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Electric Motor Effciency (Density / Watt / Losses)

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klugesmith

Wed Oct 16 2013, 11:31PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Real (manned) helicopters have an important extra consideration: reliability.

Multiple engines will generally mean a shorter mean time between failures.
If lives depend on ALL engines always working, then multi is probably bad.
If the aircraft can be controlled and landed with one engine out, then multi could be good.

Do tri- and quad-rotor RC flying toys generally have an onboard controller making differential adjustments to the ESC settings? Do users expect controllable flight with _one_ motor dead?

Patrick

Thu Oct 17 2013, 01:50AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

BigBad wrote ...

Just had a thought.

Isn't the performance-optimum design for this kind of aircraft going to be a large main rotor giving all the lift, maybe a tail rotor to stop it spinning and something to control the attitude; like small puffer jets or small rotors?

Dividing up the lift into multiple rotors is highly likely to be a net loss.

In other words, more like a standard helicopter. They are presumably designed for efficiency, after all.

yes in principle and in philosophic terms, a single infinitely large, infinately slowly turning rotor would have maximum efficiency for any lift generated.

towards your idea BigBad, ive considered a huge single prop with 3 or 4 tiny props to act a "thrusters" to tilt the disc. and yes, multiple motors and up losses a single motor and rotor would not see.

klugesmith wrote ...

Real (manned) helicopters have an important extra consideration: reliability.

Multiple engines will generally mean a shorter mean time between failures.
If lives depend on ALL engines always working, then multi is probably bad.
If the aircraft can be controlled and landed with one engine out, then multi could be good.

Do tri- and quad-rotor RC flying toys generally have an onboard controller making differential adjustments to the ESC settings? Do users expect controllable flight with _one_ motor dead?

on MTBF, you are absolutely right, (there are some multirotors that use contrarotating pairs like the TU144 bear, theyed survive motor/prop failures.) I however only build single-point-failure machines. The reason is simple, Ive seen many flying robots auger in, but never have I seen one crash due to prop/motor or ESC failure. Our academic machines are perfectly happy to fly themselves into the ground and walls due to bad programming or sensor failure long before reliability matters accumulate.

The reason for avoiding normal helicopter mechanics is equally simple, all the little expensive metal pieces that make up the rotor heads, they all bend and explode in a crash, and the multirotors are cheaper and easier to fix when they eat the ground.

these are all excellent and well thought out points you make, and exactly what id expect from a group so cagey and smart... that's why I prefer 4HV.org to all other forums -- the quality of people here is just unbeatable.

in conclusion ill state this, we cant just keep trying to take efficient bites at a small cherry pie (though efficiency is a worthy and important effort). At some point we'll have to grow the pie bigger, and by this I mean abandoning batteries and moving on to tiny but powerful fuel-cells. (of one type or another)

Carbon_Rod

Thu Oct 17 2013, 02:11AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

There are some control systems that can fly a yaw flat-spinning craft with some of the engines working.
However, any organic matter on-board would be riding in a centrifuge....

A multi-rotor RC craft will usually crash if it clips a prop or a motor fails. Some larger craft mount the engine facing down to reduce the probability of spinning off a prop.

Printable parts mean airframes are now cheaper than safety systems.

Patrick

Thu Oct 17 2013, 02:29AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

nasa investigated the possibility of pseudo-thrust vectoring for emergency purposes after the Soiux city Iowa crash of a passenger aircraft (the hydraulics had bled out), the F15 did sort-of work when landed only with throttles.

Carbon_Rod

Thu Oct 17 2013, 03:33AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

The F15 is legendary,
a normal air-craft does not achieve that level of thrust-to-weight ratio.

These craft ratings denote theoretical limits:

Some foam-board quads can do 5:1, but flight times are shorter...

Patrick

Thu Oct 17 2013, 04:10AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Carbon_Rod wrote ...

The F15 is legendary,
a normal air-craft does not achieve that level of thrust-to-weight ratio.

These craft ratings denote theoretical limits:

Some foam-board quads can do 5:1, but flight times are shorter...

yeah the F-15 like the P-51 are just masterful designs. we shall see if the F-22 ever adds up, im skeptical individually or ~180 F22's will ever prove themselves as well as the previous ace makers...

Ash Small

Thu Oct 17 2013, 10:22AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

One large prop is ALWAYS more efficient, but two counter-rotating props are often used in order to counteract all the forces already discussed during this build. the 'third' stabiliser prop makes sense. Once you've decided on this configuration, the next issue is to give the props as large a diameter as possible, given space and weight restraints, taking motor efficiency into account. (Look at the Chinook)

It's a trade-off between efficiency and manouverability.

Patrick

Fri Oct 18 2013, 12:42AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

wiring it all up tonight...

Patrick

Tue Oct 22 2013, 07:02PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

I have the Whole machine operating now, should be flying in a few hours.

I need a kill switch, absolute suicide button, I think others have advised me but can't find those posts. I'm wondering if I should use a xbee 60mW version. Which will disconnect via mosfet the main battery to motor wires.

I have a an arm switch, but there have been 'episodes' where it refuses to stop the rotors... so I can't fly it out of the garage or untethered....Due to human safety concerns, and my refusal to be a wreckless idiot.

Chris Cristini

Tue Oct 22 2013, 07:09PM

Registered Member #1749 Joined: Fri Oct 10 2008, 02:04AM
Location: Claremont New Hampshire
Posts: 497

Nice hope you upload a video

Mosfet as a switch to cut off battery power simple but clever.

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