2013 already?

Wow, time does fly!..  yet it's ironic that there's never enough time to go flying.  Workload demands of my business have been extreme, and any spare time I have had was spent getting a few hang glider flights in. 

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 !!

Damping Spirits and Chasing Tail

After going through the dynamic properties of the preliminary Carbon Eagle design, it is evident that a near-plank type layout provides very little pitch damping... and this leads to some interesting design compromises.

First off, a pitch damping primer:
Pitch damping is different than pitch stability. Imagine that the suspension in your car consisted only of coil springs... no shock absorbers. You would not have any trouble on a perfectly smooth road, but roads are never perfectly smooth. Once you hit a bump, this springs-only car will continue to rock back and forth for a long time.. It has stability, but little damping, so it overshoots the neutral point and causes a lot of uncomfortable oscillations.. If the bumps are big enough and spaced properly, the overshoot from the last bump is added to the next bump, then the next... at the right bump amplitude and frequency, the car will actually flip! NOT GOOD! So we add shock absorbers to add damping, greatly reducing, if not eliminating overshoot.. Now the car will not oscillate or flip, and driving over that same series of bumps turns into a comfortable, safe experience.

The same principle applies to an aircraft. Flying in smooth air is OK with low pitch damping, but add a few bumps --that is, turbulence, thermals and wind gusts-- and the pilot will begin to loose control... perhaps even flipping over in some conditions. Again.. NOT GOOD!

To make matters worse, slow-flying aircraft are more affected by these bumps than faster aircraft. For example: A pitch stable, under-damped glider is cruising along safely at 25 mph, when it suddenly encounters a 5 mph vertical updraft. Its angle of attack, or pitch angle, increases by 11 degrees, which approximately doubles its lift force (+2Gs). Since it's a pitch-stable wing, it will pitch back down towards the neutral pitch angle, but it's under-damped, so it keeps pitching down well past past the nuetral point, just like the car with no shocks.. Now it encounters a 5 mph downdraft .. the result: sudden strong negative Gs.. A slow, under-damped glider can very quickly become uncontrollable in strong turbulence, perhaps even resulting in structural failure in extreme conditions. A faster under-damped glider cruising at 50 mph would experience only half that angle-of-attack change as the 25 mph glider in the same flying conditions, so its induced pitch oscillation is reduced, and it therefore has far fewer risks imposed by low pitch damping in turbulent flying conditions.

So, even if a higher speed plank-type sailplane design (i.e. Marske Pioneer) may be successful, you cannot conclude that the same design concept will be successful when implemented as a much slower flying hang glider.

As a result, I've decided the Carbon Eagle is going to have a tail! This is a major change to the design trade space. Now that some of the pitch stability can be shifted to a tailplane, positive pitching moment airfoil sections are no longer needed. We can now use neutral, even slightly negative pitching moment airfoil sections, which are generally higher performing, allowing a smaller wing area, so that's good news. However, since this is a foot-launched glider, static balance is very important, so the tail plane should be kept as light and small as possible. Therefore, the highly negative section pitching moments are still undesirable, as they drive the need for a larger horizontal stabilizer area and/or tail boom length.

By the way, my first hang glider in 1975, the Quicksilver B, had a large tail and could not be launched without a tail-runner. Its single surface airfoil had a highly negative pitching moment, and the somewhat crude tail structure was fairly heavy aluminum.. not optimally engineered for minimum weight. Because we will be utilizing lightweight composite materials, a lower pitching moment wing, and more engineering effort, the Carbon Eagle will have a much smaller and lighter tail in order to meet the requirement of being foot-launchable without assistance... so after 35+ years, I'm coming around full circle .. back to a tailed glider!

General Layout

The Eagle design concept has been moving ahead on paper, and even a bit in hardware. I got some ideas from studying a few old hang glider designs, especially Bill Wolf's Valkyrie, and Klaus Hill's Fledgling/Voyager. Some sweepback is desirable to get the benefit of some directional stability and yaw/roll control from a winglet & tip rudder arrangement, but keeping the sweep angle to a minimum is important for attaining a lightweight structure. A minimal amount of washout will be used to avoid tip stalls and maintain good lift distribution throughout the speed range, as seen below.



Since washout and sweep will be very modest, the main source of pitch stability must be a positive pitching moment airfoil section. This might be something like the Eppler E 335, or Hepperle MH 78... exact airfoil section(s) to be selected is TBD. I have more-or-less settled these dimensions for the Eagle:


Sweep = 5 degrees (1/4 chord)
Root Chord = 5 feet
Tip Chord = 3 feet
Wing Span = 40 feet (w/o winglets)
Wing Area = 160 sq. ft. (w/o winglets)
Winglet Height = 4 feet
Winglet Cant = 30 degrees
Winglet Tow-in = 1.5 degrees
Dihedral = 2 degrees
Washout = 2.5 degrees



Here is a picture of the planform with MAC and CG location... happy landings, all!!

The Carbon Eagle

Here is my first public disclosure of the Carbon Eagle Ultralight Aircraft development project. It is based on various ideas that have been bouncing around in my head for almost 3 decades. Now the design has evolved and matured to the point where it has become unbearable to keep it in any longer. I didn't really need to start a new project at such a busy time in my life.. it just sort of happened.

Basically, what I'm now calling the Carbon Eagle is an unpowered ultralight aircraft -- call it a rigid wing hang glider, or a foot-launchable microlift sailplane. The Carbon Eagle will be somewhat along the lines of the AIR Atos, Rupert Composites Archeopteryx, Brightstar SWIFT/Millenium, and Exulans... it's also somewhat similar to ultralight sailplanes such as Jim Maupin's Carbon Dragon, Jim Marske's Monarch, Danny Howell's Lighthawk, Klaus Hill's Ultrafloater, and D.F. Farrar's Bird Flight Machine... Google searches on these names may give you some idea of what I'm shooting for. The Carbon Eagle design goals are:

• Empty weight near 60 lbs; roughly the same as tube, cable & sailcloth flexwing hang gliders
• Glide ratio in excess of 25:1 with a minimum sink rate less than 2 ft/sec.
• Statically balanced and fully controllable in foot-launch situations.
• Readily foldable and car-top transportable, just like a regular hang glider
  
• Less than 25 foot altitude loss in 360 degree, small radius turns
• Tracks well in thermals
• Low control forces, not physically difficult to fly
  
• Stable and controllable in gusty conditions
• Stall-spin resistant
• Reasonable cost.. approximately $5,000
  
• Open air or enclosed cockpit options

Some of the glider designs mentioned above may meet some of these goals, but none can meet them all.. that's the challenge of the Carbon Eagle. I fully expect widespread skepticism from experts and amateurs alike that meeting all these design goals is even possible. So be it. The goals drive the rough design specifications of the Carbon Eagle to be:

1. Wing span in excess of 40 feet
2. Wing loading of approximately 2.0 lbs. per square foot
3. Wing area of approximately 150 square feet
4. Cable-braced "flying plank" tailless configuration
5. Mixture of metal tubing and carbon composite spars
6. Carbon composite molded wing ribs attached to wing spar
   
7. Airfoil leading edge molded to shape
8. Remainder of the wing skin is flexible sailcloth with sewn rib pockets
   
9. Aerodynamic roll & yaw controls
   
10. Weight-shift pitch control
    

I'll be posting pictures, descriptions, and progress updates as time permits. Constructive comments and offers of help are ALWAYS welcome!

Happy Landings,

Dan