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Ever wanted to get into or improve your aerobatics?


Peter Jenkins
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Hi. Having pretty much taught myself to fly aerobatics, I thought it would be helpful for others who either want to start flying aerobatics or to improve their aerobatic flying to have a thread to which they could turn for help and advice. RCM&E is currently carrying a series written by Shahid Banglawala on how to fly the Clubman Schedule and while Shahid invites you to email him any questions it may be that some folks would rather use a forum such as this one to discuss the issues they are having with achieving the desired aerobatic shapes in the sky! Many of the photos in Shahid’s articles show quite humble airframes – the pilot remains the biggest factor in achieving a good aerobatic performance.

While there are specialist forum for aerobatic pilots, I thought there might be a large number of users of this forum who would find this thread useful and that we could build up a body of information that you could dip into as you progressed. Eventually, you might decide to graduate to other more specialist forum but I would hope that you’d keep an eye on this one.

One of the key barriers to getting into aerobatics is the fear that it will be very expensive. The reality is that many sport aerobatic models can be made to perform a great deal better than you would think by careful trimming. You can, of course, buy a 50 size aerobat and you will be amazed at how well they fly – but that’s not necessary. I started with a Wot4, which I still have and still fly and it’s great fun. Specialist aerobatic aircraft don’t have to cost much more than the standard ARTF sports model. Alternatively, thanks to the resurgence in interest in vintage aerobatic aircraft under the UK CAA there is an increasing number of plans being dusted off and brought back into use or else you can buy something like Ripmax’s Bullet if you prefer the ARTF route.

So, I hope we’ll get lots of contributions from those seeking and those offering information on getting into aerobatics.

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I'm on board.

This past summer I've been trying to fly my models in a much more disciplined manner. I agree absolutely about careful trimming and set up; makes a world of difference no matter the type of model.

I taught myself to fly a few years ago, and like Peter, I've taught (trying to teach) myself aerobatics. I distinguish flying aerobatics in a disciplined way as opposed to just bashing the sticks around from corner to corner. It certainly isn't easy without an instructor and it does take some effort, determination and practice.

I currently fly an Acromaster set up between 3D and general acros as my go to plane. An E-flite Leader 480 which is still at the flight trimming/set up stage. Lastly, I have a trusty Calmato ST1400 with which I'm trying to fly pretty much as a pattern ship.

Unfortunately, due to a combination of poor weather, a myriad of other things and a little health scare, I've not been able to get out to fly for a while. So, I'm trying to learn about the diagrams/symbols for the various manouvres.

In the new year I may buy an entry level pattern model along the lines of a Sebart Angel S30e/S50e.

Steve.

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I'd be interested in improving my aerobatics but I'm not interested in participating in competition, as much as anything else because of the transport costs of getting to events these days. My main interest is in scale and I'd like to improve my rolls which are awfully barrelly and my landings which I cannot carry out consistently smoothly in the place where I want to land the model.

There used to some instructions on this site about flying the roll. It all looked a bit complicated. Perhaps I should have printed them off.

Having said that I loved flying my DSM Smart Move until I crashed it through getting disorientated and the best model I've ever had for doing rolls was my Chris Olsen Uproar which dates from 1958!

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Count me in.

Been trying to get to grips with aerobatics after passing my 'A' and after winning this years novice aerobatic comp at my flying club I am aiming at improving and moving up to clubman level.

Having fun with my wot4 practicing new moves and currently trying to master a slow roll. I do have a 90 size yak 54 in my hangar but still in the trimming setup stages but does fly well.

Just got one off Steve Dunnings Elation Models Rapture 50 kits to build as a pattern flyer and hope to make a start on that very soon.

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An excellent idea and I will certainly be following this thread.

I've been flying for a year and put in quite a lot of hours this season and decided that whilst I don't need to prop hang (not yet anyway), I do want to have a bit of discipline and structure to my flying. I regularly tap more experienced flyers for help and have collected anything and everything I can find off the web but I still need more. I hope that Shahid's series doesn't suddenly go quiet like his video series - stops when it gets interesting and no actual flying demonstrations.

There are some very helpful pilots at our club but I think modesty seems to prevent them from really explaining the details - they don't seem comfortable when in "training mode".

Edited By Masher on 29/11/2013 15:02:00

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Well, that’s an encouraging response! I should state right here that I can’t do 3D so I’d be looking to someone to chime in to help folks with that aspect. I do fly traditional aerobatics, or precision aerobatics as it is sometimes called. What differentiates these two styles is the way the aircraft is set up. However, that’s some way down the track.

As FB3 and Steve Colman have correctly identified, the first thing is trimming your model. So, let’s start there. Well, let’s start right at the servos first. The best way to ensure that you are making full use of the power and resolution of your servos is to spend a little time on setting them up correctly. By that I mean setting up the linkages to be as correct as possible.

Here’s one way. You start by looking at what you want to produce in the way of movement on your control surface. If you are intending to fly traditional, or precision, aerobatics then you do not need vast amounts of throw. For 3D, you do so if you want to have an airframe that does both you will need to make some compromises on performance. If you focus on just one discipline then you can optimise for that and get a better outcome. At the end of the day, the pilot’s skill will be the dominant factor in achieving the desired result but the right aeroplane, correctly set up don’t half help!

So, back to the servos. When you mounted your servos did you set the sub trim to 0 and then place the servo arm on (Futaba allows you to adjust the horn position in 4 steps) so that you get the servo arm at 90 deg to the servo centreline. Only use the sub trim then for fine adjustments. This prevents problems of having oodles of sub trim wound in to get the servo arm centred which could cause difficulty with non linear movement at the extremes of servo travel. Does everyone agree with this?

Next, think about how the servo generates its turning power or torque usually expressed as either Kg.cm (European) or oz in (US or imperial). The closer in to the servo shaft you connect your pushrod, the greater the power of the servo. The other end of the connection to the flying surface is then used to achieve the desired movement. Always start with 100%, or even the full 120%, of servo movement to produce the maximum control surface movement you want. What this does is reduces the effect of any backlash, or slop, in servo gears, and maximises the servo’s power. Use the Tx ATV to fine tune your set up. You also do not want to have oversize holes for your pushrod to rattle around in as you are introducing more unnecessary slop into the control run. Avoid Z bends to terminate your pushrods – yes they are quick way of connecting up but pushing them in enlarges the servo arm or control horn hole – leading to more slop! A simple 90 deg bend with a swing keeper is the way to go or else use the correct size quick link to ensure a solid slop free linkage.

For trad aeros, you’ll find that 10 deg to 15 deg is all you’ll need. That’s not much movement but it’s enough when you have the CG in the right place. So the rule is mount your pushrod as close in to the servo shaft as possible and position the pushrod in the appropriate hole on the control surface horn to give you the desired movement. Why use degrees and not mm or inches of movement? Depends on how long (measured along the direction of flight not spanwise – if you see what I mean) your control surface is how much movement you need to measure at its end. Measured in degrees it’s the same for long or short surfaces. OK but how to convert the degrees into movement at the trailing edge of the control surface. If we call the control surface length L and the movement we want M, and the angle is A then the movement we want is:

M = L x sinA (where A is in degrees)

Sin 10 = 0.17

Sin 15 = 0.26

Sin 20 = 0.34

So M = L x 01.7 (for 10 deg) and if you have a control surface of 25 mm

M = 25 x 0.17 = 4.25 mm

It’s not much movement, is it! You can, of course, have more movement and set up your low rates to give you this very small movement or else use exponential to give you the same effect over quite a lot of stick movement depending on the amount of expo you use.

For 3D, you’ll want up to 45 degs of movement each way on a huge control surface. You will probably see that there is an inherent problem with having an aeroplane set up to do both disciplines – you have to trade off servo power (or buy more powerful servos) to avoid blowback and give you the speed of movement required on a 3D surface. Not that it can’t be done, just that you have to compromise.

OK. That’s enough for the first bit. Any comments gratefully received.

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Peter, this is a good start with plenty of detail and explanation - pitched just right for me!

I don't know if you want a million questions, well you'll get 'em anyway, but regarding the bit

"You can, of course, have more movement and set up your low rates to give you this very small movement or else use exponential to give you the same effect over quite a lot of stick movement depending on the amount of expo you use."

I am doing this method with 3 sets of rates and about 30% expo. Is there a preferred method or do you just choose the method you like best?

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I'll be watching this one with interest.

Unfortunately I also fly a funflier with +/- 45 deg of throw which means on anything set up for normal aerobatics I keep running out of control authority. Basically I find it difficult to adjust between one type and another.

Shaunie

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Shaunie

Flying lots of different types of aircraft is good fun, but I found out as you have, that when you have to keep changing the way to fly several times during your day at the patch, it is difficult to get good at any one thing. I found that my flying began to improve once I focused on flying just one aircraft. I got so used to it I began to get quite good at flying consistently - not as well as I would have liked but a lot better than I had been achieving. I happened to be preparing for my B certificate so stuck with it and flew just my Wot 4 for 5 months. It paid off as I got my B and I found that I could adapt to fly my other aircraft very much better than I used to. I also found that I could now fly in weather up to a 20 mph 90 deg cross wind (not to be recommended by the way) as well as cutting down on my crashes. I thought that was a fair return to sticking with one aeroplane for 5 months. Where I see people come to the patch with 3 quite different aircraft and they are struggling to fly half decently, I recommend that they just fly one until their competence improves significantly. Sadly, most of them ignore that advice and continue to struggle to improve. Worth thinking about.

Peter

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OK. We’ve got our servos sorted out and the pushrods connected up so there is no slop. Oh, dear, there still is some slop. Well, take a good look at the way you have hinged your flying surfaces. If this is with mylar hinges, and you didn’t push the control surface bang up to the fixed part you may have built two issues. First, if the hinges are not particularly stiff you will have some unwanted movement of the control surface. Why not cut through the hinges, re-slot for new hinges and re-do the hinges making sure that you push the control surface up against the fixed surface. That will eliminate the slack and reduce the gap between the two surfaces. If you have Robart style pin hinges and have this problem then the solution is the same. Do remember that if you have mounted your control horn with its pushrod holes level with the leading edge of the control surface you have introduced some non-linear movement into the control surface. It may not have mattered a great deal before, but if you want to fly accurate manoeuvres little things like this will make your life much harder!

If you want to make things even better, you can always seal the gap with tape. What sort of tape? Well, I quite like a tape called Blenderm, available from chemists. But you could also use Solartrim or equivalent trim applied using the soapy water trick. The main thing is not to introduce any stiffness to the control surface hinging by using these techniques.

Once you’ve done these things and re-checked your control throws, you’re ready for the next bit – CG.

I’ve noticed that many club pilots set the CG for their first flight and never think about it again. I used to do that as well until I was discussing the fact that I needed more elevator movement for the final flare for landing. My mentor suggested moving the CG back a bit at a time. So as in this setup I couldn’t move the Rx battery, I armed myself with some sticky weights (pop down to your local tyre fitters and see if you can blag some sticky lead off them) and set to to stick on 5 gm weights as far aft as I could on my Wot 4. You will be amazed at the difference in the model’s behaviour with the CG in the optimum position. When a club mate flew my Wot 4 he was amazed that mine was so much nicer to fly than his. So I helped him to get his set up with a similar CG and his then flew as well as mine – once he’d also turned down the amount of control throw as well!

So, where should the CG be then? It depends on where you start! You will usually get a range for CG position. I usually opt for the middle position, and if the information is accurate and your maiden flight ended with a normal landing, you have at least established one safe position. What I do to check if the CG is where I like it, is to pull the model into a 45 deg climb, hold that for a short while and then half roll to inverted. Then watch what happens with the elevator stick left at its centre position. The model will usually drop its nose indicating the model is stable. I prefer to keep moving the CG back (in 5 gm steps) until the model just starts to drop its nose when inverted. I have on one occasion found the nose gently pitching upwards which is an indication that you are getting into the unstable area and you should land and move the CG forward! If the model holds its flight path then you have found the neutral point – this generally means that what ever position in which you place your model, it will continue to hold that position. I prefer to have a stable aircraft so I avoid both the neutral and unstable behaviour.

Peter

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Excellent start Peter ! I'm in full agreement with all that you've written. I'm certainly no expert but I follow more or less the same process as you've described with great results.

I always give great care and attention when setting up my control surface throws for the first time. Ditto with the CG before the model gets anywhere near the flying field. This is my baseline starting point. I also write down all control surface throws, CG position, TX settings. Then, any subsequent change after flight trials are recorded for reference.

It's quite amazing how many times I've seen a couple of guys turn up at the field with a new model, stuff in a lipo, a cursory check of CG and then off they go and commit the model to the air. Assuming no crash ensues they then often complain that the model is rubbish. One guy, having seen how nicely my Calmato flew (I even let him take the sticks a while) went out and bought one for himself, even fitting it out with exactly the same electronics. When he turned up at the field with it for the first time, it was clear that the setup was less then ideal. Of course, I offered to redo many of his settings but, as is often the case, he was too eager (read impatient) to fly his lovely new model. I guess you can guess the outcome. Yep, model went home in far more pieces than when it arrived. Reason pilot gave for crash.....loss of signal!

Back to topic. I also use the 45 deg climb method to fine tune CG but I prefer to be as close to neutral as possible. I like the model to fly inverted with no elevator input. Now, this fine tuning process may take many flights and needs to be done with a methodical approach along with a large dose of patience.

Steve.

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Peter - can you go back a bit please and describe the set-up of your aircraft in terms of wing incidence, tail incidence and engine down and side thrust?

If you want to reduce unwanted control movement (slop/slack) use a ball link at each end of every pushrod. You can't get rid of it all, 'cos if you did the controls wouldn't be able to move (seized solid). And to the same end I find it better to use an outer hole on the servo arm, and a surface horn of at least twice that length, so that 40 degrees servo movement gives a maximum of 20 degrees control movement. Reduces the slack as a percentage of pushrod travel.

Graeme

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Hi Graeme,

In most cases, where people have built from a plan or assembled an ARTF, there is no adjustment of wing or tail incidence. Where you are talking about F3A pattern aircraft then you will find that some come with adjusters for wing or wing and tailplane incidence. In the latter case, the usual setup is as follows:

  • Decide on what to use as your datum point on the aircraft;
  • set the fuselage at 0 deg for the datum.
  • set the wings at +0.5 deg
  • set the tailplane at 0 deg
  • engine thrust line may already be set for you by the designer by angling the firewall for both down and side thrust. If not, then I would start without any up/down thrust and set 2 deg right side thrust.
  • set CG to the mid point of the range specified. If none specified then set it to 30% of mean aerodynamic chord

These are your start points and you adjust from there until you get the aircraft to fly in the way that you want/feel comfortable with.

As regards your second point, I entirely agree that ball links eliminate the slop at the servo/horn junction. The rest of the slop is accounted for by how well the servo gears have been made, how well the hinging of the control surface has been carried out and how stiff a control rod you are using. Using a snake will introduce some variability in response depending on how well it has been supported along its length and what measures have been taken to support it from the exit of the fuselage to the control horn. I have found that Futaba, JR and some Spektrum servos have very little slop but more importantly excellent centreing. If you have good hinging, a stiff pushrod, ball links at each end and a good servo, you will have a very tiny amount of slop - as you say, it's almost impossible to get rid of since there are manufacturing tolerances in servo gears. If you go to town and spend over £100 on a single servo then you would probably approach zero backlash in the gears.

One thing you don't want is a pushrod with a bend or crank in it. That introduces a degree of flexibility in the control run and can lead to flutter of the control surface. Flutter is caused by aerodynamic forces which feed back through a flexible pushrod and cause the control surface to literally flutter - it doesn't take long for the control surface to be destroyed or the servo gears to be stripped. To be avoided at all costs! If you dive your aircraft and you hear a buzzing noise, the chances are you are hearing the onset of flutter. Land and investigate as soon as possible.

As regards your second point, as you go further away from the servo shaft, any backlash is amplified. The angular movement remains constant but the further out you go the greater the linear effect. If you don't believe me, draw yourself a diagram or, better still, tape a long pointer to the servo arm and measure the linear backlash close into the servo shaft and that at the end of the pointer. True, by connecting to an even longer control horn you get back some of this unwanted inaccuracy but unless there are very good reasons for following this course, I would always go for the hole nearest to the servo shaft and use standard length control horns at the control surface. You may have to move outwards because the pushrod is fouling the servo shaft or you need to clear some part of a structure.

Incidentally, you can get rid of all backlash and still have a smoothly operating control system - it does not lock solid. You will need to pick your servos very carefully though to achieve that nirvana.

Hope that helps.

Peter

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