Do you use a Clutch or not?

[Casper]

Interesting!

[AndyB]

20 hours ago, xNateX said:

From the boating article above whether a boat propeller should be locked or not...

...Our graphs show the results, which dramatically confirm that wherever possible you should let your fixed three-blade prop spin. The drag when it was locked was nearly three times the drag when spinning...  Conclusion: ‘Let it spin’

 

But you don't need to test it....it is the laws of fluid dynamics! As I said, a free spinning blade is simply pushed away, so has little effect on the fluid flow. A fixed blade pushes the fluid around it!!!!!

[MagMen]

That is incorrect. A windmilling prop is a very high drag source. That is the reasoning for fitting either a prop brake or the more complicated solution of feathering a prop.

Edited May 22, 2018 by MagMen

[simonhunny]

seriously guys, does it matter? The difference is so insignificant surely, at the speeds we travel at. The weather must be bad & everyone is bored & wants to split hairs?? Lets just enjoy whatever we have & burn as much petrol as we can 

 

[MagMen]

Does it matter? Very much so! A couple of years ago my mate and I were flying our hang gliders with Mosquito harnesses. My prop brake was working his wouldn’t lock on. We were engine off thermaling together. When he held his on manually we climbed together but the moment he had to release it I climbed above him like a train. Another example is to look at aircraft such as the twin engine Saab 340 turboprop (I used to fly them for a couple of years in the 90’s). In the event of an engine failure we had around 5 seconds in an engine failure after take off scenario to identify, secure and feather the prop on the failed side before the disk drag could become so great that it’s asymetric position would cause you to start losing roll control when at V2. A couple of Saabs were lost with all passengers and crew in the early days so we spent an awful lot of time in the simulator refining technique.

Prop drag is well worth understanding as it can play a significant part in an engine off situation in a paramotor. I do routinely switch off and soar in my clutched paramotor however for that sort of flying I generally swap to a PG wing and after shutting down I go fairly deep into the brakes to slow my airspeed for long enough to let the windmilling reduce down to a couple of rpm. At that rotational speed the drag is similar to it being stopped. Prior to slowing it down the disking is much higher and the drag great enough to make soaring impossible.

Edited May 22, 2018 by MagMen

[AndyB]

Examples of free spinning prop causing more drag come from fixed wing planes -they will tell you a spinning prop has more drag......................but that is because it is still spinning the engine over -  as there is no clutch. Faster engine rpm, more hp required to do it, more drag.

With a clutch, we have a true windmilling prop. 

 

[MagMen]

Not so. Saab 340 is a free turbine so the prop can freewheel just like a clutched paramotor.

[AndyB]

Is that the one where an overrun bearing was added following an incident? This allows the prop to spin faster than the turbine.

[MagMen]

No, not on the Saab. The prop shaft is driven off a turbine stage that sits in the free stream behind the other turbine stages. The right engine can have a brake applied to that shaft so that the engine can be run without the prop spinning so that it can be used as a form of auxiliary power unit to provide air conditioning and electrics on the ground if no GPU is available. It’s a cheaper and lighter alternative to carrying a separate APU and is fairly common in most free turbine turboprop regional aircraft.

[wzh]

clutch is  good option,   

though my engine is  hirth F33,without clutch and  reduction. it helps landing safty and launch easily.

 

[beermaniac]

i think that one of the biggest hidden advantages the clutch provides, is the ability to start your engine without a prop. This is a huge advantage for carb tuning, and also allows you to start your engine on the ground and warm it up without a prop for added safety.