qwazzy123

I almost completed my Sig 4 star 60 ARF. I got a Super tiger 75 for the Airplane. According to instruction of the engine, you should be using 12x8 to break in the engine. That said, can anyone explain what the difference between using difference prop sizes to power an airplane. All things being equal what if I go with a 12x5 or 10x8 or a 15x8 - how does this affect thrust or speed. My NextStar is is using an OS .46 and 11x6 prop...so would not a 12x6 or 12x8 be to small for Super Tiger 75. If anyone can shed some light on this would be great..or point any readings on prop theory or selection.

If you have a Sig 4 start 60, let me know what engine and prop you are using.


Thank you.
Qwazzy!!

KidVermin

My Feedback: (3)

http://www.rmac.co.za/prop_sizes.htm
Normally the mfgr's. suggest the smaller size on the scale for break-in. I've used 13/6 or 8 on my 75. ended up using a 12/8 three blade on my older 63" span Hobbico Extra.

JimTrainor

This is useful for two stroke prop selection:

http://www.top-flite.com/accys/topq5000a.html

The chart suggests something between an 11x8 and 13x8 for a 0.75 ci engine.

Normally, one would lightly load the engine during break in. So if the manual suggests a 12x8 for break in, then a 13x8 or equivalent might be about right after break in. I see on the Super Tigre's web site they suggest a 12x6 or 13x6.

To guesstimate prop "equivalence" just add the diameter and pitch to get a single number that represents the prop's load. A 13x8 is 21. So, a 14x7 would be about the same, as would a 12x9. For a more refined analysis, there is a program floating around the net called "ThrustHP" that is interesting.

Generally, one tries to match the prop pitch to the speed of the plane so that the prop operates most effeciently. Slow flying planes use low pitch props, high speed planes use high pitch props. The engines speed (rpm) factors into the equation as well. There is no hard rule, however. It is a matter of preference to a certain extent. You may have to try a couple to see what you like. Once you get used to the plane, you will notice the difference between different props and will likely have no problem identifying which one "feels" best.

The Ghost

Another site to check out is [link=http://www.apcprop.com]APC Props[/link] Lots of info on prop selection.

Cheers,

joeb102072

My Feedback: (9)

Theres an intersteing article in this months issue of Model Aviation. IMO its a must read for anyone with prop issues or questions. It's called "From the Ground Up" and it can be found on their website if you dont have the magazine. Most of what was written I agreed with, but I had issues with prop loading in the air. They claim that an engine will lean approx 500 rpms in the air I think is more like 1000 but in either case I would rather over estiamte that number. Prop size is discussed in detail what should be looked for what type of flying do you want to do and how your engine should respond given the different prop sizes for your size engine. Peak RPM as opposed to Peak power out put RPM both different. Where your engine revs to peak RPMs but you get no more power then say 1500 (as an example it may be less) rpms before. Its just wasted RPMs. I found it to be very very informative IMO. Hope this helps.