Tracy or Ed,
Did you lose a little “top end”
when you trimmed the prop?
Bryan
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Bill Bradburry
Sent: Wednesday, March 03, 2010
5:01 PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re: single
rotor
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