Does a sail act as a turbine blade or a propeller blade?

For the purposes of the above poll, here are the definitions:

"Sail" excludes spinnaker but includes any typical/traditional sailing rig on any typical/traditional sailing vessel.

A turbine blade has work done on it by the air.

A propeller blade does work on the air.

JB

 

Both . . .

 

Both simultaneously. The sail uses the inertia of the air to effect the direction of travel of the air. So, in effect, it harvests energy from the air to move that very same air in a different direction & produce thrust.

Turbine, and propeller, in one piece & with no moving parts! How's that for ingenious?

 

Did I win a prize?

 

Sails act in two fashions.

#1 - as a lift producing device (wing). Sails act to generate lift on MOST points of sail. Lift from the sails works in conjunction with lift from the keel (centerboard or hull) to produce forward motion.

#2 - as a drag producing device. Sails act to produce drag once the wind is so far astern that you no longer have attached flow, and your sails (wings) are stalled.

 

There is two types of turbine blades that cover your 2 examples.

 

Most sailboats i normally see, have sails stacked down/ wrapped up, probably to dampen the blow you get on your head when you pass the boom or when the boom passes you....

 

Hey all
Call it new-guy syndrome...but I don't really get the question:

An exhaust turbine acts like a pinwheel: the blades tend to be shaped more like a scoop or a bucket than a foil. When the exhaust gasses hit it, Bernoulli's principle isn't the primary force responsible for turning the shaft (more Newtonian I guess). Compressor blades on the other hand are foil shaped, but compressor design maximizes pressure buildup by the use of alternating rotor and stator blades to minimize the velocity of the compressed air being fed into the can. The fan blades (in a bypass turbofan like an airliner) act as many propellor blades taking small 'bites' of the air vs a conventional propellor taking fewer, bigger 'bites (per revolution).

My question is here:
When you say 'turbine blades' I assume you mean fan blades because that is what most people see on the type of aircraft most people see. What difference between fan blades and propellor blades are you trying to compare a sail to ?
Not trying to be preachy or argumentative, just curious...

 

Deadeye

By "turbine blades" he means the actual turbine stages, not the fan/compressor stages as those are "propeller blades" if you read his first post:

Read: Turbine blades harvest energy from the air, Prop. blades use energy to move the air.

PAR-

I'll say you did...first one with the truly correct answer

 

I can't vote until there is a fourth choice "none of the above"

A turbine compresses air using mechanical source of energy or maybe not if it is extracting mechanical energy from a stream of moving gas. A propeller uses mechanical energy to move a vessel through a fluid.

A sail has very little to do with mechanical energy unless it is on a windmill in which case it is part of a turbine. It would be just as sensible to compare a sail with a windshield, since both deflect air.

 

Rob, I'm not sure what you mean by 'actual turbine stages' but your response did definitely answer my question.

WIND turbines, i.e. 'aeroelectric' generators...I was thinking generally about turbofan engines and specifically about the Allison 250 turboshaft engine - that's why I wasn't sure.

Neglecting apparent wind, I'd say turbine. Once apparent wind is factored in, it's both. But I guess that's what you meant, Paul?

Never thought about like that...interesting.

 

It's worse. To answer rationally one also needs to:
A) Define a "propeller" and "turbine".
Are they defined in terms of propulsive force? (propeller has force with motion, turbine has force against motion).
Or are they defined in terms of fluid energy? (propeller puts energy into fluid, turbine takes energy out of fluid).

B) Define which observer we're talking about.
The sailor on the boat? A swimmer treading water? An observer in a hot-air balloon?

Different observers and different propeller/turbine definitions will have different correct answers to the poll.

 

Mark, I agree with your concerns about definition - particularly when it relates to work done on air by the blade of vice versa. It occurs to me that that spinning device on the back of the prop cart is almost necessarily a propeller in the cases we discuss, while the overall cart could be considered a turbine. Afterall, the cart has the effect of extracting energy from the air in the frame of the ground (although adding energy to the air from the frame of the cart).

For the less ambiguous case of a bladed structure spinning on an axis I'd propose one or both of the following definitions:

1) if the structure is turning in the same direction which its shaft is torquing it, that structure is a propeller. If it's turning opposite that direction (as a result of flow over the structure) it's a turbine.

2) If we consider the flow through the actuator disk a propeller will always have higher pressure downstream of the disk, while a turbine will have higher pressure upstream of the disk.

A possible 3rd definition (perhaps redundant with the second), a turbine is subject to the Betz limitation, while a propeller is not.

Considering only the limited case of a structure spinning on a shaft would you agree with these definitions? Are there cases (for such a structure) where those definitions would still be ambiguous?

 

OK, that works for the cart or a similar water vehicle.
But this is about a sail, to which these do concepts not apply. Here, instead of the cart's torque and rotation, you can try to use force and velocity. But rotation is the same for all inertial observers, while velocity is not. To some observers the sail is giving a thrust force along the apparent velocity (its a propeller blade?), and to other observers it's giving a drag force (it's a turbine blade?).

 

I agree with these definitions.
However the original posts was about common sail, and these definitions tell us nothing about that as far as I can see, as there is no axis & torque.

One could possibly compare pressures with apparent airflow directions, and conclude there is a higher pressure in front on average, but not necessarily on leading edge. Thinking about aerofoil here. Jib hasn't this effect having so thin LE, so, it has always high pressure upstream like actuator disk of a turbine, but not like turbineblade alone which has suction peak at LE.
Confusing really, bnetter leave the sail out of this.

My concluson would therefore be that a sail is neither a turbineblade nor a propeller blade as it doesn't fit these sensible definitions at all.
As far as the poll is conserned I still answered assuming the original post definitions as given regardless.