AEROBATIC SCENE – 03.21 Part 2

Top competition pilot, Keith Jackson sets about keeping us better informed of developments with pattern style model aircraft.

This is the second part of Keith’s column from RCM&E March 2021.

CURARE XXL

Continuing with the classic pattern theme, I recently saw a build thread on Facebook for this extra-large Hanno Prettner Curare, expanded and built by French pilot, Jean Louis Bossard. The model spans 2m and is 1.8m long, which is roughly 1.25 times larger than the original airframe. Ready to fly, the Curare XXL weighs 6.5 kg and is powered by an NGH 25cc two stroke petrol engine. Jean Louis has reported that due to current restrictions in France the model has yet to fly, though I am sure it will go well, and I wish him the best of luck.

SLOW ROLL

A few months ago, we received an email from a reader asking about slow rolls and whether I’d be covering them in the future. Well, in short, the answer is yes and for the very good reason that they are a fundamental component of most F3A schedules.

In fact, at the highest level of F3A virtually every roll can be counted as a slow roll as the top pilots aspire to use a relatively slow but constant roll rate throughout their schedules. The reason for this is to gain the small number of points awarded for smoothness and gracefulness, which amounts to 20% of your mark per manoeuvre (Section 5B.4 #2 of section 4 of the FAI Sporting Code, and also 5B.9).

Several times in the very limited competition season we have had this year, I have witnessed pilots attempting slow rolls with some difficulty. It’s worth looking in more depth at what is actually happening to the model during a roll before attempting to work out what inputs the pilot may need to use.

When the model is in trimmed straight and level flight at some cruising speed the weight of the model is balanced against the lift force provided by the wing and an equilibrium is formed. When aileron is applied and the roll begins the component of lift force from the wings acting against the weight of the model is steadily reduced until at knife edge, where none of the wings lift force is available to counteract the model weight. If the roll is stopped at this point the model would dive towards the ground and eventually crash unless some action was taken by the pilot.

As the roll is continued until the model is inverted the normal trim state of the model would actually cause the model to dive as the wings lifting force is now in the same direction as the model’s weight. In fact, no further component of lift force to counteract the model weight would be available from the wing until the model has rolled past the 3/4 roll point, whereupon this would steadily increase until an equilibrium was restored when the wings were level again. I say equilibrium, though this isn’t quite true in this case as the model would exit the roll in a diving attitude due to the lack of positive lift force throughout the roll.

This may all sound pretty obvious and perhaps a bit familiar to some. Whether the model is conventionally trimmed or uses other trim conventions, such as the more recent zero gravity arrangement, the model will always require some input from the pilot to prevent the model’s flight path from deviating during a roll.

The simplest and most common control input to use is down elevator when the model is inverted and during the roll this will only take momentary application to prevent the model from diving when inverted. In fact, I guess most pilots would generally stop at this correction as the timely application of down elevator will be sufficient for most rolls in sport flying.

Returning to the elevator only correction, the timing and amount of elevator input is crucial to avoid under/overcontrolling, causing spiralling rolls. Several pilots from my home club have asked about slow rolls and, in my opinion, the best way to start to do these is to do a series of two-point rolls i.e., roll to inverted and hold, then roll back to upright. Once you’ve rolled to inverted, apply down elevator until the model tracks level. This is the point where I adjust my elevator response using the rate function in the transmitter software.

Assuming that the elevator throw is comfortable for all other types of manoeuvres you can now adjust the exponential and, if necessary, the linear rate so that when you apply the down elevator to hold inverted the control input you use is comfortably achieved without the model porpoising up and down. It will take several, if not many attempts to get this correct, and will vary from day to day depending on many factors such as air temperature, model speed, your mood, etc. Essentially you are trying to develop muscle memory in your fingers or thumbs so that you will automatically know how far to push the elevator down to comfortably achieve level inverted flight. Clearly you should not be using full down elevator nor should you be able to ‘breathe on the stick’ for this to be sufficient. Find a comfortable amount of stick throw, probably in the region of 10 – 20% down elevator and adjust the rates until this allows inverted flight.

ELEVATOR SET UP

I know I’ve emphasised this process, but it is the basis on how you setup your model to achieve fuss free inverted flight and then onto rolls, slow rolls etc. You cannot expect the initial setup in your transmitter to achieve this so be prepared to adjust your rate settings accordingly.

The aim of this initial step is to get used to comfortably rolling (at a normal pace) into inverted flight, holding this with down elevator without the model deviating, and then rolling back to up right. This step needs to be practised until it is normal and not frightening! A test of this is to do two or three consecutive, two-point rolls without any visible deviation from the flight path.

Once you can do this practice getting the whole manoeuvre centred in front of you and not performed just anywhere in the sky. For a single two-point roll the inverted section should be right in front of you and the whole manoeuvre should last about 3-5 seconds to begin with. For two consecutive two-point rolls each roll should be equally spaced either side of centre and so on.

TOP RUDDER

Once you have mastered the two point roll it is time to start considering how the model’s attitude can be maintained in knife edge flight by using rudder control.

As previously stated, when the model is in the knife edge position the lift force from the wings does not act to oppose the force of gravity and theoretically the model should go into a dive. However, the fuselage also contributes to the total lift generated by the model, although this may or may not act in a way we expect and is dependent on the geometry and its incidence into the prevailing flow of air.

In knife edge the fuselage will generate lift that opposes the weight of the model as long as it presents a positive incidence to the airflow. This may be achieved by the pilot by applying ‘top rudder’ i.e., by positioning the rudder in a direction away from the ground, and this serves to force the fuselage into a positive incidence relative to the oncoming airflow and hence generate a lift force.

This is where theory and real-world experience diverge as most pilots will know that application of rudder in knife edge leads to a number of outcomes and is rarely as the theoretical case would have us believe. The model may well roll and pitch, or do both, when rudder is applied, and this is due to a phenomenon known as rudder coupling.

This opens a very large can of worms, which I attempted address in the June 2020 edition, so I won’t go over this again. But for now, let’s make the assumption that when you apply rudder in knife edge position no other significant effects occur. Also, it is true to say that this is a very good indicator of how suitable your model is for aerobatics; a thoroughbred F3A design will react with almost pure yaw when rudder is applied but, clearly, a high wing trainer will struggle after this point!

Returning to our ideal case of the model not exhibiting control coupling and only ‘yawing’ on application of rudder, our aim is to counteract the force of gravity when in the knife edge position by applying top rudder. The ideal way to learn this is similar to our two-point roll exercise from earlier, but instead of stopping the roll when inverted, stop when in the first knife edge position after a 1/4 roll. For this exercise you would 1/4 roll to knife edge and hold, and then 1/4 roll back to upright.

Please take some time to work out in your head which direction to apply rudder as this depends on which direction you roll in initially. For example, roll the model to knife edge by applying right aileron. After the 1/4 roll the left-hand side of the fuselage will be upper most in knife edge so apply left rudder and try to hold this attitude by varying the degree of rudder input.

You will now have to go through the same exercise as previous with the elevator i.e., by tailoring the rudder throw until 10 – 20% of rudder stick movement will allow the model to hold knife edge without porpoising. I have seen numerous guides on this subject where full rudder control is advocated. Modern F3A designs will not need anything like full rudder to hold knife edge and the extra throw will be needed for other manoeuvres such as the Stall Turn or Spins.

The key to the success of this process is to become comfortable when rolling the model into knife edge and holding this position without panic setting in; Setting the model control throws up correctly will help enormously with this.

FOUR POINT ROLL

The next step is to repeat this exercise but extend the manoeuvre into the four-point roll. Again, it is crucial to be clear which direction you want to roll in and in what direction top rudder will be. To summarise:

It may help to practise this initially with a helper to remind you of the rudder input required for the direction of roll. Alternatively, this is an ideal reason to use a flight simulator and takes all the stress out of putting the wrong rudder in!

BLEND THE CONTROLS

The final extension to this is the slow roll, which itself is just an extension of the four-point roll. Ideally you should aim for this to last at least five seconds or more and requires the pilot to ‘blend the rudder and elevator movements together’.

Going from upright to knife edge should require the rudder to be slowly fed in until the model is in knife edge and ending when the rudder is in the holding position for level knife edge flight. Rolling round to inverted needs the rudder to be slowly withdrawn, whilst the down elevator required for inverted flight is slowly applied. In Mode 1 (throttle/aileron on right stick) the left-hand transmitter stick will be seen to move in a circular direction as one control is slowly fed in with the other being slowly taken out.

This is why it is crucial to setup the throw to 10 – 20% stick movement for the various holding positions; it would be quite difficult to go smoothly from 10% down elevator for inverted to 100% rudder movement for knife edge.
To do this comfortably and without stress may take a couple of afternoons or possibly last many weeks but it is important to be methodical and patient as getting this right will help you all the way up to the top class in F3A.