Savannah ADV: The Secret Is In The Wing
By Rodolfo Biancorosso
Translated From the Italian Magazine
Aviazione Spotiva We have tested in flight the Savannah 100 Hp with the new tapered wing, with electrically activated flaps and slats that offers a good handful of extra kts 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 Savannah 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 of the construction and the controls. Well, I assure that looking at this strongly tapered wing, plucky and clean, with enormous flaps deflected of 45°, I thought at first that it would have been difficult to manage the centre of pressure variations in taking off and landing, at least 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, with the possibility of replacing the wing on already flying models, and in the future the creation of a series of high performance airplanes. Today, getting down from the Savannah ADV that we have flown without any problems in something like 30 kts winds, the first impression is the amazing balance that has been achieved, much better than many other popular aircrafts: it is enough to know that all the slats-flaps system can be managed with absolute ease, 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 performance (it is enough to know that the stall speed at a MTOW of 560 Kg, in clean configuration, is up to 49 kts and we have achieved 44 kts 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 aerodynamically stressed it looks in the full flap position. It could seem a critical condition to be managed, and yet this is not so, both in piloting and in the activating the controls. In particular, the adopted solution is that of a single electrical control, therefore, for every degree of flaps deflection there is a defined extension of the front slats: the pilot has just one switch to operate and the whole mechanism is automatic. This made it 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 motor and the mechanical activation system is 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 motor shaft with a cotter pin. This solution does not seem to be really satisfying; in fact the production aircrafts will be fitted with a permanent winch connected to the motor shaft, in order to allow for immediate activation in case of electrical failure. The new wing has just one strut for each side with a jury strut. The connections are the same as the Savannah, thus the upgrading of former models will need only modification of a few pieces. Obviously the wing tanks remain the same, with a central tank situated behind the cockpit. This tank has a low level warning light that can be tested from the instrument panel. The rest of the airplane is absolutely conventional and does not differ in anything at all from the Savannah we already know, except for the tail surfaces that instead of the usual single trim tab, there are two big winglets. These are so efficient and sensitive, that only the left one is used to trim. On Board The first thing to be done, for the 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 interior 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 gapes and avoiding frictions of metal-to-metal vibrations. Brackets and control rods are well sized and easy to check, anyway, the inspection needs to be more thorough than usual. For the rest, the inspection is similar to the Savannah. Once on board, the eye is caught by the control mechanism at the root of the right wing, the indicator is well in sight. The activation is progressive and not very fast; this is not bad for ease of piloting. What is not acceptable, at least in our opinion, is the relative crowded left part of the instrument panel, where the switches for the slat/flap controls, propeller pitch, trim and both ignition switches seem to be too close 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 indecision in the rapid adjustments that are often needed in flight. For the rest, there are no differences in the location of the other controls and instruments. We just note the presence of separate brakes on the pedals: on a machine like this that could be manoeuvred 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 stick is an usual central “Y” shape there are two separate throttles. The instruments are clear and easy to read. Practical and clean is the ceiling panel for the electrical switches. Anyway, the production ADVs will have a completely new panel. Flight impressions Taxiing and take off First of all, we performed two complete cycles of extension and retraction of the wing surfaces, just to get familiar with the control 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 optimal ground handling. The main gear correctly absorbs the ground roughness. The front suspension could be stiffer. We note on this aircraft the lack of a display indicating the trim-tab winglet position, and this could be a problem, considering its high efficacy. But the reason is quickly explained: this ADV is the testing model, thus the fuselage comes from the Savannah production in its simpler version. All the production ADVs will have both trim indicators 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 26 kts IAS, the rotation is done gently with an excellent directional stability. It is hardly 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 48 kts IAS We return the flaps to zero and … here comes the first big surprise: nothing happens, absolutely nothing … apart from the immediate rising of both the speed and climb rate! 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 trim-tab winglet has strongly reduced the stick efforts and the control is now extremely light and responsive: a slight variation in attitude can be neutralized by slight pressure on the stick. We must say, that we took off with neutral trim from the previous flight. In flight The air speed indicator of this aircraft is a bit optimistic and declares some miles in excess. 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 87 kts (needing a little bit of nose-up trim) and a real fast cruise at about 100 kts. If you want to see the air speed indicator up to 110 kts and beyond, you just have to push the throttle further. But this has no sense at all: apart from the fuel consumption, such high speeds reveal the aerodynamics limits of the fuselage. Moreover, the airflow reaching the tail surfaces becomes rather turbulent, and the Savannah dances beautifully in turbulent air if flown at high speed. Anyway, it is sufficient to trim and slow down to 88 kts to stop the dancing, even in the sensitive turbulence of a downwind leg flown close to a hilly ridge. The trim is extremely responsive: a little input is enough to achieve a definite attitude variation. Probably, the trim control 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 very efficient, the adverse yaw is minimal, and the efforts on the stick is acceptable, even if higher than those required for the normal Savannah. Overall, the airplane behaves lively and always well coordinated. The stall in clean configuration arrives at about 44 kts (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 48 kts. Deflecting the flaps, we enter in another world of performances; once again we note that with the extension, the nose does not drop to the earth and trimming is not required, nor pulling the stick. The speed, 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 controls remain effective even in these conditions, ailerons included. What about the landing performances? Surprise! The ADV lands shorter, even if just a little shorter than the already excellent Savannah. Conclusions Setting up such a sophisticate aerodynamics aircraft and achieving such a level of balance is the result 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 performance, the strength and basic simplicity of the airplane, second allowing retrofitting to the conventional Savannah, by changing the wing and some pieces, obtains an airplane with even more brilliant STOL performances and cruise speeds comparable to other popular tourers. And last the price, which is about $ 58,800 on the Italian market (45,000 Euros, January 2005) for the complete and ready-to-fly aircraft. Considering the performances and the great ease of control, we assume that the ADV could be able to compete with airplanes of higher class and prices. Summary Savannah ADV 100 hp Wing span ----- 8m
Wing surface ----- 9.4 sq/m
Wing cord at root ----- 1.5m
Wing cord at tip ----- 0.75m
Stall speed clean ----- 43 kts
Stall speed flaps ----- 30 kts
Cruise speed ----- 100 kts
Maximum speed ----- 112 kts
0 Take off run ----- 35m
Climbing ratio ----- 1200 ft/min at 52 kts
Load factor ----- +6 -3 g
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