Vertical tail volume coefficients - what am I missing?

[lr27]

Yaw damping is usually more critical, though.

 

[Grimace]

Yaw damping is usually more critical, though.

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.

 

[Lendo]

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.
George

 

[Grimace]

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.
George

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.

I mean, anybody who has been around aviation for a few years can look at weight, power, size, and shape and tell you within a few percent how it probably performs. That's where I'm at right now - just kind of starting to round the edges off the square to make it a circle. There's plenty of grinding left to be done though.

 

[lr27]

I meant that yaw damping is usually more,critical than pitch damping, because the moment of inertia in yaw is likely to be much higher than in pitch. Usually, all of the wing will be close,to the pitch axis, but the wing tips will be very far from the yaw axis.

--------
It might be a good idea to figure out what "adequate" performance is, in actual numbers. For instance, you could decide that the performance numbers of a 1-26 are a good goal.

 

[Doran Jaffas]

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.

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.

 

[Jerry Lytle]

Something that might have bearing on this discussion: why the vertical tail size increase so much from the early Stinson 108-1 Voyager to the huge tail on the 108-3. As far as I could see the side area of the planes were the same and the horsepower difference was only 10 horsepower

 

[Lendo]

Grimace,
Try thinking all Aircraft require the same control Areas- relatively speaking.
1. VT Area =VC *S*b/LA
VC= Volume Coefficient (O.04).
S = Wing Area
b= Wing Span
C =Cord
LA = lever arm (25%C MAC Wing to 25%C MAC VT for prliminary estimate)

HT Area =VC * S* MAC (w)/ LA
VC = (0.45 to 0.5)
MAC(w) =MAC Wing.
LA= 25%C Mac w to 25%C MAC HT.

Hundreds if not thousands of Aircraft have been assessed and that where the VC and formula have been develop.
Want smaller Tail Feathers have longer LA

 

[lr27]

Long wings INCREASE the need for damping in yaw, unless they are compensated with enough negative mass to reduce the moment of inertia*. ;-p

*Negative mass sure would be handy in many engineering applications. Too bad no one's invented it yet

 

[lr27]

P.S. Making a wing longer without increasing the yaw damping is like using shocks from a Smart Car in your Escalade.

 

[davidjgall]

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.

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.

 

[davidjgall]

It might be worthwhile to collect vertical tail volume coefficients for some flying wings like Backstrom's and Marske's efforts. That might give short tail limit-case insights and some alternative ideas that might mitigate the need for a huge vertical tail.

 

[lr27]

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? ;-)

 

[Speedboat100]

Norton was pretty modest in tail volume too.

 

[lr27]

It may have a small vertical stab, but it also has a deep rear fuselage with a sharp corner on top.

 

[davidjgall]

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? ;-)

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.

Good things to think about when contemplating tail volume coefficient, certainly!

 

[Speedboat100]

It may have a small vertical stab, but it also has a deep rear fuselage with a sharp corner on top.

Norton was well aware what he was doing.

 

[Speedboat100]

Does all flying elevator reduce the need for elevator area ? Why jets getaway with just Cht of 0.4 ?

 

[wsimpso1]

Does all flying elevator reduce the need for elevator area ? Why jets getaway with just Cht of 0.4 ?

Yes, all flying tails do fine with reduced tail volume coefficients. Paz showed that in his book.

I suspect there are other issues for jetliners. Most of their flap area is well forward (swept wings) and thus contribute little to overall airplane pitching moment. While they look long, I would bet that their range of CG in percent of MAC is not huge. They have no tractor prop issues to counter. I am certain their control responsiveness needs are lower. They might have other moderating issues as compared to EAB.

Billski

 

[Dusan]

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.

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.