Been noodling on something for a long while.
High cruise speed, high aspect ratio, low wing loading, and turbulence are all sort of mutually exclusive. With a high cruise speed and a low wing loading occupant comfort in turbulence is poor. Obvious. High aspect ratio wings absorb some gusts but amplify others due to adding a sine wave function to a large rate change in altitude curve making peak gusts even larger. Aspect ratio is efficient at any speed but really more pronounced at lower speeds. So how to have a high cruise speed in a very low drag, high aspect ratio design and have occupant comfort in gusts so you don't have to slow down for turbulence. And yes, this is about a motorglider.
Thinking about this I started looking at lift distribution on a different wings and realized that the conspiring variable is CL vs. Alpha. Using Schrenk or any other method you can compute a lift distribution. For a high aspect ratio airframe light airframe you get a normal distribution that is not very high but long laterally. Shorten the span for the same takeoff weight and you get a taller narrower version of the same basic shape distribution. Recent discussions and articles of smaller span versions of racing sailplanes being competitive against their longer spanned brothers and also reading a bunch about the potential gusts that affected the Indonesian airliner I started visualizing being able to control the lift distribution of the wing en route. If CL vs. Alpha is the enemy in turbulence then being able to manipulate it would help in absorbing turbulence at higher cruise speeds.
Other discussions have always headed in the direction of complexity like variable span and area wings, or removable wing tip extensions like you install to go from standard class to open class in a glider to increase performance at lower speeds. Stowing wing tip extensions en route is an issue. Complexity of variable span seems not worth pursuing. So that has me thinking of how to get to the goal without the complexity or logistics issues. RC gliders have had this landing mode called "crow" for quite a while, like 30 years at least. This mode is ailerons up and flaps down both drastically but also both on the flap lever so modulatable just like a horizontal version of a split rudder drag brake. This doesn't interrupt normal control. It is mixed through it. This method is too drastic for a high speed cruise and is really designed to kill lift while increasing drag but as a starting point it got me thinking.
How to get almost no CL vs. Alpha reaction out of a portion of the span while keeping the other portion of the span in cruise trim while all the while keeping the whole span in the drag bucket. It seems there are two possibilities:
First, just plain mechanical mixed flaps and ailerons, possibly with multiple panels each but with a wing design such that when the ailerons are retracted upward the airfoil is a very low or minimal CL vs. Alpha airfoil for the outer part of the span, say 25 or even up to 50% of the span. Deploying such a flap would change the loading of the outer panels to near nil while transferring the load inboard thereby making the long wing look like a smaller, high aspect ratio span. This could be limited to be only deployed above say 125 knots so that no one would try to land or emergency maneuver in that mode. The question is is there a selection of airfoils and flap positions that could do all that and still remained trimmed into the drag bucket comfortably. The unloaded tips would create a sort of paddle damper for the inner part of the wing and would also ward off tip stall.
Second is active damping with fast acting servos only associated with pitch damping. There are existing mechanical solutions that allow passing through of mechanical controls while superimposing a servo driven control augmentation for reasons like autopilot integration. So assuming we are talking a relatively small control input at high speed cruise it would always be able to be overruled by the amount of mechanical control throws in case of failure of the system. Combine this with the autopilot system and you have a live inertial damping system. I am not suggesting FBW. I am suggesting a system that allows the airframe to safely cruise past where the actual measured wing loading would allow with pilot comfort. Now if the system failed in this mode you could get a pretty big bump on the head or bruises but probably not lethal or incapacitating as long as the airframe can take the hit structurally. So not a bad compromise. I believe that elevator and flaps should be mixed together in a system like this to provide complete pitch and gust damping at once.
Right now I am looking at this conceptual thought for show stoppers and things I may be looking at wrong. But it seems like a very possible alternative to variable span solutions. I would tentatively name this an active variable lift distribution system. And it seem a potential way to fake a wing into acting smaller than it is.
What have I missed? Is there no such thing as a low or no CL vs. Alpha foil that can be change to match the root flapped airfoil for low speed ops? Is there some mode of flaps that I have not thought of that creates a hazardous operating condition? Or any other sane criticisms....
High cruise speed, high aspect ratio, low wing loading, and turbulence are all sort of mutually exclusive. With a high cruise speed and a low wing loading occupant comfort in turbulence is poor. Obvious. High aspect ratio wings absorb some gusts but amplify others due to adding a sine wave function to a large rate change in altitude curve making peak gusts even larger. Aspect ratio is efficient at any speed but really more pronounced at lower speeds. So how to have a high cruise speed in a very low drag, high aspect ratio design and have occupant comfort in gusts so you don't have to slow down for turbulence. And yes, this is about a motorglider.
Thinking about this I started looking at lift distribution on a different wings and realized that the conspiring variable is CL vs. Alpha. Using Schrenk or any other method you can compute a lift distribution. For a high aspect ratio airframe light airframe you get a normal distribution that is not very high but long laterally. Shorten the span for the same takeoff weight and you get a taller narrower version of the same basic shape distribution. Recent discussions and articles of smaller span versions of racing sailplanes being competitive against their longer spanned brothers and also reading a bunch about the potential gusts that affected the Indonesian airliner I started visualizing being able to control the lift distribution of the wing en route. If CL vs. Alpha is the enemy in turbulence then being able to manipulate it would help in absorbing turbulence at higher cruise speeds.
Other discussions have always headed in the direction of complexity like variable span and area wings, or removable wing tip extensions like you install to go from standard class to open class in a glider to increase performance at lower speeds. Stowing wing tip extensions en route is an issue. Complexity of variable span seems not worth pursuing. So that has me thinking of how to get to the goal without the complexity or logistics issues. RC gliders have had this landing mode called "crow" for quite a while, like 30 years at least. This mode is ailerons up and flaps down both drastically but also both on the flap lever so modulatable just like a horizontal version of a split rudder drag brake. This doesn't interrupt normal control. It is mixed through it. This method is too drastic for a high speed cruise and is really designed to kill lift while increasing drag but as a starting point it got me thinking.
How to get almost no CL vs. Alpha reaction out of a portion of the span while keeping the other portion of the span in cruise trim while all the while keeping the whole span in the drag bucket. It seems there are two possibilities:
First, just plain mechanical mixed flaps and ailerons, possibly with multiple panels each but with a wing design such that when the ailerons are retracted upward the airfoil is a very low or minimal CL vs. Alpha airfoil for the outer part of the span, say 25 or even up to 50% of the span. Deploying such a flap would change the loading of the outer panels to near nil while transferring the load inboard thereby making the long wing look like a smaller, high aspect ratio span. This could be limited to be only deployed above say 125 knots so that no one would try to land or emergency maneuver in that mode. The question is is there a selection of airfoils and flap positions that could do all that and still remained trimmed into the drag bucket comfortably. The unloaded tips would create a sort of paddle damper for the inner part of the wing and would also ward off tip stall.
Second is active damping with fast acting servos only associated with pitch damping. There are existing mechanical solutions that allow passing through of mechanical controls while superimposing a servo driven control augmentation for reasons like autopilot integration. So assuming we are talking a relatively small control input at high speed cruise it would always be able to be overruled by the amount of mechanical control throws in case of failure of the system. Combine this with the autopilot system and you have a live inertial damping system. I am not suggesting FBW. I am suggesting a system that allows the airframe to safely cruise past where the actual measured wing loading would allow with pilot comfort. Now if the system failed in this mode you could get a pretty big bump on the head or bruises but probably not lethal or incapacitating as long as the airframe can take the hit structurally. So not a bad compromise. I believe that elevator and flaps should be mixed together in a system like this to provide complete pitch and gust damping at once.
Right now I am looking at this conceptual thought for show stoppers and things I may be looking at wrong. But it seems like a very possible alternative to variable span solutions. I would tentatively name this an active variable lift distribution system. And it seem a potential way to fake a wing into acting smaller than it is.
What have I missed? Is there no such thing as a low or no CL vs. Alpha foil that can be change to match the root flapped airfoil for low speed ops? Is there some mode of flaps that I have not thought of that creates a hazardous operating condition? Or any other sane criticisms....
