Torque and Angular acceleration

[egroeg93]

Hi all, first time posting so let's see how it goes!

My problems are with a piece of physics coursework I'm doing for my A-level courses. I chose to investigate the 'torque effect' on single propeller planes. (If you are unsure of what this is, its basically Newtons third law, reasoning that when a plane makes a propeller spin, the propeller makes the plane spin).

So i need a way to calculate torque so i can compare it to forces produced on the plane. The setup i used for the experiment is show in the picture:

[PLAIN]http://img827.imageshack.us/img827/1451/physicsforum1.png

Basically, its a suspended mock up of a plane, with a propeller on the front. I'm using a tachometer to measure rpm and scales to measure the 'weight' induced on the wing, which i could convert into a moment.

How can i find the torque of the propeller from this? I've spent ages trawling the internet. The variables i know are: rpm, length and mass of propeller. I also changed the distance alone the 'wings' of the mockup which i measured the force from, using the scales to measure the 'force' the torque effect applied on them.

So what i understand, is:

ω = 2*(pi)*f

a_tangential = Angular acceleration*R

τ = I*α

But how do i link angular frequency with angular acceleration. How could i do this? Could i just use the ideas in the picture and trigonometry?
[URL]http://upload.wikimedia.org/wikipedia/commons/5/56/Acceleration.JPG[/URL]

Many thanks to all!

 

[tiny-tim]

welcome to pf!

hi egroeg93! welcome to pf!

… How can i find the torque of the propeller from this? I've spent ages trawling the internet. The variables i know are: rpm, length and mass of propeller. I also changed the distance alone the 'wings' of the mockup which i measured the force from, using the scales to measure the 'force' the torque effect applied on them. …

i think you're misunderstanding both the word "torque" and the way the propellor is connected to the rest of the plane

"torque" can mean a couple (a pair of equal and opposite forces, not in-line), or it can mean the moment of a force

the torque of a propellor is the first type, but your distance x force calculation is the second type

each propellor exerts both a forward horizontal force on the wing, and a couple (a torque) on the propellor shaft

the moments (torques) of the forward horizontal forces of the two propellors (assuming the plane is symmetrical!) cancel out, so those forces don't turn the plane

the propellor shaft isn't connected to the wing or the body, it's connected to the engine … i don't know where that is or how it works, but i assume that either there's something in the engine that "absorbs the recoil", or that there is a torque, but its turning effect is very small since the moment of inertia of the plane is very large, and that that's catered for by a very slight adjustment to the rudder or to the plane's geometry

anyway, i think you'll need an apparatus that incorporates a torsion balance rather than the usual linear one

 

[egroeg93]

i was told i could calculate a torque force by my teacher :P

Its modeled on a single propeller plane, so there is no cancelation of forces.

This is a real scenario, and I'm just trying to experiment with it :)

 

[tiny-tim]

i was told i could calculate a torque force by my teacher :P

Its modeled on a single propeller plane, so there is no cancelation of forces.

This is a real scenario, and I'm just trying to experiment with it :)

If there's a single propeller, it's presumably in line with the centre of mass of the plane, so again there should be no moment (torque) about the centre of mass.

I think either you're misunderstanding your teacher's answer, or (s)he's misunderstanding your question.

 

[rwisz]

Hello,

It's great to see that you are working on a physics coursework and exploring the concept of torque and angular acceleration. Let me try to provide some guidance on your questions.

Firstly, torque is defined as the force applied times the distance from the pivot point. In your experiment, the pivot point would be the center of the propeller. So, to calculate the torque of the propeller, you would need to measure the force applied by the propeller and the distance from the center of the propeller to the pivot point.

In order to measure the force applied by the propeller, you can use the scales as you mentioned. However, you would also need to consider the direction of the force. As the propeller spins, it will produce a force in a specific direction. You can use a force sensor or a force plate to measure the magnitude and direction of this force.

Once you have measured the force, you can use trigonometry to calculate the distance from the center of the propeller to the pivot point. This will give you the lever arm, which is the distance at which the force is applied. Then, you can simply multiply the force by the lever arm to calculate the torque.

As for the relationship between angular frequency and angular acceleration, you are correct in your understanding. Angular frequency (ω) is directly related to angular acceleration (α) through the equation ω = α * t, where t is the time. In your experiment, you can measure the time it takes for the propeller to make a complete rotation (period) and then use this equation to calculate the angular acceleration.

I hope this helps and good luck with your experiment! Remember to always double check your calculations and make sure to consider all the variables involved. Keep up the good work!