WE SET OUT TOdesign an aircraftwhich on 100 hp

would be genuinely capable ofcarrying four people, their luggageand adequate fuel, withperformance equivalent to some ofthe best cross country aircraft onthe market. Furthermore, it had tobe capable of operating from shortgrass airfields.

space for fuel, and making thewing strong enough forundercarriage loads, yet light; ourtarget weight for the equippedwing was just 70 kg.

Much of the work had been doneand tested on the ultra-lightversion of the MCR two-seater in1998; the MCR ULC wing wasdesigned in 1996 in conjunctionwith Monsieur Colomban.The

Basically, once you’ve chosenyour engine (a Rotax) and madespace for four people, you haveyour fuselage. From then on, it’sall down to the wing.

The main problem is resolvingan impossible dilemma: you need asmall wing for low drag, but alarge one for STOL performanceon little power.Then there are thestructural questions—finding

164 | Pilot’s 100 years of flying

The MCR 4S wingInnovations that changed aviation

How can you cruise at 130 kt, land at 45 kt, andlift four adults, fuel and luggage on just 100 hp? The answer lies in a very clever wing design thatmight well point to the future for GA.By Christophe Robin.

When you fly it, theMCR 4S feels asthough a muchsmaller aeroplanehas been graftedonto a roomy fourseat cabin——and thesecret’s in the wing.

Pilot’s 100 years of flying | 165

The

MCR

4S

wingIn order to start with some good

solid wind-tunnel-tested data, welooked at existing profiles like thefour- and five-digit NACA series.At an early stage we decided toavoid laminar profiles because,though low drag in cruise, theydon’t work well with largesecondary surfaces—flaps andailerons—generating a strongnose-down pitching moment andgiving rise to load difficulties.

Once having chosen a likelyNACA five-digit profile, we workedon improving it with step by stepmodifications.The hundreds ofwind tunnel tests that had beencarried out provided empiric ratiosthat enabled us to predictmathematically the effect of changesin dimensions or shape.We alsoworked by eye, less scientifically,but ‘what looks right, flies right’.

The solution to ourrequirements was to have twoflaps, one contained inside theother.The main flap would be theFowler type that when retractedforms part of the wing profile, butwhen lowered also moves back,leaving a slot between the wingand the flap and increaseingeffective wing area. Because air isforced through the slot, it is ameans of enhancing airflow overthe upper surface of the wing,lowering relative pressure, andincreasing lift. By ‘blowing’ thetrailing edge, the airflow at thetrailing edge is accelerated, whichdelays the stall on the main body.Nothing is free, of course, and thedownside is a considerable increasein drag, but this is acceptableduring take-off when the airspeedis relatively low.

The second, intermediary flapwould only come into operation inthe landing phase, when the mainflap was fully lowered.Theintermediary flap then interposeditself in the enlarged slot betweenthe main flap and wing, making

kt—and achieve this ratio by usinghigh lift devices.These providefirst, high lift and high drag forlanding, second, high lift and lowdrag for take-off, and third, lowdrag and moderate lift at cruise.

It was clear that the key to ourproblem would be a very efficienthigh lift device which wouldsomehow provide all these threevery different combinations.

wing for the 4S was a developmentof this earlier wing, and part ofDyn’Aero’s continuous programfor transforming the efficiency andperformance of small, light aircraft.

To resolve the small/large wingdilemma we copied the solution onBoeing airliners.The big jet makersaim for a cruise speed four timeshigher than stall speed—in ourlittle four-seater, 140 kt and 35

166 | Pilot’s 100 years of flying

two slots and increasing lift evenfurther, and also dramaticallyincreasing drag and helping to slowthe aircraft down.

As this double-slotted flapstructure took shape it becameapparent that we would need aneven thicker wing section tosupport the flaps, which weregoing to be exceptionally large.Even with this thicker wing, wewould have to design it carefully tomake it adequately strong and stiff.

Before leaving slotted flap theory,I might mention one more benefit.Normally you increase lift byraising the angle of attack, but thiscreates a big requirement fordownload from the tailplane andelevator, which is very inefficient. Itcreates drag and necessitates largesurfaces that must be built strongand therefore heavy. A much moreelegant solution is to increase liftwithout raising the angle of attack,which is where the slotted flapscome in. Actually, the cruise load

on the horizontal tail surfaces inthe 4S is just about neutral, so theycreate very little drag.

Finally, a small wing behavesmuch better in turbulence—another benefit of slotted flaps, andthe high aspect ratio wingimproves roll rate.

The double-slotted flaps gave us amaximum lift coefficient for the

wing in landing configuration 2.5times greater than in its cruiseconfiguration—an enormousimprovement on the usual figureon similar aircraft of around 1.5.Also, we achieved a ratio of 1.5(usually 1.15) with the flap in thetake-off position together with alift-to-drag ratio equivalent to thecruising position.This gives the

The sculptedwingtips help, butare not essential——early MCR 4Saircraft flew wellwithout them.

The trick lies in theFowler flaps.

Pilot’s 100 years of flying | 167

The

MCR

4S

wingsame rate of climb with better

climbing angle with the flap attake-off position as without theflap. Usually, on light aircraft, thefirst position of flap creates a lot ofdrag and reduces the rate of climb.(This advantage allowed us to makea very efficient glider towingversion of the MCR ULC in 1999.)

Once you have the profile, youstill have to install it on a wing.Keeping the characteristics of awing section on a completed wingis not the easiest thing to do.

The flaps were so integral to oursolution, we wanted to install asmuch flap as possible on the wingspan.That meant designing anefficient aileron with the smallestpossible span. Having a short span,long-chord aileron would haveimplications for the structure andthe lateral characteristics of theaircraft.

We designed a very deepaileron—more that 40% of thewing chord—with a tiny span ofless than 3 feet.The whole designhad to be optimized for a very highefficiency, so to prevent adverseyaw the leading edge of the aileronwas designed to protrudeunderneath the wing when thetrailing edge was raised and webuilt in a differential between theup and the down aileron.

Three-quarters of the wingtrailing edge is fitted with flaps, avery high proportion.

The aspect ratio of 9.4 is also isalso quite high for a light aircraft.The wingspan of 28 ft 7 in ismodest enough to make parkingand hangarage easy.

As the wing was first designedfor the MCR ULC with a muchlower take-off weight (450kg) thanthe 4S, we added washout to thetips of the four-seater.This wasprimarily to improve the stallcharacteristics for what is intendedto be a family aircraft, but it hasalso had a beneficial effect on the

Two flaps doublethe slot effect, andmassively increasewing area. The flapsare huge in relationto the wing andleave little room forthe ailerons.

a good thing for cross-countryaircraft.The winglets are nowstandard on the MCR 4S.

The wing design doubles the liftcoefficient from the cruisingconfiguration to the landingconfiguration.The lift coefficienton the landing configuration issimilar to a B737 or A320.

We had to be very careful intesting all new aerodynamicfeatures of this wing especially nearand through the stall, includingfully developed spin testing.(Mostly pioneered, as was much ofthe wing’s development, on theMCR ULC that preceded the 4S).

One risk of our design was thevery high pitch down effect of theflap that could give rise to anelevator stall, with a complete lossof control of the aircraft on thehorizontal axis.We chose a T-tailconfiguration partly to minimizethis risk. In the 4S the pilot doesnot even feels the pitch-downeffect of the flaps because it iscompensated by an automaticdeflection of the tailplane.

The wing, both in the MCRULC and in the 4S has beensuccessfully tested through theJAR 23 standard, includingspinning, and more than 150aircraft have flown successfully. Atthe time of writing 12 MCR 4Ssare flying in France, Germany,Italy, and Holland.

Our aim was that the aircraftshould weigh less than 350kgempty, when all the four-seaterson the market were at least 75%heavier than this. It was importantthat the wing was strong, but also

168 | Pilot’s 100 years of flying

rate of climb.A lot of development went into

designing the winglets (sculptedwing tips), and this came late inthe wing’s design.The first year offlight (2000) was made with thestandard MCR ULC wing tip thatwas designed more for cruisingconditions. Meanwhile we wereworking with ONERA (OfficeNational d’Etude et de RechercheAéronautique, the mainaerodynamic office of France) on aspecial winglet.The objective was,while keeping the wingspan, tolower the drag in the climbcondition without creating anyadditional drag in cruise.

This was done with three-dimensional aerodynamicoptimisation numerical methods,then tested in a wind tunnel and inflight trials.We ended up with an8% gain of the drag coefficient inclimb and no penalty in cruise. Ithas also added a lot of directionaland lateral stability, which is always

Note the small tailsurface to minimisedrag. The wing isdesigned to keepdown pitch momentfrom extendingflaps. Even so, thetailplane isautomaticallydeflected to relievethe load, andbenefits from beingset on top of the fin,away from turbulentairflow.

Pilot’s 100 years of flying | 169

The

MCR

4S

wingvery stiff because of its high

cruising speed, so it was clear thatwe would have to make extensiveuse of carbon fibre.The wing isskinned with an unsymmetricalsandwich of carbon fibre. Its innerlayer is designed to take thetortional load, and the ‘meat’ is 6mm PVC foam.The fibres in theinner skin are laid at opposing 45°angles to line of flight, but in theouter layer the fibres are laidchordwise.The inner skin thickensprogressively towards the root.

Inside each side of a wing thereare eight equally spaced ribs.Structural ribs and those near thefuel tanks are beefed up withcarbon fibre, but those outboard aremade from foam.The tanks are inthree compartments, each holding42 litres. (A long range alternativehas five compartments per wing.)

The spar technology had beeninitially designed in 1993 for theDyn’Aero aerobatics aircraft,especially the CR100 (the two seat,side by side, 180hp training andcompetition aircraft that startedthe company in 1992). It is a wood,foam and carbon fibre technologythat ends up with very light wingspar, and very easy qualitycontrolled to ensure safety.Thespar caps are wood and fibre, bothwith opposing 45° grain, separatedby wood at attach points, and foamelsewhere.This technology hasbeen continuously improvedthrough all Dyn’Aéro aircraft(1996: MCR01, 1998 MCR ULCand MCR CLUB, 2000 MCR 4S).

On the MCR 4S we chose a one-piece wing with a double spar.Thelanding gear is mounted on thewing, and the aircraft is notdesigned to be de-rigged veryeasily like the two-seater versions.The two spars were required to fitthe feet of the passengers inbetween, and made a thinner,more aerodynamic and lighterfuselage. Also, we wanted to be

able to put as much as 200 liters offuel (10 hours of flight at cruisesetting), which meant filling up theentire wing.

We ended up with a 52 kg (118-lb) wing without equipment and 70kg (159 lb) with flaps, ailerons, fueltank, winglet and landing gear

attach structure, which met ourdesign criteria.The MCR 4Sreached all its objectives with a 130kt cruising speed on 100 hp (145kt in altitude with the turbo), astall speed of 45 kt, and an emptyweight of 350 kg for a maximumtake off weight of 750 kg.

To be effective, theailerons have tomake up in chordwhat they lose inspan.

The massive controlsurfaces putadditional strain onthe wing, butcarbon fibre hasenabled it to copewithout becoming inthe least heavy.