Prop for low power drive?

This is my first post here, but I'm an rc airplane guy and have to wonder why for a low power " commuter booster " drive why not just use an rc motor and prop setup?

It wouldnt be terribly difficult to make keep the rider safe from the prop and would sure be a simple system.

As an example, I tried out a cheapo geared brushed setup that made 19.75 oz thrust at only 73 watts input. I strapped it onto a bike and found it made the bike feel like it coasted REALLY well, and the whole rig only weighed around 10oz. Getting more power of course would be quite easy.

So, whats the verdict on these kinds of systems?

Dean

Efficiency.

You mean like this, but electric? Clearly it would "work", but...everyone has their preferences.



I've thought it would be fun to use one of these ( http://endless-sphere.com/forums/viewtopic.php?f=1&t=22874 ) to electrify one of these:

It's been done by a few folk. A quick search of Youtube will find bikes like this one: http://www.youtube.com/watch?v=f_-9OwOM4cc

Jeremy

also i ran across this one, but it looks like a larger turnigy motor:
[youtube]xmv7EOP0NF0[/youtube]

I like seeing people with crazy ideas, but anything fan driven that could otherwise be mechanically driven will never take off. :lol:

Cons: Noise pollution, poor efficiency, poor acceleration, poor safety.

Fans have practical uses in the air or on marshland, but not when surface drive is an option. Even lunar rovers where the gravity is ~83.3% less than earth and the surface is powdery use wheels, cool wheels at that.

neuraxon77 said:

I like seeing people with crazy ideas, but anything fan driven that could otherwise be mechanically driven will never take off. :lol:

Cons: Noise pollution, poor efficiency, poor acceleration, poor safety.

Fans have practical uses in the air or on marshland, but not when surface drive is an option. Even lunar rovers where the gravity is ~83.3% less than earth and the surface is powdery use wheels, cool wheels at that.

Click to expand...

Wholeheartedly agree with that, using a prop makes no sense, apart from being a fun thing to do.

BTW, the lunar rovers had to use wheel drive, as there's no atmosphere on the moon, so a prop wouldn't have worked...........

Jeremy

BTW, the lunar rovers had to use wheel drive, as there's no atmosphere on the moon, so a prop wouldn't have worked...........

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Thanks Jeremy. I remembered I should have mentioned that after I wrote the damn post... :lol:

Hindsight! It's always behind you.

not long ago the world record for human powered water craft was via a prop...
yeah - surprised me too - so it can't be all that inefficient!
http://lancet.mit.edu/decavitator/
Bob

It may not be practical, but it is tempting to add a small pusher prop and motor to the ebike just for fun. The hubmotor would be doing the heavy lifting, but folks would be really impressed by the prop. It would keep those tailgating lycras at a distance, too. Especially if it was a nitro motor... :lol:

Actually, at speeds above what the hubmotor works, a pusher prop might be useful. If it ran at the main pack voltage it would not take much gear - a castle controller and turnigy motor and prop. Pretty small addition to the bike. Call it the "afterburner". Ducted fan, anyone?? Really neat jet-like sound there.

bobc said:

not long ago the world record for human powered water craft was via a prop...
yeah - surprised me too - so it can't be all that inefficient!
http://lancet.mit.edu/decavitator/
Bob

Click to expand...

Well, they didnt have many options did they ? .... air prop ,..or water prop !
If they had put more effort into a wet prop design they may have had a different result ?
Or...if they had used a pedal powered winch cable across the lake ...... :lol: :lol:

Question for the aerodynamics experts - the largest drag is supposedly the turbulence behind the rider. If we put a prop there and send a stream of high velocity air through that zone can we lower the drag enough to matter? Seems like we might be able to improve the airflow there. Probably not enough, but interesting question.

Hillhater said:

bobc said:

not long ago the world record for human powered water craft was via a prop...
yeah - surprised me too - so it can't be all that inefficient!
http://lancet.mit.edu/decavitator/
Bob

Click to expand...

Well, they didnt have many options did they ? .... air prop ,..or water prop !
If they had put more effort into a wet prop design they may have had a different result ?
Or...if they had used a pedal powered winch cable across the lake ...... :lol: :lol:

Click to expand...

Spot on. The issue the MIT guys faced was one of getting the best efficiency from either a water prop or an air prop. Both can deliver pretty much the same efficiency when optimised, in the mid to high 80% range. Decavitator was a special case because it was a hydrofoil, so they wanted to get rid of as much parasitic drag as they could. It turned out that the air prop gave them a solution that minimised the parasitic drag from the propulsion system to a greater degree than a water prop. Since then there's been a lot of work done on optimising props for low power use in water (see my electric boat thread for some examples) so later HPBs have tended to be water prop driven. Decavitator worked OK on the river, where wind speeds were low, but was apparently a real handful in any sort of a cross wind.

The guy that helped me a great deal, Rick Willoughby, has made some very fast HPBs, including the props for a few record attempt HPBs, like Greg Kolodziejzyks (http://adventuresofgreg.com/blog/ ). One thing interesting is that it turns out that very thin bladed props, rather like aircraft ones, are much more efficient on low power boats than the conventional broad blade designs that dominate the boating world.

Jeremy

Alan B said:

Question for the aerodynamics experts - the largest drag is supposedly the turbulence behind the rider. If we put a prop there and send a stream of high velocity air through that zone can we lower the drag enough to matter? Seems like we might be able to improve the airflow there. Probably not enough, but interesting question.

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The snag is that the prop efficiency will be very heavily reduced by the poor inflow from the turbulence behind the rider. Props like to work in even flow conditions, where the direction of inflow across the whole prop disc is constant. If the angle of flow is uneven across the disc, then the prop blades see wildly varying angles of attack, resulting in noise and very poor efficiency. The airflow coming out of the prop will have the turbulence amplified, in effect, due to the added energy from the prop.

The upside of a pusher prop, when fed from relatively clean air, is that it adds a very hefty stabilising force. Anyone who has flown a microlight trike will know that applying power stiffens up the whole aircraft, letting it off makes it far more "wobbly". The effect is used to good effect in a lot of pusher aircraft, where the tail area moments can be reduced (reducing parasitic drag) because of the stabilising effect of the aft mounted prop.

The simplest and most effective way to reduce the drag coefficient of the rider is to fit a tail fairing, but this has an adverse effect on stability, because it moves the lateral aerodynamic centre aft. If the lateral aerodynamic centre ends up behind the rider, then gusts form the side will tend to turn the bike into wind, a counter-intuitive reaction and one that tends to lead to instability. Ideally, the lateral AC needs to be more or less in line, or perhaps slightly in front of, the centre of gravity of the bike, That way the impact of side winds is fairly neutral, or in the "right" sense (i.e. a side wind pushes the bike away from the wind direction, which feels more intuitive).

Jeremy

What caught my eye was the fact that for best efficiency at low speeds (very similar to bike speeds) they have an enormous prop spinning slow. All the model plane motors have (are designed for) scale size props and for rather higher speed operation with not too much worry about efficiency...
There are other options they could have (and probably did) consider - water jet drive? (can't imagine that working, or being efficient....)
How about paddlewheels? History is not kind about the efficiency of paddlewheels... but they could have provided lift as well. If the craft were run up to speed on land before hitting the water (by rolling on the paddle wheels), there would be no need for floats - now THATs a craft I would not want to set off across a wide stretch of water at the pedals of!!!

The efficiency of props has greatly to do with:

a) prop design. ( all are not created equally!)
b) rpm. A large prop spinning slow is much better than a small prop spinning fast.
c) disk loading. The more power you put to a given prop, ( raising rpm, see b), the less efficient it becomes.

In the low power realm of a commuter booster, an 11" or 12" prop is turning barely 5k rpm, remains fairly efficient.

I guess the true answer would require figuring how much forward thrust, ( in oz's), a low power wheel driven system can provide, and compare it with my benchmark prop systems output of 19.75 oz at 73 watts. (Any of you math savey guys here please chime in.)
Please keep in mind my motor was a cheapo brushed calc'ed at only 60%eff and could definately be improved upon. ( it is pretty quiet BTW)

My thought is that a wheel driven system will also have a fair bit of losses with gearing and what seems to typically be a friction drive, and may wind up being compareable, at least at low power levels.

As for mounting, my thought was a 12" square box, open front and back and disguised to look like a regular bike bag, mounted on a rear bike rack and containing the entire power system. It could pop off in a second to take with you, and most people would have no idea that its something other than a simple bag. ( reducing geek factor. )

Dean

DeaninMilwaukee said:

The efficiency of props has greatly to do with:

a) prop design. ( all are not created equally!)
b) rpm. A large prop spinning slow is much better than a small prop spinning fast.
c) disk loading. The more power you put to a given prop, ( raising rpm, see b), the less efficient it becomes.

In the low power realm of a commuter booster, an 11" or 12" prop is turning barely 5k rpm, remains fairly efficient.

I guess the true answer would require figuring how much forward thrust, ( in oz's), a low power wheel driven system can provide, and compare it with my benchmark prop systems output of 19.75 oz at 73 watts. (Any of you math savey guys here please chime in.)
Please keep in mind my motor was a cheapo brushed calc'ed at only 60%eff and could definately be improved upon. ( it is pretty quiet BTW)

My thought is that a wheel driven system will also have a fair bit of losses with gearing and what seems to typically be a friction drive, and may wind up being compareable, at least at low power levels.

As for mounting, my thought was a 12" square box, open front and back and disguised to look like a regular bike bag, mounted on a rear bike rack and containing the entire power system. It could pop off in a second to take with you, and most people would have no idea that its something other than a simple bag. ( reducing geek factor. )

Dean

Click to expand...

The easy way to do this comparison is to just look at efficiency.

A really great prop, of the largest practical diameter and spinning at the slowest possible rpm commensurate with maintaining high efficiency, might possibly be as much as 85% efficient. The vast majority of props struggle to get better than about 75% efficient. Javaprop, from Martin Hepperle, is pretty accurate at estimating prop performance, in my experience, and is worth a play if you want to experiment.

A bike chain drive is around 98% efficient.

The rolling resistance from the tyres is the same for both prop driven and wheel driven methods, as there is very little power loss through the tyre/road interface of a driven wheel.

You could gear a motor right down and drive an extremely large, slow turning prop to get the low speed thrust up and maybe get as high as 90% efficiency from it, but I doubt it'd then fit on a bike.

Jeremy

DeaninMilwaukee,

Were you at the IHPVA Championships, in Milwaukee, in 1991? It was terrific. What a hopeful time.

http://www.ihpva.org/water.htm

Driving an ebike with a prop is not as efficient as chain drive, is incredibly noisy, and potentially very dangerous...e.g....blade coming off.

Jeremy,

Jeez! Not only are you an electronics whiz, but you clearly know aero/hydrodynamics, as well. Please take a moment to address my nagging question over here.

http://endless-sphere.com/forums/viewtopic.php?f=28&t=23259&start=90

Thanks, Warren

Dean,

I can do a trial for you, but from my experience, a 12" prop isn't really gonna make it. I recently converted a RC Plane to electric, and now it does 300W with a bigger prop.

I had a .25 ci nitro engine running a 10" propeller at 10k RPM or so, and it couldn't even pull my bike. Either I was doing it wrong, or the motor just isn't powerful enough.

That said, that engine produced less than 300W or so...

I was'nt in town for that event, although I live only about 4 miles from there.

I understand that one of these low watt systems is'nt going to push the bike without pedalling at a decent speed, the point of a " commuter booster " as I understand it is to still need to pedal but your speed, range and hill climbing ability are improved in a very lightweight and unobtrusive system.

My <80 watt system was far from strong, but was enough to be noticable and would in fact keep the bike from stopping completely on level ground. I was able to go around and around in circles without pedalling untill the battery ran down.

Dean