I am building one of AK Models 3D Freedoms and I am trying to select a gas motor for the plane.

wingspan:67"
wingarea:1120"^2
OAL:64"
weight RTF: 9-10.5 lbs

They tested the airframe with a YS120NC and said that they had no performance issues and could hover no problem. Here are the motors that I am considering for this plane. They are all gassers with one end maybe a little small and the other too big. But what is too big really?

RCS 1.40 2lbs w/ign 3hp
MVVS 1.60 2.1lbs w/ign 3-3.8hp
RCS 1.80 3.15lbs w/ign 4hp
ZDZ 2.40 3.15lbs w/ign 4.8hp

They are all within $90 of each other also. I want the plane to have all the power it would ever need, but still be responsive to the controls.

Thanks,
LeRoy

The zdz 40 will be too much an engine.

The balance 3 engines will do the job of developing enough power. I would pick either the Mvvs 1.60 or the RCS 1.80. Check the ground clearance as you would need 18-19 inch props for these engines.

That is along the lines that I was thinking, but is the "too much engine" mean that it has too much power for the size of the aircraft? For illustration...what happens when you put too powerful of an engine on a plane? Worst case senario of couse. Would the torque cause ground loops, torque rolls in horizontal flight, etc?

#2 Here is a new question, the model has the crank line shown to be dead center of the firewall with the motor in the upright position. What effect would it have if I were to invert the engine and move it closer to the top of the firewall? I'd pick up alot of ground clearance and move the head away from the ground also. Let me hear your thoughts on that one also.

thanks again,
LeRoy

a moki/rcs 1.80 will easily pull unlimited vertical on a 15lb plane .
a mvvs 1.60 will pull tall verticals on a 15 lb plane. since your plane is only 10 lbs there is no need of putting too large an engine.

putting a big engine will be heavy .... then you have the CG to deal with. doing manoeuvres with too large an engine means generating more speed/torque which means you need stronger frame/body to deal with the engine.

the rcs 1.40 gas is a good engine and can hover upto a 12 lb plane if i remember correctly. will run 16 inch props in the 8000 rpm range.
the mvvs 1.60 will generate more thrust and run upto 18 inch props at 7000+ rpms.

check these forums itself for more info on the engines you have mentioned.

Go with the ZDZ engine and remember to not fly at full throttle. You should only want Full Power on uplines. You will only need full power when you are doing 3D.