help with prop sizes
 

Hi, i have just bought a PAW 35 diesel engine and the rec. prop size is 11/6 to 14/6 i bought a 11/8 prop for the engine. I am a model engineer and will be making twin engines, so the total engine size will be 70ci. What prop size should i use for this engine, the propellor is mainly to give some torque behind the engine and may not be used in a plane, also this engine will be diesel so will have higher torque and lower rpms compared to the nitro engines. So please dont rec. a prop size from a table like this as the table below is for nitro
http://www.rc-airplane-world.com/ima...size-chart.jpg
help will be appreciated, im new to the aero stuff and just need the right proprange for the engines im going to make

RE: help with prop sizes
 

Hi, use this programme ThrustHP V 2.0d to learn out which propeller will give most thrust or Hp in measured rpm and the propeller are selected.. [sm=thumbup.gif]

The link: http://www.hoppenbrouwer-home.nl/ika...rusthpv20d.htm

RE: help with prop sizes
 

Hi, thanks for the link to the program but my computer will not install it, is there any other software that you know of that does the same. thanks

RE: help with prop sizes
 

Or use Excel Spreadsheet calculator for engine power and prop rpm power calculations, go to this link: http://mvvs.nl/utilities.htm ;)

RE: help with prop sizes
 

I don't have any diesels that are that large but the recommendations from PAW are usually good. For running in a new engine then start with something near the highest recommended prop, this gives the lowest load since it requires a lower compression setting and the rpm's will be kept low.

RE: help with prop sizes
 

The twin engine with 13x8 prop will have the same load as the single with 11x8

RE: help with prop sizes
 

You have to remember that you will have a slightly less efficient engine as a twin over the same size single. I believe PAW makes a 60 so I would arm myself with at least the biggest they recommend for that engine and a tach and you will have to decide if it too much or too little a prop. It will probably take some trial and error and many props because you have a different dynamic for the engine and it will have a different red line rpm than a single.

RE: help with prop sizes
 

The PAW .35 I had was a dog. An 11x7 was more than enough load for the power it could muster. It turned that prop about 8000 RPM.

Pe, is right, adding 2" to diameter in this size range will virtually double torque required to turn the prop at the same RPM.

RE: help with prop sizes
 

thanks for the replies, , i was thinking of about 9000rpm so looking at the spreadsheet that motorboy provided i think that the 13/8 prop is a good idea because im aiming for around 1hp 1.2hp to give the engines a good run to make sure there worthy.

RE: help with prop sizes
 

My PAW .35 made about .34HP.

Greg