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HomeAircraftHelicopterLow Head Speed Helicopters › Low Speed Rotor Horse Power In Watts
08-20-2014 04:11 AM  4 years agoPost 1
icanfly

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ontario

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Taking a look at the fact of the matter, if your going to dedicate a heli to low head speed in the range of 900 to 1600 rpm, just what amount of horse power is needed to spin the rotor, starting with a 700, then an 800, and a 900?

Could it be that the supplied motors in a kit or that recommended are way over hp'd to send the heli to the moon, when you could take the hp of a nitro and find the suitable comparison in an electric motor, of course that being lighter by perhaps half of a full speed and torque monster the heli was built to withstand.

Looking at a few options I see a 4000watt 575kva motor with a 90 amp esc has about 4hp. A 7200watt 550kv giant having about 9hp, and so on. Interesting thing to note was both had an efficiency best near 3300 watts and the 7k peak wattage of the second choice came at the expense of another lb of motor mass.

If I were to take a 450 and equate what .5hp 3500kv motor can lift, almost 2lbs is stock all up with battery, multiplying that up to 10lbs it would seem fair to say that a 2.5hp motor will lift 10lbs and react to swash inputs no differently than the 2lb mini chopper. A 90 series nitro is about 5hp for example.

Power to weight and kv/watt overkill is what its all about,

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08-20-2014 03:13 PM  4 years agoPost 2
gwright

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Champaign Il

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Agree in essence but I think your numbers are a little off. A 91 is in the 3.5~3.6hp range, not 5hp, and the 105 just a couple tenths more (OS published ratings). One horsepower is 745.699872 watts,.. so we'll just say 746. that means 3.5 to 3.7 hp would be 2611 to 2760 watts. Being very generous to the glow engine, say the exhaust system and a specific fuel formulation "helps" it and adds 10%, that's still only 3000 watts. An electric motor peaking at 4000 watts is 5.36HP (a very mild electric setup by todays standards).

I think the various manuf suggest (or include) electric power system combinations that are far more powerfull than glow engines simply because that sells product. "Hey, look at our heli, it has double the performance of that glow powered one". They don't however, advertise that it only flies half as long when using that sort of power.
If all you want is nitro level performance you can easily get that, for longer flights than nitro. Totally agree with your motor sizing comment also.
Many people are using a popular 744gram motor in their 800 machines on 14S. I'm using a 445gram motor and 12S. that's a bit over half a pound savings in battery weight, and just over 10 ounces in motor weight savings,.. so a pound lighter than the norm. 12~14 minute flights, and GPS telemetry has recorded over 100 mph,.. so it's no slouch.
Back to your topic,.. when flying in JUST the 1100rpm mode on the machine mentioned above (smooth 3D, not just hovering), I see average wattage in the 600~650 range with peaks of just under 2KW. Takes a LOT less power at the lower revs than most think. At 1600 those numbers more than double. I think due to the torque characteristics of an electric motor (max torque at 0 rpms) you can do a lot better with less HP than the glow equivalent, so maybe hp per pound isn't the right comparison. Definately a place to start though.

Gary Wright

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08-20-2014 05:49 PM  4 years agoPost 3
icanfly

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ontario

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perfect, that's exactly it gright.
12~14 minute flights, and GPS telemetry has recorded over 100 mph,.. so it's no slouch.
and the lighter it stays the less hp is needed to toss it around.

A curious experiment I have is to add positive pitch to the heli on spool up and see what rpm it takes to raise it up, this is all that is needed to lift it, extra rpm is all juice but I imagine in 3d you want lots of headroom to handle bogging, I don't see those 3.5hp 105 nitro motors bogging on an 800 do you? Something about how the hp is used to maintain rpm at all pitch angles to the point of stall.

So if the concept is correct the motor would have to maintain max torque all the way to its highest efficiency.

I'm going to put a laptop cooler fan rotor on top of one of my heli motors, enclosed in an outer POP CAN housing, heat is an area of loss in electric systems, why going to high volt LOW AMP is the best way. The motor and esc loose efficiency when run hot I understand.

The goal is low rpm, high efficiency, long flight times, no real loss of agility.

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08-20-2014 06:55 PM  4 years agoPost 4
gwright

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Champaign Il

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good idea but not really needed. My setups land under 130 degrees even when pushed hard. Blades don't actually stall due to pitch, it's due to angle of attack (most airfoils in the 16 to 20 degree range for that). The 800 is using 16 degrees collective + and - and 11+ and - for cyclic. As you input pitch (coll or cyclic) the heli accelerates in that direction, so the angle of attack lessens during acceleration and you never really reach 16 angle of attack.

Gary Wright

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08-20-2014 11:40 PM  4 years agoPost 5
icanfly

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ontario

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now imagine that 800 is half the all up weight,

ni-eece

ohh, and hear 3 blades would be prime with that math, whole-ee.

Now imagine blades weighing half too, geesh.

I think I know how to make a 8 or 900 with a 1500kva motor and be super efficient. Maybe even the motor of a 450 on 2 6s in series, I think.

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