Duration

jander14indoor
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Re: Duration

Post by jander14indoor »

bwy wrote:<SNIP>I'm just wondering, if you reduced the friction, wouldn't the rotors spin faster? I thought slower rotors were better (as long as they can still keep the helicopter flying, of course) since they unwind the rubber band slower and thus the helicopter flies longer. That was what I saw at states, at least.
OK, this is a fundamental but common misunderstanding common to Wright Stuff too. People see a really good helicopter or plane fly slow think this is somehow the secret to long flight. That's just backwards.

Making it efficient is the secret to long flight, slow is just a side effect of efficiency!! If you have an efficient helicopter (or airplane), it means you get more lift for less effort. That means the rotors can turn slower to provide the same lift as an inefficient one, and take less torque to do so. Allowing thinner rubber, which means longer, which means more winds, which means a longer flight.

Note, efficiency is NOT a side effect of slowness. If you slow things down by creating useless drag you just need more torque for the same lift, fatter rubber, which means shorter, which means less winds, which means a shorter flight.

So, if you reduce friction and don't change the motor, yes the rotor will spin faster. You'll also get more lift. You don't need more lift than to hold the copter up, so use thinner rubber, slow down the prop, and fly longer.

Note, this was specifically addressed to forms of drag unrelated to lift. Now lift inherently creates drag and that's unavoidable, but someone already mentioned the key concept there and that's the lift/drag ratio and the need to maximize it. That's where concepts like pitch/diameter ratio, curved ribs, elliptical plan form, etc come in. And again high lift/drag allows slower speeds, it doesn't COME from lower speeds.

So, you all have heard me harp on weight, weight, weight. That's because I first want to get you flying. To those who want to win guess I should add, reduce drag, reduce drag, reduce drag!! Course that's the really hard part, but that's the end goal of what you are trying to do.

Jeff Anderson
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sj
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Re: Duration

Post by sj »

illusionist wrote:Hey sj, if you don't mind, could you share your vertical spacing and tip-to-tip distances (for the pitch)? ;)
If not, I completely understand. You've done the work and clearly achieved results. I've tried many pitches to get it to work on 1/8, and I have one last pitch left that I want to try.
Im really sorry but I'd rather not seeing as we made nationals and this info may distinguish us from other teams... Also my partner would murder me in my sleep. But i can say that we have 7-8 more props to try which should get us to our max time (idk near 3 min hopefully).

Also as for the kit we bought one but decided not to build it because of the fact that it is harder to experiment with different pitches due to the fact that the ribs are harder to make.

And as Jeff stated efficiency is key. Our heli lands with almost no winds left on 1/8 inch rubber wound to 1200 winds.
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lllazar
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Re: Duration

Post by lllazar »

The elliptical blades seem like an interesting idea, i don't think it'd be extremely hard to construct if you have some balsa know how...but state is in 3 days so i probably won't have time to try that :)

And efficiency really is key, my copter still comes down with a lot of winds left and that's most likely because my rotors aren't providing enough lift. If everyone could get a copter that flew with no winds left, we'd all be hitting 2:30, that's the whole challenge of this event, testing different rotor designs and rubber combinations to maximize efficiency and decrease drag.
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mrsteven
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Re: Duration

Post by mrsteven »

lllazar wrote:The elliptical blades seem like an interesting idea, i don't think it'd be extremely hard to construct if you have some balsa know how...but state is in 3 days so i probably won't have time to try that :)

And efficiency really is key, my copter still comes down with a lot of winds left and that's most likely because my rotors aren't providing enough lift. If everyone could get a copter that flew with no winds left, we'd all be hitting 2:30, that's the whole challenge of this event, testing different rotor designs and rubber combinations to maximize efficiency and decrease drag.
still not sure what 'elliptical blades' are... and google is unhelpful lol
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Re: Duration

Post by chalker7 »

mrsteven wrote: still not sure what 'elliptical blades' are... and google is unhelpful lol
Here is a link that might help. Download the "J Richmond" helicopter plan. The lower blades are elliptical.
http://www.indoornews.com/index.php?opt ... &Itemid=58
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Re: Duration

Post by mrsteven »

chalker7 wrote:
mrsteven wrote: still not sure what 'elliptical blades' are... and google is unhelpful lol
Here is a link that might help. Download the "J Richmond" helicopter plan. The lower blades are elliptical.
http://www.indoornews.com/index.php?opt ... &Itemid=58
So like, oval/egg shaped?
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Re: Duration

Post by illusionist »

Essentially, they look like a Wright Stuff plane's propeller. Like this one: http://www.aeroracers.com/cgi-bin/shopp ... add=action
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Re: Duration

Post by chalker7 »

Oval/egg shape yes. True elliptical props/rotors are symmetrical.
That link was to an ikara prop, which actually is flared a bit (it has more surface area on one side of the spar than the other), but it is similar enough for you to get the idea.
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aubrey048
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Re: Duration

Post by aubrey048 »

Which would be better, the elliptical wing shape, or a bent straight wing? My already-built helicopter has straight angled wings, but I'm looking to learn for next year.
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jander14indoor
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Re: Duration

Post by jander14indoor »

OK, this may be sending you all of deeper than you wished, but I bet someone will find it useful.

The example chalker7 gave of an elliptical prop uses a matematically correct ellipse. An ellipse is a conic section with a specific, fairly simple formula that you can think of as a stretched circle. It has length and width not equal, but is symetrical around the x and y axis of that length and width. Excercise left to the student to find out more about ellipses and how to construct them.

While it is a 'better' prop shape than the easier to build straight sided ones, it is only a better approximation of the ideal.

If you want an 'ideally' shaped prop, the correct shape is a distorted ellipse. Eugene Larrabee spent a lot of time developing 'ideal' design theory for low speed helical props like these free flight helicopter rotors. Search on "Larrabee Propellor" in google and you get a lot of good references. Some are VERY advanced mathematically, some more practical. Here's one specifically written in a free flight magazine that's fairly practical and easy to apply:
http://pensacolafreeflight.org/page5/as ... Design.pdf

Side comment, a Larrabee prop can be flaring too, the way we're using the term here is to mean there's more area in the front of the spar than behind, and in flight when motor torque is high such a prop tends to twist to higher pitches, slowing it down, until torque drops. A trick with rubber props that aren't allowed to be variable pitch that can be helpful in extending flight duration in some circumstances.

Have fun!

Jeff Anderson
Livonia, MI

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