Air resistance
Discussion
Austin Prefect said:
Given an object of known frontal area, and assuming sea level air pressure and a CD factor of 1. How do I calculate deceleration due to air resistance?
You can't, without knowing the mass of the object. Also, unless it's in free air, you need to know the amount of friction. And that's before you know the overall shape of the object because the amount of turbulence behind also makes a difference (see long-tail Le Mans cars, rear diffusers etc.)Panamax said:
You can't, without knowing the mass of the object. Also, unless it's in free air, you need to know the amount of friction. And that's before you know the overall shape of the object because the amount of turbulence behind also makes a difference (see long-tail Le Mans cars, rear diffusers etc.)
I was thinking in terms of free air, EG a bullet that's left the barrel and I figured I could plug the mass into a formula. The shape surprises me, I figured given a frontal area and a drag factor (EG 1) would cover this.Austin Prefect said:
I was thinking in terms of free air, EG a bullet that's left the barrel and I figured I could plug the mass into a formula. The shape surprises me, I figured given a frontal area and a drag factor (EG 1) would cover this.
A bullet has a lot of mass for its size and also has a narrow cross-section. It's no accident that bullets used to be made out of lead (plenty of density) and no accident that depleted uranium is favoured for munitions these days (almost twice as dense as lead).Compare ships. The crazy thing about ships is the energy it takes to push them forwards is mainly all about the shape of the front. That strange bubble thing on the bow below the water line enhances efficiency etc. And after that the length of the ship is almost completely irrelevant - the greater the length the greater the energy efficiency.
It's all rather weird.
Panamax said:
A bullet has a lot of mass for its size and also has a narrow cross-section. It's no accident that bullets used to be made out of lead (plenty of density) and no accident that depleted uranium is favoured for munitions these days (almost twice as dense as lead).
Compare ships. The crazy thing about ships is the energy it takes to push them forwards is mainly all about the shape of the front. That strange bubble thing on the bow below the water line enhances efficiency etc. And after that the length of the ship is almost completely irrelevant - the greater the length the greater the energy efficiency.
It's all rather weird.
With boats you have the additional complication of bow and stern waves, almost analogous to the compressibility effects of something in air approaching Mach 1.Compare ships. The crazy thing about ships is the energy it takes to push them forwards is mainly all about the shape of the front. That strange bubble thing on the bow below the water line enhances efficiency etc. And after that the length of the ship is almost completely irrelevant - the greater the length the greater the energy efficiency.
It's all rather weird.
Aside from the complications of fluid dynamics already discussed, you could have a good stab at it using the drag equation.
Fd = 0.5*rho*Cd*A*v^2
Fd is the drag force
rho is the density of the air
Cd is the drag coefficient
A is the frontal area
v is the initial velocity
Problem is that Fd (the drag Force) isn't constant so you then can't calculate the deceleration time or distance using a simple equation like F=ma then v^2=u^2+2as, you either need a differential equation or to solve it numerically.
Fd = 0.5*rho*Cd*A*v^2
Fd is the drag force
rho is the density of the air
Cd is the drag coefficient
A is the frontal area
v is the initial velocity
Problem is that Fd (the drag Force) isn't constant so you then can't calculate the deceleration time or distance using a simple equation like F=ma then v^2=u^2+2as, you either need a differential equation or to solve it numerically.
My brother is a fluid dynamics engineer (water) and my son is an acoustics engineer, so both a specialist's in fluids.
I asked them both the OP's question yesterday, they looked at each other almost said in unison, "how long have you got, its complicated"
I asked them both the OP's question yesterday, they looked at each other almost said in unison, "how long have you got, its complicated"
Edited by blueg33 on Monday 13th October 10:55
blueg33 said:
I asked them both the OP's question yesterday, they looked at each other also said in unison, "how long have you got, its complicated"
Didn't Heisenberg say "When I meet God, I'm going to ask him two questions: why relativity? And why turbulence? I really believe he'll have an answer for the first"?Krikkit said:
Partly - velocity has a directional component which airspeed doesn't
Airspeed is measured by a directional sensor - the pitot head. If you stuck a pitot head at 90 degrees to the airflow, you'd get some form of reading, but it would be b
ks (which is why whilst tumbling aerobatics are such fun, the ASI isn't something of any use whatsoever once you start going arse-over-tit up or down the sky. Important your airspeed prior to starting though is suitably low, otherwise things fall off and it hurts. Hence the reason you don't see European Swallows flying Lomcevaks)Edited by eharding on Wednesday 12th November 00:49
Panamax said:
A bullet has a lot of mass for its size and also has a narrow cross-section. It's no accident that bullets used to be made out of lead (plenty of density) and no accident that depleted uranium is favoured for munitions these days (almost twice as dense as lead).
Compare ships. The crazy thing about ships is the energy it takes to push them forwards is mainly all about the shape of the front. That strange bubble thing on the bow below the water line enhances efficiency etc. And after that the length of the ship is almost completely irrelevant - the greater the length the greater the energy efficiency.
It's all rather weird.
Not sure how you've formed the idea length isn't important, it's the sole decider of hull speed and thus the maximum practical speed of displacement vessels.Compare ships. The crazy thing about ships is the energy it takes to push them forwards is mainly all about the shape of the front. That strange bubble thing on the bow below the water line enhances efficiency etc. And after that the length of the ship is almost completely irrelevant - the greater the length the greater the energy efficiency.
It's all rather weird.
eharding said:
Krikkit said:
Partly - velocity has a directional component which airspeed doesn't
Airspeed is measured by a directional sensor - the pitot head. If you stuck a pitot head at 90 degrees to the airflow, you'd get some form of reading, but it would be bks (which is why whilst tumbling aerobatics are such fun, the ASI isn't something of any use whatsoever once you start going arse-over-tit up or down the sky. Important your airspeed prior to starting though is suitably low, otherwise things fall off and it hurts. Hence the reason you don't see European Swallows flying Lomcevaks)Edited by eharding on Wednesday 12th November 00:49
hidetheelephants said:
Panamax said:
A bullet has a lot of mass for its size and also has a narrow cross-section. It's no accident that bullets used to be made out of lead (plenty of density) and no accident that depleted uranium is favoured for munitions these days (almost twice as dense as lead).
Compare ships. The crazy thing about ships is the energy it takes to push them forwards is mainly all about the shape of the front. That strange bubble thing on the bow below the water line enhances efficiency etc. And after that the length of the ship is almost completely irrelevant - the greater the length the greater the energy efficiency.
It's all rather weird.
Not sure how you've formed the idea length isn't important, it's the sole decider of hull speed and thus the maximum practical speed of displacement vessels.Compare ships. The crazy thing about ships is the energy it takes to push them forwards is mainly all about the shape of the front. That strange bubble thing on the bow below the water line enhances efficiency etc. And after that the length of the ship is almost completely irrelevant - the greater the length the greater the energy efficiency.
It's all rather weird.
I don't think i'll ever forget looking at the Alderney race on the tide maps showing a speed of up to 15 knots whilst our absolute maximum hull speed was 10 knots. Timing arrival for slack water was imperative!
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