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Old 01-25-2016, 02:39 PM
  #1
aaronr123
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Default Need help with electronics!

Hi guys,

Im currently an apprentice and a group of us have been tasked with creating a display to take to events such as recruiting exhibitions etc. We have decided on having a working scale model aircraft to display physics of flight etc. (and so the kids have something to play with). We're only really in the design phases but have got a very tight timescale (mid march) and have a lot to do before then.

We're going to have a flying model within a cage so obviously it has to be tethered. We've also decided that the best route to go would be to have the power cable permanently connected as well to defeat the need for batteries and to make it lighter.

Now, the problem. As this is an exhibition, we need to showcase different things, so were gonna be making some of the components using 3D printing so its not gonna be the lightest. i was wondering, what kind of electronics will we need to have a decent amount of thrust and control but will also keep the weight right down.

Were only going to need control of the flaps and elevators to demonstrate lift etc. so I know that we'll need at least 2 servo motors, but what kind ?

Roughly i know that well need a motor, servo motors and a receiver but ive never messed about with rc aircraft so im not entirely clued up on the subject and need a point in the right direction.

Any help would be much appreciated

Aaron
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Old 01-30-2016, 03:06 PM
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Quorneng
 
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aaronr123
What you are talking about is "Round The Pole" (RTP) flying but with radio control to move the flying surfaces.
I am sure it can be done but you will have to decide how big you want the plane to be as this will largely determine how long the tethering wires will have to be.
Unless someone in your group has quite some experience in building RC planes I am afraid your time scale is really much too tight.
I can assure you even flying an RTP plane will require some skill and in the course of getting things to work correctly it will be crashed and damaged. Perhaps you can see why you need someone who can build a plane!
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Old 02-01-2016, 11:33 AM
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Yeah, we have tried explaining that the time scale is a bit too tight but we have to run with it. Im not too sure about the size of the aircraft that we will be using but our cage is only 3m diameter, due to the costs involved with buying plots at events. I know that this will mean that our aircraft will need to be fairly small, but how small would you guess that it will need to be?
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Old 02-01-2016, 05:11 PM
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A 3 m cage will means lines of about 1 m which will leave you a plane of about 400 mm span.
With modern motors you can get all the power you need but on such a short lines it will also have to be very light to fly at a reasonable speed. Even 10 mph (4.5 m/s) which is very slow for an RC plane means it will be complete a lap every 1.5 seconds.
In the past I flew many RTP (power down the wires) planes on exactly this sort of line length and believe you me they needed to be light.
For instance this RTP Fairey Gannet is based on a Keil Kraft rubber powered 'balsa and tissue' kit.

It has a wing span of 450 mm and weighs just 52g ready to go with a brushed motor.
You would be hard put to equal that even with a lighter brushless motor but having to include a servo, a speed controller and a radio.
A possibility would be to use a micro indoor type RC plane something like this.
http://www.gearbest.com/other-rc-toy...FQEFwwodjdoHdQ
Disconnect the rudder and simply use a piece of thread from the wing tip to a central pole. It might take a bit of experimenting to find exactly where to attach the thread to get it to work properly.
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Old 02-02-2016, 11:10 AM
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Wow, we did not know that it would have to be this light! Our intentions were to build a very light fuselage but then try and incorporate 3D printing, so that we can make scale replicas of the wings that our company actually makes. Then we was hoping to have different profiles to show the differences in lift and drag etc. but obviously this option would be far too heavy given what you said.

We were going to have a single motor (or maybe 2 EDF, 1 providing power and 1 dummy for aesthetics) and a single servo motor to control the elevators. We was also going to run the power cables for these motors down the tether to allow us to use a constant power supply and a control board, removing the need for a PCB and transmitter.

One idea that was suggested was to produce a hollow fuselage and place a helium filled balloon inside. Or maybe just source a larger cage (5m is our max unfortunately) what would be your take on this? given that we would be probably looking at an easy 300g for the model.
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Old 02-02-2016, 12:07 PM
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I think if you do some sums you will find the lift created by helium in a typical aircraft fuselage is not worth the trouble!

Even a 5 m cage would still be quite small. A typical RTP model would be run on 3 m lines in a 7 or 8 m wide room/hall.
https://www.youtube.com/watch?v=cHrtHqV7bSE This video of 'tethered' RC planes might give you some ideas.
https://www.youtube.com/watch?v=BZ1c9twV9Zk The planes are 'off the shelf' RC 'ready to fly' jobs.

My own RTP planes were actually electric control line. The two wires providing the electricity for the DC motor also moved relative to each other to control the elevator. As the 'pilot' sat outside the circle it did require some quite complex linkage at the centre pylon to achieve it.
The big advantage of doing this was the elevator control system added very little weight so allowed some quite complex and true scale planes to be flown within a 4 m room but it took far longer to develop (and to build!) than you have available!

The first picture (The Martin Baker MB5) is the biggest at 560 mm span and has two motors driving the contra rotating props is all sheet balsa but it still only weighs 127g.
I hope this helps.
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Old 02-02-2016, 01:02 PM
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Yeah, we would love to do a proper job like what you said but because its being taken to events, it needs to be able to fit in a car (including the cage) with all of our other gear. Plus, pitch space is priced very heavily at the larger events. We have to design our own plane unfortunately as we need it to look as close to our own aircraft as possible, we was hoping to make something similar to the A400M as this design would allow us to make interchangeable wing profiles.

But still, massive thank you for your help and time. Been a massive help. None of us are experienced in RC aircraft so this kind of information is really appreciated!

Ill take this information back to the team, and hopefully well get something up and running. We've got a contingency plan just in case though , but ill post the results here!

Big Thanks,
Aaron
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