Is a 13-4 to little prop for a Magnum 61RFS?, all of the charts I have seen recommend a 13-5 6 or 7. I am looking for a good pulling prop and not interested in speed (excessive speed). The engine is on a U-Can-Do 46.

Go to a 14-4. A 13-4 is not enough prop based on what you listed above as recommended props. If you want to have good thrust and lower speed, take a recommended prop and increase the diameter and lower the pitch.

For example, 13-5 --> 14-4.

John,

As far as load on the engine goes, going 1" up in diameter is roughly equal to going 2" up in pitch.

Going down in pitch and in diameter have the exact opposite effect, so if you go up 1" in diameter and down 2" in pitch; like when changing from a 12x6 to a 13x4, you are not changing the amount of static prop drag imposed upon the engine.

A given engine will probably spin both sizes at roughly the same RPM.

But (and it is a very big "but"), the static load is not the only one.
A larger prop has a higher inertial load than a smaller prop.
This means increasing the rotational speed of a larger prop take a lot more effort.

Your model is a 3-D capable plane, designed to fly well with a .40-.53 engine and with a 11.5x5 - 12.25x3.75 prop.
Changing the engine to your .60 increased the weight by a significant margin (needing more tail weight does not help either), but this is completely counteracted by increasing the prop disk to 13".

If you increase the prop diameter to 14", the engine will spin at a lower RPM, closer to its peak torque, so thrust is increased.
But in 3-D plane you need an immediate RPM change, which will be harder to get with the larger prop.
Your engine could also overheat, due to a decrease in cooling airflow.

The larger UCD .60 uses a 14x6 with a two-stroke .91. Your smaller UCD doesn't need such a large diameter prop.
I will have to disagree with Joe.

In 3-D more power is great, but not if it comes at such a weight penalty. And it cannot be countered with an even larger prop.

But who knows, maybe the four-stroke will transition the larger prop.

I think the 13X4 would work fine. The 13X4W may be better for your 4 stroke. I am guessing anything in the 14 inch would be a bit much for a Mag 61. I suspect the weight will not be an issue for you. Although I do not know the weight of the 61, it probably isn't much more than a 2 stroke. In addition, you'll probably need the weight of the 61 to help balance.

The plane actually balanced very well with no extra weight needed, I am using a JR extra battery pack which is a little heavier but was able to use it to balance the plane at about 5 inches back. I looked again today for a 13-5 with no luck, but did pick up a 13-4W and will try both it and the 13-4 this weekend. I also picked up a 14-4 and may give it a shot to see how the engine reacts with it. If need be I will order a couple 13-5's to try out next week.


As a side note: This will be the first flight of this engine since I discovered (with the help of the RC Universe forum, extra thanks to William) the cam gear was 1 tooth off from the factory, needless to say I am excited to see how much better if flies with full power, I do hope I did not hurt anything flying it with the timing off. I wrongly assumed things would pick up after a little more break in.