The slow stick is built very light with very low wing loading. Low wing loading allows it to fly very slowly.

Glow planes, even trainers often fly 40 MPH+. That would be a fairly fast electric. That would be pushing the park flyer definition.

My Aerobird moves at about 25 MPH top apeed, uses a 6 or 7 cell pack of 900mAh 2/3 AA cells. The whole plane weighs 16 oz with a 42" wing span. Loading is about 12oz/sq foot of wing. Sort of medium for a park flyer.

The Tiger moth and the slow stick are probably under 6 OZ wing loading. Of course that means they can't take wind. To a Tiger Moth, 5 MPH is strong wind and 10 MPH is like a hurricane.

My Aerobird handles 10 MPH winds OK, but not for a beginner.

Typical glow trainiers are probably in the 30-45 oz range with wing loadings in the 15-20 oz/sq ft wing loading. Better in wind, but need more speed to stay in the air.

Typically battery capacity runs with weight. So an eight cell 900 mAh pack will be 75-125% heavier than an 8 cell 450 mAh pack. However it imparts no more power to the motor, just more duration. Of course if you have to fly faster to support the weight, you use up some of that duration flying faster.

So if the plane calls for a 600 mAh pack that probably weighs 4-5 oz and you put in an 1800 mAh pack that probaly weighs 11-14 ozs, that is a big weight difference in a plane that weighs about 20 ounces. About 30-50% of the weight of the plane is the difference in the weight of the packs. That is a lot, so you have to fly faster to stay in the air with the extra weight.

This is true of any plane, but because electrics tend to be lighter, the difference is more pronounced. It would be like taking your .40 trainer at 36 ozs and adding enough fuel to increase the weight of the plane by 50%. and expecting the same flight characteristics.

I hope this is helpful.

Electric 101.