Anyone tried planes with interchangable airfoils on demand for stunts and cruising?

[jas_Qfix]

Spilled from the miniGeeBee thread. I was wandering:...

What does it takes to convert these acrobatics 3D planes into fuel efficient low drag cargo movers? So we can get rid of AOA on demand? Has anyone try inflatable airfoil that gives us regular air foil during cruising speed and deflat the top membrane of the wing area for dog fight stunts? It should be relatively very easy to do.

If you want to stay away from Pneumatic controls for now, we can let the 4th cruise servo (5th ch) flips up a few vertical balsa bars on both side of the wings, to bend the top saran wrap or monokote to have a near perfect airfoil for proper lift generation. That should reduces or completely eliminates the AOA.

What do you all experts think?!


ps. That should be a near imitation of an octopus that change its skin colors, textures and shapes depend on its mood and external treats. Should we call this study Octo-plane?!

[Captain Ed]

Intriguing idea. One thought that comes to mind immediately would be a moveable leading edge. It could either extend forward or droop a few degrees. That would give you a new airfoil without much additional drag like you get from flaps.

[Sparky Paul]

It's possible to do this, but not too economically, or effieciently.
The old adage..." A jack of all trades is master of none" has to be kept in mind.
A "one-size-does-all" will be beat by special purpose designs which are intended to each of the "alls" the one size does.
With this in mind, it's worthwhile to play with this kind of thing, it only that it results in improvements to parts and ideas.

[Ollie]

Carrying cargo efficiently requires high lift and low drag. A low aspect ratio wing can't do this because of induced drag. Induced drag is reduced by increasing the aspect ratio but increased aspect ratio reduces roll rate. This is a compromise that has to be made in addition to any airfoil changes.

The simplest and most effective way to achieve the performance objectives you are looking for is to have a thin, low-mean-camber airfoil with inboard flaps and out board ailerons controlled by a computer radio that changes the camber of the airfoil all along the span in conjunction with changing the decalage to maintain pitch trim when going from one flight mode to another. With this set up you can also achieve very rapid deceleration by adding a crow mode where the ailerons go up as the flaps go down.

Problem solved except for the desirability of variable aspect ratio. BTW, modern jet transports achieve variable aspect ratio by multiple flaps that widen the effective chord and increase the effective wing area as the flaps are deployed. That's an awful complication for a model.

Take an F3B model glider, increase the crossection of the front of the fuselage to accomodate a neatly cowled engine, fuel tank and retractable landing gear and you will have everything you are looking for except an extremely high roll rate. Such a model already has provisions to carry a couple of pounds of ballast (potential pay load). I have seen one do multiple vertical rolls on the up-line even without an engine.

[jas_Qfix]

This is truely a collection of talented flight design experts in these forums. I think I follow the need for a variable aspect ratio wing and how the modern jet transports achieve variable aspect ratio by multiple flaps that widen the effective chord and increase the effective wing area.

Our flat wing based 3D planes with flat air foils need quite a lot of AOA to fly at cuirsing speed. That causes drag.

I was wandering that using inflatable airfoil on flat pancake shaped wing that gives us regular air foil during cruising speed and deflat the top membrane of the wing area for dog fight stunts. It should be relatively very easy to do. Not competing with the jets anytime soon, but easier for us model to implement these inflatable airfoil on flat pancake wings.

[Ollie]

High angle of attack before stall largely results from low aspect rqtio. The lower the aspect ratio the greater the induced angle of atack, no matter what airfoil is used. Induced drag and induced angle of attack arise from the proportion of the wing that is operating in the tip vortex where the airflow is three dimensional rather than two dimensional. Air flow on airfoils measured in most wind tunnels is two dimensional simulating infinite aspect ratio. When applying the wind tunnel data for the airfoil to a real wing, the induced drag and induced angle of attack must be accounted for.

Low aspect ratio wings operate largely in their own downwash. The down wash increases the geometric angle of attack necessary to generate high lift. Both down wash angle and induced drag increase in proportion to the square of the coefficient of lift.

See:
http://www.geocities.com/CapeCanaveral/Hangar/6875/aspectratio.html

[Sparky Paul]

Inflating, deflating means carrying a pumping system.
Also flex joints in the structure.
How much gain in performance is offset by the weight/cost of the pumping system?
What will the airframe life of the inflateable parts?
Bottom line stuff like this keeps planes simple/cheap to operate.

[gjs]

A micro hydrolic system would sure be fun. Wouldn't a few 1/32nd pistons Push a drooped LE of say a balloon type membrane into the airstream (powered by a servo) enough to affect some advantage. Or rotate a sub LE torque tube/ cam shaped into the slipstream, may create havoc or work.

[Sparky Paul]

There's a thread on another site discussing a "morphing" control system. Uses a flexible wing structure ala Blerior/Fokker E-III for lateral control.
Pumping air into and out of a bladder requires a storage vessel for the air and a pump, or some sort of pump to suck/blow it back and forth.

[Lancair]

Have a look for details on the FOX glider. The model Ive seen on Paul Natons "Just Wanna Fly" has a section in the underside of the wing that is flexed up into the wing to create undercamber for higher lift. The section is flexed via a torque tube arrangment as suggest above by GJS. Such design could surely be reversed to create the additional camber on top in your theory. You would probably need to have some sort of either very flexible yet ridig sheeting for the top surface and possibly an expansion joint as well. ARGHHHH its all too much for me.

[BMatthews]

The big problem with any sort of flexible membrane used for reshaping is that the pressure gradient changes as the airfoil moves through various angle of attacks and other load changes will ensure that it's impossible to control the shape of the airfoil.

If someone wanted to play with this I suspect splitting the airfoil into mulitple span wise elements that are hinged together and allow the overall camber line to change smoothly would be a better option. This would require some very complex model building practices with finely nested joints that are effectively sealed to prevent boundry layer tripping and air leakage.

Think of a NACA 0010 and about 6 chordwise pivot locations joining 7 ridgid small box elements that allow the camber to be altered and you can see what I mean.

BUT.... it's going to be heavy, complicated and complex to design. All in all I think it would be better to build more airplanes. Certainly the aerobatic potential would be zip thanks to the extra weight and the load carrying potential would be greatly hampered by the weight of the mechanism. I won't even go into the harshness of the aerodynamics that such a segmented airfoil would have when pulled out of it's natural shape.

Consider that flaps and drooped leading edges are only used for short times where the added drag that accompanies the added lift is actually a benefit. For any designs that are meant to operate at higher lifts for their normal function it's much more efficient to pick the airfoil that does the job directly.

[Ollie]

We already have things that behave like airfoils increasing thickness and changing camber. They are laminar seperation bubbles. They wax, wane and change position with changes in airspeed and angle of attack. They are almost never useful. Airfoil designers do their best to prevent or limit their formation and growth.

[Pook]

Hmm just an idea to add to the pot....

If you were to build a flexible membrane wing like used on some of the MAVs and added a torque rod to say the middle of the wing then twisted this so the trailing edge moved down i would have thought would be a relativly simple way to do this..

~Piers

hmm hope that made sence.. haha...