Here is vid to give you idea of scenario.
https://www.youtube.com/watch?v=zP53h5yrE48
And here is the question :-
The car in the picture is lifted by eight jets of water issuing from nozzles with outlet diameters of 50mm. The delivery rate from two fire tenders is 88 gallons per minute per tender, at a pressure of 15 psi through 80mm hose. What is the combined mass of the rig?
How the fook do you work that out?
2004 - on : 1999 S14a 398bhp 378lb/ft
2010 - on : 2007 RX8 PZ
1998 - 2004 : 1991 S13
42
You can't do it exactly because there are too many estimates and variables I think. There's not really an allowance for how high the car is actually being lifted for a start.
It's basically rocket science. You're overcoming the force of gravity which has a constant value. You're given some numbers so you can work out the actual force produced by the jets, and I'm thinking if you fill in that as F, and gravity as A in the equation F=MA you can then work out an estimate of M as the weight of the rig.
That's where my brain is taking me anyway, I'm sure it's a gross oversimplification, and I'm way too rusty at physics to try to work if through
power-crazed Head-Mod
search integral momentum equation in google and its relevance to to an aircraft engine. it boils down to the net change in momentum between whats entering and leaving the system. there is is an influence for the differences in pressure between the inlet and the exit.
you have the inlet and outlet diameters - you can work out the area.
you have the flow rate from this you can work out the change in velocity
you have to make an assumption about exhaust pressure but you have the inlet and the areas so can use that.
from what I understand thrust = FR*Ve - FR*Vi + (Pe - Pi)*Ae
quick calc gives me 0.01986594kg thrust per jet hence the rig weighs 0.1589kg
Thanks for the fishOriginally Posted by sideways14a
Do you account for the fish in the water?
Sent from my iPhone using Tapatalk
Have to ask the Dolphins....or i could just get my towel
https://www.grc.nasa.gov/www/k-12/airplane/thrsteq.html
Ignoring the small acceleration of the car and assuming it's holding stationary at a height.
V car = 0 (it's hovering stationary), A acceleration due to gravity = 1 G = 9.81 m/s^2.
Total thrust is momentum change of water + nozzle pressure term.
Force (thrust) = (M dot * V)exit - (M dot * V)0 + (pexit - p0) * Aexit
M dot = density rho x nozzle exit velocity V x nozzle area Aexit
Velocity Vexit = M dot / (rho x Aexit)
(M dot * V)exit = M dot x M dot / rho x area
M dot = 2 x 88 gal / min x density of water = 2 x 400 L/min x 1 Kg/L = 2 x 6.666 L/sec * 1 Kg/L = 13.33 Kg/s.
Pexit = 14.7 + 15
Po = 14.7
Pexit - Po = 15 psi = 103421 pascal (N/m^2)
Ae = 8 x pi (Dia/2)^2 = 8 x pi (0.05 / 2)^2 = 0.0157 m^2
rho density of water = 998 Kg/m^3 (20 deg C)
Force (thrust) = 13.33 x 13.33 / (988 x 0.0157) + 103421 * 0.0157 = 11.4 + 1623 = 1635 N
Mass = Force / Acceleration = 1635 / 9.81 = 166 Kg.
166 Kg does not seem a sensible weight for lifting a car even if it's just a shell with wheels to me. I think you need a lot more pressure and pump power (it's mainly the nozzle pressure that is doing the lifting). Maybe 5-7x if the car is stripped. Which is about what Mythbusters found.
https://www.youtube.com/watch?v=XFEOctt9-UE
88 gall/min seems a bit poor for a fire tender. Jet skis used to power personal water jets have flow rates around 1000 (US?) gall/min.
Delivery pressure is also a load of tosh.
A 5.5 hp Honda fire pump delivers a lot more pressure at 88 gall/min than the claims made for a full size Dutch fire engine.
Why? They become wet fish paste in the pump.
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