Charlie is right.  Consider that an airplane flying with the wing at 0 deg. AOA does not fall out of the sky :-)

My average speed in the 2004 Sun 100 race was 217.58 mph which includes standing start takeoff and climbout.  Prop was a 74 x 88, RPM was 7250 with a 2.85 : 1 drive.  If you calculate that out it comes to 212 mph with zero 'slippage'.   Draw your own conclusions!

Tracy

Sent from my iPad

On Jul 23, 2014, at 22:59, "Charlie England" <flyrotary@lancaironline.net> wrote:

Ice sails, desert sails, and now, even unlimited class sailboats can sail faster than the wind. 'Negative slippage'. :-)

A more significant point might be that pitch numbers are virtually meaningless, unless you're comparing two props from the same prop maker with the same blade plan form. Even then, it just tells you which has a finer pitch than the other.

Variables can be: whether the pitch is measured on the back side of the blade or through the chord line, where along the diameter the pitch angle is measured (due to blade twist), and no doubt others I'm not smart enough to think of at the moment.

Bottom line is that unless there's an identical airframe flying an identical prop, the pitch number isn't a reliable indicator of speed.

FWIW,

Charlie



On 7/23/2014 7:45 PM, James Osborn wrote:

I don't know jack about slippage, but I think it is the percentage difference between actual distance traveled and theoretical distance traveled if your propeller corkscrewed through the air with no thrust.  I found a prop slip calculator online and for 86 inch pitch, 2.85 gear ratio, 7000 rpm, 180 mph, I get 10% slip.  Granted the calculator was for boat propellers, but I don't think it matters as long as the units are correct.  There has to be some slip because there would be no thrust otherwise.  So what is considered a reasonable or good amount of slip?  Using Bill's numbers 86 inch pitch, 2.85 gear ratio, 7000 rpm, 200 mph, I get 0% slip.  That can't be right!

On Fri, Jul 4, 2014 at 7:19 AM, Bill Bradburry <flyrotary@lancaironline.net> wrote:

Christian,

It seems that you have a lot of prop slippage at cruise.  I think that at

that prop rpm you should be getting 200mph if you had no slippage.

Bill

-----Original Message-----
From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net]
Sent: Thursday, July 03, 2014 4:01 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Rv7 renises p port

Well hi all
Just thought I'd throw out there the mods I,ve done to the renises in an
rv7'.
Well the p ported engine is now back in the plane and running well, over the
standard short manifold that was originally in the plane I have gained
around 400 static rpm, same prop and gearbox combo, 2.85 ratio, this equates
to allot more hp at takeoff, just shy of 2300 prop rpm, I'm running a prince
p tip prop at 68" x 86" pitch,
At 8000 ft it is turning 7000 at 180 mph tas which is an improvement of 25
mph on previous tests, . So next plan is bigger prop and less pitch to let
it rev to 7500 in strait and level.

Cheers
Christian
Rv7 renises Aus


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