prop rpm question

On Thu, 26 Jan 2006 at 19:29:21 in message
, Brian Whatcott
wrote:

Hi Brian,

Hi David,

let's forget about jets and recips. Let's imagine a vehicle that is
provided with a constant thrust device.
Then, the faster it goes, the more hose power it provides.
You can take it to the bank


No don't let us forget the basic ideas of propulsion.

A constant thrust device is doing one of two things;

1. Accelerating. In which case it is adding to its kinetic energy
and its power is going into that or

2. It reaches a constant speed against a constant drag and a steady
state occurs..

I presume you are not claiming that a constant thrust motor can generate
infinite power? In that case you would be right!

What do you have in mind as a constant thrust device? Newton's laws are
pretty good and I am not aware of any means of getting around them. They
only need adjustments at velocities and masses far beyond normal
terrestrial transport activities.

It is of course true that the efficiency of propulsion devices does vary
with speed and many other conditions.

--
David CL Francis

On Mon, 30 Jan 2006 00:26:39 GMT, David CL Francis
wrote:

[Brian]
let's forget about jets and recips. Let's imagine a vehicle that is
provided with a constant thrust device.
Then, the faster it goes, the more hose power it provides.
You can take it to the bank


[David]
No don't let us forget the basic ideas of propulsion.

A constant thrust device is doing one of two things;

1. Accelerating. In which case it is adding to its kinetic energy
and its power is going into that or

...Good

2. It reaches a constant speed against a constant drag and a steady
state occurs..


....Good

I presume you are not claiming that a constant thrust motor can generate
infinite power? In that case you would be right!

I am claiming that engineers are familiar with two Newtonian equations
1) force times distance (in the direction of the force) = work
2) force times velocity in the direction of the force = power

I understand that it is non-intuitive to non-engineers that the
arrangement of
eqn 2) as
3) force = power / velocity is always true until large fractions of
c.

Though I cannot offer any further debate with you on this topic,
(unless you wish to pay me) it is helpful for you to know that thrust
is an equivalent term to force, and so for a vehicle with thrust
5 units and velocity 10 units, its power is 50 units.
5 = 50 / 10

Moreover, I am pretty sure you can work out the missing term in this
question
What is the power of a vehicle with thrust 5 units, and velocity 20
units? 5 = P / 20





(power = 100 units)

I urge you to contemplate the great simplicity of Newton's laws - and
their interesting practical applications.

Respectfully

Brian Whatcott Altus OK

On Thu, 2 Feb 2006 at 17:59:33 in message
, Brian Whatcott
wrote:

I am claiming that engineers are familiar with two Newtonian equations
1) force times distance (in the direction of the force) = work
2) force times velocity in the direction of the force = power


The above statements are true but not the whole story.

Are you considering a situation on a vehicle passing through an
atmosphere or a rocket in empty space? If there is no drag then the body
will accelerate as long as the thrust is present. Rockets can produce a
constant thrust but by their very nature the mass of the body reduces as
fuel is used up and the acceleration increases. What is the measure of
the power developed? The momentum change taking place as the fuel and
working fluids are expelled through the jet at their inherent velocity
or the distance the rocket moves? In space there is no resistance and
therefore the distance moved is irrelevant to power. The power is
transformed into a velocity change and kinetic energy. In an atmosphere
speed will increase to a steady state when thrust and drag are equal. Of
course they are both forces.

I understand that it is non-intuitive to non-engineers that the
arrangement of
eqn 2) as
3) force = power / velocity is always true until large fractions of
c.

Though I cannot offer any further debate with you on this topic,
(unless you wish to pay me) it is helpful for you to know that thrust
is an equivalent term to force, and so for a vehicle with thrust
5 units and velocity 10 units, its power is 50 units.
5 = 50 / 10

I do not feel inclined to continue this against such patronising
elementary statements.
--
David CL Francis