Designing for Slope Aerobatics
And other aerial gyrations.
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 own 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. — JH
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 with whatever they could get their hands on — something we will always do, I hope.
Basically, the models of old have changed quite a bit in the last three decades or so but the raw, exciting, enthusiasm and sheer unmatched craziness of slopeheads remains exactly the same — and that is another good thing!
Me? As well as quite a few slope soarers in other discipline types, I have designed and put into limited or full-scale production, a series of seven ‘Big Air’ aerobatic slope soarers, namely: Vector 2 (1984), Vector 3 (1986), Dorado (was going to be called Vector IV, 1990), Mini Vector (Minivec, 2005), Aresti 80 (2008), Aresti 108 (2010). It should be noted that every one of these models was designed using all the good bits from the previous design, and omitting the bad bits, while adding new ideas. This is a true development and in fact I’m happy to say that every model was in some ways better than its predecessor.
So, let’s get into it: Lets design an aerobatic slope soarer — it’s not as hard as it may seem. Things to consider…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, namely:
VTPR (Voltige Très Près du Relief)
These are broadly the ‘in your face’, close to the slope, milliseconds from disaster, tumbling type of plane ranging through the original Sonic and now encompassing such designs as Le Fish, Coquillaj, Ahi, and many more like those. Mostly, though not always, these types 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 VTPR has been going on for a long time elsewhere too — sometimes by accident!
Precision Maneuver ‘Big Air’ Aerobats.
From my stable, planes such as my own Vector III and Minivec, plus the later Aresti 80 and its big brother the Aresti 108 are Big Air type, with of course many others such Phase 6, Voltij, Wasabi, Sagitta, and indeed more like them that fit this sector. These planes are more suited to the ‘formal’ Big Air scheduled maneuvers such as you might find in slope aerobatic competitions, and less suitable for the low level, VTPR “edge of the slope” freestyle flipping type of cycles.
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.
Takeaway: You need to decide at the onset which type you are going for.
What are the basic ingredients of a well performing slope aerobatic plane?
Size — does it really matter? — well, not that much:
As hinted above, the Big Air aerobatic planes tend to be larger for performing large open maneuvers, while the VTPR planes tend to be smaller, more agile, as they fly much closer to the slope — and some might say DISASTER. Again, it’s true to say that both types can fill some of the roles of the other, but maybe not quite as well as a dedicated airframe. If you want to see huge, ballistic aerobatics, take a look at a good aerobatic pilot flying my Aresti 108. There are lots of videos out there on the public video channels — just type in “Aresti 108”.
Takeaway: Make the size fit the application.
Weight:
VTPR Here there does tend to be a difference. The VTPR type planes are made as light as possible — a requirement of the high-speed rapidly changing attitudes and directions needed for this type of flight. Lightness however does not always mean only flyable in light conditions, as with their larger, Big Air brothers, most 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 Big Air type, so look for, or at least consider some sort of ballast ability.
Big Air Though lightness is also important on the Big Air type models, it is not as critical. Some Big Air type models — especially mine — though constructed very light, can be made extremely heavy so as to fly in hurricane conditions if needed, but without the added weight, will fly very well in light lift.
Takeaway: Think about ballast placement and build it light but as strong as you can!
Wing area and aspect ratio:
Typically, both VTPR and Big Air 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 from the roll axis. In an ideal situation the maximum chord thickness point will remain in the same 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, but there’s not much point in putting too much out at the tips where it’s not needed. This is why I choose an elliptical shaped planform for lift, but with the maximum wing thickness point at 90 degrees to the fuselage in my designs. I cut off the tips, as with our models an elliptical “Spitfire” tip can cause a lot of unwanted effects at the most unwelcome times.
Takeaway: We need lower aspect ratio wings for fast roll response, and as much wing area as we can get — within reason, but put it in the right places.
Horizontal stabilizer and elevator area:
Oddly, stability is a requirement of aerobatic models. The model needs to go where its pointed, immediately and without any protest and then stay put. This means that the wing and tail area need to be well matched. There is no point in having a horizontal stabilizer that is too small in order to, say, reduce drag because it won’t work and the model will always require constant control inputs. In contrast, having a stabilizer that is too large can be a drag — literally — and will cause its own trouble in flight by forcing constant control inputs to counteract its damping effects in a similar way to an undersized Stab.
20 to 25% of the wing area is a good place to have a model aerobatic glider tailplane with the elevator area at 25% of the of the total Stabilizer area.
Takeaway: Balance the wing and tailplane areas.
Fuselage shape and side area:
Now here is an interesting topic for discussion.
The fuselage, what does it do? It’s a stick to contain all the radio bits and to hold the wing and back end, apart right? Well, maybe a bit more than that, at least on an aerobatic model. As usual with any sailplane, we want to make it do its job well as the primary consideration, then after that make it look good as good as we can within the design performance envelope. 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, Big Air or VTPR because for reasons totally beyond me, ‘knife edge’ flight — a regime that is 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 concede that our aerobatic planes need to have a larger than strictly needed side area. However as usual there is a caveat: In fact, flying with a side wind — which effectively the model is doing when making a pass along the slope — also needs a nicely balanced side area to prevent unwanted yaw attitude changes. Needless to say, a large side area is not of much use if the fin and rudder are too small.
Takeaway: What we need is a fish shaped fuselage. Basically, a deep body where it matters — forward of the CG, that is balanced by a large enough fin and rudder control surface to be able (in theory) to achieve some semblance of knife-edge flight.
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 — whatever the 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 light wind soaring performance!” or protests like “I don’t want to compromise my light wind soaring performance with a symmetrical section!” for example.
Point is that the object of the exercise when designing an aerobatic airframe is performance so I design for pilots who are way better than I will ever be.
If the flying speed of a good symmetrical 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.
Takeaway: If you are serious, use a fully symmetrical section. If you want a REALLY good one, use mine(!)
Which fully symmetrical section?
OK here we can make a few rules, but not many. The best fully symmetrical 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 I was contracted to design some new ones that were not for model use.
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 deflected and the front of the section when flying. This is a big consideration for F3F Horizontal Stabs for example where great fast handling is needed without the section letting go in a stall. Also, a big consideration for almost any slope soaring application is the alpha 𝜶 performance of the section (i.e. when the section is not actually flying parallel to the airflow across it, but at a positive or negative angle)
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 good situation.
What thickness?
Thickness plays a large part in our choice for aerobatic planes too — basically we need some, and for many years, in fact for the last series of five designs I opted for 10% as it seemed to be a good compromise between carrying energy and converting that energy to speed, however my latest design uses a 9% section — of my own devising as usual — I feel the need for a little more speed.
For better drag performance and also improved control response and tolerance, I have found, and evidence shows that the double cusped sections offer possibly the best of both worlds.
Takeaway: Try to find sections which offer low drag and have been tested at thicknesses of at least 10%.
A personal word on model aircraft aerofoil sections:
Even though I am in theory a professional model aircraft designer, I am not a believer in ‘secret’ or ‘special’ model aircraft aerofoil sections, so all of mine are public domain — use them as you wish — all I ask is to be given the credit for designing them. The aerofoil police are already on my trail, so a few more flying won’t make too much difference.
Takeaway: We need to use 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.
Note that any of my own designed sections are available from me. As mentioned all I ask is to be given the design credit.
Control volumes:
Typically, the control volumes, i.e. 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 areas and crazy volumes than you would for a Big Air slope aerobat, due to the extreme nature of the maneuvers anticipated.
Big Air models 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 model. It’s good to remember that it’s easier to get a good response at a given speed from a larger control surface that moves the minimum amount, than it is from a smaller surface with larger travel.
Takeaway: Make the control surfaces as large as you can without being ridiculous and also consider retaining the strength that is normally robbed by over-large control surfaces.
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 use a kind of step by step evolution for any plane I design — aerobatic or otherwise. It goes like this:
Preliminary questions:
- What work will the envisaged plane have to do?
- What will be its flight limitations?
- What are the practical construction limitations?
- What will be the 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? 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?
Takeaway: Get all the sizes and parameters roughly figured out and write them down.
Next step, the sketch up:
- Sketch — just hard lines on the back of a napkin when the inspiration strikes are good enough. I know not why, but ‘inspiration’ always seems to take me when sitting on the big white throne. So, I keep a pad and a pencil on the cistern, as I have found toilet paper is sadly lacking in strength.
- 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 and empennages 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.
Takeaway: Keep playing with your design — it only costs time.
Last, the drawing…the über beast comes out of its lair:
I tend to do formal line drawings on actual paper. One reason for this is that my CAD ability is not as good, or more importantly not as fast as my draftsmanship. Then I plead with my super-fast super-good CAD guru to render the whole caboose to digital format.
Make the CAD or line drawing but remember…it’s yours…you can do what you like with it. If you don’t like it, change it until you do. There is nothing worse than seeing the complete model and then the dreaded “Wish I’d’s…” come out. If you are anything like me you will never make what you can utterly claim to be your masterpiece. There will always be something to make better next time and that’s the absolute beauty of it.
Takeaway: Get creative…forget the minnows…let’s see fully toothed barracudas!!
Cheers!
©2021 Dr. James Hammond PhD, DBA
This is the first part of a four part series. You may also want to read the next instalment, Designing for a Slope Allrounder in the April 2021 issue of RCSD. In that author James Hammond provides his take on designing for a medium-sized (80" to 100”) slope capable of performing well in a variety of roles. All figures and photos are by the author unless otherwise indicated. Now, read the next article in this issue, return to the previous article or go to the table of contents. Downloadable PDFS: article issue
