I regret not having updated this blog in so long.. but here's what has been going on with my Carbon Eagle design efforts:
I've kicked around dozens of different designs and configurations in search of the ultimate ultralight soaring machine... countless sketches, calculations, cost estimates. Then, a little over a year ago, a friend of mine found me a good deal on a Bright Star SWIFT tailless ultralight sailplane... also known as a joystick-controlled rigid wing hang glider. If you're not familiar with this design, here's a link with information:
http://aero.stanford.edu/reports/swiftarticle1991.html
This was a distraction from the Carbon Eagle design, yet at the same time, it actually help sharpen my thinking on it. I spent a lot of my spare time in the last year working to get my SWIFT (S/N 037) ready to fly.. and finally flew it in Oct. 2012. I've got to say, it was an absolute blast! Picking it up off the ground is tough.. it weighs about 130 pounds!.. This is a serious drawback, as I have calculated that a similar glider could be made weighing only about half that much. In any case, once it's considerable weight is supported on the shoulder straps, it's very well balanced and with a little wind, the joystick is very responsive and it can be ground handled fairly easily without any assistance, even in very strong winds. This is the opposite of a flexwing hang glider, which is easy to pick up, but difficult to ground handle in a moderate to strong winds without some helpers. Once the SWIFT is launched, you swing your feet up to the front bar and retract the sling seat.. you'll find the reclined flying position is extremely comfortable! No more craning your neck like you do flying prone in a flexwing. You can cruise around comfortably for hours with only fingertip pressures on the side stick.. it's an incredibly fun way to fly! Here's a YouTube video of my 1st flight: http://www.youtube.com/watch?v=NYtA4Tush44
The original Bright Star SWIFT design (like mine) has no rudder, though tip rudders have been installed on the Swift-light, now produced by Aeriane in Belgium. I found rudderless turn coordination to be a bit tricky at first.. there is no adverse yaw problem per se, but it hesitates to start turning after banking.. I eventually got used to it.. you just carry a little extra speed into the turn, roll to the desired bank angle, then pull back a bit on the stick to get the yaw rate started.. still, I think it would be nice to have tip rudders or differential spoilers to get the yaw rate started more directly.. but once in the turn, it carves very nicely.. roll and pitch response are excellent! This SWIFT flying experience has begun to affect my Carbon Eagle design decisions.. how? Well, for one thing, I like the sling seat setup very much.. I'd only change a couple of small details on that. Also, I became very comfortable with the notion of launching without direct rudder control. On a takeoff run, you really only need to have pitch & roll control authority.. that's fortunate, since foot launching has your feet fully occupied.. not able to push pedals and run at the same time. With a decent degree of directional stability that should already be inherent in any proper glider design, your wing should automatically track into the relative wind like a weather vane.... the act of foot launching can be summarized as "increase airspeed, then angle of attack until feet have nothing to step on.. all while keeping wings level".. it happens so fast in a foot-launched glider, your exact compass heading usually doesn't matter.. unlike a 747 takeoff.
With flexwing hang gliders, you have no direct control of roll.. only indirectly by means of your weight shifting, which also compels the frame and sail to warp. On my old Quicksilver, there was no roll control at all... it was rudder only, so you yaw first, which causes a roll. While this is yaw-to-roll control method is manageable once flying, it is a terrible situation during the crucial takeoff phase. Two side wire helpers and a tail runner are needed to launch into mildly ridge soarable conditions.. these 3 helpers are essentially controlling pitch and roll during takeoff.. not the pilot!!
It is a simlar situation with the old Fledgling tailless hang gliders.. the Fledges had rudder only control and side wire launch helpers were common, though it had better balance than the Quicksilvers and did not require a tailrunner.
Being tailless, the SWIFT will probably exceed the performance of ANY tailed glider of equivalent span, due mostly to the added drag of tail surfaces. Also, the SWIFT has very good static weight balance.. something a tailed aircraft cannot hope to match, although unassisted takeoffs are indeed possible with a lightweight tail, as is demonstrated regularly on the Archaeopteryx... an aircraft that costs approximately 100 times as much as my Quicksilver did!
I did notice that the SWIFT tends to wander a bit in pitch.. sort of a very mild bucking or pecking motion. Constantly correcting for those quick motions seems to be a futile excercise.. after a while, you just get used to it and merely let it hunt for whatever angle of attack it wants. The SWIFT has an aspect ratio of 11.3. The tailless SB-13 sailplane has an aspect ratio of 19.4, and its performance is excellent. However, SB-13 test pilots reported a strong pecking tendency, which they found to be quite annoying. This makes me think that the SWIFT may be near the acceptable maximum aspect ratio for a tailless aircraft with good handling properties. The ATOS series rigid wing hang gliders have less sweep and have stretched their aspect ratios to exceed that of the SWIFT, but they have also sprouted little horizontal tails to control pitch oscillation problems. In general, the narrower the chord, the stronger the severity of the pecking. This is not true of tailed aircraft.
So where does that leave the Carbon Eagle design concept? A much lighter SWIFT-inspired design with similar dimensions & performance is definitely possible, though I'd want it to be more transportable with a collapsing frame & foldable sail... similar to the Bright Star Millennium. But if you want to exceed the performance of the SWIFT while preserving good handling properties, a much larger span AND tail are probably needed. Something like this:
Fully cantilvered structure
Collapsible, foldable wings & tails
Overall length = 12 feet
Wing span = 49.2 feet
Wing area = 127 ft^2
Taper ratio = 0.62
Root chord = 3.2 feet
Tip chord = 2.0 feet
Aspect ratio = 19
Horizontal Tail:
HT span = 10.6 feet
HT area = 19 ft^2
HT taper ratio = 0.5
HT root chord = 2.4 feet
HT tip chord = 1.2 feet
HT aspect ratio = 6
Vertical Tail:
VT span = 5.4 feet
VT area = 15 ft^2
VT taper ratio = 0.5
VT root chord = 3.6 feet
VT tip chord = 1.8 feet
VT aspect ratio = 2
Empty weight = 83 pounds
Pilot weight = 140-240 pounds
Max. glide ratio @ 37 mph = 29:1
Est. glide ratio @ 60 mph = 19:1
Min. sink rate @ 30 mph = 100 feet per minute
Stall speed = 25 mph
Max. Speed = 80 mph
More later.. happy 2013 !!
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