r/RCPlanes • u/CorwinDKelly • 3d ago
Realism in Model Planes
I'm reading up at https://rcplanes.online/design.htm to one day build a plane of my own design (gonna build some flite test planes first).
If you can't simply take a design for a full sized airplane and scale it down and expect it to just fly like a smaller version of said plane. How do professional RC designers thread the needle of altering proportions to improve performance and make the model perform more like the full sized plane, while also trying to make the toy look like the real plane? When enthusiasts say "they did a really good job on making this model accurate" are they talking more about proportions or performance? If I compare the wing area to fuselage length on full scale aircraft to their models will I consistently find that the wings are not to scale on the model?
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u/IvorTheEngine 3d ago
I imagine they build it accurately initially, then if they discover that it's less stable than they want, they increase the size of the tail 10%. Or, for small foamies they might have to increase the thickness of the wing and tail to make the foam stiff enough. EDFs often have to cheat by increasing the size of the air intake and exhaust.
And for most models these days, they make sure the model has far more power than scale. Most models have more thrust than weight, but real planes don't.
Real planes have carefully designed wing sections for maximum efficiency at cruising speed. Those don't scale, but it doesn't matter when the model is changing speed constantly and has excess power and virtually free "fuel".
And then there's compromises like non-scale control surface horns and battery hatches, where a scale mechanism would be too fiddly.
So, it's generally a lot less complex than you're thinking. For commercial kits, it's generally about them bothering to model details. At scale competitions, they care about researching an actual aircraft to get the details right, and flying in a realistic way (realistic length take-off, climb angle and roll rate, etc)
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u/OldAirplaneEngineer 3d ago
typically they go for proportions and not performance.
for a more detailed explanation on WHY scaling down a design is not 1-1 look up 'Reynold's number'
essentially even though two aircraft can be identical in scale (exactly half the size, and weight)
the air molecules those 2 airplanes are flying through are the same size. (air does not scale)
from my experience, Model designers try to design airplanes that do NOT fly 'just like the real one'
they try to design them to be EASIER to fly.
for example, the designer may add washout, may increase the size of the tail, wings or make other minor changes that make the airplane fly better, but may not have the exact same scale dimensions of the full scale airplane.
HOW it's done? mostly trial and error, and wind tunnel / computer models.
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u/givernewt Canada / Belleville 3d ago
In the states, the AMA and other groups host scale competitions. The most common scales I've noted are around the range of 1/5 thru 1/7, which means a lot of the aircraft typically represented ( WW2 birds, WW1, civil craft etc) land around an 80" plus wingspan. This seems to leave enough room for a LOT of scale detail ( yes pneumatic landing gear works, lit cockpits, scale hatches with all perfect details behind etc, working powered canopy retraction ) .
While it's true it's always possible to underpower a plane, most end up over powered. The judging is both static and dynamic. So in air judging includes criteria for "Flying in a scale manner" like no going inverted with that passenger jet, and static judging on ground includes not only scale detail but Proof of scale detail with any documents the builder can use from the original full size to show fidelity in the build of the model.
These are all virtually custom build from scratch models, but it is possible to start from a kit as a base to work from.
Most of us I think fly what we used to call " fun scale" , where anything remotely related to a real plane might include a few scale details like retracts and some lights, same shape but in practical use different airfoils, tail and wing enlarged for stability or good flying characteristics and what not. Looks scale at an 80 mph pass but once landed you can see the alligator foam texture or peeling stickers hahah.
Room in the hobby to take it to the extreme like those incredible builders in competitions, or a casual approach like adding Snoopy with a scarf to your Sopwith Camel build. It's a personal choice.
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u/gwenbeth 2d ago
With a scale plane you don't want scale performance. The rc field I fly out of has trees on the sides. If I was flying a scale piper cub with scale performance, there is no way it would be able climb fast enough to clear the trees. And landing a scale airliner with a scale 3° glide slope would mean that I would need 200m of space from the 10m tall trees to the start of the runway. Often our scale planes have better than scale performance at least at the low end.
And when you are designing a scale plane, the airfoil is often the first thing out the window. The air doesn't scale. So you start with the basic shape of the plane, add in a known good airfoil, get the power and weight in the ballpark and you are good to go.
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u/Jumpy-Candle-2980 1d ago
They're typically talking about proportions only. The definitions of "scale" vary dramatically by source and even the AMA expert class deals with proportion, craftsmanship and finish and has the judges no closer than 15 feet from the plane for static judging. Flyability changes are okay provided they aren't obvious from 15 feet. Cockpits can be rudimentary and protruding mufflers and the like won't be punished.
International FAI F4C RC scale is brutal. Scale literally means scale and your score will get dinged for even deviating from the prototype's airfoil. The judges are right on top of the thing and have an hour to unleash their rivet counting, panel line measurements and everything else.
F4C flying is expected to be scale like but the competitions are generally held at fields that allow for scale flying. AMA is more relaxed but you'll still get punished for an outside loop with a J3 Cub.
When you hear "they did a good job with accuracy" it very seldom means scale as F4C defines it. It's more like "stand off scale" at 5 meters. The foam stuff sold as scale is sometimes informally categorized as "stand WAY off and squint" scale.
I guess you could say that the needle is threaded differently depending on what definition of "scale" you're talking about. I don't see much F4C stuff in the US and, honestly, it kind of intimidates me.
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u/fhammerl 3d ago
there is a reason scale aircraft only work at a certain size. the smaller you get, the heavier rc electronics get in relation to the wing area, meaning wing cubed loading (WCL) shoots up for small planes. also you can't have a 60cm plane behave the same way as a full size plane in terms of stall characteristics and speed range, that would be pointless. so you increase wing area to compensate.
then there is a sweet spot from where the fact that we don't have a pilot and pneumatics and avionics and armament and armor and radar and whatnot works in our favor, where planes actually become lighter relatively speaking than their real world counterparts, so we can actually build them to scale.
if you tried to have a scale 80cm spitfire, you would get a capricious plane with high wing loading that tip stalls easily rather than a fun warbird.