I am sure I am not the first one facing the dilemma – plastic vs. carbon – when choosing multicopter props. Carbon fiber is twice as expensive. On the other hand, props are the key moving part and may significantly affect your rig’s performance, battery life, and most of all video quality. To put the issue to rest once and for all I did my own test using an iPhone vibrometer app and two sets of 10x45 plastic and carbon fiber props, 12 vs 8 grams a piece respectively.

Both sets (hexacopter) have never been balanced before and ran at the same minimum throttle speed. Plastic props vibrated at 1.6-1.8 m/s2. They also produced more noise.  

Carbon fiber props ran at healthy 0.2 - 0.3 m/s2 and sounded much better. Also, they are significantly lighter and stronger than plastic.

Cautionary note – carbon fiber props are very stiff and sharp. Carbon fiber is 100 times stronger than aluminum and can cause bodily harm at high speeds. You should handle them with extra care. 

See for yourself 

http://www.youtube.com/watch?v=dkQqD96AEd4

Tags: carbon, fiber, multicopter, plastic, propeller, props, test

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My goal was to test the props out of the box without any adjustment. I realize that a lot depends on the manufacturer, but at the same time, I think the findings are valuable for a beginner... and certainly a major reason for less vibration is the weight...CFs are 4 grams lighter and no balancing can change this. 

Hi Guys,

Check this out.

https://vimeo.com/63383728

This shows what only one unbalanced prop dose to your video.

I broke a prop on my copter and replaced it with a new one out of the packet. I had no balancer at hand so I had to leave it as is. 

Yesterday I balanced only that one prop that was replaced. (the other three where still balanced) The difference is huge as you can see. This is the same quad same settings same camera but of course a different location.

Props used are APC 12 X 3.8 SF

Thats exactly my point. When you are out in the field, possibly in harsh conditions, you have no time for balancing. That's why I am looking for an out of the box solutions. Balancing CF props is an added bonus. 

The CF props I got on Ebay used in this test cost me only $9 per pair, weight 8 grams, and worked right out of the box. No plastic can beat that. 

Which plastic props did you use?  If you didn't use genuine APC's, or Graupners then you've proved nothing.  All the no-name props are junk and barely suitable for a rubber-band powered airplane.

Yeah, no time to balance props in the field.  That's why you balance them ahead of time if you are serious.

And Carbon Fiber absolutely is not 100 times as strong as Aluminum.  Not even close.  Maybe 5 times.  And due to the way these things are actually made, I suspect CF props are actually not much stronger than plastic.  Carbon Fiber can be a very gimmicky material, and misused in many cases.

Somewhat spiteful comment, but I stand corrected on the CF tensile strength. Here is correct data:

The modulus of carbon fiber is typically 20 msi (138 Gpa) and its ultimate tensile strength is typically 500 ksi (3.5 Gpa). High stiffness and strength carbon fiber materials are also available through specialized heat treatment processes with much higher values. Compare this with 2024-T3 Aluminum, which has a modulus of only 10 msi and ultimate tensile strength of 65 ksi, and 4130 Steel, which has a modulus of 30 msi and ultimate tensile strength of 125 ksi. from http://dragonplate.com/sections/technology.asp

You are also right that CFs can be quite brittle. The CFs are used in the test are as far no-name as they can be so mute point re Graupners which are also extremely overpriced. 

Sorry, didn't really intend the tone to be harsh.  Though I am starting to harbour a resentment for CF being used in applications that don't suit it... which is a lot these days.  And I think the testing you showed is supposed to come across as an authoritative source which, well, it's just not.  It could steer people to buy the wrong thing.

2024 is not the be-all end-all for aluminum allows.  7075 is common, and has a UTS of up to 85ksi.  So my 5 to 1 not far off.  Certainly nowhere near 100 times.

Further, I suspect that the properties of that Dragonplate quote are a misrepresentation of the truth.  I have not gotten to the bottom of this yet, but my testing is showing that the typical CF plate available to us is actually less stiff in bending than aluminum plate of the same thickness.  I think what may be happening is that the numbers quoted for CF are actually via testing straight strands pulled directly.  Not from actually pulling on a woven sheet, where half of the mass of fiber is oriented in the orthogonal direction and actually does nothing for tensile strength. And you also aren't considering the "filler" that is the epoxy that has little strength. Then also when considering a sheet in bending, since you are stressing a material where the fibers are essentially in the shape of a Wave, there is quite a lot less stiffness than one would expect.

Don't get me wrong, I'm not saying the whole world is wrong.  Carbon Fiber monocoque structures are very strong (ie: 3-dimensional shapes moulded onto a foam core or similar).  But carbon fiber plate may not be nearly as good of an engineering material as people think.  And CF propellers would be similar.

McMaster Carr, whom I would tend to trust more than DragonPlate, actually list the strength of their Carbon Fiber plate as only 120-175ksi.  In tension.  In bending, they only list is as 89-174ksi, and I'd tend to believe the 89ksi is much more relevant for thin sheets, based on my experience.  What I know for sure is that I was surprised when I held both aluminum and CF parts, of the exact same design and thickness in my hands.  And flexed them. The aluminum was stiffer. 

I'm not sure I'd agree that Graupners are overpriced.  High quality engineering plastics are not cheap.  Nor is the work involved in R&D to create an efficient propeller, which is saved by Chinese manufacturers who simply copy the shape.

I agree with most of what you are saying here. But it's not just about strength, other factors also come into play. For example if you take a thin aluminum plate and bent it, it will bend easily and stay at that angle. And if you straighten it out it will be severely weakened. In comparison a similar CF plate will only flex momentarily and be not worse for wear. But of course as with everything, bend it to far and it will snap. So it's kinda obvious. Select the material that fits the task.

And as you say, often CF is selected just because of the "cool" sales factor.

Absolutely agree John.  I've said it before, "The nice thing about Carbon Fiber is that it's either straight, or it's broken."  I've had many instances where an aircraft has taken a bump, and I don't realize that something aluminum is actually bent.  Heli tail booms are a perfect example of this.

But that actually reinforces my main point here.  I have CF heli tail booms I cannot use, because they are far less stiff than the aluminum booms.  They are unusable.  They allow massive, massive vibrations in the airframe.  Took me a while to figure this out.  Change back over to an aluminum boom, problem gone.  So I ask WTF on the heli forums and everybody says "Oh yeah, that's how it is, CF booms suck".  So why is anybody selling them?  WTF indeed.

However, I now *really* have to check the straightness of the aluminum boom if I have even a minor bump, because it doesn't take much.  So I'm still messing around with CF tail booms to see if I can use them.  On a 450, it's hopeless.  I have a noname CF boom for my 600 that *may* work.  And a genuine Align boom for a 700 that does seem quite a bit stiffer.

Part of the issue here is that 6061 has a fairly low Yield Strength, and a Ultimate Tensile of 2-3 times higher.  So there's a huge window where you get "bent".  CF by comparison, has a UTS that is only marginally higher than the plastic limit.  So it generally holds, holds, holds, then pow, it breaks.

I don't fly helicopters but I suspect the issue there is one of scale. CF should be much stiffer in general than Aluminum regardless of the alloy you choose. But the stiffness of a rod like a tailboom is as much about material choice as it is about physical geometry. I suspect that the CF tailboom was made to look like carbon fiber and have a similar size to still attach to the same fittings.

The problem with CF is that the amount of resin in your finished part robs you of the pure fiber stiffness and strength. So if you're not pressing as much of the resin out as possible then you're shooting yourself in the foot. You might as well have a plastic boom for all the good the carbon fibers in it are doing. So if someone doesn't make a high-quality CF boom with a very high fiber-to-resin ratio, then it's definitely going to have terrible stiffness qualities.

The thing is, if you can't get the stiffness you want because there's too much resin in the part, then you have to increase the diameter of the tube to get the stiffness back. But if the tube has to fit in a chassis with a preset size that option's out the window. Ideally you should use tapered CF tail booms -- stiff at the root and light at the tip.

It could also be a problem of scale. I looked at using a titanium tailboom on a competition plane to save weight a while back. But to match the stiffness of the aluminum tailbooms we had, the wall thickness had to be so thin that buckling became a concern. Titanium would have cost more (it was okay we had the money) and in the end it would have been heavier to select a boom that wouldn't buckle in-flight. Aluminum was just the better choice (fiber composites weren't allowed).

IMO CF parts on small RC fixed-wing planes (anything with a wingspan less than about 10 ft) is a marginal decision. Things like fiberglass/foam, CF spars and the like are nice, but at the end of the day they don't buy you a lot of weight savings to justify the added cost (or labor to build it unless you're explicitly set up to do it well). I like glassing foam wings a lot but I don't believe or tell others that it offers any real performance gains.

Great post.  I think you're right on.  I imagine that it makes a difference if you are using high-gloss CF (lots of resin) or matte finish (low resin). I think most people don't think matte looks nice, so the manufacturers only interested in appearance will use glossy material, which is actually weaker.

Rob, I think you are onto something close to what I have felt for a while now about CF props in comparison to good quality plastic props. The one main dislike I have for CF props is they can fracture at the motor shaft collar while tightening the nuts and you wont be able to tell this until after it breaks off completely while in flight.

I have a theory that the rigidity of the CF props might in fact be less energy efficient. Ive yet to put this to some good testing to prove it, but since Ive switch a lot of my copters back over to plastic props, they do seem to fly more easily and I get slightly better flight times out of them.

Here is where the foundation of my theory comes from. There is a big Eagles nest in the forest just out front of my apartment building and I often watch them catch thermals out the front of the building to go of hunting for the day. They often come within 5-10 meters from my balcony.

I noticed while watching them, they have long flexible wing tip feathers that seem to act like shock absorbers as they hit uneven pockets of air as they fly in and out of the thermals.

I could imagine how much easier this would be for them to manage those sudden changes in air movement and density and it would require much less energy fighting this say than if they had very rigid wings. Sudden gusts and change in air movement would be very tiring and hard to mange much like riding in a car without shock absorbers.

I feel the flexibility of plastic props might work in the same way where they give a slightly easier forgiving ride and as such likely they don't expend quite as much energy in doing so.

When I have some more time I'm going to work out a method to put this theory to test and see if it proves me right or not. .

Interesting, while doing battery testing on my basic 450 quad - hovering in one place - i noticed I get consistently slightly less flight time with CF props than I do with plastic props - 16mins vs 18mins and it's more exaggerated in a breeze/wind.

Also interesting that airplane manufacturers interested in efficiency make wings that are disconcertingly flexible (to a nervous flyer!).

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