From: svoboda@rtsg.mot.com (David Svoboda)
Subject: Re: Props
Organization: Motorola Inc., Cellular Infrastructure Group
Date: Tue, 28 Jul 1992 23:04:34 GMT

In article <1992Jul27.225145.1783@iex.uucp> djones@iex.iex.com writes:
|In article <1992Jul23.153612.20836@rtsg.mot.com> svoboda@rtsg.mot.com (David
|Svoboda) writes:
|>
|>"Double the diameter and add the pitch" is an old rule of thumb that I use to
|>determine engine loading.  So an 11x6 prop produces a "load index" of 28, as
|>does a 10x8, so those props will give approximately the same engine load. 
|Note
|>that a lower pitch prop will unload in the air more than a higher pitch 
|>prop (with the same load index), and will make the airplane fly more slowly,
|>and climb more positively.
|>

|Does this hold for both 2c and 4c engines? 

Prop load is prop load.  I would use it in any case.

|And would it be safe to say
|that if a plane is a little underpowered, that a higher pitch for a given load
|index would be preferable? For example if the manufacturer suggests a 11x6 as
|the optimum
|prop, and the plane is a little heavy, then a 10x8 would be better. Or are
|really loading then engine down MORE and thus making the plane even more
|difficult
|to fly?

A 11x6 prop would be similar in engine loading to a 10x8.  That is, you would
realize approximately the same engine rpm *at a given airspeed*.  Now, remember
that a lower pitch will give you a lower airspeed, but more static thrust.  Think
of it as a "lower gear".  Thus, a low pitched large diameter prop is good for
a naturally slow airplane, and will give you a good climb.  A higher pitch prop
will have less climb or acceleration, but will pull the airplane with a faster
top speed (and a faster cruise speed, for that matter).  If you are underpowered, 
a larger, lower pitch propeller would be a better use of the engine's horsepower, 
but you have to limit that becuase that airplane will have a minimum speed, and 
an extremely low pitched prop would not pull it faster than that (stall on takeoff).

An example of this principle is my Sig Skybolt, with an OS longstroke 61 with
Perry micro-oscillating pump.  I started out using the 11x8 prop, like I did on
my last airplane (an old Super Kaos (a Bridi kit, if anyone remembers them)).  Now,
the Skybolt only has a 50 inch wingspan, and the 11x8 pulled it around so fast
that I thought I would shed some wings.  So I bought a great big 12x6 for it,
and it still turns at about 10,000 rpm (remember, it's a longstroke; it's supposed
to lug), but it flies considerably slower, but now climbs like a rocket (a slow
rocket, but a rocket).  I fly around at half throttle most of the time, but
between half and full throttle won't speed the airplane up appreciably, it just
makes it climb better.  (Can you say "Consecutive Vertical Snaps"? :-)

|Do you set a lower pitch prop a little richer on the ground or does it really
|matter. 

A higher loading prop will lug the engine, which makes the mixture very hard to
adjust.  You can easily have a lean run, and have it masked.

|I was taught to fill the tank 1/2 way, warm the engine, peek the
|RPM on a tach and then back off about 500 RPM. Fill the tank and go fly. This
|technique was taught to me more for 4c than 2c but I expect it would work 
|in both cases.

I'm not any sort of 4-stroke guru (well, with motorcycles, maybe, but not model
airplanes :-), but that sounds like a good method of protecting your engine.  I
would further suggest that you peak the engine with the nose pointed at the
sky, rather than level.  And listen and look at it flying--some engine just seem
to change mixture in the air when they unload.

|>Changing prop brands will skew this rule a bit, because different airfoils,
|>blade widths and planforms will produce different drag.
|>
|There appears to be no easy answer!

Sure there is.  Try an alternative, and see if it works.

			Dave Svoboda, Palatine, IL
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