The Student Room Group

Physics challenge!

I have devised a simple method to calculate the thrust of a fan. In order to see if it's better than the current methods I have gotten the stats for a random ducted fan off of the internet and anyone who can work out the hidden thrust values of A B and C more accurately than my formula I'll deposit £10 into their paypal. You must show your working.

physics challenge.JPG

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Reply 1
Thought this place was full of physics geniuses?
Reply 2
It's because you are asking a poor question. You haven't got enough information to calculate it. If you have what you think is a good method, post it and let us judge if it's good or not.
Reply 3
Original post by Rubgish
It's because you are asking a poor question. You haven't got enough information to calculate it. If you have what you think is a good method, post it and let us judge if it's good or not.


Yes you do. You have the fan radius, the power input, you can assume the air density, what more do you need? I'll post my method once others have attempted to answer the question.
Reply 4
Original post by Plutonian
Yes you do. You have the fan radius, the power input, you can assume the air density, what more do you need? I'll post my method once others have attempted to answer the question.


You'd want to know how efficient the rotor is, how heavy the blades are, what angle the blades are placed at, what motor you are using to power it etc.

There is a ton of stuff you need to take into consideration if you want an actual accurate estimation of thrust.
Reply 5
Original post by Rubgish
You'd want to know how efficient the rotor is, how heavy the blades are, what angle the blades are placed at, what motor you are using to power it etc.

There is a ton of stuff you need to take into consideration if you want an actual accurate estimation of thrust.


For starters nowhere did I ask for the result to be spot on, simply more accurate than whatever I got and second of all I took none of that into consideration and got it correct to within 6 grams. If you can't answer the question just leave the thread. I don't see why you are hanging around making excuses.
Reply 6
Original post by Plutonian
For starters nowhere did I ask for the result to be spot on, simply more accurate than whatever I got and second of all I took none of that into consideration and got it correct to within 6 grams. If you can't answer the question just leave the thread. I don't see why you are hanging around making excuses.


Consider the following:

What would happen if each of the fan blades weighed 1000kg? They would barely move and so you'd get basically 0 thrust.

What would happen if the fan blades were angled so that they sliced through the air without pushing it up or down? You'd get 0 thrust again.

What if the rotor was really poorly made and had a load of friction, so that all the power was wasted as heat? Again, you'd get very little thrust.



Some of this can probably be considered negligible in a real world system, but only given some rudimentary testing first. As I said before, please do post your method. Feel free even to PM me it if you want to leave the challenge for other people.
Reply 7
Original post by Rubgish
Consider the following:

What would happen if each of the fan blades weighed 1000kg? They would barely move and so you'd get basically 0 thrust.

What would happen if the fan blades were angled so that they sliced through the air without pushing it up or down? You'd get 0 thrust again.

What if the rotor was really poorly made and had a load of friction, so that all the power was wasted as heat? Again, you'd get very little thrust.


Some of this can probably be considered negligible in a real world system, but only given some rudimentary testing first. As I said before, please do post your method. Feel free even to PM me it if you want to leave the challenge for other people.


I assure you that the ducted fan in question was not designed by retards. If the fan is designed to be efficient as possible then a basic model can describe it well.
Reply 8
Is this question too hard for TSR? You guys have no chance of getting into Oxbridge if you can't answer simple questions like this
Original post by Plutonian
Is this question too hard for TSR? You guys have no chance of getting into Oxbridge if you can't answer simple questions like this


Please stop saying stupid things.
Reply 10
Original post by PrimeLime
Please stop saying stupid things.


Having a go at me doesn't change the fact that you can't answer the question.
Original post by Plutonian
Having a go at me doesn't change the fact that you can't answer the question.


I didn't even try to answer the question. Why are you having a go at everybody else on TSR for 'not being able' to answer the question? Hasn't it crossed your mind that the 'physics geniuses' you speak of might just not be bothered to try your question?
Reply 12
Original post by PrimeLime
I didn't even try to answer the question. Why are you having a go at everybody else on TSR for 'not being able' to answer the question? Hasn't it crossed your mind that the 'physics geniuses' you speak of might just not be bothered to try your question?


For starters I was just bumping the thread with a little competitive talk, it is you who chose to take it too seriously and second of all the "it's beneath me" claim is a bit of a cop out. Just take part in the competition if you can, otherwise leave the thread.
Reply 13
Original post by PrimeLime
What you were saying was not "competitive talk", it was just an expression of annoyance that no one was participating in your competition.


Dude go away. FYI the guy who posted before you was interested and said he couldn't do it but was interested in my method and asked for it via PM. But oh no you didn't read the thread you just came here to troll.
Original post by Plutonian
Dude go away. FYI the guy who posted before you was interested and said he couldn't do it but was interested in my method and asked for it via PM. But oh no you didn't read the thread you just came here to troll.


"Is this question too hard for TSR? You guys have no chance of getting into Oxbridge if you can't answer simple questions like this"

Now, that's trolling.
Reply 15
Original post by PrimeLime
"Is this question too hard for TSR? You guys have no chance of getting into Oxbridge if you can't answer simple questions like this"

Now, that's trolling.


Are you done yet?
Original post by Plutonian
Dude go away. FYI the guy who posted before you was interested and said he couldn't do it but was interested in my method and asked for it via PM. But oh no you didn't read the thread you just came here to troll.


I didn't say I couldn't do it, I just said I couldn't do it *accurately*, there is a big difference.

As it is, I looked at your method. I even plugged some numbers into your final equation. Didn't match up with the correct answers. Not sure what you are doing tbh, especially with that whole angle thing in the middle that doesn't seem to have any impact on your final formula.

As for the method itself, it's pretty much exactly what anyone would do if that was all the information they had. You did f=ma and then put that into p=fv, not exactly ground breaking stuff.
Original post by Plutonian
Are you done yet?


I love how you have no response to that...
There are 16 aliens on a house. The house is orange and weighs 10000kg. The aliens' height range is 120cm - 190cm as a Gaussian distribution.

Using the above numbers and physics, calculate how many strands of hair the aliens have on their head.

P.S. If you can't do this simple calculation you have no chance of getting into Oxbridge.
(edited 8 years ago)
Reply 19
Is this for your prototype? You want somebody here to improve your formula so that you can take their answer (well, pay them for it) and improve your drone?

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