Do bigger scales fly better
 

Having just watched some Guillows conversions on you tube i noticed how twitchy they looked in the air. Am i right in thinking that the same plane will fly better in a bigger scale? I'm sure theres more to it but i thought i'd ask the question.

Everything I heard says yes. Easier to see, more stable. But the pucker factor still goes exponentially higher with the $$$!

Short answer is, yes, for a number of reasons.

The smaller the plane is, the "twitchier" it is and the more it gets bounced around in the air by wind and turbulence.

Yep, all things being equal bigger flies better. There is a good scientific reason for it, a thing called Reynolds Number which basically means aerodynamics works better at larger scale, in addition to being less effected by wind and turbulence.

Small planes do tend to survive crashes better though.

That one's got a good scientific reason too, the inverse cube law. :)

Yup
Bigger generally flies better in model airplanes. Along with that, the power required to fly the model goes up with perhaps the cube of the wing span. And, along with that, the cost of the model goes up the same way.

A larger plane will perform slower then a smaller one, but it will be more stable and less effected by wind and "micro burst" and changes in wind. Almost without exception.

But it has to do with a lot more then just weight and wing area. A million things come into play. If you did the wing loading of a large airframe, it would seem very heavy and like it would have to fly very fast without stalling. Part of it is also the size of the model makes it look like its flying slow just due to scale.

Someone once checked the wing loading of those military fighter jets. Seems they have pretty much the same wing loading as a cast iron manhole cover, tossed like a Frisbee!

Enough power, and anything can fly.

Responsiveness drops off with size. A maneuver that would take a full-sized plane several seconds (e.g. loop, roll, etc.) might take a tenth-scale plane (36"-48") a second to do, but will only take an ultra-micro (12"-18") a fraction of a second to do. You can have an ultra-micro upside down in the time it would take a full-sized plane to roll a few degrees.

Yes... within limits.

You can get amazing response rates with larger models by using larger control surfaces with large deflection angles.

But if you just scale up a model you will typically get something that can fly slower and/or carry far more weight. Slower is generally good for beginners.

Bigger planes also tend to have more mass to resist the effects of minor wind gusts.

The wing loading issue is a simple matter of physics. Lift is a function of the CUBE of scale. Double the wingspan and you have multiplied the wing area by 4 and multiplied the air volume affected by the wing (at any given speed) by 8.
You can't quite carry 8 times the weight at the same speed... but its close.


The wing loading of the full scale aircraft may seem huge compared to our models... but scale factors make it so its not really all that bad.
There have been a few airliners now that have GLIDED to safe landings in excess of 100 miles after lost of all engines.

If you scale up everything, though, your mass/weight will also be four times as large.
//edit eight times as large

But the larger point is that, yes, bigger planes tend to be more stable.

a) You cannot scale the air and wind - so that stays same whatever model size. Therefore a larger model can suffer air characteristics more than a small model. Gives appearance of more stable.
b) Orientation and sight of model being bigger makes it appear easier to detect what model is doing and correct accordingly. Gives appearance of being more stable.
c) Size of model as it increases gives appearance of flying slower, even though it's likely flying at same or even higher speed than the smaller. Gives appearance of being more stable.

So final result ? They give appearance of being more stable.

;)
Nigel

Then I would say appearance is everything LOL. All my biggies fly better than my park or micros. Of course "better" can mean different things. Stable? yes. Maneuverable? no.

neither is better,,,,,,I can't fly mid-sized either,,, But Try I Will!!:oops:
My two fave's are the Ultra Micro E-Fite Extra 300, and my six foot Gentle Lady:roll::red::Q
bubsteve

One thing about those little foamies. They can be much harder to fly than the "Big" ones. The required reaction time on an acrobatic high performance foamie is much shorter than a giant scale model.

So if you can fly one of these foamies under any acrobatic position, that really helps when you bring out your $$$$ larger models.

Its closer to 8 times the weight for double the scale
The larger models tend to have 2/3*(scale^3) relationship to the small ones, resulting in REAL lower minimum speed for sustained flight. (if using similar construction technique)


The structure, if directly scaled up, would cause the larger model's weight to go up with the cube of scale. (just like the wing's lift capability... its not exact)
The structure on very small models tends to be beefed up to withstand being handled. They don't need as much structure to fly as to put up with being picked up and cleaned and to have batteries inserted/strapped in.

So we don't scale the structure up directly.
We commonly use 1/16 inch balsa sheet ribs for a ".90 glow" (appx 1500-2000 watt electric) size model.
For flight a 1/128 inch thick rib would be fine for a 22/10 outrunner powered plane, but you'd shatter the ribs just wiping off dust from storage.
We use the same thickness "Monokote" on the small models as the large ones too...

On larger models we commonly taper the spars or use other methods to vary strength with with distance from the wing root to save weight where in the small ones we leave the spars full dimension from tip to tip, again to withstand handling for transport/cleaning/maintenance.

This wasn't quite so true a few years ago... I have an old Goldberg Yak 54 ARF that is appx 74 inch span (forgot exactly) and the airframe weighs more than the modern 88 inch ARF by ExtremeFlight. (glow, gas or electric are possible with these... they do well with 4000 to 6000 watt e-power systems. 5000 watts is more power than a DLE 55)

Yeah
I've got two Hacker A50 motors. The one that gets the most flying is the Hacker A50-16M, powered by a 12S2P A123 battery pack, running an APC-E 19X12 prop at 6750 RPM on the ground and 7200 in the air. This motor is running about 2700 Watts on the ground. That's about 3.5 Horsepower.

A recent Fly RC magazine did a review of the OS GT33 gasser engine. Interesting, running that GT33 engine through a fake motor design in www.motocalc.com, my Hacker has some 25% more horses on the propeller.

The problem with the giant electric conversions, not only does the models weight increase around the third power of the wingspan, so does the horsepower required to fly it. And with big electric motors, three or four horsepower means a lot more $$$$.

Here's one that'll fill the bill:D,
bubsteve

While I agree larger is better it is not without some different challenges.

The UM airplanes are kinda magic. They are so light - than they don't destroy themselves when they crash (not always at least). The have such low stall speeds they are pretty amazing at times.

The large scale planes generally (almost always) have higher wing loadings than some of the small parkflyer type models. Some have many times the wing loading of their small counterparts.

One thing I am always careful of when flyign my large scale models (especially WW2 warbirds) you really have to pay attention to airspeed. I guess it is safe to say that is important on all airplanes but especially so with the big guys. I have watched pilots transition from smaller electric models to large heavy scale models and watch them fly far too slow and then are rewarded with a nice stall, spin then crash.

I have flown a couple that exceed 30oz/sq foot and you flat have to mind those planes ALL the time. But those are also the models under full power that just command the air, beat it into submission. They are impressive and really don't see much impact from the wind.

But that challenge can be fun too. :)

Mike

True RCers, I land the monkeyChopper twin at around 20MPH and the big ones don't just settle in , you must fly-um in all the way,,bubsteve

Yeah
Some 15 years ago I had a giant scale Piper Cub with a 10 foot wingspan, powered by an Astroflight geared 90 and 38 Nicad cells.

One day I was running out of battery power, and flew it to slow, stalling it. Got it back in one piece, but never saw a model that big suddenly get weird at about 20 feet off the ground. What saved the model was ignoring the ailerons, and hitting the big rudder.

But isn't it true that you should take wing cube loading into account instead of wing loading?

That really depends on what you want it for.
Normal 'wing loading' is what is used in aerodynamics and is what you need for figuring out stuff like how fast the plane's stall speed will be. Generally speaking a light wing loading plane will be able to fly slower without stalling than a heavy loaded plane.

'cube loading' is a pretty much abstract concept used only by RC modellers, but does come in handy in figuring out how a plane will 'feel' to fly.

Yup, that's true.
What would be a fairly light wing loading on a wet turbine jet model would be so heavy it would not fly on a typical Small foamie with a wingspan on the order of 10 or 15 inches.

Looking over some of the models I've either built, or ran the numbers on (The F7F) are listed below:

A few of the models I've built.
Glider, Wingspan 72 inches, area 450 Sq In, Wing loading 7.6 Oz/Sq Ft. Power, 300 Watts.
Cessna 178, Wingspan 78 inches, area 1118 Sq In, Wing Loading 38 Oz/Sq Ft. Power 2200 Watts.
Giant Big Stick, Wing Span 80 Inches, Area 1520 Sq In, Wing Loading 24 Oz/Sq Ft. Power 2800 Watts.
Edge 540T, Wingspan 50 inches, Area 490 Sq In, Wing Loading 18 Oz/Sq Ft. Power 470 Watts.

A model I ran the numbers on for a club member.
F7F Tigercat: Wingspan 132 inches, Area 980 Sq In, Wing Loading 81 Oz/Sq Ft. Power over 5000 Watts

That Tigercat was for a club member who wanted to convert this big heavy twin to electric power. I talked him out of it. That same model was designed for a pair of BIG glow engines, and even with that, its wing loading was some 55 ounces per square foot, not counting the pair of 20 ounce fuel tanks those glow engines would require.

Compare the Edge 540T and the Giant Big Stick. Going from a 50 inch wingspan and 490 Sq In to an 80 Inch Wingspan and 1540 Sq In requires 2800/470 or nearly 6 times more watts on the motor. Electrifying Giant Scale model airplanes can get expensive real fast.

Not sure i agree there.

The foamy would fly if it had enough power but it would have to fly at a similar speed to the turbine model, which would appear incredibly fast for a small foamy. But it would actually be no faster in real mph terms than the turbine.
This is because small models (all things being equal) 'look like' they are flying faster than big ones and are 'twitchier' on the controls.

Wing loading determines the actual minimum flying speed and manoeuvrability in absolute terms. Cube loading gives a guide to how the model will 'feel' to fly.