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left or right thrust ?

propellers on a twin

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Den Moran18/02/2019 09:14:43
141 forum posts
18 photos


I'm putting together a Guanli Catalina which has been in its box for a few years, and have found out that it is supplied with reversed props (unlike the motor cowls, struts, and rods !).

Can anyone tell me the 'correct' placement of the CW and CCW props on the left or right wing please?

Thanks, Den

Jon - Laser Engines18/02/2019 09:29:56
4765 forum posts
179 photos

On my twins i always mount the engines dead straight and i dont bother messing about with opposite rotation. Having owned 5 twins and a 4 engine Hercules the torque argument just dosent stand up. All of those have been i/c.

If you have opposite rotation props and intend to use them then in theory torque is zero so there should be no side thrust needed.

Edited By Jon - Laser Engines on 18/02/2019 09:30:43

Denis Watkins18/02/2019 09:38:03
3799 forum posts
50 photos

There is loads of information on the net Den.

One guy sang the models praises set it up looking from the front

CW Port

CCW Starboard

Martin Harris18/02/2019 10:07:37
8728 forum posts
214 photos

The convention is to have the propellers moving towards each other at the top. It means that you don't have a "critical" engine issue where failure of one particular engine (usually the port one with conventional rotation) means everything is working against the pilot due to airflow, torque and P factor issues.

A few WW2 aircraft (including the P38) had 2 critical engines - it was found that having the engines rotating the opposite way improved the gunnery accuracy for some reason...

Edited By Martin Harris on 18/02/2019 10:12:04

Den Moran18/02/2019 10:24:57
141 forum posts
18 photos

So - tomayto, tomahto!

The engines/motors are mounted on the fuselage parallel axis, Jon, and I have no argument with that.

But now I have two suggestions for completely opposites! The counter rotation towards each other makes sense, any variation in torque effect being thrown 'inboard', so to speak. I'll probably go with that one.(after I make up some cowls, wingstruts, and aileron rods!). Thanks, everyone,

Den Swansea.

Piers Bowlan18/02/2019 15:45:28
1823 forum posts
44 photos

Den, only the prototype had props that turned towards the fuselage and then tests revealed that, as Martin says, the aircraft was a better gun platform with the props turning outwards. There was also a vibration issue with the props turning inwards, something to do with the spiral slipstream interfering with the twin tails.

No doubt you know Den but the P38s ordered for the RAF had twin right-hand turning props and no turbo- supercharger.

Another aircraft that had the unusual outward turning props was Dr. Charles Zimmermans Vought V173. With this, the rotating slipstream from the propellers mounted at the wingtips would negate the tip vortex, greatly improving the wings efficiency and endowing it with benign handling characteristics (until one engine stopped!) surprise

I don't think anyone will notice if the props rotate the 'wrong way', you can always tell them that it is a model of the prototype!

Edited By Piers Bowlan on 18/02/2019 16:01:03

John Dennier19/02/2019 07:27:51
35 forum posts
7 photos

I have flown two twins both had contra rotating props and both tend to yaw to the right on initial take off power application requiring left rudder input until speed builds up. I cannot recall whether the tips moved inboard or out board at 12 o clock.

J D 819/02/2019 09:09:56
1226 forum posts
74 photos

With both engine's near the center line the Catalina is one of the least "critical" type's regarding engine configuration. Many a Cat made it home long distances on one engine during the war years.

The Guanli model seams to have motors further out than scale but saying that the one's I have seen have both motors going the same way.

Den Moran19/02/2019 11:30:14
141 forum posts
18 photos

I'm convinced! Outward it is! I may need a stable gun platform if we loose at the Arms Park on Saturday!

Thanks again, everyone, really helpful, and as usual, an impressive knowledge of our hobby shown by all, I can't count the number of times everyone has given me sound advice, makes it a great hobby!

Piers Bowlan19/02/2019 13:33:17
1823 forum posts
44 photos

I better come clean here, The PBY Catalina had P & W twin wasp radials that rotated the same way, as JD 8 tactfully pointed out. I think I was getting a little bit carried away by talking about the P38 counter rotating props blush. Well, it is an interesting subject at any rate!

So sorry, Den, it is back to the drawing board for you; inward rotating, outward rotating or, for the scale aficionados, ones that rotate the same way. Don't ask me which way though crying.

Edited By Piers Bowlan on 19/02/2019 13:34:59

Martin Harris19/02/2019 14:59:51
8728 forum posts
214 photos
Posted by J D 8 on 19/02/2019 09:09:56:

With both engine's near the center line the Catalina is one of the least "critical" type's regarding engine configuration. Many a Cat made it home long distances on one engine during the war years.

I'm in the middle of creating a Ripmax Extra Slim Twin from an Extra Slim kit and some additional copied components - these are so good with their twin fuselages set very close together and excellent airflow over the rudder that I often flew almost an entire aerobatic flight on one engine with my original - good practice for rudder control of course...

Slim Twin

My original version

Its engines will be rotating the same way.

Hopefully with better engines (and more experience) than I was using on my previous example, losing an engine inadvertently won't be such a common experience with this one!

Edited By Martin Harris on 19/02/2019 15:03:03

Den Moran19/02/2019 15:35:56
141 forum posts
18 photos

That's honest of you, Piers! Thanks. That's a fast looking aircraft, Martin - good luck with it. Den

Simon Chaddock19/02/2019 17:57:10
5415 forum posts
2828 photos

My own view is that counter rotating on most twins only makes a difference at the extremes of the low speed end of the flight envelope where the elimination of motor torque will likely delay the advent of wing drop and the 'spiral of death'.

Of course if one engine stops your must not be anywhere near the low speed end of the envelope anyway!

Another issue with the P38 was the problem of an engine failure at the point of take off. Although there was sufficient thrust to continue the take off 'on one' the problem was the single engine torque overcame the ailerons at normal take off speed. At one stage this caused more deaths than from combat..

The Lockheed test pilots found the solution - quickly throttle back the good engine a bit and wait for the speed build up.

One can imagine it took some training to do that!

J D 819/02/2019 18:00:06
1226 forum posts
74 photos

I remember those slim twin's Martin,they were bloomin quick.

Going back to the first post Den you say your Gaunli model has opposit rotation props,just that the Guanli ones I have seen have both motors going the same way. John.

Edited By J D 8 on 19/02/2019 18:01:15

Martin Harris19/02/2019 18:15:35
8728 forum posts
214 photos

My old one started life on a pair of SC25s (one good, one not so good) which were adequate and progressed to a pair of mismatched ball raced OS 40s which made it sparkle, via some 40 FPs. A pair of as new OS 46 FXs have been waiting patiently in my spare room for several years, earmarked for this project.

Where's the maniacal laughter smilie? devil

Den Moran20/02/2019 07:52:51
141 forum posts
18 photos

...and that reminds me of the 'Dual Ace' that's sitting on my shelf, waiting to be flown - that has 'normal' rotation! devil

eflightray20/02/2019 10:02:03
574 forum posts
128 photos

If you intend flying off water, differential throttle comes in handy for better water steering.

I fly my Sunderland, (avatar) with counter rotating props, but only because I wasn't sure during the build how it would handle on water, it also gets flown off grass.

All my other multi-motor models use the same direction rotation.

I do have a HK Catalina, but rarely flown as I just don't like the way it flies.


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