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Old Oct 04, 2010, 02:24 PM
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Originally Posted by NoFlyZone View Post
For APC Slow Flyer props, the magic number is 90,000/diam instead of 190,000/diam

For GWS RS props (the ones commonly referred to as slo-flyer), the magic number is somewhere between 40,000/diam and 50,000/diam

A 10" GWS RS prop would have an absolute maximum of around 4000-5000 rpm

Chuck
Thank you Chuck, going to add this to my notes. Learned something new today.
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Old Oct 04, 2010, 06:07 PM
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Originally Posted by RENATOA View Post
What about the folding props, are they 100% equivalent to a fixed blade prop ?
Hi Renatoa,

The folding props are close enough in efficiency to fixed blades that by simply switching to one, you would be able to use a much larger diameter prop, drastically reduce the current draw from your battery, use a much smaller motor, and have a top speed more in line with a sailplane.

If you read the tutorial, you'll recall that we want to use the biggest prop we can get on the plane, that gives us the desired thrust we're after. And then we want to spin that prop so it gives us a top speed more in line with the plane we are flying.

This is the beauty of WebOCalc. It does this for us.
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Old Oct 05, 2010, 07:04 PM
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It is VERY unrealistic for a glider! But the problem is we are working within the 7" prop you said was the physical limit. In order to generate your required 1300+ grams of thrust, we have to spin that 7" prop like crazy. And spinning it like crazy means the plane is going to have a wicked top speed. Very inefficient!
Beautifully explained, Chuck! Nice work! Thank you again for starting and maintaining this thread - you've helped out a lot of people here.

Renatoa - I can add a little more to what Chuck said: the way a propeller makes thrust is by pushing air. To make more thrust, you have to either push a larger volume of air at the same speed, or push the same volume of air faster (accelerate it more). The more efficient way is the first one. The second way is *always* going to waste energy because you churn up the air more.

The easiest way to move more air is to use a bigger prop and push a larger quantity of air at the same speed as before. If you do this just right, you can often make more thrust than before, without using any more power than before. WebOCalc does all the complex math behind the scenes to make this easy for the user. The one lesson I hope everyone learns from WebOCalc is this: 99% of the time, bigger really *is* better when it comes to propellers!

The other way is to use a small prop and spin it faster. As Chuck said, it works, but is inefficient. You end up with way too much pitch speed, and this wastes a lot of power.

There is also a way to move more air without using a bigger propeller - you can add more blades to the propeller instead. For example, the full-size RAF Spitfire started life with a two-bladed propeller in the prototype, a huge propeller that almost touched the ground - no way to make it bigger. Soon power went up and a 3-blade prop became standard. Later in the war power went up again, and the Spitties got 4-blade propellers. By the time the last marks of the Spitfire were being made, they were up to five propeller blades - that was the only way for the prop to absorb the tremendous power from the last generation of Spitfire engines!

So, if you could come up with a low pitched six-bladed seven-inch propeller, it might generate the 1300 gm of thrust you want without making way too much pitch speed. It still wouldn't be as efficient as a bigger, slower-turning propeller - but it might do better than the two-blade 7" prop, which is truly awful!

Unfortunately, most of us RC hobbyists are stuck with off-the-shelf propellers, and don't have the knowledge or skills to design and build our own. Also, obviously, even if you could design your own prop, WebOCalc doesn't know anything about it, and can't tell you if it's good for your model or not. I'm gonna guess anyone with the skills to design a propeller doesn't need any help from WebOCalc anyway!

But if you can find 3-blade or 4-blade 7" props, I would encourage you to experiment with them on that glider. Without knowing the prop constants, WebOCalc can't help - but if you have a wattmeter and some 2S and 3S lipos, you can do some bench-testing yourself.

By the way, you mentioned the EZ Star as a comparison - you hit the nail on the head there! The Multiplex EZ Star is a wonderful glider with a horrible power system, because the darn propeller is so small. I know the designers put it there to make the model more durable for new RC pilots - but they sacrificed a *lot* of efficiency by making this choice.

Just ask Chuck - after I suggested the idea on another thread, Chuck built an EZ Star with a big propeller mounted in the nose instead of the usual little pusher prop in the back. Just ask Chuck how his creation performed compared to the usual EZ Star!

I have to say, it seems to me the glider you're discussing is an even crazier design than the EZ Star. I'm left scratching my head trying to understand why anyone in their right mind would design a big 2.8 metre glider with a 0.18" propeller diameter. Was this intended as a cruel joke??

-Flieslikeabeagle
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Old Oct 05, 2010, 07:31 PM
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Not to be totally off topic, but are you sure the 3-bladed one is really a Spitfire? It looks like the wings are all wrong...
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Old Oct 05, 2010, 07:32 PM
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Nice explanation of the multi blade props, Beagle! I wonder if there's a chart somewhere that shows how each successive power boost from the engines benefitted the thrust and top speeds of the resulting planes?
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Old Oct 05, 2010, 07:35 PM
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Originally Posted by girmann View Post
Not to be totally off topic, but are you sure the 3-bladed one is really a Spitfire? It looks like the wings are all wrong...
It looks like a clipped wing variant....
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Old Oct 05, 2010, 07:38 PM
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Beagle was right.... here it is....

SpitFire
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Old Oct 05, 2010, 08:02 PM
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Originally Posted by flieslikeabeagle View Post

By the way, you mentioned the EZ Star as a comparison - you hit the nail on the head there! The Multiplex EZ Star is a wonderful glider with a horrible power system, because the darn propeller is so small. I know the designers put it there to make the model more durable for new RC pilots - but they sacrificed a *lot* of efficiency by making this choice.

Just ask Chuck - after I suggested the idea on another thread, Chuck built an EZ Star with a big propeller mounted in the nose instead of the usual little pusher prop in the back. Just ask Chuck how his creation performed compared to the usual EZ Star!
It's cruising speed uses 1.5 amps and a grand total of 16 watts of power. And it will slow down to the point of ridiculousness.

This is just a very short clip of it flying at 16 watts of power. I was testing out my new helmet cam and not trying to showcase the EZ. The stall speed is around 10 mph if I'm not mistaken.

Oh, by the way, when I crank up the power, it will launch straight out of my hand at a slow speed. This is at maximum power of 88 watts...

Chuck

http://www.youtube.com/watch?v=MibvUMtpvpo
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Old Oct 06, 2010, 03:39 AM
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Sorry for late reply, Europe here
This page explanations were indeed excelent, and is my turn now to explain what is this mistery "joke" plane...
Well... is the grandfather of EZ*, a balsa EZ* from 1985, the first commercial pusher plane... need more hints ?
...
I amazed you didn't guessed yet, is the Aero-naut Aerofly, a Graupner design, from which derived Graupner Tipsy, then MPX EZ*. The current model is Aero-naut Aero-master, with a much generous 10" three blade prop, and glass fuselage, with change the game a bit, don't you think ?!
Ok, now my question is: where to get a 4 blades 7" prop ? In this moment my bet choice is a 3 blades 7" foldable prop solution. Checked all known stores for a 4 blade prop and nothing... and, believe me, Google takes links from me, not viceversa
Does a 7" EDF 5-7 blades prop exists? Is this a good idea ?
What about an heresy: 2 normal props mounted at 90 degrees and glued together ?
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Old Oct 06, 2010, 08:38 AM
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Europe online

Sorry to push in but I love this thread
Looking at the Aerofly and its vintage (stopped production in 1985) suggested to me that it must have flown with equipment from that time.
So I dropped some old fashioned numbers into FB’s great program and below is what I got. Marginal electric power for a nice sailplane.

In the second WebOCalc screen shot I brought things up to date with a 2S LiPo and a 1’800 rpm/V brushless motor. That is still marginal but should fly on a calm day.

All that said, I agree with Chuck & FB, a bigger prop would be nicer!

Happy flying
Mark
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Old Oct 06, 2010, 09:19 AM
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Not to be totally off topic, but are you sure the 3-bladed one is really a Spitfire? It looks like the wings are all wrong...
Galaxiex has it, it's a clipped-wing Spitfire. You can see enough of the contours of the nose and vertical stab to recognize the fuselage, though the signature elliptical wing has had its tips crudely hacked off.

I'll admit every time I see a pic of a clipped-wing Spitfire it almost hurts - kinda like looking at a picture of Heather Graham with the tip of her nose hacked off or something. Shudder.

-Flieslikeabeagle
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Old Oct 06, 2010, 09:36 AM
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Originally Posted by RENATOA View Post
I amazed you didn't guessed yet, is the Aero-naut Aerofly, a Graupner design, from which derived Graupner Tipsy, then MPX EZ*.
Cool bit of history there! Let me guess, it originally flew with a direct-drive Speed 500 or Speed 600 brushed motor? That would explain the 7" prop, and I'm guessing those were the WebOCalc numbers Mark (Swissflyer) just showed us.
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Originally Posted by RENATOA View Post
Does a 7" EDF 5-7 blades prop exists? Is this a good idea ?
What about an heresy: 2 normal props mounted at 90 degrees and glued together ?
Prop design is way beyond my limited knowledge, but as far as I know, EDF impellers only work with a duct around them - the design is different than a propeller blade.

Your second idea is an interesting one, and I've seen it done here on RC groups. If you can find a long enough hub to mount two propellers, it may be worth a try.

Years ago I found a thread (I think here on RC Groups) where someone described making a five-bladed propeller for his electric Spitfire. From memory, he bought five APC Electric propellers, cut one blade off each one, leaving one blade and the hub together, then bolted these five blades between two metal discs that acted as the new hub. He ran the mounting bolts through the original mounting hole of each propeller hub. I think he did reshape the original prop hubs so he could pack five of them together in the middle of his new five-blade propeller. And after that I think he used carbon fibre tow and epoxy to lock the blades together. I expect he'd have had to balance the whole thing after all that.

It's a lot of work (and expense!) for a propeller that might not work well in the end, though I recall the person who created that thread was happy with the five-blade propeller he made.

-Flieslikeabeagle
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Old Oct 06, 2010, 10:22 AM
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Aerofly and AXI 2808-20

FB is probably on track, I did check a few German forums but there weren’t so many when the Aerofly was flying
Since it is a nice model, probably very valuable to someone, I would put a nice motor in it. Drivecalc tells me that an AXI 2808-20 (great on 2S) will give you the pitch speed that WeboCalc suggests and 580g of static thrust using an APC E 7x4. Current consumption would be around 10 Amps.
That should be plenty for that model.

Why not start there? I am sure it will look great on a fine sunny day

Mark
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Old Oct 06, 2010, 11:09 AM
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My choice was for a similar motor, KEDA 1500kv, test results using 7*5 on 3s here:

http://www.rcgroups.com/forums/attac...mentid=2474978

Will try also a foldable 3 blades 7.5*4.
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