Prop direction

[C Norton]

I'm currently fettling a Twinstar for 2 brushless motors and want them to counter rotate to reduce torque effects. Is there a preferred way for the props to spin? ie top of the props towards or away from the fuselage? I suspect it's towards but not quite sure why.

 

I'm also trying to do a mix on a Spektrum DX8 so that I can have normal control of both motors with the throttle stick but also control 1 motor independently using the rotary control. What little hair I have left has been torn out trying to do this! Has anyone managed such a thing?

 

Many thanks,

 

Chris

[Steve Hargreaves - Moderator]

The biggest problem I think you will find is trying to source suitable props. Most propellers are designed to rotate anti clockwise as seen from the front.....spin one clockwise & it becomes a (very inefficient) pusher....

[Chris Bott - Moderator]

Hi Chris

 

I have a 3D model which terra firma converted to many pieces. So it was rebuilt as a twin.

They are contra rotating which removes all torque effects

I have a DX8 and have made two throttle channels. I have also mixed rudder to these channels, so one speeds up and one slows down when I put in some rudder. This option can be switched in and out.

 

 

What I found first that with one motor going faster than the other, I get much more lift on that wing and therefore a strong roll effect.

So, the first thing I did was change the rotation directions so that the torque of the faster motor opposes this roll effect.

 

I did wonder if the combined rotating propwash from two motors over the tail would produce a pitch effect, but I can't detect any.

 

If you are interested, I'll try to describe the mixes involved in making rudder affect both motors. It certainly makes taxiing, stall turns and knife edge really easy. However in other manoevres it has all sorts of yet untamed results.

Cheers

Chris 

 

 

 

 

Edited By Chris Bott on 02/08/2011 16:53:28

[Steve Hargreaves - Moderator]

Looks a fun project Chris....what props did you use?

 

I recall a small foamy flying wing called the Goolie I think that had a similar arrangement.....

[Chris Bott - Moderator]

First of all a pair of APC-E 9x6's (one normal one pusher.

Now I have a pair of Master Airscrew 10x7 three bladers on it.

You are right, it is a bit of a hobsons choice when it comes to finding a matching pusher prop.

 

Lot's of fun to play with yes, not much real practical use though.

I really need lots more air time to perfect some other things that it may be able to do..

 

Really its a test bed for my bigger twin, the 88inch DH88 Comet.

[PatMc]

On full size, for instance the DH Hornet , it was downgoing tip towards the fuselage. It means that on single engine power the rotating airflow tends to push the "live" side down.

[Peter Beeney]

Usually full size twins that have counter-rotating propellers are configured with in-spin, the prop tips turning toward the fuz at the top. There are some out-spin types however, the Lockheed P-38 Lightning was one such, something to do with the way the guns fired, as I remember reading somewhere.

So I guess it would be fine for a model. A friend has built an own design glider tug, this is an electric twin with counter-rotating props; but there are two more, which are contra-rotating, this is two props on the same shaft, and turning in opposite directions. Thus it has 4 props.

We are looking at possibly constructing a model of the Cri Cri, the French micro-light, one of which has been adapted by EADS, the French aviation company, for testing electric flight. Two brushless motors, each with a prop on the front, and another one on the back.

By looking at the props in one of static photos I deduced the front two were counter-rotating and the rear two were pushers of course and also contra-rotating to the front two. Model wise, finding the right propellers might be quite difficult, sorting out the motor direction is the easy bit. If the Cri Cri indeed only has two electric motors then there would have to be a direction changing gearbox on one end at least. If I’ve got my prop turning direction dead to rights, that is! And that is by no means a certainty, it looks a though I’ve got about four chances of being wrong!

Another friend built an i/c twin, own design again, not counter-rotating this time, that’s a bit more difficult, but I popped my crystals in and as his rx had sufficient channels I could utilise two throttle channels and some mixing with rudder for asymmetrical steering. Plus a bit of flying practise with one engine out. This was a trail dragger, and the ground handling was not all that clever, it had to be going quite fast before it would steer straight. The first thing that I found that with the grass on the strip a little bit long, and a bit wet, when I tried to steer it with the rudder one one motor would decrease revs and the model would slow down! Fortunately, like Chris, I could switch this out so we were able to eventually take off….

The wing was from an old Aircore Trainer, 1980’s, I think that’s what it was called, and ultimately it started to fold under a bit of to much uncalled for G, but we hope to eventually resurrect her with two nice new Jen 37’s in place of the ancient Thunder Tiger 40’s.

Although this was a complete ‘hack’ of a model it was surprising how realistic it looked in the air; and also just how fast it was, too!

PB

[Chris Bott - Moderator]

Chris here are the mixes I have set up.

 

(NOTE: This does not do what you asked for, it makes rudder inputs change the speed of both motors. Flight Mode Switch has the mix Off, operating at 50% and operating at 100%)

OOOH, don't the photos make it look tatty 'cos I still have the protective film on?

 

Switch setup to make sure none do anything unexpected.

 

Mix 1 to make Thr operate the Gear channel, (it only has a 6Ch Rx)

 

Mix 2 isn't used.

 

Mix 3 and 4 add 50% rudder to Thr and Gear but only with FM switch in posn 1

Mix 5 and 6 add 100% rudder to Thr and Gear but only with FM switch in posn 2

 

 

[C Norton]

Hi and many thanks to all for your input. First off the props are no problem; I'm using GWS 3 blade 9X6 and they come in CW and CCW versions.

 

I have a Goolie which works as intended but that isn't what I'm trying to do. I want to be able to have both motors controlled by the throttle channel but also to be able to have full control of one of the motors from zero up to whatever speed has been set by the throttle channel using the rotary control on the DX8. I know it sounds similar but it seems to be subtly different to the Goolie/rudder scenario.

[Chris Bott - Moderator]

That should be no problem Chris,

Use my Mix 1 to provide a second throttle channel.

Then mix Aux 3 (that's the knob isn't it) to the Gear channel too.

 

Try it on the monitor screen to see if it has the required effect.

You can also make it switched like I have.

Edited By Chris Bott on 02/08/2011 23:03:35

[Chris Bott - Moderator]

OK, just had a play.

 

Now I'm assuming you want both throttles to move together all the way from bottom to top.

Then you want to be able to slow one of the motors from whatever the other motor is doing. Maybe to simulate a sick engine or something??
Now the Knob has to be fully clockwise for the throttles to match. Turn it anticlockwise and the motor on the GER channel will slow down.

 

I have achieved this with the following:-

 

Sw Setup as above.

 

Mix 1 - as my mix above to make two throttle channels.

 

Mix 2 - Ax3 > GER

Rate : -100% (minus 100)

-100% (minus 100)

Offset -100% (minus 100)

Trim: Inh

Sw: ON

 

That last entry means this is always functional.

CHRIS - IF you try this, please have a really good play with it with no propellers fitted. I've just thought of one scenario with certain types of speed controller that may cause a SAFETY ISSUE.

 

With the throttle low, and the knob turned left, the extra throttle channel goes below -100%. This could reset the endpoint of some ESC's meaning that when you move the knob back to the right, and expect normal throttle function, one of the motors might fire up unexpectedly. Just give it a good old test in all conceivable stick combinations before you fit any props.

Edited By Chris Bott on 02/08/2011 23:22:25

[Steve Hargreaves - Moderator]

Not a Spekky user so not sure about the actual mixes used but would I be right in saying that the above will only allow you to operate one motor independantly...the one attached to the GER (Gear...as in retracts??) channel?

 

In actual use then, assuming the motor on the right hand side is connected to the GER channel, then with the left motor at full throttle you can set the right motor to anything you like but you then can't have the motor on the right at full throttle & vary the speed of the left hand motor?

 

Might another way be to use the throttle channel as MASTER & slave two independantly controlled channels to the throttle stick.....then you could control each motor independantly via their own knobs or sliders.....

 

Or have I mis-understood something...

[C Norton]

Hi again and many thanks for all this great info. Steve I would love to do as you suggest but there is only one useful control on the DX8 to vary the speed of one of the motors. I suppose I could use the slider switches but not very intuitive or quick to get back up to max should I have problems at a low setting.

 

Chris, that is exactly what I wanted thank you! It was the offset bit that I was missing. I take your point re throttle calibration and it's something I will have to be aware of. I owe you one!

[Steve Hargreaves - Moderator]

I say again I'm not a Spekky user so not sure how the mixes are used but I wondered if this might work (I use similar in my Fuby 10c for other things).

 

Could you use a seperate switch to mix each channel controlling each ESC back into itself thus giving a single speed setting for each motor.....thus switch A controls the THROTTLE channel (left hand motor) & is mixes the THROTTLE into itself by, say, 50%. switch B controls the GER channel (right hand motor) & is mixes GER into itself by, again say 50%.

 

Operating switch A would give you half speed (or whatever you set) on the L/H motor which still allowing the R/H motor to be controlled via the throttle stick. Operating switch B would do the same for the R/H motor......

 

Any use??

[Chris Bott - Moderator]

Chris if you want to switch the effect from off, to one motor, or the other motor, then just change the SW: ON in the text above to SW: FM1

 

This makes it only operate when the Flight mode switch is in posn1.

 

Then make another mix from the knob to the Throttle channel, but make that only operate in Flight mode position 2.

 

I can't test this as I'm away from home, but I'm pretty sure it will work.

If not, then I do have another idea, but would like to test it first..

[C Norton]

Thanks again Chris, like you I am away from home and don't have my DX8 to play with but I like the sound of being able to switch the effect from one motor to the other.

 

I was also toying with the idea of using the left and right sliders to be able to individually control the relevant motor but will need to play with that. I'd probably have that switched but that might mean that low to high throttle is applied instantaneously to the motors.... not sure if that's too clever?

[Chris Bott - Moderator]

I think experimentation is the next thing.. best of luck, let us know how it goes.

 

Chris