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Author Topic: Twins - Double the Fun, or Twice the Trouble?  (Read 309 times)

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Offline Deerslayer

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Twins - Double the Fun, or Twice the Trouble?
« on: July 28, 2017, 08:03:49 PM »
Let me tell a tale or two and offer something that may help someone else.

I have limited experience with twin-engined planes. My first was a Corroplast contraption, inverted V-tail, two OS LA15 engines. I created this to learn something about twins at lowest possible cost and minimal pain. It worked out very well, I spent many hours flying and hovering my effort. All I had ever heard about twins was that it was SO important that they be synchronized at all times! Well, it ain't all that easy, even if you break out 2 brand new engines, run them a couple of minutes (my normal break-in procedure), strap 'em onto somethin' and go flyin'.

In this case, the engines had very different histories. So, I fired them up and began to play. I could get them to fast idle reasonably close and even reach near synchronization at high end. I could either try to get them matched in mid-range or at the other two ends, but not across the whole range. It turned out to be of little issue, as I had designed the machine with great rudder (ruddervators, actually) authority in order to strongarm out of any unwanted yaw due to mis-matched power systems. It turned out that even that wasn't a big deal. What happens is that the engines seem to approach closer to sync under flight loading.
Good stuff!

On to electrickery! My first twin electric was/is my own design - Tailsitter Mk.1 and Mk.2 The concept is a VTOL flying wing with twin engines, not using a Flight Controller, just relying upon my DX9, brute force and dumb luck to get the job done.

In my flight testing of these 2 Tailsitters, I had very hairy vertical takeoffs, and hovering or vertical landings were way beyond their capabilities.  Hand-launched horizontal takeoffs were OK. I knew what the problem was, but not the root cause. At cruise, everything was fine and you could hear that distinctive tone of well-synched motors along with no rudder requirements. (There are no rudders, yaw is controlled by differential thrust of the motors, as mixed from the Throttle stick). Changing from a low or medium speed to higher Throttle caused severe yawing, due to unwanted differential thrust from the motors.

How do you know if 2 engines are in synch? Use a tachometer, or perhaps just listen. You will learn to detect the beat frequency when two sources are out of synch; it gradually decreases as the sources approach synchronization, and disappears if the frequencies match. On the ground, you can use a tach. Or, as I also did, mount your plane on a turntable and see how it responds to Throttle inputs. I have a turntable from an old microwave oven; an old phonograph turntable would be great, as well.

The big problem here was that the 2 engines would not respond identically as they are run up. I went through a lot of effort to to understand what was going on. ESCs? identical. Props? as identical as can be claimed.   It should have been obvious. Some dumbass had two slightly different motors on the thing! Now, I wonder who that could be?They were both 2208-14 motors and looked pretty much identical. Finally ... finally ... I realized what was wrong. One was a Cheetah 2208-24 (laser engraved onto the rotor) while the other was a BHP 2208-14T (it said so on a little sticker on the rotor). The former responded quicker  and more smoothly to power change than the latter. D'oh!

As part of this effort, I set out to test motors. In hindsight, I would do this before installation on an airplane, as I always do for my servos.

Here is the simple approach:
1. Bolt the motors down onto a piece of wood or your workbench, with suitable props attached.
2. Power the two ESCs via a Y-connection from a common LiPo.
3. Use a Y-connector to join the Throttle leads from the ESC. In my airplane(s), I avoid Y-connectors like the plague and on a twin you want/need separate channels for your throttles.
4.At some point, pull the Positive (RED) wire out of one of the connectors and tape it back out of the way. This is very important, both for the test setup and in the airplane.
5. Connect your servo tester to this common Throttle. Mine has a digital read-out of pulse width and a potentiometer to control that output. Mine can provide outputs of the typical range - 1000 through 2000 microseconds - which represent -100% through +100% on most systems.

Now, you have a means to control, hold and record RPM at any point in the power range while you check and record the two motors' RPM with your tach. As well, you can listen for synchronization (no beat frequency) or imperfect synch.  As others have discovered, even if you buy identical motors and ESCs, there may be some minor variations between the two systems. You could also measure current at various settings if you have an ammeter setup handy.

In my case, the Cheetah motor was very smooth in acceleration and held its RPMs close to constant at any setting. Not so for the (cheaper) BHP motor - it accelerated unevenly and wandered quite a bit. These motors are AC but  NOT synchronous motors.
The End
(at last)
My purpose in Life is to serve as a Warning to others

Offline Deerslayer

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Re: Twins - Double the Fun, or Twice the Trouble?
« Reply #1 on: August 01, 2017, 06:08:03 AM »
I set up my  2 Cheetah 2208-14 motors for testing. As I cranked them up and down quickly in speed, they sounded great. When I did the same thing slowly and then paused at various speeds, particularly in the under 4000 rpm range which is most critical to my use in the VTOL plane, here was no audible beat and the tach showed that they were perfectly synched! Good little fellows, methinks  ;D

This takes one possible variable out of my earlier concerns that the manufacturing tolerances in ESCs can lead to synchronization issues  ;D
I do think it is a good idea to use the same brand and model ESCs on any twin, if for nothing more than piece of mind - especially if you ever get into re-calibrating one or both of them, either deliberately or accidentally.

Incidentally, I had one good APC 8x4E prop. The other was the same but had tip samage, so I chopped off about 1/4 inch (the Flypaper Method for Propeller Modification/Improvement). Despite this disparity, all went perfectly well!

A couple of precautions:
1. When doing any kind of bench testing with props installed, make absolutely sure that your test system is totally secured and that the associated wires are kept well clear.
 2. As you may, like me, be operating a servo tester and a handheld tachometer  and ammeter while checking their readings, it would be very easy to get distracted or careless and ... enough said!

3. Do the testing outdoors. The 60 Hz lights inside will render your optical tach useless.

My test setup is a square chunk of wood large enough to bolt the motors onto, with about an inch or so of clearance between prop tips and clamped  in a Workmate type of stand.
« Last Edit: August 01, 2017, 06:13:34 AM by Deerslayer »
My purpose in Life is to serve as a Warning to others