A lovely "little" sailplane...

The topic of this article is a "tiny" Nimbus 4D model. I got my Nimbus as a second owner, but as far as I know he was produced in Tchec republic. He is really well built in glass fiber, the wing is really strong and even a little bit too stiff. Span 4000 (mm) Wing area 44.9 (dm²) Weight 2850 (g) Chords 0 150-115-100 75-60-40(mm) Airfoil Modified HQ ? Aspect Ratio 35 Wing Loading 63 (g/dm²) I love the aspect ratio and the general sight of this glider ! In flight, I was expecting difficult turning and yaw handling, but in fact this was not the main concern...

... with serious longitudinal handling problem

The first takeoff was made behind a tow plane model. Everything got well during the tow phase, but once gliding I entered phugoïd oscillation, with difficulties to damp it. The Nimbus was really high, and his pitch attitude really hard to evaluate    As altitude was decreasing, the Nimbus could be better seen and then handled, for I saw it better. I could note that indeed, turning was quite a challenge, but I was aware of this before flying it ;-) .    Other flight were performed in similar condition. Changing the CG improved slightly the behaviour, but not that much.    Then I made some flights over a slope, it is really nice to see it flying close. Really a wonderful sight. But the Nimbus still suffered strange longitudinal behavior, such as violent pitch down in gust and things like this.    This flight session did not end really happily, since I have to land out in the bottom of the slope, into trees in fact... Now I know this nimbus is really strong !    So, the lateral handling was quite as I expected it. But concerning longitudinal behaviour, I was really disappointed. But the sight of this Nimbus in the sky worth some digging into that problem.

Analysis of the failure

I was surprised by the thickness of the wing the first time I saw them. According to experience, as you decrease Reynolds number, you have to select thinner airfoil. Thick airfoil with low Reynolds leads to really strong laminar bubble and high (pressure) drag level. This kind of thick airfoil with strong laminar bubble may also cause some problem with the Cm0. Indeed the airfoil pitching moment is the result of the pressure distribution over the chord : the laminar bubble influences greatly the pressure distribution form, hence the pitching moment. On top of this, if the laminar bubble moves, this can create some Cm0 strong variation. This was the supposed, and now confirmed, cause of the longitudinal behavior.

Click !

Click !	This can be analysed at first glanced with xfoil result : you see that the Cm curve is quite strange, with a step on the alpha-to-Cm0 curve.
Click !	More in details, this phenomenon is to consider over the full wing, with each station along span working at the right local alpha.    Calculations were done with MIAReX to evaluate this. This is even worse, since as a result of the tridimensionnal flow, you can see a double step on the alpha-to-Cm0 curve ! This may cause neutral point movement, hence longitudinal oddities.
As a result of the Cm0 being dependant of incidence alpha, two phenomenon can be encountered : The trimming of the model may change a lot with speed. The neutral point may move in a strange way, then causing longitudinal instability due to variable static margin. Indeed, the Nimbus was very difficult to trim and to find a correct CG location.
Two ideas concerning origin and/or curing of the handling problem were studied : Work may be done on the tail. indeed variation in Cm0 may create (instationnary) tail load that the tail cannot cope with. The variation in Cm0 may be corrected directly on the wing.

First curative action : acting on the stabilizator

The stabilizator looks really tiny, even if its volume is quite fair (Vstab=0.47). This may have been a cause of problems. The first curative action was to put turbulators on the original stabilizator. this was not really successful. Then the full stabilizator was rebuilt larger. The scale ratio was 1.25. But longitudinal oddities were still encountered Something else was to be investigated...

The original and the rebuilt tail.

Second curative action : an easy way for high lift weather

On a windy day, I intended to fly the Nimbus anyway. In order to speed it up, I decided to use negative flaps camber.    As a result, I could see that the longitudinal behavior was really improved ! As soon as I tried to set flaps at neutral position, once again strange pitch phenomenon as usual. My interpretation of this fact is that flaps setting modify pressure distribution over the airfoil, particularly in trailing edge region. Negative flaps may in fact weaken the laminar bubble, due to a less steep recompression.

Click ! Negative flaps setting decrease recompression steepness

Wind tunnel session & last curative action

    As a student I had the opportunity to put my wing into one of SUPAERO wind tunnel. Thanks a lot to the LAP staff for allowing me to use the wind tunnel and the

    I wanted to visualize how the laminar bubble would look. I did only qualitative wind tunnel cession, using vaseline plus TiO2 mixing as a surface liquid for visualization. The result could be clearly seen thanks to UV light.
   Aerodynamics parameter were the following

• alpha = 4deg
• V=14m/s
• Re=160 000

   Here are the most interessant photographs that were taken during those wind tunnel tests.

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The nimbus wing is "slightly" too large to fit the wind tunnel....
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With no device, the laminar bubble is quite clear.
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Basic tape as a turbulator is quite ineffective in transitioning BL...
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Whereas Zigzag tape is really effective !

In flight

Click ! How to make ZigZag tape...

Since I put Zizag tape all allong the extrados, then "killing" the laminar bubble, the flight is really transformed ! I have got a new sailplane, it is almost magical... There is no more strange behavior on pitch axis : that was the main goal, and it works nicelly. Spinning is now much more difficult than before, and circling is really easier. Flaps action is different : negative setting is now useless, the nimbus sinks without accelerating. You can see here a video of the flight (thanks to michel !), on a quite windy day. Other photographs of the nimbus, and Wind tunnel session, are available here.

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