I'm scratch-building a new Octocopter, in the style of the Hashcopter, using square aluminum tube, and eight motors ganged as pairs with (Quad) Arducopter software.

 

Where is a good place to learn the factors and trade-offs in Quadcopter frame design?  How long should the arms be, and what are the pros/cons of different lengths?

 

Should the craft be as tight/small as possible (props near, spaced evenly), or is there a benefit to having the motors out further from center?

 

Any tips for optimizing arm length?

 

Thanks!

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More directly, is there a preferred ratio for prop diameter to motor spacing?

 

If I'm swinging a 10" prop, how much space needs to be between prop tips?  I've read that tight (close) motor spacing gives superior yaw control.  Is there a limit to this?

i had a quad with 10" props, motor to motor was 40cm

perfectly control and efficiency.

I would put them very close together (just 1cm or something space between the props)

I've been looking at this recently - the Hashcopter design is alluring.  There are two variables here on a symmetrical hashcopter: motor to motor, and crossbar to crossbar.  Making c2c about 40% of m2m allows you to maximize prop diameter by equalizing the spacing between diagonal props with the spacing between adjacent props.  Coincidentally, 40% of m2m is about the maximum prop diameter that is physically possible.

 

I wouldn't advise spacing the props *too* closely though, I would expect all sorts of adverse vibrations and drag to develop.  What is the optimal spacing?  Nobody knows.  Unlike fixed wing planes with their wind tunnels though, this *is* something you could test using an accurate digital scale, and a pair of ESC/motor/prop/watt meters on an adjustable jig.

 

40% is not correct....

70.7% of motor to motor is the maximum propeller diameter (theorem of Pythagoras)

Right on, Sebastian.

 

I'm building mine this way, with equi-distant prop hubs.  The total length of the 'hashcopter' bars is then divided into

Left + Center + Right

Where Left & Right are each Center divided by the square root of two (your 70%).

 

I like the idea of doing some testing on prop-to-prop spacing, to find the distance where they are close enough to rob power.  Having motor mounts on a 'sliding' carriage would also be cool, so that they were effectively adjustable inward/outward, but then the crossbars would also have to slide.

 

One thing I can't figure out is the flight effect of having longer rods (excess motor spacing).  It seems like you would get more 'torque' about the center for cyclic control.  Would this be offset by the larger radius to make it more stable overall?

I imagine yaw moment would be reduced, since the props are proportionally smaller compared to the whole mass.

 

The motivation for putting the props as close together as possible is of course reducing overall weight, and also overall size for mobility and transport.  What other advantages?

In a quad, yes.  In a hashcopter, which is a type of octocopter composed of two perpendicular pairs of parallel arms with motors at both end, no.

 

Quad:  X


Hashcopter:    #

 

Check your math on the Hashcopter.  Space the parallel arms such that all adjacent motor pairs are equidistant, arranged in an equilateral octagon.  There's no 40%.
Aside from spacing, I still haven't found any pro/con comparison of the Hashcopter "paired motors" Quadcopter control, vs the more traditional Octocopter (eight motor control).  I'm still looking for the catch, since it seems Hashcopter is simpler and more fault tolerant.

So, using the geometry, Google Calc says optimal crossbar-to-crossbar spacing is:


1 / ((2^0.5) + 1) = 0.414213562

As long as you're keeping the arms of equal length, excess motor spacing is proportionately equivalent to simply reducing the size of the props.  Or were you talking about using rods longer than m2m?

The props being larger and slower allows for increased efficiency.  The entire airframe being larger allows the props to be larger.  My question is, since the Young's modulus of carbon fiber rods is so high but the geometry (actual stiffness increases as thickness cubed) is questionable, whether an oversized, hollow truss system would work significantly better than simple square tubing.

See math above.  Wording it differently:

Distance between parallel arms should be about 41.4% of distance between opposite motors on a single arm.

 

Edit:
It seems that motor to motor distance traditionally goes through the center of the frame, not just the center of the rod.  It seems I was using the term incorrectly.

I do know aluminum square tubing is incredibly cheap. :)  I've got enough to make about a dozen experimental airframes.  Rigidity should be superior to the center-hub designs.

 

Having the cross-bars mount somewhere OTHER than 1/3 the length should help with potential harmonic vibrations as well.  I worry that I would bind them right at a node and end up with some intense vibration.

I see.. yes, we agree.. 41.42% of the rod length is the space between parallel rods.

 

Length = (1 + sqrt(2)) * spacing

Spacing = (sqrt(2) - 1) * Length

 

Segment lengths are:

space / (sqrt(2))

space

space / (sqrt(2))

 

I think we've officially worn Pythagoras to the nub. :)

 

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