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Old Aug 17, 2012, 01:46 PM
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DOCFIGHT's Avatar
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Hello flyers !

Multiplex Twinstar rocks ! So powerful with his 2 Dymond 340w 1100kv, ESC 35A & Lipo 3S 3600mAh.

Let's see it in our last video from the june_session :

Twinstar - MKR swiss design

Have Fun & Fly. Over.

www.matterhorns.ch
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Old Aug 17, 2012, 02:15 PM
Gravity gives sinking feeling
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Originally Posted by flieslikeabeagle View Post
That higher cruise speed was probably the main reason why the TS-II needed more power to stay in the air.

Back in the 1970's, a bunch of misfit MIT graduate students solved the problem of how to design an aircraft that would fly using the tiny amount of power a human pilot could put out. Their brilliant solution was based on the one thing everybody before them had missed: the slower you fly, the less power you need to fly. Rather than try to build an ultra-streamlined aircraft (everybody else had been trying to do this, and failing), they built an aircraft that was crude and draggy, but absurdly light with huge wings, so that it flew at a walking pace. .....


-Flieslikeabeagle
a). ... something to do with 0.5 x m x V(squared).... it's not surprising that cleaver people sometime don't look at the obvious answer and go for complex solutions.

b) but it's also true, drag = waste of energy. So by reducing drag, you'd also make it more efficient i.e. you'll need less energy to fly it. That's why IMO, a single route approach does not always give the best solution but need to make a more holistic approach. Some scientists find it difficult to handle multivariables hence we find one day salt is good and another day someone says "Salt is really bad" for our health.

BTW, (a) is my argument too why wind powered electricity generators are inefficient...

Cheers
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Old Aug 18, 2012, 01:08 AM
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Originally Posted by CurrentDude View Post
... something to do with 0.5 x m x V(squared).... it's not surprising that cleaver people sometime don't look at the obvious answer and go for complex solutions.
That's the equation for kinetic energy, what we want here is the equations for aerodynamic lift, aerodynamic drag, and power needed to overcome that drag.

You could say the obvious answer was to do what people had been doing for gliders for decades - reducing the drag coefficient so that they would glide better and better, and therefore require less power to maintain level flight.

That obvious answer was the approach everybody took before those MIT grad students came up with the idea for Chrysalis. They all worked to improve streamlining as much as possible, reducing drag. Unfortunately, that method didn't work.

That's because the drag force is 1/2 rho Cd v^2. If you work very hard and reduce the drag coefficient (Cd) by, say, 10%, that will get you the same 10% reduction in drag force. That in turn will get you a 10% reduction in the power needed to fly.

This is good, but not anywhere near enough. Humans are terrible engines; we're heavy and very, very, very underpowered compared to birds. So reducing the power needed to fly by a few percent was not helpful. It was still impossible to get a human powered aircraft off the ground. And there was no realistic way to cut the drag by, say 90%.

On the other hand, if you leave Cd along and instead reduce v (the velocity needed to fly) by 10%, the drag force reduces by about 20% because of the squared term (v^2). You got twice the benefit that you would have gotten from reducing Cd.

On top of that, because the speed is lower as well as the drag, you get another 10% reduction in power required - power is force x velocity. So you end up needing about 27% less power than before.

Better yet, why stop at reducing v by only 10%? If you double the area of the wings, you reduce v by about 30%. If you can quadruple the area of the wings, you can reduce v to half of its previous speed (and power required to fly to one-eighth!). You gain enormous reductions in power needed to fly by following this approach.

So that's why trying to reduce the drag coefficient was a dead end. There was very little improvement to be had, and often it came at the cost of more weight (more rigid structure was needed to maintain the low-drag shape).

On the other hand once those geniuses at MIT figured it out, it was relatively easy to reduce flight speed (just make bigger wings), and you could gain far more benefits by taking that route. Basically they built a gigantic free-flight model aircraft - something several of them were familiar with, having been competitive model pilots when they were boys. The drag coefficient was relatively high, but the very light structure and very low flight speed more than compensated for the poor drag coefficient.

And that's what those MIT graduate students (very smart people all) figured out. The (smart, but not quite as smart) people who came before never figured that out.

A popular myth in the USA is that smart people are really stupid and never get anything right, but instead ignorant and/or dumb people are the ones who get the right answers. Let's call it the Forrest Gump myth.
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Originally Posted by CurrentDude View Post
That's why IMO, a single route approach does not always give the best solution but need to make a more holistic approach.
True, but sometimes identifying the one key variable and focussing on that single one really pays off. That's why Chrysalis and Monarch were successful, as were the other human-powered aircraft they inspired. (Including Gossamer Condor, Gossamer Albatross, Daedalus, etc).

To this day, nobody has solved the problem of human-powered flight by any other method. Even after the Kremer prizes were won, other teams continued to work on their ultra-low-drag human powered aircraft. Not one of them was successful.
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Originally Posted by CurrentDude View Post
Some scientists find it difficult to handle multivariables hence we find one day salt is good and another day someone says "Salt is really bad" for our health.
Well, soft sciences are different, based on statistical studies, inaccurate data, and sometimes too many variables to really pin down things. And what reaches the popular media is usually not at all what top-level researchers know.

My doctor, for example, tells me that most people of European descent can lower their blood pressure by cutting back on salt - but for many Asian people, reducing salt has little benefit to their blood pressure.

So you see, it may be quite accurate that salt is really bad (for people of European descent with high blood pressure) and at the same time really good (an essential nutrient for all of us, and not a factor in blood pressure for many people of Asian descent)!

The physics needed to get a human-powered aircraft to fly is a lot less controversial: either the thing flies, or it does not. If it flies, the engineers approach worked. If it doesn't fly, their approach failed. It's hard to argue with the facts in this case!
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Originally Posted by CurrentDude View Post
BTW, (a) is my argument too why wind powered electricity generators are inefficient...
Well, I don't know much about wind turbines, but I do know that we need to be working on getting off our oil dependency as soon as possible. Turbines may be part of that process, even if they have their flaws.

-Flieslikeabeagle
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Old Aug 18, 2012, 01:39 AM
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Originally Posted by flieslikeabeagle View Post
...
On the other hand once those geniuses at MIT figured it out ...
Well, you don't need to be a genius to figure these things out. The pioneers of electric model flight were faced with the same problem (low power, heavy motor and batteries) and figured it out: big models with light structures.

Jürgen
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Old Aug 21, 2012, 12:59 PM
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Carlsbad, CA
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Originally Posted by DOCFIGHT View Post
Hello flyers !

Multiplex Twinstar rocks ! So powerful with his 2 Dymond 340w 1100kv, ESC 35A & Lipo 3S 3600mAh.

Let's see it in our last video from the june_session :

Twinstar - MKR swiss design

Have Fun & Fly. Over.

www.matterhorns.ch
@MFlyers
www.Facebook.com/matterhornsflyers
Nice video. Here is my old TSII. Still flying

Twinstar II OBV Over Dave's Beach (6 min 30 sec)
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Old Aug 22, 2012, 03:42 AM
Gravity gives sinking feeling
Joined Jun 2011
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Originally Posted by flieslikeabeagle View Post

..... popular myth in the USA is that smart people are really stupid and never get anything right,

BTW, this myth is not restricted to USA alone but the history of science is littered with numerous examples - name springs to mind is Fraser Green, a British inventor of moving pictures but died virtually pennyless (or centless).

....Well, I don't know much about wind turbines, but I do know that we need to be working on getting off our oil dependency as soon as possible. Turbines may be part of that process, even if they have their flaws.

-Flieslikeabeagle
Your last point is very interesting, specially coming from someone who lives in CA, USA !!!! I believe that the black stuff buried underground for millions of years, should be left alone where it belongs.

I've re-branded CO2 as Carbon-Die-Oxide.

Well I think, interesting tho our discussion (which I'd like to continue to much greater depth) but I think we better start discussing about TSII and not get carried away.

Cheers
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Old Aug 27, 2012, 09:59 PM
gravity is strong where I fly
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United States, NY, Rochester
Joined Jul 2011
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Man, do I love this plane!

I replaced the stock motors with e-flight 370 series brushless, along with a pair of 12A ESC's. I'm running with one 2200 3S and I have the CG set per spec....what a great combo!

I started with a set of 1 counter-rotating and 1 regular prop. First try was the 7x5 APCs and they work well, Then I tried a set of 8x3.8 slow flyer and slow flyer pusher APCs and they gave me much better acceleration but not as much speed (predictable). Then I tried a set of regular rotating 3 blade 7x3.5 GWS props. I thought it felt under powered altogether (like back to the factory 400 motors). So I'm back to the 7x5's and I'm getting around 10 minutes flying time with mixed throttle positions. I still have the torque to accelerate outta trouble too, and all these combos of props still keep me under the 12A mark (per motor) under full power.

I painted the whole plane in silver, and then clear coated it. It looks awesome and fly's just as nice with the added power. Unfortunately the added weight of the paint equaled the loss of weight I removed when I took out the brushed motors

My biggest issue is when I hand launch it; it wants to take off to my right until I get some speed going.....I'm assuming this is because the motor mounts have both motors pointing to the right (and down)? If I remove the right side motor mount and re-glue it angled to the left, will this situation get better? Yes, the right side motor is the one I have set as the counter-rotating motor.

One last question; is this plane modeled after an existing plane?
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Old Aug 27, 2012, 10:08 PM
Gig 'Em!
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United States, TX, College Station
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Originally Posted by Air Cooled View Post
One last question; is this plane modeled after an existing plane?
The Twin Otter is probably the plane the Twin Star most closely matches.

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Old Aug 28, 2012, 07:15 AM
Gravity gives sinking feeling
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Originally Posted by Air Cooled View Post
Man, do I love this plane!
...... My biggest issue is when I hand launch it; it wants to take off to my right until I get some speed going.....I'm assuming this is because the motor mounts have both motors pointing to the right (and down)? If I remove the right side motor mount and re-glue it angled to the left, will this situation get better? Yes, the right side motor is the one I have set as the counter-rotating motor.

One last question; is this plane modeled after an existing plane?
I wouldn't change anything re : motor side thrust. This happens with real thing as well. Be prepared to give it opposite rudder and it will soon correct itself and my advice wud be to learn to live with it. However if the right turn is sever than you cud add 1/16 inch washer (even ply is good enough) on the right hand side of top motor mount..

A small model like TS II doesn't have much effect if one of the motors runs anticlockwise. Hope this help.

To my mind, TS II may be based on Islander as well - just use your imagination on most twin engine.

@ TaxasAggie - is this your HorizonHobby Twin Otter? Looks beautiful.
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Old Aug 28, 2012, 07:17 AM
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United States, TX, College Station
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No, that's a full-size one I found on the net. The HH one has the sharper nose and is blue and white.
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Old Aug 28, 2012, 07:25 AM
Gravity gives sinking feeling
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No, that's a full-size one I found on the net. The HH one has the sharper nose and is blue and white.
That was lightening flash quick - haha.

I've a Twin Otter. Not flown very much this year - poor weather.
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Old Aug 29, 2012, 06:45 AM
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United States, NY, Rochester
Joined Jul 2011
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Thanx for the replys! That real Twin Otter pix is impressive!

Another question I have is can I control each motors throttle separately with the one throttle stick? Hopefully continue to control rudder travel with the motor throttle too?

I have the 2 E-Flight 370 brushless motors, each controlled by their own 12A Electrifly ESC, and I have a DX8 with a 6 channel Spektrum receiver. Right now I have one of red the bec wires unplugged from one of the motors, then a Y harness to the receiver. My plan is to put floats on this plane to use it on snow, so I don't want a rudder extension paddle.

I’m not sure if this is the right Forum to ask this question. Please advise me if it’s not.
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Old Aug 29, 2012, 08:16 AM
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you'd have to check your manual (or search these forums/google), but you should be able to mix rudder with throttle. it won't work with a y-harness, though, and you may need another receiver with more channels.
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Old Aug 30, 2012, 11:58 AM
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United States, NY, Rochester
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I know I can setup mixes with the DX8, but is there a way to setup the plane to use the throttle stick to control each motor separately?

You know; throttle up and right rudder side provide power to left motor and reduces power to right motor......like that?
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Old Aug 30, 2012, 01:07 PM
Electrif(r)ied nitroman
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België, Vlaams Gewest, Putte
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Originally Posted by Air Cooled View Post
I know I can setup mixes with the DX8, but is there a way to setup the plane to use the throttle stick to control each motor separately?

You know; throttle up and right rudder side provide power to left motor and reduces power to right motor......like that?
DIFFERENTIAL THROTTLE take a look on these threads:

DX8 settings: http://www.rcgroups.com/forums/showp...43&postcount=2

General info on diff.throttle:

http://www.rcgroups.com/forums/showthread.php?t=869018

http://www.rcgroups.com/forums/showthread.php?t=1446453

http://www.rcgroups.com/forums/showthread.php?t=1018727

http://www.rcgroups.com/forums/showp...0&postcount=44

regards,
Danny
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