Understanding Glide Path Controls - flaps, spoilers, flapperons, spoilerons - RCCanada - Canada Radio Controlled Hobby Forum
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Old 09-16-2014, 03:31 PM   #1
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Understanding Glide Path Controls - flaps, spoilers, flapperons, spoilerons

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by Ed Anderson

Both power planes and gliders can incorporate glide path controls to add to the options a pilot has when flying his plane. We have all seen commercial airliners dropping flaps during landings. Likewise gliders, which are flown without the use of a motor, also benefit from glide path controls though they may be used in slightly different ways on a glider in order to gain the best control during flight and during landing.

While a power plane has the option to “go around” if the approach is not good, when a glider comes in to land, it is going to land. Without a motor there is no going around. And gliders are often flown in thermals where the lift can become so strong we may feel we need help to get them out of the lift. That is why we may choose to incorporate glide path controls into our aircraft.

For the rest of the discussion I will focus on the controls and their use in the context of gliders as this takes the effect and option of raising or lowering power out of the discussion and focuses on the controls themselves.

Even on gliders, these glide path controls are not absolutely necessary as many glider pilots fly without the use of glide path controls, but they can be helpful. Hopefully this will clarify the terms and the use of these controls.

First, lets make sure we are using the same terms.

Spoilers and Spoilerons

Spoilers sit on top of the wing, typically near the spar. This is how the Mystique RES is designed, for example.

Spoilers will reduce lift while inducing drag, but their primary value is not as brakes but as lift reducers. This raises the stall speed, the speed at which the wing can no longer produce enough lift to support the aircraft. So the wing will stall at a higher speed. Some rise like barn doors. This is what we typically call spoilers. Some come straight up like a blade. These we often call air brakes.

When spoilers are deployed typically the nose will drop, due to reduced lift, and we compensate with some up elevator to keep the glider from going into a dive. The best effect is that the glider remains fairly flat but the sink rate is raised so it comes down more quickly but without gaining a lot of speed.

Spoilerons are trailing edge devices that go up. This might be based on ailerons, full span ailerons, flaps or flaps and ailerons in combination. Spoilerons will reduce camber, reducing lift. Think of this as raising the trailing edge like an aileron but on both sides at once.

The wind hits the raised trailing edge and induces a pressure down at the trailing edge of the wing. Whether you think of it as induced down force or reduction of lift the effect is the same. When done on both wings at the same time this often induces a rise in angle of attack, it picks up the nose since the down force is behind the CG. This may require some down elevator compensation to keep the glider from stalling.

Spoilerons induce drag as you raise them but their primary purpose is not as brakes. This also raises stall speed meaning the wing will stall at a higher speed. So spoilers and spoilerons are different but their overall purpose for use is he same. The elevator compensation needed may be opposite.

Flaps and Flapperons

Flaps go down and increase the wing’s camber, makes the wing bottom more concave. This causes more drag and more lift while reducing stall speed allowing the plane to fly more slowly without stalling. That is why they help fast planes land more slowly.

What we are looking for is not the breaking effect of the drag as much as it is the lowering of the stall speed which allows the plane to land more slowly. When flaps are deployed the increased lift will typically bring the nose up so we compensate with down elevator to avoid stalling.

Flaps might be set to as much as 90 degree down angle. However 60 degrees is typically all you need for enhanced, slower landing speed.

If you have a glider with ailerons you can use them as flapperons. This works best when the ailerons are full span, like many discus launched gliders. Here you get full trailing edge control with only one control surface on each wing. You get thermal camber (trailing edge slightly down) and reflex (trailing edge slightly up) and you get flap behavior and spoileron (not spoiler) behavior if you can get enough upward motion. This works best on a straight wing where any dihedral is at the root rather than a curved wing or a polyhedral wing where dihedral is introduce gradually over the length of the wing.


All of these controls induce increased drag at sufficiently high angles of deflection, so they can be used as brakes to either slow the aircraft down or to prevent it from gaining speed. For example, imagine you are flying in a thermal duration contest. You are in a thermal, are quite high and want to get down fast so you can land at the exact time that will get you the best score. You can extend your flaps or spoilers and push the nose over to go into a dive to lose altitude quickly.

Both will add drag which will reduce the amount of speed you will gain in the dive. In my experience with RC gliders this works better with flaps as flaps are usually larger than spoilers but it works for spoilers too.

Likewise you can use these controls as you come near to whatever you intended landing spot may be to dump energy and cut the glide short. Going to 90 degree flaps or 90 degree spoilers will induce drag to dump energy, but again, because flaps tend to be larger they do a better job of this.

Spoilers do add the ability to “pin” the plane to the ground on landing. Because they reduce lift, if you deploy spoilers on landing the wing will have reduce lift and therefore the plane will have less of a tendency to rise off the ground or hop as you land near the stall speed of the airplane or glider, especially in gusty landing conditions. The next time you ride that big airliner watch the wings on landing. Once they touch, many will raise spoilers or airbrakes to help slow the plane and reduce the lift of the wing so the plane has less chance of going back into the air.


In my opinion, flaps are more versatile than spoilers as you can use them to lower stall speed or increase it. However flaps are also more dangerous.

If you lower flaps, reducing the stall speed you can slow the plane down and keep flying below the normal stall speed. If you pull the flaps in because you realize you are not going to make the field you MAY stall because as you pull the flaps in the stall speed goes back up. If you are flying too slowly the plane will drop like a rock.

Spoilers on the other hand are more forgiving. If you apply spoilers too soon and realize you want to extend the glide you can retract the spoilers which lowers the stall speed. You will not stall and you will not drop out of the sky. The glide is extended. Personally I prefer spoilers for new pilots for this reason. Gliders with R/E and spoilers are called RES gliders.

Each approach has advantages. Flaps more versatile but more prone to error. Spoilers are less versatile but more forgiving of errors.

If I was modifying the trailing edge, I would make them flaps rather than spoilerons. However if I was a new glider pilot and wanted to avoid those stalling errors, I would install spoilers, not spoilerons.

I learned to fly gliders on a RES glider, a Great Planes Spirit. I fly RE, RES, REA and full house REAF gliders and I enjoy them all. But you fly them all a little differently. The more controls, the more versatile and the greater propensity for mistakes.

Of course, your smileage will vary.
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Old 09-16-2014, 07:20 PM   #2
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Re: Understanding Glide Path Controls - flaps, spoilers, flapperons, spoilerons

Thank you. Very useful information.
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Old 09-17-2014, 07:00 AM   #3
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Re: Understanding Glide Path Controls - flaps, spoilers, flapperons, spoilerons

Thank Ed, a good read.

I have installed spoilerons/flaperons on my Radian this year. They have enought throw to use them as spoilerons and flaperons. I found myself preferring to use spoilerons to get back from altitude (e.g. from 300m to 100m or so) using spoilerons because it is easier to control yaw, thus speed, from far high. I've usually done my landings with flaperons.

One might have a different experience with another plane, I guess these things don't behave exactly the same on all models.
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Old 09-17-2014, 09:57 AM   #4
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Re: Understanding Glide Path Controls - flaps, spoilers, flapperons, spoilerons

I like this one. Boils it down to practical applications. I saw a nice video one day showing the differences when trying to spot land on a dune with a very stiff breeze. Makes the glider behave very different depending on what you deploy.
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Old 09-17-2014, 11:09 AM   #5
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Re: Understanding Glide Path Controls - flaps, spoilers, flapperons, spoilerons

Because spoilers tend to be more forgiving for new pilot mistakes I often take new pilots along the R/E and RES path if they are at all tentative. With flaps it is so easy to make a bone head mistake and drop the glider out of the sky. That typically does not happen with spoilers.

And, naturally, since I started on a RES glider spoilers are a natural thing to me.

But, as in all things, your smileage will vary.
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Old 09-17-2014, 11:35 AM   #6
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Re: Understanding Glide Path Controls - flaps, spoilers, flapperons, spoilerons

Great explanes Ed.
Had been sloping and thermaling a couple of RE's for the last few years and just got into a full house MPX cularis this year. Just a blast.....Your explanes are right on with my experiences with the MPX. Good read. Thanks.
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