Reverse rotation for a twin?

[David Ovenden]

Having completed my Curtiss Condor twin project I'm planning my next model. I'm going to do a Cessna T-50 Bobcat twin. My intention is to design it to have interchangeable I/C and electric power modules. With electric its easy to have one motor with standard rotation and the other reversed with a "CCW/pusher" prop to avoid the torque issues. I wondered about doing the same with the I/C powerplants so the flying dynamics of the model would be the same with either electric or I/C module fitted.

So her is the question. I now have 2 Enya 53-4C engines that I will use in the model. I found online the following diagram for retiming the Enya for reverse rotation. Anyone tried it? Is the re-timed reversed rotation engine likely to run very differently from the standard rotation engine? Would it be better to to have them both on standard rotation and concentrate on reliable engine set-up so neither of them stops?

There is a video of someone who has done this on YouTube but the engine is only seen running on the bench and is not properly set up in the video so its hard to tell how successful it was in reality.

 

[Jon H]

I have owned 6 twin and one 4 engine model over the years and flown 4 other twins. All had engines in the same rotation and there was no issue with torque. 

 

My advice is not to waste your time on it. You are more likely to run into trouble with differences in performance between the reverse/normal engine and prop than you are with torque. You also need to find suitable and identical reverse props, reverse your starter and all that. Its just needless hassle. 

 

Most torque issues come from just punching the throttle from idle to full. In the case of i/c, one engine can out accelerate the other and cause a swing. Torque is then blamed for this without any real thought into what is actually going on. 

 

My current twin has laser 70's on it mounted dead straight and there are no problems at all. 

[Nigel R]

I have limited twin experience, but, this sounds like a lot of hassle.

 

27 minutes ago, David Ovenden said:

Would it be better to to have them both on standard rotation and concentrate on reliable engine set-up so neither of them stops?

 

That was certainly top of my list.

 

Getting the servo / throttle arm ball link set up so that the throttle action was physically as identical as I could make it on both sides was something I took a little time over during building and setting up. I also spent a short while fine tuning the ball link on one engine to get both engines idling nicely at the same speed. This probably took about fifteen minutes.

 

Then I left them alone and just flew it. I've not noticed any torque effects.

 

[Jon H]

Reliability is key in my view. if the engines are 100rpm apart at full power who cares. Its half throttle where you need them matched if possible. 

 

If an engine does stop its not game over by the way. Get the nose down a few degrees, nail the other one at just shy of full power and maintain the speed. Correct the yaw/roll with rudder manually (no trim as you will be out of trim again when you close the throttle), call for landing and bring it in. Ignore all the cut and glide suggestions as twins glide like bricks and you still have complete control with one engine as long as you keep the speed up. If you slow down you loose rudder authority and then its the twin spin of death when you try to accelerate/climb again. 

 

My 70 powered twin will still roll on one engine if i am careful with it. My old Ripmax harmony would as well but i never tried it with the hercules ? 

[David Ovenden]

Thanks Jon & Nigel for your very helpful input.,

That is what I was tempted to do. It means I will set up both electric motors and I/C engines with standard rotation. Then I don't have to find matching opposite rotation props and don't have to re-time the Enya. Whilst I imagine it will work in terms of allowing it to run "backward", I can't believe it will run exactly the same as the standard motor would. As I now have three Enya 53-4C engines to choose from, I will concentrate on finding which 2 run closest to each in terms of max rpm and transition across the mid-range.

 

[Chris Walby]

Oooorr a chance to reply with some (limited) experience all be it 6 years of flying and having 4 ic twins and 7 electric twins!

 

Suggestions and experience

1. Power plants need to be as reliable as possible, I have had more electric one out in flight than IC, I was advised to always have a plan B in the event and as Jon says manage the situation accordingly 
2. Keep it simple, one throttle channel, Y lead, two servos and mechanically adjust to suit. 
3. I have twins (Woohoo and Extra Slim Twin) with differential throttle control as it increases flight performance, but if you don't need it, don't set it up as one day you might regret it
4. If its not 100% right either IC or electric never commit to flight
5. As a well respected model designer and flyer says regarding not using one counter rotating motor and with adverse yaw on takeoff it can be countered with the rudder, that's what its there for.

Notes

Two BH electric mossies one with counter rotating prop, yes the other yaws a bit but rudder sorts that out

The Whoohoo has both the same direction and no rudder control and does not yaw/roll at all on hand launch or near stall even if the stick is slammed from zero to 100% - someone please explain that?

The IC Dual Ace and Mossie being ic do yaw, but with gentle use of the throttle and rudder, its not an issue (another bit of advice never WOT on take off as its the most likely time to quit on you) and if it needs 100% WOT for the length of the runway things are unlikely to improve once airborne.

 

PS One thing to consider, the choice of props may be very limited so before committing to a lot of engineering work worth a check first

 

 

[Nigel R]

A servo slow setting on your throttle channel can help avoid a panic snap-open throttle cut situation. Worth doing if your TX does it.

 

2 hours ago, Jon - Laser Engines said:

twins glide like bricks

 

Having two nacelles makes for a fair bit of extra drag. I was quite surprised how quick mine slows down when the motors are at idle. Almost like landing a bipe.

 

[Jon H]

yep, twins tend to be heavy too. Also David is building a twin, fixed gear biplane so there will be no shortage of drag. Fine pitch props of large dia are called for me thinks. 

[David Ovenden]

4 hours ago, Jon - Laser Engines said:

yep, twins tend to be heavy too. Also David is building a twin, fixed gear biplane so there will be no shortage of drag. Fine pitch props of large dia are called for me thinks. 

I have managed a first flight with the Curtiss Condor. I used 10x6 3-blade props. Both motors were standard rotation with no sidethrust. There was a small torque pull to the left on take-off but easily manageable with a bit of rudder.  As Chris rightly says "it's what it's there for".

 

Given the advice here, and my experience with the Condor, I will stick with the same standard rotation and no sidethrust for the new Bobcat project.

Edited April 14, 2021 by David Ovenden

[David Ovenden]

Pic deleted.

Edited April 14, 2021 by David Ovenden
Wrong item posted

[Andy Gates]

I have 4 electric twins, all with standard rotation motors and standard props.

Fly as per full size with gentle throttle acceleration on take off and you will be fine.

Once the plane has flying speed the rudder should be capable of doing all the straightening for you.

 

I have 2 of my machines set up with differential throttle which is fun, one is a Catalina so that helps with water steering. Both the Catalina and Twin Otter can do amazing stall turns using this function, but for normal flight I have this switched off as it is not necessary.

 

Never yet had an electric motor issue on a twin, not too sure why that would occur as the motors are normally used less hard than on a single motored machine.

I also do not really suffer any real noticable drag issues with my twins with the exception of the Beaufighter which has 2 huge great cowls on it, but really that is no issue with practice.

 

What I would suggest you do which is what I do, is that if you possibly can - parallel your supply batteries to both of your ESC's, that way any supply differences from the individual batteries will be sent to both motors so they should be synchronised.

[David Ovenden]

Thanks for sharing your twin experience Andy.  It is always helpful to have input from those with prior experience. I do like the idea of parallel wiring the lipo packs.