Flap Happy?

Talk about flaps in R/C aeromodelling circles and there’ll no doubt be a fair amount of puzzlement and head scratching as to their true function and application. There’s no doubt that flaps are becoming more common in the modelling sphere with the pushing of ARTF boundaries to include scale warbirds, bombers and jets. Model turbines are shoving airframes ever faster, and moulded composite technology is making gliders ever slicker. Surely, then, flaps are much more than just an inconvenience that we need to buy extra servos for?

Full-size aircraft design has always been something of a compromise. For the sake of the pilot, designers have always striven to bring the aircraft back to earth at a speed where he’ll be able to land it in one piece. A wing designed to facilitate a controlled, slow speed approach at a moderate angle (so the pilot can see over the nose) is never going to be efficient at the higher speeds needed for supersonic or other operational flight requirements.
A well-proven solution to the problem is to design the wing for its primary function and add flaps for use on the landing approach. We modellers mimic this, but bring simpler functionality to our airframes than the complicated systems employed by the big boys.

Flaps are hinged control surfaces on the trailing edge of the wings inboard of the ailerons (see Fig) and are operated in parallel by the pilot. In the unused (up) position they lie flush with the surface and form part of the wing skin. When deployed (down) they can deflect as much as 90° to the wing surface.

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There are many different types of flap, and each have their own characteristics, but when drooped they all increase the effective camber of the wing and hence its lift. However, deploying flaps also increases drag. There’s generally a huge gain in lift when flaps are lowered to angles of 30-60° degrees, and very little more at 60-90°. Drag, on the other hand, increases only moderately through the initial small angles but greatly at large angles of deflection (see Fig).
Due to the greatly increased lift provided at small angles of flap the stalling speed of the aircraft is reduced, and consequently it’s possible to fly a slower, safer landing approach with a correspondingly slower landing speed. And because little extra drag is being produced with the flaps at this shallow angle, the aircraft’s landing approach angle is scarcely affected. At greater throws, however, there’s that desirable and significant increase in drag that can provide yet slower landing speeds, a steeper approach and a shorter landing run. Using a small amount of flap (15° or so) will also improve the wing’s lift at take-off speeds, with such a small corresponding drag penalty that a shortened take off-run is possible.

Next time you jet off on holiday, ponder the benefits that flaps bring when the aircraft you’re sitting in is on finals. Imagine the landing speeds of a modern day airliner without its huge Fowler flaps slipping backwards out of the wing, allowing the aircraft to land far slower than it otherwise would. You’d surely need that first beer by the pool to calm your nerves!

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Flaps used in model applications are usually simple hinged affairs, with scale modellers occasionally opting for ‘split’ flaps where the need dictates (see Fig).
In most cases driving these control surfaces is no more onerous than driving a set of ailerons. Moreover, with modern radios bristling with mixing features that can match the rate of deployment of one surface to another, there’s accuracy in adjustment and control. At the transmitter it’s not unusual to find modellers selecting to control flaps by way of a three-position switch:

  • Position 1: Flap fully up.
  • Position 2 (central): Flap slightly drooped (for take-off).
  • Position 3: Flap fully deployed (for landing).

This method does assist in easy mixing of compensatory elevator trim but may not give the flexibility some purists require, i.e. that which is available using the transmitter’s auxiliary controls (be they knobs, sliders or, in the case of gliders, the ‘throttle’ stick).

Mind you, courtesy of ‘flaperons’ you don’t necessarily need flaps to provide the same aerodynamic function. The flaperon function is a defined transmitter mix where the ailerons also act as flaps, moving up and down simultaneously. Now, whilst it’s quite common to mix the names of two control surfaces where just one moving surface performs two functions, be careful not to confuse the flaperon function with ‘spoileron’, where both ailerons are suddenly deflected upwards to ‘dump’ lift on the landing approach. This function is more commonly used on higher aspect ratio wings, electric hotliners and simple slope soaring gliders. Flaperon, on the other hand, is more commonly used in conjunction with the elevator control to provide coupled flaps and elevator, or ‘snap-flap’.

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Control line stunt pilots have long realised the advantages that a small flap deflection can have when used in opposition to the elevator (i.e. up elevator, down flap and vice versa); the increase in lift afforded by this small deployment gives crispness and complimentary authority to the master elevator control. The flaps lift the wing whilst the elevator pushes the tail down, resulting in nice square corners to looping manoeuvres, tighter looping and bunting radii, and on landing, a nice floaty flare as the model approaches the stall and elevator is increased. Competitive R/C fun-fly models have used flaperons to great effect over many years, and it’s rare to find one these days that doesn’t utilise this mix. And with the advent of 3D flight, modellers are using spoilerons and flaperons in scenarios where previously they wouldn’t have been considered.

It is, perhaps, on model sailplanes where flaps really come into their own, tweaking the wing’s camber by finite amounts to produce almost magical performance changes across the flight envelope.
A four servo glider wing generally incorporates two inner flaps and two outer ailerons that cover over 90% of the model’s total span. These can be harmonised through computer mixing to enhance the specially designed wing sections across differing flight modes, providing:

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  • Camber for minimum sink in lift or while thermalling.
  • Reflex for speed.
  • Snap-flap coupled with elevator for optimising race turning, or aerobatics.
  • Crow braking with flaps fully deployed and ailerons raised (or ‘spoiled’) for landing.
  • Flap-to-aileron mixing for increased roll rate.
  • Aileron-to-flap mixing to camber or reflex the ailerons with the flaps across the whole span or, if flying off the winch, droop the whole trailing edge a more significant amount for maximum lift and launch height.

Out of these optional mixes it’s perhaps reflex that goes against everything we’ve so far learnt about flaps. In this case instead of drooping down from the wing, the flaps will actually deploy slightly upwards. Reflex is used to tweak the glider wing’s camber from its usual semi-symmetrical polars and fool it into thinking it’s a faster, full (or near) symmetrical section. Not all wing sections take well to this kind of abuse, and reflex has to be used at the right time under the right conditions to be effective. However, with the advent of computer-based aerofoil plotting software like Compufoil, X-Foil and Profili, designers like Quabeck, Herrig and Hepperle are able to design around the intent to use flaps in this way, generating new aerofoils to obtain the best performance across the expected flight envelope of the model, improving performance in even relatively inexperienced hands.

So, maybe there’s much more to flaps, then, than you initially thought. A useful tool, even in the sport flier’s arsenal. But how would we go about using them where it matters, i.e. down at the patch during an average Sunday afternoon session?

If you’re experimenting with flaps for the first time there are a few things you’ll need to take on board. As we’ve established, flaps will slow a model on landing approach to a speed far below that at which the wing of a model without flaps is happy to fly. One very common mistake model pilots make comes about when overshooting the landing.
Let’s consider the model ‘hanging’ on the flaps on short finals. The motor’s running near to tick-over, the model is cruising down to the strip and you’re starting to raise the nose for a landing flare. For whatever reason you decide you’re going to go around again. Do not immediately put the flaps away, as they’re just about the only thing keeping the model airborne! Throttle-up as you would normally and re-establish a decent air speed as if climbing out after take-off, and then slowly stow the flaps as you re-trim for straight and level flight. If your flaps are activated by a switch (i.e. on or off) then be doubly sure that you have adequate flying speed before putting them away. So, given this important piece of information, how do you go about flying a flap-enhanced approach?

Until you’re used to their effect it’s much safer to start finals over the strip with a circuit to go and at moderate air speed before steadily reducing power and deploying the flaps. This should be done gradually, and as you fly the upwind leg and upwind turn of the landing circuit you’ll begin to get a feel for how the model behaves under deployment. As the drag increases you’ll notice that the throttle setting needs to be higher than on a landing approach without flaps in order to maintain progress at a reasonable rate. As you approach the strip the extra drag of the aircraft will usually require compensation of perhaps 1/4 – 1/3 throttle, rather than a steady tick-over with the throttle stick against the stop. The only exception to this would be when using flap to fly a very steep approach to land, where gravity is helping to maintain flying speed in the steep descent (see Fig).

When the flaps are fully lowered you may chose to either battle against their effect with some down elevator held in, re-trim accordingly, or employ a compensatory elevator mixer to take care of it all. The final stages of the landing approach should be made as normal, though you might discover that it’s easier to get the model nose-high for a roll-out on the main legs (or a three-pointer, as appropriate).

With flaperons, other rules come into play. It’s usual to fly an approach with flaperons acting as the slave in a coupled flaps and elevator mix. In this scenario the approach can be made as normal, constantly adjusting elevator (and hence flap) during the whole approach. At the point of flare the model may balloon slightly but it will reach a far lower touchdown speed due to increased flap deployment as the elevator is brought in. Avoid drooping flaperons alone unless you intend to fly a very steep, nose-down approach to prevent the wing from stalling. It’s not usual to see this method utilised on power models, generally being the preserve of lightly-loaded ‘discus launch’ gliders and the like. Spoileron is much more effective (in most cases) where some level of air braking is required. It’s also much safer, de-cambering the wing away from a stalled condition.

The last point to note regarding the use of flaps is more often seen under crow braking conditions with gliders, but the principle is applicable to every model. It concerns the use of that compensatory elevator mix that can cause some anxiety where the application is deployed suddenly to its full extent (see Fig).
Both the flap and elevator servos will move to their predetermined position at roughly the same speed, however the limit of flap servo movement will be a good number of degrees further than the compensatory elevator servo needs to travel, with the result that the elevator will reach its end position first before the flap has reached the position that the elevator is set to compensate. If the flaps are ‘dumped’ quickly and the model has decent airspeed, this servo lag will produce a pronounced dive, checked only when the flap servos reach their set end position. Flying near to the ground at high speeds, the result of such an action will call for a broom and fresh bin liner…

I hope I’ve demonstrated that flaps are not to be feared. In truth, they can actually make an R/C pilot’s life much easier, and if they’re good enough for the big boys they should be good enough for us. All we have to do is learn how to use them!

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