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
Edward L. Anderson
Anderson Electronic Enterprises LLC
305 Reefton Road
Weddington, NC 28104
http://www.andersonee.com
http://www.eicommander.com
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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