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Old Jul 16, 2015, 10:44 PM
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a Q about rotors

Full discloser here, I'm not interested in choppers.

Over in the scratch built multirotor forum a guy came up with the idea of using chopper blades for aerodynamic arms.... great idea, but he is working on small quads.... I'm more interested in things that will carry a payload (read 4108 690kv on 4s swinging 13x6 acp....2.5 to 3 kg auw)

I've been running the hind legs off the electrons over at google and this is what I've found.

Most rotor blades are NACA 0013.(????) On a 800mm blade with a cord of 81mm (are they the same cord full length?) that would give you a max cord thickness of 10.53mm. Just how stiff are these blades on a horizontal plane? Say if you hung a 5 lb. weight off a the end of a 150mm length, how much deflection would you get?

OK.... I lied... not just one Q.

I'm assuming that rotor blades are hollow. On the above blade, how big is the hollow? (don't do no good to make the arm aerodynamic if you're going to run wires all over the surfaces)

Thanx for any help you can give,

cbc
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Old Jul 17, 2015, 12:42 AM
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If you hung a 5Lbs weight off the end of a 150mm blade, it could just break. Depends on what the blade is made of I'd expect. In that size, you might find wood, plastic (nylon or ABS) or CF. I have some blades for a 450 (325mm) that are Fiberglass.

Blades don't always have internal voids. If they do, they won't be continuous, along the full span (they'll be like rib bays in a wing). To some extent, you want a heavy blade. Rotor disc inertia is a key factor in flight performance. Blades are often weighted, sometimes at the tip, more usually along the chord (it's a wing, after all, it has a CG. You want it to balance on the CG).

The blades on real helicopters are indeed hollow. They'd weigh too much if they were solid, the rotor hub wouldn't be able to hold on to them.

Your best bet, find a local hobby shop and buy a selection of blades and see for yourself. I think you're going to need a much larger blade than 150mm. I think you'll want a blade from a 500 or 600 class heli.
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Old Jul 17, 2015, 02:05 AM
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Thanx for the response.

I'm not looking at blades that are only 150mm long. I'm looking at just a section of a 800mm blade made out of CF that is 150mm long.

If they are NACA 0013, and have a cord of 81mm, they would have a max cord thickness of just over 10.5mm... that's fairly thick, and if hollow, should be fairly strong.... I'm just wondering what the deflection would be if you did this...
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Old Jul 17, 2015, 02:18 AM
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So just to make it clear.. you are talking about using an 800 size heli blades and cutting them down to make the arms on a multi rotor?

Hanging a 5lb load off the end of a 150mm length of an 800mm heli blade would have no effect on it and no perceptible 'bend'... they are incredibly strong and stiff.

They are carbon skin and solid foam core, there is no internal void/hollow. You could possibly poke a hole through the foam core but that's not something I've ever tried.

They are all constant chord though a few designs taper near the tip. I'd be surprised if you would find a blade with a chord as wide as 81mm, 800 blades would typically be around 70mm chord.
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Old Jul 17, 2015, 02:42 AM
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Originally Posted by JetPlaneFlyer View Post
So just to make it clear.. you are talking about using an 800 size heli blades and cutting them down to make the arms on a multi rotor?

Hanging a 5lb load off the end of a 150mm length of an 800mm heli blade would have no effect on it and no perceptible 'bend'... they are incredibly strong and stiff.

They are carbon skin and solid foam core, there is no internal void/hollow. You could possibly poke a hole through the foam core but that's not something I've ever tried.

They are all constant chord though a few designs taper near the tip. I'd be surprised if you would find a blade with a chord as wide as 81mm, 800 blades would typically be around 70mm chord.
I can't thank you enough... that is just what I was looking for.

Running those legs off of the electrons at google, I did find a blade with a root of 81mm, and that is what I was looking for, ... I'm looking for an easy wing.

Do you know if most blades are NACA 0013?

Thanx,

cbc
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Old Jul 17, 2015, 03:25 AM
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I've just checked a bunch of the blades on my larger helis and their thickness % ranges from 13.5% to 14.5%... so say on average 14% thick.

They arent all NACA airfoils but not a million miles away. The thickness I measured indicates something similar to a NACA 0014
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Old Jul 17, 2015, 01:35 PM
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If you're using these as the arms on a quad, you're never going to be flying fast enough for the airfoil to matter. Also, unless you set the arms at the right angle, in forward flight the quad will tilt nose down and your arms (if they were horizontal) will now be in negative pitch generating down-force opposite of what you're looking to achieve...
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Old Jul 17, 2015, 11:19 PM
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Quote:
Originally Posted by JetPlaneFlyer View Post
I've just checked a bunch of the blades on my larger helis and their thickness % ranges from 13.5% to 14.5%... so say on average 14% thick.

They arent all NACA airfoils but not a million miles away. The thickness I measured indicates something similar to a NACA 0014
Thanx again JPF, good info.

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Originally Posted by rocketsled666 View Post
If you're using these as the arms on a quad, you're never going to be flying fast enough for the airfoil to matter. Also, unless you set the arms at the right angle, in forward flight the quad will tilt nose down and your arms (if they were horizontal) will now be in negative pitch generating down-force opposite of what you're looking to achieve...
Not looking for lift (yet)... that's what we have all those spinning props for.

Most quads use 16mm CF tubes for the arms, some use a rectangular box with the largest dimension in the horizontal plane, some even have the props overlap the frame .... I'm just going to address the 16mm tubes vs heli blades here.

Even with a round tube you get an obstruction in the air flow (see pic 1), once past the largest diameter you get a low pressure bubble behind the arm....= loss of lift.

With an heli blade you have a smaller obstruction (see pic 2), and it doesn't get the low pressure bubble behind it....= more lift

I'm not after speed, just lift, so the wind resistance in forward flight of the cord of the blade doesn't concern me.

cbc
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Old Jul 18, 2015, 04:28 PM
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Hmmm... your reasoning is certainly valid. But I question the "return on investment". I took your picture and copied it in to Photoshop, and with a little Ruler usage determined that the rotor disk you picture has a 160mm radius. 80425 sq. mm total swept blade area. So the CF tube at 1760 sq. mm. accounts for 2.2% of the rotor disk. Assuming it blocks the propwash to the point that the blade is not able to generate any lift as it passes over the tube, the tube is costing you 2.2% of your optimal efficiency.

In reality it's almost certainly costing much less than that. I'd expect that much of the propwash will flow around the tube with little induced drag (since that CF tube is nice and smooth) transferring only a small percentage of its force in the "down" direction to the tube. And I think there's unlikely to be much of a low pressure "bubble" behind the tube, the airflow isn't fast enough and may be too chaotic coming off the prop. I would expect turbulent flow off the back of the tube. Turbulent flow is actually what you want. For the same reasons cars often have a "spoiler". In some cases they generate downforce. But it's more generally called a spoiler because it "spoils" the laminar flow off the back of the car, inducing turbulence, filling in the low pressure area behind the car and in so doing, reducing drag and increasing MPG or MPH.

If you were working on an ultra-efficient super-low-power-high-duration copter I would think this exercise might be worth while. But at 320mm diameter, 12" props aren't in the right range for a high-power/flight-efficiency standpoint. In this size copter, I would bet money there'd be no measurable difference in hover performance from this kind of arm design. And in forward flight, those blades are going to be operating at very high AoA compared to the airflow. That's a high-drag scenario.

Not to rain on your parade, but I'm thinking a multi with heli blade motor arms is likely to be slower in forward flight while delivering no measurable/usable benefit in hover performance.
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Old Jul 18, 2015, 04:45 PM
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Just post an ad in the heli classifieds here, asking for 800mm blades from a crash, buyer pays shipping. You probably won't get those 81mm chord blades you found on google, but at least you'll get something to look at and mess around with before spending money on a new set of blades - they are by no means cheap.
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Old Jul 19, 2015, 10:28 PM
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Quote:
Originally Posted by rocketsled666 View Post
Hmmm... your reasoning is certainly valid. But I question the "return on investment". I took your picture and copied it in to Photoshop, and with a little Ruler usage determined that the rotor disk you picture has a 160mm radius. 80425 sq. mm total swept blade area. So the CF tube at 1760 sq. mm. accounts for 2.2% of the rotor disk. Assuming it blocks the propwash to the point that the blade is not able to generate any lift as it passes over the tube, the tube is costing you 2.2% of your optimal efficiency.

In reality it's almost certainly costing much less than that. I'd expect that much of the propwash will flow around the tube with little induced drag (since that CF tube is nice and smooth) transferring only a small percentage of its force in the "down" direction to the tube. And I think there's unlikely to be much of a low pressure "bubble" behind the tube, the airflow isn't fast enough and may be too chaotic coming off the prop. I would expect turbulent flow off the back of the tube. Turbulent flow is actually what you want. For the same reasons cars often have a "spoiler". In some cases they generate downforce. But it's more generally called a spoiler because it "spoils" the laminar flow off the back of the car, inducing turbulence, filling in the low pressure area behind the car and in so doing, reducing drag and increasing MPG or MPH.

If you were working on an ultra-efficient super-low-power-high-duration copter I would think this exercise might be worth while. But at 320mm diameter, 12" props aren't in the right range for a high-power/flight-efficiency standpoint. In this size copter, I would bet money there'd be no measurable difference in hover performance from this kind of arm design. And in forward flight, those blades are going to be operating at very high AoA compared to the airflow. That's a high-drag scenario.

Not to rain on your parade, but I'm thinking a multi with heli blade motor arms is likely to be slower in forward flight while delivering no measurable/usable benefit in hover performance.
Read your post this morning and have been thinking about it all day (no, that's not all I've been doing, but building fence is fairly mindless work)

An airfoil shaped arm would improve efficiency, but when coupled with the weight penalty of actually building it, coupled with your point of drag in FFF are added together, you are most likely right.

The drawing was done on Turbocad at 1/2mm resolution based on Sunnysky X4108 690kv swinging 13x6 APC, pushed by 2- 8000 mha 4s 10c lipos.... thinking seriously about moving up to Sunnysky X4110S 580KV with 14x 4.7 APC but I haven't run it though drivecalc yet, and haven't run anything through Ecalc as I still haven't settled on a final design.

Anything bigger than 14 inch props will require me to build a new carport for it and the wife just might put her foot down at that ... so that is the biggest restraint for my long duration FPV flyer.

Thanx for your input,

cbc
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Old Jul 20, 2015, 01:01 PM
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I get 25 minutes on my Hexcopter, using T-Motor 3508-700KV motors spinning 12x4 T-Motor CF props on a 4S 10000mAh 25C pack. So I'm doing, what, about 19A in flight. Give an Amp or so to my avionics (a bit more, probably) and I'm pulling less than 3A per motor.

The battery is almost 800g I think, but I'm also hoisting a DJI 2-axis Brushless Gimbal with a Hero4 Camera, plus a DJI 5.8GHz vTX (which has a *fan* in it, so it's not light), Naza2/GPS+iOSD+2.4GHz ground-station data-link, as well as a Spektrum RX and telemetry with altimeter. And it flies with authority, maybe manages about 7.5M/s straight up at WOT and will do about 15M/s in FFF. It's not aerobatic, but it's not supposed to be. It's a camera ship, what I wanted was duration, stability and a reasonable degree of maneuverability. I'd like to have been able to do this at a lower battery cost, of course. But the benefit I'd get from any streamlining efforts would I think be so minimal as to not be worth the effort. I'd do better focusing on weight, but even there the gains would be measured in seconds of added flight time, hardly worth the effort given I can just buy a bigger battery.
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Old Jul 27, 2015, 10:30 PM
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Originally Posted by JetPlaneFlyer View Post

They are carbon skin and solid foam core, there is no internal void/hollow. You could possibly poke a hole through the foam core but that's not something I've ever tried.
Some of them have a wooden spar in addition to the foam. Also some will have a wire that ties the bolt hole to the tip weight.
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