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Motor and Propeller

01/08/2010 11:31 AM

I have been reading around the internet, and can't find to find whether a motor and propeller is better going as a push or a pull. All the forums I have been reading have been writing in reference to people bailing from planes and potentially being decapitated by the propellers. I am making a fan driven remote control car, so what I have read has no reference to me. Also, clean air was talked about going over the wings, which a car has none, so that was no use either. Are there any specific advantages of a 'push or pull' fan in regards to car design? The propeller is one of those which are identical either way, so the propeller is not an issue. I have it set up currently as a push motor which seems to be working fine and will push the car even without the front wheels attached. Should I change it round to pull the air? Thanks, Bondy

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#1

Re: Propellers

01/08/2010 12:02 PM

Tractor propellers bite into undisturbed air for more efficient forward propulsion.

Pushers have to bite disturbed air so may require a more aggressive pitch for the same forward motion.

You need to consider the effect a tractor prop will have on handling, as it will tend to pull the car along a straight line.

I'd stick with the pusher prop. It's safer too in case you go off the track and into the crowd.

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#2

Re: Motor and Propeller

01/08/2010 6:14 PM

There's only a minor issue to add regarding a pusher or puller scheme. In planes, a pusher one has the potential to go forward and hit the vehicle and/or occupants in case of failure. So, a puller is always safer, because it will just go forward. Ok, safer for the vehicle, bad for those not wise enough to keep (a good) distance. Dynamically, the puller is always more stable, has more undisturbed air as mentioned. But I'm used just to airplanes, so, ... , why don't you just try? Ah! Ok! A pusher will look better, I guess. So, go pushing!

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#3

Re: Motor and Propeller

01/08/2010 10:30 PM

Does anyone know why there are no pusher boats (airboats excepted)?

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#4
In reply to #3

Re: Motor and Propeller

01/09/2010 12:23 AM

Cruise ships and some pleasure craft use, "Pods," that use propellers at the front of the pod. The pod itself can be steered (Rotated) to allow very exact steering.

Volvo markets a system. They use a joystick for control.

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#6
In reply to #4

Re: Motor and Propeller

01/09/2010 1:27 AM

I've seen those systems used for docking and other close maneuvering, but not for primary propulsion.

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#14
In reply to #6

Re: Motor and Propeller

01/09/2010 12:43 PM

Harry:

These systems are generally called 'Azipods.' Azipod is trademarked by ABB and ABB is the principal supplier. There are other similar systems developed from the so-called Z-Drive which is used on Harbor Tugs. Azipods are widely used on Icebreakers, Cruise Ships and Off-Shore Anchor Handling Tugs. The pods are steerable through 360°

The widest use to date has been Cruise Ships. Because the motor is mounted in the pod with a very short shaft there is very low noise or vibration. However, design assumptions seem to have been too low. There have been a number of bearing and shaft failures and the maintenance burden has been much higher than anticipated.

Sorry for the digression - back to the original question. The first Azipod installations had the propellers at the rear of the pod; i.e., a 'pusher' configuration. All new installations have the propeller in front of the pod because the water flow is less disturbed. The latest versions add a counter rotating propeller.

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#20
In reply to #14

Re: Motor and Propeller

01/09/2010 5:04 PM

GA!

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#22
In reply to #20

Re: Motor and Propeller

01/09/2010 6:55 PM

Thanks Chris.

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#23
In reply to #6

Re: Motor and Propeller

01/09/2010 11:53 PM

Do a search for Volve Penta IPS / Forward facing, swiveling system Lots of You Tube videos from various boat companies.

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#9
In reply to #3

Re: Motor and Propeller

01/09/2010 7:36 AM

Have you never been to Florida everglades? There are 1000s of pusher boats....

Here is one for example:-

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#17
In reply to #3

Re: Motor and Propeller

01/09/2010 3:33 PM

??? Virtually all boats have pusher props (if you are referring to the underwater props).

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#18
In reply to #3

Re: Motor and Propeller

01/09/2010 3:36 PM

There are! Have a look, I prepared this one for the kids earlier.

The cage was to pick up a propeller engine but the girls in the team convinced the petrol head that less can be more. It ended up being a space for a beach ball. In the surf it would be pushed by waves coming from behind and by this, the tip would be raised.

In a road situation I think pulling could be an advantage but weight distribution would come into it, I think. Experimenting would be on my cards.

Have fun, Ky.

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#30
In reply to #3

Re: Motor and Propeller

01/11/2010 12:42 PM

I'm not sure I understand your question.

Nearly all small boats are pushers. The prop is at the rear of the hull, below the keel. Most larger vessels also have props at the rear.

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#5

Re: Motor and Propeller

01/09/2010 1:05 AM

Interestingly, I came across this beauty today, while browsing the web...

but as I consider my own fan powered roadable craft, I think that the air flow causes turbulence for the vehicles following it.

Chris

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#7
In reply to #5

Re: Motor and Propeller

01/09/2010 3:43 AM

Nice. I suppose it's just like a little RC hovercraft with proper hulls. I think a full sized fan powered car would be a great idea. Mine is only a model and is only A3. I hate to think what the price of it all would be though, mine is already costing me £50 for a decent motor and RC system etc. If it were full size you wouldn't have the RC system but would probably find some other costs. Try making a dune buggy sort of thing.

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#8

Re: Motor and Propeller

01/09/2010 7:29 AM

Check out the cost of using a motor off a hand held brush cutter. They have strong crank shafts and with a bit of tuning, ie inlet port, exhaust porting and a tuned exhaust they put out good power and are very reliable, specially off a STIHL brush cutter or weed eater. Over here the RC Model shops are very expensive.

I don't know how you are steering, mine worked best with front wheel steering and an angled rudder to provide high speed cornering stability. You are also going to learn all about gyroscopic precession.......quickly. And like a hovercraft you will find that the braking from the prop is virtually non existent. So you will need brakes as well. Mine did about 65mph and made a big enough dent in the side of my dad's Dodge for him to notice and.....take corrective action against my backside!

Building and sorting out my R/C and full scale Hovercrafts was a lot less hassle than the darn prop driven car. So good luck and happy engineering.

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#11
In reply to #8

Re: Motor and Propeller

01/09/2010 7:42 AM

Getting old....

Go with pusher, less thrust but better control, you want cleanest air off the prop against the rudders. Also install a good size cowling around the prop with vanes to straighten the air as much as possible. A swirling wash off the prop wrecks any handling you may dream of achieving. I also sacrificed weight and installed bigger battery pack for the R/C as low as possible to help compensate for the high centre of gravity the prop motor causes. My mate who built off my mistakes installed his motor low in the chassis and used a little toothed belt to drive the prop. (Off his mother's vacuum cleaner) Have fun!

PS. Of all the things I built, that darn car got me into the most trouble with everybody of authority within a two mile radius!!

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#10

Re: Motor and Propeller

01/09/2010 7:38 AM

Why do you want to use a propeller on a car. This is most inefficient.

In France about 70 years ago they were made, but even for thoise times too inefficient!!!

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#12
In reply to #10

Re: Motor and Propeller

01/09/2010 7:49 AM

Andy if I had not had so much fun annoying my parents with mine , I would support you comment. This was my best-est-est makes no logic adolescent project that made a lot of noise and at my command on the joystick did exactly as it liked. That sir, is all the reason you need to do it.

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#16
In reply to #12

Re: Motor and Propeller

01/09/2010 2:52 PM

OK!

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#13

Re: Motor and Propeller

01/09/2010 8:12 AM

Hi Bondy111,

A tractor propeller location will have the advantage of clean air for the propeller, but the overriding disadvantage of high forward thrust line. A more practical location is at the rear of the 2nd axle where the thrust line will tend to keep the front axle planted on the ground for better vehicle control.

Regards,

Luther M

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#15

Re: Motor and Propeller

01/09/2010 2:36 PM

You are using air as a reaction mass so you have to use newtons third law.

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#19

Re: Motor and Propeller

01/09/2010 4:13 PM

There is not a simple, generalized answer.

The actual motive force of a good propeller relies heavily on the low pressure side (just as a good wing does). If a prop is mounted on the front of an aircraft, it nevertheless is said to "push" air rearward (and thus propel the aircraft forward). If it is mounted at the rear (as in a small number of airplanes and a large number of air boats) it also pushes air rearward.

In common usage tractor props are at the front of the vehicle, and pusher props are at the rear (even though both push air). The best placement from a pure efficiency standpoint depends upon the aerodynamics of the vehicle. As shown by the Rutan Vari-eze, a pusher prop can be very efficient if the aerodynamics of the vehicle are such that the flow through the prop is largely undisturbed. In a tractor-style small plane (the vast majority of those seen in small airports) the propeller blast hits the planes cowling and structure, and disturbs airflow over the wings -- all leading to inefficiency. DC9's, Fokker 100's, DC 10's, Lears, Citations, etc, etc, etc, etc have rear mounted engines which have largely undisturbed flow both in and out. These days, most of these are fan jets, meaning that they are very similar in operation to a propeller (and of course identical in the basic function of pushing a mass of air rearward, causing a reaction in the rest of the vehicle.)

So, if you were starting to manufacture a vehicle to be propelled by a prop, you would do the CFD modeling to see where it should be placed, and then analyze all the pragmatic aspects, and make compromises. In aircraft, you have both alternatives, and in small aircraft, the tendency is to put the prop at the front. But the Vari-eze is far more efficient than the typical small plane (even though its structure is heavy), and has the prop at the rear... but there are additional reasons for its efficiency.

In airboats, the decision is simple -- no one wants to be behind the prop blast. (This would be true in air cars too.) If efficiency is the guiding principal in a particular airboat, then having the people in front also helps, because the airflow speed is lower there, so there is less total drag.

Mounting the prop directly in front of a fuselage (as is done in most small planes) is about the worst place for efficiency, but the best place for weight and balance issues, simplicity, etc. etc. Airliners and corporate jets have the engines mounted either below the wings or at the tail, both of which are efficient -- and as a result you see about equal numbers of each.

Powered sailplanes are usually pusher props, so the entire power unit can be fairly easily retracted into the fuselage.

But for a model car -- put it in back.

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#21
In reply to #19

Re: Motor and Propeller

01/09/2010 5:13 PM

GA!

on another note, I have question... If turbo fans use many blades in a ducted propeller configuration, why do we not see piston powered many-blade propellers? and maybe have a slim duct integral, attached, and rotating with the blades?

similar to the following (engine fan and coooling fan)

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#24
In reply to #21

Re: Motor and Propeller

01/10/2010 1:04 AM

Quite simply it's about the aspect ratio that affects the amount of drag and then also maintaining the unit, the more blades the higher the frequency of maintenance, failure rate, weight, balancing & vibration problems.

Then there are the subtle differences in whether you are "moving the propeller" as in an aeroplane, or "moving the air" as in a fan/air handling system.

Bottom line, as mentioned in one of the previous posts, you have to do the sums to know the best solution.

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#25
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Re: Motor and Propeller

01/10/2010 1:34 AM

thank you.

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#26
In reply to #24

Re: Motor and Propeller

01/10/2010 3:36 AM

GA. Thanks

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#28
In reply to #21

Re: Motor and Propeller

01/10/2010 3:11 PM

Thanks Chris. I see that the answer to your question has been provided by Bushdriver. A single long slender wing (a la a high performance sailplane) is hard to beat for efficiency. But just try to fly your glider through a barn.

You've probably seen the Moller Sky Car. Overall efficiency is not its strong suit (although ducting a small multi-bladed prop helps reduce tip losses). (The inefficiency even extends to the engines, which are small, but not fuel-efficient.)

In a future world in which fuel costs nothing, and in which the population has dropped to a very low figure (so there are not 100 mid-air collisions each day during rush hour) flying cars will be just what we need to keep the population trending down.

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#29
In reply to #28

Re: Motor and Propeller

01/10/2010 3:16 PM

Thanks K,

... I guess I can't help dreaming the dream...

Chris

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#27
In reply to #19

Re: Motor and Propeller

01/10/2010 3:37 AM

GA

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#31

Re: Motor and Propeller

01/11/2010 12:48 PM

in your case a ducted fan PUSHING would be more efficient, but frankly with a car, having the motor drive the wheels rather then a propeller would be even more efficient. I do not know wether you are building a full size car or a model, if a model I suggest going to some of the model vendor sites such as Hobby Lobby or Hobby People and see what they offer ( if a full size vehicle you are more or less on your own I can't help you there but frankly, you are going to get a lot more efficiency and control having the motor drive the wheels rather then using a propellor )

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#32
In reply to #31

Re: Motor and Propeller

01/11/2010 10:10 PM

He wants to feel the wind in his hair again

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#33

Re: Motor and Propeller

01/12/2010 7:05 AM

shows the experimental Bennie Railplane.

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#34
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Re: Motor and Propeller

01/12/2010 2:17 PM

Nice!

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#35

Re: Motor and Propeller

01/13/2010 1:37 PM

my gut says pulling from the front lends better straight-line motion (horse/carriage). Pulling would also produce additional drag on vehicle. A pusher prop will be faster but possibly unstable.

shade tree

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