Sunspot - Chuckie Design.

From Nigel Bennett

More imperfect thoughts on the perfect Chuckie design.

Since writing the last article on this subject more information has come my way on how design in the USA and Germany has developed over the last few years.

I have now finished the fuselage and tail for "Sunspot", -sound like someone's dog; but one has to call it something!
I decided to go for a fully moulded fuz, mainly out of curiosity to see how difficult it was, but also there are now a set of moulds, available for anyone to use. I have to say while not up to the best professional standards, it is at least as good as the Monarch fuz as far as finish, and exactly the same weight as well at 2.5oz, including liteply frames, and ali tube pushrods. I won't go into the whole process of making the fuz; some of it was satisfying and some of it was bloody awful!
The fin, rudder and tailplane are of built up construction and covered in translucent film. Using the interim Monarch wing, and balanced to the same C.of G, the weight is nearly an ounce lighter than the Monarch at 13 oz which pleases me. Whether I can make its own wing any lighter than the Monarch one is doubtful, but we will try. The weight saving is due to lighter radio gear being installed as far forward as possible. I used two Hitec 55, a Jeti mini receiver and a 300 mah Nmh battery .
Back to the design choices. Whether to go for a fast low drag section, or a floater. As light as possible or more weight for penetration and zoom at the top of the bungee?
Certainly the current trend is for very light weight, which makes sense when one considers our winter thermals, or ridge soaring off a hedge with few leaves!. For two-channel competition HLG, 10oz is the norm for hand launch in the U.S.A., and under 9 oz has been achieved. How I am not sure, but certainly the specialists take everything to extremes such as using a pushrod for the aerial, stripped down radio gear and even using lithium camera batteries. I could certainly do a lighter fuz and tail, from my moulds, but it would not stand the multi purpose use I hope my model will have, as I take my 60" model around in the car to chuck off any hills that look suitable. Anyway I think that using a bungee, a little more mass may be of benefit at the launch phase. The "proper" wing will have ballast capability built in.

When we come to wing sections, the trend is to thinner sections, often around 7.5%. it seems that the favourite section for these small models have their maximum thickness further forward than those used for F3J. I believe this is to try and keep the wing with a turbulent boundary layer at these lower Reynolds numbers, as a laminar layer is likely to have a separation bubble and subsequent high drag.
HN 1033 is a popular section for HLG, as is Selig 4083. Comparing these with
SD 7037 and MH32, both popular for F3J, is interesting, particularly in terms of thickness, camber and location of maximum thickness and camber.
Section Thickness Camber Location max thickness. Location max camber
SD 7037 9.2% 3.0% 29% 40%
MH32 8.7% 2.4% 31% 43%
HN 1033 7.8% 2.5% 29% 44%
S4083 8% 3.5% 25% 36%
Monarch HLG. 8% 2.5% 25% 36%
(measured)

So it appears the Monarch uses S4083 with reduced camber. As I want Sunspot to launch higher than the Monarch I am going for the following concoction, modified from HN 1033, courtesy of Compufoil.
Sunspot 7.5% 2.4% 28% 38%

The tip will have slightly more camber and be thinned some more again to encourage it to hang on a low speeds.
This is not the "proper" way to design an airfoil, but the airofoils above show such variation, that I am confident that this will work quite well. Its really a slightly thinned HN 1033 with the thickness and camber location moved forward a bit. The nose profile is unchanged
Another consideration is that of the tip design, where I think an elliptical lift distribution is tied up with the dihedral distribution. Here is where I produce my own theory, as I like to try to understand why certain trends gain popularity.
As in small tight thermal turns the inner wingtip is travelling slower than the outer tip, some sort of wash out is desirable to prevent tip stalling. At high lift coefficient (ie low speed) there is considerable diagonal drift of the flow lines towards the wing tip, (more on the underside than the top, which is why there are trailing edge vortices). These vortices are strongest at the tip, and virtually nil at the central span. If we have considerable tip dihedral, this diagonal flow gives a form of aerodynamic washout, without the less efficient method of twisting the wing. (If you picture the wingtip end on, with the flow lines angling outwards, you will understand my drift). This, I believe is the main reason why tip dihedral, rather than straight dihedral is more stable when banking in slow turns. So the proper Sunspot wing will have polyhedral such that it approximates to an ellipse in front view. The actual tips will be swept back and be almost tiplets at 45 deg angle.
However the proper wing is for the future. I have test flown the model with the Monarch wing and achieved around 1 minute in still air from a mini bungee, similar to the vee tailed Monarch I guess. However it is easier to fly which was the point in going to a crux tail.

Part Two - the final version:

Over the winter I built up a new wing in the belief that I could get a higher launch, and have a lower sink rate. These two aims to come from a thinner section and lighter weight. The section is a modified HN 1033 with 7.5% thickness and 2.4% camber as detailed in my previous article. I have to say that my standard of building meant that the finished wing has considerable deviations from the Compufoil derived profile.

The wing is built up using 1/16" balsa ribs, carbon rod 2mm dia spar caps, a 2 mm balsa vertical sheer web with the grain spanwise, and 1/32" balsa leading edge sheeting top and bottom and trailing edge strips. In retrospect 1 mm balsa sheeting would be more robust and allow for more sanding to profile. It is 2 piece (as was the converted Monarch wing) and came out 2 oz lighter, so the finished Sunspot flew at 11 oz all up weight.

The spar construction is a little different to the norm. With such a thin wing bending strength is important and I went for 2mm carbon rods (available at Westborne Models) sandwiching a 2mm thick medium balsa vertical spar. The bottom rod was glued to the balsa spar, then the spar and the ribs slotted to half depth and the ribs assembled on the spar and glued. Then 2mm deep slots were filed in the top of the ribs and the upper carbon rod glued to the ribs and the top of the spar. Ideally the spar grain should be vertical but for a model of this size longitudinal grain was deemed sufficient. Obviously the depth that the balsa sheer web is cut to is critical. I really like these carbon rods, (and have successfully used them doubled up on a 3.2 metre wing as they provide good stiffness and resistance to buckling; the normal mode of failure in the top wing surface). Carbon is immensely strong in tension, but nothing special in compression.

The leading edge is two laminations of 1/16" hard balsa. The first is glued to the front of the ribs, sanded to profile and then the top and bottom sheeting added covering the first 1/16" strip. Then the 2^nd strip is added and sanded to profile. I use this method of leading edges as it allows the leading edge to be curved, and interlocks nicely with the top and bottom sheeting. The trailing edge is two strips of 1/32" hard balsa, sandwiching some carbon tows. This is the least successful aspect of the wing, being so slim it is difficult to make stiff. It really requires a carbon vertical member between the top and bottom strips. It is amazing how much torsional stiffness is added when the D box is completed. The joiner system uses carbon tubes and ali and or brass tubing.

It uses multi- break dihedral in the belief that with rudder only steering the outer panels do most of the work. Also I believe that a lot of tip dihedral adds to the aerodynamic washout.

I flew the finished Sunspot in the penultimate winter series at Chilbridge. The launches were spectacular, certainly an improvement. I am not sure about the lower sink rate; in the windy conditions it was difficult to press forward to the ridge lift off the upwind hedge. Probably some ballast and a little further forward C.G. might help. Certainly the model hangs around very well and will be good in less windy conditions.

Some construction views showing carbon rod spar and ribs slotted onto spar.

The finished Product: