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Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
“Weed eater†2-cycle engines usually have a diaphragm carburetor.
And are therefore the easiest 1.3 hp motor you can buy used for under $50 that will work for flying.
Indeed, the drag losses from more rotors make them less efficient, but the lift is ultimately limited by the sectional area the blades cover. Thus 2 blades may generally improve flight times, but has the same limitation of the mass it can actually lift off the ground. Therefore a quad can lift slightly more than a tri-copter with the same diameter props, but a quad with 3 blade props will not necessarily improve lift depending on the machine.
Typically we also buy the plastic 1045/1045R (10â€x4.5†Props) in bulk, as they work well for inexpensive quads. Note, while a $2k 1.5kg ATJ-120Ti RC turbine can produce over 12kg thrust, it is not really useful for VTOL craft.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
but carbon rod, those engines are enormous blocks of milled aluminum...
yeah the RC turbines are expensive. Ive crashed electrics many times, but recovered almost everything of value but the airframe. with jet turbines in a crash you tend to have kerosene fires that destroy everything as circuit boards seem to burn well above 1200F. (crashing into the dirt at 220mph contributes to the destruction too.)
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Patrick wrote ...
first, i menat to post this earlier the .91nx will produce 80% power for a little more than 1minute with 1 fl oz of nitro-alcohol.
ooh, ouch, an ounce a minute? That's 2kg and hour, give or take. Which is not the order-of-magnitude better than small electric props I had hoped.
But maybe, if you use a 4 stroke, and use less power through lower pitch heli blades rather than power through a known flaky propulsion blade calculator ...
I was kind of assuming not to use ducts, to just let it all hang out and look more like a copter. A 30" duct would weigh a lot. However, the difference between accidentally flying too close to an object and nudging it with a duct, and nudging it with the tip of a 30" diameter heli rotor, would be spectacular.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
i came up with 1.8kg per hour. not great, so im thinking the variable collective pitch tri-prop maybe the next step/
Dr. Slack wrote ...
However, the difference between accidentally flying too close to an object and nudging it with a duct, and nudging it with the tip of a 30" diameter heli rotor, would be spectacular.
I watched the following machine descend into the tree and it tumbled and threw blades like you wouldnt believe.
my most recent machine (Gen 3), destroyed in scientific action. watch at least the last 14 seconds.
Dr. Slack and others, would finding an effcient "power band" of RPM and torque and holding constant RPM while torque and power out vary be useful? (for a specific motor and prop.) In so using a variable pitch propeller. most of the power i think we lose is from rapid acceleration/decelerations from the PID loops. (steve connor and i discussed conservation of momentum in a related thread.) it just seems like at the least id gain faster response, with less overshoot, if not more lift for the same size battery.
i say this, because commercial companies seem to put a lot of thought into the drive train, but we home tinker'ers seem to just randomly pick a motor, then randomly pick a prop and throw it in the air, hoping for the best.
i also think: a first step is power storage or conversion. (battery or chemistry) a second step is coupling so as to be useful. (drive train ending with a prop)
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
It might be theoretically a good idea to do a hybrid; you drive the motor with an electric motor, which spins the main rotors at virtually constant speed, and then have a gas turbine which has more power than you need to fly, and then there's a small battery to buffer it.
So the gas turbine would turn on and charge the battery, and then shut down and the vehicle would fly on the battery for a while, and then turn back on again.
The reason it might make sense is that the energy density of the fuel is so much higher than a lithium ion battery, and provided the gas turbine/generator isn't too heavy it will still weigh less.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
BigBad wrote ...
It might be theoretically a good idea to do a hybrid; you drive the motor with an electric motor, which spins the main rotors at virtually constant speed, and then have a gas turbine which has more power than you need to fly, and then there's a small battery to buffer it.
So the gas turbine would turn on and charge the battery, and then shut down and the vehicle would fly on the battery for a while, and then turn back on again.
The reason it might make sense is that the energy density of the fuel is so much higher than a lithium ion battery, and provided the gas turbine/generator isn't too heavy it will still weigh less.
This is worth researching, but if it requires 34 mosfets and diodes, and adds 200g to a 2kg machine were still screwed.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
It's theoretically good, but I don't think it works nearly as well in practice. Gas turbines don't scale down very well, they lose efficiency and gain weight.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I'd be interested to see a bunch of graphs, if anybody has the bench facilities, or the data, of ...
a) the drive power requirements and weight of a system giving say 100N thrust, for where the lifting element is 3 or 4 ducted fans, or one open heli rotor
b) the weight of an energy store to rotary shaft system versus running time, for say 1kw shaft power, for energy stores of different battery chemistries, gas, nitro alcohol, and various 2 stroke, 4 stroke, series hybrid, in/outrunner motors. I would expect that for minutes, LiFePo would dominate, and for 10s of minutes, 4 stroke, but the graphs would tell.
Then with all of those curves, some usable machines ought to lie at the intersections of the highest curves.
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Novel flying machines
