Replies to This Discussion

ModeratorPermalink Reply by Graham Dyer on March 16, 2012 at 12:27am

What brand for the composite props? I don't think the manufacturers take 'copters into account when designing props so usually it's just a RPM limitation. I've had a few props break in flight but it's always been the cheap ones.

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Permalink Reply by Karla Jean Fripp on March 16, 2012 at 12:41am

They were the grey composite ones from J Drones, 12 x 4.5 epp. I don't say it's the manufacturer but someone should put together some specs and say which props can be used for which weight ect ect.

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ModeratorPermalink Reply by Graham Dyer on March 16, 2012 at 6:59am

APC quote "Slow Flyer props - Maximum RPM=65,000/prop diameter (inches)" (from this link) so 5416rpm max for 12", I doubt they would ever quote a max weight though. A full power climb at 600W is a massively different "weight" experienced by the prop than in a gentle hover at 180W.

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Permalink Reply by Karla Jean Fripp on March 16, 2012 at 8:29am

The plane was in a flat hover when the prop broke.

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ModeratorPermalink Reply by Graham Dyer on March 16, 2012 at 11:07am

Maybe use a prop tachometer to check the RPM?

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Permalink Reply by Karla Jean Fripp on March 16, 2012 at 11:24am

How will measuring the speed of the props help to know whether they will handle the weight of your copter?

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ModeratorPermalink Reply by Graham Dyer on March 16, 2012 at 12:45pm

Normally max RPM will be exceeded long before max carrying weight (or thrust, to give it it's correct term) is exceeded. If a prop breaks before max RPM is achieved then there's something wrong with the prop. 

A 12" prop spinning at 5400 RPM should theoretically be able to produce about 770g of thrust so for 8 props that's 6.1kg, how heavy is your Octo?

So the only way to tell if the known specs of the prop were exceeded is to measure it's RPM.

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ModeratorPermalink Reply by Graham Dyer on March 16, 2012 at 12:48pm

Here's a prop thrust calculator: http://personal.osi.hu/fuzesisz/strc_eng/index.htm

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Permalink Reply by Brad Hughey on March 18, 2012 at 8:34am

Graham, you're spot on.  They would never quote a maximum weight because it's an irrelevant number.  They might quote a maximum thrust, however, at a given RPM, but that too might be meaningless, because the thrust will change based on the inflow. 

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Permalink Reply by Brad Hughey on March 16, 2012 at 4:28pm

@Karla: Based on what I've seen, much of the knowledge base in this area is garnered from trial-and-error.  Most of the effort (and frankly, the glamor) is in the control system and not so much in the aerodynamics.  Experience does rule, of course, but static thrust testing of model airplane propellers is a bit esoteric.  Perhaps one should either go completely with a proven solution (i.e. get a kit) or be prepared to experiment. 

@Graham: No offense intended, but those calculators don't provide good results for static thrust.  They all tend to over-predict because they're using mathematical models that fall apart at low advance ratios (no forward speed).  If you want to do some comparisons, here's a database of model airplane propellers tested in a windtunnel by Dr. Selig's crew at the University of Illinois:

University of Illinois Wind Tunnel Small Prop Test Database

The props tested are just the sort most multicopter enthusiasts employ.  Unfortunately, there's a bit of math involved to get to the point of comparing the static thrust performance of one to another.

The term "figure of merit" or FM is used by helicopter aerodynamic engineers to compare the hovering efficiency of one rotor to another.  I'll spare you the long description, but it's a ratio of performance of an ideal rotor to the one in question, so it's always going to be >1.  Here's the formula (in spreadsheet notation) which you'll need to use to calculate it from the data in the "static" table:  =((CT)^1.5)/(CP)*0.707))

You'll know you've done it correctly when the FM numbers range from 0.25 to 0.55 or so.  Enjoy!

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Permalink Reply by Karla Jean Fripp on March 16, 2012 at 5:06pm

@Brad. Thanks for the info, very interesting. As I am still learning I didn't take much notice of the props and only started to ask questions when props just kept breaking and they were supposed to be the good ones!

Why don't they have specific props for the specific weight of the copter.

This is all new but I know they set specs for other parts of the copter like the thrust of the motors in proportion to the weight of the copter so they should do that with the props??

Someone should really take the time to do it. I'm sure it would help everyone and alot of money and time would be saved. I'm sure in the next few years some engineer will do it. In the meantime I guess it will just have to be trial and error.

It is all good and well getting a kit but you don't learn as much and sometimes you need it tweaked for the job you doing.

It's all part of the learning process :)

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Permalink Reply by Ellison Chan on March 16, 2012 at 6:59pm

You guys do realize that none of the prop stress ratings apply to multicopters. The rpm ratings refer to how much centrifugal force that the props can handle, which is all fine.  But the major stress on multicopters is along the axis of the prop, which is perpendicular to that.  Mounted horizontally, that force is just equivalent to the force of inertia, which is smaller and only significant at take off.  Mounted vertically on a multcopter, the prop has to be able to bear the full weight of the quad, which is several pounds for a Octo, and increase during hard manuevers .  Most props simply are not built for that kind of stress.

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