Doc James Hammond
Very Strong User
Hi potential Aerofoil Criminals and possibly Criminalettes!
I can tell you right now that It's a really good thing this thread is not on RC Gropes...otherwise the Armchair Aerodynamicists and the Aerofoil Troll police would be onto us!
Airfoils - or as we Blimy Brits call them Aerofoils.
Konrad having gone so deep into this on the Gremlin thread, - as he always seems to - as its a vast and sometimes pretty interesting subject, I feel the need to explain a bit about these aerofoils here in a new thread.
A few years ago I was given the brief by the people I often work for - let's call them "The Entity" - to try to develop a completely new series of aerofoils with "well understood characteristics optimised for the moving control surfaces". Talking further with the inventors of the brief, I began to understand that what they meant was a set of unique aerofoils actually designed for control surfaces with as little as possible drag from deflection while giving the largest force while deployed.
In other words - not much deflection angle = good pressure - therefore lower drag.
Looking at a lot of aerofoils - I have quite a library - led me to think about how they were actually generated and why. Anyway it turns out that a lot of the aerofoils, maybe most but, with some notable exceptions (Dr Quabeck's HQW series for example) were designed the wrong way round.
Instead of being designed to a specific job under a specific set of conditions, they started life as one of a large series of re-iterations of one basic design. From this long series of shapes, a few good ones will emerge. In the model world, one German gentlemen is quite famous for this - which is why he has so many sections to choose from. If you make enough, thousands - some will be OK, maybe even pretty good.
I wanted to do something a bit different (because I had to) so I wanted the normal wants from a new design: Lowest drag/highest lift.
BUT (BIG BUT) I ALSO wanted the control surface to be situated in exactly the right place, and with the right profile to give the best response. So not only would the basic aerofoil do the best job it could do at the lowest drag, it would also give the best control surface response for the angle of deflection. Basically angle Vs pressure.
So by that time I had a lot of test foils from previous projects and I began to test them for control deflection responses - control surface proportion/position and pressure. In some cases, I can tell you it was surprising that people would actually consider using them since, while the accumulated (undeflected) drag was not bad, the defection pressures were so abysmal. Realising this, I'm sure thats why Smart cookie Dr Quabeck made his HQW series (Helmut Quabeck Wolbklappen)
Me being me I concluded quite quickly that making an all-singing, all-dancing aerofoil at super low drag from someone else's coordinates and then expecting it to work OK with a control surface would be a fruitless and frustrating quest. Yes I could bugger about with a lot of aerofoils and control surface volumes and angles and measurements etc etc, but I would be lucky if I actually found ANY aerofoil that would do what I wanted to do before losing what remains of my hair.
The solution, I decided, was to try make the good old isobars do what I wanted them to do instead of trying to compensate for what they wanted to do.
Undercambered wings and the effects of the cusp are well understood, but I could find little information one what would happen if you used the cusp on the top surface -though there are aerofoils that have this feature.
I had the feeling you see, that if I could persuade the airflow boundary layer to depart a bit later, then the pressure at the back of the airfoil - which would normally be starting to rise as the boundary layer departed, would maintain just a bit lower pressure, which would not only help with the lift, it would also help with control response. Maybe I could call it cusp induced pressure differential...hehe..."Delta Cusp" for short.
There it is in a nutshell.
Because the top surface boundary layer departure is just a bit later due the isobars being forced down a bit by the cusp, then the lift is increased, but not only that - the control surface is far more effective for any given angle of deflection. On the bottom surface it works in the same way. The "curl" or Delta Cusp increases the pressure available to the control surface.
Its actually quite simple but took a long time to get right!
Doc.
I can tell you right now that It's a really good thing this thread is not on RC Gropes...otherwise the Armchair Aerodynamicists and the Aerofoil Troll police would be onto us!
Airfoils - or as we Blimy Brits call them Aerofoils.
Konrad having gone so deep into this on the Gremlin thread, - as he always seems to - as its a vast and sometimes pretty interesting subject, I feel the need to explain a bit about these aerofoils here in a new thread.
A few years ago I was given the brief by the people I often work for - let's call them "The Entity" - to try to develop a completely new series of aerofoils with "well understood characteristics optimised for the moving control surfaces". Talking further with the inventors of the brief, I began to understand that what they meant was a set of unique aerofoils actually designed for control surfaces with as little as possible drag from deflection while giving the largest force while deployed.
In other words - not much deflection angle = good pressure - therefore lower drag.
Looking at a lot of aerofoils - I have quite a library - led me to think about how they were actually generated and why. Anyway it turns out that a lot of the aerofoils, maybe most but, with some notable exceptions (Dr Quabeck's HQW series for example) were designed the wrong way round.
Instead of being designed to a specific job under a specific set of conditions, they started life as one of a large series of re-iterations of one basic design. From this long series of shapes, a few good ones will emerge. In the model world, one German gentlemen is quite famous for this - which is why he has so many sections to choose from. If you make enough, thousands - some will be OK, maybe even pretty good.
I wanted to do something a bit different (because I had to) so I wanted the normal wants from a new design: Lowest drag/highest lift.
BUT (BIG BUT) I ALSO wanted the control surface to be situated in exactly the right place, and with the right profile to give the best response. So not only would the basic aerofoil do the best job it could do at the lowest drag, it would also give the best control surface response for the angle of deflection. Basically angle Vs pressure.
So by that time I had a lot of test foils from previous projects and I began to test them for control deflection responses - control surface proportion/position and pressure. In some cases, I can tell you it was surprising that people would actually consider using them since, while the accumulated (undeflected) drag was not bad, the defection pressures were so abysmal. Realising this, I'm sure thats why Smart cookie Dr Quabeck made his HQW series (Helmut Quabeck Wolbklappen)
Me being me I concluded quite quickly that making an all-singing, all-dancing aerofoil at super low drag from someone else's coordinates and then expecting it to work OK with a control surface would be a fruitless and frustrating quest. Yes I could bugger about with a lot of aerofoils and control surface volumes and angles and measurements etc etc, but I would be lucky if I actually found ANY aerofoil that would do what I wanted to do before losing what remains of my hair.
The solution, I decided, was to try make the good old isobars do what I wanted them to do instead of trying to compensate for what they wanted to do.
Undercambered wings and the effects of the cusp are well understood, but I could find little information one what would happen if you used the cusp on the top surface -though there are aerofoils that have this feature.
I had the feeling you see, that if I could persuade the airflow boundary layer to depart a bit later, then the pressure at the back of the airfoil - which would normally be starting to rise as the boundary layer departed, would maintain just a bit lower pressure, which would not only help with the lift, it would also help with control response. Maybe I could call it cusp induced pressure differential...hehe..."Delta Cusp" for short.
There it is in a nutshell.
Because the top surface boundary layer departure is just a bit later due the isobars being forced down a bit by the cusp, then the lift is increased, but not only that - the control surface is far more effective for any given angle of deflection. On the bottom surface it works in the same way. The "curl" or Delta Cusp increases the pressure available to the control surface.
Its actually quite simple but took a long time to get right!
Doc.
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