Performance glider scratch-build: delta wing or traditional/swept wing planform?

[Rktman]

As a sport flier I've always built my gliders for strength and durability instead of performance (i.e. low weight), which I realize most glider enthusiasts would say is almost a contradiction in terms. So this time I'd like to focus on performance and lower weight.

My immediate thought was using a delta wing planform because of its larger surface area to provide lift, its lower wing loading, and greater strength due to its shape vs traditionally shaped wings. Also a delta wing is naturally stable in pitch so it doesn’t require a separate tail, which further saves weight.

The counter argument is that a delta has more viscous drag compared to a high aspect ratio wing, and swept wings have a better lift to drag ratio, which is why you don't see delta wings in full size conventional aircraft except in supersonic jets, where they have an advantage.

Having said that, at the smaller scale and Reynolds Numbers of BGs and RGs, how much of an advantage or disadvantage would a delta wing have?

 

[watheyak]

At slow speed Delta wings make exponentially more induced drag.

What kind of performance are you looking for? Will this be R/C? Will you be trying to catch thermals and stay in them by making tight, efficient turns?

If that's the case, then a Delta planform is not the most efficient shape. They require a large angle of attack at slow speed which creates lots of induced drag. Add a high bank turn while trying to core a tiny thermal and drag is through the roof.

 

[Rktman]

At slow speed Delta wings make exponentially more induced drag.

What kind of performance are you looking for? Will this be R/C? Will you be trying to catch thermals and stay in them by making tight, efficient turns?

If that's the case, then a Delta planform is not the most efficient shape. They require a large angle of attack at slow speed which creates lots of induced drag. Add a high bank turn while trying to core a tiny thermal and drag is through the roof.

Valuable feedback, thanks! This would be a free flight glider, not RC controlled. Part of what got me thinking about deltas was the Edmonds Deltie, which glides surprisingly well...though in retrospect it's probably not a true delta wing, but more of a canard. The other glider kit that came to mind was the Delta Katt, though I don't know if they were ever used in competition like the Delties were.

 

[GuyNoir]

So this time I'd like to focus on performance and lower weight.

You might want to take a look at the plans at the bottom of this page:

https://www.nar.org/contest-flying/competition-guide/duration-events/boostglide-duration/
As for weight, the lighter balsa you use in gliders, generally the better the performance. You have to trade the weight off vs. the strength required to keep the wings together for the power you're planning to use.

 

[Rktman]

You might want to take a look at the plans at the bottom of this page:

https://www.nar.org/contest-flying/competition-guide/duration-events/boostglide-duration/
As for weight, the lighter balsa you use in gliders, generally the better the performance. You have to trade the weight off vs. the strength required to keep the wings together for the power you're planning to use.

Thanks that was my first stop. I'm planning on designing for a 13mm motor, or an 18mm A at most. Since this is my first foray into building something for duration instead of durability (but still for sport flying) I'm open to experimentation, and may even try a built-up wing. I hope to get enough feedback from the experts here to decide on a planform.
I don't have the tools or experience to carve/form anything exotic in terms of an airfoil or employing composites like FG or CF, which is why I was looking at alternative wing and glider shapes like deltas that aren't often seen.

 

[GuyNoir]

may even try a built-up wing.

Those can be tricky. Using a structure similar to any free flight model airplane, I was never able to get enough strength in the wing to keep it from shreading under power. I did have success with a structure I called "semi-built up" I used solid balsa from the leading edge to the high point of the airfoil, ribs from the high point to the trailing edge and a solid trailing edge. If you make the trailing edge out of REALLY hard balsa, you can get those to work. In the end, I decided the weight savings weren't enough to justify the increased building time and risk of a shread.

 

[Rktman]

Those can be tricky. Using a structure similar to any free flight model airplane, I was never able to get enough strength in the wing to keep it from shreading under power. I did have success with a structure I called "semi-built up" I used solid balsa from the leading edge to the high point of the airfoil, ribs from the high point to the trailing edge and a solid trailing edge. If you make the trailing edge out of REALLY hard balsa, you can get those to work. In the end, I decided the weight savings weren't enough to justify the increased building time and risk of a shread.

Are swept wings any less vulnerable to shredding?

That was one reason why I started looking into a Delta planform, but if it generates more drag in comparison to other wing shapes, then I'll drop it from consideration.

 

[GuyNoir]

Are swept wings any less vulnerable to shredding?

Not that I can notice.

That was one reason why I started looking into a Delta planform, but if it generates more drag in comparison to other wing shapes, then I'll drop it from consideration.

These little models fly in aerodynamic regions that are not well studied. The better glider, in my experience, :

(a) have the right strength to weight ratio in their flying surfaces;
(b) are built straight, without warps;, and
(c) have been carefully and properly trimmed.

The rest of the glider discussions I see re: finishing techniques, carbon fiber or not carbon fiber reinforcements, etc. are simply window dressing.

My $0.02; YMMV.

 

[Rktman]

Not that I can notice.



These little models fly in aerodynamic regions that are not well studied. The better glider, in my experience, :

(a) have the right strength to weight ratio in their flying surfaces;
(b) are built straight, without warps;, and
(c) have been carefully and properly trimmed.

The rest of the glider discussions I see re: finishing techniques, carbon fiber or not carbon fiber reinforcements, etc. are simply window dressing.

My $0.02; YMMV.

Appreciate your feedback. I'm sure anyone responding has far more experience designing and flying free flight and RC planes/gliders than me.

I've built a lot of kits and clones and tried to soak up as much as I could from free flight design publications and online articles and forums, but only flown a handful of my own design (all optimized for strength/durability) but since I want to now explore design for performance, I'm pretty much in uncharted territory. So input, opinion, guidance is welcome.

 

[BABAR]

Intuitively (which as we know is not always confirmed in REALITY) I would expect deltas to be less likely to shred for two reasons. Relative to the same surface area compared to more “arm-like” non-delta wings

1. Delta wings have a shorter semi-span, so less moment arm of force or leverage on the lateral edge of the fin to break it off

2. Much longer attachment root would expect to be stronger.







Nomenclature from

https://www.rocketryforum.com/threads/fin-surface-nomenclature.10393/
Post 3. I guess they use “semi-span” instead of just “span” under expectation that there would be pair fins or wings, on opposite sides.

1. Root Chord
2. Sweep Angle
3. Sweep Length
4. Tip Chord
5. Body Tube Diameter
6. Semi Span
LE Leading edge
TE Trailing Edge

 

[Rktman]

Intuitively (which as we know is not always confirmed in REALITY) I would expect deltas to be less likely to shred for two reasons. Relative to the same surface area compared to more “arm-like” non-delta wings

1. Delta wings have a shorter semi-span, so less moment arm of force or leverage on the lateral edge of the fin to break it off

2. Much longer attachment root would expect to be stronger.






View attachment 420461
Nomenclature from

https://www.rocketryforum.com/threads/fin-surface-nomenclature.10393/
Post 3. I guess they use “semi-span” instead of just “span” under expectation that there would be pair fins or wings, on opposite sides.

1. Root Chord
2. Sweep Angle
3. Sweep Length
4. Tip Chord
5. Body Tube Diameter
6. Semi Span
LE Leading edge
TE Trailing Edge

Thanks for the definitions list BABAR. As I'm a visual type (a "show me" vs a "tell me" person) this really helps, and will be going into my library of glider definitions.

 

[fulldec]

Eric, I know you've build a lot of gliders (including the FAI S4A swing/flop wing). I'd suggest just building competition designs published in the past, or at least using them as guides for your own designs. The NAR website has some plans for BGs (including Gregorek's design rules for gliders): https://www.nar.org/contest-flying/competition-guide/duration-events/boostglide-duration/
and RGs: https://www.nar.org/contest-flying/competition-guide/duration-events/rocketglider-duration/

While having some variations, they will tend to be lighter, lower drag, and more fragile in design. They will tend to be more optimized for a particular motor class. There will also be a big variety of pop pod attachment schemes. Try a few, or if you are happy with a particular design, adapt it to other glider designs.

I honestly don't think there is a killer design (the FAI S4A design may be up there). One of the best things you can do is find a design you like and build it a lot. This will get you: improving your construction techniques (alignment, jigs for repeatability in the build); consistent trimming; picking good, light balsa; and then picking good air to fly in.

I'd suggest building for 1/2 A BG, time the flights and enter the flights in the upcoming Virtual NARAM 1/2A BG event (https://www.nar.org/site/virtual-naram/) and see how they fare against others. Also, the NAR's ongoing official NRC 1/2A BG event is another way to benchmark your glider's performance.

Good luck, I'm happy to see you going down this path.

Don

 

[Rktman]

Intuitively (which as we know is not always confirmed in REALITY) I would expect deltas to be less likely to shred for two reasons. Relative to the same surface area compared to more “arm-like” non-delta wings

1. Delta wings have a shorter semi-span, so less moment arm of force or leverage on the lateral edge of the fin to break it off

2. Much longer attachment root would expect to be stronger.






View attachment 420461
Nomenclature from

https://www.rocketryforum.com/threads/fin-surface-nomenclature.10393/
Post 3. I guess they use “semi-span” instead of just “span” under expectation that there would be pair fins or wings, on opposite sides.

1. Root Chord
2. Sweep Angle
3. Sweep Length
4. Tip Chord
5. Body Tube Diameter
6. Semi Span
LE Leading edge
TE Trailing Edge

While the feedback so far doesnt seem to favor deltas for high-performance use in BGs, I'm going to do a "just-for-the-fun-of-it" side project using a delta wing, (as it appeals to my habit of building for strength) for the reasons you mentioned: that delta planforms have a longer root attachment edge and overall greater strength due to shape vs traditionally shaped wings. Also it doesn’t require a tail, which further saves weight.

That being said, my main project focus is still on building a lightweight glider optimized for performance/duration.

 

[BABAR]

From a sport flying perspective (especially free flight) there are DEFINITELY disadvantages to have TOO good a performance, especially if you don’t have infinite field size and infinitely long legs!

 

[BABAR]

And delta wings are intrinsically cooler!

 

[Rktman]

Eric, I know you've build a lot of gliders (including the FAI S4A swing/flop wing). I'd suggest just building competition designs published in the past, or at least using them as guides for your own designs. The NAR website has some plans for BGs (including Gregorek's design rules for gliders): https://www.nar.org/contest-flying/competition-guide/duration-events/boostglide-duration/
and RGs: https://www.nar.org/contest-flying/competition-guide/duration-events/rocketglider-duration/

While having some variations, they will tend to be lighter, lower drag, and more fragile in design. They will tend to be more optimized for a particular motor class. There will also be a big variety of pop pod attachment schemes. Try a few, or if you are happy with a particular design, adapt it to other glider designs.

I honestly don't think there is a killer design (the FAI S4A design may be up there). One of the best things you can do is find a design you like and build it a lot. This will get you: improving your construction techniques (alignment, jigs for repeatability in the build); consistent trimming; picking good, light balsa; and then picking good air to fly in.

I'd suggest building for 1/2 A BG, time the flights and enter the flights in the upcoming Virtual NARAM 1/2A BG event (https://www.nar.org/site/virtual-naram/) and see how they fare against others. Also, the NAR's ongoing official NRC 1/2A BG event is another way to benchmark your glider's performance.

Good luck, I'm happy to see you going down this path.

Don

Thanks for the helpful/useful advice, all definitely actionable. The Virtual NARAM is a really interesting development, making it far easier to participate for those of us who find travel prohibitive or difficult for one reason or another. Although I favor the "old school" tradititional balsa glider designs, my personal favorite for most efficient, effective and clever design are the swing/flops. Love 'em. They're not all that difficult to build or source materials for, and certainly easier to construct and fly than the RC controlled S8E gliders, which are above my skill level and pocketbook at the moment). At any rate, this is my attempt to step away from my comfort zone of building strictly for durability in favor of performance (even if I initially shred a few in the learning/experience process). Curious to see where and how far I can take this.

 

[Rktman]

And delta wings are intrinsically cooler!

True dat!

 

[Rktman]

Follow up question: my understanding is that polyhedral wings perform slightly better than dihedral, though their structural integrity is also a tiny bit less. Given this, does the advantage still outweigh the disadvantage? Or is the difference so small it's not worth considering? Just asking because I don't recall seeing any polyhedral wings in any of the glider plans I've looked at on the NAR website, in club newsletters, or in past publications like Sport Rocketry, Model Rocketeer, American Spacemodeling, etc.

 

[fulldec]

Follow up question: my understanding is that polyhedral wings perform slightly better than dihedral, though their structural integrity is also a tiny bit less. Given this, does the advantage still outweigh the disadvantage? Or is the difference so small it's not worth considering? Just asking because I don't recall seeing any polyhedral wings in any of the glider plans I've looked at on the NAR website, in club newsletters, or in past publications like Sport Rocketry, Model Rocketeer, American Spacemodeling, etc.

There are plans out there with polyhedral wings, I've built lots with polyhedral. I couldn't tell if it was better or not. It looks better to me. It is more glue joints which adds weight and adds a little more work with potential misalignment opportunities.

Put a well built glider of either type into good air and it will go up.

Don

 

[mikec]

Given this, does the advantage still outweigh the disadvantage?

Who knows? Polyhedral wings are still commonly used in the R/C glider community, but not always. https://www.charlesriverrc.org/articles.htm

At some point conditions and construction (and for R/C, pilot skill) dominate pure aerodynamics.

All I can say is that 1) I'm no good at building FF rocket gliders that fly well and stay within my field limits, so I fly R/C; 2) my delta-wing R/C gliders have mediocre performance relative to their difficulty of construction (e.g., full built-up airfoil in a Gamma Star), and 3) polyhedral is a PITA and I don't see the need (I built my Vectoraero 'cuda three-panel instead of five-panel and it seemed fine, I am now building a Zuni with just dihedral.)

 

[Rktman]

Eric, I know you've build a lot of gliders (including the FAI S4A swing/flop wing). I'd suggest just building competition designs published in the past, or at least using them as guides for your own designs. The NAR website has some plans for BGs (including Gregorek's design rules for gliders): https://www.nar.org/contest-flying/competition-guide/duration-events/boostglide-duration/
and RGs: https://www.nar.org/contest-flying/competition-guide/duration-events/rocketglider-duration/

While having some variations, they will tend to be lighter, lower drag, and more fragile in design. They will tend to be more optimized for a particular motor class. There will also be a big variety of pop pod attachment schemes. Try a few, or if you are happy with a particular design, adapt it to other glider designs.

I honestly don't think there is a killer design (the FAI S4A design may be up there). One of the best things you can do is find a design you like and build it a lot. This will get you: improving your construction techniques (alignment, jigs for repeatability in the build); consistent trimming; picking good, light balsa; and then picking good air to fly in.

I'd suggest building for 1/2 A BG, time the flights and enter the flights in the upcoming Virtual NARAM 1/2A BG event (https://www.nar.org/site/virtual-naram/) and see how they fare against others. Also, the NAR's ongoing official NRC 1/2A BG event is another way to benchmark your glider's performance.

Good luck, I'm happy to see you going down this path.

Don

Quick question: does 1/2A refer to 13mm or 18mm motors? I've never been able to ascertain, not even by looking over the NAR Model Rocket Sporting Code.

 

[mikec]

Either as long as the motor is contest-certified. Usually there would be no advantage to using an 18mm motor, though.

 

[Rktman]

Either as long as the motor is contest-certified. Usually there would be no advantage to using an 18mm motor, though.

I should clarify: I was referencing Don's comment re: NAR competition. There's no size definition of "1/2A" (there's both 13mm and 18mm motors with that designation), not even in their Sporting Code. If I wanted to design/build something for the Virtual NAR competition, I couldn't without knowing what size motor to design it for. Or...does NAR allow use of either in competition?

 

[mikec]

Or...does NAR allow use of either in competition?

Yes. It's an impulse constraint, not a size constraint. But impulse being equal, the smaller motor has a competitive advantage since it's lighter and allows a thinner rocket, for most competition types.

For example, in D eggloft you see some people using 18mm Aerotech motors and some using 24mm Estes motors.

I know nothing about any different virtual rules.

 

[Aeronerd]

Yes. It's an impulse constraint, not a size constraint. But impulse being equal, the smaller motor has a competitive advantage since it's lighter and allows a thinner rocket, for most competition types.

For example, in D eggloft you see some people using 18mm Aerotech motors and some using 24mm Estes motors.

I know nothing about any different virtual rules.

Interesting. I was wondering about that question myself. Learned something new.

 

[Rktman]

Yes. It's an impulse constraint, not a size constraint. But impulse being equal, the smaller motor has a competitive advantage since it's lighter and allows a thinner rocket, for most competition types.

For example, in D eggloft you see some people using 18mm Aerotech motors and some using 24mm Estes motors.

I know nothing about any different virtual rules.

Thanks for clarifying that! It's been something I've wondered about for quite some time but never thought to ask.

 

[fulldec]

I'll just chime in that mikec is correct the designation is an impulse class not a motor physical-size class. The NAR competition rules don't care what physical size motors you use(that's why you don't see a reference to it in the rules). For that matter, in a 1/2A event, you could cluster or stage two 1/4A's or 4MicroMax motors if you like*!

For NAR contests, all motors used have to be on the Certified Motors list found here. There are some motors certified for sport use that are not certified for contest use. The list shows you which is and isn't. Also, it can usually be found in Sport Rocketry every year (1st issue of the year?).

As far as the unofficial Virtual NARAM we are holding this year (https://www.nar.org/site/virtual-naram/), we are following the rules in the sporting code, for the most part, and making no changes that affect your selection of motors.

Don
*As long as the event rules allow, for instance, no staging in RG.

 

[Rktman]

I'll just chime in that mikec is correct the designation is an impulse class not a motor physical-size class. The NAR competition rules don't care what physical size motors you use(that's why you don't see a reference to it in the rules). For that matter, in a 1/2A event, you could cluster or stage two 1/4A's or 4MicroMax motors if you like*!

For NAR contests, all motors used have to be on the Certified Motors list found here. There are some motors certified for sport use that are not certified for contest use. The list shows you which is and isn't. Also, it can usually be found in Sport Rocketry every year (1st issue of the year?).

As far as the unofficial Virtual NARAM we are holding this year (https://www.nar.org/site/virtual-naram/), we are following the rules in the sporting code, for the most part, and making no changes that affect your selection of motors.

Don
*As long as the event rules allow, for instance, no staging in RG.

Appreciate the links Don, and for expanding on the answer.

 

[Sooner Boomer]

While the feedback so far doesnt seem to favor deltas for high-performance use in BGs,

My buddy Braz (James Brazzel) holds the G duration record for rocket gliders. His design is a foamboard delta with canards that flip up on motor ejection. So, yes, they CAN work.

 

[Rktman]

My buddy Braz (James Brazzel) holds the G duration record for rocket gliders. His design is a foamboard delta with canards that flip up on motor ejection. So, yes, they CAN work.

I recall reading that in the "golden age" they had problems building D powered gliders that wouldn't shred. I'm amazed to hear that foam board can survive a G motor.