Prop Brake

[Nicholson58]

Originally Posted by iancoombe

There's been talk of reduced drag if the prop is allowed to spin, I'm not sure about water but if the engine in a helicopter stops I want the rotors to keep turning to slow down the decent rate, known as auto rotation.

This is counter-intuitive but totally correct. There is more turbulence and more work being done if its rotating - more drag. Locking it is less bad.

[Wotname]

Originally Posted by vinayfm

Actually a revolving prop applies more brake than one that is locked and capitate. In my humble opinion



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Originally Posted by GILow

And here we go again...

Originally Posted by Nicholson58

This is counter-intuitive but totally correct. There is more turbulence and more work being done if its rotating - more drag. Locking it is less bad.

And you need even more effort (work) stop it rotating which means stopping it increases drag.

The OP wouldn't need to apply any effort to stop the shaft rotating if it produced less drag stopped rather than spinning.

Let's not worry about what is intutive or not, let's just just use some science and think the issue through from first principles and oh, let's leave aside airplanes and helicopters; their engines and transmissions systems are quire different and the fluid dynamics are not the same.

So cutting to the chase, fixed blade props create more drag when braked than when allowed to free wheel.

[Stumble]

Originally Posted by vinayfm

Actually a revolving prop applies more brake than one that is locked and capitate. In my humble opinion

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Then you opinion is wrong. This question has been debated ad nausium on sailing forums, but the scientific debate is settled. The only real open question is how much additional drag is produced. This is a function of the specific prop in question, and boat speed, but the results are always the same. Allowing a prop to spin creates less drag.

NOTE: less drag is nice, but not destroying your transmission is likely more important. So only allow your prop to spin if your transmission allows for it.

The best study i have seen recently below. But it is in line with the rest of them.
http://www.plaisance-pratique.com/IM..._sdarticle.pdf

"The experimental results confirm that a locked propeller produces greater drag than does a freewheeling screw (up to 100% more drag was observed, this being at higher speeds). Furthermore, for the freewheeling case, the magnitude of the hydrodynamic resistance is significantly affected by the amount of frictional torque on the shaft, low torque being accompanied by low drag."

[phipseml]

Hi there,

The setup with the belt breaking and stopping the prop is very interesting since possible to remote control and simple and I'm considering copying that system. I wonder if that doesn't introduce a lot of pull upward and can misalign or otherwise do something to the gearbox or shaft seal? The same goes for belt driven alternators?

We used to have a bike disc brake from the previous owner. However, the disk was so thin and didn't hold up, so we used the string around a bolt solution as described by others and it works perfectly and is cheap. It is awkward to climb down to the engine room each time, and a remote controlled solution is most preferable.

Phil

[RaymondR]

It appears that having a split disk with a split bushing to fasten it onto the shaft would not be much of an engineering problem to implement and even do yourself.

On the caliper part it appears that if you could find the pads to mount on a couple of pieces of flat bar which were fixed to an engine bed with a pin with the brake cable fixed onto the other end, you could implement the calipers very cheaply. You are not trying to brake a heavy vehicle by dissipating a lot of kinetic energy just one little old prop and shaft. Might even be simpler than making a mount for a motor vehicle caliper?

[RaymondR]

PS

Doesn't a Roberts 53 have room for another bunk away from the prop?