Thrust / Drag question
Thrust / Drag question
(OP)
hi all, i'm new here...
I'm a computer nerd who enjoy getting into trouble making hobby programming projects, and this time i'm working on a crude aerodynamics engine. problem is, my physics background is, shall we say, limited. Although i do enjoy the learning experience.
Here's my problem.
I'm trying to calculate the resulting force for an object, given the thrust of the jet engine, the mass of the object, and the drag coefficient. The values i'm working with, to make it simpler, are:
Weight = 3800 kg
Thrust = 17600 N
Air density = 1.225 kg/m^3
I'm dumbing it down a whole lot, using constants for some things ... so my resulting formula looks something like this:
F(t) = (17600/3800) - (0.5 * 1.225 * D * v(t)^2)
Now my big issue is with the drag coefficient D (i'm assuming the reference area is 1 m^2 for simplicity)...
If this craft is to obtain something even close to it's speed at sea level, which is 870 kph, the drag coefficient needs to be 0.0000129.. and that just seems WAY too small if you compare to some common values. If i'm using the drag coefficient that seems about right for this craft (0.25) it achieves only 5.71 m/s, which is just way off.
What am i doing wrong here?
I'm a computer nerd who enjoy getting into trouble making hobby programming projects, and this time i'm working on a crude aerodynamics engine. problem is, my physics background is, shall we say, limited. Although i do enjoy the learning experience.
Here's my problem.
I'm trying to calculate the resulting force for an object, given the thrust of the jet engine, the mass of the object, and the drag coefficient. The values i'm working with, to make it simpler, are:
Weight = 3800 kg
Thrust = 17600 N
Air density = 1.225 kg/m^3
I'm dumbing it down a whole lot, using constants for some things ... so my resulting formula looks something like this:
F(t) = (17600/3800) - (0.5 * 1.225 * D * v(t)^2)
Now my big issue is with the drag coefficient D (i'm assuming the reference area is 1 m^2 for simplicity)...
If this craft is to obtain something even close to it's speed at sea level, which is 870 kph, the drag coefficient needs to be 0.0000129.. and that just seems WAY too small if you compare to some common values. If i'm using the drag coefficient that seems about right for this craft (0.25) it achieves only 5.71 m/s, which is just way off.
What am i doing wrong here?





RE: Thrust / Drag question
TTFN
RE: Thrust / Drag question
the airframe weighs 3800 kg, and the thrust of the engine is 17600 N. That means that the engine can accelerate a 3800 kg object at 0.46 m/s^2 ... is that correct? no?
RE: Thrust / Drag question
The drag equation results in Newtons, which has the same units as your thrust.
TTFN
RE: Thrust / Drag question
So, a truck-size object with a very low Cd may develop a lot more drag force than a brick- sized object in the shape of a brick.
Mike Halloran
Pembroke Pines, FL, USA
RE: Thrust / Drag question
RE: Thrust / Drag question
but nevertheless, the resulting Drag coefficient (assuming a reference area of 1 m^2) is still just way too low, 0.001294 just seems strange when i'm looking at the drag coefficient on an airfoil. I know it's nondimensional, and maybe that's my huge mistake in this whole thing.
RE: Thrust / Drag question
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Thrust / Drag question
If thrust (17600 N) / mass (3800 kg) = 4.63 m/s^2, and
drag is: (0.5 * 1.225 * 0.02 * v(t)^2)
Am i incorrect to say that the resulting vector is:
4.63 - (0.5 * 1.225 * 0.02 * v(t)^2) ???
I acknowledge that IRStuff said i'm mixing units when dividing force by mass, but was that b/c i got .463 instead of the correct 4.63?
I thought a thrust of 17600 N means that it can accelerate a mass of 17600 kg at a rate of 1 m/s^2, or a 3800 kg mass at 4.63 m/s^2
So if the output of the drag equation is in Newtons, i.e. m/s^2, i am *not* mixing units, right? wrong?
sorry for being so blissfully ignorant, just hoping someone can alleviate my case of blockhead.
RE: Thrust / Drag question
"If thrust (17600 N) / mass (3800 kg) = 4.63 m/s^2, and
drag is: (0.5 * 1.225 * 0.02 * v(t)^2) "
NO!
Thrust is a force (units of Newtons), not an acceleration (units of m/s2).
Thrust you defined as = 17600 N
Let net thrust = FN = Engine thrust - drag
Then vehicle acceleration = dv/dt = FN/mass
You will "need to" numerically integrate the acceleration, since the FN term is dependent on the velocity^2.
RE: Thrust / Drag question
RE: Thrust / Drag question
A simple test of whether Fn is positive will tell whether you can achieve the speed required. Since there's plenty of excess thrust at the speed specified, you can accelerate to the speed desired. A numerical integration is required only to determine how long it will take. But, as indicated earlier, since the drag coefficient is not constant, the numerical integration is of academic interest without the correct function for drag.
TTFN
RE: Thrust / Drag question
net force on the plane is Thrust-Drag (atleast in the longitudinal direction). Your Thrust is very simplistic, in that it isn't constant with speed. Your Drag is also very simplistic ...
1) drag is based on the wing area (not the frontal area)
2) as noted above CD is a function of speed
it would be easier to express acceleration as (T-D)/m
good luck
RE: Thrust / Drag question