Note to the reader: Slope aerobatics, like all model aircraft disciplines is a highly opinionated subject and this is a good thing – it leads to constant development.
What I have tried to do here is simply explain my approach. Many may not agree with what I have written here but nevertheless it’s the way I do it.
If you can get some good stuff out of these meanderings then I am happy, if not then maybe you can tell me the error of my ways…
– James Hammond
So, what’s it all about? Back in the day…
I guess that slope aerobatics has been around as long as slope flying itself. Even in those days of yore (That I remember 🙂 ) when kit instructions came in Latin, planes were made of balsa, tissue and dope and the only form of control was bang-bang rudder only, enthusiastic pilots have always tried to push the envelope as far as possible – something we will always do I hope.
Basically the kippers have changed a bit in the last two decades or so but the raw, exciting, enthusiasm and sheer unmatched craziness of slopeheads remains exactly the same – and that is also a good thing!
Are all aerobatic airframes the same?
Well, yes and no…I guess that today the slope aerobatic airframes might be largely, though not completely divided into two types.
Le VTPR (Voltige Très Près du Relief)
These are broadly the “in your face” close to the slope, milliseconds from disaster tumbling type of plane such as Madslide, Kulbutin, Le Fish, Coquillaj, etc. Mostly, though not always, these type of plane tend to be around or less than two meters (80”) span.
As the definition of the acronym might suggest, this type of flying was formalized in France – though it has to be said that it possibly been going on for a long time elsewhere too – sometimes by accident!
Versus, or rather compared to: The precision maneuver type “large air” or as I will call it, the “Open type”
Planes such as my own Vector III and Minivec, Phase 6, Voltij, Wasabi, Sagitta, etc – plus a copious amount of non commercial home brews fit this sector. These planes are more suited to the more “formal” scheduled maneuvers such as you might find in slope aerobatic competitions.
Note that it has to be emphasized that there are many grey areas of cross performance and maneuvers that can be well performed by both types.
- So you need to decide at the onset which type you are going for.
So, what are the basic ingredients of a well performing slope aerobatic plane?
Size – does it really matter? – well not much:
As hinted above, the Open air aerobatic planes tend to be larger for performing big Open maneuvers, while the VTPR planes tend to be smaller as they fly much closer to the slope – and some might say DISASTER! Again it’s true to say that both types can fill the roles of the other but maybe not quite as well.
- So make the size fit the application.
Here there does tend to be a difference. The VTPR type planes are made specifically as light as possible. As with their larger, Open maneuver brothers though, many can have weight added in the form of ballast.
Key point is that for violently direction changing stunts, the airframe will not perform well if it is too heavy and has a high inertia. Also the smaller VTPR types tend to be affected by very strong winds more than the Open type.
Though lightness is also important on the Open air type, it is not as critical. Some Open air type panes – especially mine – can be made extremely heavy so as to fly in hurricane conditions.
- So build it light but as strong as you can!
Wing area and aspect ratio:
Typically both VTPR and Open type planes will have a lower aspect ratio than their thermal, racing, or scale cousins. Simply put, the higher the aspect ratio, the slower the roll rate because you have to move more bits about that are further away. In an ideal situation the maximum chord thickness will remain in the same chord position along the length of the wings so as to give a symmetrical MAC and nice roll/pitch response. Wing area follows but in the opposite way; within reason we need as much wing area as we can carry.
- So we need a lower aspect ratio for fast roll response and as much wing area as we can get…within reason. Balance the wing and tailplane areas!
Fuselage shape and side area:
Now here is a topic for discussion!
OK for the fuselage, what does it do? It’s a stick to contain all the radio bits and to hold the wing and tail apart right? Well, maybe a bit more than that. As usual with any sailplane, we want to make it do its job well first then make it look good as a secondary consideration. This means that we do need to consider side area sometimes simply from the point of balance of the plane around the three dimensional CG.
Much is made of side area for our aerobatic airframes of whatever type because for reasons totally beyond me, knife edge flight – a regime totally alien to any aircraft, let alone un-powered ones – is a consideration for most if not the majority of slope aerobatic flyers.
So…as a commercial model plane designer I have to conceded that our aerobatic planes need to have an over-reasonable side area. Needless to say a large side area is not of much use if the fin and rudder are too small.
- So we need a fish! Basically a deep fuselage where it matters – forward of the CG, balanced by a large enough fin and control surface to be able (in theory) to achieve some semblance of knife-edge flight.
Typically the control volumes (Control surface area versus the actual size/aspect ratio of the wings, tailplanes, and fin) will be larger than on our not-so-aerobatic slope cousins. It is also probably true to say that for VTPR you would typically design-in even larger control volumes than you would for an Open type slope aerobat due to the nature of the maneuvers anticipated.
Open type aerobats would typically (though I stress again not always!) have a greater turn of speed in order to perform their work than a slower, and yes maybe more maneuverable VTPR type plane. It is easier to get a good response at a given speed from a large control surface that moves the minimum amount than it is from a smaller surface with larger travel.
- So make the control surfaces as large as you can without being ridiculous and also consider the strength that is normally robbed by over-large control surfaces.
Choice of aerofoil section type:
Here again a very sticky wicket…
For me, a modeler who always tries to get the very last drop of potential performance out of any airframe I design – of whatever discipline – there is only one choice for slope aerobatics and that is the fully symmetrical section – period.
I have heard cries of “Symmetrical sections have no soaring performance!” or “I don’t want to compromise my soaring performance with a symmetrical section!” etc. etc. Point is that the object of the exercise is PERFORMANCE. If the speed of a good section is maintained then while a slope aerobat will never outsoar say an F3F plane – it will still have a more than reasonable performance, even in light air.
- So if you are serious, use a fully symmetrical section.
But which one?
OK here we can make a few rules, but not many. The best aerofoil sections for slope aerobatics typically will be those that have a low drag when compared to their chord thickness. I had used the good old SD8020 for many years and many models mostly because it had the lowest drag for its 10% thickness that I could find until recently.
Nowadays there are lots of people working on low drag symmetrical section for various applications and not only for slope aerobatics.
A consideration which we now have the luxury of studying is the relationship between the control surface when defected and the front of the section when flying. This is a big consideration for F3F tailplanes for example where great handling is needed for example. There you would need the maximum effect for the smallest movement. Also a big consideration for almost any application is the alpha (when the section is not actually flying parallel to the airflow across it) performance.
If your chosen section will tolerate a nice bunch of sudden attitude changes without suddenly giving up on you, then you are clearly in a better situation. In truth my development of symmetrical sections was driven entirely by F3F design criteria. It was only later that I had the brainwave that they might just be not too bad for slope aerobatics!
Thickness plays a large part in your choice for aerobatic planes too. Basically you need some!
For better drag performance and also improved control response and tolerance, I have found and evidence shows that others also have concluded that the double cusped sections offer possibly the best of both worlds. Aerofoil sections such as the superb TP (Thierry Platon) 42 are typical of these.
For me, I have adapted my F3F tailplane section for use on my Open slope aerobats too with the JHSYM-10, 9, and 8. Even though I am now a professional model aircraft designer, like Thierry, I am not a believer in “Secret” or “Special” model aircraft aerofoil sections so mine are public domain – use ‘em as you wish!
- So we need a symmetrical section with the lowest drag for its thickness, a good alpha tolerance and as good a control response spectrum as we can get.
Getting it right but also making it look cool:
Yeah…come on…we can all put together a few planks, a bagged wing, and some bits that we have had laying around from previous re-kitting events and get something that will fly, and probably tolerably well. But for those who do not live in the Ugly Tree, we’d like to have a little bit of a cool factor – right? What I tend to do is to have a kind of step by step for any plane I design – aerobatic or otherwise. It goes like this:
- What work will the envisaged plane have to do?
- What will be its limitations?
- What are the practical limitations?
- What approximate size?
- What will it have to carry and where? – not only radio considerations which now are small because of the advances made in the last few years, but also ballast carriage etc.
- How strong will it have to be? So what materials will be best?
- Is it an experimental one-off development type, or with the knowledge available could it go to a production model with small changes?
Next step, the sketch up:
- Sketch – just hard lines on the back of a napkin when the inspiration strikes are good enough.
- Refine the sketch maybe by doing something as simple as superimposing another sheet of paper (or napkin) over the first…give it some curves maybe – change the proportions?
- Further refinement – further paper.
- Emergence of the concept – put the plane in proportion – make the wings etc right for the fuselage – use other designs for reference maybe.
- Remember you don’t have to be Da Vinci to get a good design – Bugger about with it!…just keep wasting paper until you think you are feeling good! Then do it again until you are SURE you are feeling good!
Last, the drawing…the über beast comes out of its lair!:
I tend to do formal line drawings. One reason that my CAD ability is not as good or more importantly as fast as my draftsmanship! Then I plead with one of my countless assistants to render the whole caboosh to CAD…
Make the CAD or line drawing but remember…it’s yours…you can do what you like with it!
Get creative…forget the wimpo minnows…lets see fully armed barracudas!!
I love the sound of clacking gnashing fish teeth in the morning…
James D. Hammond PhD, DBA – who has really been abducted by ailerons.
AKA: “Hambone the Impaler”, “Witch Doctor Hammond (Holidays only)”, “Bonio”, “Le Bone” “Hambone, lighter of dark corners”, and “Boneham the utterly fantastic”.
Editor’s Note: HUGE thanks to James for sharing this article. There’s not a tremendous amount of dedicated aerobatics glider design theory online, and this is a quite welcome and insightful addition. Here’s looking towards the future and pushing our shared passion forward! – Steve ‘Surfimp’ Lange