On Jan 18, 1:08*pm, bildan wrote:
On Jan 18, 9:40*am, T8 wrote:



There's an old rule of thumb that says that you need take off thrust
of about 1/4 your gross take off weight for satisfactory performance.
This holds true for a remarkably diverse range of aircraft, from J-3s
to jets. *You can struggle off on less under favorable conditions, but
not a great deal less.

There's another rule of thumb that says that you get roughly 4.5 lbf
_static_ thrust for every hp in a typical propeller driven light
plane.

At 60 kts, a reasonable efficiency estimate for a light plane
propeller is 75%, yielding right around 4 lbf thrust per actual
developed hp at takeoff. *That's about 800 lbf thrust for an L-19 or
any other 200 hp tow plane on a warm but not hot day. *Plug in the
weight of your tow plane (fueled, with pilot) and various gliders it
could be towing and now you have some good semi-quantitative insight
into the relationship between thrust, weight and take off
performance.

-Evan Ludeman / T8

(some of you may see double post -- sorry about that: posted on wrong
account)

There are a number of propeller calculators on the web. Entering the
prop diameter, pitch, airfoil etc. and the engine power and RPM
suggests a 235 Pawnee generates 400 Lbs of thrust at towing speeds.

Well, that's just not correct.

Some useful relations:

1 hp = 550 ft*lbf/sec

60 kts = 101 ft/sec

Apparent power = thrust * speed = brake hp * efficiency

Real world efficiency numbers are below 80%, typically 65 - 75% in
climb. Most light planes hit their best propeller efficiency in climb
or cruise/climb conditions.

-Evan Ludeman / T8