abufletcher

What are the guidelines or rules of thumb you use to decide what size engine you'll use in your scratch-built scale projects? Obviously if you're working from plans, the plans give a recommended engine size.

It there some sort of wingspan (single or bi) to weight to engine size chart somewhere out in the world. And to what extent does a/c type influence you decision. For the smaller sized models the three major engine types seem to be the standard 2 and 4 strokes and RCV engines. I must say the shape of the RCV 90 seems awfully appealing for inline WWI types.

Also as long as I'm asking I'm wondering how people make weight estimates for their finished birds.

sirrob01

I havent built a scale bird, but I do scratch build a bit from plans/3views or adjusted plans via paintshop , or adjust others designs.

On motors I go by the table here, its set for trainers but its a guide to start with.
http://adamone.rchomepage.com/design.htm

it works on a wing area, seems to work out about right

hope thats of some help

S01

ps as to the other part yes I est my weights of projects as I like to keep wing loads down.

khodges

My Feedback: (1)

I look at what the scale prop diameter is for the plane I am going to build, and try to use an engine that will turn that prop without overloading it. take into consideration the type flying you will do. If it's a high performance plane and you will be turning at high throttle settings, go with more engine than you would for, say, a Cub, where you would rarely be running the engine wide open; but also consider where the engine's power band is. If it's a 2 -stroke, you'll have to turn higher rpms to make the power required to turn a bigger prop, if it's a 4-stroke, you can get by with a more marginal engine because the powerband is at lower rpms (if you don't need the speed, as in a Cub, etc). If it's a gasser, then you have even more latitude because of the torque curve.

Richard L.

My Feedback: (24)

For a normally aspirated 4-stroke such as a Saito or an OS, my general rule of thumb is one pound of weight per 0.1 cubic inch of engine displacement. So a 6.5 lb plane would require a 0.65, a 9 lb plane would require a .91, a 12 lb plane would require a 120 four stroke and so forth. Sorry, I don't fly 2-stroke.