Scaling Up?

[NitroDad]

Is there a real easy method to figuring out how much power I need after I scale up a plane? For instance If I scaled up a 50" bird to say 100" if the 50" bird uses a .40, surelly a 100" plane would need more than a .90? So I guess what I'm wondering is there a formula to find the right Power to weight ratio?

[Dave Robelen]

Hi Nitro,
There sure is a formula. You will need the pocket calculator to use it, but the result is right on. The formula for power is based on horsepower, and the term is the scale factor taken to the 3.5 power. Unless you have an engineering calculator, a bit of a "fudge factor" is useful. For example- If you double the size, the scale factor is 2. Take this to the 3rd power, and the result is 8. All of this assumes that you would build the new model 8 times as heavy, and really need all that power. Experience shows that models in the 100" range normally do fine with half that power or less, depending on the flight characteristics desired.
Regards, Dave

[NitroDad]

Thanks Dave, I think I get it. So if the original plane had a .40 size, then I would take that and do something like .40X2 to the 3rd power? That would be like .160 right? Or am I still missing it? So if the 100" planes fly ok with half that a .90 would proly work. Iguess if its not to heavy. Or just go with "What ever the biggest engine fits in the cowl" line of thought. :D

[Dave Robelen]

Hi Nitro,
The formula is actually for horsepower, not displacement. The actual power required will vary with quite a few things. The wing loading, total weight, airfoil, flying style, and engine efficiency. In other words, a 100" Telemaster that weighed 12 pounds would require a whole lot less engine than a 100" Mustang at 40 pounds. My suggestion would be to check out some similar models in the meet reports, etc., and maybe use that as a rough guide. Meanwhile, it is always nice to have a little too much, than not quite enough.
Cheers, Dave

[NitroDad]

Thanks Dave,
So is there a rule of thumb as a starting place? Like a power to weight ratio? Like a 10lb plane should have X amount of HP, a 20lb should start with X amount and so on, Just as a starting point? And see thats what screws me up, because take the LT 40 I have, Its a .40 size with a 70in wing span, But the Sig Seinor is a .25 but has a 80" span and phisically is alot bigger than the LT40. I'm not trying to be a pain I am just trying to learn what goes into these things.
Nitro

[Dave Robelen]

Hi Nitro,
No pain, I can understand your confusion. The problem is still apples and oranges with the Sig models you mentioned. The Seniorita is lightly built, with a low wing loading, is non-aerobatic, and therefore needs less power to do an adequate job. Meawhile, the LT-40 would fly with a .25, but the desired performance would not be there.
Using wingspan as a guideline is possibly the toughest way to compare, just based on the two examples above. I would consider the desired mission of a new model, the likely weight and wing loading, and see if there were some comparable examples in models that have been published or or advertised as kits or ARF projects. Just generally, a lightly loaded model like the Seniorita with a flat bottom wing will need a lot less power than a scaled up aerobatic model with a symetrical wing and higher wing loading.
It is actually easier with the electric models because there are commercial programs that provide for all of these comparisons. The one that I use is www.motocalc.com which is available for a 30 day free trial. If you spend a little time with this program, tossing in different model sizes, weights, and wing designs, you will see the changes in required power pretty quickly.
Cheers, Dave

[NitroDad]

Thanks Dave that program is somethin, electric stuff is like trying to read Greek but I'll give it a whorl.