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REMOTE CONTROL By BOB YOUNG Aerodynamics & stall conditions In our discussion so far on aerodynamics, we have seen that the formulas for lift and drag follow a square law so that if you travel twice as fast, you generate four times as much force. This causes a lot of problems in airframe design and means that to fly very fast, wings must be small. Because weight follows a relatively linear law, there is much to be gained by increasing the speed. With modern engine technology squeezing more thrust out of less weight and modern engineering and material technology reducing airframe weight, we can arrive at the curious situation where we now have too much wing. We are thus flying so fast and the wing is generating so much lift that we must fly at slight negative angles of attack to get rid of it. This is inefficient as we really need to get the wing working at or near its best lift/drag ratio. The obvious solution here is to reduce the wing area and force the wing to work at a higher angle of attack. Thus we arrive at modern experimental aircraft such as the X-3 which has the tiniest wing imaginable but is designed to fly very fast. The compromise here is always speed against manoeuvrability. The test pilot slated to fly the X-3 was said to have admired the fuselage shape greatly when he first saw it and enquired when the wings were to be fitted. When advised that the stub already fitted was the wing, he was somewhat taken aback. The modern military designer works at this problem in another 74 SILICON CHIP way. He just keeps cramming more and more equipment into the airframe until it is grossly overweight and needs all the wing it can get. Now we come back to the interactive problem again. Just how do you get this thing off the ground at low airspeeds? In some instances, NASA cheates and launches these X-aircraft from a mother ship. It still must be landed however, much to the pilot's concern. How do you get this machine back on the ground safely when lift is falling with the square of the velocity reduction as it slows for landing. This, as any pilot will tell you, is the really crucial question. Very early in the piece, pilots in the US Army-Airforce strongly objected when first introduced to the old Peashooter, the first low wing monoplane fighter introduced into service. They were very perturbed by the extremely high landing speed of this aircraft. The old biplanes used to drift over the fence at 40 to 50 knots whereas the Peashooter roared over at 65 knots. Compare this to the Spitfire (90 knots) and the X-3 (240 knots). Times certainly have changed. Wing loading One of the critical factors in landing speed is wing or surface area loading, which is an expression of EFFECT OF VARIOUS LIFT INCREASING DEVICES ON WING CHARACTERISTICS BASIC AIRFOIL 15° - - 12° 51% 51¾ 'SLOTTED FLAP 12° 53¾ 42¾ FRONT SLOT (AUTOMATIC) 28° 26¾ 35¾ 19• 69¾ 7% 19° 75¾ 10% SPLIT FLAP 14° 70¾ 63¾ ZAP FLAP 13° 85 ¾ 77¾ FOWLER FLAP 15• 90¾ 83 ¾ ' ,.. SIMPLE FLAP FRONT SLOT AND SIMPLE FLAP FRONT SLOT AND SLOTTED FLAP ' \ ' \ '\. ' Fig.1: modern aircraft rely a variety of lift increasing devices so that the aircraft can be brought to a manageable speed for landings. This table shows the effect of some of these devices. the gross weight of the aircraft over the lifting surface area. This is expressed in lbs/sq ft or kg/sq metre. Thus, if we want to fly fast (ie, we reduce the wing area), we suffer some penalties and one of them is increased wing loading and higher landing speeds. Many compromises have been devised to overcome this problem, the most exotic of these being the variable sweep wing (eg, the F-111). This mechanism serves a twofold purpose but the one of interest to us is the increase in wing area (and thus lift) it offers for low speed flight. Model aircraft typically fly with wing loadings between 1.51b and 2lbs per square foot. In a recent survey of full size flying boats, the wing loadings ranged from 30-60lbs per square foot. Modern fighters can run as high as 150lb/sq ft. Variable geometry The other fact of importance, as we have already seen, is CL which can be greatly influenced by the shape of the airfoil and the angle of attack at which the airfoil is presented to the airflow. Thus, by using variable geometry wing sections (flaps, slots etc) and flying slower and thus forcing a higher angle of attack and therefore CL, we can bring the aircraft to a manageable speed and attitude. Fig.1 shows a variety of lift increasing devices and their relative effectiveness. Modern high speed aircraft rely heavily on these devices and on even more modern and exotic systems developed since this chart was prepared. Forcing a higher angle of attack also increases the drag markedly and further slows the aircraft. In this manner we can eventually put it down safely, albeit at a somewhat high speed, for the prime factor is always that V2 law. Small increases of speed give big increasef! in lift. Thus, it is always cheaper and easier to train pilots to land at higher speeds than to squeeze more lift out of linear devices such as airfoils and flaps. Interestingly enough, the power setting which gives the slowest airspeed is maximum power. There are several reasons for this but put briefly the thrust vector becomes an increasingly important lift component, until you reach the VTOL (Harrier) style aircraft. In this case, thrust is the only lift component. This type of aircraft is a special case and breaks all of the rules of flight. The VIFF (vector in forward flight) ability of the Harrier gave the Argentinian pilots quite a headache. Stalling As we saw last month, there is a definite relationship between angle of attack and C1. As the angle of attack is increased, there is a corresponding increase in C1 until the streamlined airflow breaks down into turbulent flow. At this point, the airfoil is said to have stalled and the wing losses nearly all lift and the nose of the aircraft drops. Depending upon the cleverness of the aircraft design and the task it is designed for , this stall can be violent or very gentle. In some cases, the aircraft can fall sideways and spin. Stalls are very dangerous in piloted aircraft and all manner of safety devices have Fig.2: the Boeing P-26 Peashooter was the first monoplane fighter produced for the US Army Air Corps, as well as being the first all-metal production fighter. It first flew in March 1932. been devised and fitted to aircraft over the years. Stall warning hooters can give the nervous passenger quite a fright. Landing is a dangerous time in an aircraft for the art of landing is to fly as slowly and therefore as close to the stall as safely possible. Errors in judgement can have serious consequences. This is particularly so in bad weather. Such factors as wind gusts, wind shear, bad visibility, icing and wind gradient can have very powerful effects on an aircraft. Wind shear The last factor is very important in model flying. Briefly, the viscosity of air results in a slowing of the wind speed as it gets closer to ground level (Fig.3). This effect is more pronounced the closer we approach the surface until within 20cm of the ground the wind speed almost ceases. Full size aircraft experience this to some degree but their wings never get to within centimetres of the ground; those on model aircraft do. As the aircraft settles for landing, sinking deeper and deeper inMAY 1990 75 SURFACE VISCOSITY ~ 77777777777777777777777,, to this wind gradient, lift begins to fall and the model starts to sink more quickly. The pilot pulls back on the stick, increasing angle of attack and drag and further reducing airspeed. The result is a stall or a thumped in landing with possible airframe damage. The moral? Keep some speed in hand for this possibiltynr use throttle instead of elevator to increase lift. I learned long ago to think of an aircraft as a 4-dimensional or 4axis vehicle, with throttle (therefore speed) as the fourth axis for this reason. The final interactive factor we will discuss is the relationship between thrust, drag, lift and the ability of an aircraft to turn very tightly. As we have already discovered, an aircraft requires enormous amounts of thrust to achieve high flight speeds. With drag increasing at the square of velocity, the total drag figure for even a very clean airframe is extraordinarily high but what happens when this aircraft is travelling at low speeds? 76 SILICON CHIP Fig.3: landing can be a dangerous time for model aircraft due to wind shear - an effect whereby the wind speed decreases near ground level This can lead to a sudden reduction in lift so it is important to always lceep some throttle in reserve. Reserve power The result is an enormous amount of excess thrust which can be put to many interesting uses, not the least being take-off. Watch a modern jet airliner blast off the runway and pull up into a steep climb. Because it climbs out at much lower speed than its maximum level flight speed, an airliner has much more thrust to channel into its lift/thrust vector. Compare this to the poor old DC-3 tottering off the same runway. The climb angle is very low because the top speed of the DC-3 is only about twice its take-off speed. There is very little reserve horsepower for climbing. Loss of an engine in a DC-3 was a serious business for reasons already discussed. The situation in a modern jet fighter is even more pronounced and a Mach 2 fighter manouevering at 400 knots has an enormous amount of reserve thrust for use in turning and climbing. The ability of a fighter to turn tightly is a complex issue but is related to the stalling speed of the aircraft which is related to wing loading and thrust, as we have already encountered. The radius of the turn is given by the formula: r = (Vs 2/g)(CL maxlCL) To quote Kermode, "this shows that the radius of turn will be least when: C1 = C1 max ie, when the angle of attack is the stalling angle and the radius of turn equals Vs2/g. It is rather interesting to note that the minimum radius of turn is quite independent of the actual speed during the vertical banks. It is settled only by the stalling speed of the particular aeroplane. Thus, to turn at minimum radius, one must fly at the stalling angle, but any speed may be employed providing the engine power is sufficient to maintain it. In practice, the engine power is the deciding factor in settling the minimum radius of turn whether in a vertical bank or any other bank, and it must be admitted that it is not usually possible to turn on such a small radius as the above formulae would indicate". That was written in 1932. Air has not changed much since then but engine technology certainly has. Kermode then goes on to sum up the turning ability of aircraft: "The formula above applies to some extent to all steep turns and shows that the aeroplane with the lower stalling speed can make a tighter turn than one with a higher stalling speed. But in order to take advantage of this we must be able to stand the g's involved in the steep banks and we must have engine power sufficient to maintain turns at such angles of bank" . Correct balance The ramifications for modellers is that the correct balance of weight, wing section, wing area and power loading is vital for a successful aerobatic model. Underpowered models are very poor performers. Keep in mind here that loops are only a special case of a vertical bank. As stated previously, models do not carry pilots and thus do not need to consider human comfort during manoeuvres. The model designer must however consider the structural forces involved for models can easily pull 10-15 g's. I have seen models snap wings in flight and this is very dangerous for those standing on the ground! Once again however, the central fact is the power required to keep the airspeed constant in spite of the increase in drag brought about by the increase in angle of attack. And again, the scenario described last month of an aircraft falling into the drag bucket during take-off also applies to an aircraft when turning. As the aircraft begins to bank, an increase in the angle of attack is required in order to provide the increased lift which is needed to provide the acceleration towards the centre of the turning circle. This calls for more power. If this power is not available the model will slow down, losing lift and forcing the pilot to increase elevator deflection in an attempt to maintain altitude. This is futile, for the increased angle of attack will only increase drag and cause further reductions in airspeed. The most probable outcome of this situation is a stall and spin into the ground. The stall and spin on final approach was a common cause of crashes in the early days of aviation. Equally dangerous is the stall and spin after loss of power on take off. This is most commonly brought about by pilots turning quickly in an attempt to make the airfield while some altitude remains. Land straight ahead The golden rule in this situation is, if in any doubt, land straight ahead. Without power the nose must be pushed down to maintain speed during a turn and a lot of altitude can be lost, leaving the pilot with a downwind landing on his hands. Runways shrink in length dramatically in this situation and those barbed wire fences at the boundary do awful things to aeroplanes. Fig.4: this diagram shows the forces acting on an airplane during a properly banked turn. The centripetal force is provided by banking the aircraft so that the wings can supply a component towards the centre of the turn. CENTRIPET Al FORCE wv2 time were unsound. I may have landed it straight ahead, only to hit a rock or a treestump hidden in the long grass. That would have been bad luck. From hundreds of yards away it is impossible to see hidden objects. But to stall and spin was bad airmanship. The real issue is that all of these situations call for split second decision making and a sound grasp of the fundamentals involved. Full size aviation schools teach their pupils these badly needed facts of aviation life but the tyro model flyer is rarely taught even a rudimentary knowledge of aerodynamics. The result is more broken models than necessary. Worse still is the loss of valuable but discouraged people from the R/C model movement. The true worth of a pilot is measured in his ability to analyse lbs gr W lbs Power can be lost for any number of reasons, some quite odd. I once crashed a model in the above situation when I lost a blade off my propellor. I made all of the above mistakes and tried to nurse it home on reduced power. The result was a stall and spin. The correct procedure was to throttle back and land straight ahead. In my own defence, I did not know that I had lost a blade at that time and there was one of Charlie Brown's kite eating trees stretching out its limbs directly in front of me, with rough ground underneath. But the fact remains I lost the model and so the decisions I made at the and initiate the correct remedial actions for all in-flight problems. A pilot's ability to land an aircraft under the most trying conditions is paramount to the safety of both himself and his passengers. Beginners seem to feel that aerobatics are the most difficult and rewarding of in-flight activities. Let me state right here and now that aerobatics are easy and unimportant. The most difficult and rewarding manoeuvre is landing that aeroplane safely every time you take off. For this reason landings must be practised continuously and from every possible angle and contingency. ~ MAY1990 77