Calculating Speed From Thrust

Hi,

Once you know the thrust your motor and prop setup provides, can you then use 'F=ma' to obtain a rough idea of speed? Obviously you need to calculate resistive forces....

Thanks,

Michael

F=MA

A=dV/dt

F/M=dV/dt

(F/M)x(dt)=dV

This is only useable in a theoretical sense. The geometry of the body you are applying the thrust to has a HUGE effect. The mass only applies an effect while under acceleration or in varying the velocity which means both the magnitude of speed and direction. In a steady state, steady flow condition the mass quantity falls out, or in Carnival Rubber Duck Driver terms-constant speed and with no change in direction.

You could be applying this thrust to a low drag, torpedo shaped object and arrive at one velocity, or a flat plat with the large surface exposed to the slip stream. You must first calculate the reyolds number from which you can determine the form drag factor (Cf). You must also understand, then, the dynamics of that body in the medium to which it is moving in.

Of course I have no experience in the above, I have no idea what I am talking about, and my math could not stand the light of day. LOL

Back to the lab.

[Aeroengineer1]

Simply stated no. unless you know the drag of your body for a given velocity and your thrust for a given velocity (thrust is not constant) you cannot do it. if you were to try and calculate the drag, drag is estimated through a taylor series expansion , but neglecting terms can be approximated to the square of velocity. This is not a linear function and is hard to get an exact solution. There are other methods for estimating drag, but they are only valid for higher reynolds numbers than we operate. I am in a class right now and we are using some of these methods to calculate drag on a glider. In my opinion, for this scale it is not worth the effort, use the "tlr" method combined with the "swag" method and go forward.

Adam

Adam,

I believe I said that!

It simply is not easy and not worth the effort for sure! NASTRAN makes it easy, though. I did not like this class. It gave me a headache!

[Aeroengineer1]

Yes you did. As I wrote it I remembered back to the day, but now I can calculate the drag on my model if I can get the Rn high enough. But who cares? Not me. This is my engineering sanity check.

Adam

[Subculture]

Have you finished that Deep dive VI yet, Michael?

Thanks Guys,

Andy,
No, not yet, snowed under at the moment with coursework, it's the busiest time of the year for me.

Michael

Andy,

Does your deep quest work?
Michael

[Subculture]

Very nearly, aye.

Just ordered some merkel seals for the piston tank.

Andy