Mathcad not using seconds properly

[ptc-4729593]

Hello,

Whenever I use a unit such as Newton or velocity.... anything that has a "per second", MathCad multiplies it incorrectly. If I type out m*sec^-1 it works. Why is this and how do I fix it? It is very cumbersome when working with lbf and newtons.

[StuartBruff]

Can you post a worksheet or image with an example of what you mean?

[ptc-4729593]

I attached the file. The top part of the problem is worked by saying m*sec^-1 while the bottom is not.

If you look at the value for B... it is apparent that the seconds did not get moved to the top like they should have.

[ValeryOchkov]

Dwight Temple wrote:

I attached the file. The top part of the problem is worked by saying m*sec^-1 while the bottom is not.

If you look at the value for B... it is apparent that the seconds did not get moved to the top like they should have.

See the picture:

[RichardJ]

You defined Tsl as a force, and ro as a density. So the units of B, kg^2*m^-2*s^-2, are correct. Perhaps if you told us what the variables are supposed to represent we might be able to tell you where the mistake is.

[ptc-4729593]

edit: the units on B^2 are correct. but 4*A*C should have m^4 on the bottom and it doesn't

[RichardJ]

Q1: A is the product of a density and area (according to your units, anyway). C is the product of two dimensionless quantities, W and k. the variable k is defined depending on AR and eO, both of which are dimensionless. What do W, k, AR, and eO represent?

Q2: Where does the quadratic formula come from? Is it empirical? Empirical formulas require very specific unit handling.

Mathcad is not wrong. You either have the wrong units on one or more of your quantities, or your formula is empirical.

[ptc-4729593]

A1: W is the weight in lbf - AR = b^2/S (dimensionless) eO is dimensionless aircraft constant. so k is dimensionless.

A2: the quadratic is a solution to a very lengthy equations... 1/2(ro*S)^2-T*S+2kW^2=0 the "X" associated with the quadratic formula that I am solving for = v^2 so. my final result should be m/s

[Fred_Kohlhepp]

Looks like you've got a unit balance problem.

If W is a weight, then it should have units lbf, your problem 6 sated "W:= 75000", no units.

1/2 rho S^2 looks suspiciously like dynamic pressure to me, I've never seen air density times an area, quantity squared. (Could be wrong . . .)

I've also never seen force (t_sl = sea level thrust?) times density.

Can you cite a reference?

[RichardJ · Accepted solution]

You don't have W defined as a weight. You have it defined as a dimensionless quantity, which is not a weight.

Are you saying the quadratic is

where x=v^2? Where did this come from? Either the units in all the terms must match, or it is empirical (i.e. not derived from physics, but calculated by fitting a quadratic to measured data). Or, of course, it could just be wrong. Is this an empirical formula?

In the Mathcad worksheet it says "Problem 6", and I assume this is the answer. In which case it might help if you posted the problem.

[ptc-4729593]

The equation comes from steady level flight conditions.

Thrust = 1/2 roV^2*S(Cdo +k*Cl^2)

1/2 (ro*S)^2*CdoV^4 - Thust*S*V^2+2kW^2 = 0 ---> make edit

While wokring through this problem I solved my problem. I had x representing v^2 and something was not working out properly. So... I did no substitutions and left v^4 and v^2 in the equations and all was fine

I apologize for my incessant questions! I am just a beginner and have much to learn ll!

Thanks a

[Fred_Kohlhepp]

Good for you!

One of the really helpful things about Mathcad: If the units don't balance there's usually an error in the equation. (Saved my behind more times than I care to admit.)

[RichardJ]

Asking questons is no problem.

[ptc-4729593]

So what am I supposed to do exactly? I have the same menu values checked as you do, but nothing has changed.