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Back a few years ago, there was a variable pitch prop hub
designed by a genus of a mechanical engineer. It had a pitch control
mechanism similar to the IVO adjustable pitch prop. I designed a control
circuit which among other things monitored the current draw of the pitch
motor.
While the prop project was not completed (to the best of
my knowledge), I found it was possible by conducting some test
measurements to correlate the current draw curve to propeller pitch. Now
the missing part was - we never got to the point of installing and running
the prop using the controller. Its highly likely that the current curve
would be different under air loads than not. We did not use a meter but
instead had the current turn on an LED (red) when the limit was hit.
One of the things that I had planned to do was incorporate
a manifold pressure input (as well as prop rpm) to automatically adjust to prop
pitch to maintain rpm under various loading. The hardware to do so was
completed and the software - just never got tested.
Just found some of the old code
begin //Main
RPM_Limit_Low := 3000;
RPM_Limit_High:= 7000;
RPM_TO := 5800;
RPM_CC := 5600;
RPM_CR := 5200;
RPM_DC := 5500;
//Put address of varible RPM_TO into
Pointer variable
PORTB.RB0 := 1; //set to enter while loop
in procedure
While testbit(INTCON,RBIF) = 0 do
I now recall that we actually had several target settings
such as TO (Take off), CC (Cruise Climb), CR (Cruise Range), and others for
various flight regimes. So you could choose TO, CC, CR or DC from a menu
and the prop was pitch was positioned/adjusted to maintain the rpm.
Manifold pressure was also a factor. Also it had direction LEDs so you
could select to manuall/electrically increase or decrease pitch and a bunch of
other things I have now forgotten. You could adjust those rpm values that
best suited your particular aircraft/engine combination - the preset values were
intended to reduce the pilot work load.
I was programming a PIC 18F450 chip to handle the control
and sensor inputs and provide a user interface on an LCD display with
buttons.
It would have been great had the prop project gone on to
completion, but alas despite the best efforts of a number of good folks it did
not.
Ed
Sent: Wednesday, June 13, 2012 5:52 PM
Subject: [FlyRotary] Re: IVO Prop controls
I think there's a miscommunication about how this device works.
It's quite simple: two commutator brushes on a fixed arm transfer current into
the prop hub to engage the motor and twist the prop blades. Like most
motors/actuators, you wire it such that positive voltage is applied to turn it
one way, negative turns it the other, and while not moving there is no voltage
applied. A simple (ON)-OFF-(ON) switch drives this.
There are only two
brushes so there's no way to transfer an at-limit signal, and although I'm just
guessing here, a reliable pair of limit switches that can operate properly
inside the prop at full RPM would have been complicated/maybe unreliable? They'd
also be a pain to adjust.
Anyway, the motor draws only a moderate amount
of current in the middle of its travel, and this increases as you approach the
limits. Standard practice is to install a meter to indicate this draw and it
tells you when you're near the limit.
You could install a PTC instead of
a breaker, but it's hardly an emergency. It's not actually SOP to run it that
far - in the times I've flown with John I don't recall him ever doing it except
perhaps once to show me what happens. You don't "run it until it pops." You "run
it until it's where you want it to be." So PTC or breaker, it doesn't really
make that much of a difference. Choose the safety device you prefer.
I
don't understand the bit about the shorted switch. That's pretty rare, and the
breaker would deal with it just fine. And I can't speak for anybody else, but
every car I've ever owned doesn't use a PTC to set the travel limit on the
window, it uses a limit switch on the actuator. I recall having to adjust mine
one time in a Subaru, just like the nose gear travel switches in a
Cozy.
Besides, what's the down side here? You short your switch and the
breaker will deal with it, and the prop will stop twisting. No matter WHICH
device you use, in this case you now have no way to move the prop because a
short would hold a PTC open and also keep tripping a breaker when you manually
reset that. Both devices have the same failure mode if it's the switch that's
the problem. But you still have a working prop even if it's not at the optimal
pitch. See if you can jiggle the switch to clear the short...
To each his
own.
Regards,
Chad
On 6/13/12 5:15 PM, Lehanover@aol.com wrote:
Why in the world would IVO use a device that is designed to
fail critical flight gear in the case of improper control
manipulation when
they don't have to? Isn't this the classical and proper application for a
polyfuse? Polyfuses are
used in power windows for this exact reason. You're
kids can pull on that switch all day without damaging the window
motor. I'm
thinking of the case where a switch gets shorted (like my belt sander's switch
is right now...the power cord
is serving as a temp fix until I get time),
or someone accidentally leans something against the switch.
Why does the motor draw current after the pitch has been changed?
Lynn E. Hanover
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