Several weeks ago, a couple of gentlemen from the RC Powers forum Ian Bott (UK) and Bimo Adi Prakoso (Indonesia) approached me about building, testing and providing feedback on their co-designed F16XL which they called "Garuda" which has significant representation in the national emblem of Indonesia.
I was humbled and excited that they would want my input on a plane still in development and also saw it as a great opportunity to learn the aerodynamic properties and performance qualities of a cranked arrow delta wing plane with a single vertical stabilizer and rudder. A win/win for both of us and I still feel a bigger win for me as I got to fly this very cool looking and flying before it was available to the public. Here are some pictures both before and after paint.
- Plans for the F16XL Garuda - proceeds for purchasing the plans are a donation to help out under privileged children and their families at a school where Bimo helps out in Indonesia;
- RC Powers Build thread where you can read the history of this park jet (I built MK 3);
- Wikipedia link for more info on the real F16XL;
- Some raw flight video I shot before plans were released to the public; and
- A detailed walkaround video I shot discussing my build and the plane's performance after accumulating 25+ flights on the plane.
- KF4 airfoils. Also as part of this, I would use experience from "dialing in" the KFs on the NAMC Mig-35 and Mig-FA to determine best KF size in relation to wing chord;
- More powerful motor. I decided to go with the 2212/6 2700 Kv motor and 6x3 EMP prop combo for a bit more speed as previously they had only tested with the 2826/6 2200 Kv motor and 6x4 APC prop combo; and
- Rudder. I decided to try a modified rudder setup of my own design and dimensions.
The cranked arrow delta wing design is a great combination for slow speed handling and high speed runs and manoeuvring which is what General Dynamics was shooting for when they designed it. The wing is shaped like an arrow head (hence the arrow) and having the last part of the leading edge "cranked" away from being a pure delta, gives good slow speed handling and stability while still maintaining good high speed characteristics that one would find in a pure delta wing. So a really good compromise that translated very well over to this model park jet. I will say that Ian and Bimo did an exceptional job of capturing not only the scale looks, but also the scale performance of the F16XL.
KF airfoil size/CG relationship
The original KF dimensions from the plans were approximately 59% of wing chord at the wing root and 52% of wing chord at the wing tip. These percentages are quite large compared to what I am used to, but I had never flown a delta wing plane with KFs before, so I wanted to see how it worked out.
With these larger KFs, the plane was definitely super stable, but I could feel that it was a bit sluggish and slow in the pitch and at top speed and would sometimes fight itself a bit in turns. Before flying, I had marked the wing root and wing tips of the KF where 50/45/40% were so I would have an idea of how much I would be removing.
The CG not moving was a bit unexpected for me. When testing the Mig-35 and Mig-FA, as I removed foam from the trailing edge of the KF, normally the CG would move back. It wouldn't move back in proportion to what I removed from the KF however. For example if I removed 1/4" from the back of the KF, the CG might only move about 1/16", but it did move.
So using what I had learned from the Mig-35 and Mig-FA for my next adjustment I marked the side of the fuselage and KF where the CG would intersect the fuselage and then using essentially the same angle, tapered the trailing edge of the KF removing more foam all along the back. This made for a marked improvement in the plane's agility and responsiveness with no noticeable loss of slow speed handling or high speed stability. I ended up with KFs that were 47.5% at the wing root and 42% at the wing tip. Yet the CG remained in exactly the same spot again, even though I had removed 4.8 cm/1.9" of foam at the root and 1.3 cm/0.5" at the wing tip.
So I am wondering if a delta style wing does not have it's CG and CP (centre of pressure) affected as much through KF manipulation as a standard wing configuration with a separate tail set up? Perhaps it is possible as the wing chord is so long in relation to the overall length of the plane. Either way, I found it very interesting that even with as much KF material as I removed, the CG never moved.
I'm also wondering if a more pure delta wing like the F16XL likes a larger KF in relation to wing chord than say a Mig-35 for example. Our magic percentages with both the Mig-35 and Mig-FA were approximately 40% of wing chord at the root and 35% of wing chord at the tip. Perhaps more testing is needed, but when I modified the KF4 on the F16XL to the numbers above, it just seemed to find it's overall "sweet spot" of best compromise of stability and agility.
Single fin rudder performance
Almost every park jet I have ever flown with a couple of exceptions have all been twin tail airplanes with dual rudders. So this was one of the first times I had a chance to really experience and test a single tail vertical stabilizer/rudder combo. Of course mounting the rudder servo and fashioning a good pushrod and running the servo wire is a bit of a challenge with a mid motor prop in slot park jet. The rudder servo pretty much has to go on the back of the plane and the servo wire routed around the prop slot, normally requiring at least one servo extension wire. Here is a picture of the linkage I made using a combination of carbon fibre rod and a piece of metal pushrod made into a "Z". This is definitely not the most efficient or effective setup as I am not pushing the control surface in a straight line from the servo and I do lose some effectiveness due to flex in the pushrod. If I was to do it again, I would put the rudder servo up higher and mount it in the actual fin above the yellow reinforcement formers so that I could create a short, straight pushrod.
So overall, it was a real learning experience for me to fly this very unique cranked arrow delta wing plane with the single tail. Ian and Bimo have certainly created a very unique looking and exceptionally flying plane in their F16XL and I humbly thank them again for the opportunity to build, fly and test this awesome park jet and also for their great friendship and support :)