Hobby: the propeller arrived

So wife is better, my health looks good, and the bills are paid. My IVOprop, inflight, electrically adjustable propeller arrived:


It works by twisting a steel rod in the propeller with a lead-screw driving by a two-stage, planetary gear and motor:







So one of my first series of tests was to look at the electrical load on the aircraft, 12V system. My aircraft engine uses fuel-injection, which means an electrical pump for fuel pressure and the fuel injector controller. It is not nice to turn off the engine because the prop pitch needs a change:


It is also important to understand the propeller aerodynamic characteristics:
The original 52" diameter propeller was driven by a 60hp, 3200 rpm, modified VW engine. The 52" IVOprop will be driven by a 60hp, 2314 rpm (prop)/5000 rpm (engine), Hirth 3502. Knowing the area of the propeller blade and how much is sweeps lets me know there will be no problem . . . might even need a lower gear ratio!

So this is what the new propeller will do:

	Column 1
0	[tr][th]original[th]IVOprop[th]metric
1	[tr][td2]5.0 lbs[td2]14.2 lbs[td]52" propeller weight
2	[tr][td2]858 in**2/rev[td2]2073 in**/rev[td]blade area swept per revolution
3	[tr][td2]13.4 deg.[td2]12.7-26.8 deg.[td]blade tip angle(s)

Adding 9.2 lbs to the very front of any airplane requires paying attention to the weight and balance. However, the planned engine is ~45 lbs lighter than the original VW. But more important, this propeller will let the engine run at full power at all speeds.

When the engine is at full power before releasing the brakes, the minimum airspeed, the propeller has to go into a finer pitch so the engine can turn fast enough to generate full power. Releasing the brakes, the airplane picks up speed and this 'unloads' the propeller unless the pitch is increased. This gives the maximum acceleration for the shortest takeoff.

Once the plane leaves the ground, there are two common climb speeds: maximum angle and maximum rate. The maximum angle of climb is used to clear trees and power lines at the end of the runway. At maximum rate, a faster speed, the plane climbs to altitude as fast as possible. Each speed has a different, optimum propeller pitch.

When the plane reaches cruise altitude, the nose is pushed over and the energy used to climb accelerates the plane to a higher speed. Again the propeller pitch needs to change so the engine won't 'overspeed.' Plane can cruise at different speeds depending upon whether one needs to fight a head-wind, exploit a tail-wind, reach maximum safe range, or get somewhere before bad weather gets there. Regardless of which speed is used and the altitude (aka., density altitude), the propeller pitch needs to change to let the engine operate at the best power or best fuel efficiency range.

It is nice to have a hobby like this especially as soon enough, in three more years, I'll reach my Social Security age and will need something to keep me distracted.

Bob Wilson

ps. The IVOprop is to shorten takeoff and improve climb performance. Top speed is address by drag reduction, which is different part of this project.

 

Actually I am rebuilding N19WT (ask Google):


N19WT is a two-seat, formerly converted VW, 60hp, 1800cc engined airplane. I am upgrading to a Hirth 3502, liquid cooled, two-stroke. The IVOprop is key to making the oil-injected, fuel-injected engine work on this plane.

There are technical challenges, which makes this project worthwhile. For example, I can't let he propeller put a such a load on the 12V electrical system that will shutdown or affect the fuel-injection controller. That might cause the engine to stop, not a good thing.

Bob Wilson

 

The wires, aluminum plate are what comes out of the box:

But you are right about the risks only the functional equivalent happens at each mechanical stop.

When the motor first starts, it is not moving and other than the switch ON meeting the inductance, nothing keeps the motor from drawing a huge but short lived, current spike. It is worse when the blades and gearing have traveled to the mechanical stop, in effect, a brake effect. Once the motor starts spinning, the back EMF reduces the current to a reasonable value until the mechanical stop at the other end. Using a known, weak 12V battery, I plotted the voltage and current moving the prop from minimum (low speed) to maximum pitch:

	Column 1	Column 2	Column 3
0	[tr][th]seconds[th]amps[th]volts[th]description
1	[tr][td2]0-0.7[td2]27-6[td2]3-10.4[td]initial stiction and startup current
2	[tr][td2]0.7-4.5[td2]6-3[td2]10.4-11.4[td]weak battery	lower voltage events	higher current
3	[tr][td2]4.5-7.7[td2]3-8[td2]11.4-10.4[td]torquing the blades	more load	more current
4	[tr][td2]7.7-9.5[td2]8-30[td2]10.4-2.6[td]driven to mechanical stops	the motor stops	overloading battery

My solution is to use a buck-boost, DC-to-DC converter, rated at 80W, 6A. As the electrical load exceeds 6A, the DC-to-DC converter stops drawing more than 80W and the prop has a 'soft stop.' Since the blade torque increases on both ends, I can adjust the current limit lower, say 5A, and limit the propeller pitch range without hitting the mechanical stops. Then with reverse current, the prop torque helps the motor start and unwind. The electro-mechanical model is a little more complex than this simple description but bench testing is how to cut through the Gordian knot.

I am planning an independent 12V system for the control electronics and displays. So the failure modes:

• alternator and regulator failure - Schottky diode isolation, continue engine operation on starter battery and independent pilot display for glider operation from control electronics
• add engine mechanical, ignition, or fuel failure - continue glider operation using starter battery and control electronics
• add starter battery failure - Schottky diode isolation, continue glider operation using pilot display and control electronics battery

Something like this:

• 80W DC-to-DC converter
• "H" dual-motor controller, lower left
• MSP430 micro controller, lower middle to operate "H" controller, and programming USB, lower right
• Raspberry PI, upper right, to monitor MSP430(s), record and display engineering data

The MSP430(s) handle realtime control with secondary communications to the Raspberry PI to record operation. The Raspberry PI plays a passive role except to let the human flying the glider have some insights about the engine and prop operation and later, analyze the flight.

Bob Wilson

 

The electronics will be on the back-side of the engine firewall but there will be plenty of cooling air available.

In my glider that happens to have an engine that will stop following Murphy's law? <grins>

The first step is to know the engine will stop when it will do the most damage and greatest surprise.

That was my practice too. But I had a surprise one night when there was a small difference between the dew point and temperature on a clear night over Alabama. Radiant cooling caused fog to form over the whole area in a very short period of time. I noticed the city lights ahead started to look like 'angle hair' had been laid on them. No problem, we had just passed Tuskegee so I did a 180 . . . all I could see was the rotating beacon glow in the fog.

In a few minutes, I was over the field and directly down, could see the beacon and keying UNICOM, got the landing lights on. But you couldn't see them at an angle so I did a non-standard circular pattern: full flaps and a slip to keep the lights in view. Once on the deck, I could not see the buildings but following the taxiways, got close enough to park. Could not find tie-downs but no wind, no problem.

Holly and I camped at the plane that night. It did not become VFR until after 10:00 AM that morning.

Bob Wilson

 

Funny you should mention this. A first flight of a re-engined plane, I would love to have an RC option to give everyone confidence that everything is solid enough for human flight.

Bob Wilson

 

Compared to the standard, 170 lb, actuator? <grins>

Not in this case. Too many dang drones around already although the Predator would make nice LSA.

Bob Wilson