Vance Jaqua wrote:

fixed pitch 70X78             const spd   var gear
   mph  rpm   bhp f lbs eff    f lbs eff  f lbs eff
    0   1940  108  407   0      603   0    525   0
   30   1950  109  393  28      530  26    505  25
   60   1960  110  353  51      480  50    460  47
   90   2080  118  327  66      425  63    420  62
  120   2400  130  307  76      366  73    365  73
  160   2500  149  283  81      299  80    299  80
  190   2720  160  268  83      268  83    268  83

Vance why would your fixed pitch and constant speed
props have about the
same eff. at 30, 60 and 90  MPH? I think you are off
here.
I think you are way off.  I think you forgot to
increase the eff.
for the constant speed prop. Put a HP column in
there for the constant
speed prop.

Let us define our terms here.

Theory thrust is = HP * 550/FPS.

Prop eff is = actual thrust/ theory thrust

Paul Lamar

Paul - What I am trying to stress is that constant
speed props are not more "efficient" than fixed pitch
props. Horsepower for both the constant speed and the
variable gear box is 160 over the entire range - they
let you operate the engine at full power, and that is
where they get more thrust - NOT with better
efficiency . Efficiency is output power - thrust times
speed versus shaft INPUT power at that point - not
rated power - Vance

With two people in my 182 and a full fuel weight of 2600 pounds
I can almost slide the main tires at low pitch and full throttle
with the brakes locked up.

Figuring only 50% of 2600 pounds on the mains and a coeff of friction of 0.8
that is over a thousand pounds of thrust.

A prop is just like a wing as it has a best angle of attack for the lowest lift
to drag ratio. In a prop the best lift to drag ratio gives the best
ratio of HP out over HP in or prop efficiency.

Fixed pitch props operate on the back side of the lift to drag curve until they get
to cruse speed. If you add power with a gear box you merely move further
up on the back side of the lift to drag curve. Yes.. you will get a little  more
thrust but
you will dissipate an inordinate amount of additional HP generating a monster vortex. 
This will merely heat the air dissipating that additional HP. 

Constant speed props on the other hand  are always changing their angle of attack to 
optimize the lift to drag ratio for the forward speed of the aircraft.

I will agree the max efficiency is the same for both props.

Paul Lamar
 
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