vjaqua wrote:

Paul - To put some numbers on this diameter thing, I used your potential
Cruiser
case with 250 hp. With good cleaning, this would put you in the top speed
full
power sea level 215 mph range 75 percent for cruise would be about 180 hp

and about the low 190's TAS. I ran a couple of programs, one based on
absolute
max prop efficiency as related to hp perfectly translated into air flow
powe
and the result for different diameters based on perfect momentum thrust.
the table below shows that case

speed        60in       72in        84in      96in
100          .695       .742       .780     .811
125          .780        .821       .853     .877
150          .840        .875       .900      .918
175          .883        .911       . 930     .944
200          .913        .935        .950     .961

So at cruise speed and power loading, even going to an incredible 8ft dia

prop can only gain a couple of theoretical percent efficiency

A more real case with my prop sizing program, which has real lift/drag
expressions in it would predict - geared for 2700 prop rpn and 72"dia
sea level cruise eff is .838  - gear for 2400 rated and an 84in prop
and eff only comes up to .849, and you would be fighting over 100 inches
of pitch during take off - full stall over most of the blade
Vance

Yes. The real question is what is the thrust at static (stall) and how does it
vary with speed up to cruise. If you can answer that I can calculate the
takeoff distance and the climb rate. Don said the same but he had
no numbers either. 

BTW do you have any takeoff distance numbers for the Cruiser with 
a 180 Lyc or the six cylinder 210 HP Cont. IO-360 in Bill Grote's Cruiser?

Also if anybody has that data for any of the RV series 
with the 160 or 180 HP Lycs I would like to have it as well.

Tracy Saylor gets airborne in about 350 feet with his 180 HP? Lyc
and constant speed prop in his RV6. This exceptional take off performance 
is kind of useless because the gear won't handle rough fields. That is the 
crux of my point. Most high performance home builts have such a high power 
to weight ratio the takeoff distances are unnecessarily short in a world
with 3500 foot and longer runways. 

Tracy Crook gets airborne in his 13B RV4 on his grass strip in about 500 
feet I would estimate just from watching. I think we can sacrifice
a bit of this for more HP by running the engine up to 7000 RPM or
above with a 2.85:1 PSRU ratio instead of 2.17:1 psru ratio and more pitch
in the prop.

Paul Lamar
 
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