stephan, namc chief designer
park jet Design fundamentals
A park jet should easily be identifiable as to the aircraft type. A great deal of emphasis is placed on the profile views of the plane paying close attention to promotions of the scale aircraft. Because these are build it yourself park jets, the design must remain simple and light and not be complicated with lots of pieces, tabs, slots and lots of foam overlays that additionally add weight. But I don't want a simple profile plane either. If I wanted a super realistic Mig-29, I'd buy the SebArt Mig-29 or the HobbyKing Mig-29. In the past I've crashed expensive balsa and RTF planes putting me into a RC funk and quitting the hobby. I am forever grateful to Dave Powers of RCPowers for getting me back into the hobby with his carbon and foam park jets. Now if I crash I just recycle the parts and build another one for the cost of two sheets of foam, some glue and my time.
So how to do you design a scale looking park jet? Readily available on the web are engineering plans and measurements for fighter jets. I import these plans into my drawing software and use them as a template for my plane. Later in the blog you will see examples.
What Scale Size for a Park Jet?
What size to make a plane is actually very simple. The two most commonly used foams (United States) are Depron and Model Plane Foam (MPF). The maximum width of a wing plate to fit longitudinally on Depron is 27" and the maximum length to fit transversely on MPF is 24". I never liked building a two piece wing plate so I use a single piece wing plate. The wing surface area of a 27" wingspan provides the perfect amount of lift for the thrust to weight ratio for 2200-2700kv readily available motors. Finally, this maximizes the available area on the foam sheets while minimizing waste.
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wing & Tail plate, elevon design
I’m liking having a larger than scale wing without making it too big similar to Mig-35B. Interestingly when I looked at Wiki, the Su-27 has 667 sq ft of wing area, the Mig-35 409 sq ft, but who knows how they measure that. Using authentic diagrams like this to place the plans over gives more realistic wing angles and shape. Cg coming forward is good as well as we learned from 35B, gives more flexibility in the electronics bay. With having overall optimum balance designed in by adding in the vertical balance piece, even though the PMI on 35B is over an inch longer than 35A, it is a more agile and responsive plane. I concur about the wing tip pieces, I build one plane (Dassault Rafale) with them, they last two flights, I know I can do without them. Out of curiousity, what will the length be? I think we figured with a 27” wingspan we would be in the neighborhood of 40” from the tip of the nose to the tip of the tail boom? The Su27 is an interesting design as based on some rough measurements and calculations I did, I think that tail boom accounts for approximately 8% of the plane’s length?
I pulled the starting length and width from your think document. I then scaled the original engineering plans down to a 27” wing width. The scaled length including the boom was 40.462”. Note I included the wing tip probes/pods to gain wing surface area. You’re right, the boom accounts for 8.6% of the overall length. Like the Concordesque droopy nose, the boom is integral to the Su-27 design. Here is a new drawing with measurements without and with overlay.
Ok, cool, that puts things into perspective even more for me to see the diagram below. When I measured the motor mount location on the other planes I mentioned, I measured to where the wooden piece would attach to the plane. I just measured the distance between that and the leading edge of the prop slot on 35B plans, it is 13/16” (0.8125”), so if I subtract that from the 23.646”, I get 22.8”, so we are most definitely in the right ball park there. Everything will still be tight to the Cg as with the previous planes. My calculations from the think document were just using what Wiki had, the way you are going will definitely be the most scale way to do it and with that tail boom just like on the Mig-FA, it will be the most bad a$$ looking Su27 out there! And like with the Mig-FA, having that tail boom as part of the moving elevons will make not only for an awesome look, but a little “softer” feel in the pitch control, keep it from being too twitchy and probably allow for lower overall throw. If I remember from previous Sus I have flown, since the LERX curves/tapers in towards the nose more dramatically than on the Mig, it helps it be even snappier in the pitch, perhaps less area to get in the way?
To make the rudders more scale, I decreased the rudder wedge from 3 to 2 degrees. I do not think this will make a difference in yaw stability, perhaps with the less drag the top speed will be higher than a 3 degree wedge. The elevon hinge line is one inch shorter than the Mig-35B though the moveable surface area of the elevon is the same.
Looking awesome thus far, Stephan! Where do you anticipate the motor mount (Centre of thrust CT) will end up in relation to the nose? I think we have found thus far with Mig-35/Mig-FA that 60% from the tip of the nose was a good location. However, in the case of the Su27 maybe we don’t count the tail boom length for this calculation? Again, my measurements will not be as accurate as yours, but if the tail boom is 8% of the plane’s length 40” - 8% = 36.8” x .6 = 22.1”. So the CT would be around 22” from the tip of the nose? Maybe that is too far forward structurally? I’m just brainstorming ideas mostly, I don’t want to make the whole thing even more difficult.
I’m loving the scale look of the tail booms, no other park jet designer has had the courage to do that, and I think it will make your Su27 so unique compared to the rest of the crowd. Add in the larger wing and good vertical balance and it will be amazing I’m sure!
Great question Scott, I was wondering the same. To answer your question about the CT in relation with the nose I thought it would be best to start by calculating the COP (center of pressure) of the wing. You already know all of this, so for our readers I'll explain. From Wiki, The center of pressure of an aircraft is the point where all of the aerodynamic pressure field may be represented by a single force vector with no moment. The sum of these forces create lift allowing flight. Based on the shape of Su-27 wing and even though it is smaller than the Mig-35B wing, my prediction was a similar calculated COP for the Su-27 wing based on MAC (mean area chord). I was amazed at how close they actually were.
With confirmatory flight testing, we optimize the position of the battery to bring the COP and COM/Cg into alignment. We call this final point of alignment the “Cg.” Most pilots prefer the COP range to be behind the COM/Cg. This way when the wing stalls, the COM/Cg is ahead of the COP causing the nose to drop, the plane gains speed and lift/flight is restored. A tail heavy plane with the COM/Cg behind the COP is incredibly difficult to recover after a stall. With electric planes versus gas planes, we don’t have to worry about a changing COM/Cg as the fuel burns off.
With this understanding of COP and COM/Cg let me address your question concerning the CT placement on the Su-27. The CT as measured from the motor mount to the furthest point of the elevon (not the boom) is 1.25” shorter on the Su-27. I don’t believe this will affect the COM. The moment force (torque) for pitch and yaw rotation will be greater than the Mig-35B as the lever arm is shorter. The lever arm is actually measured from the Cg to the hinge line and on the Su-27 is 1.1” and ½” shorter than the Mig-35B and Mig-35A respectively. If we find the Su-27 too “snappy” in pitch we can lengthen the PMI by moving the motor back. We have to ask ourselves, do want a plane that looks like the Su-27 and has it own flight characteristics, or do we want it to fly the same as the Mig-35B. Sounds like my chief test pilot is going to be flying two Su-27s with different PMIs.
Now that I understand the process better, I think the best thing is to leave the motor where it is, keep the mass closer to the Cg and keep the PMI shorter. After the battery, the motor is the second heaviest component in the power system, so my thought is to keep it closer to the Cg. I watched some more video of the real Su-27 and Mig-35 in action. I think without a doubt the Su-27 has much better pitch authority than the Mig-35, so capturing that performance would be important to me anyway. I certainly do want it to have Su-27 flight characteristics, pilots can adjust the pitch responsiveness in their planes, both in build and transmitter set ups. In my experience, better to have a responsive airplane by design and have the lowest amount of control surface deflection possible to generate scale maneuverability.