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Air News On when An Aeroplane Tells You Something -Listen To It! & Learning To Fly
At times an aircraft will raise your hackles and you won't know why. Brian Lecomber has been there………..
There is little to be said in favour of retiring from professional aerobatics. I am constantly being asked, with the solicitude of an undertaker enquiring how your unmentionable social disease is coming along, whether I miss display flying.
Daft question. Of course I bloody miss it. Er, well, most of it, anyway.
Some things I do not miss. I quietly grin at the thought that never again will I have to live with some obscure and prolonged technical fault which defies all attempts at logical resolution.
For, in along career, I have learned one astonishing fact. That it is not major emergencies which sear a pilots soul, but the little cancerous tumours which neither a surgeons knife nor an engineers spanners seem able to cure.
I have had a Pitts Special engine explode on me. I have had a propeller shatter. I have had a Warner Scarab fail so many times that the search for possible landing-grounds within gliding reach became by far the most important preoccupation of every flight.
I have had another Pitt's engine consume a piston and a few seconds later obliterate- as usual in these happenstances- all forward visibility when the front oil-seal blew out and dumped the contents of the sump over the windscreen, rendering my immediate landing in a ploughed field an excitingly blind experience.
I have gad Gipsy engines fail. I have had Lycomings fail- I have had partial structural failures, and terrifying control jams.
All of these events were highly attention-grabbing at the time, and I will be the first to admit to adrenaline squirting out of my ears and hands which trembled for hours afterwards.
But….they were all honest failures, if that makes any sense.
A crankshaft breaks, a piston fractures, some idiot (quite possibly me) drops a coin down a control torque –tube…..and as a result you, the pilot, have a short but intensely interesting time placing the aero plane and your own frangible neck back on the ground without further mayhem.
This is an Endeavour which all who have been through it will admit is as much presided over by Lady Luck as it is by the quivering human jelly allegedly in control.
But these are instant emergencies .You react .You survive or do not survive. Assuming the former, they do not haunt your sleep. They are honest failures, over and done with.
No, the things you lose sleep over are the odd things-often apparently little things-which you cannot for the life of you, explain. When I was a tiny child I imagined our house was infested with beasts which for some reason I called "Bosas".As an adult, these Bosas took on the form of aircraft snags. All your flying instincts know the aero plane is trying to tell you something-but what? Why is this happening……….
The following are three small things which have happened to me. Tell me if you would have done a better job of resolving them.
The Pitts That Lost Power
Over a three-month period a certain Pitts Special entrusted to my keeping steadily lost power. Nothing sudden or dramatic, but a slow decline .What had been a 1600 fpm climb-rate decayed to 1500 fpm.
In my solo display it meant I was forever fighting for energy .In formation display it meant I was forever ramming on full power two or even three seconds before I should have needed it, so as to get an intraformation acceleration started.
Acceleration within a formation is often essential but always attended by a certain pucker-factor. The Boss calls "Blue,pullying up NOW", and I ,sitting a few feet behind and below him, have to go instantly to full power on the B….of "Blue" in order to build up the acceleration I need to stay in place around the loop. If I am too late applying full power I will suffer the indignity of sliding back around the top of the loop. If I am too early I will saw the Boss' undercarriage off as I accelerate.
So an engine losing interest becomes an issue. A Bosa in night. Big time.
MP and RPM showed up fine on air tests. Cylinder compression checks showed up fine.Mag checks showed up fine. Timing checks showed up fine. Changing the fuel injector made no difference .Ditto the injector nozzles. Ditto the spark plugs.
Why is this happening……….?
Then came the big clue. Back-checking the maintenance records, an engineer noticed that the C/S prop governor had had its high-speed stop adjusted during each of the last two checks because the engine was going a little over its 2700 redline. Bingo!
I expect you're ahead of me now .A strobe-light check on actual propeller rpm-atest which was a bit of an exercise back then-revealed max rpm to be a tad under 2600, while the aircraft rev-counter said 2700.The rev-counter had been slowly failing over a period of time, said failure being in the form of an over-read .The engineers had seen that as an over-speed and wound the prop governor down accordingly.
So with one hand they'd been going over the engine with a fine toothcomb, while with the other they'd been gradually reducing the max rpm. A new rev-counter and re-setting the governor solved the problem instantly.
With hindsight, it should have been obvious. But it wasn't obvious at the time…………
The Extra That Ate Starter
I am firing up to fly the first sequence in the British Aerobatic Championships. Every other aircraft in the competition has cranked up quite normally-starter motors going chir-chir and then the snarl as the engine starts.
I wince as I turn the key to start, because I know what is going to happen. And it does. My starter goes chir……CHANG….. CHIR… DRING……chir – and then the engine catches.
I breathe out and try to re-focus my tiny mind on the aerobatic sequence to come.
I know what the problem is. The problem, is that the gear-teeth on the starter motor and the engine starter –ring are chewed up all to Hell-so much so that the starter frequently jumps several teeth on the ring and produces the CHANG which shudders both the airframe and my sol alike.
I also know full well that this will shortly prove fatal to both starter-motor and starter-ring. Moreover I know to the exact cent what each will cost to replace-which is a lot-because I replaced both two weeks ago and am now holding new spares to replace them again.Oh, I know what the problem is. What I don't know is: WHY?
Why should an Extra 230, a lovely aero plane, suddenly develop a massive appetite for starter motors and starter –rings? Why?
Apart from the changes and chings, the actual engine-start is perfectly normal. The starter-motor is properly mounted. The electrical system is wholly sound. We have been over it again and again. So why….?
A week later I stumble on the answer. In the maintenance records an entry says: "Smoke tank removed from aft of cockpit. Starter solenoid removed for access. Union replaced on smoke tank. Tank remounted .Starter solenoid re-fitted".
I pull the cockpit cushion out of the way. The starter solenoid is clamped to a fuselage cross-tube and mounted vertically, just like it is in any car.
But not in an Extra. Or any other aerobatic aero plane.
About +7G is enough to pull the guts of the solenoid down. Which dutifully activates the starter motor? Which then mindlessly engages with the engine –at 2700 rpm……?
This, inevitably, is not a happy union.
Yes, there would have been a considerable DRING noise. Yes, the high speed and the 7G you would not have seen or heard any of it, your attention being very much elsewhere at the time.
We turned the solenoid 90 degrees, to lie fore-and –aft. The problem disappeared.
The Pitts That Refused To Start
When you turn a propeller on a traditional aero-engine, by hand or by starter-motor, you hear a click .There is a good reason for this click. Old generation engines use magnetos for ignition.Magntos have many things going for them, but a powerful spark at engine-cranking speed is not one of them.
So, welcome the impulse-mag, which has a sort of clockwork device which whaps the mag round at three times engine revs during start-up and then drops out to leave the mag running normally once the engine is running. The click you hear is the impulse mag winding-up and releasing.
Except on this particular Pitts engine. Normally you hit Start and the thing goes chir-click-chir-click, chir-clickand then the engine, fires.
This one developed a habit of going chir……..maybe-click……chir……no-click……chir ……chir –which significantly failed to fire up anything.
Okay. An impulse unit snag. Unusual in a Slick or Bendix mag on a Lycoming, but not unknown.
So we replaced the impulse unit. No change.
Blinking in surprise, we changed the whole magneto. No change.
We took the back of the engine apart, found nosnags, and put it back together again. No change.
In desperation we raised a CAA Modification to fit two impulse mags instead of the normal one.
Instantly, the problem went away.
Now, as you turned the prop, it went chir-click-click as both impulses fired together. It became the easiest-starting aero plane I have ever had. But…………………….
From that point on both mags always worked perfectly. Fitting the second impulse-mag appeared to have conjured up some mysterious genii who cleared the problem with the first mag.
Which is, of course, impossible?
I never did find the answer to this. In the dark of the night, now in my retirement, I still hear that chir-click-click and wonder why the hell that worked…….
Turning: "Toppling the Cockroach"
The next in the series of learning to fly by Johan Lottering:
Turning seemingly provides at least one way to prove that the human brain has a hundred billion neurons with one quadrillion synaptic connections.
Notwithstanding, the application of Aviation Legislation in South Africa, VOl. 4,,Appendix 1.1,Exercises nine (Turning) and 15 (Advanced Turning) in practice tends to rather support "local" popular belief about brain processes involving a cockroach paddling on a cork to flip the switches inside ones skull.
Applied in proper context the legislative unit standards for turning are by no means lacking. However, popping an off-the-cuff question to some flying instructors about the effect of weight on bank angle on any turn of a given radius and velocity produces a few laughs.
Without the aid of Commercial Pilots License (CPL) notes, few are able to recall that the formula for bank angle has weight above the line being cancelled by weight below the line. Now what about our hapless students?
Aviators often keenly resort to Newton's second law dealing with circular motion to provide explanations why and how an aircraft turns. Some start off with the seasoned argument of the stone tied to a string swung around a centre point; invariably a fictitious hand. An aircraft in flight conveniently produces a lift vector partly balancing weight in the vertical plane of motion while ,due to its inclination, also produces centripetal force to the centre of the turn.
But few can explain why the stone on the string with arguably no aerodynamic lifting component can do the same, namely: to remain in a horizontal plane of motion.
Physicists reason that presumably "outward force" is an illusion derived from our experiences in which our view as independent observers was changing within a "non-inertial frame" or viewpoint. It appears the vertical component of the inward force is derived from an angular acceleration towards the centre of the turn. While this may hold true mathematically and even graphically, it suggests the popular concepts employed in flying tuition may be overly simplified (despite its complexities) and very convenient.
Furthermore, during the respective entry and exit phases of turning man oeuvres, a period of non-uniform motion would involve inward acceleration to the centre of the turn, but the vector will have both radial and tangential acceleration. Resolving the radius of turn for a given bank angle(the angle between the respective aircrafts and earths vertical axes) according to some sources produce a "constant" factor of 11,29; where ‘k' =9,8 m/s/s x360 se/hour divided by 1852mx6076,12 feet/nm.
The formula for turn radius would be: Radius (ft) =True Airspeed divided by 11.29 (k) x Tan Bank angle. The point is in view of all the intricacies,"old".A.C kermodes textbook Mechanics of Flight(pitman,1972; pp.247-248) suddenly merges as a welcome simplification of turning theory.
The aim of this series is not to duplicate theory, but to provide a different vantage point about training. So, considering all the above plus the fact that most students and some, if not many, instructors tend to invariably reveal insufficient understanding of turning theory, the human brain is truly amazing.
Turning is in essence a transitional man oeuvre. Unlike other exercises the whole range of variations of underlying principles are difficult to resolve and to visualize in practice. Yet, most pilots develop a sense of pre-empting and anticipation of the amount of force necessary to smoothly enter, maintain and recover from various rates and radii of turns.
Some students from practical background and sometime without even the faintest clue what is being explained on a whiteboard about turning theory in the air, show a natural feel for turning, transitions and rates of changes. Others seemingly understand all the ground theory and totally mess up in the air.
Advanced turning attracts exponentially increasing load factors with increasing in bank angle. The load factors are directly and inversely proportional to bank angle, depending on the direction of reasoning.
A stressed Airline Transport Pilot with intricate knowledge of all the theory and inter-relationships on one day cannot maintain altitude or attitude for the life of him or her, only to execute the man oeuvre perfectly the next day.
Theory is indeed necessary, but evidently provides no guarantee or absolute safeguard when it comes to proficiency in turning exercises.
Effective control of altitude, turn rate and radius during a turn is a function of both load factor induced by simultaneous speed and altitude manipulations by elevator and bank angle by means of ailerons. Human factors produce various linear and circular acceleration and deceleration errors with various disturbances in the vestibular systems.
External and varying G-forces continuously affect the individual. How the brain copes with these is truly amazing. There is simply no proven method or scientific way to predict how a candidate will fare beforehand.
The "hands-on" types often do better as they invariably resort to outside visual cues for attitude indications. This is maybe where the key to good performance lies. Some take their reference from cycles and motor cycles leaning into the direction of the turn.
Mathematically we see an increase in velocity has more effect on the bank angle than on the radius of turn. Lift on the wings is considerably greater during a turn than during level flight. But, lift would increase considerably with increase in bank angle. At 60 degree bank angle the amount of lift produced will have to be double that of level flight.
The stalling speed is considerably increased. A mere 15 degree increase in bank angle would require four times the lifting force and nearly double the straight and level flying stalling speed. But, the outside wing traveling on the greater radius will be producing more lift relative to the inside wing.
Consider, therefore, an aircraft at nearly a quadrupled load factor stressed to 3,8G in the factory.Futhermore, at least in theory and not allowing for auto-correcting side-slip, at extremely steep bank angles the rudder will start to partly function as an elevator and the thrust vector will have a vertical component of lift.
In the shallow bank angle regimes of flying like in a climbing turn the aircraft will tend to over-bank into the turn. During descending turns especially aircraft with low wings and dihedral (upswing of wings) ,will produce different angles of attack due to the relative wind component from below; and the aircraft will tend to bank out of the turn.
Teaching turns initially presents an anomaly in the sense that students are briefed to essentially look outside for visual cues and other aircraft, while having to observe at least five different instruments inside the cockpit.
Sophistication is therefore best phased in and quite a few exercises may be needed to accomplish a good balance.
Trying to simplify a complex exercise most aviators resort to rules of thumb such as ‘rate one' turns of 360 degrees per two minutes correspond to bank angles one tenth of indicated airspeed in knots plus seven.
Despite all the above, the easiest way to accomplish a turn is to look at the position of the nose and merely cross-check inside on the altimeter and position of the ball. The prescriptions are to also scan the attitude and directional gyro indicators, the airspeed indicators as well as the ‘ball' of the turn and bank indicator.
But, this invariably detracts from the man oeuvres .So, next time you contemplate whether or not flying is really the ideal vocation for a particular student, pat him or her on the back once you fly through your slipstream at the end of the manoeuvre.Students generally tend to have ‘slow' and ‘quick' sides in adapting to turns in different directions. In a side –by-side trainer the left-seated student would end up apparently ‘below' and ‘above' the turns and tend to over- or under-compensate.
About the Author
Anthony Juma is the Editor and Senior Aviation Director at Wings Over Africa Aviation.
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