What Props do we need ? 14 x 4 light weight Carbon ?

[BorisS]

Hi Guys,

since there are not to many prop sizes pitch etc options out there and mainly a problem for HL configs, since the bigger props are very heavy XOAR etc comparing to CF 12 3.8 porps out there with 11 grams. I wanted to ask addressing the HL pilots what prop size pitch size based on a light weight CF would be needed and demanded.

Thanks

Boris

 

[jetforce]

I think 14x5 would be a good choise, here 3100 grams trust with Axi 2814/22 (765KV) is max, according to holco`s test.

http://www.rcgroups.com/forums/showthread.php?t=1388583

 

[BorisS]

Up to know getting response that 14 or 15 with low pitch would be of interest. Important would be to know what weight they can aim at!

Boris

 

[Bartman]

Boris,
What's the total weight of your helicopter? How many motors? Which motors? Which FC system?

For what it's worth, I'm using a coaxial-XY quad configuration with 8 Avroto motors. On the tops I have Xoar PJA 12x5's spinning clockwise when viewed from above. On the bottoms I'm using Xoar PJA 12x7's spinning CCW. I get great throttle response (not too touchy but not slow either) and practically no wobbling in descending flight. All up weight is 5.5kg with the T2i/lens and dual 5000 mah batteries although that weight will be less with the new frame and mount mods that are being cut next week.
Bart

 

[BorisS]

Hi Bart,

The approach I am following with the thread and the question I am asking is less directed towards me and my setup, but the try to close the prop gap when it comes to light weight 14 15 *props. Lars hooked up a connection to one of the prop makers that would be willing to listen to our needs to design CF props accordingly. Out of my experience all 14 *+ *props are heavily overweight. There is no proportional match to 11 gram 12 3.8 prop maybe lcc or xoar but those again are only available till 13.

Boris

 

[Windbreaker]

I'd be interested in 15" x 4" props.

 

[BorisS]

Japp from the mails i got the interest is going to 14 x 4 or 5, 15 x 4 or 5 and 16 x *. Gonna do some ecalc.ch setups.

@Lars any chance that Mejzlik can predict which weight they can reach ?

Boris

 

[jetforce]

Japp from the mails i got the interest is going to 14 x 4 or 5, 15 x 4 or 5 and 16 x *. Gonna do some ecalc.ch setups.

@Lars any chance that Mejzlik can predict which weight they can reach ?

Boris

Hi Boris,

just asked them, lets see what they`ll come back with.

 

[jes1111]

14x4 and 14x5

 

[DennyR]

I was making 14 and 15 x4 carbon props. last year and then I made an important discovery. Now I make a very low aspect ratio 11x4 with a wide blade and very different section.

 

[jetforce]

I was making 14 and 15 x4 carbon props. last year and then I made an important discovery. Now I make a very low aspect ratio 11x4 with a wide blade and very different section.

What we are trying to have Mejzlik produce a High performance prop feature hollow blades, lightweight fabricated from uni-directional and bi-directional carbon fiber and epoxy.

They have them, but 12x4.5 are the biggest. ( weight 13 grams ) I use them my self, and I`m impressed.

But we would like to have Mejzlik to go for 14x* to 16x* inch for HL.

Thats why we are asking what size other pilots and builders would go for.

 

[DennyR]

Lars
The problem with wet lay-up epoxy is the weight. Carbon prepreg has a much better strength to weight ratio. Carbon nano tube filler is the highest strength/weight of any known material and could be molded with the prepreg using the infusion method. A square meter of 200gram twill prepreg carbon cloth is about one hundred British pounds so expect to pay a lot of money. There is a trade off with these large dia. props. because they are not as fast to respond to rpm change. You may be able to lift a few grams more at max. power but you are not likely to be running your motors that close to max. lift. Better to go for more response and stability.

 

[BorisS]

Well i guess the point is that there are a lot of setups out there CS AD HL Skyjib etc. that are already running 14 or 15 etc. Most of the time with APCs or XOAR which are all on the heavy side. So we will see what Mejlik will come up with and they will probably indicated what weight they will reach and what the price might be before so we can see if it makes sense. If they are capable of producing 14 x * 15 x * several grams under the usual suspects than for those already settled with bigger props it can only be positive. The general discussion if the super size me approach is really worth it when it comes to multis is something different.

Boris

 

[DennyR]

When you study the lift curves of some of these very Large props they often are less efficient at the lower end of the lift scale, so unless you are running them at the very high end of the scale it is pointless to have them. i.e. if your motor prop. combination can lift 3 kilos + then what is the point if you are only lifting 1 kilo per motor during hover. Heavy motors are a bad scenario.

 

[BorisS]

I havent even wrapped my head around this to much into detail to be honest. The only thing that I am interesting in at the moment and could be that i am naive since i am following eclac more or less blindly and a little out of subjective experience is that i want this extra 1.5 mins flight time I would get extra with my fully loaded 5,5 kg CS8 with 14 x 4 on instead of 12 x 3.8 or 11 even worse. Out of experience the 14 x 4.7 APC are not usable on a WKM setup. So i would be up for trying light weight 14 x 4 CF props on the one side because of their minimal lower pitch increasing the rpm and the lighter the easier for the props to change their speed. On the other side of the story the MK pilots suffering less or not at all from using bigger props since their results seem to be better with larger props can only benefit additionally from a lighter CF prop.

Although if i look at the S800 frame it seem like DJI is also changing their approach to bigger props. Otherwise i am not sure what their approach is to reach these numbers.

Technical Spec：S800
1: Diagonal axis length: 800mm.
2: Takeoff Weight: 6kg.
3: Load Weight: 2kg.
4: Flight time with 1.2kg payload: 20 minutes.

but maybe i am wrong and they are 12 ":

attachment

By the way I am anything but impressed by this hard mounted cam on a S800 footage, besides the vibration, it dosent look stable !

http://v.youku.com/v_show/id_XMzM0NTEyMTc2.html

 

[BorisS]

not saying DJI is the trend to follow

 

[jes1111]

Denny,

I appreciate that ones thinking moves on, and obviously we're not privy to every step. But I'm curious. Smaller props now?

You have in the past laid out what I believe to be the "correct" approach: regard the airframe as the outer axes of the gimbal. Its job is only to keep rotational error within the boundaries of the inner axes whilst minimising translational errors and vibrational input. Keeping translation errors under control is as much about the craft's mass and aerodynamics as it is about the FC/ESC/motor/prop chain. Smaller props would need more pitch and higher RPM (to lift the same payload) - both of which could be regarded as "negative" introductions. Once we have the ability to lift the weight and keep things within the inner axes' limits, bigger props will allow longer flight times (the next priority). So why smaller props?

Regarding prop design, I'm fascinated by the whale fin "tubercles" ideas currently doing the rounds. The water is muddied, as usual, by patents but it does look promising for us - allowing higher pitch props without the negative side-effects.

 

[DennyR]

Jes
In my former life designing race cars and engines I have like most had to do the odd uturn and admit I was wrong. Along with considerable success comes the usual failures.
In the last year a lot of experiments change my thinking. I was one of the pioneers of large models with theoretically correct aerodynamics. Great until the wind blows. What I have now is a huge step forward in that area. Cameras are getting better and smaller so the need for such heavy lift capability is over. Legislation in the US looks set to call for models that weigh only 2 kilos for commercial work. Camera mounts are also going to get smaller and lighter with different thinking. I just watched Droiders new Vid. shot with a GoPro solidly mounted and apart from a little jellow that can now be fixed in post, it is yet more conformation that my smaller is better theory is right. I'm not opting for a fixed mount BTW but something that moves just a few degrees with super high sensitivity. i.e. a proper inner axis system with an override on pitch angle.

I was using 14x4.7 APC's on a shoot about six months ago and I had a stone flick up into the prop. and damage the tip. All I could do was cut the blades down to eleven inches and roughly balance them to finish the job. I was amazed at how much better it was. which started the process of rethinking the design. My new props are almost silent which should tell you something about the efficiency. They also lift as much as the APC's did at full power.

The idea that smaller dia needs more pitch to generate the same lift is wrong. What you need is higher rpm or/and more blade area. More pitch can only work efficiently if the upstream airflow increases, which doesn't happen. Higher rpm produces less angle of attack in the descent which is one of the key factors to stopping the wobble that is often seen.

I now understand the full story behind model stability, and I can tell you for sure that does not come from being larger in the same way that an aircraft does. A computer decides how stable it can be based of the response in the feedback loop. The feedback loop is no longer a black art.
D.

 

[DKTek]

I was using 14x4.7 APC's on a shoot about six months ago and I had a stone flick up into the prop. and damage the tip. All I could do was cut the blades down to eleven inches and roughly balance them to finish the job. I was amazed at how much better it was. which started the process of rethinking the design. My new props are almost silent which should tell you something about the efficiency. They also lift as much as the APC's did at full power.
D.

Reminds me of the "Q-tip" story of how they started from an accident. Gear up landing but the pilot caught it and added power to "go-around" only to find that his climb rate was higher than usual. After a proper landing, he found he had hit his props on the runway and bent the tips over. The beginning of Q-tip prop exploration.

Some of the newer full scale helis are sporting rotors with the tips angled down. I'm told this doesn't affect the forward speed handling but does give additional lift in ground effect hover.

I'm definitely not a prop expert but there seems to be something here worth checking out for our application. Additionally, aren't winglets supposed to give a shorter wing, stability characteristics of a longer wing?

 

[DennyR]

Reminds me of the "Q-tip" story of how they started from an accident. Gear up landing but the pilot caught it and added power to "go-around" only to find that his climb rate was higher than usual. After a proper landing, he found he had hit his props on the runway and bent the tips over. The beginning of Q-tip prop exploration.

Some of the newer full scale helis are sporting rotors with the tips angled down. I'm told this doesn't affect the forward speed handling but does give additional lift in ground effect hover.

I'm definitely not a prop expert but there seems to be something here worth checking out for our application. Additionally, aren't winglets supposed to give a shorter wing, stability characteristics of a longer wing?

About two years ago I made a series of props that were in the 14 - 15 dia. with downturned tips. It had to work because the typical airflow into a prop follows a line that comes in from all angles (mostly from the sides) and converges towards the hub on the downstream side. However the tests were giving poor results. Eventually I removed the tips and the efficiency shot up. I ended up with a tip shape that was not at all unlike that which is seen on modern competition gliders. It was upswept. These guys do know a bit about aerodynamic drag. That shape gave the best results.

In ground effect some strange things can happen. Our friend with the bellmouth shrouds would have seen an advantage in that phase of flight because he would have been generating pressure recovery on the backside of the shroud. but away from the ground almost zilch improvement.

It should be remembered that any tip vortex drag acts at the furthest point from the hub so it has a dramatic effect on power consumption.
I find that the theory very often does not match up with test results. I have learned to base all of my final designs on proven facts alone.

Mclaren last year made a wrong aerodynamics decision and it took the whole season for it to come right. One thousand of the best engineers and brains collectively do make bad mistakes.