Scaling up Free Flight Plans

[rc capo]

I have tons of free flight plans scoured from the web and want to electrify some.

A lot of them have wingspans less than 30 inches.

Besides just scaling the plans up to the correct size...are there any hidden gremlins that need to be addressed when enlarging a set of plans?

My intentions are to fly them out doors as a Park Flyer.

thanks

rc

[steve lewin]

It sounds like you are talking about fitting RC as well as scaling up.

Obviously you'll need to adjust the material sizes and depending on how far you scale up you may want to add more wing ribs and possibly more stringers etc in the fus. You can't just just make the ribs etc. thicker but with huge gaps between them.

The incidences and CG position etc will be different as RC models because the stability requirements are not the same as for FF. FF models you want as stable as possible. RC you probably want the controls to have some effect ;).

Steve

[rc capo]

Yeah, you're right...I forgot to include that I was going to fit it with RC gear, esc, motor etc.

thanks

rc

[Ollie]

Depending on what the scaling-up factor is, the various parts may not result in stock material thicknesses, etc. this raises the question of whether to go to the next larger or smaller stock size. When scaling up a structure, the strength increases as the square of the linear dimension but the weight increases as the cube of the linear dimension. The larger the model, the more damage that occurs in a crash. A small rubber model might easily survive a cart wheel arrival with little damage but an electric model two or three times larger in linear dimensions may be totaled in a similar landing. You may want to consider breakaway or rubber banded wing mounting to limit failure propagation. Also, some consideration should be given to limiting the damage a loose battery can do in a sudden arrival.

[BMatthews]

I too have been blowing up a lot of plans for converting rubber scale models to parkfliers.

Ollie raises some good points. Obviously the load on the structure will be higher with the gear in place even if you DON"T have to scale up the plans. Wing spars will need to be increased in size and/or number and fuselage longerons will need some upsizing as well as the tail structures to allow for hinge lines and a little more structural strength.

For simple box structures 1/8 sq will work fine as long as you don't have any hard "arrivals". Half shell types with lots of stringers can use 3/32. Wingspars will need to be bumped up to 1/8 x 1/4 and perhaps some carbon tow added to the top and bottom for 6 to8 inches out from the root. In some cases it may be more prudent to use spruce. Or if it lends itself to top and bottom spars hard balsa with 1/32 webbing will work great. Rib spacing will often need some adjusting with extra ribs put in to combat the "starved dog" look. But strangely enough if the scale spacing is used and it's only an inch or so apart then I'd have no concerns about using 1/32 for the ribs. The tail surfaces will either be from 3/32 by 3/16 or perhaps 1/8 sq depending on the shape you want.

What I'm describing will result in a very light model that is only really suited for the small GWS and similar motors where the model has an AUW of around 6 to 8 oz. If you're intending on useing stronger motors like the 280's then you'll want it to be a bit stronger but not by much.

Good luck.

[Russell2]

I'm wanting to build a simple, slow R/C electric 6-8 oz AUW powered with one of the smaller GWS geared motors to take on extended business trips. I'm thinking 28" hi wing sheet balsa, ~ 20 watts input. I've seen a minisport in RC MicroFlight, but would need to scale that 17" span up, have plans for a PA-18, but would need to scale that 48" down. I build mostly from scratch, but have never adjusted size on a scratch-build, appreciate any advice regarding balsa sizes for this project.

[BMatthews]

Russel, by all means make the fuselage and tail out of the lightest 1/16 sheet that still offers some stiffness and strength but do consider a built up wing. There is a lot less drag and a lot more efficiency compared to a curved all sheet wing and, done right, it will probably build up lighter to boot. You'll need to add a small doubler of additional 1/16 sheet or 1/64 plywood at key points like the landing gear mount area to spread the loads out into the wood but other than these specifics light 1/16 will do ya for this size range.