Had a great day at the field today, managed to get 10 flights completed with my F18 V3M. It had been a couple weeks since I had flown, so I did a couple "warm up" flights with my NAMC Mig-35 to get my thumbs and brain limber :)
It was a great day for testing as there was little to no wind, maybe 5 kts maximum, so it gave me a chance to get started on the "dialing in" process. Today I focused primarily on getting the battery positioned just right to find the neutral CG and although I think I still need to tweak it just a bit more, I'm almost there.
I also wanted to work on getting the plane trimmed and determine what my throws should be. My whole philosophy on how I set up my planes has changed dramatically since I last flew an F18 V3, before this I was using 3D type throws, 50%+ expo, not in tune with the more precise way I like to set up and fly my planes now.
CG/battery position/PMI
Before getting too far into the CG and PMI, I must say there is something very unique with the F18 V3 and how it balances in the roll axis in my experience. This is the 4th F18 V3 I have built and for some reason, no matter how careful I am with aligning everything during the build and setting up the control surfaces, it always wants to roll right, not left as you would normally expect you would have to compensate for due to torque roll. As a result, I maxed out my left trim and it still wanted to roll right. I checked and double checked my alignment of everything, my control surfaces, etc and still had the same problem. So although I would prefer to fly with my battery dead centre along the centreline of the plane, I had to put it to the far left of the battery bay so that I was not dragging so much trim deflection around. Not ideal, but I guess I will just have to live with it and hopefully as the dialing in continues it won't have too much impact on how the plane performs.
As mentioned in a previous post, I started off with the plane balancing at a point 1" behind stock or 1" behind where the wing breaks from the LERX. As a reminder, this was where the battery was located to balance on that point before I left the house.
This gave me a PMI of 12.5". As I started flying it, it was quite nose heavy. I began doing several "arc" tests to determine proper balance. Essentially, I take the plane up in about a 30 degree nose up climb, 60-70% throttle, then chop the power and watch what it does. One thing I did note with the F18 is that because it slows down much quicker than other planes I have tested using this method, the nose still would drop a little quicker because the momentum bled off quite quickly. So I would cross check by rolling the plane inverted and seeing how nose heavy it still was when flying upside down.
By the end of the 10 flights, this is where my battery is located and I think it could still go back a bit further, maybe another 1/4" at least, but I will confirm that during the next test session.
My PMI is now 10"! Wow, big difference and as mentioned, this might be tightened up a bit more still.
Here is where it gets interesting. I would have thought that moving the battery back 2.5" would have a dramatic effect on where the CG balance point would be. In fact, it only moved back about 1/8", so my CG is now 1 and 1/8" behind the wing break. What tells me I am getting close to dialing in to neutral CG however is that if I put my fingers even 1/16" behind or ahead of that point, the plane either pitches up or down significantly, there is very little tolerance. I found this to be the case as we dialed in the Mig-35 and Mig-FA, so I know I am getting very close to being bang on the neutral CG of this plane, at least in the pitch.
Control surface throws
This shortened PMI has made a significant impact with how this plane is balanced and flies, much more responsive and requiring very little control surface deflection to get it to change direction. In my opinion to suit the way I like to fly, this is important, less deflection means less drag and turbulence and greater stability and less energy loss as the plane changes direction.
I fly with very little expo, only 15% on each axes, so this gives me not only a better feel for the plane, but small stick inputs translate to more instantaneous control surface movement without the lag that a large amount of expo can cause. I try to set my planes up with "just enough" throw to allow me to fly scale type manoeuvres with a little extra emergency throw in the pitch in case I need to avoid something quickly, like the ground :/
My current surface deflections are as follows -
- elevons, 1 and 1/4" in one direction, 2 and 1/2" total, measured in both the pitch and roll position;
- ailerons, 1/2" in one direction, 1" total. I may tweak this up about 10-20%, the long straight wing of the F18 needs a bit of extra help to make the rolls crisp and snappy;
- rudders, 1 and 1/8" in one direction, 2 and 1/4" total. This is sufficient for good scale stall turns, it might need to be tweaked a bit more as I explore high alpha performance.
KF adjustment
About halfway through the CG determination process, I noticed that the plane started to feel a bit "mushy" and at times was fighting itself in turns. From my experience with the Mig-35 and Mig-FA, this indicated to me that the trailing edge of the KF was too far back, causing an imbalance with the CG and CP (centre of pressure). I trimmed about 1/8" off the back of the KF top and bottom and it smoothed this out. I am thinking that I will probably need to trim just a bit more off as I continue to test, especially if I move the battery back more as during the last flight I was starting to feel that "mushiness" (I know not exactly a scientific term :/) again.
Overall performance
Ok, enough of the technical stuff, what happened in the air?!
I must say I am extremely happy with the effect these mods have made on one of my favourite park jets of all time. This plane just feels tighter, crisper and far more precise than the stock F18s I have built in the past. It tracks even truer, is more balanced and responds to inputs with just the lightest of touches.
Nothing of what I have done has done much to make the plane any faster, but it is far more agile and quicker to change direction with less control input and it does so without any hesitation, bobbles or sloppiness, just quick, precise movement. Although it is still "bottom heavy" because the battery is so far below the wing plate, it still has some self correction qualities, but far less than the stock setup, meaning it is still very stable, but more responsive for someone looking for more intermediate to advanced performance from this tried and tested park jet. This is something I will also assess and test to see if I can raise the battery up somewhat once I know exactly where it will be located for best balance in the pitch and roll axes.
I still have considerable more testing to do to get the CG really dialed in, KF adjusted just right to how I want the plane to fly and perhaps more adjustment to the throws and to assess it through the slow flight and high alpha flight regimes. So much more to follow as I can get to the field and keep blasting batteries through this plane :)
One final note concerning the wing reinforcement plan I am testing, as a reminder here is what it looks like.
I did several full throttle hard turns in front of myself and several full power pull outs from dives/loops and saw no flex in the wing, so I am very pleased with how this is working out thus far. It does add a bit more weight to the plane, but gives me the confidence that my wing is rock solid and strong.
More to come very soon I hope :)
Cheers,
Scott
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