The SU27 Project

Some time ago I started a new project, ambitious but that's how I like them.

I wanted to design a slope "jet". Before I tackled my favorite jets (F16, F14, F18E, some are already in progress, design stage) I stumbled upon the SU37 thread as a parkjet so it inspired me to design a SU 27 (no canards yet, that may become a later project, baby steps see)

So here I went and designed from scratch.

I used MPF (Model Plane Foam) as it is the best medium out there in my opinion.

The basic design objectives were:
1 - Light plane
2 - Enough lift so that it would fly correctly
3 - Simplified overall shape, yet good enough to look good and "scale"
4 - Smart enough aero design in the hope that it would perform some "post stall" maneuvers although I knew that was borderline impossible considering the format (light, lack of engines, has to be stable (CG forward of AC))
4- Test bed for some ideas in design (direct drive control surfaces)
and hopefully it would fly well and be fun to fly too.

So a list of design choices were made (to be tested):
- Foam, painted in acrylic and covered with laminating film
- Some LE reinforcement, 3/16 wood round
- Direct drive tailerons, no other surfaces for the time being
- Airbrake, for fun, and also as a access hatch for the electronic, more on that later
- Subsonic wings, based on KFm2 airfoil for gentle stall characteristics and ease of building. No flat plank here, this thing needs to soar.
- Lifting "engine nacelles" to contribute to the lift since they are hollow, more on that later too
- Sharp leading edge on the wing cheeks to encourage localized abrupt stall conditions in order to promote "post stall maneuvers" possibilities

For those that like numbers and tech studies, here are some of the design data:
Wingspan: 24"
Wing area: 1.3 sqft
AUW: 11.1oz
Wing Loading: 8.8 oz/sqft
Wing Cube Loading: 7.8
Aspect Ratio: 3.2
Dihedral: 0
Wing twist: 0
Airfoil: Flat bottom KFm2, increasing thickness ratio
Aileron ratio: 18% (tailerons)
Htail Volume coeff: 0.14; Low but full surface and high throws
Vtail volume coeff: 0.064; low also, but significant fuse and nacelles tail stability influence, not computed here.

So here is the finished prototype, paint scheme is based on real design. Easy for the proto, further versions will have more fancy paint schemes (like camo and so on)

Test campaign:
I am not a proficient enough pilot at this point to be able to test this with any validity, but I will rely on other experienced pilot, more to come.

Things that need to be validated, and possible issues and solutions:
- Balance (CG location) due to difficulty to estimate real lifting contribution of fuselage and nacelles.
- Amount of lift; Are the wing providing enough, or do I meed to evolve the nacelle into lifting bodies, as made they are fairly neutral, like flat planks, but I can offset the lift up or down through some aero trickery
- Handling characteristics. Is KFm2 good enough, in particular with inverted flight, or would a symmetrical airfoil such as KFm4 be better
- Airbrake; I expect this one to be way difficult to tune, and to have an influence that may be hard to counterbalance just with mixes. I also know that it's probably not needed, and just there for fun.
- Durability: the tailerons may be too weak as designed (the pivot point) they may have to be changed to indirect drive and axis for crash resistance
- Throws: there may be too much as consequence of direct drive and not enough definition

First test flights were very very promising.

- Balance seems to be spot on
- Glide ratio is not great (compared to "floaters" but very decent almost as good as my slope floater
- Controls are just fine as far as it can be seen from short flights. I had to introduces limits on the down movement otherwise the tailerons were sticking out underneath the fuse, not good.
- It may have a tendency to pitch left or right upon launch (not unexpected, hard to hold by the middle), but it is easily corrected quickly and then it flies fine
- It shows a propensity to be able to hover, yes yes, expected by design, but not really confident it could be achieved without motors. When slowing down full up stalls it flat and it hovers down (parachute move, known by some parkjet flyers). So it can be landed flat down.

So overall very satisfactory first test, was subsequently tested on slope.
So far what is established is that it flies (always good, but not always guaranteed), that is controllable and that all settings were OK.
It also show a propensity to be able to do "post stall maneuvers" like the original, which is what I hope when designing. It can do the cobra, it can do the top of the loop vertical flip, and it can land by "parachuting", all of these can be repeated although not totally mastered yet (3 flights, and it's pilot learning curve).
I am working on CG refining, and there seem to be a need for quite a bit more weight in the airplane (energy acquisition and retention). There is also some work that need to be perfected on the mixes and throws.