I managed to get another great testing session done yesterday with my modified F18 V3M. I managed another 10 flights for a running total of 28 flights.
I am pretty satisfied that the main "dialing in" process is complete, I think there will still be a bit of tweaking for good high alpha flight, but since that is a rather extreme portion of the flight envelope, it will take some time. Also, it relies on dead calm winds and me getting my "high alpha thumbs and brain" back which could be the longest part of the process :/.
As I mentioned in my last flight report, I did increase the deflection on my ailerons, increasing it now to 5/8" travel one way (1 and 1/4" total) which gives me noticeably crisper and snappier rolls.
KF airfoils
I have decided that the KF airfoils are just fine the way they are, I think if I did any more tweaking there, I might be chasing my own tail, as it flies now, I think that the CG and CP (centre of pressure which can be greatly affected by KF dimensions I learned from experience with the Mig-35/Mig-FA) are in excellent harmony :)
When testing the Mig-35/Mig-FA, we found we got a really "locked in" feeling when the trailing edge of the KF intersected at the point where the CG met the fuselage. As we were testing and making the final adjustments to the CG and KF, you could almost feel as the CG and CP started to be in harmony.
With a more straight wing plane like the F18, this does not seem to be the case. As you can see in this picture, I made a mark on the fuselage where the CG would intersect it. The trailing edge of the KF is about 1 and 1/2" behind this point and the plane feels solid and very well behaved. To help understand what I mean by the CG and CP being in harmony, as we were testing the Mig-35, on one of the first prototypes the trailing edge of the KF was almost 1/2" behind the CG and as I would try to pull out of an aggressive loop, split S or power dive, there was a definite lag in rotation and required much more up elevator to prevent union with terra firma! Also, quite often in a turn, the nose would wander or the plane would feel rough like it was fighting itself, throwing a tantrum like toddler. As these two came closer together, it became a much happier park jet. Anyway, those were the behaviours I was looking for as I dialed in the KF dimensions.
As you can read from previous flight reports, I moved the CG back 3/4" from where I originally suspected it might be, so I did start to notice some of these adverse behaviours, but after trimming 1/8" off the back of the KF top and bottom, things smoothed out again. Expressing the KF size in percentage of wing chord, it is 40% of chord when measured at the root along the fuselage and about 38% at the wing tip.
This is a far less tapered type of KF as you might expect from a straighter wing. By comparison, the KF on the Mig-35 which has a much more swept wing is 41.5% at the wing root and 35% at the wing tip.
If just for testing sake I put the trailing edge of the KF on the F18 at the point where the CG meets the fuselage, I would only have a 21% KF, I think I might as well not have KF at that point as I would lose the stability they provide if they were that small.
High alpha performance
If you read through the RCP F18 V3 thread on the RC Powers forum, you will notice that many folks, including me, tended to fly high alpha with the battery quite far forward for best high alpha performance and stability on the stock F18 V3. It almost felt that once you got the nose up there, the plane was "leaning" on something with that long moment of inertia.
As you might expect with a much shorter moment of inertia like I now have, this plane still performs very good scale high alpha (40-45 degree AOA), but the "slot" for staying stable is smaller. The plane feels like it is balanced on a much smaller spot and when you have it on that spot, it stays very stable, but as you get close to the edge, it does "fall off" a bit more quickly. So as my "high alpha thumbs/brain" come back I will continue to practice more and work to get it down lower. Right now, lots of altitude is my friend as I practice :) Like many things as you change things around on a design, there are points of compromise. As my skills come back, I think it will still be an excellent high alpha platform.
I could of course put the battery further forward, fly it nose heavy just to do high alpha, but I tend to like to get one setup that suits the largest scope of the flight envelope and then work on my skills to get better at the more extreme flight regimes.
With the better balance and CG being back 1 and 3/4" from the stock plane, it does require much less up elevator to keep the nose up at 40 degrees AOA. I do like to fly high alpha with spoilers deployed, so that also helps.
My elevons are deflected about 1/2 to 3/4" during high alpha. My spoilerons were initially set at 1/2", the right one stayed that way and using "differential spoilerons", I have my left spoileron deflected to 3/4" to try and help with the tendency of this plane wanting to pull right. As I have mentioned before, either I have built all 4 of my F18 V3s wrong or it just has a tendency to want to pull right instead of left which is what it should do due to prop wash/prop torque.
I did find that increasing my rudder expo from 15 to 25% did help with my smoothness and as I continue to practice and get my high alpha skills back, I will will consider reducing this back down, but the rudders are nice and responsive with this expo and the throw I have set now.
So now that I have the plane about 95% dialed in for how I like to fly, it is time to do some more "homework" and assess how much I should raise the battery to get the right vertical balance to see how the plane performs without being so "bottom heavy". Stephan did this excellent video awhile ago that explains how he used these principles that led to the great vertical balance on the NAMC Mig-FA.
More to follow as I get more testing done :)
Cheers,
Scott
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