the piston for incoming air, spark, and fuel, but what about propulsion?

can a turbine turn a propeller?

Of course! They are called turboprops. Can a turbine turn a fan? These are fan jets.

Which brings us to the ducted fan, powered by ... an electric motor?

Electric motors do not use hot gasses from combustion to produce mechanical force or thrust, so it can't be a turbine or a reciprocating engine. It's ... an electromagnetic engine.

no matter how mechanical energy is produced, fans function as propellers

They are subject to the same benefits and limitations as propellers.

Ducted fans improve propulsive efficiency at low airspeeds compared to unducted propellers.

Running the numbers on your ducted fan:

1665 W = 1.665 kW = 2.23 horsepower
12.5 cm diameter prop = around 5 inches
30 N = 6.7 lb force of thrust

Not bad, really. Can we do better for lifting?

fewer, longer, slower blades

That is the approach the designers of this aircraft took.

For the amount of horsepower you have, at low airspeeds, a 12 inch propeller may be more efficient at generating thrust.

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