I find this interesting, but confusing.
While the amperage draw, of the prop) will give some indication of
prop blade pitch due to the fact that as the pitch changes, either positive or
negative from "neutral" and thus increases the current draw, Other than giving
one some sort of number to return to, for whatever reason, it does not directly
relate to the power that the engine is producing.
The important information, with a variable or C/S prop is not the angle of
the blade, but, only, the combination of MP and RPM.
Using these two indications, one can either calculate power, or
set consistent power values. So much more accurate to climb at 24"mp and
2400 RPM (hypothetical value for example only), or whatever than to set a pitch
and hunt around for some obscure MP, to give you a desired
RPM, never really knowing what the engine is actually producing.
If you look at a C/S prop, the blade angle/pitch of the prop is constantly
changing dependant on the load the propeller is experiencing in the various
phases of flight and the MP that is given to the engine.
The paddle blades that he referenced are produced for airboats, not
aircraft. Probably the main reason that he found them unable to absorb the power
was that they were considerably under pitched with respect to the original
Magnum blades.--30-90 inch for the paddle vs 45-105 inch for the magnum.
Additionally the fact that -----"Static rpm at 39 inches was 4750 at 6 amps
fine however as soon as speed increased, rpm shot up to 5250 at only 80
knots"---- was probably due to the phenomenon that his blades were initially
partially stalled at static and as the aircraft moved forward and the angle of
attack of the blades changed, they became unstalled.
A further point of confusion is the following statement--" I kept
retarding MAP to 25 inches in the climb to hold engine rpm under 5000 and I
finally had to increase prop pitch to neutral to stabilize the rpm. Needless to
say, the climb peformance is not so good at 25 inches.
It seems like he was trying to maintain a certain RPM, rather than a
certain power by controlling the MP, rather than using the pitch of the
prop and its power absorption/transmission capabilities. to control the
RPM This is akin to starting your car in first gear uphill, full throttle
and when your RPMs reach your limit, as the hill starts to level off,
reducing the throttle rather than shifting to a higher gear and, all the
time, wondering why your performance is steadily decreasing.
His comment that he had to increase the pitch to neutral indicated that he
was initially climbing with the blades at a rather flat pitch (on a relatively
flat pitch adjustability prop to begin with) and would coarsen no further even
though the twist of the blade was only at 1/2 max pitch.
My comments, are, by no means to be construed as criticism of
Ross, and what he did and published, nor to Dis the IVO--(I have one) rather my
intent was to shed some light on prop/engine control basics, as I know them,
(having 7 C/S aircraft with a total of 9 engines, in the past), and
possibly start a conversation that might increase the knowledge of handling a
C/S or controllable prop, for all of us------ including me..
I don't know Ross but would really appreciate his replying to this reply
(if he is on the list)
Rich
In a message dated 6/12/2012 10:53:18 P.M. Central Daylight Time,
DLOMHEIM@aol.com writes:
I would highly recommend that any of you running an IVO in-flight
adjustable prop to take a few minutes of your time and read Ross's write up on
his SDS site...some great information found at:
One thing I found interesting for instance was his use of a
dedicated AMP meter on his IVO circuit which then allows him to set
his pitch based on the amount of amps he is pulling...
Doug Lomheim
RV-9A, 13B /
FWF