|Apr 09, 2014, 09:28 AM|
Speed prop airfoil?
I'm starting to mold my own carbon props to use for high speed.
And the sort.
The tips of these props will be around 70-85% Mach.
Question is, what would be a decent airfoil to try and replicate for these props?
The APC Sport props I'm using for templates have a very thick, flat-bottom foil. Even slightly undercambered. So I'll be looking to modify that to be thinner.
Is this high lift foil needed in a speed prop?
Or should I be looking at something more semisymmetrical?
Thanks for thoughts.
|Apr 09, 2014, 03:57 PM|
Yes, even get his book if you can.
Clark Y is s horrible airfoil for prop tips, yet you see it or something like it a lot.
Thickness to chord ratio is important when running at high mach numbers. That's another reason not to copy some of the APC designs. Many of them have narrow tips, making the % thickness way to much. Some of their q40ish pylon props are much wider and work pretty good.
|Apr 09, 2014, 04:37 PM|
Been doing some reading on a few articles on SuperCool's site. Heavy stuff!
Enough to make me feel like I got hit in the head with his book.
Biggest thing I took away was the tip chord being very thin, and undercambered.
Did not expect the undercambered part.
Steve, I definitely see what you're saying with it not being a good thing to have the tips super pointy. Even blunting the tips on an APC Sport still leaves you with a thick airfoil at the tip.
I could try to whittle down the APC, and come up with something like a good airfoil for the whole length of the chord.
The Q40 props ARE designed for high power. So it does seem they'd be a better starting point if I'm to try modifying an existing blade.
I even had one out and ready to mount and hack up to make a mold plug!
The thing that concerns me on them is achieving uniform pitch through the blade when increasing it from 7.8 to 13 inches pitch.
If I twist the root by, say, 20 degrees, that will give a larger percentage increase at the tip than at the root. So that's why I've initially used an APC 8x10 for the mold template. Figured since 10" pitch is closer to 13 than the 8" Q40, it'll be less of an error when increasing the angle by a fixed amount across the blade.
With the combination of changing the twist to better suit a the pitch increase, and modifying an existing prop for a better, thinner airfoil, it's looking like I may be better off making a blade from scratch.
I could make a bunch of small sections where I can control the airfoil, then put them together into a prop and blend them all in so it's a smooth transition from one section to another.
|Apr 09, 2014, 04:39 PM|
Seems simple enough to progress from MH113 to MH121 to get a good foil across the prop
Thanks for the tip, Plastik
|Apr 11, 2014, 04:36 PM|
Joined May 2011
Please note, the main issue is not the section cut its the precision the surface being done.
Example, with 10% thickness and 20mm width only 0,1mm mismatching by polishing the milled mold or mismatching parting by thick trailing edge is already 5% deformed profile. 0,1mm is nothing for manual operation on molds.
Also not to forget to apply an intelligent shape of the blade to control load warpage.
|Apr 11, 2014, 06:39 PM|
Very good point, Plastik
I can see the value of the cnc metal molds. No muss, no fuss.
Right now I have a Q40 prop sliced up into thin ribs every 1/2 inch, and a twisted cut wood blank to lay them up on for filling and blending into a prop.
That's the goal, at least.
Only way I could figure to get a truly accurate 14 degrees of pitch across the whole prop length.
A Q40 blade repitched at the hub by +15 degrees gives 12in pitch at 50%, and 15in pitch at 90%.
It'd fly for sure, just not as efficient as I'd hope.
I wish I could figure a way to keep a Q40 blade intact, but untwist it some to give less pitch at the tips, allowing repitching the hub but still allowing for even pitch from mid prop to tip.
|Apr 12, 2014, 12:46 AM|
Joined May 2011
You should definetly give a chance to javaprop, download the msi or use the online calc to check what happens with which parameters modified.
btw. nice effect on this - you´re forced to have some thoughts done on boundary conditions as it shows and learns the idea not being so easy only taking sections and blending them. Of course it works as every angled surface rotatig working in some way, but - it makes always sense to see some effects on geometries modified or always interesting, ehat happens with twisting experiments.
And, of course any experience is a good experience .... e.g. I´m just playin around with some balsa gliders for now and happy - cause i got also some details from prop design in it and thats nice, cause its working for the dlg .... got just no motivation to start molding again big fast things
|Apr 12, 2014, 12:31 PM|
I've played around with it just a little bit. Mostly enough to see I have no idea what the parameters' changes mean
I still have the limitation, though, of what I can achieve with the resources I have.
As I type, the Q40 sections are laying up with some filler between them to be blended in. Not because I see it as the ideal solution, but the best solution I have available to re-pitch a prop and still retain uniform pitch through the length by changing the twist from the stock blade.
Javaprop, combined with a CNC polished mold, would be the ideal solution.
But I can't get one of tho$e.
So the blended sections is the second best I can see.
If there's a better second best, I'd LOVE to hear about it!
|Apr 12, 2014, 03:01 PM|
Joined Jul 2007
Good effort murdnunoc
I have been thinking about a similar way.
So far I used reshaped APC 7x10, but your method has a lot more freedom.
My carbon copies are within 0.05 mm accurate in thickness, so it can work really well.
A folder has the great advantage both blades are equal. So I hope and at least suggest you go for a folder.
Could you please post some pictures?
Would be good for my inspiration to get a new prop project started.
|Apr 12, 2014, 11:24 PM|
Thanks for the encouragement, Tjarko.
I've started to form a couple of 7x13 plugs from APC 7x10 blades myself.
I'll probably move ahead on them too, just to see if there's a performance difference from my re-twisted Q40 prop.
I just had to pursue a re-twist when I measured and found highly varying pitch across the blade length.
Yes, I will for sure be building folder props. Identical blades, simplicity, and prop protection on landing are a big value.
I've got some initial layup done on the repitched, re-twisted Q40 blade.
It'll come out to an 8x14.
I cut a twisted board with inboard and outboard pitch angles at the tips.
That's the base to set the pitch for the ribs to be laid up into the new prop. Ribs are tack glued to the slick, taped surface. Then epoxy filled with milled fiberglass is applied between the ribs.
After it dried, I took a thin sanding block that would just straddle the ribs and sanded away the excess filler.
The plug won't be strong, so got to handle it very carefully taking it off the former board and sanding to final shape.
The back side of the prop will have to be filled and formed in a second step.
|Apr 12, 2014, 11:39 PM|
Took the other half of my Q40 prop that hadn't been sliced up, and held the Monokote heat gun on the last inch of the prop for about 20 seconds.
Grabbed it with a gloved hand and twisted.
It twisted! I figured there was too much "carbon" in these props to twist and hold the shape.
It took a couple of tries, but I managed to add more twist to the tip of the prop.
Now it measures out to a nearly even 14 inches of pitch across the length of the prop.
Wish I'd have tried this at first!
The nearly scratch built plug may turn out better, but there's a very good chance of inaccuracies or breakage creeping in.
With just heating and re-twisting the prop, it'll be at least as good as the Q40's aifoil will allow.
|Apr 13, 2014, 06:45 AM|
Joined Jul 2007
Good work murdnunoc!
do you manage to get the root sections with increased pitch as Well?
I have tried twisting APCs the standaard grey ones, but had to make them so hot they start to bubble on the surface and turn slightly
Brown. A bit scary at 20.000 RPM.
Do the black Q500 props work beter?
I hope they are thinner too.
PS thans for the pictures. The twisted mold reminds me of my F1 B years
|Apr 13, 2014, 09:39 AM|
I did add pitch to the root section. 15 degrees, which brings an additional 5 inches of pitch to the 2" radius point.
The problem came when the tip was increased by 8 inches of pitch by the same 15 degree increase at the root.
Thus the effort to modify the blade twist to give it more twist.
When laying up the prop on the twisted form, the actual pitches won't matter since they're not laid up relative to a hub.
The 14 inch pitch prop requires a certain twist, just as a 9 inch pitch prop requires a different certain twist.
Once the plug is completed, I'll set it up relative to a hub.
Here's the method I'm using to form a mold using an existing blade as a plug.
It involves adding material to form the folder root, and then casting the mold around the plug.
The angle that the folder bolt shaft is installed relative to the prop blade is what will set the pitch for the prop.
I think I'll move ahead with the heat twisted Q40 blade since it has the most promise of being fool-proof. If the custom formed blade works out, great, but if not I think the twisted Q40 will work out pretty well.
The black carbon Q40 props have a much better tip profile than a standard APC Sport. That's why I'm leaning toward it for the mold base.
They're designed for the high revs and airspeeds of a Q40 racer, so I would think there would be a little more engineering put into them with the focus on speed efficiency.
That said, though, the 75% airfoil section still looks quite a bit like a Clark Y
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