PilotSpike

Just got back into the hobby after a couple of decades out of it. My first building project for the winter is a dynaflite fly baby 1/4 scale 84”. Manual says target weight is 11 - 13 lbs. I want to go with a 4 cycle gas engine. Dynaflight recommended up to a 1.2 four stroke but I’m sure that was assuming a glow engine. They also suggested a us engines 25cc. Obviously an old kit.

On the gas side, an FG-21 is roughly equivalent displacement, but from what I’m reading it’s likely down on power from a nitro 4 cycle of the same displacement. Any thought on whether I should go up to the FG-30? I expect to fly this thing mostly on the wing, scale like speeds and gentleman’s aerobatics.

I’d thought about just going forward with the 30 now as it’s in stock and can be found on sale for the same price as the 21. I just don’t want it To be grossly overpowered or heavier than is reasonable.

Any help would be appreciated.

J

1967brutus

I think they both can do the job, but the FG21 might result in a pretty sedate flying style, and the FG30 might be on the wild side. Although I don't think it will be "grossly overpowered".
Weightwise, I don't think the 30 is that much heavier than the 21, and remember, you can fit a tank half the size of what is recommended for glow, so there will be a considerable weight saving there, at least "all fuelled up".
Had there been an FG25, then that would IMHO be the sweet spot.

kmeyers

From your lips to Saito's ears.

PilotSpike

I went ahead and put an order in for the FG30. Might as well, its currently on sale for $180 off and is at the same price as the 21 and its actually in stock. Ill just fly really big loops and throttle way back on the downline.

J

CW-(ClippedWings)

The Saito FG-30 is always a good choice, certainly at around the same price of the FG-21, but take note of the following:
1. The FG-30 is probably around 260g (over 9Oz) heavier than the FG-21.
2. For a "floater" the difference in power between the FA-125 and FG-21 are probably smaller than you think. Saito claims that the FA-100 and FG-17 produce the same power, it's an interesting statement/observation it appears that the ability to spin a larger prop (gasser) makes up most of the differences in power/thrust.
3. Assuming that the FA-125 and FG-21 have a comparable useable power, the FG-21 would have been sufficient. The heavier FG-30 may not improve much (if any) of the flying characteristics.

khodges

My Feedback: (1)

I'm looking at the same two engines for my build, a 1/4 scale BUSA J-3. I felt the same way you did, why not get more power for the same price. I had carefully measured to see if the FG-30 would fit, and yes, but just barely. I received it this Tuesday, and it is BIG. Way too much weight up front for the Cub, and the size, although the numbers said it will fit, it is just too big. I returned it today, and will wait on the FG-21 to be back in stock. I can't do any more on my build until I figure out a couple of discrepancies in the plans, and I want to measure and drill the firewall before I mount it to the plane. I don't think the FG-21 will be down on power from the 1.20 glow engine, it should be about the same, and will certainly use less fuel, so you can get by with a smaller tank than you would for a glow engine. Another thing about the FG-30, the manual says it burns about 25cc/minute, so a 16 oz. tank will give you about 12 minutes of flight with not much reserve. The Fg-21 is 15cc/minute. That's over 15 minutes from a 16 ounce tank with a decent reserve. Those figures are supposedly at full throttle, medium load.

I hope you can make the FG-30 work for you. It is a beautiful engine; I wish it was about 8/10 the size dimensionally, I'd have kept it.