Should we still use electric props only?

[Erfolg]

In the past I have used specifically electric props.

As time went on, I came to realise that using cheap blades on folders was not on. This was a consequence of two models at least shedding blades. I then only fitted Graupner or Aeronauht Cad types.

I have also noted that a number of other modellers in our club, have had damaged models from apparently shed blades, from conventional SF and electric props.

I think this is a consequence of the power being put through electric models is now approaching that of IC models, albeit without the pulsing/vibration.

I have recently been using these JM and JXF propellers.

I have been thinking about the propellers today, mainly because they are on back order, and the delay this will incur to my building programme, when it suddenly dawned on me, they are probably intended for IC models. Yet in the discussion associated with these propellers I see others are using them to on electric models.

The blades on order are 9 * 4 with 590 watts showing on the amp meter, I am not sure i would trust APC type props with this level of power.

What are others doing?

Edited By Erfolg on 20/05/2013 16:26:28

[Chris Bott - Moderator]

From the APC website here Erfolg, there's a simple formula for the max RPM of their props.

For the thin electric props, the formula is 145000 divided by diameter in inches.
So a 9x4 is safe up to 16000 RPM.

If you can measure it, then you know where you are.

590 watts does sound high for a 9x4. At these sort of powers I'm tending to use a minimum of a 12" prop. One consequence being, that rpm is much lower for the same power.

[Stephen Jones]

 

The difference with ic props as to electric props

 

we all know that we can match an ic engine with an equivalent electric motor by watts .

 

but most ic engines run at a lower rpm than the electric motors do , until you start to get bigger then electric motors become much slower .

But ic props are a lot thicker and more robust than the electric ones , due to vibration from the ic engines ? .

 

So maybe this is why the question has been asked ?

 

the faster the prop is spining the stronger the prop needs to be ? .

The more vibration from the engine the stronger the prop needs to be ? .

Edited By Stephen Jones on 20/05/2013 18:41:35

[Dylan Reynolds LaserCraft Services]

Ok I have run into this problem this very weekend. I ordered an APC 17x8E prop and the order came as an i/c version instead. So rather than wait I decided to try it.

To put it mildly the motor struggled to turn the prop from standing, but once running (after a flick in the right direction by hand) it ran perfectly ok upto about 2/3rds throttle, after that the esc and motor became very warm to the touch. The wattmeter showed the amps drawn as you would expect to see from a leccy prop untill, you guessed it, 2.3rd throttle then they jumped up rather rapidly.

Ok, this is a big prop and the weight difference between the leccy and the i/c verisions is probably far greater at that size as opposed to the sizes you are using, but I would still guess there is going to be more strain on the motor/esc setup whatever the size.

We did fly it, but max half throttle at all times, and everything did seem fine, but thats not what a 3d model is about, so I will certainly be getting leccy ones for this model so I can exploit her to her full potential.

[PatMc]

Stephen, in my experience electric motors run at lower rpm than ic engines across the whole power range, except perhaps for a few vintage diesel engines.

Elfolg, I've been using APC type copies for a while now with no problems. In fact they seem to have more consistent quality than the genuine APC - to wit I've come across some APC props with the hole off centre in the chord-wise direction.
Also I've used some of the folder blades from GS that are Graupner copies without any issues. They have been well matched (i.e. they balance) & equal in performance to the genuine Graupner ones.

OTOH, I've found that some folders supplied with ARTF models have very poor performance & had a couple of blade failures at moderately high rpm.

[Erfolg]

This is almost the motor i am running excepting the Kv which is 1250kv.

I can understand that the starting torque being higher to accelerate the prop. I do not see why it would be higher though at top speed. I would then anticipate a constant torque value, which is lower than the accelerating torque to increase the revs.

I would therefore expect the current drawn during accelerating periods to be higher with a heavier propeller than a light one.

The current drawn at full revs was as per the data sheet.

I have been looking at the various weights of propellers @ 9 * 6

• The APC is 20g
• Master electric 19g
• Master electric/gas 28g
• JXR 23g

and I have just weighed a old Tornado 9 * 4 @ 20g

I have just checked these weights, the results have really, really surprised me.

I am a lot easier about the weight thing now, as from a weight aspect I am fine with the JXRs I have just ordered or even the old Tornado.

Is it something else though that differentiates electric from IC propeller blades

Having a tach and endeavouring to record all aspects of my set ups, I am both ashamed and embarrassed that I have not recorded the revs in this instance. Although, I will admit, I am not happy messing at this level of power.

Again another confession, I have recently fitted a APC to a model after reassurance from BEB that it would be fine. Yet looking around the models I am flying, it is not obvious, which model it is. Although I am sceptical about APC above about 250w.

I hope you recognise I have no firm opinion at present, as i have a poor understanding of what are the facts at the moment, but bags of prejudice based on look and feel.

[Chris Bott - Moderator]

Have a read of this Erfolg APC Safety Considerations

[Allan Bennett]

I'm a puzzled by your reluctance to use APC electric props on setups putting out more than 250 watts: I have over a dozen electric models -- most of them conversions from glow -- and they all use APC E props. Power per motor ranges from 110 to 2500, with 3S A123 to 12S LiPo for power.

I'm aware of APC's limits on rpm, and choose my motors with low kv so that they're well within the limits. I stick with APC E because I believe they're designed to suit the characteristics of electric motors, and I've had no prop issues whatsoever in almost 10 years of flying electric.

[Erfolg]

Allan

It is the sharing of experiences that both reassure and warn modellers, including me.

I have seen and had a few blade failures and also seen some crashes which could have been a blade failure. The failure always seems to result in the motor tearing itself free from the model. In one case, where I suspect a blade failure on a Ripmax 109, the motor shaft was sheared.

I did note that Patmac thinks that the revs are much lower than with IC motors. I am not sure this is correct I could produce a long list of my models (or motors) and revs. Rather than do that, I will just say I currently fly about 10 models most rev about 10,000-12,000 with one at 23.700, and a smaller number at about 5,000-6,00 revs, these are all parkflyer types. I must admit I have no recorded data on Parkzone Albatross or Reliant, I have just trusted Parkzone

In many respects I am trying to get my head around what are the issues, and am I being sensible. The weight thing has taken me aback, I really thought that weight was the issue, or the difference. yet the variation seems to indicate it is not, other than the low powered type, which i have a box full, which came with various kits.

[Biggles' Elder Brother - Moderator]

I have a 78" span Cub putting out about 900W - that's on an APC-E 14x7 (if I remember correctly). Its had that prop from new and I never had a problem.

On the other hand I have 55" span Sbach 352 putting out about 1.1kW on which I'm using a Xoar wooden IC prop.

Both seem to perform well. Which is better? Well I'd say the Xoar (but so should be at the price!) mainly I suspect because its very stiff. I tend to think that wooden IC props out-peform the rather more flimsy (in stiffness, not strength) APC-E's. BUt I not found the APC-E's to be suspecable to failure or unsatisfactory - I just think the wooden Xoar is better.

I've not come across the effect Dylan describes. All my IC props on electric spin up fine and power consumption increases smoothly and progressively as you would expect. The IC props do tend to keep on spinning longer on throttle back, I guess as they have rather more inertia.

BEB

[Erfolg]

Some may wonder how this question has arisen in my mind.

As with many views, it is the consequence of numerous events and opinions.

On Sunday, on landing a model I broke the propeller. In itself just another event of no great importance. I had one spare left.

So back at home, I immediately started the ordering process, only to find, what I wanted are on back order, I then looked at an alternative to find the same situation. Today i was thinking, are these propellers suitable for electric models, realising that I had used previously Master electric propellers.yet I have been using these JXR propellers. But I had fitted a non electric propeller, and the motor did not seem to be adversely affected at all.

Yet my view starts some 7-8 years back, when I restarted modelling. Being attracted back, with a vision of a small electric model, having no interest in IC and pure gliders to boring. At that time there was really only 480 and 540 motors, both very limited in power.

With this limited power came rather puny propellers, just for electrics.

Almost immediately, brushless motors were appearing and Lipos. There was a step change in performance, although the power was still relatively low, by todays standard, but still an increase. Propellers did not change and it was now i suffered my first blade failure.

Today the power economically available is very much greater.

Another major influence, was that when I graduated, I was taught about material technology, not metallurgy as when taking HNC. One of my first jobs was in the plastics moulding industry and then BICC cable making, all using polymers.

One of the major aspects that was not lost on a engineer who had been taught about metals, was the rather limited range of viability of plastics. At a practical level I saw and heard etc that Polypropylene was both cheap and its properties were also affected greatly not just by temp, but moisture, That a moulded item, was thrown into a barrel of water, as the as moulded item was brittle, yet left in the barrel it became pliable. I saw how nylon was prone to absorbing water in its sacks if left open prior to moulding/extruding affecting product quality. Perhaps another aspect, which is not seen as important with metals, is that of loading rate. The same polymer can fail by ductile failure or a brittle failure all dependant on the rate of loading and temperature etc. Then there is Environmental Stress Cracking, the issues can go on and on.

I was aware that very little pure polymer was used, Typically the polymer having plasticers, colouring and extenders, flame retardants all added by the Alchemist, otherwise known as the Chemist.

At that time I was aware of the mechanisms of composite ratios and how stresses are absorbed as cracks are arrested by the fibres as the crack energy is dissipated around the fibres as per Grithis crack propagation theories.

So when I look at a propeller and see a soft waxy material, I am asking myself, is it rubbish or the cats whiskers?

I had assumed that the IC propeller was heavier as Dylan also suggested. It was only on his prompting, that I decided to investigate how much heavier are the IC the propellers I have ordered than the electric types. I then found, they are slap bang in the middle of the electric propeller ranges. So to me it is not certain that there is this relationship, as some electric props are heavier.

You then think, are there other differences? Then the next obvious question do the differences matter and how.

In many respects I am reassured in that the propellers on order should be fine. Yet it would seem that some of the types I have been wary of also seem to be fine. Not withstanding, the increase in both power and speeds of eclectic set ups.

I think I am a little further on, yet many aspects are not understood by me.

[PatMc]

Posted by Erfolg on 20/05/2013 20:32:13:

I did note that Patmac thinks that the revs are much lower than with IC motors. I am not sure this is correct I could produce a long list of my models (or motors) and revs. Rather than do that, I will just say I currently fly about 10 models most rev about 10,000-12,000 with one at 23.700, and a smaller number at about 5,000-6,00 revs, these are all parkflyer types. I must admit I have no recorded data on Parkzone Albatross or Reliant, I have just trusted Parkzone

I din't say "much lower" I said "lower".

I have a record of model data kept in a spreadsheet - this includes the rpm of 18 motors at WOT in various models. 12 motors run between 5900 - 8700rpm, 5 run between 9600 - 10800rpm & the only inrunner (1600Kv) at 13500rpm. Only 2 motors used 4s the rest used 3s lipos. They all used bigger props running at revs lower than ic would have done in the same models except the inrunner. It was at about the same rpm as an ic engine would have been in the same model.

[Erfolg]

Patmac

I think you suspect I am being critical, that is not the intention.

Like many discussions, we say things, which have is a view, or maybe a suggestion, of what the differences might be. I thought this was your tack. This prompt me to try and get a comparative handle on these suggestion.

In my case i find that about 10,00-12,000 revs is a typical range.

As you suggest that older IC engines would often be quoted as reving at about 12,000-15,000 revs. Yet it was just recently I read, that the figures often quoted by Peter Chin and the other famous engine tester were almost in every case higher than would be typically used by modellers, @ about the same as my electrics.

I have an impression that todays IC engines are much more powerful, which I suspect is a consequence of higher usable revs, which does give more power and perhaps more torque.

Yet not withstanding these changes, i am guessing that from a rev point of view, most current electrics do the same revs as old IC motors. The big difference being the general lack of impulses from the firing process.

Plus I do balance propellers now, which i did not bother with IC, as it seemed a waste of time, with all that vibration from the firing cycle.

It has surprised me that IC props and electric props seem to be in the same weight range. Yet is it a single instance for 9 * 4, or is it replicated across all sizes?

I think you can guess, I am not clear what is different with electric props.

However from my own records, I can see that I am using ever more powerful electric motors and now moving from 3s towards 4s. From casual observation, this also appears to be a trend within our club. Even gliders, are now expected to streak skywards, no longer is a gentle climb out being seen as normal, more of an issue. Sports planes are harder to distinguish in performance from their IC counterparts, as modellers up the watts used.

So may be the differences in duty between an electric prop and IC prop are out of necessity converging?

Edited By Erfolg on 21/05/2013 05:12:46

[Allan Bennett]

As I said earlier, I believe (though I can't remember where I read it) that E props are shaped differently (note the narrow tips on the APC ones), to better suit the torque and/or rpm characteristics of electric motors.

But in my early days of electric I did use "normal" glow props on my first model. The main reason I switched to E props is their lighter weight. The reason I stick with them is they work for me.

[Chris Bott - Moderator]

Erfolf I don't see your issue at all.

APC make sizes up to 27x13 in the elctric prop range. Are you proposing that we should abandon these props above some arbitrary size, and switch to using propellers designed to take the stresses of instataneous torque being applied once per revolution by an I/C engine?

Have a good surf around that APC site, there is more technical info than you can shake a stick at.

For example, quote:-

Ideally, a product can be designed with credible knowledge of the environment (loads acting on the product) and capabilities of the product to withstand that environment (not fail). There is nothing ideal about designing a model airplane propeller because some major components of propeller loads are very uncertain. The principle load components acting on a propeller are:

Centrifugal (from circular motion causing radial load)

Thrust/drag (from lift and drag acting on blade sections)

Torsional acceleration ( from engine combustion and/or pre-ignition)

Vibration (from resonant frequencies or forced excitation)

Another potential source of loading is aero elastic tip flutter. This may be caused by self exciting aerodynamic loads at a resonant frequency.

These loads are discussed next in order.

Centrifugal loads are very predictable, given rotational speed and mass density distribution of a blade. Their contribution to total stress is relatively small.

Thrust/drag loads are somewhat uncertain due to complexities of aerodynamic environments. The relative axial speed at the prop (at any radial station) is aircraft speed plus the amount the air in front of the blade is accelerated by the mechanics creating thrust. The latter may be approximated using first order classical theory. Much empirical lift/drag data (from wind tunnel tests) exists to quantify lift/drag loads, once relative velocity and angle of attack distributions are established.

Torsional acceleration loads are generally not known. Analytical estimating technique used by Landing Products to quantify torsional acceleration loads suggests that they can become dominant when pre-ignition or detonation occurs. These analytical observations are supported by test experience with very high performance engines running at elevated temperatures. The latter causes a high torsional load (about the engine shaft) which creates high bending stresses, adding to those from centrifugal force and lift/drag effects. These torsional acceleration loads depend on unique conditions for specific engines. Engines "hopped up" for racing appear to be especially prone to create high torsional loads when lean mixtures lead to high cylinder temperatures and pre-ignition/detonation.

Vibration causes additional loads from cyclic motions. These motions occur when resonant frequencies are excited or when cyclic load variations exist on the blade. The magnitude of these variations depends on how close the driving frequency is to the resonant frequency and the level of damping in the propeller material. Engine combustion frequency is an obvious excitation. Obstructions in front of or behind the blade can cause cyclic variations in thrust load. Once a blade starts to flutter, those motions alter the flow, causing variations in loading. High performance engines have caused propeller tips to break, presumably due to fatigue failure from vibration.

Aero-elastic flutter is speculated to be a dominant mechanism causing rapid fatigue failure near a tip when insufficient or destabilizing tip stiffness exists. The interaction between variable loading and deflection induces a high frequency vibration with unpredictable magnitude.

Efficient propeller design practice utilizes analytical/computational models to predict propeller performance and stresses. However, the uncertainty in impressed and inertial loading from complex phenomena requires testing to assure safe performance. Unfortunately, it is not possible to assure testing that convincingly replicates worst case conditions. The large combinations of engines, fuels, temperature, humidity, propeller selection, aircraft performance and pilot practices creates an endless variety of conditions. If the origins of severe loads were well understood, quantified, and measurable, structured testing might be feasible that focuses on worst case stack up of adverse conditions. However, since the origins of severe loads are really not well understood, it is essential to provide sufficient margins in material properties and design to assure safe performance. Propellers that are used in fairly routine and widespread applications (sport and pattern) lend themselves reasonably well to test procedures that provide reasonable confidence. In time, a sufficient data base develops that can be used to empirically quantify performance and "anchor" or "tune" assumptions used in analytical models.

[Chris Bott - Moderator]

continued...

However, propellers that are used for increasingly extreme performance applications do not benefit from the large empirical data base sport and pattern propellers enjoy. Assumptions and design practices developed for current generations of engines may not be valid for emerging engines whose technologies continue to push engine performance to greater extremes. Consequently, propellers that are used in applications where performance is already relatively high (and expanding) must be used with great caution.

An adverse cascading effect occurs when propellers are permitted to absorb moisture in high humidity environments. Composite strength, stiffness and fatigue endurance all reduce with increased moisture content. Reduction in stiffness typically causes resonant frequencies to move toward the driving frequency (increasing torsional loads) and, the reduction in strength reduces fatigue endurance. Composite propellers should be kept dry.

In summary, please abide by the safety practices recommended by propeller manufactures. This is especially important for high performance propellers. Assume that propellers can fail at any time, especially during full power adjustments on the ground. Never stand in or expose others to the plane of the propeller arc.

[Erfolg]

I am not proposing anything at all.

Chris, I have read the APC blurb, in many ways, it tells you what the issues are, not how they have been managed. It reminds me of some of the conclusions I have written in reports, before asking for more money to investigate further.

Perhaps the only thing I am challenging is what is the difference, in that it does not appear to be a weight issue.

Allan suggests that the tips of APC types are narrower than IC props. Which may be true. But if so why? At the same time a narrower tip is potentially counter intuitive, given the similar wight of the two applications. I would expect the IC blades to have a heavier root, where the forces due to acceleration/deceleration I would anticipate being the greatest. Following from this, the remaining mass would be less towards the tip.

I do have a few electric props in my propeller storage. I also have a number of IC propellers. I will do a visible inspection to try and determine if there is a difference in shape, that is visible to the naked eye.

I am certainly not trying to suggest that we should not use a particular type or brand. It it does seem that the differentiator of weight, is not one of the parameters. Following from this, the question I ask myself, what is different, in a material manner.

I will however suggest that I have preferred Fibreglass or CF blades on the basis of the mechanism of arresting cracks and stress. However all the blades I have examines recently, purporting to be CF, show no signs of fibres. This suggests that these are not a resin fibre matrix. Perhaps if I had a microscope, i could find micro fibres, but to the naked eye nothing.

Perhaps my other prejudice is towards polypropylene. Which i would avoid for an application such as a propeller where stress is involved. However I have no evidence that it is used.

The trouble with many polymers they are not immediately obvious in many cases by casual inspection. Perhaps Nylon is one that does have a look and feel that gives a good clue.

Again I emphasis I do not know the answers, and I am seeking

[Steve Hargreaves - Moderator]

Materials science is such a massive subject Erfolg.....I just don't see how "Gentlemen Amateurs" such as we aeromodellers can make any valued judgement at all unless we have indepth knowledge of the materials involved. We talk of "Nylon" & "Glass Filled Nylon" as generic terms but there are so many grades available how can we know what actual material was used? Or indeed SHOULD be used....unless you happen to be a Materials Engineer

A prop designer is faced with many conflicting issues & dilemmas....a prop must be stiff & rigid to work well as a propellor but tough enough to survive the odd knock & lets face it dubious handling by incompetents like us, the general public.

Trust the design engineers I say.....

As to APC "telling you what the issues are, not how they have been managed" wel why would you expect them to....? This is the sort of knowledge Companies keep to themselves....its often what makes them what they are.....as far as I know James Dyson hasn't shared details of his Cyclone vacumm cleaner.....Coke Cola don't publish their "recipe" for their vegetable based soft drink.....its called "Intellectual Property" & is jealously guarded.....

[Erfolg]

Steve

The question I am asking is quite simple in principle is what is the difference between an IC and electric prop.

You certainly have a point with respect to any materials, including polymers. A few years before retirement I was the project manager on a project where polymers were used extensively in a number of prime roles. It became apparent that one of our contractors was not able to demonstrate that traceability of base materials was being exercised. This was evident from failed routine testing, which came to high light a number of issues. Non of tis was evident from a visual inspection and labelling.

At the same time I am not seeking the solutions to issues, just what the issues are respecting electric versus IC blades.

Alan does note the thinner or is it narrower tips on electric props, which could fit with some of APC observations. Yet why should a narrower/shorter chord be beneficial with a electric prop?

I am really surprised that electric props seem to have the same mass as IC props, I could easily live with lighter electric props, in that the observations by APC would seem totally vindicated.

At present, I am not totally convinced that using a IC prop on an electric model is such a no no. I am really seeking what the penalty of doing so is?

We should not be afraid to ask questions, nor discouraged from doing so, nor admitting we do not know the answer (which is certainly my present position).

Please do not think i have an issue with APC propellers, they are just one of a number of propeller manufacturers as far as I am concerned.

As far as Dyson is concerned, there are many text books written on the subject of cyclone devices, in all manner of combinations, cascaded tails etc., complete with theory, methods of computation. Very much a triumph of self promotion over fact, although we have two in our house and they work extremely well

 

 

Edited By Erfolg on 21/05/2013 10:41:59

[Erfolg]

Whilst out and about i have been thinking of what could be different and just as importantly make a difference.

When I got home I got my collection of present propellers and those of yore.

The ones on the right being current props, with a couple of APC types. On the Left IC, mostly Tornado, and a Taipan,

As far as I can tell the weights between IC and electric are not very different, I feel almost confident to say in a range of variability that you might expect between manufactures, rather than being very different by type. There is more to do though to have confidence in this matter.

I have been thinking by a comment made by I think Patmac. That is that electric model motors normally rotate more slowly than IC. I feel that this was absolutely true when 480s were used, particularly with big propellers, often with gearboxes. I have considered that my motors are probably mainly at the bottom end of the IC rev range as used. I have also thought that many high Kv inrunners never reach the speed (that of the model) that the revs would indicate.

I recollect that it has been suggested that propellers seldom have the true pitch as indicated on the prop. The reason being that there is slippage (or what ever you want to call it) dependant on the revs of the motor and the anticipated speed of the model.

Could this be part of the story, or maybe the whole story? Electric props have a indicated pitch which anticipates a lower max rev, than an IC.

However I do note that APC propeller plan shape is very different than an IC prop, yet Master electric props, look very similar to their IC cousins. Slow fly even more extreme. Could this be a fashion thing, or could it help/matter?

Tip shape is something again, I note that both Graupner and Aeronaut have a up swept scimitar style on ther folders. This is not replicated by any other manufacturer as far as I can tell. In the case of my IC tip shape, most are similar to electric props, but the Taipan and a couple of small master electric props are squre tipped. I am guessing that there is some fashion, branding thing going on here.

I am now guessing, that the nearer that you get to both IC revs and model speeds, the less it matters that the prop is specifically for electric flight. If this were to be true, it suggests that electric propellers are best suited to slow flying, low rev applications.

I am not saying I am correct, it is a suggestion.

[Steve Hargreaves - Moderator]

Maybe electric motors accelerate faster than their IC counterparts & this affects the blade design.....a higher rate of change of RPM for a given AoA of the blade may cause the blade to be more prone to stalling.....especially considering the higher pitch we tend to use on EF props....

Again I don't know...I am only wondering.....

[Erfolg]

I am interested in the comment on the higher pitch.

I tend to stick with the small amount of data that I can find on the HK description and discussion. This has lead me to a pitch which is typically 6" (150mm).

Are others using higher pitches?

I certainly have heard what I think is a stalling blade on my trainer

Which uses a 3542 which you have as well from memory. This occurred at half throttle, when opening up to full throttle, a definite change in propeller note. I have tended to put it down to a very draggy airframe, which does not want to go much faster.

As the model has been pressed into service to suite a purpose, I have not tended to worry, in that acceleration on the ground is like a scolded cat (not that I have seen a scolded cat), and the climb out is impressive, and does seem to be tolerably fast.

I have just noted (when clicking on the picture) that it started out with a APC style propeller before having a JXK fitted (after breakage) without any apparent affect

Edited By Erfolg on 21/05/2013 15:27:27

[Simon Chaddock]

To the best of my knowledge and by inspection the main difference between E and IC is in the hub and the blade root section.

The IC hub is more massive to trasmit the big cyclic torque from the prop driver.

For the same reason an IC prop has a thicker, less than ideal aerodynaimc blade section at its root.

The result is an IC prop will require more power to produce any given level of thrust. How significant the difference is could only be identified by some accurate testing.

I would expect the difference to become less significant with increasing diameter as the whole blade design is more constrianed by the rotational and thrust forces than the torque variation per rev.

[Steve Hargreaves - Moderator]

Also worth remembering that the rotational speed of an IC prop will vary owing to the power pulses from the combustion process....during the first 120 or 130 degrees or so of a complete revolution the prop will accelerate due to the expanding gases above the piston.....for the next 230 degrees or so the prop will be slowing down as its energy is absorbed by frictional losses & the compression of the next charge....indeed in a 4 stroke the prop will be slowing down for around 600 degrees of rotation owing to the extra cycles involved.

With electric power the rotational torque is more constant hence the generally "lighter" construction of electric props....

[Erfolg]

Steve

I had accepted the premise of lighter construction of electric blades, until this latest hiccup, where I seem to find that both prop types (electric & IC) appear to be pretty much the same weight.

Although I have not looked at APC electric props, which I will do this evening. This probably because i have prefered CF or composite electric props, so just ignored APC, when making comparisons.