Scott really got my brain churning with his new NAMCv3. I started reading and realized I had ignored the importance of the polar moment. Simply put, the less torque needed to rotate the pitch/roll/yaw axis around the the Cg, the plane will rotate easier, faster, and more efficiently requiring less control surface deflection. I can't believe I have ignored this basic law of physics.
So today at the flying field, I tested the plane with the same Cg but with different polar moments. How did I do this? I usually placed the ESC wherever it would fit and "looked" right. This usually meant as forward as possible. Today, after a couple of flights, I moved the ESC as far back as possible which then allowed me to bring the battery aft and closer to the Cg. I could feel a difference and the plane was more responsive in all 3 vectors of rotation. The bigger the ESC, the more noticeable the difference will be.
After "centralizing" as much as possible the battery and the ESC, I then started to play with Cg. As mentioned in a prior post, I test the Cg by flying at 40-50% throttle nose up at 30 degrees and cut the throttle. I want to see the plane continue to climb and then slowly and gently drop the nose completing a nice arc. Note: I read about this but can't remember the citation. So I did this today and kept moving the battery aft. A tail plane is a crashfest, so I stopped before it got tail heavy. Wow, what a difference. The plane in pitch, yaw, and roll is so much smoother and looks more scale like. Induced spins and recovery were better as well. I used to like nose heavy for high alpha, but today it was much easier and wing top rock onset was more gentle and not so violent. Best way I can describe it is that as speed bleeds off in a tight turn or climb out the plane appears to be sliding through the air as the lift decreases. Make sense because as lift decreases, you are not adding more control surface input to counter act a forward Cg.
I really felt like I saw the "light" today flying. Gasser RC day habits of nose heavy planes is a hard habit to break.
My Cg is now 22 3/8 inches from the nose, 2 1/8 forward of the spar, which is about 3/4 aft of stock. Remember this plane does have a higher wing load than stock with a downsized LERX and leading edge.
Video will come, one day, I promise.
Thanks you Scott for my epiphany today! Song of the day from Archie Bell & the Drells "Tighten Up". For us that means your battery and ESC!
Stephan
Hi Stephan -
ReplyDeleteExcellent flight report as always, always makes me happy to hear when you are able to replicate and validate my findings, brings a lot more credibility to what I am seeing and reporting... :)
I must admit I do feel a bit sheepish :/ for not paying close attention to the polar moment in so many of my builds. I knew it was a consideration, but for some reason I kept overlooking it until it was right there in my face as I first started testing the NAMCV3.
I'm also wondering if having as many of the "Cs" (centers of mass, gravity, thrust and lift) as close together as possible also factors into the equation, I believe that it does. We have definitely seen how having the center of mass (CM) much closer to CG has impacted our planes. I will be interested to hear what your experiences are with moving the motor further forward on the Mig as you shorten the back end and tighten things up even more.
With these "mid mount" prop in slot planes we also have more flexibility with where the origin or center of thrust occurs. With tail pushers or nose puller planes, the rest of the bulk of the weight ends up much further from CG just to balance the plane for normal flight. In our park jets within reason we can move our center or point of thrust around to bring it closer to CG which I think also helps with more precise, efficient rotation around the CG. I did some interesting measuring yesterday at the field between the Mig29 NAMCV3 and the Su35 MK2 NAMCV2 which I will discuss in a new post that I think lends a bit more credence to this argument.
I'm also wondering after having seen the response in the pitch and roll in both my NAMCV2 and V3 Migs if having the aileron and elevon servos closer to CG also helps the cause as now the origin of torque for the ailerons and elevons is also at the CG (I have the aileron and elevon servos on both these planes balanced on either side of the CG point pretty much).
One thing I do notice as perhaps you did as well yesterday is that with everything centralized closer to CG, you have more flexibility in moving the battery to dial in that perfect "feel" before the plane starts to feel unbalanced. With a plane where the mass is spread over a much larger area, the CG is far more finite, if you are off by just a bit, you are either nose or tail heavy with very little forgiveness or flexibility. Makes sense as you are now trying to balance things over a longer "lever" rather than a shorter one.
The center of lift will end up being where it ends up being, but I think that since we have the opportunity to control and play around more with where the centers of mass, thrust and gravity are, we hopefully are achieving even more control over dialing our planes in exactly how we want them since we are focusing more on removing three of the variables from the equation.
With all this great testing and reporting, it is giving me more to think about for my next Su35 MK2 build, will be interesting to try these theories on a completely different airframe to see what happens.
Great work, my friend... :)
Cheers,
Scott
Thank you Scott and Stephen for sharing your meticulous work! Fun videos too!
ReplyDeleteOur pleasure Dave, we hope you find it useful, we are learning a lot and having lots of fun doing it...:)
DeleteThanks again.
Cheers,
Scott