The Importance of Wing Loading The wing loading of a model aircraft is vitally important if you are to attain the performance you expect. Most of us have seen the effects of too much weight on a model when airspeed is lost. Instead of the gentle mush at the stall we see instead a quick wing drop which the pilot, for some reason, usually tries to correct with full up elevator! This, of course, results in an instant flick and spin into the deck! Another symptom often displayed by the overweight aircraft is when lots of elevator is applied in normal flight. Instead of the nice loop he expected the pilot is treated to a flick roll! The pilot usually releases the sticks immediately but by this time the model is inverted, panic sets in and the pilot is unable to correct the situation, usually because he's putting in massive control corrections which are themselves causing further stall/spin events. Soon all is lost and the model explodes into the ground at high speed in a shower of shattered balsa! We've all seen it too many times. What we've also seen is the guy flying a fun fly type model, you know, the Panic/ Limbo Dancer type, and recovering from extreme low level "situations" with ease. How can this be? Well, it's all down to the wing loading. Most fun fly types have very light wing loadings that are more usually found on gliders! Something in the region of 10 to 12 ounces per square foot, and it's this that enables them to recover from a stall situation almost immediately with only a tiny increase in airspeed or decrease in angle of attack. Add to this a high power to weight ratio and thick airfoil section and you have a machine which can be safely flown into ridiculous situations at low level. Of course, a lot of this is horses for courses, as they say. Even if it were possible to build a Spitfire with a 10 ounce per square foot loading, it wouldn't be a lot of use unless it were very small. It would be bounced around by the merest zephyr and wouldn't display the fast, smooth trajectory of the full size at all. A Spit' would require a loading of 20 to 25 oz sq ft to perform as you would expect, and the pilot would have to be fairly experienced and be able to detect the onset of a stall instinctively. A trainer, for instance, would want a wing loading of around 16 to 18 oz sq ft. This is so it's heavy enough to not be blown around by the wind too much, and also to fly at a reasonable speed for good penetration and control surface effect. It would also be able to fly quite slowly at this loading and recover from a stall fairly quickly and so be forgiving to the newcomer. So we can see that the wing loading has a lot to do with the way in which a model flies. I once loaned a plan of something I designed to a friend. I told him from the start to keep it light because if the wing loading got higher than around 18 ozs sq ft he'd have a handful. He duly built the model and was soon complaining that he couldn't get it in the air. The model was designed for a .26 to .40 four stroke and he'd fitted a .15 two stroke! The next thing I heard was that he did get it aloft but it flew around nose high until eventually it tip stalled and went in. I asked if the center of gravity was in the correct position. He said he wasn't sure, so I asked if he'd added any weight to the nose, he said not. So, he was flying it with the CG of about 50% chord! No wonder it spun in. Next he repaired it and fitted a .20 two stroke and had added weight to the nose. Now it took off and flew fairly well until he tried a tight turn or loop whereupon it would try to flick. Ring any bells? You guessed huh? He bought it round one night and I put it on the scales. It should have weighed 3lbs 4ozs but his weighed 4lbs 4 ozs! Small wonder he was having problems, it was a massive 30% overweight! That worked out at 5 ozs sq ft extra wing loading. An awful lot for a 42" span model. That was an extreme case to illustrate how the extra weight had turned a sweet flying model into one that was barely flyable at all! Mine is still flying and has been one of my all time favorites. A 10% increase in wing loading will mean 10% longer take off roll, higher landing speeds, sluggish loops, vertical climbs will be shorter etc etc and recovery from any nasty situation is going to take much longer to achieve. So always do your best to achieve the designers target weight, it can make a big difference. It's difficult to say what the wing loading of such and such should be but as a general rule don't go much over 25 ozs sq ft for WW2 type aircraft and aerobatic types. Sport types and 3D models will want to stay below the 20 ozs sq ft mark. Vintage types, both scale and model, will be well under 16 ozs sq ft and the fun fly types too. A lot also depends on the size of the wing too. The bigger a wing the more efficient it becomes. For instance, if I were to build a 60" span model of a Robin Alpha it would have to have a wing loading of around 20 ozs sq ft to perform well, whereas the full size Robin Alpha has a wing loading in aerobatic mode of 12.6 pounds a square foot! It's all due to a thing called Reynolds Numbers. I've built a 80" wingspan CAP 21 which weighs 14 pounds and has a wing loading of 30 ozs sq ft. This should fly fine because the wing is so much bigger, but that kind of loading would be far to much for, say, a 54" span model of the same. I imagine there are a lot of people who were terrified to fly their first large model because of the seemingly massive wing loading! There must also be an awful lot of annoyed designers getting complaints off people who think that their design doesn't fly very well, when, in fact, it's all down to the builder who's beefed up the structure etc and totally vandalised the in flight behavior of the model! So do try to meet the designers target weight for the model, as I said before, it can make a big difference. G :-) FF-02-11-01