Science Buddies: "Ask an Expert"

Hi,
I am doing an experiment on propellers, and i was wondering how they work.
Can someone please tell me the terms and concpets i should study?
please look at: http://www.all-science-fair-projects.co ... _57_4.html for any additional info, as this is the site i am using.
BTW, my project is due on 2/6/14.

Thanks,
Manav

Hi manavthegreat-

If the site you were looking it isn't providing with what you need, you might take a look at one of the propeller projects on this site, https://www.sciencebuddies.org/science- ... p018.shtml and see what you think. It has a good detailed introduction and a nice list of terms.

Once you get a sense of what makes them tick, I think you'll have fun playing with them.

Howard

[POST DELETED]

Hi,
On the site that Howard has recomended there are 5 sites, but i cannot use wikepedia for my report,
and the java application sites, which leaves me with about 3 sites. HOwever, i must have 4-5 sources for my science fair.
Could someone suggest books or articles that might help me.
This is my first time wokring with propellers and i am an 8th grader.

Wikipedia can be a good source for general background information - to help your understanding - but I agree that's not a reliable source. Would you consider explanations by Boeing or NASA credible? Depending on what you use and how you use it, I would.

I did a Google search for "how do propellers work" and found hits on those sites, a number of university course sites and flight schools as well. Rather than give you clickable links and spoil the fun of your reading about these marvelous devices, try https://www.google.com/search?q=how+do+propellers+work and look through some of the results. Doing the basic research is part of preparing for an experiment. You'll find that locating and recognizing a *good* source is going to be a valuable skill for high school and beyond.

HI,
I just had a question about the experiment itself.
ON the site from which i ordered the propeller it said that i should trim the length accordingly.
Should I use a scissor to trim?
Also, are the radio shack motor and the Jameco motor similar or the exact same?

Hi manavthegreat,
Scissor should be fine to use for trimming to the correct length.
Looking at the link, the motors looks to be similar but not identical, with the radioshack one has a no load speed of 8300 RPM whereas Jameco one (Part #FE260-2670) have a no load speed of 5100 RPM.
Best of Luck with your experiment!
Willey

Hi,
I am still very confused about what I should research.
When I searched "how does a propeller work" I got a lot of sites which stray
into aerodynamic forces, but first I want to understand how a propeller works.
Please help me, I only have a couple days left.

Hi,
I am so sorry for the delayed response.
Wikipedia have a nice write-up on this topic but I am wondering if you are more interested in the engine that's driving the propellers?

Please let us know what question you are trying to address.
For example, are you trying to find out how propeller generate thrust? (Bernoulli's principle and Newton's third law.)

Good Luck with your project!
Willey

Explaining propellers without reference to aerodynamics is tricky, but I'll give it a try.

First, remember a propeller can work in water (like a motorboat engine) or in air (prop-driven aircraft), but what they're both doing is the same from a physics standpoint. They are allowing the vehicle to exert force on either the water (boat) or air (aircraft), and they do it by interacting with air or water. (this is why you keep ending up with Aerodynamics - that's the science of "how do you interact with air if using a solid object).

So all propellers have a few things in common
1. They are solid objects that interact with something not solid (air, water, or some other similar substance)
2. They rotate (it's easy with electricity or internal combustion engines to spin a shaft - much easier than to simulate bird wings or a dolphin tail)
3. They are shaped in such a way that rotating the propeller causes the fluid (gas or liquid) that they interact with to move in a controllable direction.

Why does a rotating propeller push air or water efficiently in one direction? That's aerodynamics. But there is a certain amount of common sense if you've ever rowed a boat, or even tried swimming.

A flat, solid surface pushes water better than one that "leaks" or is narrow (you hold fingers together when swimming, and stroke with your palms, not the edge of your hand). Something like a paddle wheel was the first way people tried to turn rotation into force - for hundreds of years people did it the other way around (had water ior wind interact with a flat surface, which spun a crank that provided energy for things like grinding grain with a big round stone, or lifting hammers for pounding steel).

When efficient, portable power sources became available (steam engines in 19th century) they just reversed the process. But gigantic paddle wheels are fragile, bulky, heavy etc. People knew that if you turn the flat surface, it shunts the air or water at an angle. Windmills used this principle to most efficiently catch the wind - so instead of attaching two water-wheels to a boat and spinning them, it is reasonable to try putting the wheel behind the boat and angling the paddles, more like a windmill. With high quality metals becoming available you could exert a lot more force than you could with wood in a small area, so propeller-driven boats did not end up looking like somebody had put a windmill on their rear. The tricky part is working out the specific shape - just angling the paddle isn't enough to work well for pushing a boat or pulling an aircraft. The kind of twisted shapes that turned out to work best are quite challenging to manufacture, which is one reason paddle boats arrived first and lasted quite a while before propeller-driven boats became dominant.

Aircraft obviously can not use giant paddles to push through air, you can not move enough air with a paddle to push or pull something as heavy as an aircraft. But there are shapes that gather up air and force it all in one direction efficiently (an electric fan is an example you are probably familiar with). Early propellers were made out of wood, because it is relatively lightweight, strong and you can make complex shapes with it. Eventually engines and propellers got good enough to move a land vehicle. Getting it to fly was another application of Aerodynamics entirely, making use of the airflow over the entire vehicle to cause it to rise. That's a whole different topic.

The important concept out of all of this is "propellers move fluids when they rotate, because of their shape, and moving the fluid exerts force on whatever the propeller is attached to", and a fluid can be either gas or liquid, but you need different shapes of propellers to work in gas than in water.