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General control line discussion => Open Forum => Topic started by: Perry Rose on April 11, 2019, 12:43:11 PM
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Why is it that i.c. props are stiff and electric props are floppy flexible? Shouldn't they all be stiff?
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Why is it that i.c. props are stiff and electric props are floppy flexible? Shouldn't they all be stiff?
I would like to know why as well. I just got some MA combination E/IC and they are much stiffer than any of my APC Electrics but a bit more flexible that the MA IC props I have. I like them better so far. They have a different sound as well. More of a Humm then a Whur.
ken
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The issues is that even though electrics can turn the IC props, the IC props draw a lot of current because they have thick blades and hub. The electric props are built with very thin blades and a reduced hub thickness. They do this because with electric you get smoother power pulses and manufactures feel they can reduce the hub section (to save weight) and not have blades fly off. Most of the cracking issues come from people over tightening the prop bolt which stresses the hub and can develop a stress point and a crack. In order to get a bit better pulling power from the electric props (because the blades are thin) they have some undercamber in the blades.
Best, DennisT
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This is because the tourque impulses of a single cylinder , into the crankshaft , are a bit like a person pretending to be a Kangaroo .
(http://wweb.uta.edu/faculty/ricard/Classes/KINE-3301/Notes/Images-13/Angular-Impulse-4a.gif)
Thus the rotational speed through each revolution is not constant , but ' spiked ' as it fires .
Unlike electrickery where it is smooth , like a V-12 or V-16 Packard . Due to all the things inside it .
Was actually looking for the Super Tigre .46 ' Tourque Graph ' which shows the B M E P throughout a few revolutions of the crank ,
But generally , theres some resemblance to the above description . Not deriding Kangaroos of course .
Theyre actually quite smooth and gracefull in full flight . Unless they dont see the fence .
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Why is it that i.c. props are stiff and electric props are floppy flexible? Shouldn't they all be stiff?
Because the torqrue impulses are much, much higher on an IC engine. That makes the hub get ahead of, then behind, the tips. For instance, an ST46 puts out around 70 in-ounces of torque on average, but the positive torque peaks are something like 1700 in-oz (peak during the firing stroke), and the negative torque peaks (as it comes up on compression) at -650 in-ln once per rev in a 2-stroke, and once ever other rev when in a 4-stroke. That's a cyclic torque of *2350 in-oz* peak to peak, for a relatively wimpy engine at 70 in-oz If you get the blade whipping back and forth "in-plane", that will incur lots of fatigue. Try holding the tips while someone applies 12 ft-lb of torque to it, an electric prop might deflect an inch on a 12-in prop, the figure it's doing the same thing and building up, 220 times a second.
An electric has a 7, 9, or more poles, the torque (called "cogging torque", usually) goes up and down depending on how the rotor and stator lines up, 7,9, etc times per rev, but each one is much lower in torque,
Based on experience, you can get away with running "electric" props on some IC engines for at least a while. But when I first did this calculation, it scared a bunch of aerospace engineers away from doing it, just because the torque spikes were just so large.
Brett
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Why is it that i.c. props are stiff and electric props are floppy flexible? Shouldn't they all be stiff?
Yes they should be stiff, many of the electric props people use are VERY stiff, most of the plastic ones are floppy, and you are right they would be better if they were much stiffer
Randy