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

[Maxout]

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.

For reasons beyond understanding, my flight that exceeded his record never got ratified, but here's proof that a conventional, lightly built glider can handle that power just fine. All you have to do is match model and motor:


I follow a simple philosophy that Stan Buddenbohm, Lee Hines, and Jim Buxton already figured out all we need to know about optimal wing platform. You'll not find anything that proves significantly better than a Sweepette or its derivatives. The 12" indoor versions for high ceiling flying hit about 160 mph on catapult launch. That's more than enough for our rocket powered work. Just scale the plane to generate enough drag to keep your chosen motor from overspending the airframe. End of problem.

 

[Rktman]

I seem to recall reading somewhere that swept wings have a better lift to drag ratio then non-swept, but since I haven't seen a whole lot of BG or RG plans that employ swept wings, I felt the erudite gliders pros here could verify whether that's correct or not.

 

[burkefj]

I've flown a ton of g and h motor rg flights with foam board construction.

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.

 

[Rktman]

I've flown a ton of g and h motor rg flights with foam board construction.

I stand corrected. Assumed that all your glider flights were maxed at E motors since your kits are designed to use Aerotech's E6.

 

[burkefj]

Yeah, that's just for the kits since they need to fit into a certain size box, for my own rg's I have four that I regularly fly on G motors, g-12 or G-38/40's and the Dynasoar Stack that flies on H-97's with the rg on the top.

I stand corrected. Assumed that all your glider flights were maxed at E motors since your kits are designed to use Aerotech's E6.

 

[o1d_dude]

For reasons beyond understanding, my flight that exceeded his record never got ratified, but here's proof that a conventional, lightly built glider can handle that power just fine. All you have to do is match model and motor:


I follow a simple philosophy that Stan Buddenbohm, Lee Hines, and Jim Buxton already figured out all we need to know about optimal wing platform. You'll not find anything that proves significantly better than a Sweepette or its derivatives. The 12" indoor versions for high ceiling flying hit about 160 mph on catapult launch. That's more than enough for our rocket powered work. Just scale the plane to generate enough drag to keep your chosen motor from overspending the airframe. End of problem.

Awesome video. Thanks for putting up the link to it.

I used to fly HLG and Catapult with Stan B, Lee Hines, and Bob De Shields down at Taft and Lost Hills years ago. Even wrote a column in the Digest of the NFFS called "The Tao of HLG" for a year or so. I may or may not have flown the first 26" HLG at the FF Champs...came in second in the Dawn HLG (dead air) event two years running. Bill Moody had my number, tho, and took first both times IIRC. Bill was built like a string bean with a great arm while Stan and I looked like ex-football players...or a baseball player in my case.

 

[Rktman]

Anyways, getting back on topic:
1. I recall reading somewhere that swept wings have a better lift to drag ratio then non-swept. Correct or not?

2. High aspect ratio vs low aspect ratio wings: which has a better lift to drag ratio?

3. Wouldn't a low aspect ratio wing be somewhat stronger and less apt to shred due to its wider chord?

 

[Rktman]

At the preliminary design stage. Decided to go with a lightweight design for 13mm 2-newton second class. As swept and delta wings are optimal only for transonic and supersonic flight, I elected to go with an elliptical wing planform (its shape tends to minimizes induced drag). Keeping the wingtips sharp to reduce tip vortex effects, kinda like a narrower version of the British Spitfire's wings.

The overall chord is not exactly high aspect, but not low aspect either, kind of a compromise, as high aspect ratio wings have lower induced drag (but higher parasite drag than low ratio). Kept the span reasonable to avoid shredding. Will use contest balsa if I can get some, 3/32” sanded to the best airfoil I can manage, and 1/16” for the stab and rudder, with the stab either airfoiled and mounted upside down, or with just a uniform taper mounted with a 1 degree incidence. Fuselage is 1/8” hard balsa, but if that proves to be not strong enough, I might try laminating two 1/16” balsa pieces with kevlar or tyvek cloth sandwiched in between to increase strength. Or reinforce both fuse sides with carbon fiber strip.

Used Dr. Gregorek's glider design guidelines on this one, with a bit of Frank Zaic and online research thrown in. Granted, not a substitute for experience, but I hope this project will supply some of that.

Okay, open to thoughts, tips, feedback on the direction so far?

"Icarus" Glider specs:
Wingspan: 12”
Fuselage: 10.75”
Stab span: 2.50”

 

[dosco]

Anyways, getting back on topic:
1. I recall reading somewhere that swept wings have a better lift to drag ratio then non-swept. Correct or not?

2. High aspect ratio vs low aspect ratio wings: which has a better lift to drag ratio?

3. Wouldn't a low aspect ratio wing be somewhat stronger and less apt to shred due to its wider chord?

1. No, and it's more complicated than that.

2. High AR is generally "better" in terms of L/D, however there are structural tradeoffs.

3. More or less, yes. But the answers to the questions you've been asking get into the difference between "strength" and "stiffness" which are related but not the same thing (and the importance of either is a complex discussion).

You're delving into the world of aircraft design, which is basically a giant tradespace with no optimal solutions … for every decision that seems good, there is an inevitable downside.

You must begin with a requirements statement, which you articulated in one of the earlier posts.

Your preliminary layout looks appropriate, and you may want to consider building a proven design first so as to avoid mistakes in moment arms and area distributions. If I were shooting for maximal duration, I'd stay away from flying wings and deltas.

The advice that was given about building light and straight is spot on. You also need to learn about "picking air" which is to say "how to catch a thermal." If your plane is built well enough, the thermal will take it away (which is the point, more or less, right?).

-Dave

 

[BABAR]

At the preliminary design stage. Decided to go with a lightweight design for 13mm 2-newton second class. As swept and delta wings are optimal only for transonic and supersonic flight, I elected to go with an elliptical wing planform (its shape tends to minimizes induced drag). Keeping the wingtips sharp to reduce tip vortex effects, kinda like a narrower version of the British Spitfire's wings.

The overall chord is not exactly high aspect, but not low aspect either, kind of a compromise, as high aspect ratio wings have lower induced drag (but higher parasite drag than low ratio). Kept the span reasonable to avoid shredding. Will use contest balsa if I can get some, 3/32” sanded to the best airfoil I can manage, and 1/16” for the stab and rudder, with the stab either airfoiled and mounted upside down, or with just a uniform taper mounted with a 1 degree incidence. Fuselage is 1/8” hard balsa, but if that proves to be not strong enough, I might try laminating two 1/16” balsa pieces with kevlar or tyvek cloth sandwiched in between to increase strength. Or reinforce both fuse sides with carbon fiber strip.

Used Dr. Gregorek's glider design guidelines on this one, with a bit of Frank Zaic and online research thrown in. Granted, not a substitute for experience, but I hope this project will supply some of that.

Okay, open to thoughts, tips, feedback on the direction so far?

"Icarus" Glider specs:
Wingspan: 12”
Fuselage: 10.75”
Stab span: 2.50”

View attachment 421584


View attachment 421585

Don’t fly it too close to the sun

 

[Rktman]

1. No, and it's more complicated than that.

2. High AR is generally "better" in terms of L/D, however there are structural tradeoffs.

3. More or less, yes. But the answers to the questions you've been asking get into the difference between "strength" and "stiffness" which are related but not the same thing (and the importance of either is a complex discussion).

You're delving into the world of aircraft design, which is basically a giant tradespace with no optimal solutions … for every decision that seems good, there is an inevitable downside.

You must begin with a requirements statement, which you articulated in one of the earlier posts.

Your preliminary layout looks appropriate, and you may want to consider building a proven design first so as to avoid mistakes in moment arms and area distributions. If I were shooting for maximal duration, I'd stay away from flying wings and deltas.

The advice that was given about building light and straight is spot on. You also need to learn about "picking air" which is to say "how to catch a thermal." If your plane is built well enough, the thermal will take it away (which is the point, more or less, right?).

-Dave

Thanks for the feedback, it's all good advice that I'll plan by. Most of my builds (95%) have been kits, proven designs and/or former NAR contest winners, so bases covered on that one. This will be my first attempt at scratch building for duration vs durability. Altered things somewhat to lighten things even more (contest balsa, aggressive airfoiling, minimal fuselage).

I was seriously considering going with a V-tail to eliminate the weight of the vertical stab but...maybe that's something to try once I get this version built and can judge it's performance.

 

[Rktman]

Don’t fly it too close to the sun

Building lightweight/fragile with contest balsa, so very good probability it'll rekit itself same way poor ole Icarus' wings did lol.

 

[dosco]

Thanks for the feedback, it's all good advice that I'll plan by. Most of my builds (95%) have been kits, proven designs and/or former NAR contest winners, so bases covered on that one. This will be my first attempt at scratch building for duration vs durability. Altered things somewhat to lighten things even more (contest balsa, aggressive airfoiling, minimal fuselage).

I was seriously considering going with a V-tail to eliminate the weight of the vertical stab but...maybe that's something to try once I get this version built and can judge it's performance.

"Scratch building" doesn't mean "designing from scratch" necessarily. You could build from a plan before making the leap to rolling your own.

I think you could probably avoid the need for contest balsa, but that's up to you. I suspect if you used judiciously selected hobby shop balsa (I'll give you that it's pretty random that a hobby shop will have anything "good") and properly select the wood for each component, that you'll be fine. Launching into a thermal is the main thing, workmanship that's "good enough" will be good enough.

V-tail versus cruciform...the weight will be about the same. The upside of the "V" is the reduction of damage in landings (and since you're considering an underslung vertical, a "traditional" upright vee may be better).

-Dave

 

[Aeronerd]

Interesting info. Learning some new things I guess alot has changed since my day.

 

[Rktman]

V-tail versus cruciform...the weight will be about the same. The upside of the "V" is the reduction of damage in landings (and since you're considering an underslung vertical, a "traditional" upright vee may be better).

-Dave

Assume that putting in incidence (say 1 degree or so) by shimming the V_tail's aft mounting edge would still work the same way as with a flat stab?

 

[dosco]

Assume that putting in incidence (say 1 degree or so) by shimming the V_tail's aft mounting edge would still work the same way as with a flat stab?

For pitch, yes.

When you start fiddling for yaw adjustments, that will be different.

Perhaps you want to use a "Y" tail like the freeflight discus launch guys do …? That way you can adjust pitch and yaw separately. The underslung vertical may sustain damage, but it might make things easier for you.

-Dave

 

[Maxout]

Some thoughts...probably redundant....

I don't like constant chord wings on fast things. All rockets are fast things. The Sweepette platform achieves this goal with exceptional efficiency, closely followed by the US Kid (basically a Spitfire wing).

Most people overbuild gliders. Shred a few and you'll start to figure out where the minimum structure is. It's far lighter than you'd think

Most people build their gliders much too small. A 12" glider on an A engine, built properly, with launch shockingly high. My radio controlled one is terrifyingly fast on A3-4T motors. Yes it's another Sweepette derivative.

Carbon tubes make excellent fuselages. There are excellent options out there, even tapered ones. You can save a ton of weight and gain a lot of durability with them. Pm me for details since it's outside the scope of this thread.

In all my years of flying gliders, I've never had an underslung fin break off from landing, only in storage and mishandling. What a lot of folks don't know is that if you have a relatively long pylon coming back from the pod (useful for getting a straight boost too), you can mount the fin directly in the path of the motor with no shielding, especially with composite motors. My S8E glider has the E6 motor pointed straight at the fin and I've gotten zero damage to it (mind you there's 25" between the nozzle and the fin). The underside actually has just as much soot as the top. The actual danger of top fins is on launch. Wires like to grab them and tear them off. Careful launcher configuration can mitigate these problems.

>back to lurk mode...

 

[mikec]

In all my years of flying gliders, I've never had an underslung fin break off from landing...

Landing on what surfaces? I have to land on sand or hard dirt, and I've snapped the inverted V-tail off my 'cuda at least once. Of course you may have perfected the art of landing correctly.

 

[Maxout]

Gotta say I'm not a fan of inverted V tails. That said, in non-contest environments I just catch my S8 model. Any landings should be nose down with full flaps. Dragging your rc model's tail on the ground is usually pretty brutal.

All the above said, free flight models are the main discussion here... they don't shed tails like Cudas apparently do.

Landing on what surfaces? I have to land on sand or hard dirt, and I've snapped the inverted V-tail off my 'cuda at least once. Of course you may have perfected the art of landing correctly.

 

[Aeronerd]

Really enjoying all the info being shared here.

 

[Rktman]

Some thoughts...probably redundant....

I don't like constant chord wings on fast things. All rockets are fast things. The Sweepette platform achieves this goal with exceptional efficiency, closely followed by the US Kid (basically a Spitfire wing).

Most people overbuild gliders. Shred a few and you'll start to figure out where the minimum structure is. It's far lighter than you'd think

Most people build their gliders much too small. A 12" glider on an A engine, built properly, with launch shockingly high. My radio controlled one is terrifyingly fast on A3-4T motors. Yes it's another Sweepette derivative.

Carbon tubes make excellent fuselages. There are excellent options out there, even tapered ones. You can save a ton of weight and gain a lot of durability with them. Pm me for details since it's outside the scope of this thread.

In all my years of flying gliders, I've never had an underslung fin break off from landing, only in storage and mishandling. What a lot of folks don't know is that if you have a relatively long pylon coming back from the pod (useful for getting a straight boost too), you can mount the fin directly in the path of the motor with no shielding, especially with composite motors. My S8E glider has the E6 motor pointed straight at the fin and I've gotten zero damage to it (mind you there's 25" between the nozzle and the fin). The underside actually has just as much soot as the top. The actual danger of top fins is on launch. Wires like to grab them and tear them off. Careful launcher configuration can mitigate these problems.

>back to lurk mode...

Since my design is still evolving, I think a carbon tube fuse would be a great idea to incorporate. A tapered hollow tube would not only benefit weightwise, I wouldn't have to fret about landings snapping the fuselage (something that's happened on more than one occasion).

 

[Aeronerd]

Some thoughts...probably redundant....

I don't like constant chord wings on fast things. All rockets are fast things. The Sweepette platform achieves this goal with exceptional efficiency, closely followed by the US Kid (basically a Spitfire wing).

Most people overbuild gliders. Shred a few and you'll start to figure out where the minimum structure is. It's far lighter than you'd think

Most people build their gliders much too small. A 12" glider on an A engine, built properly, with launch shockingly high. My radio controlled one is terrifyingly fast on A3-4T motors. Yes it's another Sweepette derivative.

Carbon tubes make excellent fuselages. There are excellent options out there, even tapered ones. You can save a ton of weight and gain a lot of durability with them. Pm me for details since it's outside the scope of this thread.

In all my years of flying gliders, I've never had an underslung fin break off from landing, only in storage and mishandling. What a lot of folks don't know is that if you have a relatively long pylon coming back from the pod (useful for getting a straight boost too), you can mount the fin directly in the path of the motor with no shielding, especially with composite motors. My S8E glider has the E6 motor pointed straight at the fin and I've gotten zero damage to it (mind you there's 25" between the nozzle and the fin). The underside actually has just as much soot as the top. The actual danger of top fins is on launch. Wires like to grab them and tear them off. Careful launcher configuration can mitigate these problems.

>back to lurk mode...

This is getting even more interesting. Back in my day we didn't have carbon fiber. All we had was all hard balsa or spruce sticks. No 3D printing or fiberglassing either. Things have really changed (for teh better).

 

[TheAviator]

Good looking design you have there. My only suggestion is to make sure you have at least 12 degrees of dihedral (I would go 15-18 myself), and make the vertical stab smaller. I know that sounds counter-intuitive, but it help with getting to a stable circling glide more quickly and also helps to prevent a spiral dive. I agree with Josh, as well. My boost/rocket-gliders are pretty much always the largest in class at NARAM.

 

[TheAviator]

Also, if you're looking for light and strong carbon shaft, pultruded works well, but you can't beat roll-wrapped. I've used Goodwinds Kites for that purpose before. I use the Allegro boom for some of my larger R/C rocket gliders. The untapered Excel Breeze would probably work very well for a glider this size. They are a tad on the expensive side though.

https://goodwinds.com/carbon-fiberglass/carbon/wrapped/straight-tubes.html

 

[Rktman]

Also, if you're looking for light and strong carbon shaft, pultruded works well, but you can't beat roll-wrapped. I've used Goodwinds Kites for that purpose before. I use the Allegro boom for some of my larger R/C rocket gliders. The untapered Excel Breeze would probably work very well for a glider this size. They are a tad on the expensive side though.

https://goodwinds.com/carbon-fiberglass/carbon/wrapped/straight-tubes.html

Appreciate the feedback and link Brian. I highly value comments/advice from more experienced builders and fliers out there (and while you can learn a lot from mistakes, I'd rather just do it right the first time...it's much faster. And less painful).

 

[Ez2cDave]

Appreciate the feedback and link Brian. I highly value comments/advice from more experienced builders and fliers out there (and while you can learn a lot from mistakes, I'd rather just do it right the first time...it's much faster. And less painful).

Eric,

The easiest and cheapest method to obtain carbon fiber shafts, for mid-size / larger gliders is the visit the Sporting Goods section in Wal-Mart.

Sort through the Carbon Fiber arrows in the Archery section . . . Select the lightest weight and smallest diameter and head for the Check-Out.

Once home, strip off the fletchings, remove the inserts with a little heat ( use caution not to get things too hot ) . . . Or, you can just cut them off.

BINGO . . . a CF fuselage boom . . . If you want "tapered", go to Flea Markets ( whenever they re-open ) and sort through damaged Fishing Rods !

Dave F.

 

[Rktman]

Modified things a bit.
First version uses a standard hard balsa fuselage. The alternate version uses a tapered carbon fiber rod. Waiting on the availability of the carbon rods before proceeding any further.

 

[Ez2cDave]

Eric,

Wow . . . That CG is really far aft !

Is the inverted, flat-bottom airfoil on the Stabilizer correct ?

According to this article by Dr. Gerald Gregorek, the farther Aft the CG, the less stable a glider is . . .

Dave F.

 

[Rktman]

Eric,

Wow . . . That CG is really far aft !

Is the inverted, flat-bottom airfoil on the Stabilizer correct ?

According to this article by Dr. Gerald Gregorek, the farther Aft the CG, the less stable a glider is . . .

Dave F.

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View attachment 426743

Stab airfoil is on the bottom to provide the necessary neg incidence. Alternately I could just use a symmetrical airfoil and shim the LE of the stab about a degree or so when bottom-mounted on the fuse, which is what I usually do. Thought I'd try this method this time to see if there's any difference.

Re: CG location, hmmm, I've heard CG rules of thumb where it should be located anywhere from 25% to 33% (and up to 40%) of the wing chord back from the leading edge. And per Gregorek's tech report above, best results were obtained when the CG was at 75% of the chord from the LE.
I'll check my sketches again. I was scaling the drawings up and down so the CG symbol and text callouts could have moved relative to where they were in the original sketches. Thanks for pointing it out. Good thing I "build" stuff on paper first, then move to cardboard prototypes before I actually start slicing balsa. Anyway, I got the CG location out of a chart from Gregorek's "Glider Design Rules" (see below) but now I'm wondering if that only applies in situations where the stab airfoil is on top (lifting tail)?

 

[Ez2cDave]

Anyway, I got the CG location out of a chart from Gregorek's "Glider Design Rules" (see below) but now I'm wondering if that's in situations where the stab airfoil is on top (lifting tail)?

This article ?

Dave F.