# Is the Suntoucher CG too far aft?

I acknowledge my very limited understanding of flight mechanics, but at first sight something tells me that the CG of the Suntoucher solar aircraft concept lies too far before the main wing and the forewing is too small to attain satisfactory stability and maneuverability.

This geometry is not OK at all.

Question: Did I miss anything?

### References:

• Questions of the form, "This thing is crazy, amirite?" never do very well on SE sites because they typically need discussion rather than a single objective answer. If you're really interested in how stable the design is you might like to reword the question to be less judgemental. Dec 13 '15 at 12:20
• To deal with your question directly, the span of the canard is much larger than normal, and I'd guess the engine is lighter than a typical piston, so the configuration doesn't look that unstable to me. Dec 13 '15 at 12:22
• I think you miss two things: stability of canard configuration and purpose of the airplane. If the goal is to stay aloft during a long period describing large circles, maneuvrability requirements are not as important as energy efficiency (really small turn rates are acceptable) Dec 13 '15 at 15:23
• A small forewing along with a shorter arm (Too advanced CG) may incur a serious penalty to the canard maneuvrability during the takeoff (Rotation). Likewise, a high AoA of the small forewing is required during flight for stability sake. Dec 13 '15 at 16:55
• This aircraft is a conceptual design. There is no real aircraft here.. move along folks: nothing to see here except rampant speculation. Dec 15 '15 at 2:35

Good catch! You are right, the center of gravity will be too much forward for a canard. If they put the prop on the sting behind the wing and the cockpit a bit ahead of the wing, things would look more credible.

The size of the foreplane is not obviously wrong, but the booms at the tips of the foreplane look strange. They serve no obvious purpose, like the sting, and the tailbooms look too thin for transmitting the loads from the vertical surfaces into the wing.

One third thing that looks odd are the control surfaces: The ailerons look too small to allow maneuvering in gusty weather at low speed. I guess the flaps on the inboard wing are for pitch control. Why are they not on the canard, where they would be more effective? Maybe because the center of gravity is so close to the foreplane, the designer decided to put them on the wing.

I agree with your assessment: This thing will remain an artist concept.

For comparison, see below a picture of a solar aircraft that actually flies and got most things right: The Icaré II:

Icaré II in flight (picture source)

• Heh, most things... don´t let the design team in Stuttgart hear that ;-) Dec 14 '15 at 11:51
• @yankeekilo: It is common knowledge that during development you collect a long wishlist of changes even before the plane flies the first time, and only a determined project leader will keep the engineers from tinkering and improving all the time. Only when the aircraft is certified do you have the knowledge to do it right the next time. Dec 14 '15 at 14:02
• I was just kidding because I used to sit next-door to said team leader ;-) Dec 14 '15 at 15:20

I think you are talking about the canards. Canard aircraft are much more common than you think and are certainly stable. For example, see this Rutan VariEze:

Image from airliners.net

(Static) stability means that the aircraft will return to its original position after a disturbance. For longitudinal stability (i.e. about the pitch axis), if some disturbance causes the aircraft angle of attack to increase, it should return t its original (lower) angle of attack by itself.

In case of the canard aircraft, the center of gravity is forward of the main wing.

Image from f-16.net

In this case, it is the main wing which is stabilizing. If for some reason, the angle of attack increases, the canard produces a moment about the c.g. that tends to increase the AoA. However, the wing also produces more lift (as AoA is higher), and this results in a stabilizing moment about the c.g.

The aircraft may still (most probably) remain a dream for various reasons, though I doubt having a canard has anything to do with it (Actually, the span is on the larger side). Without details, its just speculation.

Interestingly, the aircraft looks similar to the Rutan Voyager, which holds the world endurance record and had problems with pitch stability.

Image from nasa.gov

The Suntoucher solar aircraft design looks fine to me. A key note: this is a solar electric powered aircraft using an AC electric motor to turn the prop. Therefore the stinger section of the fuselage would contain batteries, adding much weight in the area of the wing, putting the CG near the prop on the wing side. Also with the prop mounted, and spinning on (around) the stinger part of the fuselage applying a pusher force on the crew pod, you need to strengthen the connection from the wing to the crew pod; thus the outriggers from the canards to the wing will carry the force and stabilize the wing