engine size/wing loading

[phil myers]

Hi folks,  new pilot building my first c/l (Sig Akromaster). I'm unsure about the correct engine size. I know the plans suggest a .15 but this was for the early 70s, I was advized on another thread to go for a .10 which sounds like a good idea. (also there is no guidance as to the ideal finished weight, which is frustrating!)
My question is, is there a chart or formulea I can use to determine the correct power to wing size/loading?  . Thanks for any help on these issues

Phil

[Alan Hahn]

I'll throw my 2 cents in.

I think it would be difficult to have such a chart because the performance in engines is all over the map both in power and weight.

So if you look at 15's, you have the old slant plug Fox 15 (a real light engine) and a modern 15 ball bearing Schneurle which is pretty heavy by comparison and makes a lot more power. Then add in the Asian 15's made by OS, Thunder Tiger you get another power to weight ratio. Then put in mufflers--desirable in my point of view, and the modern engines get somewhat heavier.

So to show you how screwy you can go, I flew my Akromaster with the FP10 (a delightful combination) while I fly the "35" sized Arf FlightStreak with the Fox15 Schneurle mentioned above. It flies great on 50 foot lines.

I am always amused  when people claim that sizing electric motors for planes is so confusing  ! There is nothing "obvious" about glow engines either (except the experience is already out there!).

[phil myers]

 Hi Alun, Thanks for that, what lines and prop did you use and did you ever weigh the finished Akro? I think I'll be going with a .10 to start with!
Cheers Phil

[Steve Helmick]

The old standard "Power Loading" rule of thumb is one ounce per 1/100th cubic inch displacement...for both CL Stunt and Free Flight, oddly enough. That would mean that a .60 powered model should weigh about 60 oz, .40 ~ 40 oz, etc. As engines got more powerful (and heavier), the 'rule' has become less wonderful, but if you skip to the next bigger size, you should be ok. I'd expect a .10FP powered model could weigh 15 oz and perform well, etc.

Being as you're in England, the line size would subject to your SMAE rules (for your insurance), not ours. Maybe that's the FAI rules, or maybe not, but I wouldn't make that leap. Here, you could now technically fly it on .008"  braided lines, if the plane weighs under 24 oz.!!!! Not many would actually do that, so .012" braided lines would probably get the job.  Steve   

[phil myers]

  Thanks Steve, I understood that.. I've been trying to get that information for weeks on the internet without any joy! A few voices on other forums have said finished weight for the Akromaster should be about 16-19 oz which ties in with the kit instructions for engine size i.e. .15 to .19 cu., but as you say this was for the older, lighter and less powerful engines of the good ol' days!
I got some braided lines from ss hobbies (they're brilliant they are!) 0.12s 52ft and 60ft.
Thanks again..Phil

[Alan Hahn]

Hi Alun, Thanks for that, what lines and prop did you use and did you ever weigh the finished Akro? I think I'll be going with a .10 to start with!
Cheers Phil

Phil,
Don't have the Akromaster anymore---a result of giving my brother-in-law a chance to experience a little CL flight. He was going just fine until a little dizziness crept in! I kept the parts, thinking that I might repair it, but decided to do some house cleaning a couple of years ago.

As I recall, I did hollow out the leading edge a bit with my table saw (I was practicing for future lightening projects!). Also I went with a light piece of balsa for the canopy--just didn't like the clear plastic piece.

I did pick up another kit when I was at Sig a couple of years ago. It is a fun little plane.