ADV: THE SECRET IS IN THE WING
We have tested in flight the Savannah 100 Hp mounting the new tapered wing, with electrically activated flaps and slats that offers a good handful of extra miles per hour. What about the STOL performances? Surprise …
by Rodolfo Biancorosso
I must tell the truth: about two years ago when I was at ICP while they were assembling on a fuselage the prototype of a tapered wing with slotted flaps. I felt a bit puzzled; at first they still did not know whether to adopt leading edge slats with manual or electrical activation, or maybe just abolish them thus simplifying a lot the construction and the commands. Well, I assure that looking at this strongly tapered wing, plucky and clean, with enormous flaps deflected at 45°, I thought at first that it would have been difficult to manage the centre of pressure variations in taking off and landing, at last with that wing profile and those tail surfaces. Again, truth to tell, I was told that the wing, top secret at that time, was no more than a test stand to choose among different solutions and that in the future projects there was the idea of a faster Savannah, the possibility of back-fitting the wing on already flying models, and at last, the future creation of a series of high performance airplanes. Today, getting down from the Savannah ADV that we have flown without any problems in something about 30 kts wind, the first impression is the amazing balance that has been achieved, much better than many other popular aircraft: it is enough to know that all the slats-flaps system can be managed in absolute quietness, without using any trim. But there is a secret, and this is right in the wing. Let us see why.
Just one command
The new tapered wing with reduced surface has a NACA 2315 profile. In substance, a fast wing that seems to clash with the squared and simple fuselage of the Savannah, and moreover with its STOL vocation (it is enough to know that the stall speed at a MTOW of 560 Kg, in clean configuration, is up to 56 mph and we have achieved 50 mph during our test). In front, the slats run on linear guides and perfectly re-enter in the leading edge profile, keeping it rather clean (the sheets couplings could be more cured, but remember this is still a medium budget ultralight), At the trailing edge we find two real “monsters”: two big flaps with extremely high full deflection angle and another slotted winglet extending for the whole leading edge of the flap itself. Looking at the pictures, you can see how taut looks the aerodynamic in full flaps condition. It could seem a critical condition to be managed, and yet this is not so, both in piloting and in the activation of the command itself. In particular, the adopted solution is that of a single electrical command, therefore, for every degree of flaps deflection there is a defined position of the front slats: the pilot has just one switch to operate and the whole mechanism is automatic. This made possible to establish the optimal settings during the testing phase, thus avoiding potentially dangerous “personal interpretations” by the pilots and making the airplane safe and simple to manage. So, a single switch activates the four moving surfaces and their position is indicated by a pin moving along an arch drawn out at the root of the right wing, where the electric engine and the mechanical activation system are embedded, but still in sight. The regulation is continuous and in case of emergency a little winch is provided, to be connected to the electric engine axis with a cotter pin. This solution does not seem to be really satisfying; in fact the production aircraft will be fitted with a permanent winch connected to the engine axis, in order to allow for immediate activation in case of electrical failure.
The new wing has just one strut for each side with an intermediate glazing bar. The connections are the same of the , thus the upgrading of former models will need only modification of few details. Obviously the wing tanks remain the same, with a central leading tank situated behind the cockpit. The leader tank has a low level warning light that can be tested from the panel. The rest of the airplane is absolutely conventional and does not differ in anything at all from the we already know, except for the tail surfaces that instead of the usual single trim tab, mount two big winglets. These are so efficient and sensible, that only the left one is movable.
The first consideration to be made, at an external pre-flight inspection, is that the complex winglets and flaps system needs to be inspected in both positions: completely extended to check the sliding of the anterior brackets and their fixing bolts and in clean configuration to check the correct position. In particular, the exact coupling of the slat with the leading edge of the wing is assured by an adhesive rubber profile, with the double function of sealing the clefts, avoiding frictions and metal-to-metal vibrations. Brackets and command rods are well sized and easy to check, but anyway, the inspection job needs to be more diligent than usual. For the rest, the inspection is similar to the . Once on board, the eye is caught by the activation system at the root of the right wing, the indicator is well in sight. The command is progressive and not very fast; this is not bad for easiness of piloting. What is not acceptable, at least in our opinion, is the relative disorder of the crowded left part of the panel, where the switches of the wing commands, propeller pitch, trim and both ignitions seem to be too close one to each other. In an aircraft with such performances, we think it would be better to have a well designed secondary panel, allowing an immediate identification and separation of the switches. The current position may lead to some indecisions in the rapid adjustments that are often needed in flight. For the rest, there are not differences in the location of the other commands and services. We just note the presence of separate brakes on the pedals: on a machine like this that could be conducted on the ground just with the pedals, the separate brakes could be substituted by a single lever with a by-pass for hand-brake. The bar is as usual central and “Y” shaped there are two separate throttles. The instruments are clear and easily readable. Practical and clean is the ceiling panel for the electrical appliances. Anyway, the production ADVs will have a completely new panel.
Taxiing and take off
First of all, we performed two complete cycles of extension and retraction of the wing surfaces, just to make familiar with the command and its time of activation. We set at half extension for take off and we start the engine. During taxiing, the front wheel, linked to the pedals, is more than sufficient to assure an optimal ground handling. The main gear correctly absorbs the ground roughness. The front wheel could be stiffer. We note on this aircraft the lack of a display indicating the anti-tab winglet position, and this could be a problem, considering its high efficacy. But the reason is quickly told: this ADV is the testing model, thus the fuselage comes from the production in its simpler version. All the production ADVs will have both trim indicator and a super-complete instrumentation with electrical gyro and attitude indicator. The Rotax 912 100 Hp is coupled with the classic three-bladed Ivoprop, with in-flight electric variable pitch. This is a good compromise between good performances for take off and climbing medium performances at the highest speeds. The propeller setting at minimum pitch for taking off has to be found with the switch, but this is not difficult. During the take off run, the acceleration is good; the front wheel lightens already at 30 mph IAS, the rotation is done gently with an excellent directional stability. It is nearly not necessary to use the pedals to maintain a straight climbing direction.
Climbing and levelling
The initial climbing ratio is very good, about 800 ft/min just after taking off at 55 mph IAS We bring then the flaps to zero and … here comes the first big surprise: nothing happens, absolutely nothing … apart from the immediate rising of both speed and climbing ratio! There is no need for trimming, the nose does not jump up, and the transition is extremely gentle and progressive. But that is not all: the stabilizer anti-tab winglet has strongly reduced the bar efforts and the command is now extremely light and prompt: a possible very little attitude variation can be neutralized by a minimal pressure on the bar. We must say anyway, that we took off with neutral trim from the previous flight.
The air speed indicator of this aircraft is a bit optimistic and declares some miles in excess. The cross-checking with the GPS is clear in all flight situations, with or without wind, but the results obtained in cruise are really relevant. For a STOL aircraft. We can choose between an economic cruise at 100 mph (needing a little bit of nose-up trim) and a real fast cruise at about 115 mph. In case if you want to see the air speed indicator up to 125 mph and beyond, you just have to push further the throttle. But this has no sense at all: apart from the fuel consumption, such high speeds reveal the aerodynamics limits of the fuselage. Moreover, the air flow reaching the tail surfaces becomes rather turbulent, and the dances beautifully in turbulent air, if the speed is continuing high. Anyway, it is sufficient to trim and slow down to 100 mph to stop the dances all at once, even in the sensible turbulence of a downwind leg flown close to a hilly ridge. The trim is in particular extremely authoritative: a little impulse is enough to achieve a definite attitude variation. Probably, the trim command should be geared down, considering the general features of this airplane. Last, we must say that at high speeds in level flight a little bit of right pedal is always necessary to keep the ball in the centre. The ailerons are pretty efficient, the adverse yaw is minimal, and the efforts on the bar are acceptable, even if higher of those requested by the normal . Overall, the airplane behaves lively and always well coordinated. The stall in clean configuration arrives at about 50 mph (with a weight of 1000 lbs). This speed is definitely high and proportional to the wing load. The airplane has been designed to fulfil the Canadian regulations with a MTOW of 1236 lbs and in this case the stall speed clean would be 55 mph. Deflecting the flaps, we enter in another world of performances; once again we note that with the extraction, the nose does not drop to the earth and neither trimming is requested, nor pulling the bar. The speeds, anyway, come back to ultralight values. We put up full flaps and we arrive on the ground with some power on, in quite gusty conditions. No problems, on the contrary the commands remain efficient even in these conditions, ailerons included. What about the landing performances? Surprise! The ADV lands shorter, even if just a little, than the already excellent .
Setting up such a sophisticate aerodynamics and achieving such a level of balance is a symptom of a big design job, testing and adjusting. Anyway, a lucky and gentle star has probably assisted the evolution of this project. Apparently a “monster-wing” like that of the ADV does not go well with a rough and angular fuselage. But this has its precise meaning: first not betraying the STOL vocation, the strength and basic simplicity of the airplane, second to allow the back-fitting to the conventional Savannah owners, who changing the wing and some details, obtain an airplane with even more brilliant STOL performances and cruise speeds comparable to other popular tourers. And last the price, which is about $ 58800 on the Italian market (45.000 Euros, January 2005) for the complete and ready-to-fly aircraft. Considering the performances and the great easiness of use, we assume that the ADV could be able to compete wit airplanes of higher class and prices.
ADV 100 hp
Wing span 26.24 ft
Wing surface 101.18 sq ft
Wing cord at root 4.92 ft
Wing cord at tip 2.46 ft
Stall speed clean 50 mph
Stall speed flaps 34 mph
Cruise speed 115 mph
Maximum speed 130 mph
Take off run 115 ft
Climbing ratio 1200 ft/min at 59.65 mph
Wing profile NACA 23015
Load factor +6 -3 g