Espritmodel.com Telemetry Radio
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Old May 21, 2010, 06:37 AM
Tom Bass Park RC Nonconformist
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Aahhh....


!!!!!
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Old May 22, 2010, 09:19 AM
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Tell me if I am wrong but I assume this will work with flying wings, since there is no provisions for control surfaces. I love FWs; no LG to rip out on a rough landing and few broken props (are you getting a feel for how I fly?). lol I guess my question relates more to whether this works with can motors (inrunner BL) or not.

TIA

jim
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Old May 22, 2010, 09:39 AM
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...EDIT: By the way, the reason I picked this problem is because once we see what prop I'm using, we are going to lay to waste that old adage about needing 'so many watts per pound' to fly our planes in such and such a manner. That old adage is pure, unadulterated cockapoo!
I have a cockapoo and you are correct. No matter what prop/motor combo I use, that dog don't fly
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Old May 22, 2010, 02:46 PM
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Originally Posted by Bing44 View Post
l I guess my question relates more to whether this works with can motors (inrunner BL) or not.

TIA

jim
Yeah it does, you just have to select inrunner brushless in the motor category

HT
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Old May 22, 2010, 09:03 PM
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Working through Reply #74

Here's the specs:

11.7 ounces AUW
Wingspan 35"
Wing Area 300 inē
I can use any size prop I want and have tons of ground clearance.
I'd like "Gentle scale flight" as my flight mission.
I'm using a 4100 kV motor, and a 6.66:1 gearbox. The motor would be classed as an "Average Inrunner".
I'd like to use my 2s Li-Pos in it.

What prop shall I stick on it, and how much current will I be using, and what's my expected top speed and thrust?

In Pic 1 below, all we do is enter the information like weight, wingspan, and wing area. Then we let WOC suggest a prop diameter. WOC chooses a 10" diameter prop as it's maximum size.

Then we select "Gentle scale flight" as our mission. Then we let WOC suggest a top speed and thrust value.

Then we select "Average Inrunner", then we click on Voltage Wizard and let WOC pick a 2s Li-Po and 4.2 amps of current.

Finally, we enter the kV of my inrunner, which is 4100 kV. Then we hit the Calculate button.

WOC picks a GWS 10x8 prop, with a 6.74:1 gearbox. This is very close to my 6.66:1 gearbox, so I'll zero in on my gearbox by playing with the Suggested Speed, and the current, like we learned earlier. Since I want to go down with the gear ratio (from 6.74 to 6.66) we will raise the current a tiny bit at a time.

The result of our fiddling around is in pic 2. A perfect 6.66:1 gear ratio (what I'm using), and a current draw of 4.35 amps form our 2s LiPo.
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Old May 22, 2010, 09:16 PM
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Notice the final specs of our plane. We have a thrust value (12.2 ounces) that is greater then the weight of our plane (11.7 ounces). This means our plane might actually have the ability to hover. In fact mine does.

Now "conventional wisdom" tells us that in order for a plane to hover, we need at least 150 watts per pound. It also tells us that a plane needs about 50-60 watts per pound just to fly! But look at the Power to Weight Ratio in pic 2 above.

43 watts per pound!!!!! "Conventional wisdom" is the worst way one can pick a motor and prop for planes, as you can plainly see.

This plane (shown in Reply #74) is actually a highly aerobatic little jewel capable of long vertical lines on only 43 watts per pound...
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Old May 22, 2010, 11:01 PM
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Hi Chuck,
Any ideas about my motorglider from post #62?
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Old May 25, 2010, 12:22 PM
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Chuck,
This is great stuff! I do have a question...

I've been wanting to convert some of my old control line stuff to electric. How would I modify the aircraft parameters to account for the drag produced by the lines?

Thanks,
Rob
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Old May 25, 2010, 05:08 PM
2 no or knot 2 no!
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I'm not Chuck, but I'm going to add my two bits. Drag of the control lines are the least of your worries. HEHE
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Old May 26, 2010, 09:14 AM
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Originally Posted by ommadawn View Post
Hi Chuck,
Any ideas about my motorglider from post #62?
Hi ommadawn,

I think the prop you have on there is about as efficient, given your limited use of the motor, as you're going to get. It's obvious from your amp readings that you are not running it anywhere near full throttle, and it seems at this power level to be doing a pretty fair job of getting your plane back to where you need it. Personally, for the above reasons, I'd stick with what you have. Now if for some reason yopu put the combo in another plane that will be mostly using prop power to fly it, then you would definately want to prop down, because your present prop is much too demanding on that motor for continuous use.

Chuck
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Old May 26, 2010, 09:16 AM
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Originally Posted by TheRocksKid View Post
Chuck,
This is great stuff! I do have a question...

I've been wanting to convert some of my old control line stuff to electric. How would I modify the aircraft parameters to account for the drag produced by the lines?

Thanks,
Rob
Hi Rob,

That's a good question, actually; and I must be totally honest and state here for the record that I'm just not qualified to give you an answer. I've never flown control line, and I wouldn't be able to even begin figuring out the answer for you. I apologize for not being able to help...

Chuck
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Old May 26, 2010, 12:26 PM
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Originally Posted by NoFlyZone View Post
Hi Rob,

That's a good question, actually; and I must be totally honest and state here for the record that I'm just not qualified to give you an answer. I've never flown control line, and I wouldn't be able to even begin figuring out the answer for you. I apologize for not being able to help...

Chuck
Not a problem. My first thought was to add something to the wing loading to compensate since the drag created is something that needs overcome by the thrust. Question is, how much drag is created by two 0.018 diameter 60-foot lines? I imagine that the flight speed calculations you are performing here would somehow factor in, but I'm neither an engineer or mathematician.

I'm going to try just adding 15-percent to the wing loading and give it a try. After all, what's the worst that could happen? .
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Old May 26, 2010, 12:40 PM
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I'm going to try just adding 15-percent to the wing loading and give it a try. After all, what's the worst that could happen? .
If I were forced to come up with something, that's exactly what I would do; increase the wing loading a few %. 15 sounds good to me...
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Old May 29, 2010, 05:33 PM
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Not a problem. My first thought was to add something to the wing loading to compensate
TheRocks, wing loading refers to how much weight the wing is carrying. The weight of the lines will certainly add a little to this, but my guess is this is not enough to matter - the lines aren't that heavy, are they? (I've never flown control line.)
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since the drag created is something that needs overcome by the thrust.
My thinking exactly. Therefore, what you want to do is increase the THRUST, not the wing loading. WebOCalc lets you enter any thrust figure you want, so you can just type in a number that looks good, or you can go for a thrustier "flight mission" and let WebOCalc pick a larger thrust for you. Obviously, "unlimited aerobatics" corresponds to more thrust than, say, "sport aerobatics", and "3D" corresponds to even more thrust.
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Originally Posted by TheRocksKid View Post
Question is, how much drag is created by two 0.018 diameter 60-foot lines?
I have no solid numbers on this, but I would not be at all surprised if the wires make more drag than the entire model aeroplane does. Wires are very draggy things - that's why engineers were so desperate to get rid of external wires on aircraft a hundred years ago, and did get rid of them just as soon as someone figured out how to make a wing strong enough to survive without external wire bracing.

That someone, by the way, was Hugo Junkers, a brilliant and mostly forgotten engineer.

Oh yeah, I remember reading that the huge thick wings on a Boeing 747 (they are thick enough for an adult man to stand upright inside the wing roots) have about the same drag as a 2-inch-thick cable of the same length as the wings. In other words, a two-inch-thick cable has the same drag as the seven-foot-thick wing! That's how bad the drag from a circular cross-section wire is!
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Originally Posted by TheRocksKid View Post
I imagine that the flight speed calculations you are performing here would somehow factor in, but I'm neither an engineer or mathematician.
The drag of anything moving at this sort of speed in air is given by the coefficient of drag times the square of the airspeed times a couple of constants. Since the wire is sweeping in a circle, the speed varies all along the wire, so the drag will be different at every inch along the cable. Fortunately, our friend Isaac Newton invented just the sort of mathematics we need for this sort of problem - calculus. A little integral calculus would give us the answer to your question, i.e, how much drag from cables of X diameter and Y length with the tip moving at Z mph through air?

However, I'm just not in the mood to play with math today. I'm more in the mood to go play with my guitar, and give the other half of my brain a work out. Maybe one of these days I'll revisit this.

And, in the meantime - why not just slop on some extra thrust? With brushless motors and lipo packs, it's easy to add thrust these days. Also, we know that tiny and inefficient .049 and smaller glow engines were able to pull control line models around using tiny and inefficient propellers - so clearly there isn't all that much drag from the wires, no more than a few ounces at best.
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Originally Posted by TheRocksKid View Post
I'm going to try just adding 15-percent to the wing loading and give it a try. After all, what's the worst that could happen? .
Nothing at all. That's why I wrote WebOCalc - not only to give you *an* answer, but to let you play around with "what if??" scenarios and perhaps find better answers in the process.

-Flieslikeabeagle
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Old May 29, 2010, 05:35 PM
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Chuck, a huge "Thank You!" for writing this up and helping out fellow RC pilots. I've written many threads and short tutorials on WebOCalc here on RCG over the years, but I never could find the time to keep doing it over and over - thank you so much for all the work you put into this write-up!

-Flieslikeabeagle
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