Thursday, October 2, 2014

Roll Your Own


Aerobatics is hard enough as it is, both mentally and physically, without having to overcome an evil handling aircraft. To maximise performance and minimise time spent learning, a well set up aircraft is a must.

The human brain and body is amazing at adapting and learning things, but often pilots learn bad habits in order to overcome handling issues. Once you fly a well sorted aircraft, or improve yours, you have to unlearn those habits! How much easier would the pilot progression be if the aircraft was set up correctly from the beginning?

Modern monoplanes are extremely tunable and two similar looking aircraft can handle very differently, much like race cars. And just like in motor racing, some pilots are better, or take more care, setting-up an aircraft than others. It is difficult to find good info on the subject. There is virtually nothing written down that I can find specific to aerobatic aircraft. Often we must learn the hard way, through lots of thinking and experience, and listening to those that know. This would be easier if we understood Russian or French!

When you first purchase a dedicated competition aircraft, get a knowledgeable person to help you set it up, or get them to set it up for you, before you spend time and avgas compensating for handling problems. An aerobatic aircraft should be 'easy' to fly; not too heavy or light in pitch with good harmony between controls. Controls should be powerful, but not twitchy or overbalanced. You should feel comfortable and secure in the aircraft, not on edge and unbalanced. Try and relax, and loosen the grip on the seat cushion as well as the stick.

In roll the aircraft should respond to ailerons the same to the left and to the right, apart from the known slipstream and torque effects. And, importantly, the aircraft should roll the same from inverted as when upright! It is very common to find that this is not the case – try it yourself. It is obviously more difficult to fly good high-rate rolls when the characteristics are changing significantly as you go round, requiring you to make inputs to keep it all on track.

I won't be going into spade set up here, but it is a bit of a dark art about which there is much to know. Rather, I wish to explain a little discussed phenomenon....CG effects on rolling.

The effect of Centre of Gravity (CG) on pitch feel is pretty well known – the more aft the CG the lighter the stick force per G, and the less stable in pitch the aircraft is. But the CG position also has a secondary effect when rolling.

When upright S&L the horizontal tail must have a down load on it to balance the aircraft (whether this force is trimmed out or you hold it with the stick, it is still there). Think back to your PPL theory with an aircraft drawn in profile. The aircraft mass at the CG drawn as an arrow down, Lift up through the aerodynamic centre and a smaller down load on the tail. Just like a balanced see-saw with the pivot being the Lift arrow, or vector.

Now, imagine this aircraft force diagram is rolled suddenly to 90 degrees of bank. The aircraft mass still pulls down towards earth, but the wing and tail 'lift' forces continue to act perpendicular to the flying surfaces; that is, horizontally, not vertically. The see-saw is not balanced any more. What will the nose of the aircraft want to do.....?

What was a down force on the tail, is now a horizontal force wanting to 'pitch' the nose in the direction of roll, and the Lift force, rather than supporting the aircraft weight is supporting nothing! Unbalanced forces = acceleration, said Newton, so the aircraft must want to displace and pitch 'up' in the direction of the roll.

Try it yourself next time you fly, or think back to what happened during your waggles into the box as a novice. Did the nose come off line, and by the time three waggles were done you were 30 deg off axis, and all before you even started any aerobatics! The cure? A bit of forward pressure/movement on the stick as you waggled. This reduced wing angle of attack/lift, reduced tail 'down' load and thus helped you hold your aim point. Inverted the physics is the same, so you can work out those inverted wing waggles for yourself.....

Greater problems occur, and are less easily cured, in the dynamic environment of a rapid roll. There is more to learn here.

Getting back to PPL aerodynamics, and the reason for this article, how does CG position affect the download on the tail? Thinking like a see-saw, the closer the CG (mass arrow downwards) is to the Lift (upwards), then the less download will be required on the tail to balance (aeronautical engineers please forgive me the simplification).

So it follows that an aft CG, and thus reduced tail load, will also reduce the unwanted pitching that occurs when you roll. It will also reduce the forward stick needed when inverted. This will make rapid rolls much easier as you won't have to manipulate the stick fore and aft as much to stay on axis. You still have the tilted lift vector to cope with though – can't do much about that.

Now, we can't go to extremes here and move the CG too far back, as the aircraft will become unstable and NO elevator input will be required to pitch the aircraft. Think about this, and you should realise it is a BAD THING.

What is a reasonable CG position? Personally, I think for comp aeros in a purpose built monoplane you want it at least in the aft half of the recommended envelope, say 2/3 – ¾ of the way back. Usually, this is somewhere around 26-28% of Mean Aerodynamic Chord (MAC). You may want to creep up on this if new to high performance aircraft, starting with the CG a bit further forward, and as experience grows move it back as you become comfortable with the lighter pitch forces. Don't delay too long though, as you want to start building the right muscle and visual memory as soon as possible.

I recommend re-weighing and calculating the CG position whenever you get a new aircraft or make changes such as a lightweight starter, battery, prop or alternator. Don't rely on factory figures or those in the Flight Manual. They may still be legally OK but years out of date. Don't assume, find out for yourself so you KNOW – it's your time, money, and life. We are working in the aft end of the envelope, an inch CG movement can be 2% of MAC change, with a significant affect on stability if you go too far.

You don't need an Approved Weight Control Authority to do the re-weigh. You don't need to change the Official W&B figures, you just want to check for yourself what the actual figures are. Borrow or hire some reliable scales (car clubs, SAAA chapter), research how the job is done, or get someone knowledgeable to assist. Weigh the aircraft empty, then with acro fuel, and finally with both acro fuel and you kitted to fly. That way you will be able to calculate the actual moment arms for the fuel and pilot, so if you alter either of these you will still be able to calculate what the actual CG is. A spreadsheet makes the whole job very easy.

Roll On!



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