On 5-Aug-2003, "Wayne" wrote:

I am getting the 1961 175B Cessna that I mentioned before here. I am
trying to figure out how fast it should be. The owner said that it would
go GPH. Then I looked at the original specifications from Cessna.
It
was supposed to cruise at 122 knots @ 75% power at 7500'. I had assumed
that he meant 125 MPH but that is only a little less than 109 knots. This
has
the Lycoming O360 conversion with a constant speed prop, I would think
that
would make it slightly faster and surely better on takeoff performance.
The 2003 172 that I fly is supposed to cruise at 115 knots @ 75% (I forget
the
altitude) so I wouldn't expect this one to fly faster than a new one with
the same HP and a fixed pitch prop. Anyone have a simular plane? Anyone
know what I should expect? Did Cessna stretch things that far in 1961?

Wayne


I don't have any experience with C-175s, either with the original geared
engine or with the 0-360 conversion. However, over hundreds of hours in a
C-172N (with the original 150 hp 0-320) I can state that it is a 112 kt
airplane at best. The important thing to remember is that, for a given
airframe, cruise speed will vary as the cube root of horsepower. Thus,
going from 150 hp to 180 hp, for instance, will increase cruise speed (at a
given percentage power setting) by only 6.3%. That would boost a 112 kt
cruise speed to 119 kts.

The idea that you can boost cruise speed with a modest power increase is
mostly wishful thinking. A bigger engine really helps mostly in takeoff and
climb performance (or, in some cases, allowing for more useful load). A
significant increase in cruise speed generally requires aerodynamic cleanup
of the airframe.

You will rarely meet a seller who can resist exaggerating the performance of
the plane he/she is selling. However, with the availability of handheld GPS
units, measuring cruise speed during a test flight is fairly easy. Just fly
in three headings 120 degrees apart, within a relatively small area (so the
wind will not change) and record groundspeed (from the GPS) for each
heading. From that data it is easy to compute the wind component, and then
the true airspeed. If the air is reasonably smooth you should be able to
calculate to within a knot or two.

-Elliott Drucker