For the most part, these two characteristics of size and weight are closely combined, the bigger the plane, normally the heavier. Additionally, the bigger the size and wingspan, the more drag which definitely takes away from a park jet's performance.
As mentioned, I am basing my thoughts and opinions on using the most popular park jet power setup, the 2212/6 2200 Kv motor with a 6x4 APC prop. As a baseline, in numerous tests, I have found motors in this size range to produce anywhere from 25 oz/709 gr to 30 oz/850 gr of thrust depending on the motor. Quality control and consistency is not always the best, but most are around the 27-28 oz (765-794 gr) range thrust wise. So to maximize the amount of power you have available, it is very important to consider size and weight when selecting a park jet in my experience. Too much drag and/or too much weight are two of the biggest detractors to good park jet performance.
First, I will discuss size. I have found through experience that about a 27"/685 mm wingspan is the ideal "sweet spot" for a park jet. It gives the plane good presence in the air without having too much frontal drag and just seems to work really, really well. With good build techniques, for most planes of this size, it is still relatively easy to keep them light so even with the 2212/6 2200 Kv motor setup, you can still have decent speed and plenty of acceleration and power for aerobatics. I use this 27" wingspan as the baseline to determine other scale dimensions like elevon span, etc.
For several of the recent planes I have built (Parkflyers International Su-35, RC Powers Su-27 and F-22 V5s), I sized the plans when I printed them off to ensure I had a 27" wingspan and they have all performed exceptionally well.
Another thing I like to do is determine the ratio between the length and the wingspan to see how close it is to being a scale representation of the real airplane. Not only does this confirm to me that it is a close scale size of the real plane, but lets me know if it isn't, what sort of behaviors I might expect. So first I go to Wikipedia to find the length and wingspan of the real plane to calculate a ratio.
I will use our NAMC Mig-35B as an example. Looking at Wikipedia, we find the length of the real Mig-35 to be 17.3 meters divided by the wingspan of 12 meters (I find using the metric dimensions easier to calculate a ratio rather than feet and inches) gives a ratio of 1.44:1. So multiplying 1.44 times 27" which is the wingspan of the model, to be scale, the model should be 38.9" long. It is in fact 38.5" long, so very close to scale. I have found that for best scale looks and overall performance, being within about an inch of true scale dimension to be very acceptable. Of course there are always exceptions to the rule, but having the length and wingspan ratio close to scale dimensions normally helps keep everything in good proportion and provides overall best flying characteristics.
Why is this important? Well, the F-18 V3 which I already recommended as a good starting score and fold park jet is about 2.5" too long when compared to the scale dimensions of the F-18 E/F Super Hornet on which it is loosely based. This means the overall length/lever of the plane is a bit out of proportion, which slows it's performance in the pitch down somewhat. However, again this is not a bad thing in my opinion for a starter park jet as it slows things down allowing a newer pilot a better chance to keep up with or stay ahead of the airplane.
If you look at the recent RC Powers Eurofighter V5 build I just completed, that model is almost 6" too short, yet the vertical stabilizer and canards are scale size, so this throws things out of proportion somewhat and I believe contributes to some of it's instability in any sort of wind.
So not all the dimensions of a plane will be, need to be or should be scale, sometimes to make the plane fly better, designers need to take some licence with the scale dimensions. For example, when Stephan updated the Mig-35A to the Mig-35B, one of the biggest improvements was increasing the wing area by about 10%, helping to really open up the slow speed handling of this plane. So as always, there will be aerodynamic and scale compromises, but if one or more things gets too far out of whack, it will impact how the plane flies, so it needs to be considered when trying to determine which park jet is right for your or how you might modify it to suit your flying needs and style.
Weight is also very important to consider when looking at a park jet's design. Early in my park jet career, I was a card carrying member of "overbuilder's anonymous". I was stuck in a mind set that when in doubt, add more glue or more tape, it will make the plane stronger and more survivable. I could not have been more wrong. I look back now and shake my head to think I actually had built planes that weighed over 2 lbs! No wonder they never flew well :/
It is simple science really, the heavier the plane, the higher the wing loading, the faster you need to fly it, the less responsive it is...I guess you know where this is going :/ Heavy plane, not too responsive, too much speed, crash is not going to be pretty even with all the extra glue, tape, etc. As I mentioned in the intro, most of my early crashes were spectacular if nothing else, but rarely repairable. When you have too much mass and kinetic energy coming to a sudden halt in a tree or on the ground, the foam cannot withstand the punishment. As soon as I got out of this mindset and starting building my planes lighter, I turned a huge corner in my park jet career. First, my number of mishaps was reduced immediately as my planes were easier to fly, didn't have to be flown as fast and were far more responsive when I did get myself into trouble. And when I did have a mishap, which I still do on occasion, just part of the hobby, they are far less damaging to the plane and normally easily repairable.
Although sometimes very tough to tell until you actually look at a set of the plans, doing some reading and research can help you answer a couple of important questions.
- Are there a lot of parts? Some designers have a lot of parts for scale looks or increased strength and structure. I know it seems a bit hard to believe that a few extra pieces of foam would contribute to a plane being heavy, but grams add up to ounces and so on. Starting off with a lot of foam in the build normally means a lot more glue to bond all these parts together and means you might have to make some sacrifices in equipment (servos for example) or paint you can add later in the build to remain at a decent flying weight;
- Does the design call for lots of reinforcement? Some designs by their nature require extra reinforcement in the form of carbon fibre, bamboo BBQ skewers, etc. Also, some designers recommend all kinds of reinforcement, so it important to take this into account, perhaps you don't need it all and it might be prudent to ask the designer what their experience has been to recommend all the extra reinforcement.
So hopefully this article will have given you some food for thought about how important the starting size and weight of a plane is from the beginning of the build. It is always much easier to add a little weight later to get good balance or help with wind penetration and stability, but if the plane starts off too big or too heavy for your power system right from the start, it is much harder to lighten the plane later without sacrificing performance or what you want out of your plane.
In the next article, I will discuss wing size and shape and how the properties of each wing affect the plane's performance.