Re: Team 2526 - Propeller Propulsion Prototype.
 

For teams looking into propeller propulsion here are some helpful hints:

Static thrust depends ONLY on prop diameter and RPM, Pitch only impacts power use and top speed. So you will want the largest diameter, lowest pitch prop you can mount, and spin it as fast you can.

Look at the power curve for the motor you have chosen, and select a prop that will run at the peak power point.

To find where a prop might run, here is a link to download a freeware program of great benefit. The page is in German, but the program is in English. The download link is at the top left of the page. The program is called THRUSTHP.

http://www.hoppenbrouwer-home.nl/ika...rusthpv20d.htm

Re: Team 2526 - Propeller Propulsion Prototype.
 

Here is the calculator we used to calculate the maximum thrust and size of engine required:
http://personal.osi.hu/fuzesisz/strc_eng/index.htm

As others have said the pitch of the props have no effect on static thrust, only the maximum speed at which the craft will be traveling it, and the pitch have enormous effect on the engine. To other teams attempting this method of propulsion, use the smallest pitch you can find (12.25x3.75 was the smallest we found). It will provide the same thrust and require less power.

Due to a loose belt during our first test run, we couldn't run the prop up to full speed :( Spinning that blade at 10k rpm according to our calculator (from above) we will get about 6lb of thrust (12lbs total with 2 of them). Is 12 lbs of thrust enough to move the robots?

Our other possibilities for props are using a 28x4 (still havn't found a low pitch prop this big) spinning at much lower speeds to get more thrust. and one more question if anyone knows this, besides requiring more power, is there a reason to use a 2 prop vs a 4prop blade? Using a 4 blade 28x4 prop spinning at 3500 r, according to this calculator (above) would provide nearly 35lbs of thrust

Re: Team 2526 - Propeller Propulsion Prototype.
 

I seem to recall some theoretical math stating the CIMs driving a wheel on the floor could only generate 9 lbs of thrust, so it might be feasible.

Re: Team 2526 - Propeller Propulsion Prototype.
 

A two bladed prop is more efficient than a 4 bladed one. Don't ask me how I know though because it was demonstrated to my R/C flying club one year. If you could make a 1 bladed prop that would be the most efficient.

Please, please, please do not exceed the max. rpm rating for the prop. I've seen blades break and imbed themselves into wooden benches 10 feet away.
These props are not toys and should be treated as you would a circular saw.
I know you will use proper guarding around the prop. If a prop gets knicked THROW IT AWAY!!!

Other than that it looks like it would do the job of moving your bot around. I think the only problem you will see is a slow take off (no pun intended :) )from a dead stop.

BTW I have been flying model airplanes for 30+ years.

Re: Team 2526 - Propeller Propulsion Prototype.
 

Btw, just to clear some stuff up, we plan on having two of these prop and motor systems, hooked up together with like a car driving system. Seeing as how Max power of the CIM motors occur at 2671 RPM, the RPMs on the prop would be 3.5 times that. I'd guess that been the minimum, so the RPMs would range from the max power RPMs(2671) to the free load RPMs (5342), resulting in the prop RPMs ranging from 8348.5 to 18697. But due to imperfections such as friction and air resistance, it's pretty much guaranteed that it won't be spinning nearly as fast as the free load speed.
Oh and final, there will be a steel wire safety housing around each prop. =D

Re: Team 2526 - Propeller Propulsion Prototype.
 

If the coefficient of friction numbers given to us in the manual are correct, then a full weight robot using wheel propulsion will only get 9 lbs of pushing force from the wheels before they slip. If you can get a total of 12 lbs from two propellers you should be able to outrun the other robots on the field.

One neat thing about propeller propulsion is that (unlike using wheels) your pushing force doesn't depend on the weight of your robot. While wheel driven robots will all accelerate at mostly the same rate, you could put those props on a very lightweight robot and move across the field at rediculous speeds!

Cool stuff, I can't wait to see the final robot.

Re: Team 2526 - Propeller Propulsion Prototype.
 

This was the reason we decided to attempt the whole prop idea. Our plan is to keep the robot as light as possible, 80 pounds (including battery) is our goal. That prototype prop weights around 3-4 pounds (haven't actually weighed it).

Re: Team 2526 - Propeller Propulsion Prototype.
 

It would be nice if you can post the gear ratio and also the propeller size and pitch would be nice.


I would have to argue with that;)

Re: Team 2526 - Propeller Propulsion Prototype.
 

the prop is 12.75 x 3.75 (video used a 12x7). and our gear ratio is about 1:3.5 the gear ratio was just an estimate based on how many times we had to spin the prop around to turn the base gear around.

Re: Team 2526 - Propeller Propulsion Prototype.
 

How come you didn't go with a Ducted Fan set up? You can safely turn a fan unit 25,000 - 30,000 :ahh:
Also the fan shroud would be a good safety cage.

Re: Team 2526 - Propeller Propulsion Prototype.
 

One of our early ideas was a leaf blower, but we decided the high air speed exiting the robot would be unsafe. A larger diameter, slower moving blower like the one shown would be better in this respect.

Re: Team 2526 - Propeller Propulsion Prototype.
 

Except have you noticed how ducted fans don't seem to fly away?
That's because they have really big pitches on the blades, making them move a lot of air, but provide very minimal thrust.

Re: Team 2526 - Propeller Propulsion Prototype.
 

I could be wrong here... I may have forgotten all my physics.
Since this is a near ideal surface, Ff = mu * N; the force of friction is how much "pushing force" you have, so it would depend on your weight. However, once you calculate your maximum acceleration, you have to divide out your weight again, so your actual acceleration does not depend on the weight of your robot, only the coefficient of friction and the constant acceleration of gravity.

Also, could you run through the numbers where you got 9 lb of pushing force for a fully weighted robot? Does this include bumpers and batteries, or just the 54 kg? I got slightly different numbers, but I haven't reviewed my physics for quite a while.

Re: Team 2526 - Propeller Propulsion Prototype.
 

You're right that a robot, by itself, would accelerate the same regardless of it's mass. Using Newton's second law and the formula for frictional force we have two equations:

Ff = m * a
Ff = mu * N = mu * m * g

Setting them equal, we get:

m * a = mu * m * g ---> a = mu * g

The masses cancel, showing that the maximum acceleration of the robot depends just on mu and g (which are both the same for everyone)

The difference between driving with wheels and driving via fans is that the fans provide a pushing force that doesn't decrease with mass. So we just have Newtons second law:

F(fan) = m * a ---> a = F(fan) / m

In this case, acceleration does depend on mass. The lighter you make the robot, the faster it can accelerate.

(It's a little more complicated than this because of the added mass of the trailer, but using a fan still allows you to decrease your overall mass but keep the same pushing force)

Re: Team 2526 - Propeller Propulsion Prototype.
 

HAHAHAHA thas right it took me a while to get it...don't mean to spam the thread..so I'll post


I think the amount of thrust you will need to blow a ball away will severly limit you in terms of weight and size unless u plan on just being a defense robot, in which sense it might make sense to go with a fan...is this purely for blowing balls away or for moving?