Electric powering a Biplane or twin

[i12fly]

We generally accept that the power required for a moderately aerobatic monoplane is 100 W/lb.

If the model is a twin engine would it be appropriate to drop that figure to be equivalent?

If the model is a draggy biplane, what sort of adjustment should be made to this general rule?

The models are already built and are actually a 'modified' PM Tiger Cub and a heavy Puppeteer at 9.5 lb weight, but I'd really like a rule of thumb guide to cover future projects too. The Puppeteer flies well on an SC70FS but I don't want one odd IC model in my fleet, so currently doesn't get flown. So for this do I need 950 W, more or less?

[Nigel R]

Short answer, no difference.

Longer answer, biplane drag will affect top speed much more than anything such as climb rate.

For a twin, power loading is power loading regardless of number of motors. Consider, two motors belt driving one prop.

[Peter Jenkins]

Posted by i12fly on 30/07/2019 23:13:51:

We generally accept that the power required for a moderately aerobatic monoplane is 100 W/lb.

Depends what you mean by moderately aerobatic. A competition aerobatic model (F3A) has around 250 W/Lb or more. Like I say, it depends on what you mean by moderately aerobatic but I would not consider 100 W/Lb adequate for even moderate aerobatics.

[Allan Bennett]

I've got a few electric bipes and a twin, and I don't remember using anything other than the usual guide watts-per-pound. Unfortunately I don't have my data with me, so can't give exact numbers. A bipe generally being slower flyer than a monoplane, I suspect that the same power but lower 'gearing' (i.e. larger diameter but smaller pitch prop) is the answer.

[Denis Watkins]

Your numbers are right in the ballpark

The 70FS is comfortable with a 9.5 lb draggy airframe as this motor outputs 1.1kw at top end

As Nigel says, no difference

And answer to your question, no less than 950 W

[Bob Cotsford]

When it comes to the Puppeteer I'm going to disagree with Denis to a degree as I've seen these fly just fine on a 52 4 stroke. You need to decide what sort of performance you want. With all that wing area it should fly quite happily at 70-80W per lb, but it will perform like the WW1 bipe it aims to represent, eg dive to gain speed for a loop. For a modern sport model level of performance Denis is right though it may not look the part. My 5lb 1 1/2 strutter is quite nippy on much less than 400W, on the other hand my WotsWot is 'adequate' on 1000+W.