E props

[Fly Boy 3]

Hi all do watt meters retain thier last readings after power system (Motor etc) is switched off. Thanks

[Erfolg]

As Simon has written, IC props are heavier.

 

The consequence is that more power is used in turning the prop, than a lighter prop. Which does mean there is a reduction in revs for the same watts drawn. As W=2piTN re-arranged N=W/2piT, a fancy way of saying less thrust. Also The higher camber on most if not all ic props seems to also adversely affect the thrust, possibly a drag thing. 

 

Does it matter, from experience and much to my surprise it does. IC props do not provide the same performance to an extent that is noticeable.

 

Like many returned modellers, I have quite a collection of IC props. I do have concerns about them. My own seem to have dried out (could be a storage issue), plus polymers are known to deteriorate with age, again is it so with our propellers. We know that UV damages polymers and fabrics, is it an issue with our props, they are generally not outside. The other thing with age is that the plasticisers often leach out. I have just had an issue with some plastic hinges, which were buried in an old model, the old spares were brittle, again much to my surprise and annoyance (Kavan, about 30 years old).

 

At the end of the day, I use electric props as they seem to work better than the equivalent new ic props. Plus all the ifs and buts are avoided.

[GrumpyGnome]

37 minutes ago, Fly Boy 3 said:

Hi all do watt meters retain thier last readings after power system (Motor etc) is switched off. Thanks

 

Mine reads the current/voltage in real time so can't 'store' anything......

[Martin Harris - Moderator]

Isn’t the trade name Whattmeter rather than wattmeter?

 

Strictly speaking, they don’t measure watts but calculate them from the product of measured voltage and current. 

[Philip Lewis 3]

9 hours ago, Erfolg said:

As Simon has written, IC props are heavier.

 

The consequence is that more power is used in turning the prop, than a lighter prop. 

 

 

 

That's not right, heavier prop accelerates slower but one up to speed will turn at a speed determined by the power input and resistance to it which will be aerodynamic drag.

[GaryW]

All I do with electric models is look at the motor specs for the recommended prop sizes and just bolt on what ever prop i have in my flight box that's the right size whether it be an IC prop or Electric Prop... Turns out more time than not these days to be IC Props I have more of than Electric Props as a year ago coming back in to the hobyy all my models were Electric where as now a year on only 2 are Electric and the rest are IC as I seem to have ended up going back to prodominantly flying IC Models... Just cant seem to shake the Glow Fuel Smell in my life...lol

 

Edited March 15 by GaryW

[SIMON CRAGG]

17 hours ago, Erfolg said:

As Simon has written, IC props are heavier.

 

The consequence is that more power is used in turning the prop, than a lighter prop. Which does mean there is a reduction in revs for the same watts drawn. As W=2piTN re-arranged N=W/2piT, a fancy way of saying less thrust. Also The higher camber on most if not all ic props seems to also adversely affect the thrust, possibly a drag thing. 

 

Does it matter, from experience and much to my surprise it does. IC props do not provide the same performance to an extent that is noticeable.

 

Like many returned modellers, I have quite a collection of IC props. I do have concerns about them. My own seem to have dried out (could be a storage issue), plus polymers are known to deteriorate with age, again is it so with our propellers. We know that UV damages polymers and fabrics, is it an issue with our props, they are generally not outside. The other thing with age is that the plasticisers often leach out. I have just had an issue with some plastic hinges, which were buried in an old model, the old spares were brittle, again much to my surprise and annoyance (Kavan, about 30 years old).

 

At the end of the day, I use electric props as they seem to work better than the equivalent new ic props. Plus all the ifs and buts are avoided.

Thats pretty much what I have experienced, thanks for a more technical explanation. I also used a digital set of fishing scales to measure thrust. One end attached to the model.......other to a table leg. Lots of interesting figures. Plus readings gleaned from my wot / what / watt / wat meter!.

[Erfolg]

Phillip

 

You are taught in both "A" level applied maths and General Engineering studies (at all levels) that a rotating body is subjected to acceleration, in essence an inertial force. The heavier the body the greater acceleration is required to keep it all going. I just thought that we all experience the force when going around a corner, particularly noted when in F1 talk of "G" force.

[Martin Harris - Moderator]

A heavier prop will be of benefit in manoeuvres involving torque and gyroscopic effects such as the Lomcevak. 

[PatMc]

Of course an ic prop could be a wooden one (as on Leccyflyer's Magnatilla), which would likely be lighter than an E prop. 

[leccyflyer]

Maybe I should change the wooden prop on the Magnatilla for a heavier one, to help with Lomcevaks? 😉

[Jolly Roger]

5 hours ago, Erfolg said:

Phillip

 

You are taught in both "A" level applied maths and General Engineering studies (at all levels) that a rotating body is subjected to acceleration, in essence an inertial force. The heavier the body the greater acceleration is required to keep it all going. I just thought that we all experience the force when going around a corner, particularly noted when in F1 talk of "G" force.

Sorry Erfolg but I think you’re confusing radial forces and tangential forces, which is an easy thing to do. 🙂
You are absolutely correct that a  body in circular motion requires a centripetal force. “Centripetal” means acting radially -  towards the centre of the circle.  
But the forces needed to accelerate a prop act in a different direction - they are tangential to the circle.  It is these tangential forces that are supplied by the torque of the motor, and once it is up to a constant speed, they only need to overcome the drag of the prop.  
So as Philip says, the mass of the prop is irrelevant to the motor except for when the prop is speeding up or slowing down.

i hope this helps. Circular motion is one of the harder topics to grasp in A-level physics.

ok, now I’m going to Google what a “Lomcevak” is… 😊

Edited March 15 by Jolly Roger

[Simon Chaddock]

Just out of curiosity has anyone been able to quantify the change in rotational speed due to the IC power and non power stroke?

It must have a detrimental effect on prop efficiency. For a 4 stroke the effect is even more pronounced. 

 

I believe it is this effect that supports the general consensus that an electric motor of equivalent power to IC delivers more "performance".  The better efficiency of an e  prop certainly helps.

There are a lot of variables at play so it may not be possible to measure any actual performance difference.

[Peter Jenkins]

On 14/03/2024 at 14:12, Fly Boy 3 said:

Hi all do watt meters retain thier last readings after power system (Motor etc) is switched off. Thanks

I have given up using Wattmeters and use on-board telemetry.  The system I use is called a Unisens-E and is made by SM Modelbau (link).    This unit can be set to speak Futaba, JR, Spektrum, Powerbox and a lot more.  You get both real time indication of current, voltage, power, as well as a max and min in the flight provided you don't turn off the Rx before checking the max/min figures.  You also get a height read out and a vario if you are into thermalling.

 

I found on a static ground run, full throttle gave a reading of 3,600 watts and 99 A!  I shut that down PDQ.  In the air, the full throttle reading was usually around 2,70 W and 80A.  Only once, did I see a max of 2850 W.  So, if you set up your system on the ground and, quite naturally, don't want to exceed the maximum power/current for your motor/ESC, you will always be underusing your motor's power.  In the case above, the difference between static and dynamic tests is 28.6%.  Now, I only ever check for max power in the air. 

 

I recently ran a test on a contra drive using a 10S pack and this was the outcome of the values I was prepared to inflict on the drive.  The motor has a max power of 2,800 watts and 80 A.  These figures were with the throttle stick at around 65% open.  I fully expect that the full power in flight figures will not be too far off these ones.

 

The screen shows the max values during that particular operation.  So, the Watts show 2,709.8 for an rpm of 3,789 and Amps of 71.9 A.  Ignore the voltage figure as that is showing the maximum voltage that the system saw and that would be at the initial plugging in of the flight pack.  The min values would have the voltage that corresponded to these power figures.  The props, by the way, are a pair of 22 x 20 Falcons.

 

The Unisens-E is a very small device and you don't need any other sensors other than what comes inside the less than 20 g unit that costs Euro 65.90 + shipping.  No doubt it can also fall foul of customs but it shouldn't at that price.  It is easy to swap from model to model provided you have leads to the Rx that you can plug into as well as a lead from one of the motor 3 phase wires to give you the rpm signal.  I just use servo plugs to make swapping the Unisens-E between models very easy.

 

SM Modelbau also sell a device called the Unilog 2 that has a micro SD card that records every flight and can be read off line but that costs Euro 99 plus you need to buy the SD card. 

[Tom Gaskin 1]

Some fascinating discussions here and also memories of school trauma with the physics laws!

 

I do think that we are getting bogged-down in the science though. The quick answer to the OP is 'yes you can'. In the days of brushed motors (last century) we did need all of the help we could get in order to get the blessed models to fly for any length of time! Nowadays we have so much more power and efficiency to call upon that the type of prop on your electric motor is not terribly important.

 

Tom

[Erfolg]

I tend to agree, that for most, the physics are not even of passing interest.

 

For those with a engineering background, it helps in understanding of what is happening and probably more importantly as to why. Again is mostly of interest when or if trying to optimise something. Not all aspects of physics or material science translates to the reality of the real world as expected.

 

Many will arrive at similar conclusions from/via intuition. Most of us just copy others with what works.

 

For most of us modellers it does not matter, if just plodding about the sky. For those Pylon racing, optimisation matters. Yet it is real world measurements that matter, not the predictions.