Contra rotating props

[Glasshopper]

I am about to fit a pair of contra rotating 3 bladed props 8x4 to my twinstar. Does it make any difference which side the props are fitted?  Commonsense tells me to put the standard prop on the port side and the "contra" on the st'b'd. What is the consensus?         Don't ask me why!..........

[Marko]

Hi Grasshopper l think you mean

COUNTER ROTATING PROPELLERS  for you twinstar both turning inboard as to push airflow over the rudder !        

cheers ........................mark

[Glasshopper]

contra rotating  props.................counter rotating propellors.........? Whatever next?

[Glasshopper]

Thanks Mark .You are right. I meant counter rotating propellers and I didn't know the difference. I know now.

 

This is an extract from wikipedia which seems to answer my question and confirms my thoughts.

 

" Counter-rotating propellers, are found on twin-, and multi-engine, propeller-driven aircraft and have propellers that spin in opposite directions.

Generally, the propellers on both engines of most conventional twin-engined aircraft spin clockwise (as viewed from the the rear of the aircraft). Counter-rotating propellers generally spin clockwise on the left engine, and counter-clockwise on the right. The advantage of counter-rotating propellers is to balance out the effects of torque and p-factor, eliminating the problem of the critical engine."

[Marko]

Hi grasshopper it would good with conta rotating props never seen it on a leccy though ! Ha ha .............mark

[001]

Something like this! Should make a twinstar growl.

[Bert]

Contra-Rotating Brushless Setups 

[Simon Chaddock]

Grasshopper

From a torque point of view it obviously does not make any difference which way you have the counter rotating props but it does make a difference to the aerodynamics.

The P38 Lightning and DH Hornet initially both had the props rising outward but this was reversed in production as it was found to improve the stability as a gun platform. See here.

So you pays your money and you takes your choice!

Edited By Simon Chaddock on 21/10/2009 10:15:28

[Chris Dickens]

I've recently built the Eflite Dueces wild and that has the blade nearest the fuselage going down.

So as Simon says 'you pays your money and takes your choice!'

I think that the best way will be different for each airframe layout so experiment to see which is best, it's easy to change the rotation of a leccy motor.

 

[001]

Seems that Photobucket have removed my (borrowed) picture from this forum and my album.

 

I would have thought that any  good publicity of this type would have been beneficial for the commercial concern involved.

[David perry 1]

The reason that full size aircraft have their props going the way they do (top blades towards the fuz) is because the net effect of the propeller is to throw the thrust line towards the fuselage (opposite rotation would have the opposite effect).  This is important because if an engine fails then thrust line position is what makes it easy or hard to control the sudden yaw.  Imagine an engine set out on the wing somewhere, and the yaw forces involved. Now imagine that same engine sat on the centre line (as in a "single"), there are few yaw forces to contend with.

 

 

The big powerful WW2 twins were a real problem because of the massive torque involved - uncontrollable in fact in many, many cases.  Thus pairing the engines to put the thrust lines closer to the centreline.

 

In an electric model where the single-engine case is almost unimaginable, there is no point to counter-rotating props at all.  In almost aall cases for twin engined aeroplanes, models and full sized, the best option upon engine failure is to close the other throttle and glide to a gentle crash!  This is not the case of course in "Performance A" machines which can continue on one engine...provided the pilot is trained well enough!

 

My day job is as an Airbus pilot and we still consider very carefully the option of adding thrust on the good engine when we lose one as the yaw forces are huge.

 

HTH

 

david

[Tony T]

Looks like you guys might be overanalysing the situation

I have the original sticky-backed plastic Twinstar still sitting in its original box...

 

Now assuming that the current Twinstar is pretty much the same aside from a couple of tweaks and better covering,  the Twinstar was specificlly designed as a twin trainer and as such has no adverse handling characteristics with an engine out.

 

Changing the direction of rotation of one of the propellors will affect the airflow over the airframe as mentioned above and may adversely alter the handling......

 

Of course I get the feeling you may not be talking about the Hobbico Twinstar after all...