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I have Got This Far Should I Just Fly it

Designing Your Own When You Haven't a Clue

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Andrew McKelvey 125/02/2020 17:10:07
6 forum posts
3 photos

Long Story severely shortened. I have designed my own twin engine high wing plane which I am calling a cartoon scale Bristol Freighter, only because it has roughly the same bits in the same shape but not necessarily exactly the same size.

I wanted to have a go a building a model with foam board using simple building techniques and see what I ended up with. The dimensions roughly the same as a single engine high winger I 'inherited' and put back in the sky.

I have a couple of questions still nagging in the back of my mind which someone in the know might answer for me.

I am using two brushless motors with two esc and one battery and I had it in the back of my mind I am supposed to isolate the live from one 'BEC' so It works, well I didn't initially and it does, in effect the bec's from both esc is feeding a regulated 5v to the receiver, I was told I had to isolate a +ve from one so it didn't confuse the esc? the receiver? So first question why do I need to isolate one of the supplies from the esc's because both mine work ok without. I must be missing something.

The motors are configured one to run CCW and the other CW does it matter which wing they are mounted on?

At the moment neither has any side thrust they both point straight ahead so do they need side thrust and if so in which direction?

Lastly the wing the engines are mounted on is Clark Y because of simplicity as it is made from blue foam. The flat bottom has zero incidence with the tail plane so in effect aerodynamically they will have a couple of degrees down thrust.

The plane this model is based on has exactly the same setup and flys quite well ie little or no side thrust and only a cople of degrees down thrust on the engine, clark y wing bottom at zero incidence to the tail plane.

I would be interested in your thoughts otherwise I just put it down on the strip when the wind subsides and open the throttle and see what happens. Think I might take a few photos for you

Martin Harris25/02/2020 17:42:22
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9159 forum posts
229 photos

I'm led to believe that the motors may not always start properly and run in the desired direction unless the red wire is removed from one.

Flying it - give it a safe C of G and go for it if you're experienced with trimming new models.

Andy Joyce25/02/2020 18:08:58
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181 forum posts
30 photos

Andrew

Some quick responses having built lots of Depron twins myself in the past;

If both esc's have built in BEC's then yes, break the red wire of one BEC going to the RX. This is to avoid one BEC voltage trying to regulate the other ones output to the Rx

One Battery is OK but ensure that it has the capacity to drive both motors. I normally use two batteries and have a cross feed between both batteries to ensure a common supply voltage to both motors.

Running one CW and the other ACW is good approach as it removes the torque effect from the prop. No side thrust is required on both motors. Remember if the motors are mounted high on the wing some up thrust may be required, but with the exception of one model I have never found this necessary.

Andy

Ray Wood 425/02/2020 18:17:51
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161 forum posts
34 photos

Hi Andrew,

I'm sure it will fly ok, the full-size freighters had engines and tailplane at zero and the main wing at 4 degrees + they flew with a pronounced nose down attitude with 3 cars & 16 passengers. Both engines had out thrust to help the asymmetric thrust with one engine out.

Regards Ray

Peter Beeney25/02/2020 18:20:53
1587 forum posts
59 photos

The short answer then Andrew, is that in my opinion the two BECs will operate perfectly happily which ever way you connect them. This has been done in the past with industrial size kit.

With respect to other points of view; not an instruction on how to do it.

There have always been differing options on this little conundrum , it’s probably always going to be an aeromodelling bone of slight contention…

The Clark Y might also be be a good choice for a Bristol Freighter lookalike. Nice and sedate flying. I can remember as a youngster in the nineteen fifties cycling about 20 miles to an airfield perimeter fence just to watch Bristol Freighters taking off and landing.

Good Luck!

PB

Edited By Peter Beeney on 25/02/2020 18:23:24

Simon Chaddock25/02/2020 20:02:18
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5613 forum posts
2978 photos

The business with the red wire on ESCs depends on what type of BEC it has.

If a linear BEC you can have as many connected as you like feeding the rx. They won't share the load equally though.

A switched BEC is different as the high frequency pulsing from one BEC may confuse the other.one so the voltage control may not be as good as if only one was connected.

I certainly would set up counter rotating props. Remember models have a much higher thrust to weight and thus torque than full size. With counter rotating you don't have to worry about side thrust either.

Just get on and fly it. Only then will you have any idea as to whether any adjustment is required.

If it flies and lands satisfactorily do as Matt Summers said on the prototype Spitfire - leave it alone - until you have flown it a bit more. wink 2

Andrew McKelvey 126/02/2020 08:35:53
6 forum posts
3 photos

It is always a bit difficult knowing when to reply to one of your posts because I think it deters people from then adding their weight behind the thread.

However, I thank those of you have responded and I take on board all the comments, the bec issue I summarily got 4max view on the subject when I perused the site last evening and conclude that I will disconnect a +ve from one of the esc, in fact depending upon where the cd comes out I might just disconnect both and power the receiver via a nimh in the nose if it needs some weight.

When designing a scratch built it is interesting how many decisions have to be made some will affect the flying ability and some become aesthetic, like the above thread it is some times nice to have some one else's thought when you in the shed.. just to confirm your own thoughts or the opposite.

My intension is to do the normal preliminary checks put it on the strip and let it go and see what happens, most things I can probably compensate for on the sticks but if I wreck it I'll get some practise repairing a foam board model, what can do wrong!

Ps I did take some photos but do not seem to be able to add them here keeps asking me to browse a library which means nought at the moment

Andrew McKelvey 126/02/2020 08:42:29
6 forum posts
3 photos

I get there eventually

p1010003.jpg

p1010004.jpg

Lima Hotel Foxtrot26/02/2020 09:13:00
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378 forum posts
Posted by Andrew McKelvey 1 on 25/02/2020 17:10:07:

I am using two brushless motors with two esc and one battery and I had it in the back of my mind I am supposed to isolate the live from one 'BEC' so It works, well I didn't initially and it does, in effect the bec's from both esc is feeding a regulated 5v to the receiver, I was told I had to isolate a +ve from one so it didn't confuse the esc? the receiver? So first question why do I need to isolate one of the supplies from the esc's because both mine work ok without. I must be missing something.

The receiver is going to get two lots of power from the ESC's. Cutting the live wire from one should stop your RX melting from the combined voltage.

Lima Hotel Foxtrot26/02/2020 09:15:15
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378 forum posts

And, yes, you should just fly it (when the weather dies down...)! How else will you know what a good job you've done in building it?

Robin Colbourne26/02/2020 09:18:26
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502 forum posts
6 photos

With regard to propeller rotation for a twin, both the P-38 Lightning and F-82 Twin Mustang reversed the direction of rotation between the prototype and production aircraft. In the case of the F-82, the airflow over the centre section was so bad that the middle part of the wing between the two fuselages was stalled and the prototype could not leave the ground!

In other words, try both options and see which works best on your model.

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