Why do the propellers pull an aircraft, but push a ship?

[Degu]

I think that the title of the topic is very self explanatory. I want to know what the explanation is for the different implementations, I reckon that it might have
something to do with the density of air and water.

 

[Danger]

I want to know what the explanation is for the different implementations, I reckon that it might have
something to do with the density of air and water.

Hi Degu;
This is my first time in this particular forum, so I'm not sure what level of education you are at right now. Forgive me if I oversimplify my response, because I have to assume that you are in a lower year of your education.
Basically, any propeller is an airfoil. It is really a wing in motion. According to elementary aerodynamics, an airfoil creates lift by causing the air above it to travel a long path. Since the same amount of air goes under it along a shorter path, there is less air per unit length of airfoil along the top than there is along the bottom. That makes a lower pressure area ('vacuum') above, so the air below pushes the airfoil up. If you cause the airfoil to rotate in the same plane of motion as a stationary wing, you have a helicopter. Lay it down so that it's roughly at 90º to the ground, and you have an aeroplane propeller. The pitch of the propeller (how steeply the blades hit the air) determines the amount of movement the air experiences for every turn of the prop. Fine pitch is like using low gear in a car, and coarse pitch is like high gear. You can see the same effect with different types of screws that you can get at the hardware store. The more threads per inch there are, the easier it is to drive in and the longer it will take to do it.
In the case of a marine propeller, it works exactly the same way as one on an aeroplane, except that it is generally a lot smaller and turns a lot slower than one that is made to work in air. It takes a lot more energy to move water than to move air, so a boat is always in 'low gear' compared to a plane. Both aeroplane and boat propellers can be reversed so that they push the air or water the other way to act as brakes. (Not all of them; but it's available as an option.)

 

[brewnog]

Hi guys.

I understood Degu's question as "why is a propeller at the front of a plane, but the back of a ship?".

I'm actually a little stumped to provide you with an answer, other than for some practical reasons (location of engine, weight distribution, air supply to engine) which crossed my mind. Get Fred in here.

 

[kawikdx225]

In an airplane you want lots of air moving over the wing to create lift. By putting the prop at the front of the plane you create more lift during takeoff and landing when the planes velocity is low. There are exceptions to this as I have seen funny looking airplanes with the prop at the rear.

I'm not sure why boats have the prop at the rear, maybe to help protect the prop from damage as the hull will push objects to the sides as the boat cuts through the water. Typically in a boat the engine is at the rear to help isolate passengers from engine noise and exhaust fumes so it makes sense to have the prop at the rear.

 

[Danger]

In an airplane you want lots of air moving over the wing to create lift.
Typically in a boat the engine is at the rear to help isolate passengers from engine noise and exhaust fumes so it makes sense to have the prop at the rear.

I guess that I misunderstood originally. Sorry about that. A prop in front of a plane does increase airflow around the wings at low speed, but with current engine technology it doesn't make a really big difference. A pusher prop has some advantages of its own. It can be preferable, for instance, in a seaplane where a front prop can send excessive spray over the cockpit. It's also a little less susceptible to bird collisions if the area presents that hazard, since the body of the plane is in front of it as a shield.
I think that a very large part of why a boat has the prop in the back is that boat designs for centuries before the invention of motors had a pointed prow which didn't allow for a motor mount. In addition, weight transfer will force the operator toward the rear during acceleration. If he's already seated there, the efficiency of the boat is increased by letting more of the hull leave the water. Lastly, a rear mount allows for a very small turn radius which can be handy in small inlets or other confined areas. I'm not sure that the exhaust problem really exists, because a lot of outboards vent the exhaust under water.

 

[Degu]

Thank you very much for your answers, many of them brought me closer to the solution.
It is an exercise to be honest, that is why I think that the answer has nothing to do with the facts of history

 

[OlderDan]

I think that the title of the topic is very self explanatory. I want to know what the explanation is for the different implementations, I reckon that it might have
something to do with the density of air and water.

To add to some of the ideas already presented, think about where a boat propeller has to be located in relation to the center of mass of a boat, and the effect of a force applied in such a location. Why does the bow of a boat rise out of the water when being driven forward by the propeller?

Some boats have propellers in the front as well as in the rear. Watch a fishing show on TV sometime and you will see that anglers have a big engine in the back of their boat to get them where they want to go, and a small engine they usually mount to the front of their boat for trolling. Again think about the position of these relative to the center of mass of the boat. Why is it better to mount that small engine in the front instead of the back?

 

[FredGarvin]

As has been mentioned, pusher props vs. pullers is really a trade-off for certain aircraft design interests. Take for example, the B-36. The pusher set up was used because of the airfoil chosen for the wing section and it's intolerance for the wake vorticies left by the huge propellers.
http://www.proft.50megs.com/b-36-kh.jpg

As far as sea planes go, the first one that enters my mind is the Lake Buccanneer.
http://www.shanaberger.com/images/lake_buccaneer.jpg

However, one reason I believe puller (or tractors) are used mostly is because you normally get an added lift benefit from the puller prop's wash. You do get a bit of a lift advantage. That means that a pusher prop has to take a bigger bite of air to produce the same amount of lift (or a slightly higher air speed requirement). This can sometimes lead to a noise issue with some people.

There have been aircraft that have used a dual configuration. The O-2 Skymaster by Cessna was a **** hot little plane that I would love to have.
http://bleyer.de/Modellbau/skymaster/fotos/images/foto_102_g.jpg

Also, the speed record for a helicopter (loose definition thereof it was actually an autogyro) was set by a pusher prop, the AH56 Cheyenne:
http://tri.army.mil/LC/CS/csa/ah56aam.jpg

 

[cburgoon]

I have built, flown and currently own three aircraft with a pusher props. Mostly Burt Rutan designs. They are efficient and really cool! I prefer pushers.

I am also directing the construction of a large commercial vessel in China that uses a tractor configuration with props (actually thrusters) in the front.

Regards,

Chuck.

 

[cburgoon]

I also built and flew an twin engine aircraft that was an (in line) twin tandem counter rotating pusher. Not a push/pull like the Skymaster but a push/push. There are interesting beneficial effects of counter-rotating propellers (in close proximity). Lots written on the subject.

Chuck