Doc J
Very Strong User
Hi Guys, with increasing cost of ARTF models, more and more of us (Yes even me!) are designing ‘Home Built’ models, so good design information could be really useful. If you get your baby wrong in the beginning, then you might think that you are not a good designer; but armed with just a few important points to consider when you have your design pen out, can take quite a bit of the guesswork out of it and help to make a successful design. So here I have taken some good stuff on Aileron design from publications on full sized aircraft and adapted it using my experience for model aileron design.
Part 1. Design Process: Roll Control Basics
The wing with the aileron deflected up will have less lift than the opposite wing with the aileron deflected down. This asymmetry in lift causes a rolling moment in the direction of the raised aileron.
While the primary function of the wing is to generate lift, it must also incorporate control surfaces to control the aeroplane in roll. On most aeroplanes these surfaces are the ailerons.
First let’s look at the overall roll-control requirements our aeroplane will have. The flyer needs to be able to control the aeroplane about the roll axis. The roll controls serve to trim the aeroplane in roll and to generate and arrest roll rates to maneuver the aeroplane in all conditions.
Roll Acceleration
Initially, the rolling moment causes an angular acceleration around the roll axis, and the roll rate increases. The angular acceleration the flyer can command is a function of the roll-control power of the ailerons and the roll inertia of the aeroplane.
The roll inertia of the aeroplane is a function of its configuration. The longer and heavier the wings and their contents are, the greater the roll inertia and the smaller the roll acceleration will be for a given rolling moment.
The roll control power of the ailerons is a function of both the configuration of the aeroplane and of airspeed. The shape, size, deflection and spanwise position of the ailerons all affect the amount of rolling moment the ailerons can generate.
For a given configuration, the rolling moment the ailerons generate is proportional to airspeed squared. This means that the faster the aeroplane is flying, the more rapid roll acceleration the ailerons can command. This seems quite simple: ‘well I designed a fast ‘plane…so I must need
smaller ailerons for less drag, right?’ – which works while the model is at high speed, but how about landings?
Roll and Roll Damping
As the aeroplane rolls, the angle of attack across the wing’s span becomes asymmetric. The angle of attack of the down-going wing increases, and the angle of attack of the up-going wing decreases. This increases the lift of the down-going wing and decreases the lift of the up-going wing. The lift asymmetry thus generated causes a rolling moment that opposes the roll.
This phenomenon, called “roll damping,” is a major factor determining the steady-state roll rate the aeroplane can achieve.
As the aeroplane roll rate accelerates, driven by the aileron-induced rolling moment, the opposing moment due to roll damping increases, reducing roll acceleration. At some point, the moment generated by the ailerons is equal and opposite to the moment generated by roll damping. At this point, roll acceleration will be zero, and the aeroplane will be at its maximum steady-state roll rate.
To stop a roll, the flyer can simply center the stick and allow roll damping to stop the roll. Roll inertia will tend to keep the aeroplane rolling, so a bit of opposing aileron can be used for a moment to stop the roll quicker. This is why a typical roll command sequence for non-neutrally stable model would be something like: dab of up elevator to tip the nose up slightly – then right or left roll command to roll the model – then a dab of opposite aileron to stop the roll. Why the dab of up elevator? To position the aircraft so that it already has an angle of attack so as to lessen the roll damping effect.
Banking – how does it work?
In normal turning maneuvers, the flyer tries to roll the aeroplane to a specific bank angle and then capture that bank angle and hold it steady during the turn. The control inputs to do this are not constant.
The flyer initiates the maneuver by moving the stick laterally, which deflects the ailerons and generates an initial roll acceleration. As the roll rate builds up, the flyer may modulate the stick deflection to capture a desired roll rate.
As the aeroplane approaches the desired bank angle, the flyer must neutralize the stick or use a bit of opposite stick to stop the roll rate and capture the bank angle. Once the aeroplane is in a steady-state turn at the desired bank angle, it may require a small aileron deflection either way to hold the bank angle constant. After that, the elevator can be used to turn the model in bank.
To stop the turn and regain steady-state level flight, the flyer must deflect the stick to the “high side” to command a roll out of the bank and then capture the wings-level attitude, ending up wings-level with the stick neutralized.
It’s important to understand that lateral motion of the stick does not directly command a bank angle. The initial stick deflection commands a roll-rate acceleration, and after the aeroplane is rolling, a constant stick deflection commands a constant roll rate, not a constant bank angle.
Watch this space for part 2!
Doc.
Part 1. Design Process: Roll Control Basics
The wing with the aileron deflected up will have less lift than the opposite wing with the aileron deflected down. This asymmetry in lift causes a rolling moment in the direction of the raised aileron.
While the primary function of the wing is to generate lift, it must also incorporate control surfaces to control the aeroplane in roll. On most aeroplanes these surfaces are the ailerons.
First let’s look at the overall roll-control requirements our aeroplane will have. The flyer needs to be able to control the aeroplane about the roll axis. The roll controls serve to trim the aeroplane in roll and to generate and arrest roll rates to maneuver the aeroplane in all conditions.
Roll Acceleration
Initially, the rolling moment causes an angular acceleration around the roll axis, and the roll rate increases. The angular acceleration the flyer can command is a function of the roll-control power of the ailerons and the roll inertia of the aeroplane.
The roll inertia of the aeroplane is a function of its configuration. The longer and heavier the wings and their contents are, the greater the roll inertia and the smaller the roll acceleration will be for a given rolling moment.
The roll control power of the ailerons is a function of both the configuration of the aeroplane and of airspeed. The shape, size, deflection and spanwise position of the ailerons all affect the amount of rolling moment the ailerons can generate.
For a given configuration, the rolling moment the ailerons generate is proportional to airspeed squared. This means that the faster the aeroplane is flying, the more rapid roll acceleration the ailerons can command. This seems quite simple: ‘well I designed a fast ‘plane…so I must need
smaller ailerons for less drag, right?’ – which works while the model is at high speed, but how about landings?
Roll and Roll Damping
As the aeroplane rolls, the angle of attack across the wing’s span becomes asymmetric. The angle of attack of the down-going wing increases, and the angle of attack of the up-going wing decreases. This increases the lift of the down-going wing and decreases the lift of the up-going wing. The lift asymmetry thus generated causes a rolling moment that opposes the roll.
This phenomenon, called “roll damping,” is a major factor determining the steady-state roll rate the aeroplane can achieve.
As the aeroplane roll rate accelerates, driven by the aileron-induced rolling moment, the opposing moment due to roll damping increases, reducing roll acceleration. At some point, the moment generated by the ailerons is equal and opposite to the moment generated by roll damping. At this point, roll acceleration will be zero, and the aeroplane will be at its maximum steady-state roll rate.
To stop a roll, the flyer can simply center the stick and allow roll damping to stop the roll. Roll inertia will tend to keep the aeroplane rolling, so a bit of opposing aileron can be used for a moment to stop the roll quicker. This is why a typical roll command sequence for non-neutrally stable model would be something like: dab of up elevator to tip the nose up slightly – then right or left roll command to roll the model – then a dab of opposite aileron to stop the roll. Why the dab of up elevator? To position the aircraft so that it already has an angle of attack so as to lessen the roll damping effect.
Banking – how does it work?
In normal turning maneuvers, the flyer tries to roll the aeroplane to a specific bank angle and then capture that bank angle and hold it steady during the turn. The control inputs to do this are not constant.
The flyer initiates the maneuver by moving the stick laterally, which deflects the ailerons and generates an initial roll acceleration. As the roll rate builds up, the flyer may modulate the stick deflection to capture a desired roll rate.
As the aeroplane approaches the desired bank angle, the flyer must neutralize the stick or use a bit of opposite stick to stop the roll rate and capture the bank angle. Once the aeroplane is in a steady-state turn at the desired bank angle, it may require a small aileron deflection either way to hold the bank angle constant. After that, the elevator can be used to turn the model in bank.
To stop the turn and regain steady-state level flight, the flyer must deflect the stick to the “high side” to command a roll out of the bank and then capture the wings-level attitude, ending up wings-level with the stick neutralized.
It’s important to understand that lateral motion of the stick does not directly command a bank angle. The initial stick deflection commands a roll-rate acceleration, and after the aeroplane is rolling, a constant stick deflection commands a constant roll rate, not a constant bank angle.
Watch this space for part 2!
Doc.
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