Mind you, I say this while still in the preliminary stage, but the reason I don't *think* yaw damping will be the critical issue is the centralized mass. Just eyeballing, it looks like more than 50% of the aircraft's mass (including pilot) will be within about 24" of the cg. So it's more like a flying bowling ball than a barbell.Yaw damping is usually more critical, though.
Hi George. I'm not minimum sizing, I'm approximate sizing, just trying to figure out how the thing will look. And unfortunately, there aren't a whole lot of aircraft in this category, so I will definitely be erring on the side of caution. As I said above, I don't have any precise performance goals in mind, so there's no need to push the boundaries. And as an UL glider, I don't have any speed limitations, nor limitations on complexity - water ballast, retractable gear, etc are all in play. It just has to be easy to build, very light, and provide adequate performance with minimal surprises.If comparing to other aircraft, for minimum sizing, I would want to know how well they perform, No use comparing to a low performance design.
Back in the wee days of the world....well maybe not that far back, the rule of thumb was if it looks right in comparison to the rest of the aircraft then you were good. If not, then you weren't. Still applies today. When looking at most general purpose aircraft , which is what most of us are flying, the overall look of the control and flying surfaces are in harmony with each other.Mind you, I say this while still in the preliminary stage, but the reason I don't *think* yaw damping will be the critical issue is the centralized mass. Just eyeballing, it looks like more than 50% of the aircraft's mass (including pilot) will be within about 24" of the cg. So it's more like a flying bowling ball than a barbell.
And the long wings should also help with damping as well. And given my lack of professional training, I am more focused on playing it safe than squeezing an extra ounce of performance out of it. I mean, just by using wet CF, I know I'm going to have a lot of variability and huge safety margins factored in from the beginning (and a good amount of necessary materials testing and verification).
But that's what preliminary sizing is for, right? Once I know roughly what it looks like, then I'll be able to figure out the details.
You know, I've heard it said that the biggest obstacle the Wright brothers faced when designing and building an airplane was not knowing what they are supposed to look like. At least I have BUGs, GOATs, Carbon Dragons, and ULF-1s to draw inspiration from, eh?
Where this deviates from normal aircraft is that I have no real mission requirements that it has to meet. Every iteration just goes through a loop of - can it be done Lighter? Can the fabrication be simpler for a homebuilder? Can you reduce the parts count? Can a flat panel or a simple curved part be used where a complex mold would normally be required?
So in a sense, it's sort of a modern composite ultralight primary glider that just pushes the performance envelope a little bit in every direction without sacrificing the core principles. The main structure, for example, is intended to be a 2-dimensional flat sheet of steel flashing, bent into the shape of a seat, and then used as a disposable mold to make a carbon fiber sandwich core, to which the spar is bolted, and the pilot is strapped. Optimal? No. Simple, light, and easy to copy in a garage as a one-off? For sure.
Also, sweep and aspect ratio of the vertical stab + rudder will influence damping via slope of the lift curve. Hence, sailplanes employ tall, straight vertical tails capped with T-tails (to enhance end-plate effect and apparent aspect ratio) which help to mitigate the low coefficient of tail volume.As I have gotten it from Lednicer and Roncz, yaw and pitch damping are a function of tail area and arm length squared, or put another way, tail volume times arm length. If you hold tail volume constant, then longer tail arms damp much better than shorter ones, even as tail area drops.
I don't know about the Backstrom, but the Horten used a washout schedule similar to that of the Prandtl-D based on Prandtl's 1933 bell-shaped lift distribution. This would indicate that it has relatively little or no adverse yaw, likely has proverse yaw, so has very little to perturb it in the yaw axis in the first place. Don't need much damping if you never wiggle that way. There's also roll-yaw coupling that we're not even acknowledging in this discussion. And I "know nothing" of drag rudders, spoilers, etc., that might have been fitted to these craft.I've often wondered how something like Backstrom's plank copes with what must be relatively weak,yaw damping. Also, what's the vertical tail volume on a Horten sailplane? ;-)
Yes, all flying tails do fine with reduced tail volume coefficients. Paz showed that in his book.Does all flying elevator reduce the need for elevator area ? Why jets getaway with just Cht of 0.4 ?
The Light aircraft needs to cope with propeller effects, e.g rotating slipstream and p-factor. Also the propeller in front is de-stabilising, and all the prop effects need to be compensated by a larger tail coefficient. All these effects are lacking on a sailplane so its tail can be smaller.Raymer's 0.04 coefficient is correct for Light Aircraft, I'm surprised that Sailplanes have half that coefficient, although I must say I have never looked at that in Sailplanes before and I can't understand why that might be.
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