On Oct 11, 8:44 pm, "Kyle Boatright" wrote:
wrote in message

oups.com...



On Oct 10, 9:17 pm, "Kyle Boatright" wrote:

Don't confuse thrust with HP unless you're using the exact same prop and
prop RPM. An 0-360 powered Robinson R-22 generates what, 1500 lbs of
thrust
because of gearing, disc area, etc? But, that thrust is only effective
in a
narrow (vertical) speed range. The same engine, combined with an
RV-style
cruise prop, would only generate (WAG coming) 250 lbs of static thrust.

That Soob had the same diameter of prop and ran at the
same RPM. I'm aware of the diameter/RPM/pitch effects, and that's why
the results were so startling to me. The 150s performance was much
better with the Soob, confirming its output. So either the Soob
produced more power than claimed, or the O-200 was really sick. Yet
the guy said that the O-200 was OK and the 150's performance with it
was typical.

Dan

I still think it is important to have the *exact same* prop, not just the
same diameter. Depending on prop pitch, chord, etc. you could get
significantly different amounts of thrust from props with the exact same
diameter.

Not so much. Efficiencies are typically in the range of
85-90% unless the prop is a real dog, and I don't think Cessna would
have continued using the prop they did on the 150 if it was a poor
performer.
We had an Ivoprop on a 135 hp Soob in a Glastar. Didn't trust
that prop, and had problems getting it to run smoothly. We eventually
put a Warp drive on it, three Blades instead of two, but the
performance was nearly identical even though there were big
differences in chord and planform.
The guy who put the Soob in the 150 may have used the same
prop, but I can't remember. Some of those Soob conversions used the
Lycoming bolt circle instead of the Continental, and the Cont's prop
wouldn't fit, of course.
As you mentioned, a larger, slower-turning prop is far more
efficient, so a redrive is better than direct drive, especially where
an auto engine is used. The airplane that became the Helio Courier was
a much-modified Piper Super Cruiser, IIRC, with a 125 hp Lyc driving a
big, slow prop through a redrive using several V-belts. Very rapid
acceleration and spectacular climb, aided by slats and flaps and the
usual STOL wizardry.

Dan