Monday 1 January 2001

How to set-up a racing glider

Even pilots who have been racing for years will take many sessions in differing lift conditions to set a model up to their satisfaction.
Although a good distributor will supply set-up suggestions with their models most of the measurements will be subject to personal taste. It is also worthy of note that although two moulded designs should behave identically this is not always the case. In practice a sensitive pilot (one who is not afraid to cry?) may detect minor differences in lay-up (changing torsional rigidity) and weight distribution. Any differences should be small and only recognisable at extremes but serve to prove the point that there is no substitute for taking the time to set each model up on its own merits to your style of flying.
Also it's fair to say that your own stele is likely to develop, so the way you set up a model 12 months ago may not be the way you'd set it up if you started again.
There are two vital mixes that can be optimised and are not necessarily solely personal taste. Snapflap and differential. These will be looked at in detail shortly. First though a few other thoughts.
For racing do not use aileron/rudder mixing. As a matter of principle I do not believe this coupling should be used by anyone with two thumbs but for racing there is a legitimate reason not to use it. If you have coupled aileron and rudder you are only achieving what correct differential should be doing in the turns. The downside to the coupling is that if you have good speed on the straights and need to make an aileron adjustment you will also be putting rudder in, which can start an oscillation that will scrub off speed. The effect will be minor but they all add up. It would be possible to have a very fine coupling that would do little damage but why bother when you can simply use the correct differential in the first place?

What we are talking about here is programming your TX so that the ailerons are making the plane track correctly around a turn without the fuselage pointing high or low. When done correctly significant drag reductions can be achieved both in the turn and in reducing any subsequent control inputs required.
Flying different styles - reversals, energy management, bank and yank - may not need significant differences in differential but subtle changes can make the pilot feel more at home.
Set the model up with the aileron response that suits you. Once done perform a crosswind roll without using any rudder input. Your airspeed should be reasonable but not so fast as to make anything axial. When I'm doing this I leave the differential menu open on my TX so that I can adjust the settings without landing. Multiplex TX's are a doddle in this respect as you can use the digi-adjuster. You may have to be more safety conscious with other radios i.e. don't blame me!
Once you have adjusted your differential to achieve the perfect axial roll at all but the slowest of speeds you have a place to start from. I usually save these settings for the TX's aerobatics mode. Switch your TX to racing mode and dial in maybe 10% more differential than was used to attain the axial roll. On a good day with the wind square on to the hill start flying an imaginary F3F course. After you pass the pilots position and are around 15 metres from the base, lift the nose slightly and roll 3/4 to inverted watching the line of the fuselage the whole time. If the fuselage seems to sit nose high and tail low during the turn increase the differential. If the glider seems to be thrown more towards the hill with the tail high and nose pointing into the hill try easing off on the differential a little.
The following two photos are designed to illustrate the general point. Even if a plane has what I would consider less than optimum differential settings, it is still relatively simple to adjust your flying style to suite the differential. What I always aim for though is to make it simple for myself so that I can fly the plane how I want to not how the set-up dictates I need to fly it.

This Cobra is the best example I could find of not enough differential. The nose is comparatively high and the tail correspondingly low. Of course there could have been many other factors that left the plane on this trajectory but I'm trying to illustrate the point.

This Sting is in a much better position to complete the run without the need for unwanted control inputs. The nose is low and the tail is high.

The Cobra above could have achieved the same result with more differential.
After a session in good lift you should have the plane set up with an aerobatic mode of perfect axial rolls and a racing mode where the fuselage tracks around the turn totally in line with the direction that the plane is travelling in.
I also use a slightly different setting for lighter conditions using a little more differential.
The speed of aileron response is largely a matter of personal taste although it is worth bearing in mind that in turbulent air you may need enough response to level the wings rapidly.
With regard to the all important turns you need to strike a balance between having too little movement making you start the turn early thereby missing out on the best lift and having so much that you can turn very late but are creating a lot of drag and end up twitching about the sky. Also bear in find that shaky thumbs can get to anyone, so less can be more.

Snapflap is the mixing of the flaps with the elevator. Normally the mixing will involve up elevator producing an amount of down flap and vice versa. I say normally because if certain flight characterises are required (such as damped pitch response) there is nothing to stop you experimenting with the direct opposite of what I will be discussing.
From the mixing we hope to gain tighter, faster turns and better inverted performance. In the case of 4 servo wings, whether or not the ailerons will be mixed 100% with the flaps (i.e. the whole trailing edge droops equally) is down to the model. My experiences have been that the more cambered the section the less outboard aileron you want acting as flap. The reason is that to have the outboard surfaces moving fractionally less than the inboard ones mean that you are less likely to induce a tip stall and associated flick.
Setting up planes with the optimum amount of snapflap is simply a case of evaluating a number of different settings, although it is complicated slightly as conditions and ballast can also have an effect. As a general rule the more flap you have mixed with the elevator the tighter the model will turn, up to the point that you are adding too much drag. This becomes apparent when the plane will still turn in a small radius but will scrub off lots of speed. You are looking for the compromise between the tightness of the turn and the speed retention throughout the turn. A few years ago I started using the phrase 'grip' which people seem to empathise with. A good set up will give you good grip, like an F1 car glued to the road. Too much snap flap and you'll be like a rally car drifting around the corners.
The factors governing this are numerous. Wing section, wing loading, moment arms, tail area/thickness/movement, wind direction, airspeed, flap areas, flying style, aspect ratio, CG etc. So go  and have a play.
As a cheat you may want to start at 1.5 mm down flap with full up elevator on an RG15 60" model and around 3 mm on a 3 m RG15 model.
The up flap mix with down elevator is far less critical. Generally for aerobatics you are only looking at a few mm to de-camber the section and for racing many people, myself included, use none at all.
It is very tempting to use computer TXs to show that you can waggle everything but it is worth bearing in mind that it is probably better to have too little elevator/flap mix than too much.
One final thought is don't be afraid to experiment. The aileron side of things is pretty straightforward the real potential gains are when you pull in the up elevator to turn the plane so watch, think and tinker.
Some areas worth experimenting with on the set-up front, try putting your snapflap on a curve so that all the flap comes in at, say, 1/2 the available up elevator movement. Also play with elevator curves, if you like a really response elevator but that can make a model a bit flicky use negative exponential i.e. front load the elevator movement around neutral but have it fall off at the extremes in case you get a bit excited.
Try different set-ups for different lift conditions. Consider more snapflap for liftier days when ballast is on board and less when the lift is marginal and/or the wind is not square to the hill.
One of the things I enjoy about slope racing is the analytical side of getting the best from a plane in varied conditions and setting a model up to do this is a skill, the same as flying the model in different conditions is. Like all skills of any merit it takes time, experience and sometimes help to feel confident in what you are doing.

By the way you'll have noticed I've steered clear of the CG debate, go fly and find it yourself!