Fullscreen HTML5Med Res (??MB)HTML4

REMOTE CONTROL By BOB YOUNG The adverse effects of dihedral on aerobatic aircraft This month, we will conclude our considerations of the design elements in modern aerobatic pattern aircraft. This is an important topic in view of the trend towards more sophisticated electronics in R/C transmitters. In addition to the six points raised in the last column on this subject, in the June issue, there is one additional and very important point. Aerobatic manoeuvres must be presented to the judges in such a manner that the model aircraft stays inside a hypothetical "box". Thus, the size of the manoeuvres is severely restricted. The ramifications of this rule are serious and, in full size aerobatics, have tended to favour the development of the biplane, of which the Pitts Special is an outstanding example. Such a trend is developing in model aviation and we are beginning to see model biplanes appearing in aerobatic events. I feel this is a real pity, for we now live in the world of fast jets which use vast amounts of sky and which perform the most graceful and sweeping of manoeuvres. To /C::::::-, -+- - ~ c ~ SIDE-MOUNTED MOTOR return to sharp, jerky manoeuvres is very dissatisfying. I would not mind much if there was a choice but as it stands at the moment, the fast sweeping model is completely locked out of the competition scene. To me, the most spectacular and difficult manoeuvre is the low slow roll, performed at high speed and stretched out in a perfectly straight line over the entire field. This is very, very graceful indeed and exhibits complete mastery over the aircraft. Neutrally stable aircraft Back now to our analysis. To begin, the best approach is the centreline, neutrally stable aircraft. Thus, the basic parameters are as follows: wing and tailplane of symmetrical section, mounted on the thrustline. Fig.1 gives a stylised version of the design to date. /7\ ==---~~s ~~ . --===---SYMMETRICAL WING SYMMETRICAL TAILPLANE Fig.1: the best approach for aerobatics is the centreline, neutrally stable aircraft. This has symmetrical wing and tailplane sections mounted on the thrustline and will go exactly where you point it. 110 SILICON CHIP At this point, it is necessary to delve further into the secondary effects of the flight controls in order to establish some very important design parameters. One of the most destabilising effects of the controls on design considerations is the secondary effect of rudder, which is a roll in the opposite direction to the yaw. Fig.2 shows the problem. The rudder in effect acts as a mini aileron. The effect is most noticeable in a slow roll, due to the very small aileron deflection called for. Application of top rudder, applied when the model is on its side in knife-edge flight to keep the nose up (Fig.2), speeds up the roll on one half of the manoeuvre and slows it down on the other. This means that some compensation must be fed into the ailerons to keep the roll rate constant. This can be a real pain. If judges spot a change in roll rate, bang go more points. Here again we see the highly interactive nature of an aircraft between all three axes - an ideal opportunity for some computerised hocus-pocus from our encoders. The modern move to mixing of control channels drew its strength from attempting to derive electronic solutions for these problems. Knife edge flight There is also plenty of scope for some aerodynamic hocus-pocus. The obvious answer to this problem is to design an aircraft that will hold knife edge flight with a minimum of top rudder, or ideally, no rudder at all. This is an impor- RUDDER ANTI-ROLL Ct=J--=======:.:.-=.:.-:.-:.-~::---4<~r-T------c::;::b AILERON ROLL COMPONENT Fig.2: application of the rudder to keep the nose up when the aircraft is on its side speeds up the roll on one half of the manoeuvre and slows it down on the other. In effect, the rudder acts as a mini aileron. tant point and we will return to it soon. Knife edge flight is straight and level flight with the wings vertical (see Fig.3). Speed is a great help in this situation as we are relying entirely upon the lift from the fuselage side area and you will recall that lift is related to velocity squared. It also suggests a fuselage with generous side area and a plan view which is like an aerofoil section. Narrow, parallel sided fuselages are out. Let me point out here that many a designer has been bewitched by the problem of secondary rudder effects, including myself. One of the many fixes tried for this problem is the application of anhedral to the tailplane (ie, sloping downwards see Fig.4). Now we really are in it up to our nostrils. The primary and secondary effects of dihedral and anhedral when combined with the weathercock stabilisation effect provided by the fin and rudder are tremendously complex. Perhaps I had better explain. Dihedral effects The series of diagrams of Fig.5 show the primary and secondary effects of anhedral and dihedral when applied to a wing. Fig.5(a) shows the traditional effect of dihedral applied to a wing in order to provide roll stabilisation. If a gust from below upsets the air- craft and lifts one wing, the aircraft will immediately begin to fall sideways [sideslip) towards the low wing, thus producing a crosswind component striking the aircraft from the side of that low wing. This also applies if we fly one wing low, another very important point which will be discussed further. This will in turn call into play the weathercock stability of the vertical stabiliser and the result is that the aircraft will turn into this crosswind and thereby increase the deviation from the original flight path. This will ultimately lead to an ever increasing deviation and eventually a spiral dive. This is a real irony, for the better the weathercock stability, the greater the spiral instability, yet we cannot fly an aircraft without weathercock stability. As I have stated in some earlier columns, aerodynamics is one huge compromise and the balance between all of the various factors is a very delicate one. Let me tell you that there have been, and still are, good aeroplanes and bad aeroplanes. The bad ones are really dangerous to fly. How then do we get around this problem'? Fortunately, the addition of dihedral [ie, wings sloping upwards) provides a simple fix. In a side slip, which develops soon after a turn is initiated, the inside wing strikes the slipstream at a higher angle of attack than the outside wing and the aircraft rolls away from the crosswind component, thereby restoring the original flight path. This is a very important point when we come to pattern aircraft design and we will return to it shortly. Roll stability Fig.3: during knife-edge flight (ie, with the wings vertical), the aircraft relies entirely on the lift from the fuselage side area. This manoeuvre is best performed at high speed. The larger the dihedral angle, the greater the roll stability. The final tur:,ning ability and spiral stability will be a complex balance of various factors including dihedral, weathercock stability, and the power of the rudder, which in turn is a function of rudder area and angle of deflection. Now roll stability is fine in a aircraft designed to be positively stable. It is not so good in an aircraft designed to fly as straight as OCT0BER1990 111 is well earned. As stated previously, aircraft design is one great compromise. Biggest ever radio controlled model Fig.4: one approach to combatting the problem of secondary rudder effect is to use an anhedral tailplane. Another approach is to design an aircraft that will hold knife-edge flight without top rudder. an arrow and perform good slow rolls. You just simply cannot afford to have an aircraft designed for pattern flying doing its own thing and all of these factors fighting each other tend to produce an aircraft which constantly demands the pilot's attention during manoeuvres. They may fly themselves well but if you wish to deliberately disturb them, then they will fight you every inch of the way. In short, they are a pain in the neck to the aerobatic pilot. .They are, of course, a dream for the pilot on a long flight who does not wish to be constantly correcting the flight path. Dutch rolling Again too much of a good thing will produce "Dutch rolling" or excessive roll stability. The Boeing 707 suffered badly in this respect, having the twin roll stabilising effects of dihedral and sweepback, and an important autopilot function was the suppression of the Dutch roll. Here again is a prime example of electronics in action to minimise interaction between axes. There is an interesting aside here which some reader may be able to enlighten me on. I have always wondered why Boeing persevered with the low, swept wing on their jet airliners. The underslung engines called for large dihedral 112 SILICON CHIP angles as a consequence, in order to keep the outboard engines clear of the ground in a bad crosswind landing. I have seen a 707 scrape an engine on the ground (very spectacular) and the 707 has severe crosswind landing constraints imposed upon it for this reason. This dihedral, when combined with the dihedral effect of swept wings, results in excessive roll stability and thus the auto pilot fix. Flying a 707 with the autopilot unserviceable is not nice and early 707s had engines torn from the pylons in some extreme cases of Dutch rolling. The mystery is deepened when one considers that the Boeing B47, the forerunner to the 707, was a shoulder wing aircraft with anhedral. This anhedral cancels the dihedral effect of the sweep back and gives a much nicer flight characteristic, completely ellminating the Dutch roll. I find this issue an interesnng one and I have not yet had a satisfactory answer to my question. One factor does occur to me, that being the safety angle. The weight of the wing and fuel is underneath the passengers in an emergency and the solid wing spar protects them in a wheels up landing. Certainly, the Boeing airliners have a reputation for safety which In one of the most elaborate radio controlled experiments I have seen to date, NASA and the FAA combin.ed resources to produce what was known as the Controlled Impact Demonstration Flight. A full size Boeing 707/720 was crashed under radio control to test various safety features under development. One of these features was a new fuel which was believed to be much safer in a crash. At the last moment, the aircraft veered off line due to the pilot accidentally inducing a Dutch roll. As a result the experiment was somewhat flawed. The point here is that even the full size R/C operators have their share of problems and interaction between axes is one of them. To return to the world of model aviation, we can now see that an aircraft with a small dihedral angle and large fin will be directionally stable and spirally unstable. So much for the traditional approach to roll and spiral stability. By now it should be fairly obvious that the position for the neutrally stable aircraft is becoming fairly murky. All of these interactions tend to make such an aircraft an impossibility it would appear. It is also becoming obvious that the more simple the design the better. The position is compounded when we now consider knife edge flight, which is in effect only straight flight in a 'yawed position. This is an essential manoeuvre in an aerobatic aircraft, forming as it does one component of a slow roll. Now this really does set the cat amongst the pigeons. How do you make an aircraft with dihedral fly in a straight line when yawed? The answer is, of course, with great difficulty. Figs.5(b) and 5[c) show that when yawed, a model will immediately try to "weathercock" the fuselage back parallel to the direction of the airflow. At the same time, the dihedral on the wing will roll the aircraft away from the advanced wing. Thus, the aircraft banks and turns. f FL a LR t E (a) AERODYNAMIC FORCES ON A STRAIGHT WING WITH DIHEDRAL IN STRAIGHT FLIGHT LL fC) (c) STRAIGHT WING WITH ANHEORAL WR-.__ FL FL (b) STRAIGHT WING, DIHEDRAL IN YAWED OR KNIFE EDGE FLIGHT (d) STRAIGHT WING , NO DIHEDRAL , YAWED FLIGHT Fig.5: if an aircraft with dihedral wings is yawed (b), lift left (LL) will be greater than lift right (LR) & the aircraft will roll away from the advanced wing. An aircraft with anhedral wings (c) will roll in the opposite direction. If there is no dihedral (d), neither wing will lift. Interestingly enough, here we find the proof that an aircraft cannot be turned on rudder only without dihedral; it will only yaw. Fig.5(d) shows this quite clearly. If there is no dihedral, there will be no increase in lift on the advancing wing and hence no turn. Thus, if you wish to fly an aircraft such as an old-timer on rudder only, then you must have dihedral. Again, the larger the dihedral angle, the more responsive the rudder - initially! Then we run into the problem of the dihedral trying to correct the turn. Thus, we can see that a model with the correct balance of dihedral and fin/rudder area will turn beautifully on rudder only. Get this balance wrong and you will have a real pig-boat on your hands. The converse is that you cannot achieve knife edge flight with dihedral, without large control deflections to counteract the rolling effects of the dihedral. This results in a very difficult, crooked and awkward looking manoeuvre. One final word on dihedral: remember also that dihedral becomes anhedral when the aircraft is flown inverted. Thus the anhedral will try to force the model to return to the upright position, again making the pilot's task of keeping his aircraft inverted a difficult one. Finally then, after this long, drawn out analysis, we arrive at the conclusion that you apply any form of dihedral to an aerobatic aircraft at your own peril. We can now therefore add a wing with no dihedral to our list of essential parameters and stylised drawing. ~ OCT0BER1 990 113