60" Chuckies- a (futile) search for the perfect design.

Many club members took part last winter in Dave Camp's well organised Chuckie series competition. There was always a good turn out, the sun usually shone, a wide variety of models flew, and a good time was had by all. Hopefully he and Trevor can be persuaded to host another one this winter.

There were several home designs, three or four flying wings, and a few commercial kits amongst the models flown. I flew a "Monarch" which is an American kit with polyhedral, a V tail, and a reputation of being one of the best commercial models around.

Usually good piloting won the day, many of the tasks being biased towards accurate landing times. Dave is hinting that a future series might have soaring as more important than landing.

While the Monarch flew well and probably had as good a low sink rate as anything out there, I was never totally relaxed when circling, as I found the set up of the V tails very critical and the model needed flying all the time while circling. Probably the servos need to be of better quality to ensure the controls always centre, and this is difficult on such a light small model.

I decided to build a tail less design but only got it finished after the series had ended. As we know tail less models rely for longitudinal stability either on reflexed aerofoils to give a positive pitching moment, or considerable washout at the tips to achieve the same stability or a combination of both. Aerofoil data tells us, and also I know from wind tunnel experience with racing car wings that reflexed aerofoils fall far short of non reflexed ones in terms of lift coefficient and efficiency. Also washout reduces the potential lift coefficient as that part of the wing is acting at a lower angle of attack. (Lift is directly proportional to area, section coefficent, and angle of attack). So the potential of very low wing loading of a flying wing cannot be achieved as the useful area is effectively reduced or the section used is an inefficient one.

However a tail less design can be built much lighter, and does not have the drag associated with the fuz and tail. The "C wing" which originated in Germany, might be the answer. No longer a true flying wing as it uses small inwards pointing "tailplanes" mounted on top of the tip fins which provide the longitudinal stability, thus allowing a much more efficient wing to be built without as much (any?) washout or reflex. O.K. some drag is added, but it should still be lighter than a conventional design.

Here is where I made a big mistake. The original C wings had the control surfaces in the "tails", but I decided in the interests of light weight and simplicity, I would keep normal elevons with with fixed tails at an angle of "decelage" as with normal tails. To keep a long story short these tails were a disaster, though I had made them removable. Being set further aft than the control surface elevons, they almost removed all control authority. Roll control was almost nil, and a bungee launch produced a loop even with full down applied to the elevons. Reducing the tails' incidence just required more up elevon in the normal glide, reducing efficiency. Of course more nose weight was required with the elevons, so I'm not sure the idea has much merit.

Without the "tails" and with some washout twisted into the wing, it performed better. The launch phase is superb, as approximately 15% higher launches are achieved than with the Monarch. However flight time even from the higher launch is certainly no better than the Monarch. However it does circle very tightly so might be good if thermals are about.

Incidentally the basic aerofoil I chose uses an RG14a bottom surface and EH 1.5/9.0 top surface. The resultant bodge was 8.8% thick with 2 % camber. However the reflex changed throughout the span having drooped trailing edge at the root for good lift coefficient and reflexed somewhat at the tip for stability.

Here are some comparison data
Monarch:- Weight 13.75 oz, Span 58.5" Wing area of 383 sq ins. Loading of 5.17 oz per sq ft. Tail moment (1/4 chord to ¼ chord) 20" Aerofoil unknown. 7.6% thickness and 2.6% camber.
C wing. (without tails): Span 58.5" Weight 10.25 oz Wing area 345 sq ins Loading 4.28 oz per sq ft. Base airofoil EH 1.5/9.0 top-RG 14a bottom. Washout about 3 degs.

So, how to improve on the Monarch?

When one considers what is required from a 60" chuckie launched by bungee, the requirements are a little different to the "hand launch" where the model is either launched "javelin" style or now "discus" style by holding a peg in
one tip and whirling around 360 degrees and letting go. This produces higher launch speed but still not compared to the estimated 50 odd mph at some point of the bungee "catapault" launch.
So ultra low drag is required for good high speed penetration off the launch to gain maximum height.

There must be an optimum weight for height off the launch as too much weight will reduce the acceleration of the bungee and insufficient inertia will allow the drag to slow the model too quickly after the bungee has released.
So removable ballast with the lowest possible design weight might be worth something.

In the glide good handling is paramount as with any thermal soarer. For this I require a cruxiform tail as it will be a rudder elevator model for simplicity and light weight. For me v tails are fine for aileron models but not for rudder/ elevator jobs.

How to achieve the best possible L/D or glide slope? Or do we require minimum sink?

Do we go for a high lift (and high drag inevitably) floater or a faster slimmer wing.

Anyway I've decided on the fuz and tail design. Construction has started, for which details next month hopefully. It can initially use the Monarch wing. For its proper wing I am unsure what section to use, though the plan shape is decided and also the dihedral breaks.

Any theories and ideas out there please?

Just one thing to consider that is perhaps a little hard to grasp:-

Max L/D (say 15:1) also defines the minimum glide slope. i.e 15ft forward for every 1 foot of height lost. Now for any straight glide, as the speed changes with glide angle the lift produced does not change; it is only that which is required to counter the models weight. The angle of attack (incidence) changes to keep the lift constant. So Drag is the factor which effects the glide slope, which is why the Vee tail is so popular. Which brings us back to ponder on what aerofoil to use. High lift is going to be draggy, unless the loading, and thus the speed, is very low. However we want that high launch, which suggests that a faster flying, lower drag aerofoil might be better.
More of my thoughts, together with any input that you chaps can provide, coming soon

The resulting design has been called 'Sunspot'  Two articles on the design, together with construction pictures in this LINK.