flyrotary@lancaironline.net Mailing List Archive

Al is correct about it taking HP to make static thrust with a prop but the assumption about the relationship between HP and static thrust is subject to a lot of variables. There is no fixed relationship between static thrust and HP. If there were, you could not account for the ability of most helicopters to hover.

You could easily increase static thrust by 1.18 by increasing the diameter of the prop and the reduction ratio of the redrive with NO increase in HP.

But my real point was that static thrust is not a very useful measurement to us.

Tracy

On Wed, Mar 3, 2010 at 11:06 AM, Al Gietzen <ALVentures@cox.net> wrote:

Looking at the two sizes of the engine, it takes 1.6 times as much horsepower to develop 1.18 times as much static thrust! Somehow this does not compute for me….I always doubt the performance figures in a sales presentation and when they don’t make sense to me…..???

Bill B (hoping this generates an educational experience for me :>)

We’re talking about the amount of force exerted by the prop with the plane (motor) standing still.

So, it seems to make sense to me that the power needed to accelerate the air to generate the thrust would go as the cube root; and the cube root of 1.6 is very close 1.18.

To move the amount of air it takes to generate the thrust certainly does take horsepower. Very much the same as the power it takes to drive the pump (or generator) on a dyno. So I don’t know how Tracy was interpreting the question.

Al