I would like to get some educational (for me) discussions going on
this.
A prop of 76 X 88 is pretty common in our usage. Tracy, Ed, and I
have a Performance Prop in this dimension. Dennis and maybe others have a
Catto prop in this dimension. We all seem to be getting static rpm of
about 52-5400 rpm (except for Dennis with his new DIE manifold). Tracy
and Ed had their prop cut down to 74 X 88 and are getting increased static to around
6000 rpm. Higher rpm = higher HP for the rotary. We should get
higher thrust with a slightly smaller diameter prop? This has something to
do with the idea of sizing the prop to the engine. I wonder what is the
proper size? What is the proper static rpm for best performance with the
rotary? What did Tracy and Ed lose in prop performance and what did they
gain in total performance when they cut the prop down?
It seems to me that a prop sized for climb would allow around 7500 rpm
at about Vx or Vy? Max speed would require 7500 rpm at WOT sea level?
I wonder what rpm our props allow at these speeds? If you had a prop that
would do the above, I wonder what the static rpm would be? Then since
most of us have fixed pitch props, I wonder where we should try to be for the
best of both worlds (a compromise)?
We have some really good engineers in this group and they have made
these selections. I know they know why they made the selection they did.
How about sharing? :>)
Don’t worry, you can not ramble on too much for me!
Bill B
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Tracy Crook
Sent: Wednesday, March 03, 2010
2:32 PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re: single
rotor
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.
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