"Useful Power"

[Jon H]

Posted by PatMc on 30/07/2014 22:39:22:

Posted by Jon Harper on 30/07/2014 09:19:46:

And Andy, you are correct, its not always the case that you get more thrst from bigger props. the 16x6 at 9200 gives 16lbs of thrust but the 17x8 at 7900 gives 15lbs. these are calculated figures and would need to be checked as I have always found bigger props pull better than small ones due to greater efficiency. And for the difference of 1lb I think I will take the extra speed and less frantic rpm!

Static thrust is a pretty useless parameter unless you want to hover or climb verticaly.

From the point that the model is moving forward the thrust diminishes at the same time the drag increases. Once the two are equal, if the model is held at that speed, the drag & thrust will remain constant.

Static thrust is the only parameter as it tells you how much pull you are actually getting. Also when the model moves forward thrust will increase as the airflow reaching the blade as a certain speed and will unstall the prop (most props are partially stalled on the ground) and allow it to function correctly. this reduces drag and the engine speeds up in the air by 500-1000rpm. Clearly above a certain point thrust and drag balance, but you need a balance with pitch speed as well.

As for the rpm recommendation the Laser is mechanically safe to 14000rpm. They only recommend 9000 as a top limit because of noise and you get diminishing returns for the extra rpm as the propeller becomes more and more inefficient. As we all know, drag squares with speed so going half as much again on rpm needs twice the power to overcome the drag. If all your power is being soaked up by the drag of the prop its not going to propel your model very well. Also, you would have to be pretty crazy to run a large 4 stroke at 12,500rpm. Even 52 size 4 strokes are rarely much above 10000, so...its a bit pointless really

the argument about a high rpm engine being more suited to a fast aeroplane is wrong. the rpm of the engine is not the be all and end all, its all about the thrust from the prop and the pitch as I showed in my earlier post. if a 15x10 was used on the Laser @ 8700rpm it would have a pitch speed of 82mph. how fast do you want to go? The thing is, with this prop thrust comes down to 11lbs, so you may find that drag is now more than your available thrust and you actually go slower than with a lower pitch and higher diameter. A balance is required between pitch and diameter. But then we are back to torque vs power as more torque is needed for a larger diameter...

 

 

Edited By Jon Harper on 31/07/2014 08:43:43