Doc J
Very Strong User
Part 2: Aileron Sizing Criteria
Aileron sizing criteria are generally of two types: The first, and simplest, is to specify a required steady-state roll rate at specific airspeeds. i.e. For our aerobatic model design the roll rate should be as high as possible, for say an F3f model it should be lively but not ‘twitchy, while maybe for a big GPS model we would need a strong but steady response so as not to cause too much altitude-eating drag.
As we saw above, the rolling moment available from the ailerons is less at low airspeeds while the roll inertia is essentially constant. Accordingly, the aeroplane’s roll response will be slower at lower airspeeds. This is why for we modelers the most taxing roll-performance requirements are for bank-to-bank maneuvers at or near minimum speed such as during the landings.
Design Process: Ailerons
Some full-sized unlimited aerobatic aeroplanes are capable of rolling up to 400° per second, which is over 25 times the roll rate of a minimally compliant Part 23 aeroplane.
Accordingly, let’s take a closer look at options for the design of the ailerons themselves.
Size Matters
The first aileron design decision is how big to make them. The more rapid the roll response needed, the larger and more effective the ailerons need to be.
The requirements vary widely depending on the mission of the full-sized aeroplane. For example, a cruising aeroplane meeting the minimum requirements of FAR Part 23 is required to do a bank-to-bank reversal from 30° one way to 30° the other in 4 seconds at landing conditions. This 60° roll only requires an average roll rate of 15° per second. Clearly, the aileron size and design for the two types will be significantly different.
There are a wide variety of types of ailerons. Their design is a balance between roll control power, control forces, mechanical complexity and aerodynamic drag.
Hinge Moment (The amount of force that can be used to deflect the ailerons)
One of the primary concerns in the design of the ailerons used in full-sized aviation is the amount of force or moment required to deflect them. The larger the aileron and faster the aeroplane flies, the more force is needed to move the aileron and the same is true in model aviation.
This became a major concern in the era before the advent of power controls and hydraulically actuated flight control systems as the size and performance of aeroplanes increased over time. By WW-II, aeroplanes still had mechanical control systems actuated only by the flyer’s muscles, but fighters were exceeding 400 mph and bombers (the B-17 and B-29, for example) had grown to be quite large so in many cases control at high speed was an increasing concern.
WWII Fighters needed the ability to roll quickly in combat at high speed, so it was vital to keep the aileron hinge moments down to a level where the flyer’s strength could still command a large aileron deflection. Bomber flyers needed to be able to control their much larger aeroplanes and fly formation, again on muscle power alone.
Once the size of the ailerons and the speed of the aeroplane are set, the geometry of the ailerons and their hinges control hinge moment. For our purposes this means the mechanical control system moments. A long control horn for example needs less servo force to move it – but on the other side we really don’t like great long horns and messy control rods sticking out of our Baby’s nice shiny wings, do we?
The chord of the aileron is a primary factor. I’ll repeat that: The chord of the aileron is a primary factor
For a given area, our model ailerons can have different aspect ratios. The longer the span, or the narrower the wing chord, the shorter the aileron chord. For a simple “plain-flap” surface of constant area, the hinge moment required to deflect it is a linear function of the chord. The longer the chord, the higher the moment. Accordingly, a short-span, long-chord aileron will require more force to actuate than a long-span short-chord aileron. For our purposes this is simple mechanical advantage, and very happily today can be taken care of by the various IDS systems available which are more effective than the old servo/control horn setups and allow far more efficient use of a servo’s power.
Lower aspect ratio wings (shorter/fatter) can have wider chord shorter span ailerons than higher aspect ratio wings. For slope glider model use, a good rule of thumb for aileron size is about 20 to 25% of the wing chord over 60 to 70% of the wing span.
Aerodynamic Drag and Gap Flow
In addition to providing adequate roll control power and keeping hinge moments under control, the designer must address the aerodynamic effects of the aileron installation when the ailerons are in the neutral position in cruise. In perfect world the ailerons should disappear aerodynamically when in neutral position i.e. influence on the wing in terms of control and drag is zero.
The two primary sources of aerodynamic effect on the wing from un-deflected ailerons are the flow around and through the gaps at the hinge and ends of the aileron, and the effect of exposed hinges and linkages on drag.
Flow through gaps can increase drag, decrease aileron effectiveness and reduce the lift of the wing. Consequently, the ailerons should have very small, well-sealed gaps and no exposed hinges, control arms or linkages. With IDS it’s now possible to get very close, and some fast GPS and F3f type aeroplanes have aileron systems that approach the ideal in this respect.
Wing tips - this is more imprtant than you might think:
Many F3j type model aeroplane designs go to great length trying to design ailerons that get as close as possible to the wing tips, thinking I suppose that the closer the ailerons are, the better they will work. The problem is that it doesn’t always work that way. As any designer who has spent any time in wind tunnels will tell you, the wing tips re often the biggest source of trouble on any wing design - but especially on gliders. Let the wingtips – whatever the design is – do their job as well as they can, and try to keep the deflected aileron wake and vortexes away from them.
Takeaways:
Doc.
Aileron sizing criteria are generally of two types: The first, and simplest, is to specify a required steady-state roll rate at specific airspeeds. i.e. For our aerobatic model design the roll rate should be as high as possible, for say an F3f model it should be lively but not ‘twitchy, while maybe for a big GPS model we would need a strong but steady response so as not to cause too much altitude-eating drag.
As we saw above, the rolling moment available from the ailerons is less at low airspeeds while the roll inertia is essentially constant. Accordingly, the aeroplane’s roll response will be slower at lower airspeeds. This is why for we modelers the most taxing roll-performance requirements are for bank-to-bank maneuvers at or near minimum speed such as during the landings.
Design Process: Ailerons
Some full-sized unlimited aerobatic aeroplanes are capable of rolling up to 400° per second, which is over 25 times the roll rate of a minimally compliant Part 23 aeroplane.
Accordingly, let’s take a closer look at options for the design of the ailerons themselves.
Size Matters
The first aileron design decision is how big to make them. The more rapid the roll response needed, the larger and more effective the ailerons need to be.
The requirements vary widely depending on the mission of the full-sized aeroplane. For example, a cruising aeroplane meeting the minimum requirements of FAR Part 23 is required to do a bank-to-bank reversal from 30° one way to 30° the other in 4 seconds at landing conditions. This 60° roll only requires an average roll rate of 15° per second. Clearly, the aileron size and design for the two types will be significantly different.
There are a wide variety of types of ailerons. Their design is a balance between roll control power, control forces, mechanical complexity and aerodynamic drag.
Hinge Moment (The amount of force that can be used to deflect the ailerons)
One of the primary concerns in the design of the ailerons used in full-sized aviation is the amount of force or moment required to deflect them. The larger the aileron and faster the aeroplane flies, the more force is needed to move the aileron and the same is true in model aviation.
This became a major concern in the era before the advent of power controls and hydraulically actuated flight control systems as the size and performance of aeroplanes increased over time. By WW-II, aeroplanes still had mechanical control systems actuated only by the flyer’s muscles, but fighters were exceeding 400 mph and bombers (the B-17 and B-29, for example) had grown to be quite large so in many cases control at high speed was an increasing concern.
WWII Fighters needed the ability to roll quickly in combat at high speed, so it was vital to keep the aileron hinge moments down to a level where the flyer’s strength could still command a large aileron deflection. Bomber flyers needed to be able to control their much larger aeroplanes and fly formation, again on muscle power alone.
Once the size of the ailerons and the speed of the aeroplane are set, the geometry of the ailerons and their hinges control hinge moment. For our purposes this means the mechanical control system moments. A long control horn for example needs less servo force to move it – but on the other side we really don’t like great long horns and messy control rods sticking out of our Baby’s nice shiny wings, do we?
The chord of the aileron is a primary factor. I’ll repeat that: The chord of the aileron is a primary factor
For a given area, our model ailerons can have different aspect ratios. The longer the span, or the narrower the wing chord, the shorter the aileron chord. For a simple “plain-flap” surface of constant area, the hinge moment required to deflect it is a linear function of the chord. The longer the chord, the higher the moment. Accordingly, a short-span, long-chord aileron will require more force to actuate than a long-span short-chord aileron. For our purposes this is simple mechanical advantage, and very happily today can be taken care of by the various IDS systems available which are more effective than the old servo/control horn setups and allow far more efficient use of a servo’s power.
Lower aspect ratio wings (shorter/fatter) can have wider chord shorter span ailerons than higher aspect ratio wings. For slope glider model use, a good rule of thumb for aileron size is about 20 to 25% of the wing chord over 60 to 70% of the wing span.
Aerodynamic Drag and Gap Flow
In addition to providing adequate roll control power and keeping hinge moments under control, the designer must address the aerodynamic effects of the aileron installation when the ailerons are in the neutral position in cruise. In perfect world the ailerons should disappear aerodynamically when in neutral position i.e. influence on the wing in terms of control and drag is zero.
The two primary sources of aerodynamic effect on the wing from un-deflected ailerons are the flow around and through the gaps at the hinge and ends of the aileron, and the effect of exposed hinges and linkages on drag.
Flow through gaps can increase drag, decrease aileron effectiveness and reduce the lift of the wing. Consequently, the ailerons should have very small, well-sealed gaps and no exposed hinges, control arms or linkages. With IDS it’s now possible to get very close, and some fast GPS and F3f type aeroplanes have aileron systems that approach the ideal in this respect.
Wing tips - this is more imprtant than you might think:
Many F3j type model aeroplane designs go to great length trying to design ailerons that get as close as possible to the wing tips, thinking I suppose that the closer the ailerons are, the better they will work. The problem is that it doesn’t always work that way. As any designer who has spent any time in wind tunnels will tell you, the wing tips re often the biggest source of trouble on any wing design - but especially on gliders. Let the wingtips – whatever the design is – do their job as well as they can, and try to keep the deflected aileron wake and vortexes away from them.
Takeaways:
- Wider chord, shorter span ailerons work better aerodynamically than shorter chord longer span ailerons.
- Keep your ailerons away from the wingtips for at least 5% of the half span and you will see most of those tip stall/flick roll problems fly away.
- Most of the mechanical advantage problems can be solved by using really good IDS systems.
- Gap seals are more important than they appear to be – design them well.
Doc.
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