Why are props at the back of a ship but at the front on a plane?

[JasonDS]

Thread drift is interesting, it's like a documented version of Chinese whispers.
I wonder if i had posted on the 'flyers, flying forum', I would have got mainly boat answers? :-)
Thanks for the discussions and insights.

[model 10]

Originally Posted by a64pilot

They do in effect prevent a stall.

Not all the time. The model 120 AG plane fom Scaled Composites stalled and crashed empty.

[GWK417]

Prop wash over the wings aids in lift. No comparable advantage with the wetted hull.

[model 10]

Originally Posted by JasonDS

Thread drift is interesting, it's like a documented version of Chinese whispers.
I wonder if i had posted on the 'flyers, flying forum', I would have got mainly boat answers? :-)
Thanks for the discussions and insights.

You sure would. Some homebuilt aircraft are made from blue boat dock foam and fiberglass, Burt Rutan types etc. Those are not the usual aviation building materials. If not for boats, who knows? Around the world, non stop? Maybe not yet. Remember the picture of Voyager with the wing skin flapping on take off? That was blue boat dock foam showing

[buzzstar]

SailMonkey-

Where are you when we need you? Houston is where I recall you were, and we need the information you may have about what has and is happening as far as boats/hurricanes/rain there. Also, you may be the only one who knew Ambrose Bierce well enough to understand his connection to canards and fowled lines. Hope things are going well enough for you, so you can tellus wha is happening now.

[buzzstar]

Originally Posted by GWK417

Prop wash over the wings aids in lift. No comparable advantage with the wetted hull.

You mean the dynamics of the two fluids are unrelated???

[Pelagic]

The humorous reply is that Dolphins stopped dancing on the bow wave as soon as the props were put on the bow.....

Engineering wise, the impact forces on a relatively weak rotating shaft, going into a seaway would be enormous

https://youtu.be/dM3nbMbh0tw

[ZULU40]

Originally Posted by a64pilot

They do in effect prevent a stall. They way they do that is so long as the aircraft is within CG and operated within its limits, the canard will stall first, before the main wing. What that does is lower the nose and prevent a main wing stall, so the canard stalls preventing the aircraft from stalling so to speak.

People say canards are stall proof but its a myth. What is happening is the aircraft is automatically recovering, which is a safety of a sort. When it was discovered that a rear weighted canard could stall and be unrecoverable they didnt have the tools to explain why, so the things were left alone until analysis could catch up. Film evidence of the Wright Flyer suggests it was pitch unstable which the brothers quickly fixed. Even today canards give us problems, I think there are 2 Velocity aircraft that have gone in with stall related crashes.

Originally Posted by a64pilot

There have been other "stall proof" designs, the old Aircoupe comes to mind, the way it prevented a stall was the elevator simply didn't have enough authority to raise the wings angle of attack high enough to stall.
However nothing is stall proof as it requires the aircraft to be operated within limits, particularly CG limits, and sometimes stall proof designs can be a real problem to recover from a stall when the aircraft is operated in a manner inconsistent with its type design.

Lovely explanation. I met Aircoupe's designer RT Jones and his friend Martin Holmann decades ago. Still have his book 'Modern Subsonic Aerodynamics'

some useful links for the wary students who may feel misled:

https://en.wikipedia.org/wiki/Canard_%28aeronautics%29

https://en.wikipedia.org/wiki/Stall_...d_mechanics%29

http://www.flyingmag.com/technique/a...ades-minus-one
.

[ZULU40]

Originally Posted by JasonDS

Thread drift is interesting, it's like a documented version of Chinese whispers.
I wonder if i had posted on the 'flyers, flying forum', I would have got mainly boat answers? :-)
Thanks for the discussions and insights.

There is a worthwhile connection, as fluid dynamics broaches both air and water mediums. I was fascinated to watch the plane borne the Americas cup boats race with such agility and stability. These craft can be aeroplane or canard configuration hydrofoils,

... its possible we are just a little ahead of our time..

[CatNewBee]

I think the answer is easy.

Ships don't fly.

But first another (related) question - why is the rudder always astern and not in front of a vessel / aircraft?

Now to the ships:

1.) the propeller is better covered by the keel of the vessel, so it is unlikely to run a ground. If the vessel runs on a sand bank, you can try to back off using your engine. If the propeller was on front - well, it will stick in the mud.

2.) the propeller needs to be as close as possible to the rudder. again - rudder astern means propeller astern for maneuverability in close ports. You need the water stream to the rudder to steer.

3.) you want the bow of the ship out of the water and the stern below while sailing, a prop in front would go more often out of the water than a prop astern. You do not want to drag a bow into the water by a bow propeller.

So what is different with aircraft?

1.) Simply aircraft need a lot of speed in the first place to fly, so there is always sufficient air stream on the rudder to steer, at the airport you usually maneuver using the wheels.

2.) the propeller is always in the air, there is no getting out of one medium (water) to the other (air), so it can be placed on front. Jets do have rear engines btw. and larger aircraft have the props on the wings. So you usually look for the best place you can find there.

3.) when you fly into a flock of birds, you want to slice them pretty, before they hit your windshield, so less damage and a great show...

[weavis]

My favourite canard aircraft

[davewtsnape]

That will be physics

[valhalla360]

All the talk about efficiency is missing the mark

Boats/Ships: The rear position was established back in the steam era. I seriously doubt they were considering detailed analysis of efficiency with the possible exception of comparison to paddle wheels but none that I have seen use bow mounted props or paddle wheels. They also weren't considering sonar implications.
- The stern had the structure to support the forces and it's easier to design in compression, especially with lower quality steel parts.
- The bow even on large ships comes out of the water more frequently than the stern and keeping the prop in the water is useful.
- Ships run aground and it's easier to protect a stern mounted prop (plane running aground usually has more issues than just the prop getting dinged up).

Planes:
- Large planes rarely have nose mounted engines. Most are wing mounted but there are a few with a center engine in the tail.
- Small planes it is more about the cabin space and weight balance. By putting the motor in the nose, you can put the cockpit right behind where it is already wide enough for two passengers and then most of the rest back to the tail is available space for storage. If you put the motor in the rear, all that tail space gets eaten up and there is limited storage space ahead of the cockpit. (yes, there are unconventional designs the mitigate it to a degree but just like your 25' monohull almost always has the same interior layout, on a small plane, they long ago figured out the most efficient space layout.

[Cadence]

Just imagine, with a prop at the bow you could mow lobster pot warps like a lawn.

[CatNewBee]

Well, there are steam ships on the Mississippi with wheels on both sides too...