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Ever wanted to get into or improve your aerobatics?


Peter Jenkins
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Algy, I've asked around and no one else seems to have experienced this issue. The technique I used for my B test flying a Wot4 was to set the higher power as soon as I had settled on the required line, do the two rolls, fly a procedure turn and fly the two opposite rolls. I did not notice any reduction in roll rate for either sets of rolls. I think you said you flew mode 2 so is there a possibility that as you use rudder on the rolls you are inadvertently easing back on the power setting and therefore having to increase power consciously half way through the 2 rolls? Just a thought.

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All manoeuvres are a combination of loop, roll, stall and spin.

Remember that the wing stalls when it reaches the critical angle of attack – that is the angle between the wing and the direction of the airflow well ahead of the wing. You can reach this critical angle either by slowing down the aircraft gradually or by approaching it very rapidly and at a very high speed. The latter is called an accelerated stall or a high speed stall and it is beautifully illustrated if you look up the Red Bull Incident thread on this forum. The pilot gives a good description of what happens when you pull too much up elevator at speed – the wing stalls regardless of the speed at which the aircraft is travelling. Recovery from the high speed stall is immediate on releasing the elevator back pressure fractionally to get below the critical angle of attack. If you are in normal speed stall, then you need to take more deliberate action.

Well, the stall is the precursor to the spin. You must stall the aircraft in order for it to spin. In aerobatics, spin entry is watched very carefully to see if the pilot lets the aircraft fall into the spin naturally or forces it into the spin. In the latter case, he scores zero.

So, to enter a spin, we first of all need to pick the spot at which we are going to spin the aircraft. Typically this will be right in front of you and on the centreline. Once the aircraft is spinning, you have no control over it and it will drift downwind until you initiate recovery. Key points on spinning are:

  • You must maintain a straight line following a horizontal flight path i.e. allowing the aircraft to visibly climb to slow down loses you points. So the aim is to approach the centre point slowing down and creep up to the point at which you wish to initiate the spin, closing the throttle completely shortly before you begin the final smooth backward movement of the elevator stick. You may wish to carry out a spin from inverted flight in which case you obviously need to invert these instructions.
  • As the nose of the aircraft is seen to drop apply full rudder in the direction you wish to spin. What this does is to unstall the wing on the outside of the spin while keeping the inboard wing stalled as a result of the yaw induced by your rudder input. Most aircraft spin one way more readily than the other so always pick the easy way to spin. The one proviso is that if you have an aircraft that drops a different wing from time to time, go in the direction of the dropped wing.
  • I have never found the need to use aileron to commence a spin or to maintain a spin but some folks say they do need aileron to spin. It’s an interesting discussion!
  • If you cannot get the aircraft to stall, it will never spin and using full aft stick, rudder and aileron will result in a flick entry to the spin. For aerobatics, this is scored as zero. You may need to increase elevator travel to get sufficient elevator power to get the wing to stall. You may want to use the dual rates to give you this additional elevator throw. If you have trimmed your CG as I described earlier, that pretty much guarantees that the aircraft will spin once you have carried out the above actions.

The flick roll or snap is a spin carried out above the normal stalling speed. You will need to have more elevator movement than I described for normal aerobatic flying to ensure the power is there to get the wing to the critical angle. You will also need to apply rudder and aileron to achieve the desired rotation and speed of rotation. You can have positive snaps, full up elevator, or negative snaps, full negative elevator. A properly conducted snap or flick roll, will result in a loss of flight direction but you are not penalised for this but you are expected to get back on track asap.

In aerobatics, they never specify which direction, i.e. left or right, you must spin, just whether the spin is upright or inverted and with snap rolls, either positive or negative. However, they might specify that you have to change direction of the spin or snap. I don’t propose that you should go this far at this stage! For the time being just focus on the upright spin with recovery to upright flight.

More in the next post!

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Carrying on from the previous post.......

So, spin entry must be from a straight and level flight path but what about the spin recovery? Well, they normally specify the number of turns for the spin so you need to ensure that your spin stops after usually 2 or 3 turns. Stop means that you stop within 10 deg of the required heading – normally the direction of the flight path from which you entered the spin. So, you need to know when to centralise your controls to ensure that the spin stops on the required heading. Best way to do this is to wait until the aircraft is on the exit heading, centralise the controls and see how far past the desired heading the spin stops. Next time, start you spin recovery this amount before your desired heading direction and the spin should stop at exactly the right place – or not! Needs careful watching to get this right consistently.

Next, you should allow the aircraft to fall vertically for a reasonable distance before initiating a smooth pull out to level flight increasing power to cruise power as you would in a loop recovery from around 4 o’clock onwards. (That’s why I suggested you mix elevator with throttle when the throttle was fully closed to provide a small amount of, almost always, down elevator to achieve the vertical flight path.) Two reasons for this recovery profile. First, it avoids an early pull that might cause a snap spin to develop and second it looks graceful and under control! Now, if you have a wind blowing, what was a vertical flight path in still air will get blown backwards and since we are interested in the flight path and not the aircraft attitude, we may need to gently apply some up elevator to get a vertical descent of the aircraft.

When it’s done right, a spin is a beautiful manoeuvre to watch.

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Thanks Peter for your answer, you may well be right about the rudder input in Mode 2 but the reason I do it was I noticed it when I flew Mode 3 but it seams more pronounced now in Mode 2, which your idea would support.

I have had another thought that it might have been due to the anhedral on the model I had back then and the rudder would had a slight roll action so in the first 1/2 roll the different amounts of rudder are all act with the roll in the second 1/2 they would all against to the roll rate, unlike a model with dihedral such as a trainer which would be the opposite. The model was a pattern ship but I can't remember which one but I do remember we had changed it from a flat wing to anhedral to try and reduce the yaw roll coupling (which we slightly over did).

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Yes I use mixes to try to eliminate rudder effects on roll and pitch and normally use the knife edge flight as a test but I have found on some models that the mix needed changes with speed (I assume due to the side slip airflow component), so I fly the knife edge at the speed I would fly the roll at.

Is there a better way?

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Yes there certainly is Algy. In ye days of olde we had no mixes so it came down to model design. The most important factors affecting roll/yaw coupling are the dihedral and the rudder rake angle. Tailplane height above the wing and any tail anhedral also come into the equation.

It was not unusual to take a saw to a foam wing to cut a slot and modify the dihedral; rejoining with some glass tape. Rudder rake back causes a down elevator effect so this was also optimised. An anhedral tail means that you need less wing dihedral more often than not.

It is worth experimenting with an old imperfect model just to see what effect such changes have.

Prettner`s 1977 model had odd bits of Solarfilm all over the back end to cover up the very minor mods.

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Thanks Martin, You are right about the mods and I have tried a lot of them (some with the advice of Mr Prettner) and that was exactly what i did (over did) with the model a talked about above. It is good to talk about these as a lot of model fliers today just use a mix. I still build models and play with designs by changing dihedral (easy on foam core wings) and other things like rudder rake.

The question I was more about is there a better way to check the rudder effects, in the old days knife edge was harder with the shallow fuselages we had and I used to fly control line circles to see if the model piched or rolled with the rudder input, now with the deep fuselage I tend to do both control line circles and knife edge flight to see. The trouble is with the modern plug on wing it is a lot harder to change dihedral on the mainplane.

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Algy re the issue of mixing changing with speed. That's only to be expected as you are using a fixed relationship to overcome undesirable aerodynamic behaviour. Hence the need to fly at as near a constant speed as possible. Therefore, the mix also works best at one point in the speed range. Eliminating the cause of the aerodynamic deviation is undoubtedly the right way to go. However, as you pointed out, sometimes it is very difficult to eliminate the problem and so a mix is used as a palliative. I thought twice about posting something that doesn't actually help you but it does answer the question about speed effecting mixes!

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Just found this thread, very interesting. I'm just finishing off a wot4 and hopefully will use this thread as a guide to sorting it as I have also two acrowots and an Xtrawot waiting for maidens........quite surprised that you chose to put a piped 53 in, I've been agonizing over engine choice since I started the kit.

Edited By Braddock, VC on 12/12/2013 12:11:42

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Hi Peter,

Just got back from my hols, and saw your thread, there is certainly lots of interest in the topic. So I suppose we all want to be able to fly better!

Just my two cents worth:

Practice and guidance and more practice. And then practice.

By going to a GBRCAA new pilots open day, and then doing a few of their competitions I learned so, so much, The people were very friendly, helpful and approachable in my experience. I am not a competitive type, I merely try to improve my own flying.

The other thing that has been a huge help to me has been indoor flying with a Clik 2. Very intense practice provided you have a good supply of foam safe cyano, Uhu Por glue and Blenderm tape!

And finally from me: if you want to learn, set yourself a goal for each flight, and think about it afterwards. It is all about trying to steadily work at ones weak points.

Have fun,

Charles

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I would add to Charles' post which I agree with full, try to work on one thing at a time, only change one thing at a time (that way you know what did what) and stick to one aircraft (well one for improving your aerobatics, you can have a fun hack to relax to) as no two fly the same.

Indoor flying and sim's are good but not the same as flying a pattern aircraft to my mind; they do though, help you learn as lot, that can be adapted to outdoor aerobatics.

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Well, I think you have had quite enough theory for the time being. If the weather plays ball, I do hope you’ll have a go at putting some of this information to use. I’d be really interested in any feedback on how you got on.

You may wish to have a look at other websites that cover aerobatics. Here are two:

  • For precision, or F3A, aerobatics – the Great Britain Radio Control Aerobatic Association (GBR/CAA) is the body delegated by the BMFA to oversee all UK F3A (F3P – indoor) aerobatic contests and has the responsibility for picking the team to represent GB in European and International Contests. Their web address is HERE.

 

  • For IMAC aerobatics (covers both precision and 3D) but the focus is on scale aircraft based on full size aerobatic machines. IMAC UK is responsible for team selection for representation of the UK at the IMAC World Championships. The rules used for IMAC are those set out by the IMAC in the USA under their governing body the AMA. The IMAC UK web address is HERE.

 

  • Don't forget to check out the UKCAA threads on this forum for coverage of the classic aerobatic aircraft and flying styles.

Both have various resources that include articles on trimming – which I hope you see are similar to what I’ve set out. Both have a diagram of their schedules. The GBR/CAA's include videos of their schedules, sometimes using a simulator and sometimes showing a real aircraft being flown through the schedule. The IMAC UK site shows schedules but you will need to understand the Aresti notation to understand them.

If eventually your aim is to fly competitions, note that both organisations require pilots to hold a B certificate to participate in their competitions. Now, luckily, if you follow the trimming and flying tips that I have set out above, you will be well placed to start practicing for your B certificate. There is a schedule which the GBR/CAA calls the Clubman schedule which is set out on their website HERE. If you look at the Clubman manoeuvres, with the exception of the Cuban 8, they are all B certificate manoeuvres. So, if you are interested in getting your B, practicing the Clubman will help you to get your flying up to the standard required of the B certificate. Note, that you must practice the B certificate and fly it in the way it is described in the BMFA handbook and not as a schedule.

The Clubman is flown as a schedule with so called turn around manoeuvres at each end of the box whereas the B, with the exception of the stall turn, does not have any turn around manoeuvres THAT ARE MARKED, although I dare say that the way that you fly during the entire B certificate flight will be taken into account if your performance of the required manoeuvres is, shall we say, marginal!

Even if you are not interested in competing, but want to fly a schedule, the GBR/CAA Clubman schedule will stand you in good stead should you wish to go on and get your B certificate in due course.

For F3A, don’t be put off by the photographs of large aircraft in many of the picture galleries. As I’ve said before, pilots have flown 50 size aircraft up to Masters and won the class against others flying 2 mtr class aircraft! It’s the pilot’s ability more than the model;s that makes the difference until you get into the FAI schedule.

For IMAC, you can fly in different classes and I believe that different aircraft size limitations apply between the entry level and other classes.

Edited By Peter Jenkins on 13/12/2013 18:37:48

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I used to fly IMAC and was their webmaster back in the day, you can fly the "Basic" class with a smaller machine. The models are "scale" aerobats but are allowed to vary by up to 10% from true scale. Being scale the models are to a degree harder to trim perfectly as they are not purpose built like the F3A machines. A knowledge of Aresti is advantageous in IMAC as all the schedules are in Aresti just like full size aerobatics. In IMAC you have to fly "Unknowns" which are given to you on the day of the comp and if you are calling for another competitor it's a big time saver if you can call straight from the Aresti rather than translating it all into the Queen's english. Sorry to bore as I know Peter's thread is mainly aimed at the club flyer, but then again you never know

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Following an exchange of PMs, I've put a reader of this thread in touch with experienced aerobatic pilots from the GBR/CAA who have volunteered to help with his aerobatic flying. If there are others who would like to get some practical help from experienced aerobatic pilots please feel free to PM me and I'll see what I can do to arrange some help.

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Following an exchange of PMs, I've put a reader of this thread in touch with experienced aerobatic pilots from the GBR/CAA who have volunteered to help with his aerobatic flying. If there are others who would like to get some practical help from experienced aerobatic pilots please feel free to PM me and I'll see what I can do to arrange some help.

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I was reading a magazine at the weekend that had an article in it about 3D aircraft trimming for CofG. The method given was with the aircraft trimmed for level flight climb and then descend at 45deg and see it the aircraft tried to: A) Level off CofG too far forward. B) Stay on 45deg line CofG correct. C) Dive down CofG to far back. I tried it out on my Monolog which I have already set the CofG on and it flew straight down the 45deg line, so I feel that this is probably a reasonable test. What do you think?

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Hi Terence,

I have the Monolog 110 on 8S. I have the CofG 4mm behind the recommended position and the control thows reduced to around 75% on elevator and aileron using longer horns but left the rudder as per instructions. It is smooth and flies very well on less the 1/2 throttle with endless vertical and a duration of over 11mins and very clean needing very little mixing. It flies better than me!cheeky

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Hi Algy, thanks for coming back to me so quickly. The Monolog 70 instructions do not have a specific measurement for the c/g, just a photo showing fingers over the fuselage sides! My 70 also has an excess of speed with the 560kv PurplePower motor/esc/5S giving 1400 watts on a 14"x7" wooden prop. I am also using Purple Power 40C 5S4500 batteries. Not many flights yet, only a handfull.

It is also very smooth, but i had to attach a small amount of weight to the right wing, and increased the right side thrust a fraction.

At present I am dialling in 25% aileron differential to counter adverse yaw when rolling.

Not much assistance in our club for the finer points of trimming for aerobatics, hence having to try different "fixes" myself.

What would help would be if yo could give me the % c/g position @ wing root, for me to compare with my set up.

Terry M

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Hi Terry,

The CofG is 38% cord at the root but that might not work for you. On the 110 there is a cross piece above the wing joiner tube and that is the position in the insructions for it to balance, I have mine 5mm behind that. looking at the 70 manual it look like it should be just in front of that cross member, which would be the safe start point and then move it slowly back a mm at a time until you are happy with the model.

If you follow the trim system Peter has layed out here in order you should be right on the money.

Have a look at this link on the GBRCAA site on the Monolog 70 http://www.gbrcaa.org/smf/index.php?topic=2894.0

It is hard on your own but I think you will get a lot od good advice here.

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