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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
From APC company . . .
Airfoils
The airfoils may have arbitrary shapes defined with either tabular data (splined cubic fits) or analytical functions typically used for NACA airfoils. The airfoil shapes may vary with span. Capability exists to smoothly "splice" together widely different airfoil shapes. The dominant basis for the primary airfoil shape used in most APC propellers is similar to the NACA 4412 and Clark-Y airfoils, except the leading edge is somewhat lower. Also, the aft region is somewhat thicker. This alters the zero-lift angle by approximately one degree and provides greater lift without having to twist the blade even more. All blades have some washout near the tip. For applications where Mach number effects become significant near the tip, either pitch washout or camber reduction tailoring minimizes Mach drag rise.
12 x 4.5 inch prop, made specifically for multirotors. shows the planned cross cuts.
Registered Member #4266
Joined: Fri Dec 16 2011, 03:15AM
Location:
Posts: 874
The airfoils may have arbitrary shapes defined with either tabular data (splined cubic fits) or analytical functions typically used for NACA airfoils. The airfoil shapes may vary with span. Capability exists to smoothly "splice" together widely different airfoil shapes. The dominant basis for the primary airfoil shape used in most APC propellers is similar to the NACA 4412 and Clark-Y airfoils, except the leading edge is somewhat lower. Also, the aft region is somewhat thicker. This alters the zero-lift angle by approximately one degree and provides greater lift without having to twist the blade even more. All blades have some washout near the tip. For applications where Mach number effects become significant near the tip, either pitch washout or camber reduction tailoring minimizes Mach drag rise.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Andy wrote ...
The airfoils may have arbitrary shapes defined with either tabular data (splined cubic fits) or analytical functions typically used for NACA airfoils. The airfoil shapes may vary with span. Capability exists to smoothly "splice" together widely different airfoil shapes. The dominant basis for the primary airfoil shape used in most APC propellers is similar to the NACA 4412 and Clark-Y airfoils, except the leading edge is somewhat lower. Also, the aft region is somewhat thicker. This alters the zero-lift angle by approximately one degree and provides greater lift without having to twist the blade even more. All blades have some washout near the tip. For applications where Mach number effects become significant near the tip, either pitch washout or camber reduction tailoring minimizes Mach drag rise.
Flaps on plane wings
yes, but there under camber is even done differently, they have noticeably more camber at the 0.25 trailing edge chord. thats unusual and a departure from there fixed wing props. you can see it in the 2 and 3 inch radius pics.
12 x 4.5 MR prop data sheet. (i had to use the wayback machine)
From the above: 3,000 rpm, thrust becomes zero at 21 mph. 4,000 rpm, thrust becomes zero at 27.6 mph 5,000 rpm, thrust becomes zero at 34.5 mph
im not sure what all the terms mean like "J" and all.
Personal test of this prop. i realy need to know the flow speed in a hover, thats the part thats holding me back.
EDIT: now im starting to see a solution, or at least a path to one. the datasheets show a max thrust at zero speed, so clearly the measurements in each RPM "bunch" are true static thrust. so with static thrust we could pull tree stumps out. most propellers are never rated in this way, or kinda iffy at best.
EDIT: i think its 17 or so mph that i should aim for.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
The way I see it, it'#s the static thrust we're interested in for hover.
We should design for greatest efficiency at hover, but allow for higher RPM for ascent, etc. I'm assuming twice the static thrust will be plenty at max RPM, but we can confirm this once we have the 'right maths'.
Grom what I've read on prop design, those you've linked to with thicker aft sections than standard NACA profile don't achieve laminar flow over the trailing edge, leading to turbulence losses. Do we have figures for disc loading for those props? Our current plan has around 5kg/m^2 disc loading, and, as I previously mentioned, those you linked to are, I think, for much higher loadings.
I have found some useful maths and NACA profile design tools, but I still need to read a bit more. I assume we need to consider 'section lift coefficients' and stuff like that, which, I think, requires integration, or some such clever mysticism, which I'll also have to brush up on. I also found something on thin section airfoils which looked useful.
I suspect we may need some form of spreadsheet for designing a NACA profile specifically for our purposes.
When we get the sections, I'm sure I can put them into a 3D CAD package and get dxf files out, or whatever you need for the 3D printer.
EDIT: Found this regarding laminar flow NACA profiles: "Laminar flow airfoils were originally developed for the purpose of making an airplane fly faster. The laminar flow wing is usually thinner than the conventional airfoil, the leading edge is more pointed and its upper and lower surfaces are nearly symmetrical. The major and most important difference between the two types of airfoil is this, the thickest part of a laminar wing occurs at 50% chord while in the conventional design the thickest part is at 25% chord.
The effect achieved by this type of design of a wing is to maintain the laminar flow of air throughout a greater percentage of the chord of the wing and to control the transition point. Drag is therefore considerably reduced since the laminar airfoil takes less energy to slide through the air. The pressure distribution on the laminar flow wing is much more even since the camber of the wing from the leading edge to the point of maximum camber is more gradual than on the conventional airfoil. However, at the point of stall, the transition point moves more rapidly forward."
EDIT: Also found this:
And this is really interesting.
I still need to read a bit more on this.
EDIT: This, on the naca 0012 profile is pretty interesting too, especially at low angles of attack, but I think the maximum thickness should be further back, at 50% chord length, from the other stuff I've read.
Edit: I'm thinking something like a naca 16009 or 16006 profile, or something similar.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
The main question seems to be cambered blades or laminar flow.
Most copters use laminar flow, as far as I'm aware. If we can achieve laminar flow with fixed blades then this is the best solution, I think, otherwise we'd probably be better off with camber.
Ideally, we want the 'drag bucket' to begin at hover and continue up to max RPM.
There are plenty of examples of laminar flow NACA profiles with drag buckets from Mach 0.5 to Mach 0.7, which equates to 1,600m/s to 2240m/s. The max RPM we were working with, 2000m/s equates to Mach 0.625.
I still think this satisfies all our requirements, lift to drag ratio, low Reynolds number, lightweight blades, low disc loading, etc.
In order to achieve this it will require a bit more maths, though
Edit: Mach 0.5 equates to hover at 4000 RPM. Max RPM will be between 5000 and 5500 RPM. Does this give us sufficient ascent rate, manouverability, etc?....More maths required.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Found a downloadable program called XFOIL for designing subsonic airfoils
Not tried it yet, but it looks like it could be very useful. It runs on 32 bit architecture.
EDIT: Also discovered we don't want max thickness at 50% chord length at the tips. I suspect we may want to move max thickness forwards towards the centre of the rotor, but it looks like it's what we want towards the outside of the rotor.
We also want as thin a section as possible, to give sufficient strength, and as much chord length and angle of attact to give sufficient lift for hover at 4000RPM (assuming we stick with the figures above).
Looks like I need to re-read thin airfoil theory and section lift coefficient.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
tomarrow is my last class for this semster, so then ill be full time on props. i really need to make progress.
jve got the 3D printer really tuned good. So ill start making molds as soon as we have something useful. ive aslo got a BC3538/10 motor rewound so well see on my new dynomometer a slower motor with larger prop, compared to the previous high speed configurations.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
tomarrow is my last class for this semster, so then ill be full time on props. i really need to make progress.
jve got the 3D printer really tuned good. So ill start making molds as soon as we have something useful. ive aslo got a BC3538/10 motor rewound so well see on my new dynomometer a slower motor with larger prop, compared to the previous high speed configurations.
I think we either need a motor that goes to 5k or a belt drive that takes it to 5k.
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Propeller Physics and VABs Program.
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