If you start with Kilograms...STAY with kilograms.

Well I finally figured out why airspeed wasn't bleeding off correctly in a turn. I think it was partly due to my being lazy and just pluggin in a couple numbers till the aircraft handling "seemed" right. But, I was actually mixing U.S. measurement and Metric. Plus for anyone who's doing something with physics etc. It's a good idea to stick with one measurment. For instance, feet/pounds or meters/kilograms.

Or is it meters/Newtons?!?!

Actually, can someone help define the metric units used for physics?

Does this look right?

gravity: 9.81m/s
weight: kilograms
mass: kilograms
speed: meters per sec
length: meters
area: meters^2
air density: kg/cu m

weight= Newtons= mass of object* gravity
mass: kilograms
air density: kilograms per metre^3

So kg isn't a unit of weight too?

Mass and Weight are two different things....

gravity is 9.81 "metres per seconds squared" (ie ms^-2 or m/(s^2))

People use kg to measure weight, but as cygnus says, it's not technically correct. Mass is proportional to weight.

Cygnus, weight is mass * gravity. Of course mass and weight are two different things. But sometimes the line between mass and weight are skewed, especially when you get to very small amounts of weight. Per sey, the ingredients in a can of soda.

What I'm saying and obviously I wasn't clear about what I was trying to ask here; Is that you have to stick with the units you start with. Ie....meters and kg. So if you could say, tell me what is correct when it comes to using the metric system for a physics simulation. What you would use for weight, mass, length, speed. I know what each is btw. :)

Matty, so you're saying when I add gravity, instead of just adding 9.81m/sec...I should be adding 9.81 * 9.81?

Nope. Gravity is approx. 9.81m/s/s or as matty puts it m/s^2. This just means that it's an acceleration and not just a speed. i.e. Speed(m/s) due to gravity increases by 9.81(m/s) every second.

If you look up any aircraft of wikipedia under the aircraft weight it will give you 2 values. One in pounds and one in kilograms. In the U.S. we use feet and pounds and slugs/ft^3 for use in aerodynamic equations. I'm guessing in the UK you guys use meters and kilograms and kg/m^3. Is that correct?

Also, say you're using a phsyics lib like JV-ODE. When you set the mass of an object to 1.0 and 1 unit in blitz is 1 meter. Are you setting the mass to 1 kilogram? Or is it just some arbitrary value that everyone uses but don't really define?

I just noticed this statement and it's incorrect. Mass is constant. Weight is based on gravitaional pull. Ie...on the moon, mass is the same but the weight is different because the gravitational pull is less.

Force (in N) = mass (in kg) * 9.8

1 pound = 4.45 Newtons.

An F-16 weighs 26463 lb. That would be 117760 Newtons and a mass of 12016 kg.

Now if I look on wikipedia for the F-16 is says it weighs 26463 lb with (12004 kg).

Here's the kicker. The engine puts out about 14500 lb of thrust and lists 64.9 kN also. I'm assuming that's kilo-Newton. This is the part that messes me up. Brb, gonna go rip my hair out and slam a 6 pack. Back, kN is just multiplying by 1000 so that's not too bad. So the engine puts out 65000 Newtons.

So I can say my aircraft weighs 117760 N. Has a mass of 12004 kg. Has a wing area of 27m. I'll buy that for a dollar. Hope it works later.

MilliMeters = toe nail thik.

Here you go: http://en.wikipedia.org/wiki/International_System_of_Units

Well you could, but it would be wrong. Newton is a measure of force. The weight of the aircraft (when not in motion, and at the earths surface) is the same as its mass. In motion it depends on numerous factors (primarily forces working against the pull of gravity [like lift].

Technically, you're wrong. People do refer to mass as a weight casually, but not generally in scientific work.

Oh yeah, try to plug your thinking into a physics engine. Tell us all what happens. :)

The weight of the aircraft is not affected by its motion, lift, drag etc. For a flight sim, you can assume it's weight is not affected by distance from the Earth's surface as this is a tiny, unnoticable effect.

To calculate such tiny fluctuations in weight would be ludicrous imo.

You've already established that weight is the product of mass and gravitational pull. So how do you reckon that as the gravitational pull is canceled out by lift, the aircrafts weight is not diminished?

Yes. Well these people are not in the business of writing flight simulators.

Weight is indeed diminished, but only as the distance between the aircraft and the earth increases, and then only slightly for the distances we're talking about. Weight does NOT change because other forces are acting on the aircraft. Weight is the force of gravity acting on the object's mass.

The aircrafts weight is not diminished it's just being countered by lift. If weight was canceled as you're implying, there would be no inertia. I mentioned that some people refer to kilograms as a weight (you), which is wrong. And I am writing a flight simulator so I can't do that. Can we stop talking about this part? It's just worthless.

All I'm concerned about is getting the unit of measurements right in the lift/drag formulas. I found an article that states.



Air Density at sea level is rougly 1.2929 kg/m^3
We'll say velocity is 150 knots (102 m/s)
And wing Area is 27 m^2.
We'll say Cl is 0.9

Lift = 0.9 * 0.5 * (102^2) * 27 * 1.2929

In this case the lift being generated is 163433 Newtons. That's 36741 pounds of force. This is with about 4 angle of attack, meaning the Cl is .9 because the pilot has pulled back on the stick. When the AOA is close to 0 and in level flight the lift being generated will pretty much match the weight of the aircraft above stall speed.

I generally have this working, I'm just trying to get it all converted internally to SI units correctly.

Next on my list is being able to implement using NACA airfoil characteristics.

Inertia depends solely on the forces operating on an object at any given time. An object can have 0 weight (outside significant gravitational pull) yet still experience inertia (from thrusters and other such forces).

Yes. Kilograms is a measure of mass. I realize I got them mixed up in my previous post. I meant to say that that your aircraft exerts a force of 65000 N on whichever surface it is resting on, and that this force is then eventually canceled out by other forces (like lift).

Then stick to SI units religiously.

Well yes, but what did we just agree on with regards to using SI units? Particularly you should stay away from "pounds" as that can be used to mean both mass and force and the two are not immediately comparable.

In which case then yes, you are correct.

I'm sticking with SI units and I found that the air density was causing some slight problems. When looking up air density I saw .0023769 slugs/ft^3 for the english system at sea level and 1.2929 kg/m^3 for SI at sea level, but they aren't the same when converted. So, all I had to do was convert slugs/ft^3 to kg/m^3 and I got approximately 1.225225.

Then of course I look again and on a different site it lists 1.225 kg/m^3! Argh...oh well.

THIS should work and it's equivalent to the English system. I really can't see how it would falter now...will post some results when I try it later tonight.

You can use arbitrary values providing they remain consistent throughout the simulation. A mass value of 1.0 equals 1 kilogram in ODE if you're using 9.81m/s for gravity.

Thanks viper.