I converted a small 24" Cessna Park Flyer from a speed 400 gear driven small prop plane to a PJS 3D 550 outrunner motor. It flew so nicely with the geared speed 400 motor that I assumed it would fly even better with the outrunner motor. It doesn't! In fact it wants to constantly fall out of the air.

The only major change I made to the airplane was the diameter of the prop. The original speed motor was driving a small 6" prop. The outrunner motor is swinging a 10x4.7" prop. Is is possible that the larger prop is creating a huge disturbance of airflow over the wing and causing it to fly erratically? The weight of the plane is actually less with the outrunner motor and lipo battery.

Any other ideas as to why my Cessna won't fly correctly?
Thanks for any insight you can provide.
Frank

First problem is proving that your two different propulsion systems were working equally well. Unless you know that for a fact, all the other questions are just speculations.

For example, "the wieght of the plane is actually less....etc" And did that move the CG? If you didn't move stuff to compensate for the lighter nose...... you created a tail heavy airplane. Or one that's more tail heavy. And that gives you a more effective elevator if you're lucky. And a over-sensitive airplane if you aren't. Which will fly erratically if you're lucky. And crash if you're not.

The geared original was swinging only a 6" prop? and the outrunner a 10"???? Usually a gearbox is used to swing a larger prop, and does it more slowly. Sounds like the difference in power was considerable. And you mention, "the outrunner motor and lipo battery." So you also are using a lipo too? How much lighter is the new combo than the old one?

If the CG moved, it moved back. The elevator became more efficient, and if the CG went too far, the plane went unstable.

Odd though it sounds you may be underproped. At least in the pitch department.

It's hard to compare outrunners to can motors due to the wide number of options for wind patterns in the brushless motors. This is given by the Kv value for the motors. Meanwhile you replaced a Speed 400 with a small diameter prop that would have had a high Kv value.

Try switching the prop from the 10x4.7 over to something like a 8x6 or 8x7. I'll bet it smartens up the flying first of all by providing a better pitch speed (the max possible flying speed for that prop at the motor's RPM) and at the same time reducing the undesireable torque effect of such a large prop to wingspan ratio. As it sits your prop is over 1/3 the wingspan. That's a bad setup in any book other than for some of the free flight rubber power stuff I've flown in the past. And even there it's a struggle to trim the models.

The ideal situation would be to replace the brushless motor with a higher Kv value that would be proper match with a 6 inch prop. By proper match I mean one that would spin this smaller prop to the RPM's you want while drawing the required current so you have the watts of power to haul the plane the way you want it to fly. You want it to draw the same or slightly more current than the old Speed 400 in order to smarten the model up and improve on the old performance level. This means a motor that'll draw around 8 to 10 amps from a two cell Lipo when running a 6x3 or 6x4. Alternately if it draws around 7 to 8 amps from a 3 cell pack while turning a 5x4 at a zillion RPM you'll have a real hotrod on your hands.

I know it all sounds like black magic but with so many brushless options out there we need to do our homework to find the correct match for each style of model. And last I heard a Cessna scale model isn't a good 3D prop hanging harriering snap rolling fiend at the best of times....