What is special about 'electric props'?

[Nick Farrow]

Hi,

 

I was wondering what is so special about the props for electric flight.

 

I appreciate why this might be for slow fly, but not for sports type flying. So you can get an APC 9x5 on a typical foamy, that could either be and e' type or not (e.g APC 9x5e). As far as I can see these take about the same revs as non 'e' types, so is there another critical difference?

 

Just curious!

 

Thanks

 

nick

[Ultymate]

I think Nick that the "E" props are more lightly constructed than the IC versions, being that they are not subjected to the same stresses and vibrations. Therefore I would say at a pinch it's ok to fit an IC prop to an electric sports model but never the other way round. This is my understanding 

Edited By Ultymate on 11/05/2010 12:29:33

[Plummet]

Just guessing, but...

 

Vibration?  Flywheel effect?

 

An electric motor gives thrust through all 360 degrees.  An IC one only for perhaps 60 degrees on 360 or 720 for four strokes.  Also the flywheel effect of the prop is actually driving the engine during the compression stroke.  An IC prop has a lot more force on it than a leccy one.

 

Also, the aerofoil section of the two types is different, so the drag/thrust characteristics will differ.  So a 9x5 leccy prop and a 9x5 IC prop may draw different currents when both used on the same lecci setup.

 

Plummet

[Bruce Richards]

E props are more lightly constructed. This is possible because they do net need to contend with the shock loadings generated by an I/C engine with one power stroke per revolution. Electric motors deliver power in a smoother fashion.

 

Bit slow of the mark there

Edited By Bruce Richards - Moderator on 11/05/2010 12:31:41

[Tim Mackey]

Yup...thats about it.

[Plummet]

Isn't there something we could disagree about. It would be more fun!

 

P

[Biggles' Elder Brother - Moderator]

iI don't do electric flight so I might be dead wrong here -  but when I look at props on electric powered models they seem to me to have a pitch profile change very close to the hub - like a sudden increase in pitch in this area. You don't see this on ic props.

 

It could be that actually, aerodynamically, that is the absolutely most efficient way of forming a prop - ie that you need the apparent pitch increase near the hub because it is the slowest moving part of the blade - and maybe ic just doesn't do it because it would weaken the blade close to the hub. Don't know - has anyone else noticed this or got any views on it?

 

BEB

Edited By Biggles' Elder Brother on 11/05/2010 15:03:16

[Tim Mackey]

Posted by Plummet on 11/05/2010 15:01:00:

Isn't there something we could disagree about. It would be more fun!

 

P

 

Dont tempt fate - we are busy on admin work and cant afford to be throwing cold water on you lot!

[Tim Mackey]

Posted by Biggles' Elder Brother on 11/05/2010 15:02:19:

iI don't do electric flight so I might be dead wrong here -  but when I look at props on electric powered models they seem to me to have a pitch profile change very close to the hub - like a sudden increase in pitch in this area. You don't see this on ic props.

 

It could be that actually, aerodynamically, that is the absolutely most efficient way of forming a prop - ie that you need the apparent pitch increase near the hub because it is the slowest moving part of the blade - and maybe ic just doesn't do it because it would weaken the blade close to the hub. Don't know - has anyone else noticed this or got any views on it?

 

BEB

Edited By Biggles' Elder Brother on 11/05/2010 15:03:16

 

You could be right there BEB

- but just to keep Plummet happy.....dont talk such rubbish man

[Plummet]

Rubbish?  Rubbish?

 

You think that was rubbish?

 

You should see some of the other stuff I have written!

 

Stomp stomp stomp.

 

P

[Roberto Gava]

As far as I know, IC shafts are subjected to strong torsional vibrations that should be supported by the prop. Also the prop works as a flywheel to give smooth idle.

In electric motors there isn't those requirements so props can be lighter to give more acceleration and its shape could be optimized aerodynamically because there is less structural requirements.

[Nick Farrow]

Hi Guys,

 

Thanks very much some good stuff here!

 

nick

[Erfolg]

Yep

 

I am sure the consensus on the vibration/cyclic nature is correct.

 

I do not know where I saw it, on U tube, but some one, has somehow used a strobe? to show how a IC prop pulses slightly backwards and forewords on each rev, showing clearly two tracks when moving. Yet when turned by hand, both tips tracked the same path. I did think, it could be that each blade had a differing stiffness level. I just accepted it would happen, but would it be sufficiently great to practically measure.  This really surprised me, I believed the variation in angular velocity took place, again thinking the practicality of measuring the change was not practically achievable without considerable effort.

 

Measuring vibration on the 3 axis should be do able.  But who would cares?

 

One thing I have noted though, there seems to be no one design concept or material for electric props. Ranging from, slow fly, then JKX carbon to folding propeller systems.

 

Erfolg