Ever wanted to get into or improve your aerobatics?

[Peter Jenkins]

Reverse Cuban 8,

Just like a Cuban 8 only different! Again, take a look at the diagram below before I describe it.

The Reverse Cuban 8 is flown by approaching downwind. What’s different is that instead of crossing the centre of the box still flying level, you commence a 1/8 loop before you reach centre and establish a 45 deg up line. Then you perform a half roll and then perform a ¾ loop ending on another 45 deg up line. A 5/8 loop then follows and that gets you back to level after which you must track level across the box until you pass the left hand edge of the first loop.

Key points to watch are that you set the height of your loop by the length of your 45 deg climb so continue the straight line following the half roll so that you do not cramp the size of the loop for both sections of the manoeuvre. Wind corrections are as before except that you will have your 45 deg climbs going in the opposite direction to the Cuban 8. So, as the first climb is downwind, you’ll need to pull to higher angle than 45 deg and the obverse for the climb into wind.

Again, by performing a half reverse Cuban 8 you have another turnaround manoeuvre.

Diagrams courtesy of the GBR/CAA.

[Steve Colman]

Cuban 8 and Reverse Cuban 8 are probably my favourite to fly.

I often combine a half reverse Cuban 8 with a half Cuban 8 for some variety and often use them as turnaround manouvres too.

However, I do have to add that they're somewhat crudely executed by my hands on the sticks and never as precise as discussed in this great thread. However, I will continue in my attempts to improve when the weather permits some decent flying time.

Moreover, I have my recently completed, but still to maiden, Sebart Angel S30 waiting in the wings.

Steve.

[Martyn K]

Always puzzled as to why this (Cuban 8 and reverse Cuban 8) is considered/deemed to be a down wind manoeuvre as 50% of it is flown upwind and 50% downwind.

Is there a reason for this or is it simply convention due to where it sits in the schedule?

Martyn

[Kevin Caton]

It's just where it sits in the schedule Martyn. Over the years they have sometimes been downwind and sometimes upwind. As you recognise, you fly half of it upwind and half of it downwind whichever direction you start.

Kevin

[Martyn K]

Thanks Kevin

I guessed there had to be good reason for it . However, it makes a big difference - up until not I had always done a Cuban starting into wind. I know that it shouldn't make any difference...

Martyn

[Peter Jenkins]

Humpty Bump,

The first point to make here is that I don’t know why this manoeuvre has this name. If anyone can throw some light on it I’d be very interested.

Back to the manoeuvre. This is another turnaround manoeuvre. It is usually described with options and the reason for the options are to allow the pilot either to correct for wind or to position for a manoeuvre that needs more, or less, depth. Again, take a look at the manoeuvre diagram below first.

The manoeuvre is begun towards either end of the aerobatic box. It commences with a pull to the vertical, followed by vertical flight (which in due course you will need to wind correct). During the vertical flight the simplest manoeuvre you may be required to perform will be a ¼ roll. You must position this ¼ roll in the middle of your vertical flight line. Then, you would be asked to fly a ½ loop, followed by a vertical down line with again a ¼ roll so that when you pull out you will be flying level in the opposite direction to your entry direction. This would be called a Pull, Pull Pull Humpty Bump because that’s what you had to do to perform it.

If you had been told to perform a ½ bunt or outside loop, then it would have been described as a Pull, Push, Pull Humpty Bump. There are a whole host of other variations but we don’t need to know about them at the moment.

Notice that the direction of the ¼ rolls is not specified as the direction you need to roll is determined by two things: a) whether you need to push or pull; and b) whether you want to move the flight path towards you or away from you. What determines that is whether you think you have been blown in too far or out too far.

The big problem is deciding which way you need to ¼ roll to get the aeroplane in the right position.

For a Pull, Pull, Pull Humpty, what I found helps is this. With the aircraft flying left to right, which ever way I want the aircraft to go, I push the aileron to the inside of the manoeuvre – imagine swinging the Tx so that you are pointing the aerial at the model. Then on the way down, you ¼ roll in the opposite direction. That way you don’t need to think about rolling left or right. So, in this case, if you wanted to fly away from yourself, you would roll to the left (to the inside of the loop that is coming) and then roll to the right to recover in the correct direction. If you roll the wrong way you end up pointing out of the box!

For a Pull, Push, Pull Humpty, the opposite applies. You will not get both versions of the Humpty until you are into flying the harder schedules so you only need to remember one set of techniques to cope with the manoeuvre you are flying. I have to say, that until I worked out this simple rule, I would quite often end up rolling the wrong way!

You might also be given an option. Let’s say the option is ½ roll up and none down. That allows you to remain at the same distance out and is pretty simple to perform as it doesn’t matter which way you roll!

Make the ½ loop over the top nice and big and the whole manoeuvre will look elegant.

All the usual wind correction techniques will be needed to perfect the Humpty Bump and as you make the manoeuvre larger these corrections become essential.

[Martyn K]

Hi Peter

How do you get on line again after completing this manoeuvre? Assuming that you are pulling a normal sized half loop then you will be one loop diameter either nearer or further away afterwards

Regards

Martyn

[Peter Jenkins]

Hi Martyn

Say you have not countered the crosswind and you have been blown in then you fly the Humpty to space you back out - and vice versa. If you think you've gone too far out then as you fly on the inbound leg you squeeze on some rudder (either using opposite aileron or you mix out the secondary effect of rudder) to fly you back to your desired depth while keeping wings level. Don't forget to use rudder the opposite way when you have reached the correct depth.

The option allows you to conduct the Humpty without changing the depth but you will have less room in front of you for setting up the next manoeuvre. For something like a stall turn as a centre manoeuvre, the won't be a problem. However, if you have a roll combination (e.g. half roll, full roll reversed, two of four point roll reversed) you may want all the space available and opt for the sideways Humpty and sort out the depth problem at the next turn around.

OR - allow the aircraft to drift in or out and then correct with the Humpty to bring you back on line.

If the wind is blowing into your face, being further out with a centre spin manoeuvre which will result in you being blown towards yourself is good. If you made the wrong decision as to which way to roll, and you came in before the spin - then you have a problem!. Spin recovery overhead yourself is not a good idea!

The key thing is to make small changes early so that it all looks smooth and graceful. Late corrections, especially when you start off by going the wrong way, are jerky and look awful and lose you points in a competition.

I hope that answers your question. If not, try again and so will I.

[SR 71]

Hi Peter I don't know if I have missed it but I have a question for you, I am currently building an own design 2x2 F3A model, and I am new to F3a

Why do you need to be able to adjust the incidence on the wing and tailplane if they are both symmetrical, I can see the point of side thrust and down thrust

I was going to build mine with zero incidence

Don't know how to load some pics of it on here

Just found this thread and I think its fantastic the information you are giving us

Thank you

Tony

[Peter Jenkins]

Tony

Good question. It used to be that aerobatic aircraft were set up zero/zero but in those days they used to fly at much higher speed than now. So when you slow down you need to increase the angle of incidence and with a zero incidence wing your fuselage would take up a slight tail down attitude.

Generally, therefore, the starting point, for today’s F3A setup is +0.5 deg on the wing and 0 deg on the tailplane. This is just the starting point. There are a whole host of other issues such as getting the aircraft to dive vertically on recovery from spins and stall turns. This can be achieved by mixing elevator with closed throttle or by adjusting incidence (up to fix the pull away from canopy or down to fix a pull towards the canopy).

As I say, the figures I gave above are the starting point and you go from there. I have also tweaked the incidence on one wing when I had a slight roll with ailerons at neutral. One will always have slight manufacturing tolerances which cause slight imbalances. The most popular incidence adjusters are called Gator and I usually get mine from a US source (LINK).

Some designers, do not allow for any incidence change as they design a specific relationship for that design. Any Brian Hebert design has this feature. All you can then do is to adjust the CG so that there is no coupling between roll and yaw and you are done! Apart that is from adjusting engine side/downthrust.

If you want more detailed help, a better forum is that of the GBR/CAA which may bring you more help (LINK). I know of at least one member there who has built his own 2 mtr aerobatic aeroplane recently and flown it successfully in competition.  You'll find the build thread here LINK

Check out the Forum user guide section as it gives you detailed instructions on how to load pictures onto the thread. That’s what I used to find out how to do it and they are very good.

Edited By Peter Jenkins on 28/01/2014 21:18:38

[Peter Jenkins]

Inverted spin

I did wonder about covering this but here goes. As its name implies, you enter this spin from….. an inverted position! If your aircraft has dihedral and a non-symmetric wing section you will find it becomes quite difficult to perform this manoeuvre.

Inverted spin entry is similar to the erect spin with the exception that you will be using increasing amounts of down elevator to slow up the aircraft and to get it to stall. Remember that the direction of spin will now be reversed so that left rudder produces a spin to the right because the fin and rudder are now underneath the aircraft and the rudder, while moving in the same direction as before is now seen by the aeroplane, and the pilot for that matter, as moving in the other direction.

Recovery is as before, namely, centralise the controls and the aircraft will, in most cases, come out of the spin. Some aircraft will take longer to stop spinning than others while a few will require the full spin recovery: full opposite rudder and then bring the stick backwards (in this case) when the spin stops centralise rudder and elevator. Allow the aircraft to accelerate in a vertical down line and then recover to straight and level. Recovery can be either to upright or, if you have to position for an inverted entry, to inverted.

It’s quite odd the first time you try this manoeuvre but it is an interesting one. The first time you try to recover to the inverted position is also an unusual action so give your nerves a break and do this at height for the first time to convince yourself that both you and your aeroplane are happy doing this.

[Martin McIntosh]

I must agree with Peter. Zero-zero is just not on since there can be no lift other than the motor dragging the model into the air with the tail down. Also, flying inverted will mean that very little down elevator is required which can be an uncomfortable situation when you try to hit the right amount of down when exiting inverted. Everything becomes much too sensitive. 0.5 to 1 degree positive is about normal. I have used a variable incidence tail a few times which is simple to install. You will end up with totally level elevators which helps enormously to prevent coupling in knife edge. An all flying tail has the same effect and can be very smooth but gives problems with snaps and spins.

Good luck with your build, but stick with what others do.

[SR 71]

Hi Peter I already belong to the GBR/CAA I joined last year, its good but this thread is better maybe you should do one on that forum as well

Tony

[SR 71]

Hi Martin thank you for the advice, to be honest I was expecting to be told to stick with proven aircraft before attempting something of my own

Tony

[Peter Jenkins]

Hi Tony,

The reason I've used this forum for this thread is that I thought I'd reach a wider audience than doing this on the GBR/CAA forum. Most on that forum would know more than I do except for the newcomers. But I don't see any thread on this forum dealing with a building a 2 mtr aerobatic machine whereas there are threads on the Association forum that do.

Re Martin's comment, much of model aircraft design is based on the empirical process (build it and see how it performs) and it's best to make use of existing experience than to re-learn expensively on your own. We are, though, at the stage when one could design an F3A machine using computational fluid dynamics (CFD) since that is what all major aircraft manufacturers today use - but you need deep pockets to use these tools unless you can blag some time on their very expensive design tools and you know how to use them - a very tall order. I know that some Universities use CFD tools but I don't know how good they would be at producing a better design than you can buy commercially. However, the satisfaction of being able to build your own competitive 2 mtr design must be great. However, to assess how good your design is, you'll need a pretty experienced F3A pilot to give an assessment of the aircraft's capability and the analytic knowledge to know what to do to make it better.

Good luck and do let us know how you get on. How about starting a thread on it?

[Martyn K]

Back to the inverted spin, I find that my Acrowot spins better inverted than the right way up. I have never really understood why, but the wing stalls much deeper inverted than the right way up. I often wondered if the effective anhedral may be the cause but I can't visualise the reason why that would be the case

Not really looking for an explanation her but I found it easier to perform than expected (if more nerve racking/wrecking)

Martyn

["L"Plate]

I have had a few aircraft that spin better inverted than upright, I have put this down to wash out on the wings (were the wings are twisted so the incidence at the wing tip is reduced compared to the wing root).

My reasoning for this is; normally wash out is used so that the wing root stalls before the tip, helping to keep the aircraft stable at low speed, so when you are inverted (with wash out) the tips will stall before the root and if one tip stall first due to the rudder input the drag caused will assist in developing the spin. I am sure someone will correct me if I have this wrong.

[David Hardaker]

Bear

I agree with your reasoning. Also the rudder will have more authority when inverted, as it's in less-disturbed airflow...

But you'll not find washout on a true pattern model with a symmetrical wing.

d

[SR 71]

Hi Peter I will try and do a thread of my OD F3A but cannot get the pictures to load

Help please

Tony

[Peter Jenkins]

Hi Tony,

You need to create and album, load photos into it and then pick the photos from your Album to put them in your thread. Detailed instructions on how to do this, and lots of other stuff including how to paste in links without screwing up the site, are on the Forum.

Just click on the Forum in the Black Bar and then scroll down to the fourth bold heading The Site Platform, click on that and then scroll down to the 7th Sticky entitled Creating an Album and Adding Pictures. This describes how you create an Album and how you add pictures to it. They must be JPEG format.

Once you've created your Album, you then need to read the 3rd Sticky on The Site Platform section entitled Using Album Photos. This will tell you how, from within the thread you are writing, you can select and include your chosen picture.

If you get stuck PM me rather than post on this thread. You can do that by clicking on the "Message member" link immediately below this post.

Peter

[Peter Jenkins]

Hi Martyn

I think that Bear and David H have responded in exactly the way I was thinking.

I didn't quite understand your comment "the wing stalls much deeper inverted...". Did you meant that the fuselage takes up a greater angle to achieve the stall inverted than upright? If you did, then that will be a function of the wing having a positive Angle of Attack (AoA) compared to the fuselage datum. That will mean your fuselage will have to take up a slightly higher angle to the flight path to make the wing reach its critical AoA.

If you meant that the actual stall was deeper, then all I can ask is that you describe what you mean by that as a stall is a stall. Does it take longer to come out of the spin compared with spinning upright?

(There is a term "Deep Stall" in aerodynamics but it refers to high mounted T tails that get blanketed by the wing in a stall e.g. BAC 111 when you need either a drag parachute or to jettison weight from the rear to get out of a stall since the tailplane won't help you.)

Regards

Peter

[Peter Jenkins]

Knife edge Part 1

Knife edge looks deceptively simple. Roll to wings vertical and bang in rudder towards the top wing, or top rudder as it is called. Thing is, it has to look elegant and smooth to look impressive and there are three phases to Knife Edge or KE.

First, the roll into KE. As with the a normal speed roll, the moment you start rolling, the amount of lift being directed vertically begins to fall so you need to increase this vertical component and that is done by applying a little up elevator and, in order to keep going straight, an increasing amount of top rudder. (As a rule to remember, roll left, right rudder needed and vice versa). As the roll to wings vertical progresses, you need to start reducing the wing generated lift as that will just drag you off line in the direction of the wing lift. So, you need to increase your vertical component of lift by increasing the amount of engine thrust directed upwards by increasing the amount of top rudder while as the same time decreasing the elevator input to zero i.e. stick in the middle of the elevator range. But, you need to watch the aircraft’s path and be ready to compensate with down elevator or up elevator to maintain your ground track. Clearly, this will be affected by any cross wind component.

Second the steady state. With the aircraft on its side and tracking straight, the rudder deflection can also impact the direction of flight horizontally. You can get roll/pitch coupling which can pitch you either nose towards the canopy or away from the canopy. There are two ways of dealing with this.

Moving the aircraft’s CG forwards increases the tendency to pitch towards the canopy or reduces the tendency to pitch away depending on from where you are starting. The converse is true i.e. moving the CG aft reduces the tendency to pitch towards the canopy or increases the tendency to pitch away from the canopy. As you may remember from when we started the trimming process, moving the CG means you have to go through all the other trimming stages again as everything changes. I cannot stress enough that the CG position is the single most powerful mechanism for trimming your aircraft.

[Peter Jenkins]

Knife Edge Part 2

The alternative to moving the CG is to use mixing. I recommend that you put the required mixes on a switch so that you can check the effect at a safe height. It will also allow you to switch the mixes off if you feel you don’t need them for other manoeuvres. There are two mixes which you might need.

• One will be with Rudder as master and Elevator as slave. It’s best when setting up what will be very small amounts of mixing to make sure you start with better than 50% so you can see without any problems in which direction the slave is moving. Then bring it down to say 2% to 3% for your first trial. Watch carefully to see which way the aircraft pitches when in KE both with left and then right wing down – you may find it easier to get a friend to write this down as you are flying the tests. Then set up the relevant mixes to give you a small amount of elevator in the required direction. Re fly, the tests and make the necessary changes so that application of rudder, at the same manoeurvre speeds, produces no pitch interaction. The speed bit is important as the effect of mixing is non-linear i.e. the mix holds true for a very narrow range of speeds – go too much outside them and your mix may not have the exact effect you want.

• You can see the advantage of moving the CG to eliminate unwanted pitching as it is speed independent.

• The other mix will be with Rudder as master and Aileron as slave. This is really a function of how much of the rudder is above the aircraft thrust line. With pattern models the rudder area is designed to be the same above and below this datum and you may not get any interaction. With non-pattern models you are highly likely to get an induced roll usually in the direction you applied the rudder because the rudder is applying a rolling moment around the thrust line. As for the first mix, fly a few tests and make sure you know which rudder produces which roll. Then, set up the required mix, put it on a switch, and go and fly a few more tests until you get it right. Again, remember this has a very narrow speed range where the mix will be spot on.

The third stage of KE is the roll out of KE and this needs to mimic the roll in. You need to coordinate the reduction in rudder input with an increase and then decrease in elevator input to achieve the desired flight path.

There is a lot going on during KE flight and once you can get into KE, fly a decent KE and get out of KE confidently and repeatedly then you can start dealing with the effects of the cross wind. This is achieved by using the elevator to angle the fuselage into wind enough to hold a constant ground track.

So, if you have trimmed out your steed and you still have a minor roll or pitch then you will have to fly it by hand. Well, it can be easy to get confused as to which direction to apply aileron. Here’s a rule I picked up on the GBR/CAA forum.

• With the aircraft flying towards you, LOOK AT THE BOTTOM WING and push the stick in the direction you want the wing to go.

• With the aircraft flying away from you, LOOK AT THE TOP WING and push the stick in the direction you want the wing to go.

With the elevator, it depends on whether you are performing a KE with the canopy towards you or away from you. I don’t know of any easy rule of thumb here (any offers?) other than if the input you used didn’t have the desired effect go the other way!

You shouldn’t have any difficulty with the rudder other than when going into KE when you might find that you use rudder in the same direction as you roll! Best to do your first KE experiments at a decent height!

As regards maintaining height, if the aircraft’s height is decreasing, increase rudder and if the height is increasing, decrease rudder. If you reach full rudder deflection and the aircraft is still descending, then your airspeed is too low. Roll level and try again with more power. Aircraft with narrow fuselages, e.g. the WOT4 and most of the classic aerobatic aircraft, need quite a bit of power since the fuselage generates very little lift and you are relying on the vertical component of thrust to provide the force to keep you flying level. Modern pattern models with deep fuselages are much easier to fly KE with in that respect.

[Martin McIntosh]

Me again Peter.

I have never heard of moving the cg to compensate for adverse pitch in KE. Surely this has been previously set correctly for normal flight and spins?

Pitching is usually a direct function of rudder rake back angle and area above/below the fus centre line. Often this needs to be compensated for by either piloting skill or a little mixing but never moving the cg. I would say that up to 8% elevator mix would be acceptable otherwise change the rudder configuration or get a better model. An other than ideal model does teach you a lot about KE.

Regarding aileron coupling.

This can usually be negated by getting the dihedral angle correct. Not always easy to change but again a well designed model should have this right, if not then back to the mixing.

Martin Mc

[Peter Jenkins]

Hi Martin

Don't know if you have come across Brian Hebert. His last three F3A designs were the Alferma, Valiant and Shinden. He has had Oxai and CARF produce his models and he has been using a technique called Triangulation Trimming which focuses on setting CG by reference only to Knife Edge performance. Brian has had pilots like Chip Hyde adopt his technique successfully as well. You can access Brian's web site here and read his description of Triangulation Trimming where you will also see Brandon Ransley and Chad Northeast (Canadian Champion) endorse this approach.

You might also be interested in the following comment from the GBR/CAA forum:

• For the last couple of years I have been using the Brian Hebert trimming method known as Triangulation Trimming, or at least my own variation of it. Having been building and trimming pattern models for over 30 years I was a bit skeptical until I tried it but the bottom line is for me it works.
• My Asyuler has no mixing other than a couple of percent down elevator with low throttle to correct the vertical down lines. It holds dead straight indefinite knife edge on both sides with the application of rudder alone and goes absolutely straight up too.
• The basis of this trimming method is to set the CG ONLY by the knife edge performance and no other criteria. Once this is correct you move on to correcting the vertical climb by addition of wing incidence and only then worry about the correct "feel" with adjustments to throws, expo, differential etc.

I have not tried this approach myself but I'm going to with my current machine. I thought it useful to give it as an alternative.

​Peter