Firstly, i hope I am posting on the right forum.

I am busy building a Swallow EX90 and have the option for either a 2 stroke OS91FX with tuned pipe or a 4 stroke FS120S III.

I have no practical experience with these two motors and would value any input in this regard. I am not in the States and in our classes for pattern in my country, these are the two max size options one may use in the class I will be flying in (probably the States version of Basic class) and am situated at the coast in our country.

From what I have heard from others so far:
1. 4 stroke will give better throttle response over the range as opposed to a 2 stroke with tuned pipe.
2. down legs on maneauvres and landings will fair better with the "prop brake" provided by the 4 stroke.
3. since the FS120S III has a pump and is not reliant on exhaust back pressure, one can place the fuel tank on the CG point thus stabilising CG during flight.
4. some have said the 2 stroke will give better vertical performance???
5. Mounting the 4 stroke upside down can give idle problems (2 stroke to be mounted at a 60 degree angle)???

Your opinions would be appreciated since I am now in a quandry as to which direction to go. Cost difference between the two options is negligible

Thanks

JR

1. Generally, yes a good 4-stroke will give better throttle response than a piped 2-stroke. Also, the 4-stroke will have more low-RPM torque, which could be very useful. The 4-stroke will also probably be lighter.

2. The braking is almost entirely dependent on the prop you are using.

3. You can use a regulator (Cline or Iron Bay) to accomplish the same thing. Personally, I'd go with a Saito 1.25 with an Iron Bay regulator over the OS 120 pumped. You can place the tank anywhere you want with either setup.

4. Your vertical is going to be dependent on how you prop the plane, though I would tell you that the OS120 will be able to swing a larger prop than the OS91, thus allowing for more thrust.

5. I've never ever had a problem with an inverted 4-stroke.

Do a search in the Pattern forum, there is lots of info on the swallow. I would opt for the 90 2 stroke+ tuned pipe, just personal preferrence. Probably lighter as well.

Just a thought but I imagine you could also use the pump/backplate from the heli version.