KR> [CorvAircraft] Prop

[brian.kraut at eamanufacturing.com]

I spoke to Ed Sterba about that a few years ago and he told me that low
drag planes like the KR and the Midget Mustang can go faster than the
theoretical pitch will allow.

I am not a prop expert, but if I had to take a wild guess as to a theory
which may or may not be correct I would look at this.  Prop pitch is not
as simple as the angle of incidence of the airfoil.  A propeller is not
a screw that exactly moves so many inches forward for each revolution. 
For one thing, it is an airfoil, not just a flat surface.  It is not
"screwing" its way through the air, it is creating lift 90 degrees to
the axis it is spinning.  And if it were a screw you need to remember
that the root is a much higher pitch than the tips so the prop would
"screw" its way further at the root than the tip.  The effective
distance it would "screw" forward is somewhere between the pitch of the
root and the pitch of the tip and depends on the drag of the plane.

Or to sum it all up, it is PFM and black magic and the formula, as Mark
says, does not always fit, it is a rule of thumb to get an approximation
only.


 Original Message 
Subject: Re: KR> [CorvAircraft] Prop
From: Mark Langford via KRnet 
List-Post: krnet@list.krnet.org
Date: Sun, June 12, 2016 7:32 pm
To: CorvAircraft , 'KRnet'

Cc: Mark Langford 

That equation has never worked for me either. If it makes anybody feel 
any better, Joe Horton reminded me today that he can reproduce 180 mph 
TAS on a regular basis, and his ASI system is well calibrated. He also 
has a nosewheel out front. Temperature and altitude are factors, and of 
course RPM and weight of the plane.

I flew the KR2 (old wing) with a 54x50 prop today at WOT at 168 mph TAS 
at about 2500 ft, so I'm getting over 100% efficiency. Bottom line is 
this equation doesn't fit KRs. Properly built, they are simply more 
efficient than most airplanes out there, especially at the time that 
equation was probably hatched...I'm guessing the early 1940's.

Mark Langford
ML at N56ML.com
http://www.n56ml.com



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KR> [CorvAircraft] Prop

[Larry Flesner]

At 09:32 PM 6/12/2016, you wrote:
>That equation has never worked for me either.



The only way the equation can be in error is if the "inches of pitch" 
on the propeller, as stated by the manufacturer, doesn't actually 
relate to real world performance which it sounds like it doesn't.

My prop is somewhere between a 64 and 68 pitch.  It started as a 68 
and was reworked to a 64 but I couldn't tell the difference.  At 66 
inch pitch and 2600 rpm, the rpm I ran the race,  and 100 percent 
efficiency,  my speed would be 162.5 mph.  I was timed at 163.8 .  It 
seems to be a good rule of thumb if nothing else.

Larry Flesner

KR> [CorvAircraft] Prop

[Larry Flesner]

At 08:30 PM 6/12/2016, you wrote:
>Mine cruised at 155mph with a 54X54 and 2700CC at 2950RPM.
++

A 54 inch pitch propeller running at 100% efficiency would only make 
151 mph at that RPM.  Either your tach or ASI were in error or my 
math teacher in a country school was in error.

54 inches X 2950rpm = 159,300 inch per minute X 60 minutes= 9,558,000 
inches per hour /12 = 796,500 feet per hour / 5280 = 150.85 mph

Larry Flesner 

KR> [CorvAircraft] Prop

[Mark Langford]

That equation has never worked for me either.  If it makes anybody feel 
any better, Joe Horton reminded me today that he can reproduce 180 mph 
TAS on a regular basis, and his ASI system is well calibrated. He also 
has a nosewheel out front. Temperature and altitude are factors, and of 
course RPM and weight of the plane.

I flew the KR2 (old wing) with a 54x50 prop today at WOT at 168 mph TAS 
at about 2500 ft, so I'm getting over 100% efficiency.  Bottom line is 
this equation doesn't fit KRs.  Properly built, they are simply more 
efficient than most airplanes out there, especially at the time that 
equation was probably hatched...I'm guessing the early 1940's.

Mark Langford
ML at N56ML.com
http://www.n56ml.com