WHY DESIGNING (BUILDING AND FLYING) FOOT LAUNCHABLE SAILPLANE WITH SUCH POOR PERFORMANCE?
Well I am hang glider pilot since 1979 and even flying the sailplane for around 45 hrs solo, I am stil emotionally attached to foot launch comunity and hang gliders.
I think it will be fun to show up on a hill with such design and having fun simply soaring on ridge updraft or nearby thermals, beside hang gliders and paragliders, but having 3 axes aerodynamic controll over glider…
The main purpose of this glider would be to made some futuristic design with no similiraty with present foot launchable sailplanes as:ULF-1, Carbon Dragon, Super Floater, Swift and other different types of tailless rigid wings.
The design should be of simple construction adapted for the homebuilders. There is not an attempt to commercially use of this design.
To achieve safety, handling and performance goals I decided so:
They are as follows:
1. cockpit area with nose cone and two cockpit sidewalls
2. central part with two main bulkheads and wing attachment points
3. tail cone with rudder fin
-WHY ARE THE FLAPS DIVIDED IN 3 PIECES?
Because the wing will defenetly be bend up during flight, the invisible »hinge line« of flaps wil be distort from ideal straight line to some kind of arc. If flaps were hinged say 3 times per halfspan, it will be hard to rotate flaps (midle hinge not in line with outer ones), and flaps will be stressed much more, because it should be bend at unpleasant direction. At flap retracted it will bend at same manner as main wing, but at deployed flap it looks that trailing edge of the flap will be stressed in tension, and will probably break. This hinge missaligment would probably cause some extra friction in hinges thus rising the command forces to operate flap…Carbon Dragoon fight this with differential drive 1:4 (+4/-16 if I remember right)
If we take a piece of paper, and bend it at long edge (performing simplified shape of wing with deployed flap) we soon discover that if trying to bend the wing, the bended part- flap, will take almost all bending load. I dont want to flap take any necessary load in my design.
So, if I divide flap into 3 parts per halfspan and hinging each part only twice (with ball bearings) I avoid almost all unwilling stresses of the flap, regarding any wing bending (in aeroelastic range of operation). Well, the gaps between flaps panels will decrease their effectivines to some degree, but the gap can be filled with some kind of foamy material, to avoid air leaking..
-WHY ONLY OUTER PART OF FLAP ACT AS FLAPERON?
The simpliest way (for calculations and aso for manufacturing reasons) is to use so called flaperons on wing for both function (high lift device + controll over bank angle) as is done at Carbon Dragon.
But I prefferd to divide them into 3 parts and to use inboard two sections to act as flaps and outer one as flaperon. Why?
If we imagine ourself just prepared on the hill for launching, we want to have all benefits of our high lift device. So, any change of flap(eron) setting toward up, wil cause less lift. But we want all lift we can get from the wing at such low speeds during starting phase. If we must rise or lower the tip of wing (if sudden gust on start put the wing out of level attitude) we must produce a roll moment over centerline of sailplane. This is simply done by changing the lift force on wings. But the same moment we get if we produce big force somwhere at mid span, or much smaller force at the verry outboard part of wing.
I decided to use smaller force at the tip part of the wing, so there are placed ailerons. They are flaperons, because mechanically they follow the general motion of two inboart parts of flaps.
Because wing is already at high angle of attack (stil talking about take of phase) and almost the highest CL, the ailerons in this case should move up only. In flap retracted case, (talking about normal flying) ailerons attain full up and down motion - diferential of course to fight adwerse yaw.
The mechanical device is not yet taken in concern because I think it is possible to design such mechanism and it is now out of scope of this discusion and at present design stage. But will be soon actual…
With this arangement my wing will loose minimal of lift force during aileron operation.
REM:I am afraid that long chord flaperons over full span when deployed and then additionnaly deployed down (and on other wing up) for roll motion, will produce big controll forces and troubles balancing and overcoming them…
-ABOUT RUDDER CONTROLL
During my past 22 years of hang gliding I saw a lot of bad startings and many of them was when some gust push one wing up, and then glider tend to make circle and hit back to the hill. If pilot act too slow there is a danger, and also if pilot sudden »pull up« the high wing gets into stall and we are again back rotating toward the hill. Here I believe that prompt and exact respond of the pilot is crusial, and also the respond of sailplane to pilots controlls…
So, I think that all controlls should be »under controll« during take off to allow the pilot to instantly correct any deviation from planned motion of the craft. Well, sailplane is maybe of enough stability around all axes, but we can not controll nature of the wind over the microlocation of starting place.
Because pilots legs are too busy runnung at this point, the ruder pedals are free. They are forced into center position via bungee cords or springs. But in the same time they are connected with side stick mounted at left side of cockpit, so any action to rudder is available instantly. Because the ruder is aerodynamically balanced, the stick forces should not be to big. When pilot retract his legs into cockpit, they can take over ruder pedals, and side stick is not necessary to use anymore.
REM: a pilot is responsibile for the safety of his flight and on him is to choose the weather situation apropriate for his glider type and his piloting skilss and of course the type of flying he intend to do. If he made a bad decision, even fine plane can act as bad and leed to disaster.
ABOUT ANGLE OF ATTACK AT START
In my previous writting I mentioned that it is not important to controll the wing angle of attack…(I claim this only for my design and not for general at all) Well, in my configuration the pilot is really free of concernig about the angle of attack during start phase, because the design of fuselage take this responsibility. As one of my sketches show, the pilot in the cockpit is unable to produce higher angle of attack that his legs and tail resting on the ground allow.
As we remember from aerodynamical basics, the angle of attack is angle between direction of relative wind and airfoil chord line. In starting phase of foot launched sailplane (or any hang glider) the direction of relative wind is almost paralel to ground where pilot take run, so angle of attack is stable and guaranteed, until sailplane take off ground…then we fly, and angle of attack (speed) is pilot responsibility..
REM: I doubt about effectivines of elevator because of downwash produced by flaps…?!? L ! Still loot of required thinking about tail volume, location, deflections, airfoil..ooh! ..oh!!
ABOUT MOLDLESS FOAM/COMPOSITE CONSTRUCTION
It is possible to make structural parts enough lightweight, I think…
First, I say bye bye to the laminar flow around fuselage. If we want laminar flow ower fuselage, then shape must be totaly diferent than my design. In moldless construction one should achieve exact shape and streamlining without wavines, by filling, sanding, filling, sanding, filling, sanding, filling, sanding, .. and this is one of reasons for heavier structural parts. The wing in this building technique (Windrose, Rutans designs,…) is generally heavy because the foam core represent almost 1/3 of total wing weight…In my case I design hollow fuselage with inner and outer skin. Outer skin should get only such quantity of filler and micro to cover the weavines of the outer glass fabric ply and I will not bother with some shape waviness that will cause early flow separation on high performance sailplanes. We will just let them alone were they are. Let just take care, that outer skin look straight and smooth for average eye…we not intend to competite for best finish prize at one of famous Fly-Ins…we aim for pure strenght and lightweight…
The fuselage core will be reinforced with stringers of graphite (tape or rods..) and few bulkheads.
ABOUT THE FLAP DRIVE
I imagine, that flaps would be deployed only before start and at landing. It will be better (safer) for pilot to land on skid and sitting inside cockpit. It will be possible also to land with flaps in zero position. If we estimate wing loading of 13 kg/m2 and CL max =1,6 of pure airfoil, we can expect the stall speed about 40 km/h what is enough low even to land safely with this (flight) confuiguration.
The question is only how to reduce glide angle on final if field is short and it is to late to deploy flaps. I dont like to put glider into slip close to the ground…it give me strange uncorfotable feelings…Spoilers I dont like to put on the glider-one more control system and added weight…
Any solution, suggestion?
I choose electrical driven flaps simply because In my case, they wil serve only at start and at landing. Maybe some of the flaps will be needed during some thermal circling..
I think that one battery (usually used for electrical driven model airplanes, say 2000 mAh and 150W cheap can motor, with homemade gears) can serve during flight and landing demands. Before start, when preparing the sailplane for flight, pilot can mannually drive mechanism to flaps be deployed, and save some battery power for later (in flight) use. The battery can be easily recharged with use of charger and automotive battery in short time, when preparing for next flight…