The premiere issue of FlyRC magazine has an article on converting a Sig 4 Star to electric. The author mentioned a "magic wing loading rule" of 1 pound per 100 square inches of (I triple checked the units given in the article). In conventional wing loading units, this is 23 oz/sq ft.
What is so special about this number?

Nothing really... The "magic" number is HIS personal preference for how a certain size model flys.

Try that wing loading on the little 4 inch span "Fly Powered" models. (1940 Model Airplane News article on how to do them... and there was a website selling kits for them a couple of years ago... might still exist) At 1 lb per 100 sq-in... they wouldn't fly for beans. Lead brick syndrome.

Try that wing loading on a 6 meter span sailplane.. the darn thing would be a mess. First thing you'd want to do would be stick 5 lb lead in it to make it be able to penetrate a 5 mph wind.

Target wing loading has to consider the size of the model, and the way you want it to fly.


The "Magic numner" is... he's trying to mantain the same characteristics that make the Four-Star 40 a good airplane for the way it was designed to fly.

FHHuber, thanks for the reply, that makes sense. When you refer to "size" of the model are you referring to the wingspan (mostly) or some combination of the span and weight of the model?
How does one find out about the relationship between wing loading/model size/performance?
Generally, what happens to the performance of a model as wing loading is increased?

That's a wing loading from the 70's. Nowadays it's considered a heavy plane (if we're talking up to .60 size aerobats).

Back in the 70's and 80's, planes were way over-built. They had thick plywood doublers from the firewall to the rear of the wing saddle, triangle stock on all four corners of the fuselage running nose to tail and basically no weight-saving measures. They were designed trying to survive impact.

We figured out that planes like that were more likely to impact the earth because they were heavy. And to make it worse, their weight made them more susceptible to damage due to more inertia.

Now we do everything we can to strip weight from the plane so it will fly as well as it can. Lighter planes are more aerobatic (respond faster and maneuver in a smaller area).

Point being that rule of thumb is out of date.

If size is constant... and wing loading ncreases... minimum speed to prevent stall increases, because a higher angle of attack is needed at any given airspeed.

Keep the engine and propellor cnstant too... and maximum level flight airspeed decreases. Again... this is due to the higher angle of attack... effectively presenting more frontal area and producing more drag. (simplistic... but good enough.)

Keep the same structure with the increased weight and you have an airplane that can not survive as many G's in aerobatics. (Its a really bad idea to loop a bomber with a full bomb load.)

The heavier airplane will be less susceptible to wind gusting effects. More mass with the same basic area for the gusts to work on means it takes a stronger gust to bounce the airplane around as much.


When dealing with an electric model... you have a problem f having to carry a HEAVY battery. that and motor technology kept electric models from progressing well for a long time. Newer battery technologies and motor designs have changed that... yo can now get a model that with the electric motor and battery will weigh LESS than the glo power version with full fuel and have a longer flight time with the electric. (but its still a little expensive to do with planes larger than the indoor models you see advertised...)

Eventually... I expect electric power to dominate many areas where glow power dominates now.

There are some rule of thumb guidelines which are --crude --but get you into the ballpark.
The problem is that these guidelines all shift , if you look at a sport flyer type -vs a good aerobat vs a "scale type flyer.
The last set of guidelines is for the old "warbird" set.
The standard gag was that warbirds flew scalelike
The realworld was that most of them were simply overweight .
I remember looking wide eyed at "how to" columns , which explaine how to trowel on automotive primer onto plywood , in layers , etc., to make panel lines.
Also watching footage of the great builders whose scale planes simply shed their retracts on "landing".
Some very rough parameters for powered aerobatics:
electric aerobats under 300 squares -- 5-8 ozs per sq ft--10-16 ozs all up
little 40 size areobats -if they used that 1 lb per 100 squares , would be at almost 23 ozs per sq ft.
That is awfully high.
but go to a 1200 sq in 12 lb model -23 ozs per sq ftis a good loading (1 lb to 100 squares)
Now the ratio shifts the otherway
2000 squares can easily fly 28 lbs. (32 oz per sq ft.)
many older kits were simply grossly over weight.
The CAP 20 kits once sold by Great Planes earned a reputation of "snappin Caps".
Caps are NOT snappers - just heavy the heavy ones.
there ain't no magic number - but the rapid change in RN at these small sizes,really shows up.