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Special: Facet Opal

Small, but gooooooood.
Many designers want to combine four factors to create an extraordinary airplane.
  • Small
  • Low drag airframe
  • Very low weight
  • Reasonably good power for the weight and size
Now … this one is a real beauty in all 4. The Facet Opal. 
In this page I want to show you what was possible with this tiny airplane. I hope to make it clear how he achieved that goal.
It was Scott Winton’s wish to get Australia back on the map of aviation. He wanted to break some records to achieve that. To get that done he needed to make sacrifices. Comfort was no longer important. He had to fit in and it needed to fly. Fly fast. Fly high. Fly far. And … that is what it did.
Scott Winton, the designer and builder of the Opal Facet, succeeded in breaking several records for homebuilts.
  • Time to climb to 15000 feet: 6 minutes
  • Time to climb to 20000 feet: 20 minutes
  • Maximum altitude: 30100 feet
  • Max speed: cruise at 150 kts (250 km/h)
  • Longest range: 3000 Nm
(If you have data about possible others, please, tell me.)
Now look at the airplane to understand how he managed to do those records. If you look at the cockpit, you see how very small the cockpit was. He did fit in. That is all.

No comfort, Just a seat, a dashboard and controls. Very very basic. Also noticed how low the cockpit is. He was very recumbent. Small frontal area gives lesser drag. The very sleek cockpit was a arrow through the wind.
If you want to see how basic the cockpit was, just go see how Scott handles the canopy. He has to hold it in his hands. No hinges! It was simply place and lock in position. Weight reduction all over.

Flying Plank

Scott choose to design a flying wing in flying plank configuration. No weight of a fuselage behind the wing. No large vertical tail. No separate elevator. All were reductions in weight and drag. Two tiny fins were installed to keep the airplane stable in yaw.

Very basic landinggear

If you take a look at the landing gear, you see the very narrow front wheels track. Just 20 inch (50.8 cm). It made ground rolls very interesting. Sometimes it wobbled over grass, wingtips touching the ground. Once speed began to rise the wings could be kept level by using the elevons. When you look at the details of the landing gear, you see it is all very basic and light.

Wing

The wings had constant chord. The airfoil of the wing must have reflex. The airfoil Scott choose was a modified airfoil. Which one I will not tell, because Scott made it known that he would change it if he would redesign the airplane. It made the airplane a good speed flyer. But a terrible low speed flyer.

Control surfaces

He used elevons as controls. Elevons combine the function of elevators and ailerons. You need to have a mixer-system to combine both controls in a single pair of control-surfaces. I really wonder how the mixer of Scott Winton looked like. If you know that info, please contact me.
Jim Marske used elevators and ailerons on his own flying wing designs. But it was due to the forward sweep that air separation started near to the fuselage. If you place your elevators near the fuselage, you get an ideal anti-stall-system by itself. Maybe control could have been better if the Opal Facet had no elevons, but elevators and ailerons separately. But … using a single system like elevons might have been easy to build and might have been lighter. So I kind of understand why he made the choice of using elevons.

Control stick

If you see the picture of the cockpit, you might see like me that Scott used a side-stick for control over the elevons. Do i see wrong? Anyway, i am very interested in how the control-stick worked. Reason: there has to be some kind of mixer involved to steer the elevons, which are a combined control of elevator and ailerons. I have seen a few mixer already, but i wonder how this clever designer did it. If you can show me pictures of his steering system, you would help us all to better understand the genius design of this airplane.

Small but pricy ...at that time

The Facet Opal may look like a very easy airplane to build. But … it was a very expensive airplane for its time.
The structure was like the one of the airplane that flew around the world, the Voyager. It was a mixture of Divinycell rigged PVC FOAM, ACI reinforcing materials and Dow Chemicals Derakane vinylester resins. It resulted in a airplane for 7g.
I guess that some materials were pretty expensive in those days and he also used negative moulds during construction. To counter the cost, he searched and found a sponsor. After the recordflights, the sponsor asked to add his brand on top of the wings. It was not painted on. It was a sticker on top of the fabric. The thickness of the vinyl created a no longer perfect airflow over the wing. The performance went a bit down. If those letters would have been placed on the underside of the wings, the effect would have been less. Uppersurface of a wing is very important. Undersurface can be less perfect and still be ok.
I do believe that today the project might be more acceptable in price for a one-off. It might not have the same end-weight if you build it without negative moulds.

History

I found this info at Aeropedia.com.au:
"Also known as the Winton Facet Opal the aircraft was completed and underwent twelve months of intensive test flying and refinement before its attack on the world records.  In that time more than 100 hours were flown and distances of up to 1,300 km (808 miles) were achieved non-stop.  When first flown the Opal was fitted with a Rotax 532 engine, achieved a speed of 254 km/h (158 mph) and achieved a sustained rate of climb of 914 m/min (3,000 ft/min).  In order to set records approval had to be received from the FAA, the international controlling body, and the Australian Civil Aviation Authority, which at that time limited this class of aircraft to 152 m (500 ft) above sea level.
On 5 March 1989, having flown from Tyagarah airfield to Evans Head, NSW, a record time to climb to a height of 3,000 m (9,943 ft) of 6 mins 46 seconds was set. On 16 March a time to height record to 6,000 m (19,685 ft) of 20 mins 30 seconds was set; and on 8 April 1989 an altitude record of 9,189.72 m (30,150 ft) for landplanes weighing less than 299 kg (660 lb) was set. Subsequently plans were put in train to attempt to set records in the speed and un-refuelled distance categories. Calculations projected a performance of an economic cruising speed of 252 km/h (156 mph) with a fuel burn of 6.9 kg (15.2 lb) per hour, an endurance of 17½ hours, with a range of 4,214 km (2,619 miles).

The Opal team received some sponsorship in relation to the record setting, including Mils Petroleum, Mobile One, Castrol, Global Aviation Support, Bert Flood Imports and Sky Sports. At one stage the Opal team was hopeful of gaining major sponsorship to fly across Australia in an attempt at the world record non-stop, non-refuelled distance for Microlight aircraft, the record at that time of 1,028.92 km (639.35 miles) being held by Thomas Pratt in the United States in a Mitchell Silver Eagle."

Dimensions

Seats: 1
Length: 3.2 m (10 feet 6 inch)
Span: 6.6 m (21 feet 7 inch)
Wing area: 10 m2 (107.6 sq.ft.)
Chord: 1.6m
Airfoil: modified NACA 66(1)-212 12 % thick
Empty weight: 110 kg (242.5 lb)
Fuel weight 117 kg (258 lb)
Max Take-off weight: 300 kg (660 lb)
Max wing loading: 30 - 32 kg/m2
Cruise: 250 km/h
Max speed 280 km/h (174 mph)
Initial rate of climb: 610 m/min (2,000 ft/min)
Fuel consumption at cruise: 9.2l / h
Absolute range: with 280 litres (62 Imp gals) of fuel 5,600 km (3,480 miles)
Engine: 30 kw (40 hp) Rotax 447 two-cylinder two-stroke in-line engine
Registration: 10-0004

Accident

But I need to end this page about this remarkable airplane by telling that the airplane crashed and killed Scott Winton. There are several versions of the reasons to be found. It might have been a combination of many reasons. Scott made a very high flight and the composites might have become altered by the cold temperature. A bearing of the prop shaft was located in the rear spar. Some say that the bearing was overheated and made the spar fail. It was apparently not the bad landing that killed Scott. He hit a tree during that bad landing and that was the reason of his death. A very sad ending of a wonderful designer and airplane.

Restoration

Currently the airplane is in restoration. They intend to make it in display status for a museum.

Discussions about its design

I have seen that many experienced persons had discussions about the Opal Facet. I will try to gather several if they seems to be acceptable enough to non-aero-engineers. 
Elevons:
Topaz on April 14, 2006:
"I went back to my notes and texts and did a little more research on pitch control placement for flying planks. The results were:

Full-span elevons have the least effect on the induced drag with control-surface deflection, as one might expect.
Center-mounted elevators (with outboard ailerons) affect the induced drag the most, and may promote tip stall because of the change in the lift distribution with 'nose up' deflections.
Outboard elevators (with either spoilers or inboard ailerons) are less effective as pitch control devices, because of tip losses (end plates can reduce the effect)
Full-span elevons worsen the landing problem I alluded to in earlier posts: When they are deflected for 'nose-up', they immediately reduce section Cl over the entire wing, before eventually increasing the angle of attack to the point that the overall wing CL goes up and you begin to climb. If you make a sharp nose-up control motion at low altitude (such as during flare), this can potentially cause the aircraft to settle suddenly and perhaps impact the ground. Limited-span pitch controls reduce this effect *somewhat*, but it's pretty much inherent in flying planks to one degree or another.
The increase in induced drag for center-mounted elevators can be reduced by increasing the local wing chord over the span of the elevator itself, along with some small adjustments to the wing planform near the tips. Nickel & Wohlfart give a theoretical basis for this and a method to determine the required increase in chord. Done properly, they claim that this 'fix' works "independantly of the CL value and the deflection angle" of the elevator. From what I can see, both Fauvel and Marske are using this technique - or at least they both are extending the elevator chord on their later designs.

Combining this with Marske's claim about center-elevators having a 'self-limiting' pitch authority near the stall led me to go for center elevators on my own design. When I ran lift distributions on my airplane, the initial stall was just outboard of the ends of the elevator, but still more than a quarter-span away from the tips. I was hoping this would still give me fairly tame stall characteristics from the point of view of the planform (my airfoil was quite benign all along). I wasn't getting initial separation over the elevators as a result of the planform, but I was hoping for some there due to my small 'fuselage pod' that would help a little bit - if only by providing some 'shake' to the pitch controls near the stall."

Related documents

A tribute to Scott Winton: http://ultralightaircraftaustralia.com/wp-content/uploads/2014/06/Scott-Winton.pdf
Aeropedia: https://aeropedia.com.au/content/winton-opal/