How do you convert 81 newtons thrust to power output in watts?

[willib]

Lets talk (Wren) Turbines..

http://www.wren-turbines.com/specifi.htm
How would you convert 81 Newtons Thrust output to power output in watts ??

 

[FredGarvin]

Jet engines have a thrust horsepower calculation, but I don't think it could be held applicable in this case. The rule is at 375 mph, 1 Lbf of thrust is equal to 1 HP.

 

[willib]

These Wren turbines are just under three inches in diameter..
very cool stuff..

 

[Q_Goest]

Hey Fred, that works out, because if you say an aircraft is flying at 375 mph and that aircraft requires 1 lbf of thrust to keep it going at that speed, the power (force times distance per unit time) works out to 1 hp. But that's an arbitrary velocity. If the aircraft has more resistance, and can only fly at 100 mph with the same engine, then the equation results in a power output of 0.2667 hp. And if it's zipping along at 1000 mph, the power output becomes 2.667 hp.

So there doesn't seem to be a correlation between hp and thrust. But there should be since the power actually equates to energy in (ie: energy burned in the engine).

I always wondered about this one, how can you equate thrust to hp? Perhaps because it's "static thrust" one must be able to get a "thrust curve" which is analogous to a pressure curve on a centrifugal pump, along with an efficiency.

There has to be more to it than simply thrust at 375 mph equates to a given hp.

 

[FredGarvin]

Hey Fred, that works out, because if you say an aircraft is flying at 375 mph and that aircraft requires 1 lbf of thrust to keep it going at that speed, the power (force times distance per unit time) works out to 1 hp. But that's an arbitrary velocity. If the aircraft has more resistance, and can only fly at 100 mph with the same engine, then the equation results in a power output of 0.2667 hp. And if it's zipping along at 1000 mph, the power output becomes 2.667 hp.

So there doesn't seem to be a correlation between hp and thrust. But there should be since the power actually equates to energy in (ie: energy burned in the engine).

I always wondered about this one, how can you equate thrust to hp? Perhaps because it's "static thrust" one must be able to get a "thrust curve" which is analogous to a pressure curve on a centrifugal pump, along with an efficiency.

There has to be more to it than simply thrust at 375 mph equates to a given hp.

You do bring up good points. Personally I have NEVER expressed a thrust producing engine in terms of horsepower. It is always in thrust. For some (unknown to me) reason there is a small need to express HP in this fashion. The relationship $$THP = \frac{F_n * MPH}{375}$$ is actually from a Pratt and Whitney reference I have. It is the only reference I have that even states it. What it's method of derivation is I really do not know.

 

[willib]

Oops my mistake..
please disregard..

 

[Danger]

The relationship $$THP = \frac{F_n * MPH}{375}$$ is actually from a Pratt and Whitney reference I have. It is the only reference I have that even states it. What it's method of derivation is I really do not know.

As I understand it, hp:thrust ratio depends on not only speed, but also altitude. I assume that it has something to do with factoring in air resistance.
It always bugs me that a full-throttle jet bolted to a test stand produces zero horesepower.