Variable pitch prop design for electric

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Dan Thomas

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There is a limit to how "pointy it can be when subjected to wind speeds of 650mph to 450mph. The "pointy" is less drag but also less strong. The other thing is "pointy" produces little or no thrust, you have to give something to get something and you are going to have to give some drag to get some thrust. Reasonableness.

The DC-6 does 315mph @ cruise @ 25,000ft. How pointy are those propellers? (13ft 6 inches diameter)

The "6" is also very fuel efficient when carrying 15 tons of cargo and is still viable today.


View attachment 106272
I worked on Lockheed Electra fire bombers for a while. They had big square-tipped props, too. This was one of them:

1610647323369.png

Now, if you knew that those Allison turboprops produced 3750 HP each you'd realize that a lot of blade area would be necessary to absorb all that and turn it into thrust. Then you'd also know that the prop is about 11-1/2 feet in diameter so it only turns at 1020 RPM to keep the tip speed down. That means even more blade area. Then you'd see the ground clearance and understand that the designers didn't have a lot of options in making tapered blades without losing a lot of blade area, so they chopped them off square and accepted the vortex drag. While I was there the shop was also putting a bomb tank on one of Buffalo Airways' Electras (the Ice Pilots outfit). This is me with one of the props on it:

1610648220437.png

The airplane was elevated on jacks for the modification work, so the prop tips are much farther off the floor than they are normally.

Airplanes of any type are full of compromises.
 
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Marc Zeitlin

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IVO twists a composite blade with an embedded torsion rod and DC motor. Works well, reliable, durable, many thousands in service going back 20 years.
I've always been extremely skeptical of the IVO props from an aerodynamic standpoint. With standard C/S props, the whole blade twists the same angle as the prop adjusts. With the IVO, the tip twists substantially more than the root, which, unless I'm extremely confused, is the opposite of what you want to happen as the forward velocity changes - the tip is moving a lot faster (rotationally) than the root, so the relative amount of twist that needs to occur to maintain optimum AOA is less than that required at the root.

Seems to me that however advantageous the mechanism might be from a simplicity standpoint, it should be fairly sub-optimal aerodynamically.

Convince me I'm wrong...
 

rv6ejguy

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The root on most props doesn't produce much thrust being pretty club shaped on many designs. One client went from an IVO to an MT later on and picked up 6 knots in cruise so I don't think the IVO is the best choice for cruise but he lost a bit in the climb with the MT. Other than this, I don't know of any other back to back tests on the same airplane to compare performance.

Mine works well enough turning a max of 2090 rpm however I run out of pitch at altitude with the turbo engine above 30 inches and 9000 feet even with the 3 blade, 76 inch coarse blade Magnum.
 

Vigilant1

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With the IVOprop, it would seem that to get the optimum pitch at about 75-80% radius (where the chord is typically greatest on most GA props) you'd potentially be overpitching the tip. Given the V² factor in both the drag and lift equations, this could be as/ more problematic than an underpitched root segment. I suppose the answer, which IVO knows, is to shape the blade planform to d
accommodate the blade twist rate, or at least find a good approximation.

Most buyers will weigh the benefits of better aerodynamic performance of a blade that rotates on a spigot vs the IVO, and against costs (initial and maintenance) and any perceived difference in the risks of inflight failure, then make their choice. As always, compromises multifactor utility optimization.
 
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rv6ejguy

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Is the IVO optimal? From my experience and some others, no. Does it work? Yes. Is it reliable? Yes.

Light weight, relatively inexpensive, simple, low maintenance were factors for me. For auto engines and especially turbocharged ones, a variable pitch design is important to extract decent TO and climb performance. My airplane would be a dog with a FP prop.
 

rv6ejguy

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Further to the IVO design, it has an airfoil shape virtually right down to the spinner unlike most spigot type blades which must transition to a round shape.

If we compare to FP props which are generally optimized for cruise but perform fairly poorly in TO and climb regimes, the IVO has some advantages over these.

Even C/S types are not necessarily optimized with twist and pitch across the entire range of speeds and power settings. As in all things in aviation designs, everything is a compromise.

Marc, didn't John Slade eventually use an IVO on his Cozy? I can't recall what his results were.
 

blane.c

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A "normal" constant speed propeller it would seem to me is a fixed pitch blade(s) and would have "one optimal" position in its rotational sphere @ a given rpm and airspeed all other positions and rpm would be a compromise. I mean empirically it works well of course but there can be only one optimal solution. So it 's compromise is different from the Ivo's but none the less it must have compromise also.
 

Marc Zeitlin

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Thanks for the confirmation that the IVO is not optimal (at least to the extent that a standard C/S prop is optimal) aerodynamically. I agree that that's not the only criteria, and the cost and complexity of standard C/S props is the largest reason that I don't recommend them for canards except in very specific circumstances.
Marc, didn't John Slade eventually use an IVO on his Cozy? I can't recall what his results were.
IIRC, he did. But John never did enough testing, with enough rigor, to determine any real performance numbers for his plane, and in the 10 - odd years he flew it, he only put about 125 hours on it. And for reasons beyond my comprehension, he'd take off at about 75% throttle... RIP. Plus, he never put any OTHER prop on it, so as you say, no A to B comparisons.
 

Vigilant1

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Thanks for the confirmation that the IVO is not optimal (at least to the extent that a standard C/S prop is optimal) aerodynamically. I agree that that's not the only criteria, and the cost and complexity of standard C/S props is the largest reason that I don't recommend them for canards except in very specific circumstances.
IIRC, he did. But John never did enough testing, with enough rigor, to determine any real performance numbers for his plane, and in the 10 - odd years he flew it, he only put about 125 hours on it. And for reasons beyond my comprehension, he'd take off at about 75% throttle... RIP. Plus, he never put any OTHER prop on it, so as you say, no A to B comparisons.
A comment he made in his blog might give a clue to his reasoning:

"Anyway, the more pitch, the harder the prop is to turn, so if you put in too much pitch you wont get enough static RPM to reach the top of you're engine's power curve - thus take off power would be reduced. You also might get some cavitation. Apparantly, one way to get around this is to begin the take off with reduced rpm, then increase throttle as the airspeed increases."
The last part is what I don't " get'. If the prop is over pitched for takeoff, I would still give 'er all the throttle for takeoff and initial climb. For the next 30 seconds, I don't care about prop efficiency, fuel efficiency, etc. I care about making all the thrust I can during a critical phase of flight. Unless a lower throttle setting and lower rpm somehow gives me more thrust, I'm not gonna do that.

Oh, and propellers can't cavitate in air, only in a liquid.

I know people who, when driving, will leave later to "miss traffic", and they believe they will arrive at their destination earlier. Not just in less time, but at an earlier time.
 
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Marc Zeitlin

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Unless a lower throttle setting and lower rpm somehow gives me more thrust, I'm not gonna do that.
Mr. Slade was never the most logical person, although he was always very calm and civil in discussions. A very nice guy, but difficult to reason with.

Oh, and propellers can't cavitate in air, only in a liquid.
Your pet peeve too? Sheesh.
 

Dan Thomas

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With the IVOprop, it would seem that to get the optimum pitch at about 75-80% radius (where the chord is typically greatest on most GA props) you'd potentially be overpitching the tip. Given the V² factor in both the drag and lift equations, this could be as/ more problematic than an underpitched root segment. I suppose the answer, which IVO knows, is to shape the blade planform to d
accommodate the blade twist rate, or at least find a good approximation.
The Ivo's twist rod goes out to around 70% of the blade's span, IIRC, and twists that point. So the tip will move about the same amount as the 70% point, and the blade will have less and less change in twist as you move inboard from the 70% point.
 

rv7charlie

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I think that 'cavitate' should be read as 'stall' in this context. Scientifically incorrect term, but the observation is valid. With some a/c you can literally hear the effect as they begin their takeoff roll. You can hear the pitch (rpm) of the engine *decrease* below static rpm, and then go back up as speed begins to increase from the standing start. Not unlike a car spinning its tires on damp pavement and then rpm sagging as the tread hits dry pavement and gets proper 'bite'.

I've flown a couple of IVOs on Lycs. Slick hub. Knurled hub. Makes no difference. In a big 4cyl engine, the blades *will* move in the hub. There's a reason that prop hubs have drive lugs (that's why they're called *drive* lugs), and the IVO ignores that. I haven't kept up with IVO's marketing in years, but shortly after my experiences, they quit selling for Lyc use. Ignoring the blade movement issue, the blade twist (or lack of it) is real. Multiple RV drivers, including me, have hit the 'IVO wall' at around 160-165 mph with IVOs, along with worsened cooling, due to the inadequate pitch near the blade roots forming an effective air dam at higher speeds. There have been some faster a/c running IVOs, and their reported speeds are always 15-30 kts slower than similarly equipped models running conventional props. I've got an IVO on a Rotax 503 powered Kolb, but friends don't let friends fly IVOs on Lycs.

Charlie
 

Vigilant1

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Post deleted after further consideration of prop vector diagrams..
 

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