Pushy props

[Richard Purser]

Its me yet again! (a new can of worms) this time i want to know if you can use a standard prop (reversed) so instead of pulling it pushes, the model or would it have to be a special prop to do the job efficiently..?      thanks guys...... oh is there a montly prize for the most threads started in a week!??

[birdy]

In a word, no! It would work, but horribly innefficiently. Also I thought the phrase was "a different kettle of fish", or perhaps being in an age where chav dialect has taken over, it is simply an new phrase to me?

[Erfolg]

If you are flying electric, I understood in dioesnot matter. You just mount the prop with the leading edge foreword and reverse the running of the motor.

I also understood you can do the same thing with a rear induction IC motor. You just move the backplate by 90 dergees, which reverses the the running of the motor.  

Then again I could be wrong.

Erfolg

[Simon Chaddock]

It will not work!

A propeller is an "air-screw" and conventionally the blades are twisted to make a right hand "thread" through the air to match the rotation of the engine. Putting the prop on backwards just means the blades will be "flying" upside down and backwards (ugh!) but it stills goes round the same way! A normal prop can be a pusher if the engine rotation is reversed (some two stokes can do this & electric motors) or you need a left hand prop with a conventional right hand engine.

[Erfolg]

With electric, you can set the blade in exactly the same orientation as used as a tractor. You will now have to reverse the direction of the motor from the ususal if it were being used as a tractor. Many people will not realise that it is rotating in the opposite direction unless they have previously used the motor in a tractor configuration.

IC people would notice as they will be required to change the direction of rotation., if possible. Althogh possible with bolt on front inlets or rear  removable inlet timing shaft or rotor arrangements

Just ask a Zaggi type modeller what type of prop they use and how they mount it.

Erfolg

[Richard Purser]

Thanks all for the feedback, ive got something to work with now...cheers richard

[ERIC CLAPHAM]

Richard, Hi there.  Simon had the correct info  about  fitting  a normal  tractor  prop  backwards.It does  not work. In days of yore this was atrick to reduce the thrust on a model with no throttle for its first test hops. Similarly with an electric motor you can simply reverse the direction of rotation-easy on a canned motor but I dont know about the runners. Cox glow ic motors-049/051s-were symetrically timed and ran well if started in either direction. In fact did if you overprimed and got a bounce back. Very embarassing if you launched without noticing the prop blast was not in your face. Older larger engines could be reversed if the front /rear  inductioncasing assys were bolted to the c/case assy by undoing the screws and rotating those 90deg. I well remember doingthis on an E.D Racer years ago to easier fit in the cowl and tried in vain to get it running as a tractor.......doh. Nobody has mentioned that with electric reversal you must also reverse the prop on the shaft i.e turn the prop 180deg. It will work the other way but is less efficient. Rgds., E>C>

[Tim Mackey]

With electric motors, brushed or brushless, inrunners or outrunners, the prop basically needs to simply be facing "forward" IE the letters / numbers ( on almost all props ) should be at the "front" - this applies whether the model is pusher or tractor. The motor direction is then simply setup to provide the normal anti-clockwise direction of the prop, as seen when looking at the front face of the prop. Swappping the direction of a brushed motor simply requires the 2 wires swapping over, and on a brushless, just swap any 2 of the 3 wires over with themselves.

Eric's tale of fitting props on "backwards" will, I am sure, recall several memories for several people of models taking to the sky in a somewhat limp and feeble flight, followed by much cussing and complaining from the pilot, only to find after all the collective gurus have had their say....

....that he had accidentally fitted the prop on backwards

[Former Member]

[This posting has been removed]

[Richard Purser]

It looks like ive got a lot of experimentation to do! (did i spell that right?) thanks again all..richard

[Ed Kershaw]

A lot of conflicting information above, some correct, some incorrect.

Just to be clear, if you fit a standard tractor prop to the motor with the front face against the motor backplate and reverse the rotation of the motor (c/w rotation when viewed from the rear of the model), you will get a fully effective pusher prop.

Timbo is certainly taking the responsible line (as indeed our moderator should!) by recommending that props only be fitted in the orientation intended by the manufacturer.  If you do fit one in reverse, you risk running the prop out of true because the front faces are often not flat, i.e. not designed to be run against a back plate.  You therefore risk injury both to yourself and spectators if the prop breaks up.  So do take care with your experiments!

Having said that, I've found woods work fine in reverse, as do Master Airscrew electrics.  APC-Es do not, which is a shame because they are very efficient props.  Although APC do a reasonable selection of dedicated electric pushers anyway - only up to about 9 or 10 inches though (I think).

[Erfolg]

I see no reason to use an electric prop in the reverse direction,( even if a 480 type is in use) other than a twin where the modeller wants one to run the motor in a the opposite rotation ( reverse direction), yet provide thrust.

As for IC move the induction system through 90 degrees (if possible) or use a pusher prop (which have never been plentiful).

Erfolg

[Ed Kershaw]

Erfolg,

In your example of using one reversed prop to achieve a contra-rotating pair for twins, that will of course require one conventionally fitted tractor prop running in conventional rotation and one reversed pusher prop running in reverse rotation.

The other reason someone might want to use an electric prop in the reverse direction is if they want to turn a tractor prop into a pusher.

Also, I don't understand your reference to the 480.

Ed

[birdy]

nor do I...

[Richard Purser]

Ive got the ball rolling this time!!...all good stuff!

[Erfolg]

It would seem that I am not being at all clear in what I have said.

The problem with some brushed motors is that of timing, most are timed to run with a tractor prop. The reference to a 480 was about it running slightly less efficiently if running in reveres (if using a tractor prop, but running in a pusher mode). The problem with brushed motors is the lack of power relative to weight, so inefficiencies are to be avoided.

I have always tried to use a tractor prop, when the propulsive unit is mounted in a pusher mode. There are far fewer IC motors built these days where the timing can be altered, so as to run in reverse. Mainly due to the optimisation of the manufacturing process and reduction in component count. I have been told that many 4 strokes can be reversed, to run as as a pusher, with conventional prop. But i do not know that this is really true.

I feel quite chastened in my lack of precision and clarity, in my responses. Take a hundred lines that man, "must be clearer".

Erfolg

[Ed Kershaw]

GOOD POINT!  And one that I hadn't appreciated but should be noted.

I'm a brushless enthusiast.  Brushed motors seem complicated and full of - err - brushes and things.

Do the 100 lines anyway and let us know how long it took!

Ed

[birdy]

Maby I (and possibly chris clapman, judgeing by description thing) should write out any spellings we get wrong 100 times!

[Erfolg]

Re Reading some of the correspondence, I think some mis-understand, what I am trying to say.

My view is the same as Timbos (I think). To use a conventional propeller in a pusher configuration, Simply mount it as usual, as Timo states facing forward, ala Ziggy type models. Then reverse the direction of the motor.

In the case of the twin, using one pusher. Still mount both facing forward, but reverse the direction of rotation of the pusher motor. Both motors will now move the air backwards.

I hope that my writings have not been the cause of the controversy and debate.

Erfolg 

[Former Member]

[This posting has been removed]

[birdy]

Well... The phrase is true - you leaarn a new thing every day... Now what did I learn yesterday...

[Myron Beaumont]

Methinks without the forum we / electric converts  (nearly in my case) would still be struggling with nicads not just spelling

Grumpy

[birdy]

I've never even tried ic! The visions of my mum if she find out I was hiding highly flammable fuel (and no, I'me not this stupid!) in my bedroom, with the carpet ruined from engine gunk...

[Erfolg]

Birdy

You must be very young. All us old timers, used to buy and store Ether in the bedroom. As to running the deisel in the house, yes if you could get them started. As to the carpet, could only afford lino in those days.

Health and Safety was unthought of, and mothers were very tolerant.

Erfolg

[Alistair Taylor]

Ahem

to further murkify the muddy waters...

APC do electric props (as in props for electric motors) in both tractor and pusher versions.

 You would have thought that this was illogical given that electric motors (both brushed and unkempt) can be reversed (barring twin installations where you might want props that rotated in opposite directions), HOWEVER...APC props all come with a set of spacers that ensure a snug fit on whichever shaft they are fitted on. These spacers fit into a precision machined recess on the REVERSE of the tractor props, but on the FRONT of the pusher props. NB only the recess for the spacer is precision machined, the rest of the hole through the prop is done on a friday at 4pm.

Thought you might like to knote.

AlistairT