I have two good flight sessions under my belt now with this plane, very impressed with how it looks and flies :)
I'm very impressed with the quickness and speed of this plane with the drag reduction resulting from building with DTF. I have toned down my throws in the pitch and roll about 8% now compared to how I fly my other Mig-35Bs that were all built with Depron and MPF so that I don't over control it since it responds that much quicker.
So now that I have just over 20 flights on this plane in calm conditions and in conditions with winds about 10 mph, I have a few ideas why this plane is so slippery in the air which helps make it quicker to respond on the sticks and at top speed.
As mentioned in the video, all the surfaces and leading and trailing edges have been reduced in thickness. Where the paper is left on the vertical stabilizers/rudders, it is 5 mm thick, so 1 mm thinner that Depron/MPF. Where the paper was taken off like on the leading edges of the nacelles and wings, the foam is 4.5 mm thick, so my wing leading edge is 13.5 mm thick as compared to 15 mm thick with my other Depron/MPF builds. Additionally, since I used the same reference line when shaping the leading edge of my wing with the KF4s installed, I ended up with a slightly more shallow angle on the leading edge of the wing which I think also contributes to reduced drag.
With the paper off the fuselage, the width of the fuselage is 3 mm less than if it was built with MPF/Depron.
The surface of DTF with paper on or off is quite a bit smoother than Depron or MPF, it has more of a closed cell consistency, so sanded or not sanded, it is a much smoother surface. Any areas where the paper is left on of course is also very smooth.
So it is a bit hard to put a number on it, but having flown so many Mig-35B flights built with Depron/MPF, at the same weight and with the same power setup, I would say the drag reduction has increased the maneuverability and top speed of the plane between 5-10%. All these little numbers add up to making a quicker, faster and smoother flight experience.
Strength and durability
Other than the minor problem with the bottom of the nacelles which I will discuss later in this article, I am very, very impressed with the strength of the foam where the paper was left on, the edges sealed with glue and the paper treated with oil-based Minwax. Even without the addition of the reinforcement, it is much stiffer than Depron or MPF. However, I think the reinforcement is still needed in key areas. The reason being is that even with the Minwax, if the foam gets flexed too far, it crumples and wrinkles as shown in this picture. Then like soft aluminum, there is always a weak point at the wrinkle that cannot be straightened.
During both my flight sessions thus far, there has been a lot of moisture on the ground, so the plane has been wet and thus far I am very pleased to report that the paper is holding up without issue, the sealing with glue and Minwax is working as advertised, so I'm very happy with that.
As mentioned in the video, there are a couple things I would do differently in future complete DTF builds to compensate for the differences in building with this foam.
The paper does add weight behind the CG, but also adds very important strength, so for a complete DTF build, it is just part of the equation. This requires me with this build to fly with my battery about 1/2" further forward than what I have flown with in the past and my ESC about 2" further forward to get the balance right. It doesn't feel like it is nose heavy, but at times when slow, I could feel the difference in the moment of inertia over my other Mig-35B builds. With a longer moment of inertia, sometimes when slow in the vertical or in turns, the nose would wander a bit on it's own. Nothing scary, but noticeable from my other Mig-35Bs.
I knew that the plane was going to be tail heavy even before I started putting the electronics in it, as it sat on the table it always wanted to sit back on it's tail. I installed the motor with some down angle which I'm sure helps drive the nose down a little bit more without needing the battery even further forward. In the picture, the angle looks a little more severe than it is, if I extend a line from the center of the bullet nut, it would be pointing about 1 mm below the bottom of the back plate.
To solve the flexibility on the bottom of the nacelles, this would be an easy fix I think without adding a whole lot of weight and I already have thought of several solutions I might try in future builds.
- fiberglass drywall tape along the entire inside of the nacelle, kind of a "poor man's fiberglass";
- making a butt joint instead of a score and fold joint where the bottom of the nacelle meets the sides, with the bottom resting on top of the sides;
- putting another layer of paperless DTF along the bottom of the nacelle with some fiberglass tape sandwiched in between;
- reinforcing inside or outside the nacelle with very thin tongue depressors I can get from the Dollar store.
Having gone through this experience with Peter's help, DTF building will be kept as a very valuable tool in my "builder's toolbox".
I will experiment more with future builds completely of DTF to see if moving the motor forward and playing with some reinforcement on the bottom of the nacelles helps out with the minor things I have seen with this plane.
I can see me using this for other planes and test beds as the cost is so inexpensive and with the right patience and attention, the strength and durability is so far equal to Depron or MPF. I will certainly use paperless MPF for my next fuselage on an MPF/Depron build as I'm quite sure that a DTF fuselage is not only lighter than an MPF or Depron fuselage, but as mentioned before in this article, causes noticeably less drag.
I still have lots of MPF and a few sheets of hobby grade Depron I'm hoarding, so using the inexpensive, readily available (for me anyway) DTF will help stretch this supply even further/longer. The biggest "stick in the mud" for me with MPF and Depron is that I have to order it from the US. Depending on how the Canadian dollar is doing, this can be a good thing or bad thing :/ Shipping from the US certainly has not been getting cheaper and accounts for nearly 40% of my delivered cost for both Depron and MPF...yikes!
Certainly the build process to have a nice looking, strong, light and well protected plane with DTF takes longer for me than an MPF or Depron build, but being retired, time is something I have in more abundance than cash :)
As I continue to "fly the wings" off this plane over the summer, I will continue to evaluate the durability of the DTF build and report on this thread.