The Counter Rotating Props Scam

< "Angular momentum is a conserved quantity" < would that cut the ice with you?

If it was a closed system.

Bret Cahill

Ok so far, so good. Now: if the conserved momentum is lost from the airframe, where does it go?

To the airstream, no matter how?

:-)

Brian Whatcott altus, OK

< Now: if the conserved momentum is lost from the airframe, where does it go? < To the airstream,

Or the ground or a flywheel on the plane.

Bret Cahill

OK I think we have a mutual stepping stone here - a single prop, or several props turning in the same sense, react angular momentum on the airframe, which if airborne needs to shed it (assuming that like most, it does not have a flywheel on board to spin up) This is usually accomplished by rotating the largest possible mass of air by the slowest angular rate.

However - a contra rotating prop does not present appreciable angular momentum on the airframe - and this is a small point in its favor. The same benefit is shared by twins with mirror rotations or by tractor/pushers with opposed (forward view) rotation.

Yes?

Brian W

< This is usually accomplished by rotating the largest possible mass of air by the < slowest angular rate.

The lift on one wing is greater than on the other side. The angular momentum imparted to the air from the wings [large mass at slow rpm] + that of the prop [small mass at large rpm] = 0.

< However - a contra rotating prop does not present appreciable angular

< momentum on the airframe

No bending moment, no torque.

< - and this is a small point in its favor.

Probably negligible compared with the safety factor already build into the airframe but I'm not an aircraft designer.

My only focus here is saving fuel.

Bret Cahill

The literature on ship propulsion is filled with devices that claim to increase efficiency. A number of these are even used on ships (from small up to supertanker level).

Some typical devices are:

- contra rotating screws

- free rotating contra rotating screw

- stators acting in the wake of the screw (sometimes attached to the rudder)

- stators acting before the screw, giving opposite swirl before the water enters the screw

Of these the contra-rotating screw appears to be the most effective. Not only does it eliminate the swirl. but it gives an opportunity to reduce the loading of each separate screw, reducing the risk for cavitation and associated vibration levels. Of course the main reason why it hasn't gained wide acceptance is the high cost, both of installation and of maintenance, and the risk of problems.

The problem with the stator options is that the gain in efficiency is small. Much of the gain from reducing the swirl is consumed by added drag, not only of the stator itself, but also of the structure needed to hold it in place. Together with the added capital cost this leads to a situation where they are used a few times by a convinced ship owner, and thereafter discarded as they don't give the gains hoped for, specially after a few years of operation as they tend to get fouled.

With regard to bunker oil: large ships burn something called Heavy Fuel Oil, which is the heavy fracture with lots of sulphur which remains after all the useful stuff has been extracted from the crude in refineries. Something cheaper is not easy to be found. Of course ship owners like to reduce cost by improving efficiency, but it shouldn't cost too much to do so.

Timo

Unfortunately twin screw ships are less efficient than single screw ships, in bulk and cargo shipping therefore hardly any twin screw ship is built anymore. The reason they were used more often in the past was simply that the large engines and propellers were not available.

The reason why it is unfortunate that only single screw ships are built of course is that it is more safe to have twin screw ships. When one of the engines or rudders fail there is still the other left to bring it to safe harbor. Engine or rudder failure is a likely scenario leading to grounding of a vessel (for instance the engine on Braer and the rudder on Amoco Cadiz). Only a few owners are willing to invest in twin screw vessels for the sake of safety only, but sometimes it is done, as on the Millennium class tankers for the Valdez trade for Arco.

It is not likely that there will ever be any regulations requiring the use of double screw ships. The maritime industry is mainly self regulated, with an overwhelming influence from ship owners. This has let to a situation were most of the ships in the international trade are systematically undermanned, which is a much greater threat to security than any mere technical issue.

Timo de Beer

Yes!

Brian W