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Aerodynamics question:


crudboy12
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Propellers for almost all prop aircraft, constant speed or not, have fairly thin blades. What would happen if a propeller resembled a fan, or even a cork-screw? What is the aerodynamic reason that most of the "disk" occupied by propellers is empty space?

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Propellers for almost all prop aircraft, constant speed or not, have fairly thin blades. What would happen if a propeller resembled a fan, or even a cork-screw? What is the aerodynamic reason that most of the "disk" occupied by propellers is empty space?

 

Because then it would just be a fan with massive amounts of drag required to provide any thrust. You need to remember that prop blades are just wings that rotate, and as such are subject to the same aerodynamic considerations that are further complicated by their rotation and forward speed.

Just because a prop blows air out the back like a fan - they are not the same thing.

 

I am not going to try and explain the entire theory behind props and associated dynamics - there are literally millions of sources for this information on the internet already.

I would highly recommend watching this, particularly around 14min+ ..

 

 

Engines do utilise a fan thou - ducted fans are used in Turbine engines, but this is another subject again, and involving far more power than a prop scenario. Ducted and pure turbine engines also have considerably different efficiencies than a prop aircraft, even when that prop is driven by a turbine engine!

Also check the end of the video for an example of the prop-fan..so almost there lol.

 

Trent-1000 Ducted Fan

 

170415-10301452.jpg

 

This is a pretty cool reference as well - http://www.bestinflight.net/doclibrary/propellertheory.pdf


Edited by VampireNZ

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I have like 150 propellers or more for my R/C airplanes and I never find the matching one on my prop-wall for a project..I always have to go and buy a few ones with sizes I yet done have, run them, measure the load and then decide which one is best for which scenario.

 

 

The Fw-190 for example has fairly "wide" blades to have some grip in thin air up high at Angels 30 but they do hinder the aircraft at lower levels.

 

 

My buddy had a "Raketenwurm" Hotliner with a 18x19 pitch prop ( higher pitch than diameter is unusual ) that didnt do much but rattle very loud upon hand-launching from zero speed..until--until it reached it's threshold speed and the prop started to BITE hard and that thing took of 90° UP endless..out of sight in less than 3 seconds. Thin blades, very pointed and an unusual diameter/pitch ratio.

 

 

 

There is no general "best" prop, there is a best prop for a specific task at a specific speed and mass to pull.

 

One constant is, the more blades the less efficient it is, best prop eff. is a 1-blade prop with a counter weight, it is just not used anywhere that I know of. Blades are only added if you need to to take the power from the motor.

If possible, make the blades larger instead of increasing their number.

 

 

Wider blades are best for higher altitudes/thin air and thin blades are for high speed at low-medium altitude. You can use them anywhere but eff. will suffer to some extend.

 

 

Modern TurboProps have even curved blades as well ( 400er Airbus Mil Cargo ), I guess that is to minimize turbolences at the tip near the magic Mach number. No prop may ever reach Mach speed with it's tips, do it once and you know why ;)

 

 

 

 

The reason for EMPTY SPACE between the blades is efficiency. The more you fill the disk with more or wider blades the more the blades will run in the shadow of the blade in front of it. That's why the 1-blade design is king of eff.. There is no 2nd blade in the shadow/turbulences of the 1st one.

More empty space = bigger efficiency

 

 

Since most planes are multi-task airplanes you have to find a compromise for different altitudes, speeds and weights. Finding the best one is an hard to achieve art and the quest never stops as CAD evolves, composites get tougher etc...

 

 

 

 

The best prop in general is a 2-blade design with variable pitch. You would ONLY add blades if the motor has more torque than you have gorund clearance for larger blades. As soon as you add another blade you loose efficiency, no matter what. No one would add a 3rd blade if they could increase the diameter of the 2 blades mounted and still not hit the ground when the tail lifts ( Corsair and its P&W R2800 is a good example for a too big motor in asmall airplane, Gullwings to lengthen the gear struts and many many blades to absorb 2000+HP whereas a 30 feet 2-blade prop would be sooo much better..if there wouldnt be a ground clearance issue then baaahhhhh


Edited by BitMaster

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No prop may ever reach Mach speed with it's tips

comes with a host of problems, but you make it sound like it's physically impossible, which isn't true. xf-84 thunderscreech is known for its supersonic propeller, and the tu-95 is even more famous for having ridiculous supersonic propellers.

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Some light reading regarding tip speed....

 

https://www.science.gov/topicpages/p/propeller+tip+speed.html

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comes with a host of problems, but you make it sound like it's physically impossible, which isn't true. xf-84 thunderscreech is known for its supersonic propeller, and the tu-95 is even more famous for having ridiculous supersonic propellers.

 

The AT-6 Harvard/Texan is another example (although not fully supersonic at the prop tip, there are regions of supersonic flow, giving the characteristic rasping AT-6 sound).

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Sure, there are exceptions with special blade foils and shapes or counter-rotating setups.

 

As a rule of thumb, when the prop rattles = throttle back with any ordinary prop or risc disintegration

 

 

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So if I get this straight the drag caused by the "fan" prop reduces efficiency. Too much bite creates too much resistance for the engine to handle. However if you had a theoretical engine that had infinite torque at a set RPM would the "fan" design create the most thrust?

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So if I get this straight the drag caused by the "fan" prop reduces efficiency. Too much bite creates too much resistance for the engine to handle. However if you had a theoretical engine that had infinite torque at a set RPM would the "fan" design create the most thrust?

 

I guess yes,..unless the rpm is too high for that specific diameter/pitch ratio and shape.

 

Remember, the fan-blade sized shape produces some serious shadow behind it and the faster it spins the less effective it gets. So in theory, you would get a runaway scenario with infinite torque, less eff. = higher rpm = even less eff. = more rpm...and more and more... until it flies apart due to material disintegration. That is what I think would happen with infinit torque.

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I have nothing of Value to add, but I just wanted to say that reading this thread is super interesting. Hope the discussion continues! :thumbup::smartass:

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My guess is that a fan is shaped the way it is so it will be relatively quiet, but less efficient at producing thrust. Airplane propellers are shaped to be more efficient, but at the expense of being louder.

 

In the most basic terms, the type of fan pictured by the OP is purely pushing the air out in front of it by direct impact with its blades, no aerodynamic effect at all.

Obviously not efficient in an aeroplane and a prop blades utilises the same aerodynamic qualities as a wing shape to maximise the 'lift' in the forward direction with the minimum of drag.

 

The majority of prop sound is related to the tip speed approaching the transonic/supersonic region in particular applications. An excellent example of this is the modification available to the MD500 tail rotor in the form of a two-blade to four-blade change. With twice the blades the TR can rotate slower moving the tip speed away from that region, and as a result the helicopter as a whole is significantly quieter!

 

On the other end of the spectrum is the Tu-95 which is louder than all buggery! It's tip speed is supersonic, and there are eight 4-bladed props, rotating in the opposite direction to the prop right next to them. I actually don't think it would be possible to make a louder configuration lol.


Edited by VampireNZ

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So if I get this straight the drag caused by the "fan" prop reduces efficiency. Too much bite creates too much resistance for the engine to handle. However if you had a theoretical engine that had infinite torque at a set RPM would the "fan" design create the most thrust?

 

I don't think it's about the engine handling the load on the power turbine shaft, there is a lot of torque there, it's about fuel efficiency, why would you use so much fuel to do something that you can easily do changing the blade design or changing the engine, for example a turbofan, which is basically a ducted turboprop, but can't change the blade pitch. But it can go waaay faster without losing propulsive efficiency.

 

In fact, generally speaking, a turboprop starts losing propulsive efficiency at speeds above 350, 375mph. From this point, if you really need to fly faster, you'd use a turbojet or turbofan. Obviously, it will vary depending on your need.

 

Here is a little chart I use from a book.

 

ds7QUNN.png


Edited by Vitormouraa
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