Recently I have been very busy testing a modified version of the very popular RCP F18 V3 park jet.
Below I have pictures of each airplane in a rear view, I think it demonstrates more clearly the differences between the three.
- the relationship between the motor location (I will refer to this as centre of thrust or CT), CG (centre of gravity) location and centre of mass location (CM) or essentially where my battery and other electronics ended up;
- How the relationship above affected the PMI (polar moment of intertia) which in this case is measuring from the front of the battery to the tip of the bullet nut on the motor. All of the electronics are contained within this distance which essentially account for 60-65% of the weight of the plane;
- How once I had found the neutral CG on the F18 V3 it affected the CM and the PMI and therefore affected the amount of control surface deflection needed to fly with scale pitch, roll, turn rates, etc; and
- assess how KF size and angle with relation to the wing sweep differed with a straight wing plane versus a plane with considerable wing sweep.
On the Mig-35 and Mig-FA, we strived to end up with the scale ratio when dividing the length of the plane by the wingspan. This not only gives the plane a more scale look, but ensures things stay nice and compact to help with rotation in all three axes with minimum effort. So I did the same with the F18 V3 and ended up shortening it to 42.25" compared to 44.35".
Here are the current dimensions of the three planes listed above
- Mig-35 - 38.5" long, 27" wingspan;
- Mig-FA - 38" long, 27" wingspan; and
- Modified F18 V3 - 42.25" long, 30.5" wingspan.
One of the unique things about these mid motor mount "prop in slot" park jets is that you do have some leeway where you place the motor to achieve a certain look or certain performance. Obviously we were looking for best balance which leads to best performance if you want efficient and precise movement of the plane with minimal control input. If you put the motor too far back, the PMI becomes much longer as battery and other electronic placement has to go further forward to compensate. Not only does this make the PMI longer, but it takes the CM away from the CG meaning the plane is less precise and responsive in most cases. If you put the motor too far forward, you can run into issues of structure and how and where to place the battery and other electronics.
Looking at these three planes as a percentage of their length, in all three cases the motor mount is located at 60% of the total length when measured from the nose. Of course since I realized we had started off with a good thing with the Mig-35 and Mig-FA, I did the same with the F18 V3 hoping the other pieces would fall into place accordingly.
All three of these planes have been tested extensively using the "arc" test cross checked with inverted flight to try and find the optimum neutral CG.
Here is an area where it gets "scary" similar. When again expressing the plane's total length as a percentage and measuring where the CG is in relation to the nose, on all three planes it is at exactly 55%.
I fly with a 2200 3S battery in all of these planes and they are in the neighbourhood of 21-21.5 oz each, so with my battery weighing 7 oz, it makes up for 1/3rd of the total weight of the plane. Although the other electronics are concentrated around the battery, it is the most significant component affecting the balance of each of these planes.
So for fun, I put the batteries in place for how they would be in flight. Here is another "scary" similarity, when measuring each plane from the nose to the centre of the battery as it lays in the battery bay, this point on all three planes was at exactly 50% of the overall length of the plane.
The PMI of each plane is as follows
- Mig-35 - 9.25"
- Mig-FA - 9.25"
- Modified F18 V3 - 9.5"
demonstrating how tightly concentrated 60% of the total weight of each plane is arranged
- Mig-35 - 24%
- Mig-FA - 24.3%
- Modified F18 V3 - 22%
Control surface throws
If you have read any of our threads or watched some of the educational videos we have on our videos page, you know that we use Dave Scott's Airplane and Radio Setup Manual as our bible for setting up servos, linkages, throws, programming our radios and even how we handle our transmitters and fly our planes. Essentially it allows us to fly with minimal expo (about 15% in each axes) with maximum servo resolution and very low control surface deflection for optimum precision of control while reducing drag, turbulence and maximizing stability while manoeuvring the airplane.
These are the current throws I use in all these airplanes. It gives me "just enough" to fly smooth and scale with a little emergency built into the pitch in case I need a little more response to avoid hitting an immoveable object.
Elevons - 1.25" one way (2.5" total) in both the pitch and roll axes;
Ailerons - 3/8" one way (3/4" total);
Rudders - 7/8" one way, (1 and 3/4" total).
Mig - FA
- Elevons - 7/8" travel one way (1 and 3/4" total) in the pitch and 3/4" travel one way (1.5" total) in the roll;
- Ailerons - 3/8" travel one way (3/4" total);
- Rudders - 3/4" travel one way (1.5" total)
- Elevons - 1 and 1/4" travel one way (2 and 1/2" total) in both pitch and roll;
- Ailerons - 5/8" travel one way (1 and 1/4" total);
- Rudders - 1 and 1/4" travel one way (2 and 1/2" total)
One area that was not similar and I suspect it is to do with the more straight wing of the F18 was the trailing edge of the KF in relation to the CG. The dimensions at the root are all very similar on all three planes, in the neighbourhood of 40% of wing chord, the Mig-35 and Mig-FA then taper more aggressively to about 35% of chord at the tip where the F18 tapers to only 38%. The trailing edge of the KF on both the Mig-35 and Mig-FA intersects with where the CG meets the fuselage. This gives the plane great manoeuvrability and a "locked in" feeling as the CG and centre of pressure are working in harmony. You can read in detail my findings in my flight reports on the old blog about the KF adjustments that I did make.
Essentially, the trailing edge of the KF on the F18 is about 1 and 1/2" behind where the CG intersects the fuselage, but I still have that very precise "locked in" feeling. If I was to move the trailing edge forward, I would end up with only 21% KF at the root which I suspect would negate any of the stability the KF provides.
I am very excited about how similar all the numbers are above, it is almost "scary" how they ended up so much the same with three distinctly different airframes. It leads me to believe we have established a pretty secure baseline of where things need to be on future designs through our previous research, application of good scientific logic and principles validated with extensive field testing. Of course each new airplane that we design may be slightly different, but the parallels thus far are too close not to consider.
Our motto defines our approach:
Puto, Consilium, Test et Convalidandum; Think, Design, Test, and Validate
Blue skies and calm winds to everyone!